A CT scan, also known as computed tomography scan, makes use of
computer-processed combinations of many
X-ray measurements taken from
different angles to produce cross-sectional (tomographic) images
(virtual "slices") of specific areas of a scanned object, allowing the
user to see inside the object without cutting. Other terms include
computed axial tomography (CAT scan) and computer aided tomography.
Digital geometry processing is used to further generate a
three-dimensional volume of the inside of the object from a large
series of two-dimensional radiographic images taken around a single
axis of rotation.
Medical imaging is the most common application of
X-ray CT. Its cross-sectional images are used for diagnostic and
therapeutic purposes in various medical disciplines. The rest of
this article discusses medical-imaging
X-ray CT; industrial
X-ray CT are discussed at industrial computed
The term "computed tomography" (CT) is often used to refer to X-ray
CT, because it is the most commonly known form. But, many other types
of CT exist, such as positron emission tomography (PET) and
single-photon emission computed tomography (SPECT).
a predecessor of CT, is one form of radiography, along with many other
forms of tomographic and non-tomographic radiography.
CT produces data that can be manipulated in order to demonstrate
various bodily structures based on their ability to absorb the X-ray
beam. Although, historically, the images generated were in the axial
or transverse plane, perpendicular to the long axis of the body,
modern scanners allow this volume of data to be reformatted in various
planes or even as volumetric (3D) representations of structures.
Although most common in medicine, CT is also used in other fields,
such as nondestructive materials testing. Another example is
archaeological uses such as imaging the contents of sarcophagi.
Individuals responsible for performing CT exams are called
radiographers or radiologic technologists.
Use of CT has increased dramatically over the last two decades in many
countries. An estimated 72 million scans were performed in the
United States in 2007. One study estimated that as many as 0.4% of
current cancers in the United States are due to CTs performed in the
past and that this may increase to as high as 1.5 to 2% with 2007
rates of CT use; however, this estimate is disputed, as there is
not a consensus about the existence of damage from low levels of
radiation. Lower radiation doses are often used in many areas, such as
in the investigation of renal colic.Side effects from intravenous
contrast used in some types of studies include kidney problems.
1 Medical use
1.5 Abdominal and pelvic
3 Adverse effects
3.2 Contrast reactions
5 Scan dose
Radiation dose units
5.2 Effects of radiation
5.3 Excess doses
7.3 Multiplanar reconstruction
7.4 Volume rendering
8 Image quality
8.2 Dose versus image quality
9 Industrial use
12 Types of machines
13 Research directions
14 See also
16 External links
Since its introduction in the 1970s, CT has become an important tool
in medical imaging to supplement X-rays 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 high risk
of heart disease. A number of 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.
Computed tomography of the head
Computed tomography of human brain, from base of the skull to top.
Taken with intravenous contrast medium.
CT scanning of the head is typically used to detect infarction,
tumors, calcifications, haemorrhage and bone trauma. 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. Ambulances equipped with small bore multi-sliced
CT scanners respond to cases involving stroke or head trauma. 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.
Magnetic resonance imaging
Magnetic resonance imaging (MRI) of the head provides superior
information as compared to CT scans when seeking information about
headache to confirm a diagnosis of neoplasm, vascular disease,
posterior cranial fossa lesions, cervicomedullary lesions, or
intracranial pressure disorders. It also does not carry the risks
of exposing the patient to ionizing radiation. CT scans may be
used to diagnose headache when neuroimaging is indicated and MRI is
not available, or in emergency settings when hemorrhage, stroke, or
traumatic brain injury are suspected. Even in emergency
situations, when a head injury is minor as determined by a physician's
evaluation and based on established guidelines, CT of the head should
be avoided for adults and delayed pending clinical observation in the
emergency department for children.
High-resolution computed tomographs of a normal thorax, taken in the
axial, coronal and sagittal planes, respectively. Click here to scroll
through the image stacks.
CT scan can be used for detecting both acute and chronic changes in
the lung parenchyma, that is, the internals of the lungs. 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 (emphysema, fibrosis, and so forth), thin sections with high
spatial frequency reconstructions are used; often scans are performed
both in inspiration and expiration. This special technique is called
high resolution CT. Therefore, it produces a sampling of the lung and
not continuous images.
Bronchial wall thickness (T) and diameter (D).
Bronchial wall thickening can be seen on lung CTs, and generally (but
not always) impies inflammation of the bronchi. Normally, the
ratio of the bronchial wall thickness and the bronchial diameter is
between 0.17 and 0.23.
