" ALARA " ("As Low As Reasonably Achievable") should be maintained to reduce radiation doses to staff as well as patients. [ 3 ]


Radioanatomy  ( x-ray anatomy ) is an  anatomy   discipline  that involves studying anatomy through the use of  radiographic films . [ 3 ]
The x-ray film represents a two-dimensional image of a three-dimensional object due to the summary projection of different anatomical structures  onto a planar surface.


It requires certain skills for the correct interpretation of such images. Radiological anatomy is a necessary component of training for medical students and radiologists.


This  anatomy  article is a  stub . You can help Wikipedia by  expanding it .


Rarefying osteitis  is a general term for a  radiolucent  lesion on a  radiograph  usually diagnosed as a  periapical abscess  or a  periapical cyst . [ 1 ] [ 2 ]


This article about a  disease , disorder, or medical condition is a  stub . You can help Wikipedia by  expanding it .


A  scorotron  (from  screen controlled corona [ 1 ] ), also called a  corona grid , [ 2 ]  is a device which creates  corona discharge  current, used in  xerography .  Scorotrons appear in  photocopiers , in  xeroradiography  equipment, [ 3 ]  and similar applications.


A  sensitivity speck  or  sensitivity center  is an imperfection or other specific point in a  silver halide  crystal which traps  electrons , causing  photosensitivity . This can produce a  latent image  in the crystal, having applications in  photography [ 1 ]  and  dosimetry . [ 2 ]


A sensitivity speck is very often the site of shallow electron traps, such as crystalline defect (particularly edge  dislocation ) and  silver sulfide  specks created by sulfur sensitization process.


When a photon is absorbed by a silver halide crystal, a free-carrier (electron in the conduction band) is generated. This free-carrier can migrate through the  crystal lattice  of silver halide, until captured by the shallow electron trap, where the electron is likely to reduce an interstitial silver ion to form an atomic silver. Subsequent exposure can grow the size of silver cluster through the same mechanism. This forms the  latent image  where the silver cluster becomes large enough to render the entire crystal developable in developer solution.


This photography-related article is a  stub . You can help Wikipedia by  expanding it .


Spectral imaging  is an umbrella term for energy-resolved  X-ray imaging  in medicine. [ 1 ]  The technique makes use of the energy dependence of X-ray attenuation to either increase the  contrast-to-noise ratio , or to provide quantitative image data and reduce image artefacts by so-called material decomposition. Dual-energy imaging, i.e. imaging at two energy levels, is a special case of spectral imaging and is still the most widely used terminology, but the terms "spectral imaging" and "spectral CT" have been coined to acknowledge the fact that  photon-counting detectors  have the potential for measurements at a larger number of energy levels. [ 2 ] [ 3 ]


The first medical application of spectral imaging appeared in 1953 when B. Jacobson at the  Karolinska University Hospital , inspired by  X-ray absorption spectroscopy , presented a method called "dichromography" to measure the concentration of  iodine  in X-ray images. [ 4 ]  In the 70's, spectral  computed tomography  (CT) with exposures at two different voltage levels was proposed by  G.N. Hounsfield  in his landmark CT paper. [ 5 ]  The technology evolved rapidly during the 70's and 80's, [ 6 ] [ 7 ]  but technical limitations, such as motion artifacts, [ 8 ]  for long held back widespread clinical use.


In recent years, however, two fields of technological breakthrough have spurred a renewed interest in energy-resolved imaging. Firstly, single-scan energy-resolved CT was introduced for routine clinical use in 2006 and is now available by several major manufacturers, [ 9 ]  which has resulted in a large and expanding number of clinical applications. Secondly, energy-resolving  photon-counting detectors  start to become available for clinical practice; the first commercial photon-counting system was introduced for mammography in 2003, [ 10 ]  and  CT systems  are at the verge of being feasible for routine clinical use. [ 11 ]


An energy-resolved imaging system probes the object at two or more photon energy levels. In a generic imaging system, the projected signal in a detector element at energy level

 Ω
 ∈
 {

 E

 1

 ,

 E

 2

 ,

 E

 3

 ,
 …
 }

 {\textstyle \Omega \in \{E_{1},E_{2},E_{3},\ldots \}}

  is [ 1 ]


where

 q

 {\textstyle q}

  is the number of incident photons,

 Φ

 {\textstyle \Phi }

  is the normalized incident energy spectrum, and

 Γ

 {\textstyle \Gamma }

  is the detector response function.  Linear attenuation coefficients  and integrated thicknesses for materials that make up the object are denoted

 μ

 {\textstyle \mu }

  and

 t

 {\textstyle t}

  (attenuation according to  Lambert–Beers law ). Two conceivable ways of acquiring spectral information are to either vary

 q
 ×
 Φ

 {\textstyle q\times \Phi }

  with

 Ω

 {\textstyle \Omega }

 , or to have

 Ω

 {\textstyle \Omega }

 -specific

 Γ

 {\textstyle \Gamma }

 , here denoted incidence-based and detection-based methods, respectively.


Most elements appearing naturally in human bodies are of low  atomic number  and lack  absorption edges  in the diagnostic X-ray energy range. The two dominating X-ray interaction effects are then  Compton scattering  and the  photo-electric effect , which can be assumed to be smooth and with separable and independent material and energy dependences. The linear attenuation coefficients can hence be expanded as [ 6 ]


In contrast-enhanced imaging, high-atomic-number  contrast agents  with K absorption edges in the diagnostic energy range may be present in the body. K-edge energies are material specific, which means that the energy dependence of the photo-electric effect is no longer separable from the material properties, and an additional term can be added to Eq. ( 2 ) according to [ 12 ]


where

 a

 K

 {\textstyle a_{K}}

  and

 f

 K

 {\textstyle f_{K}}

  are the material coefficient and energy dependency of contrast-agent material

 K

 {\textstyle K}

 .


Summing the energy bins in Eq. ( 1 ) (

 n
 =
 ∑

 n

 Ω

 {\textstyle n=\sum n_{\Omega }}

 ) yields a conventional non-energy-resolved image, but because X-ray contrast varies with energy, a weighted sum (

 n
 =
 ∑

 w

 Ω

 ×

 n

 Ω

 {\textstyle n=\sum w_{\Omega }\times n_{\Omega }}

 ) optimizes the  contrast-to-noise-ratio  (CNR) and enables a higher CNR at a constant patient  dose  or a lower dose at a constant CNR. [ 13 ]  The benefit of energy weighting is highest where the photo-electric effect dominates and lower in high-energy regions dominated by Compton scattering (with weaker energy dependence).


Energy weighting was pioneered by Tapiovaara and Wagner [ 13 ]  and has subsequently been refined for projection imaging [ 14 ] [ 15 ]  and CT [ 16 ]  with CNR improvements ranging from a few percent up to tenth of percent for heavier elements and an ideal CT detector. [ 17 ]  An example with a realistic detector was presented by Berglund et al. who modified a photon-counting mammography system and raised the CNR of clinical images by 2.2–5.2%. [ 18 ]


Equation ( 1 ) can be treated as a system of equations with material thicknesses as unknowns, a technique broadly referred to as material decomposition. System properties and linear attenuation coefficients need to be known, either explicitly (by modelling) or implicitly (by calibration). In CT, implementing material decomposition post reconstruction (image-based decomposition) does not require coinciding projection data, but the decomposed images may suffer from beam-hardening artefacts because the reconstruction algorithm is generally non-reversible. [ 19 ]  Applying material decomposition directly in projection space instead (projection-based decomposition), [ 6 ]  can in principle eliminate beam-hardening artefacts because the decomposed projections are quantitative, but the technique requires coinciding projection data such as from a detection-based method.


In the absence of K-edge contrast agents and any other information about the object (e.g. thickness), the limited number of independent energy dependences according to Eq. ( 2 ) means that the system of equations can only be solved for two unknowns, and measurements at two energies (

 |

 Ω

 |

 =
 2

 {\textstyle |\Omega |=2}

 ) are necessary and sufficient for a unique solution of

 t

 1

 {\textstyle t_{1}}

  and

 t

 2

 {\textstyle t_{2}}

 . [ 7 ]  Materials 1 and 2 are referred to as basis materials and are assumed to make up the object; any other material present in the object will be represented by a linear combination of the two basis materials.


Material-decomposed images can be used to differentiate between healthy and malignant tissue, such as  micro calcifications in the breast , [ 20 ]  ribs and pulmonary nodules, [ 21 ]   cysts , solid  tumors  and normal breast tissue, [ 22 ]  posttraumatic  bone bruises  ( bone marrow edema ) and the bone itself, [ 23 ]  different types of  renal calculi  (stones), [ 24 ]  and  gout  in the joints. [ 25 ]  The technique can also be used to characterize healthy tissue, such as the composition of  breast tissue  (an independent risk factor for breast cancer) [ 26 ] [ 27 ] [ 28 ]  and  bone-mineral density  (an independent risk factor for fractures and all-cause mortality). [ 29 ]  Finally, virtual autopsies with spectral imaging can facilitate detection and characterization of bullets, knife tips, glass or shell fragments etc. [ 30 ]


The basis-material representation can be readily converted to images showing the amounts of photoelectric and Compton interactions by invoking Eq. ( 2 ), and to images of  effective-atomic-number  and  electron density  distributions. [ 6 ]  As the basis-material representation is sufficient to describe the linear attenuation of the object, it is possible to calculate virtual monochromatic images, which is useful for optimizing the CNR to a certain imaging task, analogous to energy weighting. For instance, the CNR between grey and white brain matter is maximized at medium energies, whereas artefacts caused by photon starvation are minimized at higher virtual energies. [ 31 ]


In  contrast-enhanced imaging , additional unknowns may be added to the system of equations according to Eq. ( 3 ) if one or several K absorption edges are present in the imaged energy range, a technique often referred to as K-edge imaging. With one K-edge contrast agent, measurements at three energies (

 |

 Ω

 |

 =
 3

 {\textstyle |\Omega |=3}

 ) are necessary and sufficient for a unique solution, two contrast agents can be differentiated with four energy bins (

 |

 Ω

 |

 =
 4

 {\textstyle |\Omega |=4}

 ), etc. K-edge imaging can be used to either enhance and quantify, or to suppress a contrast agent.


