Carbon dioxide angiography  is a diagnostic  radiographic  technique in which a  carbon dioxide  (CO 2 ) based contrast medium is used - unlike traditional  angiography  where the contrast medium normally used is iodine based – to see and study the  body vessels . [ 1 ]  Since CO 2  is a non-radio-opaque contrast medium, angiographic procedures need to be performed in  digital subtraction angiography  (DSA).


The use of carbon dioxide as a  contrast agent  goes back to 1920s when the gas was used to visualize retroperitoneal structures. In the 1950s and early 1960s, CO 2  was injected intravenously to delineate the right atrium for the detection of  pericardial effusion . This imaging technique developed from animal and clinical studies which demonstrated that CO 2  was safe and well tolerated with venous injections. In the early 1970s, Dr. Hawkins and Dr. Cho started using and studying CO 2  as a contrast agent also for peripheral vascular imaging and intervention. With the advent of  digital subtraction angiography  (DSA) technique in 1980s, CO 2  has evolved into a safe and useful alternative contrast agent in both arteriography and venography. Because of its lack of renal toxicity and allergic potential, CO 2  is a preferred contrast agent in patients with renal failure or iodinated contrast medium allergy, and particularly in patients who require large volumes of contrast medium for complex endovascular procedures . [ 2 ]


CO 2  angiography is intended only for peripheral procedures. In case of procedures in the arterious system it is allowed to inject CO 2  only below the diaphragm; while in the venous system it can also be investigated supradiaphragmatic, provided that the cerebral vessels are excluded. Taking this aspect into consideration, the practical approach follows that of the iodinated contrast procedures. The contrast injection can be carried out, similarly, both with manual devices and with automatic injectors (Automated Carbon Dioxide Angiography, ACDA). [ 3 ] [ 4 ]


Being naturally present in the human body, CO 2  is the only 100%  biocompatible  contrast agent, meaning no adverse reactions, such as allergy, nephrotoxicity, and hepatotoxicity.


Carbon dioxide is a negative contrast medium and it has a low  radiopacity  (while iodinated contrast media are defined as positive contrast media due to their high radiopacity). Contrast is caused by the different X-ray absorption coefficients between the tissue and the contrast agent. In the vascular imaging results produced using CO 2 , vessels look brighter rather than the surrounding tissues, because the contrast medium absorbs less X-ray radiations rather an iodine-based contrast medium, where the vessel are displayed in black.


The CO 2  does not mix with blood. At  atmospheric pressure  CO 2  is in gaseous form and, when it comes out from the catheter, it forms a train of bubbles which displaces blood, causing a transient  ischemia , in relation to the bloodstream ( systolic pressure ). When added together by DSA “stacking” software, [ 5 ]  the result is a composite diagnostic image of the frames.


Carbon dioxide is highly  soluble , allowing multiple injections without a maximum dosage (per procedure, while it is 100 mL per injection by the literature), but, in case of multiple injections, should be considered and adequate time interval between them, so to allow the gas to be expelled from the body. Compared with the oxygen, the most present gaseous substance in the body, CO 2  is more than 20 times more soluble, meaning the possibility of injecting high quantities in the body.


High  compressibility  and explosive delivery. More pressure is exerted to the gas, more its density increases, resulting in a decrease in gas volume and an increase in gas pressure. The effusion of the gas from the catheter orifice into a state of lower pressure, such as a blood vessel, leads to a sudden increase in the volume of the gas - the “explosive delivery” or “jet effect” - which could lead to an excessive stress in vessels walls. To avoid this, immediately prior to the injection of CO 2 , a flush is performed, injecting small amounts of CO 2  to reduce gas compression and guarantee gas delivery at a steady  flow rate .


CO 2  is 400 times less viscous than iodinated contrast medium, allowing its injection through devices with a very little inner lumen, as microcatheters, or, even, with other devices inserted in the catheter, as guidewires, balloons or as in atherectomy procedures. The  low viscosity  of CO 2  makes it easy to pass through small vessels, visualizing tight stenosis, collaterals, small bleedings and endoleaks in AAA procedures.


Expulsion: Once dissolved in the plasma, CO 2  is transported to the lungs and removed in a single pass by the alveoli, favoring the possibility of performing multiple injections without complications (in healthy patients, meaning no severe COPD or significant POF, especially in presence of pulmonary embolism).


Buoyancy  is defined as the tendency of a body to float when submerged into a fluid. CO 2  is lighter than blood and, therefore, floats above the bloodstream. The main advantage is represented by the simplicity of filling the more superficial (in transverse plane) vessels of the body, conversely the main disadvantage consists in a less ease of filling the deeper ones. [ 6 ]


Pins and needles/burning sensation, nausea and temporary discomfort are possible sensations during CO 2  angiography, mainly because the transient ischemia caused by the CO 2  bubbles flowing in the bloodstream. CO 2  is also neurotoxic, so brain injections should be avoided. The most feared complication for intravascular use is air embolism, which can result in stroke, myocardial infarction, paralysis, amputation, or death, although this risk across all patients is less than 1%. A large amount of CO 2  trapped in the pulmonary artery or right side of the heart (only of concern during venography) obstructs venous return resulting in bradycardia and hypotension. The patient should be rotated into a left lateral decubitus position if this happens to attempt to separate the CO 2  into a gas layer floating "on top of" and no longer interfering with the flow of the liquid and solid components of blood (vapor lock). Therefore, having a delivery system, which prevents air room diffusion, is a necessary safety measure for the patients. [ 7 ] [ 8 ]


Cargo scanning  or  non-intrusive inspection  ( NII ) refers to non-destructive methods of inspecting and identifying goods in transportation systems. It is often used for scanning of  intermodal freight   shipping containers . In the US, it is spearheaded by the  Department of Homeland Security  and its  Container Security Initiative  (CSI) trying to achieve one hundred percent cargo scanning by 2012 [ 1 ]  as required by the  US Congress  and recommended by the  9/11 Commission . In the US the main purpose of scanning is to detect  special nuclear materials  (SNMs), with the added bonus of detecting other types of suspicious cargo. In other countries the emphasis is on manifest verification, tariff collection and the identification of contraband. [ 2 ]  In February 2009, approximately 80% of US incoming containers were scanned. [ 3 ] [ 4 ]  To bring that number to 100% researchers are evaluating numerous technologies, described in the following sections. [ 5 ]


Gamma-ray   radiography  systems capable of scanning trucks usually use  cobalt-60  or  caesium-137 [ 6 ]  as a radioactive source and a vertical tower of gamma  detectors . This  gamma camera  is able to produce one column of an image. The horizontal dimension of the image is produced by moving either the truck or the scanning hardware. The cobalt-60 units use gamma  photons  with a mean energy 1.25  MeV , which can penetrate up to 15–18 cm of steel. [ 6 ] [ 7 ]  The systems provide good quality images which can be used for identifying cargo and comparing it with the manifest, in an attempt to detect anomalies. It can also identify high-density regions too thick to penetrate, which would be the most likely to hide nuclear threats.


X-ray  radiography is similar to gamma-ray radiography but instead of using a radioactive source, it uses a  high-energy   bremsstrahlung  spectrum with energy in the 5–10 MeV range [ 8 ] [ 9 ]  created by a  linear particle accelerator  (LINAC). Such X-ray systems can penetrate up to 30–40 cm of steel in vehicles moving with velocities up to 13 km/h. They provide higher penetration but also cost more to buy and operate. [ 7 ]  They are more suitable for the detection of  special nuclear materials  than gamma-ray systems. They also deliver about 1000 times higher dose of radiation to potential  stowaways . [ 10 ]


Dual-energy X-ray radiography [ 11 ]


Backscatter X-ray  radiography


Examples of  neutron activation  systems include: pulsed fast neutron analysis (PFNA), fast neutron analysis (FNA), and thermal neutron analysis (TNA). All three systems are based on neutron interactions with the inspected items and examining the resultant gamma rays to determine the elements being radiated. TNA uses thermal neutron capture to generate the gamma rays. FNA and PFNA use fast neutron scattering to generate the gamma rays. Additionally, PFNA uses a pulsed collimated neutron beam. With this, PFNA generates a three-dimensional elemental image of the inspected item.


Muon tomography  is a technique that uses  cosmic ray   muons  to generate three-dimensional images of volumes using information contained in the  Coulomb scattering  of the muons. Since muons are much more deeply penetrating than  X-rays , muon  tomography  can be used to image through much thicker material than x-ray based tomography such as  CT scanning . The muon  flux  at the Earth's surface is such that a single muon passes through a volume the size of a human hand per second. [ 12 ]


Muon imaging was originally proposed and demonstrated by Alvarez. [ 13 ]  The method was re-discovered and improved upon by a research team at  Los Alamos National Laboratory , [ 14 ] [ 15 ]  muon tomography is completely passive, exploiting naturally occurring  cosmic radiation . This makes the technology ideal for high throughput scanning of volume material where operators are present, such as at a marine cargo terminal. In these cases, truck drivers and customs personnel do not have to leave the vehicle or exit an exclusion zone during scanning, expediting cargo throughput.


Multi-mode passive detection systems (MMPDS), based upon  muon tomography , are currently in use by Decision Sciences International Corporation at Freeport, Bahamas, [ 16 ]  and the  Atomic Weapons Establishment  in the United Kingdom. [ 17 ]  An MMPDS system has also been contracted by Toshiba to determine the location and the condition of the nuclear fuel in the  Fukushima Daiichi Nuclear Power Plant . [ 18 ]


Radiological materials emit gamma photons, which  gamma   radiation detectors , also called radiation portal monitors (RPM), are good at detecting. Systems currently used in US ports (and  steel mills ) use several (usually 4) large  PVT  panels as  scintillators  and can be used on vehicles moving up to 16 km/h. [ 19 ]


They provide very little information on energy of detected photons, and as a result, they were criticized for their inability to distinguish gammas originating from nuclear sources from gammas originating from a large variety of benign cargo types that naturally emit radioactivity, including bananas,  cat litter ,  granite ,  porcelain ,  stoneware , etc. [ 4 ]  Those  naturally occurring radioactive materials , called NORMs account for 99% of nuisance alarms. [ 20 ]  Some radiation, like in the case of large loads of bananas is due to  potassium  and its rarely occurring (0.0117%) radioactive isotope potassium-40, other is due to  radium  or  uranium  that occur naturally in earth and rock, and cargo types made out of them, like cat litter or porcelain.