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. 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. 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. 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
Example of a CTPA, demonstrating a saddle embolus (dark horizontal
line) occluding the pulmonary arteries (bright white triangle)
Computed tomography angiography
Computed tomography angiography
Computed tomography angiography (CTA) is contrast CT to visualize
arterial and venous vessels throughout the body. 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.
CT scan of the heart is performed to gain knowledge about cardiac or
coronary anatomy. Traditionally, cardiac CT scans are used to
detect, diagnose or follow up coronary artery disease. 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.
The main forms of cardiac CT scanning are:
Coronary CT angiography
Coronary CT angiography (CTA): the use of CT to assess the coronary
arteries of the heart. The subject receives an intravenous injection
of radiocontrast and then the heart is scanned using a high speed CT
scanner, allowing radiologists to assess the extent of occlusion in
the coronary arteries, usually in order to diagnose coronary artery
Coronary CT calcium scan: also used for the assessment of severity of
coronary artery disease. Specifically, it looks for calcium deposits
in the coronary arteries that can narrow arteries and increase the
risk of heart attack. A typical coronary CT calcium scan is done
without the use of radiocontrast, but it can possibly be done from
contrast-enhanced images as well.
To better visualize the anatomy, post-processing of the images is
common. 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
Abdominal and pelvic
Main article: Abdominal and pelvic CT
CT Scan of 11 cm
Wilms' tumor of right kidney in 13-month-old.
CT is an accurate technique for diagnosis of abdominal diseases. Its
uses include diagnosis and staging of cancer, as well as follow up
after cancer treatment to assess response. It is commonly used to
investigate acute abdominal pain.
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 recognised with a 0.2 mm resolution. With
modern Dual-energy CT scanners, new areas of use have been
established, such as aiding in the diagnosis of gout.
There are several advantages that CT has over traditional 2D medical
radiography. First, CT completely eliminates the superimposition of
images of structures outside the area of interest. Second, because of
the inherent high-contrast resolution of CT, differences between
tissues that differ in physical density by less than 1% can be
distinguished. Finally, data from a single CT imaging procedure
consisting of either multiple contiguous or one helical scan can be
viewed as images in the axial, coronal, or sagittal planes, depending
on the diagnostic task. This is referred to as multiplanar reformatted
CT is regarded as a moderate- to high-radiation diagnostic technique.
The improved resolution of CT has permitted the development of new
investigations, which may have advantages; compared to conventional
radiography, for example, CT angiography avoids the invasive insertion
of a catheter. CT colonography (also known as virtual colonoscopy or
VC for short) is far more accurate than a barium enema for detection
of tumors, and uses a lower radiation dose. CT VC is increasingly
being used in the UK and US as a screening test for colon polyps and
colon cancer and can negate the need for a colonoscopy in some cases.
The radiation dose for a particular study depends on multiple factors:
volume scanned, patient build, number and type of scan sequences, and
desired resolution and image quality. In addition, two helical CT
scanning parameters that can be adjusted easily and that have a
profound effect on radiation dose are tube current and pitch. Computed
tomography (CT) scan has been shown to be more accurate than
radiographs in evaluating anterior interbody fusion but may still
over-read the extent of fusion.
The radiation used in CT scans can damage body cells, including DNA
molecules, which can lead to cancer. According to the National
Radiation Protection and Measurements, between the 1980s
and 2006, the use of CT scans has increased sixfold (+500%). 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. However, a lumbar spine
x-ray has a similar dose as a head CT. 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, the radiation dose associated with a routine abdominal CT has
a radiation dose similar to 3 years average background radiation (from
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."
Early estimates of harm from CT are partly based on similar radiation
exposures experienced by those present during the atomic bomb
explosions in Japan after the Second World War and those of nuclear
industry workers. A more recent study by the National Cancer
Institute in 2009, based on scans made in 2007, estimated that 29,000
excess cancer cases and 14,500 excess deaths would be caused over the
lifetime of the patients. Some experts project that in the future,
between three and five percent of all cancers would result from
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 1800 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 person's age plays a significant role in the subsequent risk of
cancer. Estimated lifetime cancer mortality risks from an
abdominal CT of a 1-year-old is 0.1% or 1:1000 scans. 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. The International
Commission on Radiological Protection estimates that the risk to a
fetus being exposed to 10 mGy (a unit of radiation exposure, see
Gray (unit)) increases the rate of cancer before 20 years of age from
0.03% to 0.04% (for reference a
CT pulmonary angiogram
CT pulmonary angiogram exposes a fetus
to 4 mGy). 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
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. Furthermore, certain conditions can require
children to be exposed to multiple CT scans. Studies support
informing parents of the risks of pediatric CT scanning.