Enhancement of contrast agents can be used for improved detection and diagnosis of tumors, [ 32 ]  which exhibit increased retention of contrast agents. Further, differentiation between iodine and  calcium  is often challenging in conventional CT, but energy-resolved imaging can facilitate many procedures by, for instance, suppressing bone contrast [ 33 ]  and improving characterization of  atherosclerotic plaque . [ 34 ]  Suppression of contrast agents is employed in so-called virtual unenhanced or virtual non-contrast (VNC) images. VNC images are free from iodine staining (contrast-agent residuals), [ 35 ]  can save dose to the patient by reducing the need for an additional non-contrast acquisition, [ 36 ]  can improve  radiotherapy  dose calculations from CT images, [ 37 ]  and can help in distinguishing between contrast agent and foreign objects. [ 38 ]


Most studies of contrast-enhanced spectral imaging have used iodine, which is a well-established contrast agent, but the K edge of iodine at 33.2 keV is not optimal for all applications and some patients are hypersensitive to iodine. Other contrast agents have therefore been proposed, such as  gadolinium  (K edge at 50.2 keV), [ 39 ]  nanoparticle  silver  (K edge at 25.5 keV), [ 40 ]   zirconium  (K edge at 18.0 keV), [ 41 ]  and  gold  (K edge at 80.7 keV). [ 42 ]  Some contrast agents can be targeted, [ 43 ]  which opens up possibilities for  molecular imaging , and using several contrast agents with different K-edge energies in combination with photon-counting detectors with a corresponding number of energy thresholds enable multi-agent imaging. [ 44 ]


Incidence-based methods obtain spectral information by acquiring several images at different  tube voltage  settings, possibly in combination with different filtering. Temporal differences between the exposures (e.g. patient motion, variation in contrast-agent concentration) for long limited practical implementations, [ 6 ]  but dual-source CT [ 9 ]  and subsequently rapid kV switching [ 45 ]  have now virtually eliminated the time between exposures. Splitting the incident radiation of a scanning system into two beams with different filtration is another way to quasi-simultaneously acquire data at two energy levels. [ 46 ]


Detection-based methods instead obtain spectral information by splitting the spectrum after interaction in the object. So-called sandwich detectors consist of two (or more) detector layers, where the top layer preferentially detects low-energy photons and the bottom layer detects a harder spectrum. [ 47 ] [ 48 ]  Detection-based methods enable projection-based material decomposition because the two energy levels measured by the detector represent identical ray paths. Further, spectral information is available from every scan, which has work-flow advantages. [ 49 ]


The currently most advanced detection-based method is based on  photon-counting detectors . As opposed to  conventional detectors , which integrate all photon interactions over the exposure time, photon-counting detectors are fast enough to register and measure the energy of single photon events. [ 50 ]  Hence, the number of energy bins and the spectral separation are not determined by physical properties of the system (detector layers, source / filtration etc.), but by the detector electronics, which increases efficiency and the degrees of freedom, and enable elimination of  electronic noise . The first commercial photon-counting application was the MicroDose mammography system, introduced by Sectra Mamea in 2003 (later acquired by Philips), [ 10 ]  and spectral imaging was launched on this platform in 2013. [ 51 ]


The MicroDose system was based on silicon strip detectors, [ 10 ] [ 51 ]  a technology that has subsequently been refined for CT with up to eight energy bins. [ 52 ] [ 53 ]   Silicon  as sensor material benefit from high charge-collection efficiency, ready availability of high-quality high-purity silicon crystals, and established methods for test and assembly. [ 54 ]  The relatively low photo-electric cross section can be compensated for by arranging the silicon wafers edge on, [ 55 ]  which also enables depth segments. [ 56 ]   Cadmium telluride (CdTe)  and  cadmium–zinc telluride (CZT)  are also being investigated as sensor materials. [ 57 ] [ 58 ] [ 59 ]  The higher atomic number of these materials result in a higher photo-electric cross section, which is advantageous, but the higher fluorescent yield degrades spectral response and induces cross talk. [ 60 ] [ 61 ]  Manufacturing of macro-sized crystals of these materials have so far posed practical challenges and leads to charge trapping [ 62 ]  and long-term polarization effects (build-up of space charge). [ 63 ]  Other solid-state materials, such as  gallium arsenide [ 64 ]  and  mercuric iodide , [ 65 ]  as well as gas detectors, [ 66 ]  are currently quite far from clinical implementation.


The main intrinsic challenge of photon-counting detectors for medical imaging is pulse pileup, [ 62 ]  which results in lost counts and reduced energy resolution because several pulses are counted as one. Pileup will always be present in photon-counting detectors because of the  Poisson distribution  of incident photons, but detector speeds are now so high that acceptable pileup levels at CT count rates begin to come within reach. [ 67 ]


Tudor Scan Tech  SA is a Swiss company that manufactures  aircraft  and  cargo  scanners, including  x-ray scanners  and robotic scanning systems. The company was founded in 2014 by Mircea Tudor and is currently headquartered in  Saint-Imier, Switzerland . [ 1 ]  Tudor Scan Tech incorporates  MBTelecom Ltd , a Romanian company developing high technologies for the security industry, and also heps customs and border officials to check for  illegal weapons ,  contraband , and  explosive materials . [ 2 ]  They have won The Grand Prix of the International Exhibition of Inventions thrice, 2009, 2013 and 2014. [ 3 ] [ 4 ]  The company has also won WIPO Awards in 2009, organized by  World Intellectual Property Organization . [ 5 ] [ 6 ]


Tudor Scan Tech SA (TST) was founded by Mircea Tudor in 2014, in  Switzerland , forming a group of companies with the Romanian company MBTelecom Ltd. (MBT) founded in 1994 by Mircea Tudor. The group of companies designs and manufactures inspection scanners, including x-ray, linear accelerators and  gamma ray  scanning system for aircraft, cargo, containers, occupied cars and vans, heavy vehicle and border control. [ 1 ] [ 7 ] [ 8 ]


In 2009, the group started to develop the first ever non- intrusiveness  inspection technology, Roboscan Aeria. The company was awarded in 2009 with The Grand Prix of the 37th International Exhibition of Inventions of  Geneva  for the invention of the first robotic mobile scanner for trucks and containers. During the same year, the company also won World Intellectual Property Organisation Award. The company was nominated and awarded again at The Grand Prix of the 41st International Exhibition of Inventions of Geneva for the first scanner for civil and military aircraft. [ 9 ] [ 10 ] [ 6 ]


In 2013, they launched a new plant worth 5 million euros to increase the airplane scanners production. [ 11 ]


Tudor Scan Tech is also a strategic security partner of  International Air Transport Association  (IATA). [ 12 ]


Roboscan Aeria is a patented design for aircraft security inspection developed inside the group MBT-TST. Roboscan Aeria is designed to scan and detect threats, illegal or undeclared goods within the aircraft. [ 13 ]  The scanning process is remotely operated without any human exposure to  ionizing radiations .  Radiography  detects any object with a resolution of 0.5 mm. [ 5 ] [ 9 ] [ 14 ]  The first scanner is installed at Bucharest's Henri Coanda Airport. [ 15 ]


Unsharpness  is the loss of  spatial resolution  in a  radiographic  image. There are generally considered to be three types of unsharpness: geometric unsharpness, motion unsharpness and photographic or system unsharpness. [ 1 ]


Motion unsharpness  is caused by movement of the patient, the detector or the source of  X-rays , during the exposure. Movement of the patient, either voluntary or otherwise, is the most common cause and this can be minimised in a number of ways: immobilizing the patient, asking the patient to keep still or to hold the breath and keeping exposure time short, and thereby giving them less time in which to move, are the most obvious.


System unsharpness  (previously called photographic unsharpnesss) is the result of the detector system employed. Every detector type has a limiting factor which determines its maximum spatial resolution. In film systems it is the size of the grains of photographic chemical. In  computed radiography  systems it the size of the laser used to read the phosphor plate in the cassette reader. In  digital radiography  systems it is the size of the individual  thin film transistors . Since each type has a maximum capability, there is no way to minimise this system other than using a better and probably more expensive machine.


Geometric unsharpness  is caused by aspects of the geometry of the X-ray beam. Two principal factors play simultaneously: the apparent focal spot size and the ratio between object-film distance (OFD) and focus-film distance (FFD). Fine focal spot sizes will minimise geometric unsharpness, and therefore give more detailed images, but it is often impossible to employ them due to the tube loading necessary in the exposure. Keeping the ratio FFD:OFD high will minimise geometric unsharpness. This is most easily done by keeping the OFD to a minimum, i.e., keeping the part of the body being X-rayed as close to the detector as possible. If this is not possible however, then increasing FFD beyond the normal 100–110 cm will be necessary to keep the unsharpness level acceptable.


In  urology ,  voiding cystourethrography  ( VCUG ) is a frequently performed technique for visualizing a person's  urethra  and  urinary bladder  while the person  urinates  (voids). It is used in the diagnosis of  vesicoureteral reflux  (kidney reflux), among other disorders. [ 1 ]   The technique consists of  catheterizing  the person in order to fill the bladder with a  radiocontrast  agent, typically  diatrizoic acid . Under  fluoroscopy  (real time x-rays) the  radiologist  watches the contrast enter the bladder and looks at the anatomy of the patient. If the contrast moves into the  ureters  and back into the  kidneys , the  radiologist  makes the diagnosis of  vesicoureteral reflux , and gives the degree of severity a score. The exam ends when the person voids while the radiologist is watching under fluoroscopy. Consumption of fluid promotes excretion of contrast media after the procedure. It is important to watch the contrast during voiding, because this is when the bladder has the most pressure, and it is most likely this is when reflux will occur.  Despite this detailed description of the procedure, at least as of 2016 [update]  the technique had not been standardized across practices. [ 1 ]


Some uses of this procedure are: to study the presence of  vesicoureteric reflux , study of urethra during micturition, presence of bladder leak post surgery or trauma, and is used in  urodynamic testing  to assess  urinary incontinence . [ 2 ]


Indications  for performing VCUG:


Contraindications for voiding cystourethrogram is when the subject is having:


A high osmolar contrast agent such as  diatrizoate  or a low osmolar contrast agent such as  Iotalamic acid  with a concentration of 150 mg per ml is used for the procedure. [ 2 ] [ 1 ]


The urinary bladder is catheterised under aseptic technique. The contrast medium is slowly injected or dripped in. The level of bladder filling is observed by taking intermittent images using  fluoroscopy . The early filling of the bladder should be monitored carefully to detect any accidental placement of the catheter in the distal  ureter  or  vagina  and to detect any reflux of contrast into the ureters. The bladder should be filled up with as much contrast as possible until the subject is unable to tolerate it or when there is no more contrast going into the bladder. If the subject is able to pee, then the catheter can be removed for the subject to do so. If there is no confidence that the subject is able to pee, then the urinary catheter should remain in place. It is more convenient for adults to pee in an erect position with a urine receiver. Meanwhile, children can pee while lying down on a table with a urine receiver. Infants and smaller children can lie down on a table and pee onto absorbent pads. For those children or infants with a neuropathic bladder, pressure on the  suprabic  region can help them to pee. [ 2 ]