Radiation originating from earth is also a major contributor to  background radiation .


Another limitation of gamma radiation detectors is that gamma photons can be easily suppressed by high-density shields made from lead or steel, [ 4 ]  preventing detection of nuclear sources. Those types of shields do not stop fission neutrons produced by  plutonium  sources, however. As a result, radiation detectors usually combine gamma and neutron detectors, making shielding only effective for certain uranium sources.


Fissile materials emit neutrons. Some nuclear materials, such as the weapons usable  plutonium-239 , emit large quantities of neutrons, making neutron detection a useful tool to search for such contraband. Radiation Portal Monitors often use  Helium-3  based detectors to search for neutron signatures. However, a global supply shortage of He-3 [ 21 ]  has led to the search for other technologies for neutron detection.


A  cephalogram  is an  X-ray  of the  craniofacial  area. [ 1 ]  A  cephalometric analysis  could be used as means for measuring growth in children.


The lateral cephalogram is a profile x-ray of the skull and soft tissues and is used to assess the relation of the teeth in the jaws, the relation of the jaws to the skull and the relation of the soft tissues to the teeth and jaws. In children, growth predictions can be made and we can also determine the changes that have occurred with treatment. In adults, treatment can be predicted with varying degrees of accuracy and results quantified.


The  CTX  ( C omputer  T omography  X -ray) is an explosive detection device, a family of  x-ray  devices developed by  InVision Technologies  in 1990 that uses  CAT scans  and sophisticated  image processing  software to automatically screen checked baggage for explosives.


In 1994, the CTX-5000 became the first  computed tomography  explosive detection system certified by the  US Federal Aviation Administration  (FAA). The certification of the CTX-5000 followed nine years of development. During that time the FAA invested $90 million in explosives detection and nearly $8.6 million in the specific technology. From 1995 to 1997, the CTX-5000 was tested to solve the challenges involved in integrating an explosives detection system into a baggage system and to validate the estimated costs of wide-scale deployment of the systems.


The CTX-5000 SP scanning system, an improved version of the CTX-5000 for checked baggage, was delivered to the FAA in 1997 and placed at several of the US's busiest and largest airports. From 1997 to 2000, more than 100 of the systems have been purchased by the FAA to install in US airports, according to  InVision .


The CTX-5500DS is an automated explosives detection system that uses computed tomography to characterize materials in checked bags and automatically identify objects that could be  improvised explosive devices . The CTX-5500DS is the most widely used, FAA-certified Explosives Detection System in the world. [ 1 ]  It can be used for either standalone applications or in an integrated manner with airport baggage handling systems. It can also be configured to detect other types of contraband material. The CTX-5500DS has an FAA-certified throughput of 384 bags per hour. Its Dynamic Screening (DS) capability offers flexibility by allowing manual or automatic switching between various screening modes.


The CTX-2500 is a small-sized explosives detection system that is half the length of earlier CTX models. The CTX 2500 utilizes a single rotating X-ray source to acquire positioning images and CT-slice images, thus achieving its smaller size. The CTX 2500 system is the first FAA-certified Explosives Detection System (EDS) mounted on a truck for easy mobility and access to cargo. One of the units costs approximately US$700,000. [ citation needed ]


The CTX-9000 DSi system is the world's fastest FAA-certified (Certification moved to TSA Transportation Security Lab in 2002) Explosives Detection System, handling 542 bags per hour. It features alternate operational modes yielding even higher throughputs. The CTX-9000 DSi is designed for integrated airport installations. Its 1-metre wide conveyor coordinates with standard airport baggage handling systems. The system's architecture utilizes modular components, helping to ease scanner upgrading and servicing. The scanner contains 4 active radiation-shielding curtains. In addition, the gantry rotates at 120 RPM, enabling a slice image to be generated within half a second. A high-speed  RF  data link connects the rotating gantry to the stationary part of the unit. An air-conditioning unit ensures high performance and reliability in hot, dusty and humid airport environments.


In the late 1990s,  L-3 Communications  developed a competing computerized tomography system that also met FAA approval, however it was not TSA Qualified until late 2002. In November 1999, the FAA awarded a contract worth up to US$75 million to  L-3  to purchase up to 60 of its explosive detection systems. The  eXaminer   3DX 6000  explosive detection system developed by  L-3  operates similarly to the CTX system.


In 2013  Rapiscan Systems  made the first delivery of its RTT product. [ 2 ]  This machine operates with a non-rotating 360 degree multi source X-ray tube and detector array CT imaging system so the only moving part is the conveyor belt. This allows it to achieve a throughput of 1800 bags per hour. [ 3 ]


Recent research has evaluated the use of  computer vision  based algorithms that operate on the volumetric data used collected as CT-slice images by these and other manufacturers  computed tomography  (CT) baggage scanner machines for the automatic detection of other threat types (e.g. guns, knives, liquid containers) using 3D object classification. [ 4 ] [ 5 ]


The  radiography of cultural property  is the use of  radiography  to understand intrinsic details about objects.  Most commonly this involves  X-rays  of paintings to reveal  underdrawing ,  pentimenti  alterations in the course of painting or by later restorers, and sometimes previous paintings on the support.  Many  pigments  such as  lead white  show well in radiographs.


X-ray spectromicroscopy  has also been used to analyse the reactions of pigments in paintings. For example, in analysing colour degradation in the paintings of  Vincent van Gogh . [ 1 ]


These processes can reveal various details about objects that are not visible to the naked eye. This information, which includes structural elements, aids  conservators  as they assess object condition and consider treatment plans.


For three dimensional objects, the  computed tomography (CT)  has become a common tool, which when combined with analysis can, for example, "digitally unroll" or unfold and make possible the reading of fragile scrolls, books, or  sealed correspondence . [ 2 ]


Infrared  and  ultraviolet light  are also useful tools to understand the intrinsic details of certain objects. However, X-rays tend to be more useful for denser objects. [ 3 ]  The benefit of radiography is that it is not intrusive. Radiography does expose the object to  radiation , but these levels are low. In fact, they are much lower than the radiation levels required for medical X-rays. While technicians and staff conducting the X-ray must use protective gear, the object is not damaged during the process. [ 4 ] [ 5 ]  Furthermore, the use of radiography is widely accepted by conservators, art historians, and archaeologists. [ 5 ]  Several institutions around the world conduct radiography of objects in their collections including the  Victoria & Albert Museum  in London, England and the  Smithsonian , which operates the  Museum Conservation Institute .


Conservators and art historians have used radiography to uncover technical information about paintings. Compositions of materials, previous alterations, and painting techniques have been revealed in X-rays. [ 6 ]  This data has also been used to date works and identify forgeries. [ 7 ]  Diagnostic and therapeutic X-ray systems are generally used to produce X-rays of paintings. [ 8 ]  Infrared reflectography has also been used to see  underdrawings  and previous markings on painted canvases. [ 9 ]


Paints  are produced with a variety of  elements . Depending on how much these pigments absorb X-rays affects how clear or opaque they will appear in the radiograph, this is known as X-ray  fluorescence . [ 4 ] [ 10 ]   Lead white , for example, will absorb more rays and appear much more opaque on an X-radiograph than  carbon black , which will allow most of the X-rays to pass through resulting in a clearer result on the radiograph. [ 4 ] [ 11 ]  To produce a radiograph of a painting, the radiographic film is placed on the painted surface and the X-ray tube is placed behind the canvas. [ 12 ] [ 4 ]


An X-ray of the  Ghent Altarpiece , painted by  Jan van Eyck , in  Saint Bavo Cathedral in Ghent ,  Belgium  revealed the structure and details of the large altarpiece's painting scheme. The complete radiography of the  altarpiece  was conducted between 2010 and 2011 as part of a project largely funded by the  Getty Institute . The X-rays and other technical information that was gathered were used to prepare conservation treatments. [ 13 ]


The Sampling Officials of the Amsterdam Drapers' Guild, also known as the  Syndics of the Drapers' Guild  or more simply as the Syndics, was painted by  Rembrandt  in 1662. An X-ray of the painting revealed that Rembrandt fine-tuned the composition several times, alternating the glances between the figures and slightly changing their positions before he settled on that is known today. [ 14 ]


X-ray analysis revealed alterations to the paint of a sixteenth century portrait that had been identified as a  Bronzino  portrait of  Eleanor of Toledo  at the  Carnegie Museum of Art  in  Pittsburgh ,  Pennsylvania . After a conservation treatment, which removed the added paint, the subject of the portrait was found to be  Isabella de' Medici . The painting was also attributed to Alessandro Allori. [ 15 ] [ 16 ]


Jean-François Millet 's The Wood Sawyers at  Victoria & Albert Museum  was X-rayed, revealing that Millet had reused a canvas to complete this oil painting. The artist not only painted over a previous work, but he had also added strips of canvas to enlarge the painting area. [ 7 ]


According to X-rays taken of Twilight by  Jean Baptiste Camille Corot , also at the Victoria & Albert Museum, the artist painted over a previous picture that had not even dried yet. Pigments from the lower painting appear through cracks in the surface layer. [ 7 ]


The Old Guitarist  by  Pablo Picasso  at the  Art Institute of Chicago  had been previously examined with visible and ultraviolet light, which had hinted at the possibility of an earlier composition. X-rays of the painting revealed that Picasso had originally painted two female figures behind the guitarist. The X-rays also penetrated far enough to reveal how Picasso had prepared the wooden panel for painting. [ 8 ]


X-rays can provide a better picture of  plaster casts  and other works that rely on internal supports. However, size and mobility can often affect whether or not radiography is an option for sculptural works. X-rays can also identify cracks and previous repairs to  glass  and  ceramic  materials, which is important for assessing the condition. [ 4 ]  This information can also reveal details about the manufacturing process, which may be instrumental in providing establishing place of manufacture, and possibly also reveal  inherent vices  that are not visible to the naked eye. [ 5 ]   Jewelry  and other objects with  inlaid  pieces have been X-rayed to reveal more about their structure. [ 6 ]


The plaster cast of  Michelangelo's   David  at the Victoria & Albert Museum was X-rayed revealing that the supports in David's legs were positioned similarly to that of bones in a human leg. The size of this particular piece required a portable machine to complete the X-rays. [ 17 ]


The  Museum of Applied Arts in Vienna , the  Research Centre Seibersdorf  [ de ] , and the  New York Historical Society  have used X-rays to learn more about the manufacture of  Art Nouveau  style  glass . In particular, they are investigating differences between  Tiffany glass  of New York and Austria's Loetz glass to learn more about differences in the manufacturing process. [ 18 ]


In the X-rays of a wooden power figure at the  Indianapolis Museum of Art , conservators discovered that there were hollowed out sections through the center of the sculpture that connected three filled cavities. Information about the network inside of these sculptures has aided curators as they research the function of these pieces. The findings have led to the use of radiography to compare power figures in other collections. [ 19 ]


X-rays can reveal information about layers of  textiles  and stitching patterns. For  quilts , for instance, different textile types and other materials are used. [ 20 ]  These materials are often hidden in the finished quilt. Therefore, X-radiography can provide conservators with useful information. For other textiles X-rays can also provide conservators with information about  dyes  since metallic mordant has historically been used in the dye making process. Details about stitching patterns can also appear on X-rays. [ 20 ]  The Victoria & Albert Museum has used X-rays as a tool for several textile conservation projects.


