Recent studies show that organs like the liver can also be used to estimate age and sex, because of the unique feature of liver. [ 41 ]  Liver weight increases with age and is different between males and females. Thus, the liver can be employed in special medico-legal cases of skeletal deformities or mutilation.


A table of possible causes of abnormal stature and the expected bone age associated with each condition is provided below.


Formation of the human skeletal system begins in fetal life with the development of a loosely ordered connective tissue known as  mesenchyme . [ 43 ]  The cells of the mesenchyme can become bone by one of two primary methods: (1)  intramembranous ossification  where mesenchymal cells differentiate directly into bone or (2)  endochondral ossification  where  mesenchymal cells become a cartilaginous model of chondrocytes which then become bone. [ 44 ] [ 45 ]  The bones of the limbs form and lengthen through endochondral ossification beginning by the 12th week after fertilization. [ 43 ]


At birth, only the  metaphyses  of the " long bones " are present. The long bones are those that grow primarily by elongation at an  epiphysis  at one end of the growing bone. The long bones include the  femurs ,  tibias , and  fibulas  of the lower limb, the  humeri ,  radii , and  ulnas  of the upper limb (arm + forearm), and the  phalanges  of the  fingers  and  toes . The long bones of the leg comprise nearly half of adult height. The other primary skeletal component of height is the  spine  and  skull .


As a child grows the epiphyses become calcified and appear on x-rays, as do the carpal and tarsal bones of the  hands  and feet, separated on x-rays by a layer of invisible  cartilage  where most of the growth is occurring. As  sex steroid  levels rise during puberty, bone maturation accelerates. As growth nears conclusion and attainment of adult height, bones begin to approach the size and shape of adult bones. The remaining cartilaginous portions of the epiphyses become thinner. As these cartilaginous zones become obliterated, the epiphyses are said to be " closed " and no further lengthening of the bones will occur. A small amount of spinal growth concludes an adolescent's growth.


The carpal bones arise from primary ossification centers and continue their calcification in an outward manner. The emergence of the primary ossification centers of the carpal bones appear in a predictable order that can help in determining bone age. First the capitate forms at an average age of 2 months, followed shortly by the hamate, then the triquetrum around 14 months, and so on. [ 46 ]


A  bone scan  or  bone scintigraphy   / s ɪ n ˈ t ɪ ɡ r ə f i /  is a  nuclear medicine  imaging technique used to help diagnose and assess different bone diseases. These include  cancer of the bone  or  metastasis , location of bone  inflammation  and  fractures  (that may not be visible in traditional  X-ray images ), and bone infection (osteomyelitis). [ 1 ]


Nuclear medicine provides functional imaging and allows visualisation of  bone metabolism  or  bone remodeling , which most other imaging techniques (such as X-ray  computed tomography , CT) cannot. [ 2 ] [ 3 ]  Bone  scintigraphy  competes with  positron emission tomography  (PET) for imaging of abnormal metabolism in bones, but is considerably less expensive. [ 4 ]  Bone scintigraphy has higher  sensitivity  but lower specificity than CT or MRI for diagnosis of  scaphoid fractures  following negative  plain radiography . [ 5 ]


Some of the earliest investigations into skeletal metabolism were carried out by  George de Hevesy  in the 1930s, using  phosphorus-32  and by  Charles Pecher  in the 1940s. [ 6 ] [ 7 ]


In the 1950s and 1960s calcium-45 was investigated, but as a  beta emitter  proved difficult to image. Imaging of  positron  and  gamma emitters  such as  fluorine-18  and  isotopes of strontium  with  rectilinear scanners  was more useful. [ 8 ] [ 9 ]  Use of  technetium-99m  ( 99m Tc) labelled  phosphates ,  diphosphonates  or similar agents, as in the modern technique, was first proposed in 1971. [ 10 ] [ 11 ]


The most common  radiopharmaceutical  for bone scintigraphy is  99m Tc with  methylene diphosphonate  (MDP). [ 12 ]  Other bone radiopharmaceuticals include  99m Tc with HDP, HMDP and DPD. [ 13 ] [ 14 ]  MDP  adsorbs  onto the crystalline  hydroxyapatite  mineral of bone. [ 15 ]  Mineralisation occurs at  osteoblasts , representing sites of bone growth, where MDP (and other diphosphates) "bind to the hydroxyapatite crystals in proportion to local blood flow and  osteoblastic  activity and are therefore markers of bone turnover and bone perfusion". [ 16 ] [ 17 ]


The more active the  bone turnover , the more radioactive material will be seen. Some  tumors ,  fractures  and  infections  show up as areas of increased uptake. [ 18 ]


Note that the technique depends on the osteoblastic activity during remodelling and repair processes following initial osteolytic activity. This leads to a limitation of the applicability of this imaging technique with diseases not featuring this osteoblastic (reactive) activity, for example with  multiple myeloma . Scintigraphic images remain falsely negative for a long period of time and therefore have only limited diagnostic value. In these cases CT or MRI scans are preferred for diagnosis and staging.


In a typical bone scan technique, the patient is injected (usually into a vein in the arm or hand, occasionally the foot) with up to 740  MBq  of  technetium-99m-MDP  and then scanned with a  gamma camera , which captures planar  anterior  and posterior or  single photon emission computed tomography  (SPECT) images. [ 19 ] [ 14 ]  In order to view small lesions SPECT imaging technique may be preferred over planar scintigraphy. [ 20 ]


In a single phase protocol (skeletal imaging alone), which will primarily highlight osteoblasts, images are usually acquired 2–5 hours after the injection (after four hours 50–60% of the activity will be fixed to bones). [ 19 ] [ 14 ] [ 21 ]  A two or three phase protocol utilises additional scans at different points after the injection to obtain additional diagnostic information. A dynamic (i.e. multiple acquired frames) study immediately after the injection captures  perfusion  information. [ 21 ] [ 22 ]  A second phase "blood pool" image following the perfusion (if carried out in a three phase technique) can help to diagnose inflammatory conditions or problems of blood supply. [ 23 ]


A typical  effective dose  obtained during a bone scan is 6.3  millisieverts  (mSv). [ 24 ]


Although bone scintigraphy generally refers to gamma camera imaging of  99m Tc radiopharmaceuticals, imaging with  positron emission tomography  (PET) scanners is also possible, using  fluorine-18   sodium fluoride  ([ 18 F]NaF).


For  quantitative  measurements,  99m Tc-MDP has some advantages over [ 18 F]NaF. MDP renal clearance is not affected by urine flow rate and simplified data analysis can be employed which assumes  steady state  conditions. It has negligible tracer uptake in  red blood cells , therefore correction for plasma to whole blood ratios is not required unlike [ 18 F]NaF. However, disadvantages include higher rates of protein binding (from 25% immediately after injection to 70% after 12 hours leading to the measurement of freely available MDP over time), and less  diffusibility  due to higher  molecular weight  than [ 18 F]NaF, leading to lower  capillary permeability . [ 25 ]


There are several advantages of the PET technique, which are common to PET imaging in general, including improved  spatial resolution  and more developed  attenuation  correction techniques. Patient experience is improved as imaging can be started much more quickly following radiopharmaceutical injection (30–45 minutes, compared to 2–3 hours for MDP/HDP). [ 26 ] [ 27 ]  [ 18 F]NaF PET is hampered by high demand for scanners, and limited tracer availability. [ 28 ] [ 29 ]


Caldwell's view  (or  Occipitofrontal view ) is a radiographic view of the skull where the X-ray plate is perpendicular to the  orbitomeatal line . The rays pass from behind the head and are angled at 15-20° to the radiographic plate. It is commonly used to get better view of the ethmoid and  frontal sinuses . [ 1 ]    It is named after the noted American radiologist  Eugene W. Caldwell , who described it in 1907. [ 2 ] [ 3 ]


A  collimator  is a device which narrows a beam of particles or waves. To narrow can mean either to cause the directions of motion to become more aligned in a specific direction (i.e., make  collimated light  or  parallel  rays), or to cause the spatial  cross section  of the beam to become smaller ( beam limiting device ).


The English physicist  Henry Kater  was the inventor of the  floating collimator , which rendered a great service to practical astronomy. He reported about his invention in January 1825. [ 1 ]  In his report, Kater mentioned previous work in this area by  Carl Friedrich Gauss  and  Friedrich Bessel .


In  optics , a collimator may consist of a  curved mirror  or  lens  with some type of light source and/or an image at its  focus . This can be used to replicate a target focused at  infinity  with little or no  parallax .


In  lighting , collimators are typically designed using the principles of  nonimaging optics . [ 2 ]


Optical collimators can be used to calibrate other optical devices, [ 3 ]  to check if all elements are aligned on the  optical axis , to set elements at proper focus, or to align two or more devices such as  binoculars  or  gun barrels  and  gunsights . [ 4 ]  A surveying camera may be collimated by setting its  fiduciary markers  so that they define the principal point, as in  photogrammetry .


Optical collimators are also used as  gun sights  in the  collimator sight , which is a simple optical collimator with a cross hair or some other  reticle  at its focus. The viewer only sees an image of the reticle. They have to use it either with both eyes open and one eye looking into the collimator sight, with one eye open and moving the head to alternately see the sight and the target, or with one eye to partially see the sight and target at the same time. [ 5 ] [ clarification needed ]  Adding a  beam splitter  allows the viewer to see the reticle and the  field of view , making a  reflector sight .


Collimators may be used with  laser diodes  and  CO 2  cutting lasers . Proper collimation of a laser source with long enough  coherence length  can be verified with a  shearing interferometer .


In  X-ray optics ,  gamma ray  optics, and  neutron  optics, a  collimator  is a device that filters a stream of rays so that only those traveling parallel to a specified direction are allowed through. Collimators are used for X-ray, gamma-ray, and neutron imaging because it is difficult to focus these types of radiation into an image using lenses, as is routine with  electromagnetic radiation  at optical or near-optical wavelengths. Collimators are also used in radiation detectors in  nuclear power stations  to make them directionally sensitive.


The figure to the right illustrates how a Söller collimator is used in neutron and X-ray machines. The upper panel shows a situation where a collimator is not used, while the lower panel introduces a collimator. In both panels the source of radiation is to the right, and the image is recorded on the gray plate at the left of the panels.


Without a collimator, rays from all directions will be recorded; for example, a ray that has passed through the top of the specimen (to the right of the diagram) but happens to be travelling in a downwards direction may be recorded at the bottom of the plate. The resultant image will be so blurred and indistinct as to be useless.