Fluoroscopic spot images and videos are taken during the micturition phase to detect any reflux. The lower ureter is best seen on an anterior oblique position. In males, peeing should be done in oblique or lateral positions to visualise the whole of urethra. Finally, the whole abdomen is imaged to detect any undetected reflux in previous images. Any urine left in the bladder after peeing is also recorded in this image. Lateral views are useful to evaluate any fistulas from the bladder connecting into the rectum or vagina. Oblique views are used to evaluate any leaks from the bladder or urethra. Stress views are useful in urodynamic studies. [ 2 ]  The  verumontanum  appears elongated and the proximal bulbal urethra has a less conical appearance. [ 4 ]


Children may have painful micturition after the procedure, which can lead to  urinary retention  (children afraid to pee due to pain). Some painkillers or peeing inside a warm bath may help. Those children who receive antibiotics before the procedure for urinary tract infection will double the dose for 3 days after the procedure. Those not already on antibiotics will be prescribed with 3 days of  trimethoprim . Haematuria (blood in urine) may also occur after the procedure. [ 2 ]     With respect to post-procedural urinary tract infection, the risk has been found to be sufficiently low, except in patients with a pre-existing urologic diagnosis, that pre-operative antibiotic use is not considered a necessary adjunct. [ 5 ]


The procedure is invasive and uncomfortable, and it carries a high potential for psychological trauma for both children and parents. [ 1 ] [ 6 ]  The long-term psychological effects of VCUGs on children have been compared to that of childhood  sexual abuse . [ 7 ]


Another complication is perforation of the bladder due to over-distension. Accidental catherisation of vagina or unusual urethral opening and retention of urinary catheter are also possible. [ 2 ]


An increased risk of cancer, in particular genitourinary cancer, has been observed in one study arising from the radiation exposure inherent in the procedure. [ 8 ]


An  X-ray  (also known in many languages as  Röntgen radiation ) is a form of high-energy  electromagnetic radiation  with a  wavelength  shorter than those of  ultraviolet  rays and longer than those of  gamma rays . Roughly, X-rays have a  wavelength  ranging from 10  nanometers  to 10  picometers , corresponding to  frequencies  in the range of 30  petahertz  to 30  exahertz  ( 3 × 10 16  Hz  to  3 × 10 19  Hz ) and photon energies in the range of 100  eV  to 100  keV , respectively. [ 1 ]


X-rays were discovered in  1895  by the German scientist  Wilhelm Conrad Röntgen , [ 2 ]  who named it  X-radiation  to signify an unknown type of radiation. [ 3 ]


X-rays can penetrate many solid substances such as construction materials and living tissue, [ 4 ]  so X-ray  radiography  is widely used in  medical diagnostics   (e.g., checking for  broken bones ) and  material science  (e.g., identification of some  chemical elements  and detecting weak points in construction materials). [ 5 ]   However X-rays are  ionizing radiation  and exposure can be hazardous to health, causing  DNA  damage, cancer and, at higher intensities, burns and  radiation sickness .  Their generation and use is strictly controlled by public health authorities.


X-rays were originally noticed in science as a type of unidentified  radiation  emanating from  discharge tubes  by experimenters investigating  cathode rays  produced by such tubes, which are energetic  electron  beams that were first observed in 1869. Early researchers noticed effects that were attributable to them in many of the early  Crookes tubes  (invented around  1875 ). Crookes tubes created free electrons by  ionization  of the residual air in the tube by a high  DC   voltage  of anywhere between a few  kilovolts  and 100 kV. This voltage accelerated the electrons coming from the  cathode  to a high enough velocity that they created X-rays when they struck the  anode  or the glass wall of the tube. [ 6 ]


The earliest experimenter thought to have (unknowingly) produced X-rays was  William Morgan . In  1785 , he presented a  paper  to the  Royal Society of London  describing the effects of passing  electrical currents  through a partially evacuated glass tube, producing a glow created by X-rays. [ 7 ] [ 8 ]  This work was further explored by  Humphry Davy  and his assistant  Michael Faraday . [ citation needed ]


Starting in 1888, Philipp Lenard conducted experiments to see whether cathode rays could pass out of the Crookes tube into the air. He built a Crookes tube with a "window" at the end made of thin aluminium, facing the cathode so the cathode rays would strike it (later called a "Lenard tube"). He found that something came through, that would expose photographic plates and cause fluorescence. He measured the penetrating power of these rays through various materials. It has been suggested that at least some of these "Lenard rays" were actually X-rays. [ 9 ]


Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Röntgen made his discovery and announcement. He based it on the  electromagnetic theory of light . [ 10 ] [ full citation needed ]  However, he did not work with actual X-rays.


In early 1890, photographer  William Jennings  and associate professor of the  University of Pennsylvania  Arthur W. Goodspeed were making photographs of coins with electric sparks. On 22 February after the end of their experiments two coins were left on a stack of photographic plates before Goodspeed demonstrated to Jennings the operation of  Crookes tubes . While developing the plates, Jennings noticed disks of unknown origin on some of the plates, but nobody could explain them, and they moved on. Only in 1896 they realized that they accidentally made an X-ray photograph (they didn't claim a discovery). [ 11 ]


Also in 1890, Roentgen's assistant  Ludwig Zehnder  noticed a flash of light from a fluorescent screen immediately before the covered tube he was switching on punctured. [ 12 ]


When  Stanford University  physics professor  Fernando Sanford  conducted his "electric photography" experiments in 1891–1893 by photographing coins in the light of electric sparks, [ 13 ]  like Jennings and Goodspeed, he may have unknowingly generated and detected X-rays. His letter of  6 January 1893  to the  Physical Review  was duly published [ 13 ]  and an article entitled  Without Lens or Light, Photographs Taken With Plate and Object in Darkness  appeared in the  San Francisco Examiner . [ 14 ]


In  1894 ,  Nikola Tesla  noticed damaged film in his lab that seemed to be associated with Crookes tube experiments and began investigating this invisible,  radiant energy . [ 15 ] [ 16 ]  After Röntgen identified the X-ray, Tesla began making X-ray images of his own using high voltages and tubes of his own design, [ 17 ]  as well as Crookes tubes.


On  8 November 1895 , German physics professor  Wilhelm Röntgen  stumbled on X-rays while experimenting with Lenard tubes and  Crookes tubes  and began studying them. He wrote an initial report "On a new kind of ray: A preliminary communication" and on 28 December 1895, submitted it to  Würzburg 's Physical-Medical Society journal. [ 18 ]  This was the first paper written on X-rays. Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. Some early texts refer to them as Chi-rays, having interpreted "X" as the uppercase  Greek letter Chi ,  Χ . [ 19 ] [ 20 ] [ 21 ]


There are conflicting accounts of his discovery because Röntgen had his  lab notes  burned after his death, but this is a likely reconstruction by his biographers: [ 22 ] [ 23 ]  Röntgen was investigating cathode rays from a Crookes tube which he had wrapped in black cardboard so that the visible light from the tube would not interfere, using a  fluorescent  screen painted with barium  platinocyanide . He noticed a faint green glow from the screen, about 1 meter (3.3 ft) away. Röntgen realized some invisible rays coming from the tube were passing through the cardboard to make the screen glow. He found they could also pass through books and papers on his desk. Röntgen threw himself into investigating these unknown rays systematically. Two months after his initial discovery, he published his paper. [ 24 ]


Röntgen discovered their medical use when he made a picture of his wife's hand on a photographic plate formed due to X-rays. The photograph of his wife's hand was the first photograph of a human body part using X-rays. When she saw the picture, she said "I have seen my death." [ 27 ]


The discovery of X-rays generated significant interest. Röntgen's biographer  Otto Glasser  estimated that, in  1896  alone, as many as 49 essays and 1044 articles about the new rays were published. [ 28 ]  This was probably a conservative estimate, if one considers that nearly every paper around the world extensively reported about the new discovery, with a magazine such as  Science  dedicating as many as 23 articles to it in that year alone. [ 29 ]  Sensationalist reactions to the new discovery included publications linking the new kind of rays to occult and paranormal theories, such as telepathy. [ 30 ] [ 31 ]


The name X-rays stuck, although (over Röntgen's great objections) many of his colleagues suggested calling them  Röntgen rays . They are still referred to as such in many languages, including  German ,  Hungarian ,  Ukrainian ,  Danish ,  Polish ,  Czech ,  Bulgarian ,  Swedish ,  Finnish ,  Portuguese ,  Estonian ,  Slovak ,  Slovenian ,  Turkish ,   Russian ,  Latvian ,  Lithuanian ,  Albanian ,  Japanese ,  Dutch ,  Georgian ,  Hebrew ,  Icelandic , and  Norwegian . [ original research? ]


Röntgen received the first  Nobel Prize in Physics  for his discovery. [ 32 ]


Röntgen immediately noticed X-rays could have medical applications. Along with his 28 December Physical-Medical Society submission, he sent a letter to physicians he knew around Europe (1 January 1896). [ 33 ]  News (and the creation of "shadowgrams") spread rapidly with Scottish electrical engineer  Alan Archibald Campbell-Swinton  being the first after Röntgen to create an X-ray photograph (of a hand). Through February, there were 46 experimenters taking up the technique in North America alone. [ 33 ]


The first use of X-rays under clinical conditions was by  John Hall-Edwards  in Birmingham, England on 11 January 1896, when he radiographed a needle stuck in the hand of an associate. On 14 February 1896, Hall-Edwards was also the first to use X-rays in a surgical operation. [ 34 ]


In early 1896, several weeks after Röntgen's discovery,  Ivan Romanovich Tarkhanov  irradiated frogs and insects with X-rays, concluding that the rays "not only photograph, but also affect the living function". [ 35 ]  At around the same time, the zoological illustrator James Green began to use X-rays to examine fragile specimens.  George Albert Boulenger  first mentioned this work in a paper he delivered before the  Zoological Society of London  in May 1896. The book  Sciagraphs of British Batrachians and Reptiles  (sciagraph is an obsolete name for an X-ray photograph), by Green and James H. Gardiner, with a foreword by Boulenger, was published in 1897. [ 36 ] [ 37 ]


The first medical X-ray made in the United States was obtained using a discharge tube of  Ivan Puluj 's design. [ 38 ]  In January 1896, on reading of Röntgen's discovery, Frank Austin of  Dartmouth College  tested all of the discharge tubes in the physics laboratory and found that only the Puluj tube produced X-rays. This was a result of Puluj's inclusion of an oblique "target" of  mica , used for holding samples of  fluorescent  material, within the tube. On 3 February 1896, Gilman Frost, professor of medicine at the college, and his brother Edwin Frost, professor of physics, exposed the wrist of Eddie McCarthy, whom Gilman had treated some weeks earlier for a fracture, to the X-rays and collected the resulting image of the broken bone on  gelatin photographic plates  obtained from Howard Langill, a local photographer also interested in Röntgen's work. [ 39 ]