X-rays of the  King George III  Golden Jubilee Quilt from 1810 revealed concealed  stitching  patterns and fabric dyes. [ 20 ]


Conservators learned more about the complex stitching of the Sundial Coverlet, which dates to 1797, through X-rays. [ 20 ]


X-rays were used to understand some of the stains and stitching patterns on an Egyptian tunic, dating to AD 600–799, in the Victoria & Albert collection. [ 21 ]


Hidden design and structure details were visible on X-rays of pairs of  shoes  in the V&A's collection. [ 22 ]


Archaeological materials have also benefited from X-rays. X-rays of soil segments have revealed artifacts that have eroded away, leaving them nearly undetectable to the naked eye. [ 23 ]  Worn and damaged surfaces, which appear unmarked, have yielded  inscriptions  or other markings on X-rays. [ 24 ]  Heavily corroded metal objects have also used X-rays to learn more about their original state. [ 5 ]  Industrial and medical  CT scans  have also been used by archaeologists to study a variety of artifacts. [ 25 ] [ 26 ] [ 27 ]   Underwater archaeologists  have utilized X-rays to see what is beneath layers of  concretions .


Radiography has been used with human dry bones to diagnose pathologies, demonstrate trauma and assist age estimation through dentition eruption status. [ 28 ]


X-rays have been employed to analyze what is under the wrappings of  mummies . [ 26 ]  In addition to providing images of the bones within, X-rays have revealed the location of jewelry and other objects that were buried with the body without disturbing the wrappings.


The  Herculaneum papyri  that survived the  Vesuvius  disaster were excavated and researchers have used X-rays to read their contents. Previous methods involved slowly unrolling the  papyrus , which damaged much of the  scrolls , some beyond repair. [ 29 ]


X-ray technology was used to quickly identify individual  coins  uncovered in a single container. Researchers did not have to wait for slower, traditional conservation methods to separate and decipher the coins. [ 30 ]


X-rays were among the imaging techniques used to uncover lost text on the  Archimedes Palimpsest , which is in the collection of the Walters Museum in Baltimore, Maryland. The museum spearheaded an extensive research project on the palimpsest that employed various imaging techniques including ultraviolet, infrared, and X-radiography. [ 31 ]


The heavily corroded  Antikythera mechanism , which was uncovered from a shipwreck at the beginning of the 20th century, has been X-rayed several times in an effort to understand how it works.


Digital variance angiography (DVA)  is a novel image processing method based on  kinetic imaging , which allows the visualization of motion on image sequences generated by penetrating radiations. DVA is a specific form of kinetic imaging: it requires angiographic image series, which are created by X-ray or fluoroscopic imaging and by the administration of contrast media during various medical procedures. The resulting single DVA image visualizes the path of  contrast agent  with relatively low background noise. [ 1 ]


Between 2017 and 2019, two clinical studies have been performed to investigate the clinical usability of DVA and these studies have found that it has the potential to be used for low-dose radiographic imaging and carbon-dioxide angiography in the future. [ 1 ] [ 2 ]


DVA is currently under development by Kinetic Health Ltd. and  Semmelweis University  (Budapest, Hungary).


In 2018 Gyánó M. et al. compared the quality of DVA and  DSA (digital subtraction angiography)  images in a prospective observational crossover study, which involved the analysis of 232 image pairs of 42 patients undergoing lower limb x-ray angiography (performed by using iodinated contrast agent) between February and June 2017. Methods included the measurement of SNR ( signal-to-noise ratio ) and visual quality comparison. [ 1 ]


Although other factors like spatial resolution, sharpness, and object size may contribute to image quality and object perceptibility, noise places a fundamental limitation on the ability to recognize structures on low-contrast images and that was the main reason why the SNR measurement method was chosen. The results showed 2-3 times higher SNR values in the case of DVA images compared to traditionally used DSA images, which has indicated that DVA has the potential to improve the ability to view blood vessels, since a higher SNR value indicates lower noise levels.


Qualitative comparison has been performed by three vascular surgeons and three interventional radiologists, with about 17 years of experience on the average. In an online visual questionnaire, which showed DVA and DSA image pairs of the same anatomical regions, raters were asked to choose the image which they found to be more useful for making the diagnosis. Overall, the raters judged the kinetic images better in 69% of all images. Regarding different anatomical regions, the raters agreed that the DVA was significantly better for talocrural and popliteal regions.


Since the SNR is proportional to radiation dose, the authors have concluded that the higher SNR values indicate that the DVA method has the ability to generate angiographic images which have the same quality as the currently used DSA, but the dose of the administered radiation and/or contrast media could be lowered to achieve the same vessel visibility.


In 2019 Óriás V. et al. published the results of a clinical study, which investigated the feasibility of digital variance angiography (DVA) in lower extremity  carbon-dioxide angiography  and compared the quantitative and qualitative performance of the new image processing technique to that of the current reference standard digital subtraction angiography (DSA). [ 2 ]


The study enrolled 24 patients undergoing lower limb carbon-dioxide angiography between December 2017 and April 2018 at two clinical centres in Hungary. For comparison, the signal-to-noise ratio (SNR) of DSA and DVA images were calculated and the visual quality of DSA and DVA images were also compared by independent clinical specialists using an online questionnaire.


The ratio of SNR DVA /SNR DSA was calculated and the median values for the two centres were 3.53 and 4.52. During the visual evaluation 120 DSA and DVA image pairs were compared and it was judged that the DVA provided higher quality images in both centres, in 78% and 90% of comparisons. DVA images also received consistently higher individual rating than DSA images, regardless of the research site and anatomical region.


As the authors conclude, these results have shown that in lower limb carbon-dioxide angiography DVA, regardless of the image acquisition instruments and protocols, produces higher signal-to-noise ratio and significantly better image quality than DSA, therefore this new image processing method might help the widespread use of carbon-dioxide as a safer contrast agent in clinical practice.


Several oral presentations and posters have been presented at  CIRSE  2019 conference about ongoing research projects, including the study of possible application of DVA during prostatic artery embolization and the development of new algorithms for DVA to further improve image quality, which would create a 'quality reserve' and allow the reduction of radiation and contrast media dose. [ 3 ]


A  DMSA scan  is a  radionuclide  scan that uses  dimercaptosuccinic acid  (DMSA) in assessing renal morphology, structure and function. Radioactive  technetium-99m  is combined with DMSA and injected into a patient, followed by imaging with a  gamma camera after 2-3 hours . [ 1 ]  A DMSA scan is usually static imaging, while  other radiotracers like DTPA  and MAG3 are usually used for dynamic imaging to assess renal excretion. [ 2 ]


The major clinical indications for this investigation are


Procedure: Patient is injected with 2-5 mCi of Technetium-99m DMSA intravenously and static imaging is done using Gamma camera after 2-3 hours. Imaging time is approximately 5 - 10 minutes depending on the views taken. Usually, posterior and oblique views are a must for better interpretation of the scan. Patient is asked to maintain good hydration before and after the radiotracer injection by drinking water or intravenous fluid administration, if patient cannot drink water for any reason. Usually fasting is not required for scanning purpose and patients can have light breakfast in the morning of the scan day.


The technetium-99m DMSA binds to the proximal convoluted tubules in kidney so the excretion pattern of the kidneys cannot be assessed by this for which renal dynamic scans using radiotracers like DTPA, MAG3 are used.


Flat-panel detectors  are a class of  solid-state   x-ray   digital radiography  devices similar in principle to the  image sensors  used in digital photography and video. They are used in both  projectional radiography  and as an alternative to  x-ray image intensifiers  (IIs) in  fluoroscopy  equipment.


X-rays  pass through the subject being imaged and strike one of two types of detectors.