In the lower panel of the figure, a collimator has been added (blue bars). This may be a sheet of lead or other material opaque to the incoming radiation with many tiny holes bored through it or in the case of neutrons it can be a sandwich arrangement (which can be up to several feet long; see  ENGIN-X ) with many layers alternating between neutron absorbing material (e.g.,  gadolinium ) with neutron transmitting material. This can be something simple, such as air; alternatively, if mechanical strength is needed, a material such as aluminium may be used. If this forms part of a rotating assembly, the sandwich may be curved. This allows energy selection in addition to collimation; the curvature of the collimator and its rotation will present a straight path only to one energy of neutrons. Only rays that are travelling nearly parallel to the holes will pass through them—any others will be absorbed by hitting the plate surface or the side of a hole. This ensures that rays are recorded in their proper place on the plate, producing a clear image.


For  industrial radiography  using gamma radiation sources such as  iridium-192  or  cobalt-60 , a collimator (beam limiting device) allows the radiographer to control the exposure of radiation to expose a film and create a radiograph, to inspect materials for defects. A collimator in this instance is most commonly made of  tungsten , and is rated according to how many half value layers it contains, i.e., how many times it reduces undesirable radiation by half. For instance, the thinnest walls on the sides of a 4 HVL tungsten collimator 13 mm (0.52 in) thick will reduce the intensity of radiation passing through them by 88.5%. The shape of these collimators allows emitted radiation to travel freely toward the specimen and the x-ray film, while blocking most of the radiation that is emitted in undesirable directions such as toward workers.


Although collimators improve  resolution , they also reduce  intensity  by blocking incoming radiation, which is undesirable for remote sensing instruments that require high sensitivity. For this reason, the  gamma ray spectrometer  on the  Mars Odyssey  is a non-collimated instrument. Most lead collimators let less than 1% of incident photons through. Attempts have been made to replace collimators with electronic analysis.  [ citation needed ]


Collimators (beam limiting devices) are used in  linear accelerators  used for  radiotherapy  treatments. They help to shape the beam of radiation emerging from the machine and can limit the maximum field size of a beam.


The treatment head of a linear accelerator consists of both a primary and secondary collimator. The primary collimator is positioned after the electron beam has reached a vertical orientation. When using photons, it is placed after the beam has passed through the X-ray target. The secondary collimator is positioned after either a flattening filter (for photon therapy) or a scattering foil (for electron therapy). The secondary collimator consists of two jaws which can be moved to either enlarge or minimize the size of the treatment field.


New systems involving  multileaf collimators  (MLCs) are used to further shape a beam to localise treatment fields in radiotherapy. MLCs consist of approximately 50–120 leaves of heavy, metal collimator plates which slide into place to form the desired field shape.


To find the spatial resolution of a parallel hole collimator with a hole length,

 l

 {\displaystyle l}

 , a hole diameter

 D

 {\displaystyle D}

  and a distance to the imaged object

 s

 {\displaystyle s}

 , the following formula can be used

 R

 collimator

 =
 D
 +

 D
 s

 l

 effective

 {\displaystyle R_{\text{collimator}}=D+{\frac {Ds}{l_{\text{effective}}}}}

where the effective length is defined as

 l

 effective

 =
 l
 −

 2
 μ

 {\displaystyle l_{\text{effective}}=l-{\frac {2}{\mu }}}

Where

 μ

 {\displaystyle \mu }

  is the linear attenuation coefficient of the material from which the collimator is made.


Companion shadow  is a term used in describing  radiographs  that denotes the appearance of a smooth, homogenous, radiodensity with a well-defined margin that runs parallel with a bony landmark.  Companion shadows represent  soft tissue  that overlies the respective bony landmark in profile. They are not seen in every radiograph and can be misinterpreted as  pathology .


Computational human phantoms  are models of the  human body  used in  computerized analysis . Since the 1960s, the  radiological science  community has developed and applied these models for  ionizing radiation   dosimetry  studies. These models have become increasingly accurate with respect to the internal structure of the human body.


As computing evolved, so did the  phantoms . Graduating from phantoms based on simple  quadratic equations  to  voxelized  phantoms, which were based on actual  medical images  of the human body, was a major step. The newest models are based on more advanced mathematics, such as  non-uniform rational B-spline  (NURBS) and  polygon meshes , which allow for  4-D  phantoms where simulations can take place not only  3-dimensional space  but in time as well.


Phantoms have been developed for a wide variety of humans, from children to adolescents to adults, male and female, as well as pregnant women. With such a variety of phantoms, many kinds of  simulations  can be run, from  dose  received from medical imaging procedures to  nuclear medicine . Over the years, the results of these simulations have created an assortment of standards that have been adopted in the  International Commission on Radiological Protection  (ICRP) recommendations. [ 1 ]


The very first generation computational phantoms were developed to address the need to better assess  organ   doses  from internally deposited  radioactive materials  in workers and patients.  Until the late 1950s, the ICRP still used very simple models. [ 2 ]  In these calculations, each organ of the body was assumed to be represented as a  sphere  with an "effective  radius ". The  radionuclide  of interest was assumed to be located at the center of the sphere and the "effective absorbed energy" was calculated for each organ. Phantoms such as the  Shepp-Logan Phantom  were used as models of a human head in the development and testing of  image reconstruction  algorithms. [ 3 ] [ 4 ] [ 5 ] [ 6 ]  However, scientists attempted to model individual organs of the body and ultimately the entire human body in a realistic manner, the efforts of which led to stylized  anthropomorphic  phantoms that resemble the  human anatomy .


In general, stylized computational phantom is a mathematical representation of the human body which, when coupled with a  Monte Carlo   radiation transport   computer code , can be used to track the radiation interactions and energy deposition in the body. The feature of stylized computational phantom is finely tuned by adjusting individual parameters of the  mathematical equations , which describes the volume, position, and shape of individual  organs . Stylized computational phantom has a long history of development through the 1960s to 1980s.


The MIRD phantom [ 7 ]  was developed by Fisher and Snyder at  Oak Ridge National Laboratory  (ORNL) in the 1960s with 22  internal organs  and more than 100 sub-regions. [ 8 ] [ 9 ]  It is the first anthropomorphic phantom representing a  hermaphrodite  adult for internal  dosimetry .


Based on MIRD phantom, many derivations of phantoms were developed for the following decades. The major types of phantom include: stylized "Family" phantom series developed in the 1980s by Cristy and Eckerman; "ADAM and EVA" developed by GSF, Germany; CAM (Computerized Anatomical Man) phantom developed by  NASA  unknown by the mainstream radiation protection dosimetry community, etc.


Although many efforts were undertaken to diversify and extend its applications in  radiation protection ,  radiation therapy , and  medical imaging , one cannot overcome its inborn limitation. The representation of  internal organs  in this mathematical phantom was crude, by capturing only the most general description of the position and  geometry  of each organ. With the powerful computer and  tomographic  imaging technologies became available in the late 1980s, the history launched a new era of  voxel  phantoms.


The stylized phantoms provided only basic information with a large degree of error.  More accurate methods of simulating a human body were necessary to advance.  To allow further research, the computer technology had to become more powerful and more readily available.  This did not occur until the 1980s.  The real breakthrough occurred when  computed tomography  (CT) and  magnetic resonance imaging  (MRI) devices could generate highly accurate images of internal organs in three dimensions and in digital format.  Researchers discovered that they could take that  diagnostic  data and transform it into a  voxel  (volumetric pixel) format, essentially re-creating the human body in digital form in 3D.  Today there are over 38 human phantoms in voxel format, for many different uses. [ 11 ]


Two major issues with development of the reference phantoms are difficulty in obtaining useful images and handling the large amount of  data  created from these images.  CT scans give the human body a large  dose  of  ionizing radiation  – something the computational phantom was designed to circumvent in the first place.  MRI images take a long time to process.  Furthermore, most scans of a single subject cover only a small portion of the body, whereas a full scan series is needed for useful data.  Handling this data is also difficult.  While the newer computers had hard drives large enough to store the data, the memory requirements for processing the images to the desired voxel size were often too steep. [ 1 ]


While there have been many voxel phantoms developed, they have all followed a similar path to completion.  First, they must obtain the raw data, from CT scans, MRI imaging, or direct imaging through photography.  Second, the components of the body must be segmented, or identified and separated from the rest. Third, the density of each component must be identified, along with the composition of each. Lastly, the data must be unified into a single 3D structure so it may be used for analysis.


The earliest work on voxelized phantoms occurred independently at about the same time by Dr. Gibbs, of  Vanderbilt University , and Dr. Zankl at the  National Research Center for Environment and Health  (GSF) in Germany. [ 12 ] [ 13 ]  This occurred about 1982.  Dr. Gibb's work started with  X-ray  images, not CT or MRI images, for the reconstruction of a human phantom which was used for medical dose  simulations . M. Zankl and team did use CT imaging to create 12 phantoms, ranging from BABY to VISIBLE HUMAN.


A computational framework was presented, based on statistical shape modelling, for construction of race-specific organ models for internal radionuclide dosimetry and other nuclear-medicine applications. The proposed technique used to create the race-specific statistical phantom maintains anatomic realism and provides the statistical parameters for application to radionuclide dosimetry. [ 32 ]


Boundary representation  (BREP) phantoms are computational human models that contain exterior and interior anatomical features of a human body using boundary representation method. In the realm of  health and medical physics  they are primarily used for  ionizing radiation   dosimetry .


In the development of computational human phantoms, of particular interest is the concept of a  "deformable"  phantom whose  geometry  can be conveniently transformed to fit particular physical organ shapes, volumes, or body postures. Design of this type of phantom is realized by Non-Uniform Rational B-Spline (NURBS) method or polygonal mesh method, which are usually collectively called BREP methods. Compared to the voxel phantoms, BREP phantoms are better suited for geometry deformation and adjustment,  because a larger set of computerized operations are available, such as  extrusion ,  chamfering , blending,  drafting ,  shelling  and  tweaking . A major advantage of BREP phantoms is their ability to morph into an existing reference phantom or into the anatomy of a real worker or patient, which makes individual-specific dose calculation possible. [ 33 ]


Surfaces of a  non-uniform rational B-spline  (NURBS)-based phantom are defined by NURBS equations which are formulated by a set of control points. The shape and volume of a NURBS surface vary with the coordinates of  control points . This feature is useful in designing a time-dependent  4D  human body modeling. [ 33 ]   An example is given by NCAT phantoms by Segars et al., which is used to simulate cardiac and respiratory motions with more realistic modeling of the cardiac system.