Many experimenters, including Röntgen himself in his original experiments, came up with methods to view X-ray images "live" using some form of luminescent screen. [ 33 ]  Röntgen used a screen coated with barium  platinocyanide . On 5 February 1896, live imaging devices were developed by both Italian scientist Enrico Salvioni (his "cryptoscope") and  William Francis Magie  of  Princeton University  (his "Skiascope"), both using barium platinocyanide. American inventor  Thomas Edison  started research soon after Röntgen's discovery and investigated materials' ability to fluoresce when exposed to X-rays, finding that  calcium tungstate  was the most effective substance. In May 1896, he developed the first mass-produced live imaging device, his "Vitascope", later called the  fluoroscope , which became the standard for medical X-ray examinations. [ 33 ]  Edison dropped X-ray research around 1903, before the death of  Clarence Madison Dally , one of his glassblowers. Dally had a habit of testing X-ray tubes on his own hands, developing a cancer in them so tenacious that both arms were  amputated  in a futile attempt to save his life; in 1904, he became the first known death attributed to X-ray exposure. [ 33 ]  During the time the fluoroscope was being developed, Serbian American physicist  Mihajlo Pupin , using a calcium tungstate screen developed by Edison, found that using a fluorescent screen decreased the exposure time it took to create an X-ray for medical imaging from an hour to a few minutes. [ 40 ] [ 33 ]


In 1901,  U.S. President William McKinley was shot twice  in an assassination attempt while attending the  Pan American Exposition  in  Buffalo, New York . While one bullet only grazed his  sternum , another had lodged somewhere deep inside his  abdomen  and could not be found. A worried McKinley aide sent word to inventor Thomas Edison to rush an  X-ray machine  to Buffalo to find the stray bullet. It arrived but was not used. While the shooting itself had not been lethal,  gangrene  had developed along the path of the bullet, and McKinley died of  septic shock  due to bacterial infection six days later. [ 41 ]


With the widespread experimentation with X‑rays after their discovery in  1895  by scientists, physicians, and inventors came many stories of burns, hair loss, and worse in technical journals of the time. In February 1896, Professor John Daniel and  William Lofland Dudley  of  Vanderbilt University  reported hair loss after Dudley was X-rayed. A child who had been shot in the head was brought to the Vanderbilt laboratory in 1896. Before trying to find the bullet, an experiment was attempted, for which Dudley "with his characteristic devotion to science" [ 42 ] [ 43 ] [ 44 ]  volunteered. Daniel reported that 21 days after taking a picture of Dudley's  skull  (with an exposure time of one hour), he noticed a bald spot 5 centimeters (2 in) in diameter on the part of his head nearest the X-ray tube: "A plate holder with the plates towards the side of the skull was fastened and a  coin  placed between the skull and the head. The tube was fastened at the other side at a distance of one-half-inch [1.3 cm] from the hair." [ 45 ]  Beyond burns, hair loss, and cancer, X-rays can be linked to infertility in males based on the amount of radiation used. [ citation needed ]


In August 1896, H. D. Hawks, a graduate of Columbia College, suffered severe hand and chest burns from an X-ray demonstration. It was reported in  Electrical Review  and led to many other reports of problems associated with X-rays being sent in to the publication. [ 46 ]  Many experimenters including  Elihu Thomson  at Edison's lab,  William J. Morton , and  Nikola Tesla  also reported burns. Elihu Thomson deliberately exposed a finger to an X-ray tube over a period of time and suffered pain, swelling, and blistering. [ 47 ]  Other effects were sometimes blamed for the damage including ultraviolet rays and (according to Tesla) ozone. [ 15 ]  Many physicians claimed there were no effects from X-ray exposure at all. [ 47 ]  On 3 August 1905, in San Francisco, California,  Elizabeth Fleischman , an American X-ray pioneer, died from complications as a result of her work with X-rays. [ 48 ] [ 49 ] [ 50 ]


Hall-Edwards developed a cancer (then called X-ray dermatitis) sufficiently advanced by 1904 to cause him to write papers and give public addresses on the dangers of X-rays. His left arm had to be amputated at the elbow in 1908, [ 51 ] [ 52 ]  and four fingers on his right arm soon thereafter, leaving only a thumb. He died of cancer in 1926. His left hand is kept at  Birmingham University . [ citation needed ]


The many applications of X-rays immediately generated enormous interest. Workshops began making specialized versions of Crookes tubes for generating X-rays and these first-generation  cold cathode  or Crookes X-ray tubes were used until about 1920. [ citation needed ]


A typical early 20th-century medical X-ray system consisted of a  Ruhmkorff coil  connected to a  cold cathode Crookes X-ray tube . A spark gap was typically connected to the high voltage side in parallel to the tube and used for diagnostic purposes. [ 53 ]  The spark gap allowed detecting the polarity of the sparks, measuring voltage by the length of the sparks thus determining the "hardness" of the vacuum of the tube, and it provided a load in the event the X-ray tube was disconnected. To detect the hardness of the tube, the spark gap was initially opened to the widest setting. While the coil was operating, the operator reduced the gap until sparks began to appear. A tube in which the spark gap began to spark at around 6.4 centimeters (2.5 in) was considered soft (low vacuum) and suitable for thin body parts such as hands and arms. A 13-centimeter (5 in) spark indicated the tube was suitable for shoulders and knees. An 18-to-23-centimeter (7 to 9 in) spark would indicate a higher vacuum suitable for imaging the abdomen of larger individuals. Since the spark gap was connected in parallel to the tube, the spark gap had to be opened until the sparking ceased to operate the tube for imaging. Exposure time for photographic plates was around half a minute for a hand to a couple of minutes for a thorax. The plates may have a small addition of fluorescent salt to reduce exposure times. [ 53 ]


Crookes tubes were unreliable. They had to contain a small quantity of gas (invariably air) as a current will not flow in such a tube if they are fully evacuated. However, as time passed, the X-rays caused the glass to absorb the gas, causing the tube to generate "harder" X-rays until it soon stopped operating. Larger and more frequently used tubes were provided with devices for restoring the air, known as "softeners". These often took the form of a small side tube that contained a small piece of  mica , a mineral that traps relatively large quantities of air within its structure. A small electrical heater heated the mica, causing it to release a small amount of air, thus restoring the tube's efficiency. However, the mica had a limited life, and the restoration process was difficult to control. [ citation needed ]


In  1904 ,  John Ambrose Fleming  invented the  thermionic diode , the first kind of  vacuum tube . This used a  hot cathode  that caused an  electric current  to flow in a  vacuum . This idea was quickly applied to X-ray tubes, and hence heated-cathode X-ray tubes, called "Coolidge tubes", completely replaced the troublesome cold cathode tubes by about 1920. [ citation needed ]


In about 1906, the physicist  Charles Barkla  discovered that X-rays could be scattered by gases, and that each element had a characteristic  X-ray spectrum . He won the  1917   Nobel Prize in Physics  for this discovery. [ citation needed ]


In  1912 ,  Max von Laue , Paul Knipping, and Walter Friedrich first observed the  diffraction  of X-rays by crystals. This discovery, along with the early work of  Paul Peter Ewald ,  William Henry Bragg , and  William Lawrence Bragg , gave birth to the field of  X-ray crystallography . [ 54 ]


In  1913 ,  Henry Moseley  performed crystallography experiments with X-rays emanating from various metals and formulated  Moseley's law  which relates the frequency of the X-rays to the atomic number of the metal. [ citation needed ]


The  Coolidge X-ray tube  was invented the same year by  William D. Coolidge . It made possible the continuous emissions of X-rays. Modern X-ray tubes are based on this design, often employing the use of rotating targets which allow for significantly higher heat dissipation than static targets, further allowing higher quantity X-ray output for use in high-powered applications such as rotational CT scanners. [ citation needed ]


The use of X-rays for medical purposes (which developed into the field of  radiation therapy ) was pioneered by Major  John Hall-Edwards  in  Birmingham , England. Then in 1908, he had to have his left arm amputated because of the spread of  X-ray dermatitis  on his arm. [ 55 ]


Medical science also used the motion picture to study human physiology. In 1913, a motion picture was made in Detroit showing a hard-boiled egg inside a human stomach. This early X-ray movie was recorded at a rate of one still image every four seconds. [ 56 ]  Dr Lewis Gregory Cole of New York was a pioneer of the technique, which he called "serial radiography". [ 57 ] [ 58 ]  In 1918, X-rays were used in association with  motion picture cameras  to capture the human skeleton in motion. [ 59 ] [ 60 ] [ 61 ]  In 1920, it was used to record the movements of tongue and teeth in the study of languages by the Institute of Phonetics in England. [ 62 ]


In  1914 ,  Marie Curie  developed radiological cars to support soldiers injured in  World War I . The cars would allow for rapid X-ray imaging of wounded soldiers so battlefield surgeons could quickly and more accurately operate. [ 63 ]


From the early 1920s through to the 1950s, X-ray machines were developed to assist in the fitting of shoes [ 64 ]  and were sold to commercial shoe stores. [ 65 ] [ 66 ] [ 67 ]  Concerns regarding the impact of frequent or poorly controlled use were expressed in the 1950s, [ 68 ] [ 69 ]  leading to the practice's eventual end that decade. [ 70 ]


The  X-ray microscope  was developed during the 1950s. [ citation needed ]


The  Chandra X-ray Observatory , launched on  23 July 1999 , has been allowing the exploration of the very violent processes in the  universe  that produce X-rays. Unlike  visible light , which gives a relatively stable view of the universe, the X-ray universe is unstable. It features  stars  being torn apart by  black holes ,  galactic collisions , and  novae , and  neutron stars  that build up layers of  plasma  that then  explode  into  space . [ citation needed ]


An  X-ray laser  device was proposed as part of the  Reagan Administration 's  Strategic Defense Initiative  in the 1980s, but the only test of the device (a sort of laser "blaster" or  death ray , powered by a thermonuclear explosion) gave inconclusive results. For technical and political reasons, the overall project (including the X-ray laser) was defunded (though was later revived by the second  Bush Administration  as  National Missile Defense  using different technologies). [ citation needed ]


Phase-contrast X-ray imaging  refers to a variety of techniques that use phase information of an X-ray beam to form the image. Due to its good sensitivity to density differences, it is especially useful for imaging soft tissues. It has become an important method for visualizing cellular and histological structures in a wide range of biological and medical studies. There are several technologies being used for X-ray phase-contrast imaging, all using different principles to convert phase variations in the X-rays emerging from an object into intensity variations. [ 71 ] [ 72 ]  These include propagation-based phase contrast, [ 73 ]   Talbot  interferometry, [ 72 ]  refraction-enhanced imaging, [ 74 ]  and X-ray interferometry. [ 75 ]  These methods provide higher contrast compared to normal absorption-based X-ray imaging, making it possible to distinguish from each other details that have almost similar density. A disadvantage is that these methods require more sophisticated equipment, such as  synchrotron  or  microfocus  X-ray sources,  X-ray optics , and high resolution X-ray detectors. [ citation needed ]