Indirect detectors contain a layer of  scintillator  material, typically either  gadolinium oxysulfide  or  cesium iodide , which converts the x-rays into light. Directly behind the scintillator layer is an  amorphous silicon  detector array manufactured using a process very similar to that used to make  LCD  televisions and computer monitors.  Like a  TFT-LCD  display, millions of roughly 0.2 mm  pixels  each containing a  thin-film transistor  form a grid patterned in amorphous silicon on the glass substrate. [ 1 ]  Unlike an LCD, but similar to a digital camera's image sensor chip, each pixel also contains a  photodiode  which generates an electrical signal in proportion to the light produced by the portion of scintillator layer in front of the pixel. The signals from the photodiodes are amplified and encoded by additional electronics positioned at the edges or behind the  sensor array  in order to produce an accurate and sensitive digital representation of the x-ray image. [ 2 ]


Direct conversion imagers utilize  photoconductors , such as amorphous  selenium  (a-Se), to capture and convert incident x-ray photons directly into electric charge. [ 3 ]  X-ray photons incident upon a layer of a-Se generate electron-hole pairs via the internal photoelectric effect. A  bias  voltage applied to the depth of the selenium layer draw the electrons and holes to corresponding electrodes; the generated current is thus proportional to the intensity of the irradiation. Signal is then read out using underlying readout electronics, typically by a  thin-film transistor  (TFT) array. [ 4 ] [ 5 ]


By eliminating the optical conversion step inherent to indirect conversion detectors, lateral spread of optical photons is eliminated, thus reducing blur in the resulting signal profile in direct conversion detectors. Coupled with the small pixel sizes achievable with TFT technology, a-Se direct conversion detectors can thus provide high spatial resolution. This high spatial resolution, coupled with a-Se's relative high quantum detection efficiency for low energy photons (< 30 keV), motivate the use of this detector configuration for  mammography , in which high resolution is desirable to identify  microcalcifications . [ 6 ]


Flat-panel detectors are more sensitive and faster than  film . Their sensitivity allows a lower dose of radiation for a given picture quality than film. For  fluoroscopy , they are lighter, far more durable, smaller in volume, more accurate, and have much less image distortion than  x-ray image intensifiers  and can also be produced with larger areas. [ 7 ]  Disadvantages compared to IIs can include defective image elements, higher costs and lower spatial resolution. [ 8 ]


In  general radiography , there are time and cost savings to be made over  computed radiography  and (especially) film systems. [ 9 ] [ 10 ]  In the  United States ,  digital radiography  is on course to surpass use of computed radiography and film. [ 11 ] [ 12 ]


In  mammography , direct conversion FPDs have been shown to outperform film and indirect technologies in terms of resolution [ citation needed ] , signal-to-noise ratio, and quantum efficiency. [ 13 ]  Digital mammography is commonly recommended as the minimum standard for  breast screening programmes . [ 14 ] [ 15 ]


Wolfram Conrad Fuchs  (1865–1908) was a German-born  electrical engineer  who became a pioneer in  radiography . He opened the first x-ray laboratory in the United States in  Chicago , and had completed over 1400 x-ray examinations by 1896. His work was critical to the history of radiation protection. [ citation needed ]  He was the father of  Arthur Wolfram Fuchs  (1895 - 1962), the inventor of the fixed kilovoltage technique of radiography.


Fuchs was born in  Germany  in 1865 to Julius and Wilhelmina Fuchs. The family emigrated to Chicago in 1870. He returned to Germany to study electrical engineering at the  University of Berlin  and graduated in 1889. He then went on to  Paris  to study at the  Ecole des Beaux Arts . Upon returning to the United States, Fuchs completed his post-graduate work at the  Massachusetts Institute of Technology  in Boston. [ 1 ]


Shortly after  Wilhelm Röntgen 's discovery of x-rays, Fuchs was traveling in Germany and was interested in the potential implications x-rays could have for electrical engineering.  [ citation needed ] Meanwhile, in Chicago, Dr.  Friedrich Cort Hamisch  was also becoming interested in x-ray technology and had established a correspondence with Röntgen. He set up an x-ray laboratory but eventually handed it over to Dr. Otto L. Schmidt, who placed Fuchs in charge of what was eventually known as the Fuchs X-ray Laboratory. [ 2 ]


Fuchs' son,  Arthur Wolfram Fuchs , wrote in his personal correspondence that:


"[the laboratory" was a Mecca for physicians and manufacturers who wanted information regarding the machinery to use and the technic of radiography. He was, according to the men of his time, one of the outstanding radiographers. it seemed that, for a time, no one could obtain the pictures which he was able to make routinely. He was so wrapped up in his experimental work that he would often sleep in his laboratory night after night and week-ends." [ 1 ]


Fuchs was called to  Buffalo , NY to aid the dying  President William McKinley  after his assassination in 1901, even though no x-rays were ever ultimately used. [ 3 ]


Fuchs realized the radiation damage from x-ray technology before it was acknowledged by the American Medical Association.  [ citation needed ]  His own extensive experimentation with x-rays resulted in severe Roentgen-Dermatitis, requiring the eventual amputation of his fingers and thumbs on both hands. Fuchs believed that "the damage must be seen to be insignificant compared to the good that follows from this wonderful discovery," and came up with ideas to reduce the damage. On December 12, 1896, Fuchs made the following reasonable recommendations in  Western Electrician : [ 4 ]


He also added that: "The x-ray 'burn' is no more dangerous than normal burns... when the x rays encounter the skull for a longer period, the hair falls out but it grows back without any unpleasant after-effects." [ 4 ]


Also suffering from  metastatic cancer , Fuchs passed away after several operations on April 21, 1907. [ 5 ]


Industrial radiography  is a modality of  non-destructive testing  that uses  ionizing radiation  to inspect materials and components with the objective of locating and quantifying defects and degradation in material properties that would lead to the failure of engineering structures. It plays an important role in the science and technology needed to ensure product quality and reliability. In Australia, industrial radiographic non-destructive testing is colloquially referred to as "bombing" a component with a "bomb".


Industrial Radiography uses either  X-rays , produced with  X-ray generators , or  gamma rays  generated by the natural  radioactivity  of sealed  radionuclide  sources. Neutrons can also be used. After crossing the specimen, photons are captured by a  detector , such as a silver halide film, a  phosphor plate ,  flat panel detector  or  CdTe  detector. The examination can be performed in static 2D (named  radiography ), in real time 2D ( fluoroscopy ), or in 3D after image reconstruction ( computed tomography  or CT). It is also possible to perform tomography nearly in real time ( 4-dimensional computed tomography  or 4DCT). Particular techniques such as X-ray fluorescence ( XRF ), X-ray diffractometry ( XRD ), and several other ones complete the range of tools that can be used in industrial radiography.


Inspection techniques can be portable or stationary. Industrial radiography is used in  welding ,  casting  parts or  composite  pieces inspection, in food inspection and luggage control, in sorting and recycling, in  EOD  and  IED  analysis,  aircraft maintenance ,  ballistics ,  turbine  inspection, in surface characterisation, coating thickness measurement, in  counterfeit drug  control, etc.


Radiography started in 1895 with the discovery of  X-rays  (later also called  Röntgen  rays after the man who first described their properties in detail), a type of  electromagnetic radiation . Soon after the discovery of X-rays,  radioactivity  was discovered. By using radioactive sources such as  radium , far higher  photon  energies could be obtained than those from  normal   X-ray generators . Soon these found various applications, with one of the earliest users being  Loughborough College . [ 1 ]  X-rays and gamma rays were put to use very early, before the dangers of ionizing radiation were discovered. After  World War II  new isotopes such as  caesium-137 ,  iridium-192  and  cobalt-60  became available for industrial radiography, and the use of radium and radon decreased.


Gamma radiation  sources, [ 2 ]  most commonly iridium-192 and cobalt-60, are used to inspect a variety of materials. The vast majority of radiography concerns the testing and grading of welds on piping, pressure vessels, high-capacity storage containers, pipelines, and some structural welds. Other tested materials include concrete (locating  rebar  or conduit), welder's test  coupons , machined parts, plate metal, or pipewall (locating anomalies due to corrosion or mechanical damage). Non-metal components such as ceramics used in the aerospace industries are also regularly tested. Theoretically, industrial radiographers could radiograph any solid, flat material (walls, ceilings, floors, square or rectangular containers) or any hollow cylindrical or spherical object.


The beam of radiation must  be directed to the middle of the section under examination and must be normal to the material surface at that point, except in special techniques where known defects are best revealed by a different alignment of the beam. The length of  weld  under examination for each exposure shall be such that the thickness of the material at the diagnostic extremities, measured in the direction of the incident beam, does not exceed the actual thickness at that point by more than 6%. The specimen to be inspected is placed between the source of radiation and the detecting device, usually the film in a light tight holder or cassette, and the radiation is allowed to penetrate the part for the required length of time to be adequately recorded.


The result is a two-dimensional projection of the part onto the film, producing a latent image of varying densities according to the amount of  radiation  reaching each area. It is known as a radio graph, as distinct from a photograph produced by light. Because film is cumulative in its response (the exposure increasing as it absorbs more radiation), relatively weak radiation can be detected by prolonging the exposure until the film can record an image that will be visible after development. The radiograph is examined as a  negative , without printing as a positive as in photography. This is because, in printing, some of the detail is always lost and no useful purpose is served.


Before commencing a radiographic examination, it is always advisable to examine the component with one's own eyes, to eliminate any possible external defects. If the surface of a weld is too irregular, it may be desirable to grind it to obtain a smooth finish, but this is likely to be limited to those cases in which the surface irregularities (which will be visible on the radio graph) may make detecting internal defects difficult.


After this visual examination, the operator will have a clear idea of the possibilities of access to the two faces of the weld, which is important both for the setting up of the equipment and for the choice of the most appropriate technique.


Defects such as  delaminations  and  planar  cracks are difficult to detect using radiography, particularly to the untrained eye.


Without overlooking the negatives of radiographic inspection, radiography does hold many significant benefits over ultrasonics, particularly insomuch that as a 'picture' is produced keeping a semi permanent record for the life cycle of the film, more accurate identification of the defect can be made, and by more interpreters. Very important as most construction standards permit some level of defect acceptance, depending on the type and size of the defect.


To the trained radiographer, subtle variations in visible film density provide the technician the ability to not only accurately locate a defect, but identify its type, size and location; an interpretation that can be physically reviewed and confirmed by others, possibly eliminating the need for expensive and unnecessary repairs.


For purposes of inspection, including  weld inspection , there exist several exposure arrangements.


First, there is the panoramic, one of the four single-wall exposure/single-wall view (SWE/SWV) arrangements. This exposure is created when the radiographer places the source of radiation at the center of a sphere, cone, or cylinder (including tanks, vessels, and piping). Depending upon client requirements, the radiographer would then place film cassettes on the outside of the surface to be examined. This exposure arrangement is nearly ideal – when properly arranged and exposed, all portions of all exposed film will be of the same approximate density. It also has the advantage of taking less time than other arrangements since the source must only penetrate the total wall thickness (WT) once and must only travel the radius of the inspection item, not its full diameter. The major disadvantage of the panoramic is that it may be impractical to reach the center of the item (enclosed pipe) or the source may be too weak to perform in this arrangement (large vessels or tanks).


The second SWE/SWV arrangement is an interior placement of the source in an enclosed inspection item without having the source centered up. The source does not come in direct contact with the item, but is placed a distance away, depending on client requirements. The third is an exterior placement with similar characteristics. The fourth is reserved for flat objects, such as plate metal, and is also radiographed without the source coming in direct contact with the item. In each case, the radiographic film is located on the opposite side of the inspection item from the source. In all four cases, only one wall is exposed, and only one wall is viewed on the radiograph.