A  polygonal mesh  is composed of a set of  vertices ,  edges , and  faces  that specify the shape of a  polyhedral object  in  3D space . The surfaces of the phantom are defined by a large amount of polygonal meshes, most commonly triangles. The polygonal mesh has three remarkable advantages in developing whole-body phantoms. Firstly, mesh surfaces depicting human anatomy can be conveniently obtained from real patient images or commercial human anatomy mesh models. Secondly, the polygonal mesh-based phantom has considerable flexibility in adjusting and fine-tuning its geometry, allowing the simulation of very complex anatomies. Thirdly, many commercial  computer aided design  (CAD) software, such as  Rhinoceros ,  AutoCAD ,  Visualization Toolkit  (VTK), provide built-in functions able to rapidly convert polygonal mesh into NURBS. [ 33 ]


Segars was the precursor of applying NURBS to phantom design. In 2001 his  doctoral thesis  described the method of developing a dynamic NURBS-based cardiac-torso (NCAT) phantom in detail. The phantom has a 4D beating heart model which was derived from 4D tagged (MRI) data. The remaining organs in the torso of the phantom were designed based on the  Visible Human Project  CT data set and were composed of 3D NURBS surfaces. Respiratory motion was also incorporated into this phantom.


In 2005, Xu et al. at Rensselaer Polytechnic Institute used the 3D VIP-Man phantom to simulate respiratory motions by adopting the gated respiratory motion data of the NCAT phantom. [ 35 ]  The 4D VIP-Man Chest phantom was used to study  external-beam treatment  planning for a  lung cancer  patient. [ 36 ]  In 2007, Xu's research group reported creation of a series of polygon-based phantoms representing a pregnant woman and her  fetus  at the end of 3, 6, and 9 month  gestations  (RPI Pregnant Females). [ 37 ]  The mesh data were initially obtained from separate anatomical information sources including a non-pregnant female, a 7-month pregnant woman CT data set, and a mesh model of the fetus. In 2008, two triangular mesh-based phantoms were created, named as RPI Deformable Adult Male and Female (RPI-AM, RPI-FM). [ 38 ] [ 39 ]  The anatomic parameters of the phantoms were made consistent with two datasets: the mass and density of internal organs originated from ICRP-23 and ICRP-89, and the whole-body height and weight percentile data were obtained from the  National Health and Nutrition Examination Survey  (NHANES 1999–2002). Later on, to study the relationship between breast size and lung dosimetry, a new group of phantoms were produced by altering the breast geometry of RPI-AF. [ 39 ]


From 2006 to 2009, researchers at the  University of Florida  designed a total of twelve "hybrid" male and female phantoms, representing newborn, 1-, 5-, 10-, and 15-year-old and adult male/females. [ 40 ] [ 41 ] [ 42 ]  The phantoms are addressed as " hybrid " because most organs and tissues were modeled by NURBS surfaces whereas the skeleton, brain and extra-thoracic airways were modeled by polygonal surfaces. [ 43 ]  Anatomic parameters of the phantoms were adjusted to match 4 reference datasets, i.e., standard  anthropometric  data, reference organ masses from ICRP Publication 89, reference elemental compositions provided in ICRP 89 as well as ICRU Report 46, and reference data on the alimentary tract organs given in ICRP Publications 89 and 100.


In 2008, researchers at  Vanderbilt University , in collaboration with researchers from  Duke University , developed a family of adult and pediatric phantoms by adapting the NURBS-based NCAT adult male and female phantoms. [ 43 ]  ICRP-89 reference body and organ values were used to adjust NURBS surfaces.


In 2009 Cassola et al. [ 44 ]  at the  Federal University of Pernambuco , Brazil, developed a pair of polygonal mesh-based phantoms in standing posture, FASH (Female Adult meSH) and MASH (Male Adult meSH). The methodology is very similar but not entirely identical to the one implemented in the designing of RPI-AM and RPI-FM.


In 2010, based on existing RPI-AM, researchers at  RPI  continued to create 5 more phantoms with different  body mass index  (BMI) ranging from 23 to 44 kg∙m-2. [ 45 ]  These phantoms are used to study the correlation between BMI and organ doses resulting from CT and  positron emission tomography  (PET) examinations.


In 2011 researchers at  Hanyang University , Korea, reported a polygon-surface reference Korean male phantom (PSRK-Man). [ 46 ]  This phantom was constructed by converting the Visible Korean Human-Man (VKH-man) into a polygonal mesh-based phantom. The height, weight, geometry of organs and tissues were adjusted to match the Reference Korean data. Without voxelization the PSRK-man could be directly implemented in  Geant4   Monte Carlo  simulation using a built-in function, but the  computation  time was 70~150 times longer than that required by High Definition Reference Korean-Man (HDRK-Man), a voxelized phantom derived also from VKH-man.


In 2012, researchers at  RPI  developed the Computational Human for Animated Dosimetry (CHAD) phantom, structured such that its posture could be adjusted in conjunction with data obtained using a  motion capture  system. [ 47 ]  This phantom can be used to simulate the movement of a worker involved in an occupational of nuclear accident scenario, allowing researchers to gain an understanding of the impact of changing posture in the course of worker movement on radiation dose.


A  computed tomography scan  ( CT scan ),  formerly called  computed axial tomography scan  ( CAT scan ), is a  medical imaging  technique used to obtain detailed internal images of the body. [ 2 ]  The personnel that perform CT scans are called  radiographers  or radiology technologists. [ 3 ] [ 4 ]
CT scanners use a rotating  X-ray tube  and a row of detectors placed in a  gantry  to measure X-ray  attenuations  by different tissues inside the body. The multiple  X-ray  measurements taken from different angles are then processed on a computer using  tomographic reconstruction  algorithms to produce  tomographic  (cross-sectional) images (virtual "slices") of a body. CT scans can be used in patients with metallic implants or pacemakers, for whom  magnetic resonance imaging  (MRI) is  contraindicated .


Since its development in the 1970s, CT scanning has proven to be a versatile imaging technique. While CT is most prominently used in  medical diagnosis , it can also be used to form images of non-living objects. The 1979  Nobel Prize in Physiology or Medicine  was awarded jointly to South African-American physicist  Allan MacLeod Cormack  and British electrical engineer  Godfrey Hounsfield  "for the development of computer-assisted tomography". [ 5 ] [ 6 ]


On the basis of image acquisition and procedures, various type of scanners are available in the market.


Sequential CT, also known as step-and-shoot CT, is a type of scanning method in which the CT table moves stepwise. The table increments to a particular location and then stops which is followed by the  X-ray tube  rotation and acquisition of a slice. The table then increments again, and another slice is taken. The table movement stops while taking slices. This results in an increased time of scanning. [ 7 ]


Spinning tube, commonly called  spiral CT , or helical CT, is an imaging technique in which an entire  X-ray tube  is spun around the central axis of the area being scanned. These are the dominant type of scanners on the market because they have been manufactured longer and offer a lower cost of production and purchase. The main limitation of this type of CT is the bulk and inertia of the equipment (X-ray tube assembly and detector array on the opposite side of the circle) which limits the speed at which the equipment can spin. Some designs use two X-ray sources and detector arrays offset by an angle, as a technique to improve temporal resolution. [ 8 ] [ 9 ]


Electron beam tomography  (EBT) is a specific form of CT in which a large enough X-ray tube is constructed so that only the path of the  electrons , travelling between the  cathode  and  anode  of the X-ray tube, are spun using  deflection coils . [ 10 ]  This type had a major advantage since sweep speeds can be much faster, allowing for less blurry imaging of moving structures, such as the heart and arteries. [ 11 ]  Fewer scanners of this design have been produced when compared with spinning tube types, mainly due to the higher cost associated with building a much larger X-ray tube and detector array and limited anatomical coverage. [ 12 ]


Dual Energy CT, also known as Spectral CT, is an advancement of Computed Tomography in which two energies are used to create two sets of data. [ 13 ]  A Dual Energy CT may employ Dual source, Single source with dual detector layer, Single source with energy switching methods to get two different sets of data. [ 14 ]


CT perfusion imaging is a specific form of CT to assess flow through  blood vessels  whilst injecting a  contrast agent . [ 21 ]  Blood flow, blood transit time, and organ blood volume, can all be calculated with reasonable  sensitivity and specificity . [ 21 ]  This type of CT may be used on the  heart , although sensitivity and specificity for detecting abnormalities are still lower than for other forms of CT. [ 22 ]  This may also be used on the  brain , where CT perfusion imaging can often detect poor brain perfusion well before it is detected using a conventional spiral CT scan. [ 21 ] [ 23 ]  This is better for  stroke  diagnosis than other CT types. [ 23 ]


Positron emission tomography–computed tomography is a hybrid CT modality which combines, in a single gantry, a  positron emission tomography  (PET) scanner and an X-ray computed tomography (CT) scanner, to acquire sequential images from both devices in the same session, which are combined into a single superposed ( co-registered ) image. Thus,  functional imaging  obtained by PET, which depicts the spatial distribution of metabolic or biochemical activity in the body can be more precisely aligned or correlated with anatomic imaging obtained by CT scanning. [ 24 ]


PET-CT gives both anatomical and functional details of an organ under examination and is helpful in detecting different type of cancers. [ 25 ] [ 26 ]


Since its introduction in the 1970s, [ 27 ]  CT has become an important tool in  medical imaging  to supplement conventional  X-ray  imaging and  medical ultrasonography . It has more recently been used for  preventive medicine  or  screening  for disease, for example,  CT colonography  for people with a high risk of  colon cancer , or full-motion heart scans for people with a high risk of heart disease. Several institutions offer  full-body scans  for the general population although this practice goes against the advice and official position of many professional organizations in the field primarily due to the  radiation dose  applied. [ 28 ]


The use of CT scans has increased dramatically over the last two decades in many countries. [ 29 ]  An estimated 72 million scans were performed in the United States in 2007 and more than 80 million in 2015. [ 30 ] [ 31 ]


CT scanning of the head is typically used to detect  infarction  ( stroke ),  tumors ,  calcifications ,  haemorrhage , and bone  trauma . [ 32 ]  Of the above,  hypodense  (dark) structures can indicate  edema  and infarction, hyperdense (bright) structures indicate calcifications and haemorrhage and bone trauma can be seen as disjunction in bone windows. Tumors can be detected by the swelling and anatomical distortion they cause, or by surrounding edema. CT scanning of the head is also used in CT- guided   stereotactic surgery  and  radiosurgery  for treatment of intracranial tumors,  arteriovenous malformations , and other surgically treatable conditions using a device known as the  N-localizer . [ 33 ] [ 34 ] [ 35 ] [ 36 ] [ 37 ] [ 38 ]


Contrast CT  is generally the initial study of choice for  neck masses  in adults. [ 39 ]   CT of the thyroid  plays an important role in the evaluation of  thyroid cancer . [ 40 ]  CT scan often incidentally finds thyroid abnormalities, and so is often the preferred investigation modality for thyroid abnormalities. [ 40 ]