X-rays with high  photon energies  above 5–10 keV (below 0.2–0.1 nm wavelength) are called  hard X-rays , while those with lower energy (and longer wavelength) are called  soft X-rays . [ 76 ]  The intermediate range with photon energies of several keV is often referred to as  tender X-rays . Due to their penetrating ability, hard X-rays are widely used to image the inside of objects (e.g. in  medical radiography  and  airport security ). The term  X-ray  is  metonymically  used to refer to a  radiographic  image produced using this method, in addition to the method itself. Since the wavelengths of hard X-rays are similar to the size of atoms, they are also useful for determining crystal structures by  X-ray crystallography . By contrast, soft X-rays are easily absorbed in air; the  attenuation length  of 600 eV (~2 nm) X-rays in water is less than 1 micrometer. [ 77 ]


There is no consensus for a definition distinguishing between X-rays and  gamma rays . One common practice is to distinguish between the two types of radiation based on their source: X-rays are emitted by  electrons , while gamma rays are emitted by the  atomic nucleus . [ 78 ] [ 79 ] [ 80 ] [ 81 ]  This definition has several problems: other processes can also generate these high-energy  photons , or sometimes the method of generation is not known. One common alternative is to distinguish X- and gamma radiation on the basis of wavelength (or, equivalently, frequency or photon energy), with radiation shorter than some arbitrary wavelength, such as 10 −11  m (0.1  Å ), defined as gamma radiation. [ 82 ]  This criterion assigns a photon to an unambiguous category, but is only possible if wavelength is known. (Some measurement techniques do not distinguish between detected wavelengths.) However, these two definitions often coincide since the electromagnetic radiation emitted by  X-ray tubes  generally has a longer wavelength and lower photon energy than the radiation emitted by  radioactive   nuclei . [ 78 ]  Occasionally, one term or the other is used in specific contexts due to historical precedent, based on measurement (detection) technique, or based on their intended use rather than their wavelength or source.
Thus, gamma-rays generated for medical and industrial uses, for example  radiotherapy , in the ranges of 6–20  MeV , can in this context also be referred to as X-rays. [ 83 ]


X-ray  photons  carry enough energy to  ionize  atoms and disrupt  molecular bonds . This makes it a type of  ionizing radiation , and therefore harmful to living  tissue . A very high  radiation dose  over a short period of time causes burns and  radiation sickness , while lower doses can give an increased risk of  radiation-induced cancer . In medical imaging, this increased cancer risk is generally greatly outweighed by the benefits of the examination. The ionizing capability of X-rays can be used in  cancer treatment  to kill  malignant   cells  using  radiation therapy . It is also used for material characterization using  X-ray spectroscopy . [ citation needed ]


Hard X-rays can traverse relatively thick objects without being much  absorbed  or  scattered . For this reason, X-rays are widely used to  image  the inside of visually opaque objects. The most often seen applications are in medical  radiography  and  airport security  scanners, but similar techniques are also important in industry (e.g.  industrial radiography  and  industrial CT scanning ) and research (e.g.  small animal CT ). The  penetration depth  varies with several  orders of magnitude  over the X-ray spectrum. This allows the photon energy to be adjusted for the application so as to give sufficient  transmission  through the object and at the same time provide good  contrast  in the image. [ citation needed ]


X-rays have much shorter wavelengths than visible light, which makes it possible to probe structures much smaller than can be seen using a normal  microscope . This property is used in  X-ray microscopy  to acquire high-resolution images, and also in  X-ray crystallography  to determine the positions of  atoms  in  crystals . [ citation needed ]


X-rays interact with matter in three main ways, through  photoabsorption ,  Compton scattering , and  Rayleigh scattering . The strength of these interactions depends on the energy of the X-rays and the elemental composition of the material, but not much on chemical properties, since the X-ray photon energy is much higher than chemical binding energies. Photoabsorption or photoelectric absorption is the dominant interaction mechanism in the soft X-ray regime and for the lower hard X-ray energies. At higher energies, Compton scattering dominates. [ citation needed ]


The probability of a photoelectric absorption per unit mass is approximately proportional to

 Z

 3

 /

 E

 3

 {\textstyle Z^{3}/E^{3}}

 , where

 Z

 {\textstyle Z}

  is the  atomic number  and

 E

 {\textstyle E}

  is the energy of the incident photon. [ 84 ]  This rule is not valid close to inner shell electron binding energies where there are abrupt changes in interaction probability, so called  absorption edges . However, the general trend of high  absorption coefficients  and thus short  penetration depths  for low photon energies and high atomic numbers is very strong. For soft tissue, photoabsorption dominates up to about 26 keV photon energy where Compton scattering takes over. For higher atomic number substances, this limit is higher. The high amount of  calcium  (

 Z
 =
 20

 {\textstyle Z=20}

 ) in bones, together with their high density, is what makes them show up so clearly on medical radiographs. [ citation needed ]


A photoabsorbed photon transfers all its energy to the electron with which it interacts, thus ionizing the atom to which the electron was bound and producing a photoelectron that is likely to ionize more atoms in its path. An outer electron will fill the vacant electron position and produce either a characteristic X-ray or an  Auger electron . These effects can be used for elemental detection through  X-ray spectroscopy  or  Auger electron spectroscopy . [ citation needed ]


Compton scattering is the predominant interaction between X-rays and soft tissue in medical imaging. [ 85 ]  Compton scattering is an  inelastic scattering  of the X-ray photon by an outer shell electron. Part of the energy of the photon is transferred to the scattering electron, thereby ionizing the atom and increasing the wavelength of the X-ray. The scattered photon can go in any direction, but a direction similar to the original direction is more likely, especially for  high-energy X-rays . The probability for different scattering angles is described by the  Klein–Nishina formula . The transferred energy can be directly obtained from the scattering angle from the  conservation of energy  and  momentum . [ citation needed ]


Rayleigh scattering is the dominant  elastic scattering  mechanism in the X-ray regime. [ 86 ]  Inelastic forward scattering gives rise to the refractive index, which for X-rays is only slightly below 1. [ 87 ]


Whenever charged particles (electrons or ions) of sufficient energy hit a material, X-rays are produced.


X-rays can be generated by an  X-ray tube , a  vacuum tube  that uses a high voltage to accelerate the  electrons  released by a  hot cathode  to a high velocity. The high velocity electrons collide with a metal target, the  anode , creating the X-rays. [ 90 ]  In medical X-ray tubes the target is usually  tungsten  or a more crack-resistant alloy of  rhenium  (5%) and tungsten (95%), but sometimes  molybdenum  for more specialized applications, such as when softer X-rays are needed as in mammography. In crystallography, a copper target is most common, with  cobalt  often being used when fluorescence from iron content in the sample might otherwise present a problem. When even lower energies are needed, as in  X-ray photoelectron spectroscopy , the K α  X-rays from an aluminium or magnesium target are often used. [ citation needed ]


The maximum energy of the produced X-ray  photon  is limited by the energy of the incident electron, which is equal to the voltage on the tube times the electron charge, so an 80 kV tube cannot create X-rays with an energy greater than 80 keV. When the electrons hit the target, X-rays are created by two different atomic processes: [ citation needed ]


So, the resulting output of a tube consists of a continuous  Bremsstrahlung  spectrum falling off to zero at the tube voltage, plus several spikes at the characteristic lines. The voltages used in diagnostic X-ray tubes range from roughly 20 kV to 150 kV and thus the highest energies of the X-ray photons range from roughly 20 keV to 150 keV. [ 91 ]


Both of these X-ray production processes are inefficient, with only about one percent of the electrical energy used by the tube converted into X-rays, and thus most of the  electric power  consumed by the tube is released as waste heat. When producing a usable flux of X-rays, the X-ray tube must be designed to dissipate the excess heat.


A specialized source of X-rays which is becoming widely used in research is  synchrotron radiation , which is generated by  particle accelerators . Its unique features are X-ray outputs many orders of magnitude greater than those of X-ray tubes, wide X-ray spectra, excellent  collimation , and  linear polarization . [ 92 ]


Short nanosecond bursts of X-rays peaking at 15 keV in energy may be reliably produced by peeling pressure-sensitive adhesive tape from its backing in a moderate vacuum. This is likely to be the result of recombination of electrical charges produced by  triboelectric charging . The intensity of X-ray  triboluminescence  is sufficient for it to be used as a source for X-ray imaging. [ 93 ]


X-rays can also be produced by fast protons or other positive ions. The proton-induced X-ray emission or  particle-induced X-ray emission  is widely used as an analytical procedure. For high energies, the production  cross section  is proportional to  Z 1 2 Z 2 −4 , where  Z 1  refers to the  atomic number  of the ion,  Z 2  refers to that of the target atom. [ 94 ]  An overview of these cross sections is given in the same reference.


X-rays are also produced in lightning accompanying  terrestrial gamma-ray flashes . The underlying mechanism is the acceleration of electrons in lightning related electric fields and the subsequent production of photons through  Bremsstrahlung . [ 95 ]  This produces photons with energies of some few  keV  and several tens of MeV. [ 96 ]  In laboratory discharges with a gap size of approximately 1 meter length and a peak voltage of 1 MV, X-rays with a characteristic energy of 160 keV are observed. [ 97 ]  A possible explanation is the encounter of two  streamers  and the production of high-energy  run-away electrons ; [ 98 ]  however, microscopic simulations have shown that the duration of electric field enhancement between two streamers is too short to produce a significant number of run-away electrons. [ 99 ]  Recently, it has been proposed that air perturbations in the vicinity of streamers can facilitate the production of run-away electrons and hence of X-rays from discharges. [ 100 ] [ 101 ]


X-ray detectors vary in shape and function depending on their purpose. Imaging detectors such as those used for  radiography  were originally based on  photographic plates  and later  photographic film , but are now mostly replaced by various  digital  detector types such as  image plates  and  flat panel detectors . For  radiation protection  direct exposure hazard is often evaluated using  ionization chambers , while  dosimeters  are used to measure the  radiation dose  the person has been exposed to. X-ray  spectra  can be measured either by energy dispersive or wavelength dispersive  spectrometers . For  X-ray diffraction  applications, such as  X-ray crystallography ,  hybrid photon counting detectors  are widely used. [ 102 ]


Since Röntgen's discovery that X-rays can identify bone structures, X-rays have been used for  medical imaging . [ 103 ]  The first medical use was less than a month after his paper on the subject. [ 39 ]  Up to 2010, five billion medical imaging examinations had been conducted worldwide. [ 104 ]  Radiation exposure from medical imaging in 2006 made up about 50% of total ionizing radiation exposure in the United States. [ 105 ]


Projectional radiography  is the practice of producing two-dimensional images using X-ray radiation. Bones contain a high concentration of  calcium , which, due to its relatively high  atomic number , absorbs X-rays efficiently. This reduces the amount of X-rays reaching the detector in the shadow of the bones, making them clearly visible on the radiograph. The lungs and trapped gas also show up clearly because of lower absorption compared to tissue, while differences between tissue types are harder to see. [ 106 ]


Projectional radiographs are useful in the detection of  pathology  of the  skeletal system  as well as for detecting some disease processes in  soft tissue . Some notable examples are the very common  chest X-ray , which can be used to identify lung diseases such as  pneumonia , lung cancer, or  pulmonary edema , and the  abdominal x-ray , which can detect  bowel (or intestinal) obstruction , free air (from visceral perforations), and free fluid (in  ascites ). X-rays may also be used to detect pathology such as  gallstones  (which are rarely  radiopaque ) or  kidney stones  which are often (but not always) visible. Traditional plain X-rays are less useful in the imaging of soft tissues such as the brain or  muscle . One area where projectional radiographs are used extensively is in evaluating how an orthopedic  implant , such as a knee, hip or shoulder replacement, is situated in the body with respect to the surrounding bone. This can be assessed in two dimensions from plain radiographs, or it can be assessed in three dimensions if a technique called '2D to 3D registration' is used. This technique purportedly negates projection errors associated with evaluating implant position from plain radiographs. [ 107 ]


Dental radiography  is commonly used in the diagnoses of common oral problems, such as  cavities .