Of the other exposure arrangements, only the contact shot has the source located on the inspection item. This type of radiograph exposes both walls, but only resolves the image on the wall nearest the film. This exposure arrangement takes more time than a panoramic, as the source must first penetrate the WT twice and travel the entire outside diameter of the pipe or vessel to reach the film on the opposite side. This is a double wall exposure/single wall view DWE/SWV arrangement. Another is the superimposure (wherein the source is placed on one side of the item, not in direct contact with it, with the film on the opposite side). This arrangement is usually reserved for very small diameter piping or parts. The last DWE/SWV exposure arrangement is the elliptical, in which the source is offset from the plane of the inspection item (usually a weld in pipe) and the elliptical image of the weld furthest from the source is cast onto the film.


Both hold luggage and carry-on hand luggage are normally examined by  X-ray machines  using X-ray radiography. See  airport security  for more details.


Gamma radiography and high-energy X-ray radiography are currently used to scan  intermodal freight  cargo containers in US and other countries. Also research is being done on adapting other types of radiography like  dual-energy X-ray radiography  or muon radiography for scanning  intermodal  cargo containers.


The American artist  Kathleen Gilje  has painted copies of  Artemisia Gentileschi 's  Susanna and the Elders  and  Gustave Courbet 's  Woman with a Parrot .
Before, she painted in  lead white  similar pictures with differences: Susanna fights the intrusion of the elders; [ 3 ]  there is a nude Courbet beyond the woman he paints. [ 4 ]
Then she painted over reproducing the original.
Gilje's paintings are exhibited with radiographs that show the underpaintings, simulating the study of  pentimentos  and providing a comment on the old masters' work.


Many types of ionizing radiation sources exist for use in industrial radiography.


X-ray generators  produce  X-rays  by applying a  high voltage  between the cathode and the anode of an  X-ray tube  and in heating the tube filament to start the electron emission. The electrons are then accelerated in the resulting  electric potential  and collide with the anode, which is usually made of  Tungsten . [ 5 ]


The X-rays that are emitted by this generator are directed towards the object to control. They cross it and are absorbed according to the object material's  attenuation coefficient . [ 6 ]  The attenuation coefficient is compiled from all the  cross sections  of the interactions that are happening in the material. The three most important inelastic interactions with X-rays at those energy levels are the  photoelectric effect ,  compton scattering  and  pair production . [ 7 ]  After having crossed the object, the photons are captured by a  detector , such as a silver halide film, a  phosphor plate  or  flat panel detector . [ 8 ]  When an object is too thick, too dense, or its  effective atomic number  is too high, a  linac  can be used. They work in a similar way to produce X-rays, by electron collisions on a metal anode, the difference is that they use a much more complex method to accelerate them. [ 9 ]


Radionuclides  are often used in industrial radiography. They have the advantage that they do not need a supply of electricity to function, but it also means that they can't be turned off. The two most common radionuclides used in industrial radiography are  Iridium-192  and  Cobalt-60 . But others are used in general industry as well. [ 10 ]


These isotopes emit radiation in a discrete set of energies, depending on the  decay  mechanism happening in the  atomic nucleus . Each energies will have different intensities depending on the probability of a particular decay interaction. The most prominent energies in Cobalt-60 are 1.33 and 1.17 MeV, and 0.31, 0.47 and 0.60 MeV for Iridium-192. [ 11 ]  From a  radiation safety  point of view, this makes them more difficult to handle and manage. They always need to be enclosed in a shielded container and because they are still radioactive after their normal life cycle, their ownership often requires a license and they are usually tracked by a governmental body. If this is the case, their disposal must be done in accordance with the national policies. [ 12 ] [ 13 ] [ 14 ]  The radionuclides used in industrial radiography are chosen for their high  specific activity . This high activity means that only a small sample is required to obtain a good radiation flux. However, higher activity often means higher dose in the case of an accidental exposure. [ 15 ]


A series of different designs have been developed for radiographic "cameras". Rather than the "camera" being a device that accepts photons to record a picture, the "camera" in industrial radiography is the radioactive photon source. Most industries are moving from film based radiography to a digital sensor based radiography much the same way that traditional photography has made this move. [ 16 ]
Since the amount of radiation emerging from the opposite side of the material can be detected and measured, variations in this amount (or intensity) of radiation are used to determine thickness or composition of material.


One design uses a moving shutter to expose the source. The radioactive source is placed inside a shielded box; a hinge allows part of the shielding to be opened, exposing the source and allowing photons to exit the radiography camera.


Another design for a shutter is where the source is placed in a metal wheel, which can turn inside the camera to move between the expose and storage positions.


Shutter-based devices require the entire device, including the heavy shielding, to be located at the exposure site. This can be difficult or impossible, so they have largely been replaced by cable-driven projectors.


Modern projector designs use a cable drive mechanism to move the source along a hollow guide tube to the exposure location. The source is stored in a block of  shielding that has an S-shaped tube-like hole through the block. In the safe position the source is in the center of the block. The source is attached to a flexible metal cable called a pigtail. To use the source a guide tube is attached to one side of the device while a drive cable is attached to the pigtail. Using a hand-operated control the source is then pushed out of the shield and along the source guide tube to the tip of the tube to expose the film, then cranked back into its fully shielded position.


In some rare cases, radiography is done with  neutrons . This type of radiography is called  neutron radiography  (NR, Nray, N-ray) or  neutron imaging . Neutron radiography provides different images than X-rays, because neutrons can pass with ease through lead and steel but are stopped by plastics, water and oils. Neutron sources include radioactive ( 241 Am/Be and Cf) sources, electrically driven D-T reactions in vacuum tubes and conventional critical nuclear reactors. It might be possible to use a neutron amplifier to increase the neutron flux. [ 17 ]


Radiation safety is a very important part of industrial radiography. The  International Atomic Energy Agency  has published a report describing the best practices in order to lower the amount of  radiation dose  the workers are exposed to. [ 18 ] [ 19 ]  It also provides a list of national competent authorities responsible for approvals and authorizations regarding the handling of radioactive material. [ 20 ]


Shielding  can be used to protect the user of the  harmful  properties of ionizing radiation. The type of material used for shielding depends on the type of radiation being used. National radiation safety authorities usually regulate the design, commissioning, maintenance and inspection of Industrial Radiography installations. [ 21 ]


Industrial radiographers are in many locations required by governing authorities to use certain types of safety equipment and to work in pairs. Depending on location industrial radiographers may have been required to obtain permits, licenses and/or undertake special training. Prior to conducting any testing the nearby area should always first be cleared of all other persons and measures should be taken to ensure that workers do not accidentally enter into an area that may expose them to dangerous levels of radiation.


The safety equipment usually includes four basic items: a radiation survey meter (such as a Geiger/Mueller counter), an alarming dosimeter or rate meter, a gas-charged dosimeter, and a film badge or thermoluminescent dosimeter (TLD). The easiest way to remember what each of these items does is to compare them to gauges on an automobile.


The survey meter could be compared to the speedometer, as it measures the speed, or rate, at which radiation is being picked up. When properly calibrated, used, and maintained, it allows the radiographer to see the current exposure to radiation at the meter. It can usually be set for different intensities, and is used to prevent the radiographer from being overexposed to the radioactive source, as well as for verifying the boundary that radiographers are required to maintain around the exposed source during radiographic operations.


The alarming dosimeter could be most closely compared with the tachometer, as it alarms when the radiographer "redlines" or is exposed to too much radiation. When properly calibrated, activated, and worn on the radiographer's person, it will emit an alarm when the meter measures a radiation level in excess of a preset threshold. This device is intended to prevent the radiographer from inadvertently walking up on an exposed source.


The gas-charged dosimeter is like a trip meter in that it measures the total radiation received, but can be reset. It is designed to help the radiographer measure his/her total periodic dose of radiation. When properly calibrated, recharged, and worn on the radiographer's person, it can tell the radiographer at a glance how much radiation to which the device has been exposed since it was last recharged. Radiographers in many states are required to log their radiation exposures and generate an exposure report. In many countries personal dosimeters are not required to be used by radiographers as the dose rates they show are not always correctly recorded.


The film badge or TLD is more like a car's odometer. It is actually a specialized piece of radiographic film in a rugged container. It is meant to measure the radiographer's total exposure over time (usually a month) and is used by regulating authorities to monitor the total exposure of certified radiographers in a certain jurisdiction. At the end of the month, the film badge is turned in and is processed. A report of the radiographer's total dose is generated and is kept on file.


When these safety devices are properly calibrated, maintained, and used, it is virtually impossible for a radiographer to be injured by a radioactive overexposure. The elimination of just one of these devices can jeopardize the safety of the radiographer and all those who are nearby. Without the survey meter, the radiation received may be just below the threshold of the rate alarm, and it may be several hours before the radiographer checks the dosimeter, and up to a month or more before the film badge is developed to detect a low intensity overexposure. Without the rate alarm, one radiographer may inadvertently walk up on the source exposed by the other radiographer. Without the dosimeter, the radiographer may be unaware of an overexposure, or even a radiation burn, which may take weeks to result in noticeable injury. And without the film badge, the radiographer is deprived of an important tool designed to protect him or her from the effects of a long-term overexposure to occupationally obtained radiation, and thus may suffer long-term health problems as a result.


There are three ways a radiographer will ensure they are not exposed to higher than required levels of radiation: time, distance, shielding. The less time that a person is exposed to radiation the lower their dose will be. The further a person is from a radioactive source the lower the level of radiation they receive, this is largely due to the inverse square law. Lastly the more a radioactive source is shielded by either better or greater amounts of shielding the lower the levels of radiation that will escape from the testing area. The most commonly used shielding materials in use are sand, lead (sheets or shot), steel, spent (non-radioactive uranium) tungsten and in suitable situations water.