A CT scan can be used for detecting both acute and chronic changes in the  lung parenchyma , the tissue of the  lungs . [ 41 ]  It is particularly relevant here because normal two-dimensional X-rays do not show such defects. A variety of techniques are used, depending on the suspected abnormality. For evaluation of chronic interstitial processes such as  emphysema , and  fibrosis , [ 42 ]  thin sections with high spatial frequency reconstructions are used; often scans are performed both on inspiration and expiration. This special technique is called  high resolution CT  that produces a sampling of the lung, and not continuous images. [ 43 ]


Bronchial wall thickening  can be seen on lung CTs and generally (but not always) implies inflammation of the  bronchi . [ 44 ]


An  incidentally  found nodule in the absence of symptoms (sometimes referred to as an  incidentaloma ) may raise concerns that it might represent a tumor, either  benign  or  malignant . [ 45 ]  Perhaps persuaded by fear, patients and doctors sometimes agree to an intensive schedule of CT scans, sometimes up to every three months and beyond the recommended guidelines, in an attempt to do surveillance on the nodules. [ 46 ]  However, established guidelines advise that patients without a prior history of cancer and whose solid nodules have not grown over a two-year period are unlikely to have any malignant cancer. [ 46 ]  For this reason, and because no research provides supporting evidence that intensive surveillance gives better outcomes, and because of risks associated with having CT scans, patients should not receive CT screening in excess of those recommended by established guidelines. [ 46 ]


Computed tomography angiography  (CTA) is a type of  contrast CT  to visualize the  arteries  and  veins  throughout the body. [ 47 ]  This ranges from arteries serving the  brain  to those bringing blood to the  lungs ,  kidneys ,  arms  and  legs . An example of this type of exam is  CT pulmonary angiogram  (CTPA) used to diagnose  pulmonary embolism  (PE). It employs computed tomography and an  iodine-based contrast agent  to obtain an image of the  pulmonary arteries . [ 48 ] [ 49 ] [ 50 ]  CT scans can reduce the risk of angiography by providing clinicians with more information about the positioning and number of clots prior to the procedure. [ 51 ] [ 52 ]


A CT scan of the heart is performed to gain knowledge about cardiac or coronary anatomy. [ 53 ]  Traditionally, cardiac CT scans are used to detect, diagnose, or follow up  coronary artery disease . [ 54 ]  More recently CT has played a key role in the fast-evolving field of  transcatheter structural heart interventions , more specifically in the transcatheter repair and replacement of heart valves. [ 55 ] [ 56 ] [ 57 ]


The main forms of cardiac CT scanning are:


To better visualize the anatomy, post-processing of the images is common. [ 54 ]  Most common are multiplanar reconstructions (MPR) and  volume rendering . For more complex anatomies and procedures, such as heart valve interventions, a true  3D reconstruction  or a 3D print is created based on these CT images to gain a deeper understanding. [ 62 ] [ 63 ] [ 64 ] [ 65 ]


CT is an accurate technique for diagnosis of  abdominal  diseases like  Crohn's disease , [ 66 ]  GIT bleeding, and diagnosis and staging of cancer, as well as follow-up after cancer treatment to assess response. [ 67 ]  It is commonly used to investigate  acute abdominal pain . [ 68 ]


Non-contrast-enhanced CT scans are the gold standard for diagnosing  kidney stone disease . [ 69 ]  They allow clinicians to estimate the size, volume, and density of stones, helping to guide further treatment; with size being especially important in predicting the time to spontaneous passage of a stone. [ 70 ]


For the  axial skeleton  and  extremities , CT is often used to image complex  fractures , especially ones around joints, because of its ability to reconstruct the area of interest in multiple planes. Fractures, ligamentous injuries, and  dislocations  can easily be recognized with a 0.2 mm resolution. [ 71 ] [ 72 ]  With modern dual-energy CT scanners, new areas of use have been established, such as aiding in the diagnosis of  gout . [ 73 ]


CT is used in  biomechanics  to quickly reveal the geometry, anatomy,  density  and  elastic moduli  of biological tissues. [ 74 ] [ 75 ]


Industrial CT scanning  (industrial computed tomography) is a process which uses X-ray equipment to produce 3D representations of components both externally and internally. Industrial CT scanning has been used in many areas of industry for internal inspection of components. Some of the key uses for CT scanning have been flaw detection, failure analysis, metrology, assembly analysis, image-based finite element methods [ 76 ]  and reverse engineering applications. CT scanning is also employed in the imaging and conservation of museum artifacts. [ 77 ]


CT scanning has also found an application in transport security (predominantly  airport security ) where it is currently used in a materials analysis context for explosives detection  CTX (explosive-detection device) [ 78 ] [ 79 ] [ 80 ] [ 81 ]  and is also under consideration for automated baggage/parcel security scanning using  computer vision  based object recognition algorithms that target the detection of specific threat items based on 3D appearance (e.g. guns, knives, liquid containers). [ 82 ] [ 83 ] [ 84 ]  Its usage in airport security pioneered at  Shannon Airport  in March 2022 has ended the ban on liquids over 100 ml there, a move that  Heathrow Airport  plans for a full roll-out on 1 December 2022 and the TSA spent $781.2 million on an order for over 1,000 scanners, ready to go live in the summer.


X-ray CT is used in geological studies to quickly reveal materials inside a drill core. [ 85 ]  Dense minerals such as pyrite and barite appear brighter and less dense components such as clay appear dull in CT images. [ 86 ]


Traditional methods of studying fossils are often destructive, such as the use of thin sections and physical preparation. X-ray CT is used in paleontology to non-destructively visualize fossils in 3D. [ 87 ]  This has many advantages. For example, we can look at fragile structures that might never otherwise be able to be studied. In addition, one can freely move around models of fossils in virtual 3D space to inspect it without damaging the fossil.


X-ray CT and  micro-CT  can also be used for the conservation and preservation of objects of cultural heritage. For many fragile objects, direct research and observation can be damaging and can degrade the object over time. Using CT scans, conservators and researchers are able to determine the material composition of the objects they are exploring, such as the position of ink along the layers of a scroll, without any additional harm. These scans have been optimal for research focused on the workings of the  Antikythera mechanism  or the text hidden inside the charred outer layers of the  En-Gedi Scroll . However, they are not optimal for every object subject to these kinds of research questions, as there are certain artifacts like the  Herculaneum papyri  in which the material composition has very little variation along the inside of the object. After scanning these objects, computational methods can be employed to examine the insides of these objects, as was the case with the virtual unwrapping of the  En-Gedi scroll  and the  Herculaneum papyri . [ 88 ]  Micro-CT has also proved useful for analyzing more recent artifacts such as still-sealed historic correspondence that employed the technique of  letterlocking  (complex folding and cuts) that provided a "tamper-evident locking mechanism". [ 89 ] [ 90 ]  Further examples of use cases in archaeology is imaging the contents of sarcophagi or ceramics. [ 91 ]


Recently, CWI in Amsterdam has collaborated with Rijksmuseum to investigate art object inside details in the framework called IntACT. [ 92 ]


Varied types of fungus can degrade wood to different degrees, one Belgium research group has been used X-ray CT 3 dimension with sub-micron resolution unveiled fungi can penetrate micropores of 0.6 μm [ 93 ]  under certain conditions.


Sawmills use industrial CT scanners to detect round defects, for instance knots, to improve total value of timber productions. Most sawmills are planning to incorporate this robust detection tool to improve productivity in the long run, however initial investment cost is high. [ citation needed ]


The result of a CT scan is a volume of  voxels , which may be presented to a human observer by various methods, which broadly fit into the following categories:


Technically, all volume renderings become projections when viewed on a  2-dimensional display , making the distinction between projections and volume renderings a bit vague. The epitomes of volume rendering models feature a mix of for example coloring and shading in order to create realistic and observable representations. [ 98 ] [ 99 ]


Two-dimensional CT images are conventionally rendered so that the view is as though looking up at it from the patient's feet. [ 100 ]  Hence, the left side of the image is to the patient's right and vice versa, while anterior in the image also is the patient's anterior and vice versa. This left-right interchange corresponds to the view that physicians generally have in reality when positioned in front of patients. [ 101 ]


Pixels  in an image obtained by CT scanning are displayed in terms of relative  radiodensity . The pixel itself is displayed according to the mean  attenuation  of the tissue(s) that it corresponds to on a scale from +3,071 (most attenuating) to −1,024 (least attenuating) on the  Hounsfield scale . A  pixel  is a two dimensional unit based on the matrix size and the field of view. When the CT slice thickness is also factored in, the unit is known as a  voxel , which is a three-dimensional unit. [ 102 ]  Water has an attenuation of 0  Hounsfield units  (HU), while air is −1,000 HU, cancellous bone is typically +400 HU, and cranial bone can reach 2,000 HU. [ 103 ]  The attenuation of metallic implants depends on the atomic number of the element used: Titanium usually has an amount of +1000 HU, iron steel can completely block the X-ray and is, therefore, responsible for well-known line-artifacts in computed tomograms. Artifacts are caused by abrupt transitions between low- and high-density materials, which results in data values that exceed the dynamic range of the processing electronics. [ 104 ]


CT data sets have a very high  dynamic range  which must be reduced for display or printing. This is typically done via a process of "windowing", which maps a range (the "window") of pixel values to a grayscale ramp. For example, CT images of the brain are commonly viewed with a window extending from 0 HU to 80 HU. Pixel values of 0 and lower, are displayed as black; values of 80 and higher are displayed as white; values within the window are displayed as a gray intensity proportional to position within the window. [ 105 ]  The window used for display must be matched to the X-ray density of the object of interest, in order to optimize the visible detail. [ 106 ]  Window width and window level parameters are used to control the windowing of a scan. [ 107 ]


Multiplanar reconstruction (MPR) is the process of converting data from one  anatomical plane  (usually  transverse ) to other planes. It can be used for thin slices as well as projections. Multiplanar reconstruction is possible as present CT scanners provide almost  isotropic  resolution. [ 108 ]


MPR is used almost in every scan. The spine is frequently examined with it. [ 109 ]  An image of the spine in axial plane can only show one vertebral bone at a time and cannot show its relation with other vertebral bones. By reformatting the data in other planes, visualization of the relative position can be achieved in sagittal and coronal plane. [ 110 ]


New software allows the reconstruction of data in non-orthogonal (oblique) planes, which help in the visualization of organs which are not in orthogonal planes. [ 111 ] [ 112 ]  It is better suited for visualization of the anatomical structure of the bronchi as they do not lie orthogonal to the direction of the scan. [ 113 ]


Curved-plane reconstruction (or curved planar reformation = CPR) is performed mainly for the evaluation of vessels. This type of reconstruction helps to straighten the bends in a vessel, thereby helping to visualize a whole vessel in a single image or in multiple images. After a vessel has been "straightened", measurements such as cross-sectional area and length can be made. This is helpful in preoperative assessment of a surgical procedure. [ 114 ]


For 2D projections used in  radiation therapy  for quality assurance and planning of  external beam radiotherapy , including digitally reconstructed radiographs, see  Beam's eye view .