In medical diagnostic applications, the low energy (soft) X-rays are unwanted, since they are totally absorbed by the body, increasing the radiation dose without contributing to the image. Hence, a thin metal sheet, often of aluminium, called an  X-ray filter , is usually placed over the window of the X-ray tube, absorbing the low energy part in the spectrum. This is called  hardening  the beam since it shifts the center of the spectrum towards higher energy (or harder) X-rays.


To generate an image of the  cardiovascular system , including the arteries and veins ( angiography ) an initial image is taken of the anatomical region of interest. A second image is then taken of the same region after an iodinated  contrast agent  has been injected into the blood vessels within this area. These two images are then digitally subtracted, leaving an image of only the iodinated contrast outlining the blood vessels. The  radiologist  or surgeon then compares the image obtained to normal anatomical images to determine whether there is any damage or blockage of the vessel.


Computed tomography  (CT scanning) is a medical imaging modality where  tomographic images  or slices of specific areas of the body are obtained from a large series of two-dimensional X-ray images taken in different directions. [ 108 ]  These cross-sectional images can be combined into a  three-dimensional  image of the inside of the body. [ 109 ]  CT scans are a quicker and more cost effective imaging modality that can be used for diagnostic and therapeutic purposes in various medical disciplines. [ 109 ]


Fluoroscopy  is an imaging technique commonly used by physicians or  radiation therapists  to obtain real-time moving images of the internal structures of a patient through the use of a fluoroscope. [ 110 ]  In its simplest form, a fluoroscope consists of an X-ray source and a fluorescent screen, between which a patient is placed. However, modern fluoroscopes couple the screen to an  X-ray image intensifier  and  CCD   video camera  allowing the images to be recorded and played on a monitor. This method may use a contrast material. Examples include cardiac catheterization (to examine for  coronary artery blockages ), embolization procedures (to stop bleeding during  hemorrhoidal artery embolization ), and barium swallow (to examine for  esophageal disorders  and swallowing disorders). As of recent, modern fluoroscopy utilizes short bursts of x-rays, rather than a continuous beam, to effectively lower radiation exposure for both the patient and operator. [ 110 ]


The use of X-rays as a treatment is known as  radiation therapy  and is largely used for the management (including  palliation ) of cancer; it requires higher radiation doses than those received for imaging alone. X-rays beams are used for treating skin cancers using lower energy X-ray beams while higher energy beams are used for treating cancers within the body such as brain, lung, prostate, and breast. [ 111 ] [ 112 ]


X-rays are a form of  ionizing radiation , and are classified as a  carcinogen  by both the World Health Organization's  International Agency for Research on Cancer  and the U.S. government. [ 104 ] [ 113 ]   Diagnostic X-rays (primarily from CT scans due to the large dose used) increase the risk of developmental problems and cancer in those exposed. [ 114 ] [ 115 ] [ 116 ]   It is estimated that 0.4% of current cancers in the United States are due to  computed tomography  (CT scans) performed in the past and that this may increase to as high as 1.5–2% with 2007 rates of CT usage. [ 117 ]


Experimental and epidemiological data currently do not support the proposition that there is a  threshold dose of radiation  below which there is no increased risk of cancer. [ 118 ]  However, this is under increasing doubt. [ 119 ]  Cancer risk can start at the exposure of 1100 mGy. [ 120 ]  It is estimated that the additional radiation from diagnostic X-rays will increase the average person's cumulative risk of getting cancer by age 75 by 0.6–3.0%. [ 121 ]  The amount of absorbed radiation depends upon the type of X-ray test and the body part involved. [ 117 ]  CT and fluoroscopy entail higher doses of radiation than do plain X-rays.


To place the increased risk in perspective, a plain chest X-ray will expose a person to the same amount from  background radiation  that people are exposed to (depending upon location) every day over 10 days, while exposure from a dental X-ray is approximately equivalent to 1 day of environmental background radiation. [ 122 ]  Each such X-ray would add less than 1 per 1,000,000 to the lifetime cancer risk. An abdominal or chest CT would be the equivalent to 2–3 years of background radiation to the whole body, or 4–5 years to the abdomen or chest, increasing the lifetime cancer risk between 1 per 1,000 to 1 per 10,000. [ 122 ]  This is compared to the roughly 40% chance of a US citizen developing cancer during their lifetime. [ 123 ]  For instance, the effective dose to the torso from a CT scan of the chest is about 5 mSv, and the absorbed dose is about 14 mGy. [ 124 ]  A head CT scan (1.5 mSv, 64 mGy) [ 125 ]  that is performed once with and once without contrast agent, would be equivalent to 40 years of background radiation to the head. Accurate estimation of effective doses due to CT is difficult with the estimation uncertainty range of about ±19% to ±32% for adult head scans depending upon the method used. [ 126 ]


The risk of radiation is greater to a fetus, so in pregnant patients, the benefits of the investigation (X-ray) should be balanced with the potential hazards to the fetus. [ 127 ] [ 128 ]  If there is 1 scan in 9 months, it can be harmful to the fetus. [ 129 ]  Therefore, women who are pregnant get ultrasounds as their diagnostic imaging because this does not use radiation. [ 129 ]  If there is too much radiation exposure there could be harmful effects on the fetus or the reproductive organs of the mother. [ 129 ]  In the US, there are an estimated 62 million CT scans performed annually, including more than 4 million on children. [ 117 ]  Avoiding unnecessary X-rays (especially CT scans) reduces radiation dose and any associated cancer risk. [ 130 ]


Medical X-rays are a significant source of human-made radiation exposure. In 1987, they accounted for 58% of exposure from human-made sources in the United States. Since human-made sources accounted for only 18% of the total radiation exposure, most of which came from natural sources (82%), medical X-rays only accounted for 10% of  total  American radiation exposure; medical procedures as a whole (including  nuclear medicine ) accounted for 14% of total radiation exposure. By 2006, however, medical procedures in the United States were contributing much more ionizing radiation than was the case in the early 1980s. In 2006, medical exposure constituted nearly half of the total radiation exposure of the U.S. population from all sources. The increase is traceable to the growth in the use of medical imaging procedures, in particular  computed tomography  (CT), and to the growth in the use of nuclear medicine. [ 105 ] [ 131 ]


Dosage due to dental X-rays varies significantly depending on the procedure and the technology (film or digital). Depending on the procedure and the technology, a single dental X-ray of a human results in an exposure of 5 to 40 μSv. A full mouth series of X-rays may result in an exposure of up to 60 (digital) to 180 (film) μSv, for a yearly average of up to 400 μSv. [ 132 ] [ 133 ] [ 134 ] [ 135 ] [ 136 ] [ 137 ] [ 138 ]


Financial incentives have been shown to have a significant impact on X-ray use with doctors who are paid a separate fee for each X-ray providing more X-rays. [ 139 ]


Early photon tomography or EPT [ 140 ]  (as of 2015) along with other techniques [ 141 ]  are being researched as potential alternatives to X-rays for imaging applications.


Other notable uses of X-rays include:


While generally considered invisible to the human eye, in special circumstances X-rays can be visible. Brandes, in an experiment a short time after Röntgen's landmark 1895 paper, reported after dark adaptation and placing his eye close to an X-ray tube, seeing a faint "blue-gray" glow which seemed to originate within the eye itself. [ 146 ]  Upon hearing this, Röntgen reviewed his record books and found he too had seen the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen had noted the same blue glow, seeming to emanate from the eye itself, but thought his observations to be spurious because he only saw the effect when he used one type of tube. Later he realized that the tube which had created the effect was the only one powerful enough to make the glow plainly visible and the experiment was thereafter readily repeatable. The knowledge that X-rays are actually faintly visible to the dark-adapted naked eye has largely been forgotten today; this is probably due to the desire not to repeat what would now be seen as a recklessly dangerous and potentially harmful experiment with  ionizing radiation . It is not known what exact mechanism in the eye produces the visibility: it could be due to conventional detection (excitation of  rhodopsin  molecules in the retina), direct excitation of retinal nerve cells, or secondary detection via, for instance, X-ray induction of  phosphorescence  in the eyeball with conventional retinal detection of the secondarily produced visible light. [ citation needed ]


Though X-rays are otherwise invisible, it is possible to see the  ionization  of the air molecules if the intensity of the X-ray beam is high enough. The beamline from the  wiggler  at the   European Synchrotron Radiation Facility [ 147 ]  is one example of such high intensity. [ 148 ]


The measure of X-rays  ionizing  ability is called the exposure: [ citation needed ]


However, the effect of ionizing radiation on matter (especially living tissue) is more closely related to the amount of energy deposited into them rather than the  charge  generated. This measure of energy absorbed is called the  absorbed dose : [ citation needed ]


The  equivalent dose  is the measure of the biological effect of radiation on human tissue. For X-rays it is equal to the  absorbed dose . [ citation needed ]


X-ray detectors  are devices used to measure the  flux ,  spatial  distribution,  spectrum , and/or other properties of  X-rays .


Detectors can be divided into two major categories: imaging detectors (such as  photographic plates  and X-ray film ( photographic film ), now mostly replaced by various  digitizing  devices like  image plates  or  flat panel detectors ) and dose measurement devices (such as  ionization chambers ,  Geiger counters , and  dosimeters  used to measure the local  radiation exposure ,  dose , and/or dose rate, for example, for verifying that  radiation protection  equipment and procedures are effective on an ongoing basis).