Industrial radiography appears to have one of the worst safety profiles of the radiation professions, possibly because there are many operators using strong  gamma  sources (> 2 Ci) in remote sites with little supervision when compared with workers within the  nuclear  industry or within hospitals. [ 22 ]  Due to the levels of radiation present whilst they are working many radiographers are also required to work late at night when there are few other people present as most industrial radiography is carried out 'in the open' rather than in purpose built exposure booths or rooms. Fatigue, carelessness and lack of proper training are the three most common factors attributed to industrial radiography accidents. Many of the "lost source" accidents commented on by the  International Atomic Energy Agency  involve radiography equipment. Lost source accidents have the potential to cause a considerable loss of human life. One scenario is that a passerby finds the radiography source and not knowing what it is, takes it home. [ 23 ]  The person shortly afterwards becomes ill and dies as a result of the radiation dose. The source remains in their home where it continues to irradiate other members of the household. [ 24 ]  Such an event occurred in March 1984 in  Casablanca ,  Morocco . This is related to the more famous  Goiânia accident , where a related chain of events caused members of the public to be exposed to radiation sources.


Medipix  is a family of  photon counting  and particle tracking pixel detectors developed by an international collaboration, hosted by  CERN . [ 1 ]


These are  hybrid detectors  as a  semiconductor  sensor layer is bonded to a processing electronics layer.


The sensor layer is a semiconductor, such as  silicon ,  GaAs , or  CdTe  in which the incident radiation makes an electron hole/cloud. The charge is then collected to pixel electrodes and, via bump bonds, conducted to the  CMOS  electronics layer.


The pixel electronics first amplifies the signal and then compares the signal amplitude with a pre-set discrimination level (an energy threshold). The subsequent signal processing depends on the type of device. A standard Medipix detector increases the counter in the appropriate pixel if the signal is above the discrimination level. The Medipix device also contains an upper discrimination level and hence only signals within a range of amplitude could be accepted (within an energy window).


Timepix devices offer two more modes of operation in addition to the counting. The first one is so called “Time-over-Threshold” mode ( Wilkinson type analog-to-digital converter ). It is a mode where the counter in each pixel records the number of clocks for which the pulse remains above the discrimination level. This number is proportional to the energy of detected radiation. This mode is useful for particle tracking applications or for direct spectral imaging.


The second mode of the Timepix chip is “Time-of-arrival”, in which pixel counters record time between a trigger and detection of radiation quanta with energy above the discrimination level. This mode of operation finds use in  time of flight (ToF)  applications, for instance in neutron imaging.


Every individual hit of radiation is processed by the electronics integrated in each pixel this way, therefore the device could be considered as 65 536 individual counting detectors or even spectrometers. The energy discriminators are adjustable. Therefore, scanning with their level it is possible to measure over frequency-bands of the incoming radiation; thus enabling spectroscopic x-ray imaging.


Medipix-2, Timepix, and Medipix-3 are all 256×256 pixels, each 0.055 mm (55 μm) square, forming a total area 14.08 mm × 14.08 mm. Larger area detectors can be created by bump-bonding many chips to larger monolithic sensors. Detectors of sizes from 2x2 to 2x4 chips are commonly used. Even larger, gapless areas could be created using the edgeless sensor technology. Medipix/Timepix chips each have its own sensor. These assemblies are tiled next to each other to create nearly arbitrarily sized detector arrays (the largest build using this technology has 10x10 chips, hence 14x14 cm and 2560x2560 pixels [ 2 ] ).


Photon counting pixel detectors represent the next generation of radiation imaging detectors. The photon counting technology overcomes limitations of current imaging devices. Comparison of photon counting with existing technologies is in the following table:


Medipix-1  was the first device of the Medipix family. It had 64x64 pixels of 170 μm pitch. Pixels contained one comparator (threshold) with 3-bit per-pixel offset adjustment. The minimum threshold was ~5.5 keV. The counter depth was 15-bit. The maximum count rate per pixel was 2 MHz per pixel.


Medipix-2  is the successor of Medipix-1. The pixel pitch was reduced to 55 μm and the pixel array is of 256x256 pixels. Each pixel has two discrimination levels (upper and lower threshold) each adjustable individually in pixels using a 3-bit offset. The maximum count rate is about 100 kHz per pixel (however in pixels with 9x smaller area compared to Medipix-1).


Medipix-2 MXR  is an improved version of Medipix-2 device with better temperature stability, pixel counter overflow protection, increased radiation hardness and many other improvements.


Timepix  is device conceptually originating from Medipix-2. It adds two more modes to the pixels, in addition to counting of detected signals: Time-over-Threshold (TOT) and Time-of-Arrival (TOA). The detected pulse height is recorded in the pixel counter in the TOT mode. The TOA mode measures time between trigger and arrival of the radiation into each pixel.


Medipix-3  is the latest generation of photon counting devices for X-ray imaging. The pixel pitch remains the same (55 μm) as well as the pixel array size (256x256). It has better energy resolution through real time correction of charge sharing. It also has multiple counters per pixel that can be used in several different modes. This allows for continuous readout and up to eight energy thresholds.


Timepix-3  is a successor of the Timepix chip. One of the biggest distinguishing changes is the approach to the data readout. All previous chips used the frame-based readout, i.e. the whole pixel matrix was read out at once. Timepix-3 has event-based readout where values recorded in pixels are read out immediately after the hit together with coordinates of the hit pixel. The chip therefore generates a continuous stream of data rather than a sequence of frames. The next major difference compared to the previous Timepix chip is the ability to measure the hit amplitude simultaneously with the time of arrival. Other parameters such as energy and timing resolution were also improved compared to the original Timepix chip.


Timepix-4  is the successor of the Timepix-3 chip. It has general stronger specifications for instance its time-of-arrival resolution is 195 ps, 8 times faster than Timepix-3, it also has a larger pixelmatrix of 512x448 pixels and can handle 8 times higher data rates. [ 3 ]


The digital data recorded by Medipix/Timepix devices are transferred to a computer via readout electronics. The readout electronics is also responsible for setup and control of the detector parameters. Several readout systems were developed within the Medipix collaboration


Muros was one of the first readout systems of Medipix detectors. Muros was developed at  Nikhef ,  Amsterdam ,  The Netherlands . It was relatively compact readout enabling access to all features of the detector. It allowed maximum frame rate of cca 30 frames/s with a single chip.


This electronics was developed at  IEAP - CTU ,  Czech Republic . It provides a lower frame rate compared to Muros, but the electronics was integrated into a box not larger than a pack of cigarettes. Moreover, no special PC hardware card was needed as it was in case of Muros. Therefore, the  USB  interface become quickly the most used readout within the Medipix collaboration and its partners.


Relaxd is a readout electronics developed at  Nikhef . The data is transferred to PC via 1 Gbit/s Ethernet connection. The maximum frame rate is at level of 100 frame/s.


Fitpix is the next generation of the  USB  interface developed by the group in Prague. The electronics implements the parallel Medipix/Timepix readout and therefore the maximum frame rate reaches 850 frame/s. It supports also the serial readout with frame rate of 100 frames/s.


Minipix is a miniaturized integrated chip+readout electronics device developed by  ADVACAM s.r.o.  in  Prague . The whole system has size of a  USB flash drive . Several of these devices were used on the  International Space Station  as radiation monitoring systems. [ 4 ]


Spidr3 is powerful readout electronics for the TimePix3 and MediPix3 chip. The readout rate for the MediPix3 is about 12500 frames per second and for the TimePix3 of 120 Million Hits per second. The data are transferred by a powerful 10 GB optical fiber connection. The chip and readout system is developed together with Nikhef and Amsterdam Scientific Instruments.


Both systems are developed at  Diamond Light Source , UK, for Medipix3 readout and applications at synchrotrons. Merlin is available with CdTe sensors from  Quantum Detectors  who are collaborating on further development with Diamond Light Source.


Lambda is a high-speed (2,000 fps) big area (12 chips) readout systems developed at  DESY . Lambda is available with high-Z sensor options, such as GaAs (Gallium-Arsenide) and CdTe (Cadmium-Telluride).


MARS is a gigabit Ethernet readout accommodating up to 6 Medipix 2 or Medipix 3 detectors. The electronics was developed at  University of Otago ,  Christchurch ,  New Zealand .


X-ray imaging  is the primary application field of Medipix detectors. Medipix offers to the X-ray imaging field in particular an advantage in higher dynamic range and energy sensitivity. [ 5 ]  Examples of X-ray images from selected X-ray imaging application fields are:


Timepix-based detectors from the Medipix2 Collaboration have been flown on the International Space Station since 2013, and on the first flight test (EFT-1) of NASA's new  Orion Multi-Purpose Crew Vehicle  in December 2014. Current plans call for similar devices to be flown as the primary radiation area monitors on the future initial crewed Orion missions.


The detectors may also find applications in  astronomy ,  high energy physics ,  medical imaging , and  X-ray spectroscopy .


Nuclear densitometry  is a technique used in civil construction and the  petroleum industry , as well as for mining and archaeology purposes, to measure the density and inner structure of the test material. The processes uses a  nuclear density gauge , which consists of a  radiation  source that emits particles and a  sensor  that counts the received particles that are either reflected by the test material or pass through it. By calculating the percentage of particles that return to the sensor, the gauge can be calibrated to measure the density.


In  geotechnical engineering , a  nuclear densometer  or  soil density gauge  is a field instrument used  to determine the density of a compacted material. The device uses the interaction of  gamma radiation  with matter to measure density, either through direct transmission or the "backscatter" method.  The device determines the density of material by counting the number of photons emitted by a radioactive source (cesium-137) that are read by the detector tubes in the gauge base.  A 60-second time interval is typically used for the counting period.


Different variants are used for different purposes. For density analysis of very shallow objects such as roads or walls, a  gamma  source emitter such as  137 Cesium  is used to produce gamma radiation. These isotopes are effective in analyzing the top 10 inches (25 centimeters) with high accuracy.  226 Radium  is used for depths of 328 yards (300 meters). Such instruments can help find caves or identify locations with lower density that would make tunnel construction hazardous.


Nuclear density gauges are typically operated in one of two modes: [ 1 ]


Direct transmission:  The retractable rod is lowered into the mat through a pre-drilled hole.  The source emits radiation, which then interact with  electrons  in the material and lose energy and/or are redirected ( scattered ).  Radiation that loses sufficient energy or is scattered away from the detector is not counted.  The denser the material, the higher the probability of interaction and the lower the detector count.  Therefore, the detector count is inversely proportional to material density.  A calibration factor is used to relate the count to the actual density.