A threshold value of radiodensity is set by the operator (e.g., a level that corresponds to bone). With the help of  edge detection  image processing algorithms a 3D model can be constructed from the initial data and displayed on screen. Various thresholds can be used to get multiple models, each anatomical component such as muscle, bone and cartilage can be differentiated on the basis of different colours given to them. However, this mode of operation cannot show interior structures. [ 116 ]


Surface rendering is limited technique as it displays only the surfaces that meet a particular threshold density, and which are towards the viewer. However, In volume rendering, transparency, colours and  shading  are used which makes it easy to present a volume in a single image. For example, Pelvic bones could be displayed as semi-transparent, so that, even viewing at an oblique angle one part of the image does not hide another. [ 117 ]


An important issue within radiology today is how to reduce the radiation dose during CT examinations without compromising the image quality. In general, higher radiation doses result in higher-resolution images, [ 118 ]  while lower doses lead to increased image noise and unsharp images. However, increased dosage raises the adverse side effects, including the risk of  radiation-induced cancer  – a four-phase abdominal CT gives the same radiation dose as 300 chest X-rays. [ 119 ]  Several methods that can reduce the exposure to ionizing radiation during a CT scan exist. [ 120 ]


Although images produced by CT are generally faithful representations of the scanned volume, the technique is susceptible to a number of  artifacts , such as the following: [ 124 ] [ 125 ] Chapters 3 and 5


CT scanning has several advantages over traditional  two-dimensional  medical  radiography . First, CT eliminates the superimposition of images of structures outside the area of interest. [ 140 ]  Second, CT scans have greater  image resolution , enabling examination of finer details. CT can distinguish between  tissues  that differ in   radiographic density  by 1% or less. [ 141 ]  Third, CT scanning enables multiplanar reformatted imaging: scan data can be visualized in the  transverse (or axial) ,  coronal , or  sagittal  plane, depending on the diagnostic task. [ 142 ]


The improved resolution of CT has permitted the development of new investigations. For example, CT  angiography  avoids the invasive insertion of a  catheter . CT scanning can perform a  virtual colonoscopy  with greater accuracy and less discomfort for the patient than a traditional  colonoscopy . [ 143 ] [ 144 ]  Virtual colonography is far more accurate than a  barium enema  for detection of tumors and uses a lower radiation dose. [ 145 ]


CT is a moderate-to-high  radiation  diagnostic technique. The radiation dose for a particular examination depends on multiple factors: volume scanned, patient build, number and type of scan protocol, and desired resolution and image quality. [ 146 ]  Two helical CT scanning parameters, tube current and pitch, can be adjusted easily and have a profound effect on radiation. CT scanning is more accurate than two-dimensional radiographs in evaluating anterior interbody fusion, although they may still over-read the extent of fusion. [ 147 ]


The  radiation  used in CT scans can damage body cells, including  DNA molecules , which can lead to  radiation-induced cancer . [ 148 ]  The radiation doses received from CT scans is variable. Compared to the lowest dose X-ray techniques, CT scans can have 100 to 1,000 times higher dose than conventional X-rays. [ 149 ]  However, a lumbar spine X-ray has a similar dose as a head CT. [ 150 ]  Articles in the media often exaggerate the relative dose of CT by comparing the lowest-dose X-ray techniques (chest X-ray) with the highest-dose CT techniques. In general, a routine abdominal CT has a radiation dose similar to three years of average  background radiation . [ 151 ]


Large scale population-based studies have consistently demonstrated that low dose radiation from CT scans has impacts on cancer incidence in a variety of cancers. [ 152 ] [ 153 ] [ 154 ] [ 155 ]  For example, in a large population-based Australian cohort it was found that up to 3.7% of brain cancers were caused by CT scan radiation. [ 156 ]  Some experts project that in the future, between three and five percent of all cancers would result from medical imaging. [ 149 ]  An Australian study of 10.9 million people reported that the increased incidence of cancer after CT scan exposure in this cohort was mostly due to irradiation. In this group, one in every 1,800 CT scans was followed by an excess cancer. If the lifetime risk of developing cancer is 40% then the absolute risk rises to 40.05% after a CT. The risks of CT scan radiation are especially important in patients undergoing recurrent CT scans within a short time span of one to five years. [ 157 ] [ 158 ] [ 159 ]


Some experts note that CT scans are known to be "overused," and "there is distressingly little evidence of better health outcomes associated with the current high rate of scans." [ 149 ]  On the other hand, a recent paper analyzing the data of patients who received high  cumulative doses  showed a high degree of appropriate use. [ 160 ]  This creates an important issue of cancer risk to these patients. Moreover, a highly significant finding that was previously unreported is that some patients received >100 mSv dose from CT scans in a single day, [ 158 ]  which counteracts existing criticisms some investigators may have on the effects of protracted versus acute exposure.


There are contrarian views and the debate is ongoing. Some studies have shown that publications indicating an increased risk of cancer from typical doses of body CT scans are plagued with serious methodological limitations and several highly improbable results, [ 161 ]  concluding that no evidence indicates such low doses cause any long-term harm. [ 162 ] [ 163 ] [ 164 ]
One study estimated that as many as 0.4% of cancers in the United States resulted from CT scans, and that this may have increased to as much as 1.5 to 2% based on the rate of CT use in 2007. [ 148 ]  Others dispute this estimate, [ 165 ]  as there is no consensus that the low levels of radiation used in CT scans cause damage. Lower radiation doses are used in many cases, such as in the investigation of renal colic. [ 166 ]


A person's age plays a significant role in the subsequent risk of cancer. [ 167 ]  Estimated lifetime cancer mortality risks from an abdominal CT of a one-year-old is 0.1%, or 1:1000 scans. [ 167 ]  The risk for someone who is 40 years old is half that of someone who is 20 years old with substantially less risk in the elderly. [ 167 ]  The  International Commission on Radiological Protection  estimates that the risk to a fetus being exposed to 10  mGy  (a unit of radiation exposure) increases the rate of cancer before 20 years of age from 0.03% to 0.04% (for reference a CT pulmonary angiogram exposes a fetus to 4 mGy). [ 168 ]  A 2012 review did not find an association between medical radiation and cancer risk in children noting however the existence of limitations in the evidences over which the review is based. [ 169 ]  CT scans can be performed with different settings for lower exposure in children with most manufacturers of CT scans as of 2007 having this function built in. [ 170 ]  Furthermore, certain conditions can require children to be exposed to multiple CT scans. [ 148 ]


Current recommendations are to inform patients of the risks of CT scanning. [ 171 ]  However, employees of imaging centers tend not to communicate such risks unless patients ask. [ 172 ]


In the United States half of CT scans are  contrast CTs  using intravenously injected  radiocontrast agents . [ 173 ]  The most common reactions from these agents are mild, including nausea, vomiting, and an itching rash. Severe life-threatening reactions may rarely occur. [ 174 ]  Overall reactions occur in 1 to 3% with  nonionic contrast  and 4 to 12% of people with  ionic contrast . [ 175 ]  Skin rashes may appear within a week to 3% of people. [ 174 ]


The old  radiocontrast agents  caused  anaphylaxis  in 1% of cases while the newer, low-osmolar agents cause reactions in 0.01–0.04% of cases. [ 174 ] [ 176 ]  Death occurs in about 2 to 30 people per 1,000,000 administrations, with newer agents being safer. [ 175 ] [ 177 ]
There is a higher risk of mortality in those who are female, elderly or in poor health, usually secondary to either anaphylaxis or  acute kidney injury . [ 173 ]


The contrast agent may induce  contrast-induced nephropathy . [ 178 ]  This occurs in 2 to 7% of people who receive these agents, with greater risk in those who have preexisting  kidney failure , [ 178 ]  preexisting  diabetes , or reduced intravascular volume. People with mild kidney impairment are usually advised to ensure full hydration for several hours before and after the injection. For moderate kidney failure, the use of  iodinated contrast  should be avoided; this may mean using an alternative technique instead of CT. Those with severe  kidney failure  requiring  dialysis  require less strict precautions, as their kidneys have so little function remaining that any further damage would not be noticeable and the dialysis will remove the contrast agent; it is normally recommended, however, to arrange dialysis as soon as possible following contrast administration to minimize any adverse effects of the contrast.


In addition to the use of intravenous contrast, orally administered contrast agents are frequently used when examining the abdomen. [ 179 ]  These are frequently the same as the intravenous contrast agents, merely diluted to approximately 10% of the concentration. However, oral alternatives to iodinated contrast exist, such as very dilute (0.5–1% w/v)  barium sulfate  suspensions. Dilute barium sulfate has the advantage that it does not cause allergic-type reactions or kidney failure, but cannot be used in patients with suspected bowel perforation or suspected bowel injury, as leakage of barium sulfate from damaged bowel can cause fatal  peritonitis . [ 180 ]


Side effects from  contrast agents , administered  intravenously  in some CT scans, might impair  kidney  performance in patients with  kidney disease , although this risk is now believed to be lower than previously thought. [ 181 ] [ 178 ]


The table reports average radiation exposures; however, there can be a wide variation in radiation doses between similar scan types, where the highest dose could be as much as 22 times higher than the lowest dose. [ 167 ]  A typical plain film X-ray involves radiation dose of 0.01 to 0.15 mGy, while a typical CT can involve 10–20 mGy for specific organs, and can go up to 80 mGy for certain specialized CT scans. [ 184 ]


For purposes of comparison, the world average dose rate from naturally occurring sources of  background radiation  is 2.4  mSv  per year, equal for practical purposes in this application to 2.4 mGy per year. [ 182 ]  While there is some variation, most people (99%) received less than 7 mSv per year as background radiation. [ 186 ]  Medical imaging as of 2007 accounted for half of the radiation exposure of those in the United States with CT scans making up two thirds of this amount. [ 167 ]  In the United Kingdom it accounts for 15% of radiation exposure. [ 168 ]  The average radiation dose from medical sources is ≈0.6 mSv per person globally as of 2007. [ 167 ]  Those in the nuclear industry in the United States are limited to doses of 50 mSv a year and 100 mSv every 5 years. [ 167 ]


Lead  is the main material used by radiography personnel for  shielding  against scattered X-rays.