In the United States half of CT scans are contrast CTs using
intravenously injected radiocontrast agents. The most common
reactions from these agents are mild, including nausea, vomiting and
an itching rash; however, more severe reactions may occur. Overall
reactions occur in 1 to 3% with nonionic contrast and 4 to 12% of
people with ionic contrast. Skin rashes may appear within a week
to 3% of people.
The old radiocontrast agents caused anaphylaxis in 1% of cases while
the newer, lower-osmolar agents cause reactions in 0.01–0.04% of
cases. Death occurs in about two to 30 people per 1,000,000
administrations, with newer agents being safer. There is a
higher risk of mortality in those who are female, elderly or in poor
health, usually secondary to either anaphylaxis or acute renal
The contrast agent may induce contrast-induced nephropathy. This
occurs in 2 to 7% of people who receive these agents, with greater
risk in those who have preexisting renal insufficiency,
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
renal 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. 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.
CT scanner with cover removed to show internal components. Legend:
R: Gantry rotation
Left image is a sinogram which is a graphic representation of the raw
data obtained from a CT scan. At right is an image sample derived from
the raw data.
Main article: Operation of computed tomography
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. A visual representation of the
raw data obtained is called a sinogram, yet it is not sufficient for
interpretation. 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. 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 +3071 (most attenuating) to
−1024 (least attenuating) on the Hounsfield scale.
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. The phenomenon that one part
of the detector cannot differentiate between different tissues is
called the "Partial Volume Effect". That means that a big amount of
cartilage and a thin layer of compact bone can cause the same
attenuation in a voxel as hyperdense cartilage alone. Water has an
attenuation of 0
Hounsfield units (HU), while air is −1000 HU,
cancellous bone is typically +400 HU, cranial bone can reach
2000 HU or more (os temporale) and can cause artifacts. The
attenuation of metallic implants depends on 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. 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. 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 grey intensity proportional to position
within the window. The window used for display must be matched to the
X-ray density of the object of interest, in order to optimize the
Main article: Contrast CT
Contrast media used for
X-ray CT, as well as for plain film X-ray, are
called radiocontrasts. Radiocontrasts for
X-ray CT are, in general,
iodine-based. 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.
to the whole body
to the organ in question
Annual background radiation
Chest, abdomen and pelvis CT
Cardiac CT angiogram
Neonatal abdominal CT
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. 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
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. While there is some variation, most people (99%) received
less than 7 mSv per year as background radiation. 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. In the
United Kingdom it accounts for 15% of radiation
exposure. The average radiation dose from medical sources is
≈0.6 mSv per person globally as of 2007. Those in the
nuclear industry in the United States are limited to doses of
50 mSv a year and 100 mSv every 5 years.
Lead is the main material used by radiography personnel for shielding
against scattered X-rays.
Radiation dose units
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
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. 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.
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.
Effects of radiation
Further information: Radiobiology
Most adverse health effects of radiation exposure may be grouped in
two general categories:
deterministic effects (harmful tissue reactions) due in large part to
the killing/ malfunction of cells following high doses; and
stochastic effects, i.e., cancer and heritable effects involving
either cancer development in exposed individuals owing to mutation of
somatic cells or heritable disease in their offspring owing to
mutation of reproductive (germ) cells.
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.
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.
In October, 2009, the US Food and Drug Administration (FDA) initiated
an investigation of brain perfusion CT (PCT) scans, based on overdoses
of radiation caused by incorrect settings at one particular facility
for this particular type of CT scan. Over 256 patients over an
18-month period were exposed, over 40% lost patches of hair, and
prompted the editorial to call for increased CT quality assurance
programs, while also noting 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." Similar problems have been reported at other
centers. These incidents are believed to be due to human
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
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. 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
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.
World Health Organization
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.
Use of CT has increased dramatically over the last two decades. An
estimated 72 million scans were performed in the United States in
2007. Of these, six to eleven percent are done in children, an
increase of seven to eightfold from 1980. Similar increases have
been seen in Europe and Asia. 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%. 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).