Backscatter:  The retractable rod is lowered so that it is even with the detector but still within the instrument.  The source emits radiation, which then interact with electrons in the material and lose energy and/or are redirected (scattered).  Radiation that is scattered towards the detector is counted.  The denser the material, the higher the probability that radiation will be redirected towards the detector.  Therefore, the detector count is proportional to the density.  A calibration factor is used to correlate the count to the actual density.


Many devices are built to measure both the density and  moisture content  of material. This is important to the  civil construction  industry specifically as both are essential to verifying suitable soil conditions to support structures, streets, highways, and airport runways.


A nuclear densometer is used on a compacted base to establish its percentage of compaction. [ 2 ] [ 3 ]  Before field tests are performed, the technician performs a calibration on the gauge which records the 'standard count' of the machine. Standard counts are the amount of radiation released by the two nuclear sources inside the machine, with no loss or leakage. This allows the machine to compare the amount of radiation released to the amount of radiation received. With the use of a 3/4" diameter rod a hole is created in the compacted base by hammering the rod into the base to produce a hole that the densometer's probe can be inserted into. The densometer is placed on top of the hole, and then the probe is inserted into the hole by unlocking the handle at the top of the probe. One source produces radiation that interacts with the atoms in the soil, and is then compared to the standard count, to calculate the density. The other source interacts with  hydrogen  atoms to calculate the percentage of water in the soil.


In direct transmission mode, the source extends through the base of the gauge into a predrilled hole, positioning the source at the desired depth.  The testing procedure is analogous to burying a known quantity of radioactive material at a specific depth, and then using a  Geiger counter  at the ground surface to measure how effectively the soil's density  blocks the penetration of gamma radiation through the soil.  As the soil's density increases, less radiation can pass through it, owing to dispersion from collisions with electrons in the soil being tested.


Since the soil's moisture level is partly responsible for its in-place density, the gauge also contains a  neutron moisture gauge  consisting of an  americium / beryllium  high-energy  neutron source  and a  thermal neutron  detector.  The high-energy neutrons are slowed when they collide with hydrogen atoms, and the detector then counts the "slowed" neutrons.  This count is proportional to the soil's water content, since the hydrogen in this water ( H 2 O) is responsible for almost all the hydrogen found in most soils.  The gauge calculates the moisture content, subtracts it from the soil's in-place (wet) density, and reports the soil's dry density.


Nuclear density gauges can also be used to measure the density of a liquid in a pipe.  If a source is mounted on one side of a pipe and a detector on the other, the amount of radiation seen at the detector is dependent upon the shielding provided by the liquid in the pipe.   Tracerco  pioneered the use of radiation to measure density in the 1950s and determined that the  Beer–Lambert law  also applied to radiation as well as optics.  Gauges are normally calibrated using gas and a liquid of known density to find the unknowns in the equation.  Once it has been calibrated and as long as the source detector alignment remains constant, it is possible to calculate the density of the liquid in the pipe.  One factor is the  half-life  of the radioactive source (30 years for  137 Cs), which means that the system needs to be recalibrated at regular intervals.  Modern systems incorporate correction for source decay. [ 4 ]


Another variant is to use a strong neutron source like  241 americium/beryllium to produce  neutron radiation  and then measure the energy of returning  neutron scattering . As  hydrogen  characteristically slows down neutrons,  the sensor can calculate the density of hydrogen - and find pockets of underground water, humidity up to a depth of several meters, moisture content, or asphalt content.


Neutron sources can also be used to assess the performance of a  separator (oil production)  in the same way.  Gas, oil, water and sand all have different concentrations of hydrogen atoms which reflect different amounts of slow neutrons.  Using a head which contains an  241 AmBe neutron source  and a slow  neutron detector , by scanning it up and down a separator it is possible to determine the interface levels within the separator.


The  New York City Police Department  is reported to have a number of  military-grade [ 1 ]   X-ray vans  that contain  X-ray  equipment for inspecting vehicles. [ 2 ]


They are described as being able to see into walls [ 1 ]  and other vehicles using  Z backscatter  technology. [ 3 ] [ 1 ]  They are estimated to cost between $729,000 and $825,000. [ 1 ]


The NYPD has not disclosed how this technology is used as it would reveal investigation techniques, however  Police Commissioner  William Bratton  states that they are not used to scan people for weapons. [ 1 ]


According to the  New York University School of Law  Policing Project, the manufacturer of the vans is  American Science and Engineering . [ 4 ]  The product website for the van depicts a video where the van slowly drives past empty passenger cars, and in real time generates an x-ray image. [ 5 ]


The x-ray van manufacturer found that the vans expose bystanders to a 40% larger dose of ionizing radiation than the radiation delivered by airport scanners utilizing similar technology. In airports, The European Union and United States Transportation Security Administration banned the use of this type of radiation technology citing privacy and health concerns such as cancer. [ 2 ]


On December 18, 2019, the  NYCLU  submitted testimony in support of Intro. 487, the  Public Oversight of Surveillance Technology (“POST”) Act .  In it, the NYCLU cited the example of X-ray vans as a violation of privacy, and stated in general that, "Left unchecked, police surveillance has the potential to chill the exercise of First Amendment-protected speech and religious worship, intrude on Fourth Amendment-protected privacy rights, and cast entire communities under a cloak of suspicion in contravention of the Fourteenth Amendment’s guarantee of equal protection." [ 6 ]


In 2015  ProPublica  issued an Article 78 proceeding in order to have the NYPD respond to  FOIL requests [ 7 ] [ 1 ]  to give further information about the usage and health risks of the x-ray technology. Although initially the lower court granted the request, the NYPD issued an appeal and the lower court ruling was overturned. [ 1 ]


The NYPD has refused to release details of the uses and operation of these vans. [ 8 ]  The  New York Civil Liberties Union  have filed an  amici curiae  brief in support of a legal action by the journalist Michael Grabell, who is attempting to obtain more information about these vehicles. [ 2 ]


The NYU Policing project asserts that exposure to the levels of  Ionizing radiation  that are used in these vans is linked to increased rates of cancer. [ 4 ]


Orbitomeatal line  is a positioning line used in radiography of the skull. [ 1 ]  It passes through the outer  canthus  of the eye and the center of the  external auditory meatus . It is used for positioning the patient for different radiographic views including  Water's view ,  Perorbital view ,  Lateral view , and others.


Photographic plates  preceded  photographic film  as a capture medium in photography. The light-sensitive  emulsion  of  silver salts  was coated on a  glass plate , typically thinner than common window glass. They were heavily used in the late 19th century. With the spread of photographic film, the use of plates declined through the 20th. They were still used in some communities, particularly in science and  medicine , until the late 20th century.


Glass plates were far superior to film for research-quality imaging because they were stable and less likely to bend or distort, especially in large-format frames for wide-field imaging. Early plates used the wet  collodion process . The wet plate process was replaced late in the 19th century by  gelatin   dry plates .


A  view camera  nicknamed "The Mammoth" weighing 1,400 pounds (640 kg) was built by  George R. Lawrence  in 1899, specifically to photograph "The  Alton Limited " train owned by the  Chicago & Alton Railway . It took photographs on glass plates measuring 8 feet (2.4 m) × 4.5 feet (1.4 m). [ 1 ]


Glass plate photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile films were increasingly adopted. However, photographic plates were reportedly still being used by one photography business in London until the 1970s, [ 2 ]  and by one in Bradford called the Belle Vue Studio that closed in 1975. [ 3 ]  They were in wide use by the professional  astronomical  community as late as the 1990s. Workshops on the use of glass plate photography as an alternative medium or for artistic use are still being conducted.


Many famous  astronomical surveys  were taken using photographic plates, including the first  Palomar Observatory  Sky Survey ( POSS ) of the 1950s, the follow-up POSS-II survey of the 1990s, and the UK  Schmidt Telescope   survey  of southern  declinations . A number of  observatories , including  Harvard College  and  Sonneberg , maintain large archives of photographic plates, which are used primarily for historical research on  variable stars .


Many solar system objects were discovered by using photographic plates, superseding earlier visual methods. Discovery of  minor planets  using photographic plates was pioneered by  Max Wolf  beginning with his discovery of  323 Brucia  in 1891.  The first  natural satellite  discovered using photographic plates was  Phoebe  in 1898.   Pluto  was discovered using photographic plates in a  blink comparator ; its moon  Charon  was discovered 48 years later in 1978 by  U.S. Naval Observatory  astronomer  James W. Christy   by carefully examining a bulge in Pluto's image on a photographic plate.
 [ 4 ]


Glass-backed plates, rather than film, were generally used in astronomy because they do not shrink or deform noticeably in the development process or under environmental changes. Several important applications of  astrophotography , including  astronomical spectroscopy  and  astrometry , continued using plates until  digital imaging  improved to the point where it could outmatch photographic results.  Kodak  and other manufacturers discontinued production of most kinds of plates as the market for them dwindled between 1980 and 2000, terminating most remaining astronomical use, including for sky surveys. [ 5 ]


Photographic plates were also an important tool in early  high-energy physics , as they are blackened by  ionizing radiation .  Ernest Rutherford  was one of the first to study the absorption, in various materials, of the rays produced in  radioactive decay , by using photographic plates to measure the intensity of the rays.  Development of particle detection optimised  nuclear emulsions  in the 1930s and 1940s, first in physics laboratories, then by commercial manufacturers, enabled the discovery and measurement of both the  pi-meson  and  K-meson , in 1947 and 1949, initiating a flood of new particle discoveries in the second half of the 20th century. [ 6 ]


Photographic emulsions were originally coated on thin glass plates for imaging with  electron microscopes , which provided a more rigid, stable and flatter plane compared to plastic films. [ 7 ]  Beginning in the 1970s, high-contrast, fine grain emulsions coated on thicker plastic films manufactured by Kodak, Ilford and DuPont replaced glass plates. These films have largely been replaced by digital imaging technologies. [ 8 ]


The sensitivity of certain types of photographic plates to ionizing radiation (usually  X-rays ) is also useful in  medical imaging  and  material science  applications, although they have been largely replaced with reusable and computer readable  image plate  detectors and other types of  X-ray detectors .