The radiation dose reported in the  gray or mGy  unit is proportional to the amount of energy that the irradiated body part is expected to absorb, and the physical effect (such as DNA  double strand breaks ) on the cells' chemical bonds by X-ray radiation is proportional to that energy. [ 187 ]


The  sievert  unit is used in the report of the  effective dose . The sievert unit, in the context of CT scans, does not correspond to the actual radiation dose that the scanned body part absorbs but to another radiation dose of another scenario, the whole body absorbing the other radiation dose and the other radiation dose being of a magnitude, estimated to have the same probability to induce cancer as the CT scan. [ 188 ]  Thus, as is shown in the table above, the actual radiation that is absorbed by a scanned body part is often much larger than the effective dose suggests. A specific measure, termed the  computed tomography dose index  (CTDI), is commonly used as an estimate of the radiation absorbed dose for tissue within the scan region, and is automatically computed by medical CT scanners. [ 189 ]


The  equivalent dose  is the effective dose of a case, in which the whole body would actually absorb the same radiation dose, and the sievert unit is used in its report. In the case of non-uniform radiation, or radiation given to only part of the body, which is common for CT examinations, using the local equivalent dose alone would overstate the biological risks to the entire organism. [ 190 ] [ 191 ] [ 192 ]


Most adverse health effects of radiation exposure may be grouped in two general categories:


The added lifetime risk of developing cancer by a single abdominal CT of 8 mSv is estimated to be 0.05%, or 1 one in 2,000. [ 195 ]


Because of increased susceptibility of fetuses to radiation exposure, the radiation dosage of a CT scan is an important consideration in the choice of  medical imaging in pregnancy . [ 196 ] [ 197 ]


In October, 2009, the US  Food and Drug Administration  (FDA) initiated an investigation of brain perfusion CT (PCT) scans, based on  radiation burns  caused by incorrect settings at one particular facility for this particular type of CT scan. Over 200 patients were exposed to radiation at approximately eight times the expected dose for an 18-month period; over 40% of them lost patches of hair. This event prompted a call for increased CT quality assurance programs. It was noted that "while unnecessary radiation exposure should be avoided, a medically needed CT scan obtained with appropriate acquisition parameter has benefits that outweigh the radiation risks." [ 167 ] [ 198 ]  Similar problems have been reported at other centers. [ 167 ]  These incidents are believed to be due to  human error . [ 167 ]


CT scan procedure varies according to the type of the study and the organ being imaged. The patient is made to lie on the CT table and the centering of the table is done according to the body part. The IV line is established in case of contrast-enhanced CT. After selecting proper [ clarification needed ]  and rate of contrast from the pressure injector, the scout is taken to localize and plan the scan. Once the plan is selected, the contrast is given. The raw data is processed according to the study and proper windowing is done to make scans easy to diagnose. [ 199 ]


Patient preparation may vary according to the type of scan. The general patient preparation includes. [ 199 ]


Computed tomography operates by using an  X-ray generator  that rotates around the object;  X-ray detectors  are positioned on the opposite side of the circle from the X-ray source. [ 202 ]  As the X-rays pass through the patient, they are attenuated differently by various tissues according to the tissue density. [ 203 ]  A visual representation of the raw data obtained is called a sinogram, yet it is not sufficient for interpretation. [ 204 ]  Once the scan data has been acquired, the data must be processed using a form of  tomographic reconstruction , which produces a series of cross-sectional images. [ 205 ]  These cross-sectional images are made up of small units of pixels or voxels. [ 206 ]


Pixels  in an image obtained by CT scanning are displayed in terms of relative  radiodensity . The pixel itself is displayed according to the mean  attenuation  of the tissue(s) that it corresponds to on a scale from +3,071 (most attenuating) to −1,024 (least attenuating) on the  Hounsfield scale . A  pixel  is a two dimensional unit based on the matrix size and the field of view. When the CT slice thickness is also factored in, the unit is known as a  voxel , which is a three-dimensional unit. [ 206 ]


Water has an attenuation of 0  Hounsfield units  (HU), while air is −1,000 HU, cancellous bone is typically +400 HU, and cranial bone can reach 2,000 HU or more (os temporale) and can cause  artifacts . The attenuation of metallic implants depends on the atomic number of the element used: Titanium usually has an amount of +1000 HU, iron steel can completely extinguish the X-ray and is, therefore, responsible for well-known line-artifacts in computed tomograms. Artifacts are caused by abrupt transitions between low- and high-density materials, which results in data values that exceed the dynamic range of the processing electronics. Two-dimensional CT images are conventionally rendered so that the view is as though looking up at it from the patient's feet. [ 100 ]  Hence, the left side of the image is to the patient's right and vice versa, while anterior in the image also is the patient's anterior and vice versa. This left-right interchange corresponds to the view that physicians generally have in reality when positioned in front of patients.


Initially, the images generated in CT scans were in the  transverse  (axial)  anatomical plane , perpendicular to the long axis of the body. Modern scanners allow the scan data to be reformatted as images in other  planes .  Digital geometry processing  can generate a  three-dimensional  image of an object inside the body from a series of two-dimensional  radiographic  images taken by  rotation around a fixed axis . [ 124 ]  These cross-sectional images are widely used for medical  diagnosis  and  therapy . [ 207 ]


Contrast media  used for X-ray CT, as well as for  plain film X-ray , are called  radiocontrasts . Radiocontrasts for CT are, in general, iodine-based. [ 208 ]  This is useful to highlight structures such as blood vessels that otherwise would be difficult to delineate from their surroundings. Using contrast material can also help to obtain functional information about tissues. Often, images are taken both with and without radiocontrast. [ 209 ]


The history of X-ray computed tomography goes back to at least 1917 with the mathematical theory of the  Radon transform . [ 210 ] [ 211 ]  In October 1963,  William H. Oldendorf  received a U.S. patent for a "radiant energy apparatus for investigating selected areas of interior objects obscured by dense material". [ 212 ]  The first commercially viable CT scanner was invented by  Godfrey Hounsfield  in 1972. [ 213 ]


It is often claimed that revenues from the sales of The Beatles' records in the 1960s helped fund the development of the first CT scanner at EMI. The first production X-ray CT machines were in fact called EMI scanners. [ 214 ]


The word  tomography  is derived from the  Greek   tome  'slice' and  graphein  'to write'. [ 215 ]  Computed tomography was originally known as the "EMI scan" as it was developed in the early 1970s at a research branch of  EMI , a company best known today for its music and recording business. [ 216 ]  It was later known as  computed axial tomography  ( CAT  or  CT scan ) and  body section röntgenography . [ 217 ]


The term  CAT scan  is no longer in technical use because current CT scans enable for multiplanar reconstructions. This makes  CT scan  the most appropriate term, which is used by  radiologists  in common vernacular as well as in textbooks and scientific papers. [ 218 ] [ 219 ] [ 220 ]


In  Medical Subject Headings  (MeSH),  computed axial tomography  was used from 1977 to 1979, but the current indexing explicitly includes  X-ray  in the title. [ 221 ]


The term  sinogram  was introduced by Paul Edholm and Bertil Jacobson in 1975. [ 222 ]


In response to increased concern by the public and the ongoing progress of best practices, the Alliance for Radiation Safety in Pediatric Imaging was formed within the  Society for Pediatric Radiology . In concert with the  American Society of Radiologic Technologists , the  American College of Radiology  and the  American Association of Physicists in Medicine , the Society for Pediatric Radiology developed and launched the Image Gently Campaign which is designed to maintain high-quality imaging studies while using the lowest doses and best radiation safety practices available on pediatric patients. [ 224 ]  This initiative has been endorsed and applied by a growing list of various professional medical organizations around the world and has received support and assistance from companies that manufacture equipment used in Radiology.


Following upon the success of the  Image Gently  campaign, the American College of Radiology, the Radiological Society of North America, the American Association of Physicists in Medicine and the American Society of Radiologic Technologists have launched a similar campaign to address this issue in the adult population called  Image Wisely . [ 225 ]


The  World Health Organization  and  International Atomic Energy Agency  (IAEA) of the United Nations have also been working in this area and have ongoing projects designed to broaden best practices and lower patient radiation dose. [ 226 ] [ 227 ]


Use of CT has increased dramatically over the last two decades. [ 29 ]  An estimated 72 million scans were performed in the United States in 2007, [ 30 ]  accounting for close to half of the total per-capita dose rate from radiologic and nuclear medicine procedures. [ 228 ]  Of the CT scans, six to eleven percent are done in children, [ 168 ]  an increase of seven to eightfold from 1980. [ 167 ]  Similar increases have been seen in Europe and Asia. [ 167 ]  In Calgary, Canada, 12.1% of people who present to the emergency with an urgent complaint received a CT scan, most commonly either of the head or of the abdomen. The percentage who received CT, however, varied markedly by the  emergency physician  who saw them from 1.8% to 25%. [ 229 ]  In the emergency department in the United States, CT or  MRI  imaging is done in 15% of people who present with  injuries  as of 2007 (up from 6% in 1998). [ 230 ]


The increased use of CT scans has been the greatest in two fields: screening of adults (screening CT of the lung in smokers, virtual colonoscopy, CT cardiac screening, and whole-body CT in asymptomatic patients) and CT imaging of children. Shortening of the scanning time to around 1 second, eliminating the strict need for the subject to remain still or be sedated, is one of the main reasons for the large increase in the pediatric population (especially for the diagnosis of  appendicitis ). [ 148 ]  As of 2007, in the United States a proportion of CT scans are performed unnecessarily. [ 170 ]  Some estimates place this number at 30%. [ 168 ]  There are a number of reasons for this including: legal concerns, financial incentives, and desire by the public. [ 170 ]  For example, some healthy people avidly pay to receive full-body CT scans as  screening . In that case, it is not at all clear that the benefits outweigh the risks and costs. Deciding whether and how to treat  incidentalomas  is complex, radiation exposure is not negligible, and the money for the scans involves  opportunity cost . [ 170 ]


Major manufacturers of CT scanning devices and equipment are: [ 231 ]