To obtain an image with any type of image detector the part of the patient to be X-rayed is placed between the X-ray source and the image receptor to produce a shadow of the internal structure of that particular part of the body. X-rays are partially blocked ("attenuated") by dense tissues such as bone, and pass more easily through soft tissues. Areas where the X-rays strike darken when developed, causing bones to appear lighter than the surrounding soft tissue.


Contrast compounds containing  barium  or  iodine , which are  radiopaque , can be ingested in the gastrointestinal tract (barium) or injected in the artery or veins to highlight these vessels. The contrast compounds have high atomic numbered elements in them that (like bone) essentially block the X-rays and hence the once hollow organ or vessel can be more readily seen. In the pursuit of nontoxic contrast materials, many types of high atomic number elements were evaluated. Some elements chosen proved to be harmful – for example,  thorium  was once used as a contrast medium ( Thorotrast ) – which turned out to be toxic, causing a very high incidence of cancer decades after use. Modern contrast material has improved and, while there is no way to determine who may have a sensitivity to the contrast, the incidence of serious allergic reactions is low. [ 1 ]


Typical x-ray film contains  silver halide  crystal "grains", typically primarily  silver bromide . [ 2 ]  Grain size and composition can be adjusted to affect the film properties, for example to improve  resolution  in the developed image. [ 3 ]  When the film is exposed to radiation the halide is  ionised  and free  electrons  are trapped in  crystal defects  (forming a  latent image ). Silver ions are attracted to these defects and  reduced , creating clusters of  transparent  silver  atoms . [ 4 ]  In the developing process these are converted to  opaque   silver  atoms which form the viewable image, darkest where the most radiation was detected. Further developing steps stabilise the sensitised grains and remove unsensitised grains to prevent further exposure (e.g. from  visible light ). [ 5 ] : 159  [ 6 ]


The first radiographs (X-ray images) were made by the action of X-rays on sensitized glass photographic plates. X-ray film (photographic film) soon replaced the glass plates, and film has been used for decades to acquire (and display) medical and industrial images. [ 7 ]  Gradually, digital  computers  gained the ability to store and display enough data to make digital imaging possible. Since the 1990s, computerized radiography and digital radiography have been replacing photographic film in medical and dental applications, though film technology remains in widespread use in industrial radiography processes (e.g. to inspect welded seams). The metal  silver  (formerly necessary to the radiographic & photographic industries) is a  non-renewable resource  although silver can easily be reclaimed from spent X-ray film. [ 8 ]  Where X-ray films required wet processing facilities, newer digital technologies do not. Digital archiving of images also saves physical storage space. [ 9 ]


Phosphor plate radiography [ 10 ]  is a method of recording X-rays using  photostimulated luminescence  (PSL), pioneered by  Fuji  in the 1980s. [ 11 ]  A photostimulable phosphor plate (PSP) is used in place of the photographic plate. After the plate is X-rayed, excited electrons in the phosphor material remain 'trapped' in ' colour centres ' in the crystal lattice until stimulated by a laser beam passed over the plate surface. [ 12 ]  The  light  given off during laser stimulation is collected by a  photomultiplier tube , and the resulting signal is converted into a digital image by computer technology. The PSP plate can be reused, and existing X-ray equipment requires no modification to use them. The technique may also be known as computed radiography (CR). [ 13 ]


X-rays are also used in "real-time" procedures such as  angiography  or contrast studies of the hollow organs (e.g.  barium enema  of the small or large intestine) using  fluoroscopy .  Angioplasty , medical interventions of the arterial system, rely heavily on X-ray-sensitive contrast to identify potentially treatable lesions.


Solid state detectors use  semiconductors  to detect x-rays. Direct digital detectors are so-called because they directly convert x-ray photons to electrical charge and thus a digital image. Indirect systems may have intervening steps for example first converting x-ray photons to  visible light , and then an electronic signal. Both systems typically use  thin film transistors  to read out and convert the electronic signal to a digital image. Unlike film or CR no manual scanning or development step is required to obtain a digital image, and so in this sense both systems are "direct". [ 14 ]  Both types of system have considerably higher  quantum efficiency  than CR. [ 14 ]


Since the 1970s,  silicon  or  germanium  doped with  lithium  (Si(Li) or Ge(Li))  semiconductor detectors  have been developed. [ 15 ]  X-ray photons are converted to electron-hole pairs in the semiconductor and are collected to detect the X-rays. When the temperature is low enough (the detector is cooled by  Peltier effect  or even cooler  liquid nitrogen ), it is possible to directly determine the X-ray energy spectrum; this method is called  energy-dispersive X-ray spectroscopy  (EDX or EDS); it is often used in small  X-ray fluorescence   spectrometers .  Silicon drift detectors  (SDDs), produced by conventional  semiconductor fabrication , provide a cost-effective and high resolving power radiation measurement. [ 16 ]  Unlike conventional X-ray detectors, such as Si(Li), they do not need to be cooled with liquid nitrogen. These detectors are rarely used for imaging and are only efficient at low energies. [ 17 ]


Practical application in  medical imaging  started in the early 2000s. [ 18 ]  Amorphous  selenium  is used in commercial large area flat panel X-ray detectors for  mammography  and general  radiography  due to its high spatial resolution and x-ray absorbing properties. [ 19 ]  However Selenium's low atomic number means a thick layer is required to achieve sufficient sensitivity. [ 20 ]


Cadmium telluride  ( Cd Te ), and its alloy with  zinc ,  cadmium zinc telluride , is considered one of the most promising semiconductor materials for x-ray detection due to its wide  band-gap  and high quantum number resulting in room temperature operation with high efficiency. [ 21 ] [ 22 ]  Current applications include  bone densitometry  and  SPECT  but flat panel detectors suitable for radiographic imaging are not yet in production. [ 23 ]  Current research and development is focused around energy resolving   pixel detectors , such as  CERN 's  Medipix  detector and  Science and Technology Facilities Council 's  HEXITEC  detector. [ 24 ] [ 25 ]


Common  semiconductor   diodes , such as  PIN photodiodes  or a  1N4007 , will produce a small amount of current in  photovoltaic mode  when placed in an X-ray beam. [ 26 ] [ 27 ]


Indirect detectors are made up of a  scintillator  to convert x-rays to visible light, which is read by a TFT array. This can provide sensitivity advantages over current (amorphous selenium) direct detectors, albeit with a potential trade-off in resolution. [ 20 ]  Indirect  flat panel detectors  (FPDs) are in widespread use today in medical, dental, veterinary, and industrial applications.


The TFT array consists of a sheet of glass covered with a thin layer of silicon that is in an amorphous or disordered state. At a microscopic scale, the silicon has been imprinted with millions of transistors arranged in a highly ordered array, like the grid on a sheet of graph paper. Each of these  thin-film transistors  (TFTs) is attached to a light-absorbing photodiode making up an individual  pixel  (picture element).  Photons  striking the photodiode are converted into two  carriers of electrical charge , called electron-hole pairs. Since the number of charge carriers produced will vary with the intensity of incoming light photons, an electrical pattern is created that can be swiftly converted to a voltage and then a digital signal, which is interpreted by a computer to produce a digital image. Although silicon has outstanding electronic properties, it is not a particularly good absorber of X-ray photons. For this reason, X-rays first impinge upon  scintillators  made from such materials as  gadolinium oxysulfide  or  caesium iodide . The scintillator absorbs the X-rays and converts them into visible light photons that then pass onto the photodiode array.


X-rays going through a  gas  will  ionize  it, producing positive  ions  and free  electrons . An incoming photon will create a number of such ion pairs  proportional  to its energy. If there is an  electric field  in the gas chamber ions and electrons will move in different directions and thereby cause a detectable  current . The behaviour of the gas will depend on the applied  voltage  and the geometry of the chamber. This gives rise to a few different types of gas detectors described below.


Ionization chambers  use a relatively low electric field of about 100 V/cm to extract all ions and electrons before they recombine. [ 28 ]  This gives a steady current proportional to the  dose  rate the gas is exposed to. [ 7 ]  Ion chambers are widely used as hand held radiation  survey meters  to check radiation dose levels.


Proportional counters  use a geometry with a thin positively charged  anode  wire in the center of a cylindrical chamber. Most of the gas volume will act as an ionization chamber, but in the region closest to the wire the electric field is high enough to make the electrons ionize gas molecules. This will create an  avalanche effect  greatly increasing the output signal. Since every electron cause an avalanche of approximately the same size the collected charge is proportional to the number of ion pairs created by the absorbed x-ray. This makes it possible to measure the energy of each incoming photon.


Geiger–Müller counters  use an even higher electric field so that  UV-photons  are created. [ 29 ]  These start new avalanches, eventually resulting in a total ionization of the gas around the anode wire. This makes the signal very strong, but causes a dead time after each event and makes it impossible to measure the X-ray energies. [ 30 ]


Gas detectors are usually single pixel detectors measuring only the average dose rate over the gas volume or the number of interacting photons as explained above, but they can be made spatially resolving by having many crossed wires in a  wire chamber .


It was demonstrated in the 1960s that silicon PN  solar cells  are suitable for detection of all forms of ionizing radiation including  extreme UV , soft X-rays, and hard X-rays. This form of detection operates via  photoionization , a process where ionizing radiation strikes an atom and releases a free electron. [ 31 ]  This type of  broadband ionizing radiation sensor  requires a solar cell, an  ammeter , and a visible light filter on top of the solar cell that allows the ionizing radiation to hit the solar cell while blocking unwanted wavelengths.


Self-developing radiochromic film can provide very high resolution measurements, for dosimetry and profiling purposes, particularly in radiotherapy physics. [ 32 ]


An  X-ray machine  is a device that uses  X-rays  for a variety of applications including  medicine ,  X-ray fluorescence , electronic assembly inspection, and measurement of material thickness in manufacturing operations. In medical applications, X-ray machines are used by  radiographers  to acquire x-ray images of the internal structures (e.g., bones) of living organisms, and also in  sterilization .


An X-ray generator generally contains an  X-ray tube  to produce the X-rays. Possibly,  radioisotopes  can also be used to generate X-rays. [ 1 ]


An X-ray tube is a simple  vacuum tube  that contains a  cathode , which directs a stream of electrons into a vacuum, and an  anode , which collects the electrons and is made of tungsten to evacuate the heat generated by the collision. When the electrons collide with the target, about 1% of the resulting energy is emitted as  X-rays , with the remaining 99% released as heat. Due to the high energy of the electrons that reach relativistic speeds, the target is usually made of  tungsten  even if other material can be used particularly in XRF applications. [ citation needed ]


An X-ray generator also needs to contain a cooling system to cool the anode; many X-ray generators use water or oil recirculating systems. [ 2 ]


In medical imaging applications, an X-ray machine has a control console that is used by a radiologic technologist to select X-ray techniques suitable for the specific exam, a power supply that creates and produces the desired kVp (peak kilovoltage), mA (milliamperes, sometimes referred to as mAs which is actually mA multiplied by the desired exposure length) for the X-ray tube, and the X-ray tube itself.