The earliest flexible films of the late 1880s were sold for amateur use in medium-format cameras. The plastic was not of very high optical quality and tended to curl and otherwise not provide as desirably flat a support surface as a sheet of glass. Initially, a transparent plastic base was more expensive to produce than glass. Quality was eventually improved, manufacturing costs came down, and most amateurs gladly abandoned plates for films. After large-format high quality cut films for professional photographers were introduced in the late 1910s, the use of plates for ordinary photography of any kind became increasingly rare.


The persistent use of plates in astronomical and other scientific applications started to decline in the early 1980s as they were gradually replaced by  charge-coupled devices  (CCDs), which also provide outstanding dimensional stability. CCD cameras have several advantages over glass plates, including high efficiency, linear light response, and simplified image acquisition and  processing . However, even the largest CCD formats (e.g., 8192 × 8192 pixels) still do not have the detecting area and  resolution  of most photographic plates, which has forced modern survey cameras to use large CCD arrays to obtain the same coverage.


The manufacture of photographic plates has been discontinued by Kodak, Agfa and other widely known traditional makers. Eastern European sources have subsequently catered to the minimal remaining demand, practically all of it for use in  holography , which requires a recording medium with a large surface area and a submicroscopic level of resolution that currently (2014) available electronic image sensors cannot provide. In the realm of traditional photography, a small number of historical process enthusiasts make their own wet or dry plates from raw materials and use them in vintage large-format cameras.


Several institutions have established archives to  preserve photographic plates  and prevent their valuable historical information from being lost. The emulsion on the plate can deteriorate. In addition, the glass plate medium is fragile and prone to cracking if not stored correctly. [ 9 ]


The United States  Library of Congress  has a large collection of both wet and dry plate photographic negatives, dating from 1855 through 1900, [ 10 ]  over 7,500 of which have been digitized from the period 1861 to 1865. [ 11 ]
The  George Eastman Museum  holds an extensive collection of photographic plates. [ 12 ] [ failed verification ]  In 1955, wet plate negatives measuring 4 feet 6 inches (1.37 m) × 3 feet 2 inches (0.97 m) were reported to have been discovered in 1951 as part of the  Holtermann Collection . These purportedly were the largest glass negatives discovered at that time. [ 13 ]  These images were taken in 1875 by  Charles Bayliss [ 14 ]  and formed the "Shore Tower" panorama [ 15 ]  of Sydney Harbour. [ 13 ]  Albumen contact prints made from these negatives are in the holdings of the Holtermann Collection, the negatives are listed among the current holdings of the Collection. [ 14 ] [ 16 ]


Preservation of photographic plates is a particular need in astronomy, where changes often occur slowly and the plates represent irreplaceable records of the sky and astronomical objects that extend back over 100 years. The method of digitization of astronomical plates enables free and easy access to those unique astronomical data and it is one of the most popular approaches to preserve them. This approach was applied at the  Baldone Astrophysical Observatory  where about 22,000 glass and film plates of the  Schmidt Telescope  were scanned and cataloged. [ 17 ]  Another example of an astronomical plate archive is the Astronomical Photographic Data Archive (APDA) at the  Pisgah Astronomical Research Institute  (PARI).  APDA was created in response to recommendations of a group of international scientists who gathered in 2007 to discuss how to best preserve astronomical plates (see the Osborn and Robbins reference listed under Further reading).  The discussions revealed that some observatories no longer could maintain their plate collections and needed a place to archive them. APDA is dedicated to housing and cataloging unwanted plates, with the goal to eventually catalog the plates and create a database of images that can be accessed via the Internet by the global community of scientists, researchers, and students. APDA now has a collection of more than 404,000 photographic images from over 40 observatories that are housed in a secure building with environmental control.  The facility possesses several plate scanners, including two high-precision ones, GAMMA I and GAMMA II, built for NASA and the Space Telescope Science Institute (STScI) and used by a team under the leadership of the late Barry Lasker to develop the Guide Star Catalog and Digitized Sky Survey that are used to guide and direct the  Hubble Space Telescope .  APDA's networked storage system can store and analyze more than 100 terabytes of data. [ 18 ]


A historical collection of photographic plates from  Mt. Wilson observatory  is available at the  Carnegie Observatories . [ 19 ]  Metadata is available via a searchable database, [ 20 ]  while a portion of the plates has been digitized.


Photostimulated luminescence  ( PSL ) is the release of stored energy within a  phosphor  by stimulation with visible light, to produce a luminescent signal.  X-rays  may induce such an energy storage. A plate based on this mechanism is called a  photostimulable phosphor (PSP) plate  (or  imaging plate ) and is one type of  X-ray detector  used in  projectional radiography . Creating an image requires illuminating the  plate  twice: the first exposure, to the  radiation  of interest, "writes" the image, and a later, second illumination (typically by a visible-wavelength  laser ) "reads" the image. The device to read such a plate is known as a  phosphorimager  (occasionally spelled  phosphoimager , perhaps reflecting its common application in  molecular biology  for detecting  radiolabeled   phosphorylated   proteins  and  nucleic acids ).


Projectional radiography  using a photostimulable phosphor plate as an  X-ray detector  can be called " phosphor plate radiography " [ 1 ]  or " computed radiography " [ 2 ]  (not to be confused with  computed tomography  which uses computer processing to convert multiple projectional radiographies to a  3D image ).


On photostimulable phosphor (PSP) plates, the phosphor layer is typically 0.1 to 0.3 mm thick. After the initial exposure by short- wavelength  (typically,  X-ray )  electromagnetic radiation , excited electrons in the  phosphor  material remain 'trapped' in 'colour centres' ("F-centers") in the  crystal   lattice  until stimulated by the second illumination. For example, Fuji's photostimulable phosphor is deposited on a flexible polyester film support with grain size about 5  micrometers , and is described as " barium fluorobromide  containing a trace amount of bivalent  europium  as a luminescence center". [ 3 ]  Europium is a  divalent  cation that replaces barium to create a  solid solution . When Eu 2+  ions are struck by ionizing radiation, they lose an additional electron to become Eu 3+  ions. These electrons enter the  conduction band  of the crystal and become trapped in the bromine ion empty lattice of the crystal, resulting in a  metastable state  that is higher in energy than the original condition.


A lower-frequency light source that is insufficient in energy to create more Eu 3+  ions can return the trapped electrons to the conduction band. As these mobilized electrons encounter Eu 3+  ions, they release a blue-violet 400 nm luminescence. [ 4 ]   This light is produced in proportion to the number of trapped electrons, and thus in proportion to the original X-ray signal.  It can be collected often by a  photomultiplier tube , which is clocked at a specific resolution or pixel capture frequency. The light is thereby converted to an electronic signal and significantly amplified. The electronic signal is then quantized via an  ADC  to discrete (digital) values for each pixel and placed into the image processor pixel map.


Afterwards, the plates can be "erased," by exposing the plate to room-intensity  white light . Thereby, the plate can be used over and over again. Imaging plates can theoretically be re-used thousands of times if they are handled carefully and under certain radiation exposure conditions. PSP plate handling under industrial conditions often results in damage after a few hundred uses. Mechanical damage such as scratches and abrasions are common, as well as radiation fatigue or imprinting due to high energy applications. An image can be erased by simply exposing the plate to a room-level fluorescent light - but more efficient, complete erasure is required to avoid signal carry-over and artifacts. Most laser scanners automatically erase the plate (current technology uses red LED lighting) after laser scanning is complete. The imaging plate can then be re-used.


Reusable phosphor plates are environmentally safe but need to be disposed of according to local regulations due to the composition of the phosphor, which contains the heavy metal Barium.


Computed radiography is used for both  industrial radiography  and medical  projectional radiography . Image plate detectors have also been used in numerous  crystallography  studies. [ 5 ]


In phosphor plate radiography, the imaging plate is housed in a special cassette and placed under the body part or object to be examined and the x-ray exposure is made. The imaging plate is then run through a special laser scanner, or CR reader, that reads and converts the image to a  digital radiograph . The digital image can then be viewed and enhanced using software that has functions very similar to other conventional digital image-processing software, such as contrast, brightness, filtration and zoom. CR imaging plates (IPs) can be retrofitted to existing exam rooms and used in multiple x-ray sites since IPs are processed through a CR reader (scanner) that can be shared between multiple exam rooms. [ 6 ]


PSP plate radiography is often distinguished from  Direct Radiography  (DR). Direct radiography usually refers to image capture onto an amorphous silicon or selenium  flat panel detector  (FPD), the data being directly passed electronically to the processing computer. PSP plate radiography instead uses a cassette containing the imaging plate, which stores the image until it is read out and loaded into the computer. This additional extra step, from exposing the detector to a viewable digital image, is the main difference between the two techniques. [ 7 ]


PSP plates and DR FPDs are typically used for  projectional radiography . This should not be confused with  fluoroscopy , where there is a continuous beam of radiation and the images appear on the screen in real time, for which PSP plates cannot be used. [ 8 ]


PSP plates are commonly used as x-ray detectors for measurements in  high energy-density physics .  Examples include self-emission imaging of  inertial confinement fusion  implosions, [ 9 ]  backlit radiographic microscopy, [ 9 ]  and spatially-resolved  emission spectroscopy  of  quantum dots . [ 10 ]


Image plates were pioneered for commercial medical use by  Fuji  in the 1980s. [ 11 ]


Patients  are exposed to  ionizing radiation  when they undergo  diagnostic examinations using x-rays  or  radiopharmaceuticals . Radiation emitted by radioisotopes or radiation generators is utilized in therapy for cancer or benign lesions and also in interventional procedures using  fluoroscopy . There has been a tremendous increase in the use of ionizing radiation in medicine during recent decades and health professionals and patients are concerned about the harmful effects of radiation. [ 1 ]  The  International Atomic Energy Agency  (IAEA) has established a program on  radiological protection of patients  in recognition of the increasing importance of this topic. The emphasis in the past had been on  radiation protection  of staff and this has helped to reduce radiation doses to staff at levels well below the limits prescribed by the  International Commission on Radiological Protection  (ICRP) and accepted by most countries. The recent emphasis on  radiation protection  of patients is helping in developing strategies to reduce radiation doses to patients without compromising on diagnostic or therapeutic purpose. [ 2 ]