Photon-counting computed tomography  is a CT technique currently under development. [ as of? ]  Typical CT scanners use energy integrating detectors; photons are measured as a voltage on a capacitor which is proportional to the X-rays detected. However, this technique is susceptible to noise and other factors which can affect the linearity of the voltage to X-ray intensity relationship. [ 232 ]  Photon counting detectors (PCDs) are still affected by noise but it does not change the measured counts of photons. PCDs have several potential advantages, including improving signal (and contrast) to noise ratios, reducing doses, improving spatial resolution, and through use of several energies, distinguishing multiple contrast agents. [ 233 ] [ 234 ]  PCDs have only recently become feasible in CT scanners due to improvements in detector technologies that can cope with the volume and rate of data required. As of February 2016, photon counting CT is in use at three sites. [ 235 ]  Some early research has found the dose reduction potential of photon counting CT for breast imaging to be very promising. [ 236 ]  In view of recent findings of high cumulative doses to patients from recurrent CT scans, there has been a push for scanning technologies and techniques that reduce ionising radiation doses to patients to sub- milliSievert  (sub-mSv in the literature) levels during the CT scan process, a goal that has been lingering. [ 237 ] [ 158 ] [ 159 ] [ 160 ]


Computed tomography enterography  ( CT enterography ,  CTE ) is a medical imaging technique which uses  computed tomography  scanner and  contrast media  to examine the  small bowel . [ 1 ]  It was first introduced by  Raptopoulos et al. in 1997. [ 2 ]  CT Enterography can be used to assess a variety of problems involving the small bowel, however it is mainly used to diagnose and assess severity of  Crohn's disease . [ 3 ]


CT enterography  should not be confused with  CT enteroclysis . In CT enterography contrast media is given orally, and in CT enteroclysis contrast media is administered through a  fluoroscopy -guided positioned  nasojejunal tube . [ 3 ]


CTE provides enough distention of the bowel not present during normal CT imaging to increase the ability to examine in lumen and internal lining of the small intestines. [ 4 ]  When the small bowel is not properly distended it can be difficult to see if there is a problem in that area. [ 5 ]   CTE also provides better visualization of extraenteric findings, as well as acute inflammation, of  Crohn's disease . These extraenteric findings include, but no limited to,  fistulas  and  abscesses . [ 5 ]  Additionally, compared with CT enteroclysis, the patient does not need to be sedated for CTE nor requires the invasive step of placing the nasojejunal tube. [ 3 ]


While CTE's main use is in the diagnosis and follow up in  Crohn's disease , many of the findings on Crohn's disease found on CTE can be caused by a wide variety of other conditions. [ 5 ]  Spasm and collapse of the small intestine, which can happen in Crohn's disease, can obscure imaging of that portion of the bowel even with CTE. [ 5 ]


At least four hours of no intake of solid foods, patient may have clear liquids.  Metoclopramide  (Reglan) will be administered to assist with emptying the stomach and increase movement through the small intestines. Large amounts of an oral contrast agent are given to the patient. Neutral contrast agents are preferred over positive contrast agents such as  barium . The neutral agents are vitally important for the effective visualization of the lining of the small intestine. Use of positive contrast agents could make it difficult to see any inflammation in the lining. Neutral agents include water, EG electrolyte solution,  sugar alcohols , and  methylcellulose . Patients are usually able to drink the large of amounts of these agents required for the study without major difficulty. [ 5 ]  This step is given at increments of 0, 20, 40, and 55 minutes after Reglan dose.  Glucagon  is given to patient five minutes before they enter the CT scanner to counter act the previous medication and attempt to slow down bowel activity.  Intravenous contrast is also given when the patient is on the scanner. The patient will then enter the scanner for the image to be captured. [ 3 ]


CTE is preferred for the examination of  Crohn's disease  due to its increased spatial resolution and better ability to examine the wall of the small intestine than traditional CT studies of the abdomen and pelvis. [ 5 ]  Findings on CTE that indicate active inflammation in the small bowel, possibly caused by Crohn's disease, include:


CTE is also used in examining if bowel inflammation improves after therapy and if the disease is progressing in a concerning way. [ 8 ]


Computer-aided detection  ( CADe ), also called  computer-aided diagnosis  ( CADx ), are systems that assist doctors in the interpretation of  medical images . Imaging techniques in  X-ray ,  MRI ,  endoscopy , and  ultrasound  diagnostics yield a great deal of information that the  radiologist  or other medical professional has to analyze and evaluate comprehensively in a short time. CAD systems process digital images or videos for typical appearances and to highlight conspicuous sections, such as possible diseases, in order to offer input to support a decision taken by the professional.


CAD also has potential future applications in  digital pathology  with the advent of whole-slide imaging and  machine learning  algorithms. So far its application has been limited to quantifying  immunostaining  but is also being investigated for the standard  H&E stain . [ 1 ]


CAD is an  interdisciplinary  technology combining elements of  artificial intelligence  and  computer vision  with radiological and  pathology  image processing. A typical application is the detection of a tumor. For instance, some hospitals use CAD to support preventive medical check-ups in  mammography  (diagnosis of breast cancer), the detection of polyps in  colonoscopy , and  lung cancer .


Computer-aided detection (CADe) systems are usually confined to marking conspicuous structures and sections. Computer-aided diagnosis (CADx) systems evaluate the conspicuous structures. For example, in mammography CAD highlights microcalcification clusters and hyperdense structures in the soft tissue. This allows the radiologist to draw conclusions about the condition of the pathology. Another application is CADq, which quantifies,  e.g. , the size of a tumor or the tumor's behavior in contrast medium uptake.  Computer-aided simple triage (CAST)  is another type of CAD, which performs a fully automatic initial interpretation and  triage  of studies into some meaningful categories ( e.g.  negative and positive). CAST is particularly applicable in emergency diagnostic imaging, where a prompt diagnosis of critical, life-threatening condition is required.


Although CAD has been used in clinical environments for over 40 years, CAD usually does not substitute the doctor or other professional, but rather plays a supporting role. The professional (generally a radiologist) is generally responsible for the final interpretation of a medical image. [ 2 ]  However, the goal of some CAD systems is to detect earliest signs of abnormality in patients that human professionals cannot, as in  diabetic retinopathy , architectural distortion in mammograms, [ 3 ] [ 4 ]  ground-glass nodules in thoracic CT, [ 5 ] [ 6 ]  and non-polypoid (“flat”) lesions in CT colonography. [ 7 ]


In the late 1950s, with the dawn of modern computers researchers in various fields started exploring the possibility of building computer-aided medical diagnostic (CAD) systems. [ 8 ]  These first CAD systems used flow-charts, statistical pattern-matching, probability theory, or knowledge bases to drive their decision-making process. [ 9 ]


In the early 1970s, some of the very early CAD systems in medicine, which were often referred as “ expert systems ” in medicine, were developed and used mainly for educational purposes. Examples include the  MYCIN  expert system, [ 10 ]  the  Internist-I  expert system [ 11 ]  and the  CADUCEUS (expert system) . [ 12 ]


During the beginning of the early developments, the researchers were aiming at building entirely automated CAD / expert systems. The expectated capability of computers was unrealistically optimistic among these scientists. However, after the breakthrough paper, “Reducibility among Combinatorial Problems” by  Richard M. Karp , [ 13 ]  it became clear that there were limitations but also potential opportunities when one develops algorithms to solve groups of important computational problems. [ 9 ]


As result of the new understanding of the various algorithmic limitations that Karp discovered in the early 1970s, researchers started realizing the serious limitations that CAD and expert systems in medicine have. [ 9 ]  The recognition of these limitations brought the investigators to develop new kinds of CAD systems by using advanced approaches. Thus, by the late 1980s and early 1990s the focus sifted in the use of  data mining  approaches for the purpose of using more advanced and flexible CAD systems.


In 1998, the first commercial CAD system for mammography, the ImageChecker system, was approved by the US Food and Drug Administration (FDA). In the following years several commercial CAD systems for analyzing mammography, breast MRI, medical imagining of lung, colon, and heart also received FDA approvals. Currently, CAD systems are used as a diagnostic aid to provide physicians for better medical decision-making. [ 14 ]


CAD is fundamentally based on highly complex  pattern recognition . X-ray or other types of images are scanned for suspicious structures. Normally a few thousand images are required to optimize the algorithm. Digital image data are copied to a CAD server in a  DICOM -format and are prepared and analyzed in several steps.


1. Preprocessing  for


2.  Segmentation  for


3. Structure/ROI (Region of Interest) Analyze
Every detected region is analyzed individually for special characteristics:


4. Evaluation / classification
After the structure is analyzed, every ROI is evaluated individually (scoring) for the probability of a TP. The following procedures are examples of classification algorithms.


If the detected structures have reached a certain threshold level, they are highlighted in the image for the radiologist. Depending on the CAD system these markings can be permanently or temporary saved. The latter's advantage is that only the markings which are approved by the radiologist are saved. False hits should not be saved, because an examination at a later date becomes more difficult then.


CAD systems seek to highlight suspicious structures. Today's CAD systems cannot detect 100% of pathological changes. The hit rate ( sensitivity ) can be up to 90% depending on system and application. [ 24 ]  A correct hit is termed a True Positive (TP), while the incorrect marking of healthy sections constitutes a False Positive (FP). The less FPs indicated, the higher the  specificity  is. A low specificity reduces the acceptance of the CAD system because the user has to identify all of these wrong hits. The FP-rate in lung overview examinations (CAD Chest) could be reduced to 2 per examination. In other segments ( e.g.  CT lung examinations) the FP-rate could be 25 or more. In  CAST  systems the FP rate must be extremely low (less than 1 per examination) to allow a meaningful study  triage .


The absolute detection rate of a radiologist is an alternative metric to sensitivity and specificity. Overall, results of clinical trials about sensitivity, specificity, and the absolute detection rate can vary markedly. Each study result depends on its basic conditions and has to be evaluated on those terms. The following facts have a strong influence:


Despite the many developments that CAD has achieved since the dawn of computers, there are still certain challenges that CAD systems face today. [ 25 ]


Some challenges are related to various algorithmic limitations in the procedures of a CAD system including input data collection, preprocessing, processing and system assessments. Algorithms are generally designed to select a single likely diagnosis, thus providing suboptimal results for patients with multiple, concurrent disorders. [ 26 ]  Today input data for CAD mostly come from  electronic health records  (EHR). Effective designing, implementing and analyzing for EHR is a major necessity on any CAD systems. [ 25 ]


Due to the massive availability of data and the need to analyze such data,  big data  is also one of the biggest challenges that CAD systems face today. The increasingly vast amount of patient data is a serious problem. Often the patient data are complex and can be semi-structured or  unstructured data . It requires highly developed approaches to store, retrieve and analyze them in reasonable time. [ 25 ]


During the preprocessing stage, input data must be normalized. The  normalization  of input data includes  noise reduction  and filtering.