The discovery of X-rays came from experimenting with  Crookes tubes , an early experimental electrical discharge tube invented by English physicist  William Crookes  around 1869–1875. In 1895,  Wilhelm Röntgen  discovered X-rays emanating from Crookes tubes and the many uses for X-rays were immediately apparent. One of the first X-ray photographs was made of the hand of Röntgen's wife. The image displayed both her wedding ring and bones. On January 18, 1896 an  X-ray machine  was formally displayed by  Henry Louis Smith . A fully functioning unit was introduced to the public at the  1904 World's Fair  by  Clarence Dally . [ 3 ]  The technology developed quickly: In 1909  Mónico Sánchez Moreno  had produced the first portable medical device and during World War I  Marie Curie  led the development of X-ray machines mounted in "radiological cars" to provide mobile X-ray services for military field hospitals.


In the 1940s and 1950s, X-ray machines were used in stores to help sell footwear. These were known as  Shoe-fitting fluoroscopes . However, as the harmful effects of  X-ray   radiation  were properly considered, they finally fell out of use. Shoe-fitting use of the device was first banned by the state of  Pennsylvania  in 1957. (They were more a clever marketing tool to attract customers, rather than a fitting aid.) Together with  Robert J. Van de Graaff ,  John G. Trump  developed one of the first million-volt X-ray generators.


An X-ray imaging system consists of a generator control console where the operator selects desired techniques to obtain a quality readable image(kVp, mA and exposure time), an x-ray generator which controls the x-ray tube current, x-ray tube kilovoltage and x-ray emitting exposure time, an  X-ray tube  that converts the kilovoltage and mA into actual x-rays and an image detection system which can be either a film (analog technology) or a digital capture system and a  PACS .


X-ray machines are used in  health care  for visualising bone structures, during surgeries (especially orthopedic) to assist surgeons in reattaching broken bones with screws or structural plates, assisting cardiologists in locating blocked arteries and guiding stent placements or performing angioplasties and for other dense tissues such as  tumours . Non-medicinal applications include  security  and material analysis.


The main fields in which x-ray machines are used in medicine are  radiography , radiotherapy, and  fluoroscopic-type procedures . Radiography is generally used for fast, highly penetrating images, and is usually used in areas with a high bone content but can also be used to look for tumors such as with mammography imaging.  Some forms of radiography include:


In fluoroscopy, imaging of the digestive tract is done with the help of a  radiocontrast agent  such as  barium sulfate , which is opaque to X-rays.


Radiotherapy  — the use of x-ray radiation to treat malignant and benign  cancer cells , a non-imaging application


Fluoroscopy  is used in cases where real-time visualization is necessary (and is most commonly encountered in everyday life at  airport security ). Some medical applications of fluoroscopy include:


X-rays are highly penetrating,  ionizing radiation , therefore X-ray machines are used to take pictures of dense tissues such as bones and teeth. This is because bones absorb the radiation more than the less dense  soft tissue . X-rays from a source pass through the body and onto a photographic cassette.  Areas where radiation is absorbed show up as lighter shades of grey (closer to white). This can be used to diagnose broken or fractured bones.


In 2012, European Commission of Radiation Protection set leakage radiation limit from X-ray generators such as X-ray tubes and CT machines as one mGy/hour at one metre distance from the machine. [ 4 ]


X-ray machines are used to screen objects non-invasively. Luggage at  airports  and student baggage at some  schools  are examined for possible weapons, including bombs. Prices of these Luggage X-rays vary from $50,000 to $300,000. The main parts of an X-ray Baggage Inspection System are the generator used to generate x-rays, the detector to detect radiation after passing through the baggage, signal processor unit (usually a PC) to process the incoming signal from the detector, and a conveyor system for moving baggage into the system. Portable pulsed X-ray Battery Powered X-ray Generator used in Security as shown in the figure provides EOD responders safer analysis of any possible target hazard.


When baggage is placed on the conveyor, it is moved into the machine by the operator. There is an  infrared  transmitter and receiver assembly to detect the baggage when it enters the tunnel. This assembly gives the signal to switch on the generator and signal processing system. The signal processing system processes incoming signals from the detector and reproduce an image based upon the type of material and material density inside the baggage. This image is then sent to the display unit.


The colour of the image displayed depends upon the material and material density : organic material such as paper, clothes and most explosives are displayed in orange. Mixed materials such as aluminum are displayed in green. Inorganic materials such as copper are displayed in blue and non-penetrable items are displayed in black (some machines display this as a yellowish green or red). The darkness of the color depends upon the density or thickness of the material.


The material density determination is achieved by two-layer detector. The layers of the detector pixels are separated with a strip of metal. The metal absorbs soft rays, letting the shorter, more penetrating wavelengths through to the bottom layer of detectors, turning the detector to a crude two-band spectrometer.


A film of  carbon nanotubes  (as a cathode) that emits electrons at room temperature when exposed to an electrical field has been fashioned into an X-ray device. An array of these emitters can be placed around a target item to be scanned and the images from each emitter can be assembled by computer software to provide a 3-dimensional image of the target in a fraction of the time it takes using a conventional X-ray device. The system also allows rapid, precise control, enabling prospective physiological gated imaging. [ 6 ]


Engineers at the  University of Missouri  (MU),  Columbia , have invented a compact source of x-rays and other forms of radiation.
The radiation source is the size of a stick of gum and could be used to create portable x-ray scanners. A prototype handheld x-ray scanner using the source could be manufactured in as soon as three years. [ 7 ]


An  X-ray image intensifier  (XRII) is an  image intensifier  that converts  X-rays  into visible light at higher  intensity  than the more traditional  fluorescent  screens can. Such intensifiers are used in X-ray imaging systems (such as  fluoroscopes ) to allow low-intensity X-rays to be converted to a conveniently  bright  visible light output. The device contains a low absorbency/scatter input window, typically aluminum, input fluorescent screen, photocathode, electron optics, output fluorescent screen and output window. These parts are all mounted in a high vacuum environment within glass or, more recently, metal/ceramic. By its  intensifying  effect, It allows the viewer to more easily see the structure of the object being imaged than fluorescent screens alone, whose images are dim. The XRII requires lower  absorbed doses  due to more efficient conversion of X-ray quanta to visible light. This device was originally introduced in 1948. [ 1 ]


The overall function of an image intensifier is to convert incident x-ray photons to light photons of sufficient intensity to provide a viewable image. This occurs in several stages. The input window is convex is shape, made up of aluminium to minimise the scattering of X-rays. The window is 1 mm in thickness. Once X-rays pass through the aluminium windows, it encounters input  phosphor  that converts X-rays into light photons. The thickness of input phosphor range from 300 to 450 micrometres reach a compromise between absorption efficiency of X-rays and spatial resolution. Thicker input phosphor has higher absorption efficiency but poor spatial resolution and vice versa. Sodium activated Caesium Iodide is typically used due to its higher conversion efficiency thanks to high  atomic number  and  mass attenuation coefficient , when compared to zinc-cadmium sulfide. The input phosphor are arranged into small tubes, to allow photons to pass through the tube, without scattering, this improving the spatial resolution. [ 2 ]  The light photons are then converted to  electrons  by a photocathode. The photocathode is made up of antimony caesium, which is to match the photons produced from input phosphor, thus maximise the efficiency of producing photoelectrons. The photocathode has a thickness of 20 nm with absorption efficacy of 10 to 15%. [ 2 ]


A  potential difference  (25-35 kilovolts) created between the anode and photocathode then accelerates these photoelectrons while  electron lenses  focus the beam down to the size of the output window. The output window is typically made of silver-activated zinc-cadmium sulfide and converts incident electrons back to visible light photons. [ 2 ]  At the input and output phosphors the number of photons is multiplied by several thousands, so that overall there is a large brightness gain. This gain makes image intensifiers highly sensitive to X-rays such that relatively low doses can be used for fluoroscopic procedures. [ 3 ] [ 4 ] [ 5 ] [ 6 ]


X-ray image intensifiers became available in the early 1950s and were viewed through a microscope. [ 7 ]


Viewing of the output was via mirrors and optical systems until the adaption of television systems in the 1960s. [ 8 ]  Additionally, the output was able to be captured on systems with a 100mm cut film camera using pulsed outputs from an X-ray tube similar to a normal radiographic exposure; the difference being the II rather than a film screen cassette provided the image for the film to record.


The input screens range from 15–57 cm, with the 23 cm, 33 cm and 40 cm being among the most common. Within each image intensifier, the actual field size can be changed using the voltages applied to the internal electron optics to achieve magnification and reduced viewing size. For example, the 23 cm commonly used in cardiac applications can be set to a format of 23, 17, and 13 cm. Because the output screen remains fixed in size, the output appears to "magnify" the input image.  High-speed digitalisation with analogue video signal came about in the mid-1970s, with pulsed fluoroscopy developed in the mid-1980s harnessing low dose rapid switching X-ray tubes. In the late 1990s image intensifiers began being replaced with flat panel detectors (FPDs) on fluoroscopy machines giving competition to the image intensifiers. [ 9 ]


"C-arm" mobile fluoroscopy machines are often colloquially referred to as  image intensifiers  (or IIs), [ 10 ]  however strictly speaking the image intensifier is only one part of the machine (namely the detector).


Fluoroscopy, using an X-ray machine with an image intensifier, has applications in many areas of medicine. Fluoroscopy allows live images to be viewed so that  image-guided surgery  is feasible. Common uses include  orthopedics ,  gastroenterology  and  cardiology . [ 11 ]  Less common applications can include  dentistry . [ 12 ]


A system containing an  image intensifier  may be used either as a fixed piece of equipment in a dedicated screening room or as mobile equipment for use in an  operating theatre . A mobile fluoroscopy unit generally consists of two units, the  X-ray generator  and image detector (II) on a moveable C-arm, and a separate workstation unit used to store and manipulate the images. [ 13 ]  The patient is positioned between the two arms, typically on a  radiolucent  bed. Fixed systems may have a c-arm mounted to a ceiling gantry, with a separate control area. Most systems arranged as c-arms can have the image intensifier positioned above or below the patient (with the X-ray tube below or above respectively), although some static in room systems may have fixed orientations. [ 14 ]  From a  radiation protection  standpoint, under-couch (X-ray tube) operation is preferable as it reduces the amount of  scattered  radiation on operators and workers. [ 15 ] [ 16 ]  Smaller "mini" mobile c-arms are also available, primarily used to image extremities, for example for minor  hand surgery . [ 17 ]