Processing may contain a few sub-steps depending on applications. Basic three sub-steps on medical imaging are segmentation,  feature extraction  / selection, and classification. These sub-steps require advanced techniques to analyze input data with less computational time. Although much effort has been devoted to creating innovative techniques for these procedures of CAD systems, no single best algorithm has emerged for any individual step. Ongoing studies in building innovative algorithms for all the aspects of CAD systems is essential. [ 25 ]


There is also a lack of standardized assessment measures for CAD systems. [ 25 ]  This fact may cause the difficulty for obtaining approval for commercial use from governing bodies such as the  FDA . Moreover, while many positive developments of CAD systems have been proven, studies for validating their algorithms for clinical practice have not been confirmed. [ 27 ]


Other challenges are related to the problem for healthcare providers to adopt new CAD systems in clinical practice. Some negative studies may discourage the use of CAD. In addition, the lack of training of health professionals on the use of CAD sometimes brings the incorrect interpretation of the system outcomes. [ a ]


CAD is used in the diagnosis of  breast cancer ,  lung cancer ,  colon cancer ,  prostate cancer ,  bone metastases ,  coronary artery disease ,  congenital heart defect , pathological brain detection, fracture detection,  Alzheimer's disease , and  diabetic retinopathy .


CAD is used in screening  mammography  (X-ray examination of the female breast). Screening mammography is used for the early detection of breast cancer. CAD systems are often utilized to help classify a tumor as malignant (cancerous) or benign (non-cancerous). CAD is especially established in the US and the Netherlands and is used in addition to human evaluation, usually by a radiologist.


The first CAD system for mammography was developed in a research project at the  University of Chicago . Today it is commercially offered by iCAD and  Hologic . However, while achieving high sensitivities, CAD systems tend to have very low specificity and the benefits of using CAD remain uncertain. A 2008 systematic review on computer-aided detection in screening mammography concluded that CAD does not have a significant effect on cancer detection rate, but does undesirably increase recall rate ( i.e.  the rate of false positives). However, it noted considerable heterogeneity in the impact on recall rate across studies. [ 28 ]


Recent advances in  machine learning ,  deep-learning  and  artificial intelligence  technology have enabled the development of CAD systems that are clinically proven to assist  radiologists  in addressing the challenges of reading  mammographic  images by improving cancer detection rates and reducing false positives and unnecessary patient recalls, while significantly decreasing reading times. [ 29 ]


Procedures to evaluate mammography based on  magnetic resonance imaging  (MRI) exist too.


In the diagnosis of lung cancer,  computed tomography  with special three-dimensional CAD systems are established and considered as appropriate second opinions. [ 30 ]  At this a volumetric dataset with up to 3,000 single images is prepared and analyzed. Round lesions ( lung cancer , metastases and benign changes) from 1 mm are detectable. Today all well-known vendors of medical systems offer corresponding solutions.


Early detection of lung cancer is valuable. However, the random detection of lung cancer in the early stage (stage 1) in the X-ray image is difficult. Round lesions that vary from 5–10 mm are easily overlooked. [ 31 ]  The routine application of CAD Chest Systems may help to detect small changes without initial suspicion. A number of researchers developed CAD systems for detection of lung nodules (round lesions less than 30 mm) in chest radiography [ 32 ] [ 33 ] [ 34 ]  and CT, [ 35 ] [ 36 ]  and CAD systems for diagnosis ( e.g. , distinction between malignant and benign) of lung nodules in CT. Virtual dual-energy imaging [ 37 ] [ 38 ] [ 39 ] [ 40 ]  improved the performance of CAD systems in chest radiography. [ 41 ]


CAD is available for detection of  colorectal polyps  in the  colon  in CT colonography. [ 42 ] [ 43 ]  Polyps are small growths that arise from the inner lining of the colon. CAD detects the polyps by identifying their characteristic "bump-like" shape. To avoid excessive false positives, CAD ignores the normal colon wall, including the  haustral  folds.


State-of-the-art methods in cardiovascular computing, cardiovascular informatics, and mathematical and  computational modeling  can provide valuable tools in clinical decision-making. [ 44 ]  CAD systems with novel image-analysis-based markers as input can aid vascular physicians to decide with higher confidence on best suitable treatment for  cardiovascular disease  patients.


Reliable early-detection and risk-stratification of  carotid atherosclerosis  is of outmost importance for predicting  strokes  in asymptomatic patients. [ 45 ]  To this end, various noninvasive and low-cost markers have been proposed, using  ultrasound -image-based features. [ 46 ]  These combine  echogenicity , texture, and  motion [ 47 ] [ 48 ] [ 49 ] [ 50 ]  characteristics to assist clinical decision towards improved prediction, assessment and management of cardiovascular risk. [ 51 ]


CAD is available for the automatic detection of significant (causing more than 50%  stenosis )  coronary artery disease  in coronary CT angiography (CCTA) studies. [ 52 ]


Early detection of pathology can be the difference between life and death. CADe can be done by  auscultation  with a digital stethoscope and specialized software, also known as  computer-aided auscultation . Murmurs, irregular heart sounds, caused by blood flowing through a defective heart, can be detected with high sensitivity and specificity. Computer-aided auscultation is sensitive to external noise and bodily sounds and requires an almost silent environment to function accurately.


Chaplot et al. was the first to use  Discrete Wavelet Transform  (DWT) coefficients to detect pathological brains. [ 53 ]  Maitra and Chatterjee employed the Slantlet transform, which is an improved version of DWT. Their feature vector of each image is created by considering the magnitudes of Slantlet transform outputs corresponding to six spatial positions chosen according to a specific logic. [ 54 ]


In 2010, Wang and Wu presented a forward neural network (FNN) based method to classify a given MR brain image as normal or abnormal. The parameters of FNN were optimized via adaptive chaotic particle swarm optimization (ACPSO). Results over 160 images showed that the classification accuracy was 98.75%. [ 55 ]


In 2011, Wu and Wang proposed using DWT for feature extraction, PCA for feature reduction, and FNN with scaled chaotic artificial bee colony (SCABC) as classifier. [ 56 ]


In 2013, Saritha et al. were the first to apply wavelet entropy (WE) to detect pathological brains. Saritha also suggested to use spider-web plots. [ 57 ]  Later, Zhang et al. proved removing spider-web plots did not influence the performance. [ 58 ]  Genetic pattern search method was applied to identify abnormal brain from normal controls. Its classification accuracy was reported as 95.188%. [ 59 ]  Das et al. proposed to use Ripplet transform. [ 60 ]  Zhang et al. proposed to use particle swarm optimization (PSO). [ 61 ]  Kalbkhani et al. suggested to use GARCH model. [ 62 ]


In 2014, El-Dahshan et al. suggested the use of pulse coupled neural network. [ 63 ]


In 2015, Zhou et al. suggested application of naive  Bayes classifier  to detect pathological brains. [ 64 ]


CADs can be used to identify subjects with Alzheimer's and mild cognitive impairment from normal elder controls.


In 2014, Padma  et al . used combined wavelet statistical texture features to segment and classify AD benign and malignant tumor slices. [ 57 ]  Zhang et al. found kernel support vector machine decision tree had 80% classification accuracy, with an average computation time of 0.022s for each image classification. [ 65 ]


In 2019, Signaevsky  et al . have first reported a trained Fully Convolutional Network (FCN) for detection and quantification of  neurofibrillary tangles  (NFT) in Alzheimer's disease and an array of other tauopathies. The trained FCN achieved high precision and recall in naive  digital whole slide image  (WSI) semantic segmentation, correctly identifying NFT objects using a SegNet model trained for 200 epochs. The FCN reached near-practical efficiency with average processing time of 45 min per WSI per  graphics processing unit (GPU) , enabling reliable and reproducible large-scale detection of NFTs. The measured performance on test data of eight naive WSI across various tauopathies resulted in the  recall, precision , and an  F1 score  of 0.92, 0.72, and 0.81, respectively. [ 66 ]


Eigenbrain is a novel brain feature that can help to detect AD, based on  principal component analysis (PCA) [ 67 ]  or  independent component analysis  decomposition. [ 68 ]  Polynomial kernel SVM has been shown to achieve good accuracy. The polynomial KSVM performs better than linear SVM and RBF kernel SVM. [ 69 ]  Other approaches with decent results involve the use of texture analysis, [ 70 ]  morphological features, [ 71 ]  or high-order statistical features [ 72 ]


CADx is available for nuclear medicine images. Commercial CADx systems for the diagnosis of bone metastases in whole-body bone scans and coronary artery disease in myocardial perfusion images exist. [ 73 ]


With a high sensitivity and an acceptable false lesions detection rate, computer-aided automatic lesion detection system is demonstrated as useful and will probably in the future be able to help nuclear medicine physicians to identify possible bone lesions. [ 74 ]


Diabetic retinopathy is a disease of the retina that is diagnosed predominantly by fundoscopic images. Diabetic patients in industrialised countries generally undergo regular screening for the condition. Imaging is used to recognize early signs of abnormal retinal blood vessels. Manual analysis of these images can be time-consuming and unreliable. [ 75 ] [ 76 ]  CAD has been employed to enhance the accuracy, sensitivity, and specificity of automated detection method. The use of some CAD systems to replace human graders can be safe and cost effective. [ 76 ]


Image pre-processing, and feature extraction and classification are two main stages of these CAD algorithms. [ 77 ]


Image normalization  is minimizing the variation across the entire image. Intensity variations in areas between periphery and central macular region of the eye have been reported to cause inaccuracy of vessel segmentation. [ 78 ]  Based on the 2014 review, this technique was the most frequently used and appeared in 11 out of 40 recently (since 2011) published primary research. [ 77 ]


Histogram equalization  is useful in enhancing contrast within an image. [ 80 ]  This technique is used to increase  local contrast.  At the end of the processing, areas that were dark in the input image would be brightened, greatly enhancing the contrast among the features present in the area. On the other hand, brighter areas in the input image would remain bright or be reduced in brightness to equalize with the other areas in the image. Besides vessel segmentation, other features related to diabetic retinopathy can be further separated by using this pre-processing technique. Microaneurysm and hemorrhages are red lesions, whereas exudates are yellow spots. Increasing contrast between these two groups allow better visualization of lesions on images. With this technique, 2014 review found that 10 out of the 14 recently (since 2011) published primary research. [ 77 ]


Green channel filtering  is another technique that is useful in differentiating lesions rather than vessels. This method is important because it provides the maximal contrast between diabetic retinopathy-related lesions. [ 81 ]  Microaneurysms and hemorrhages are red lesions that appear dark after application of green channel filtering. In contrast, exudates, which appear yellow in normal image, are transformed into bright white spots after green filtering. This technique is mostly used according to the 2014 review, with appearance in 27 out of 40 published articles in the past three years. [ 77 ]  In addition, green channel filtering can be used to detect center of optic disc in conjunction with double-windowing system. [ citation needed ]