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). As of 2007 in the United States a proportion of CT
scans are performed unnecessarily. Some estimates place this
number at 30%. There are a number of reasons for this including:
legal concerns, financial incentives, and desire by the public.
For example, some healthy people avidly pay to receive full-body CT
scans as screening, but it is not at all clear that the benefits
outweigh the risks and costs, because deciding whether and how to
treat incidentalomas is fraught with complexity, radiation exposure is
cumulative and not negligible, and the money for the scans involves
opportunity cost (it may have been more effectively spent on more
targeted screening or other health care strategies).
CT creates a volume of voxels.
Types of presentations of CT scans:
- Average intensity projection
- Maximum intensity projection
- Thin slice (median plane)
Volume rendering by high and low threshold for radiodensity.
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
Thin slice. This is generally regarded as planes representing a
thickness of less than 3 mm.
Projection, including maximum intensity projection and average
Volume rendering (VR)
Technically, all volume renderings become projections when viewed on a
2-dimensional display, making the distinction between projections and
volume renderings a bit vague. Still, the epitomes of volume rendering
models feature a mix of for example coloring and shading in
order to create realistic and observable representations.
Two-dimensional CT images are conventionally rendered so that the view
is as though looking up at it from the patient's feet. 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.
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 +3071 (most attenuating) to −1024 (least attenuating) on the
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. The phenomenon that one part of the detector
cannot differentiate between different tissues is called the "Partial
Volume Effect". That means that a big amount of cartilage and a thin
layer of compact bone can cause the same attenuation in a voxel as
hyperdense cartilage alone. Water has an attenuation of 0 Hounsfield
units (HU), while air is −1000 HU, cancellous bone is typically
+400 HU, cranial bone can reach 2000 HU or more (os
temporale) and can cause artifacts. The attenuation of metallic
implants depends on atomic number of the element used: Titanium
usually has an amount of +1000 HU, iron steel can completely
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
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 grey
intensity proportional to position within the window. 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.
Projections include maximum intensity projection (MIP), which enhance
areas of high radiodensity, and so are useful for angiographic
studies. MIP reconstructions tend to enhance air spaces so are useful
for assessing lung structure.
Average intensity projection essentially imitates conventional
projectional radiography, but can be used for specific volumes within
the human body.
Typical screen layout for diagnostic software, showing one volume
rendering (VR) and multiplanar view of three thin slices.
Multiplanar reconstruction (MPR) is the creation of slices in more
anatomical planes than the one (usually transverse) used for initial
tomography acquisition. It can be used for thin slices as well as
projections. Multiplanar reconstruction is feasible because
contemporary CT scanners offer isotropic or near isotropic
MPR is frequently used for examining the spine. Axial images through
the spine will only show one vertebral body at a time and cannot
reliably show the intervertebral discs. By reformatting the volume, it
becomes much easier to visualise the position of one vertebral body in
relation to the others.
Modern software allows reconstruction in non-orthogonal (oblique)
planes so that the optimal plane can be chosen to display an
anatomical structure. This may be particularly useful for visualising
the structure of the bronchi as these do not lie orthogonal to the
direction of the scan.
For vascular imaging, curved-plane reconstruction can be performed.
This allows bends in a vessel to be "straightened" so that the entire
length can be visualised on one image, or a short series of images.
Once a vessel has been "straightened" in this way, quantitative
measurements of length and cross sectional area can be made, so that
surgery or interventional treatment can be planned.
Main article: Volume rendering
A threshold value of radiodensity is set by the operator (e.g., a
level that corresponds to bone). From this, a three-dimensional model
can be constructed using edge detection image processing algorithms
and displayed on screen. Multiple models can be constructed from
various thresholds, allowing different colors to represent each
anatomical component such as bone, muscle, and cartilage. However, the
interior structure of each element is not visible in this mode of
Surface rendering is limited in that it will display only surfaces
that meet a threshold density, and will display only the surface that
is closest to the imaginary viewer. In volume rendering, transparency,
colors and shading are used to allow a better representation of the
volume to be shown in a single image. For example, the bones of the
pelvis could be displayed as semi-transparent, so that, even at an
oblique angle, one part of the image does not conceal another.
Reduced size 3D printed human skull from computed tomography data.
A series of CT scans converted into an animated image using Photoshop
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:Chapters 3 and 5
Streaks are often seen around materials that block most X-rays, such
as metal or bone. Numerous factors contribute to these streaks:
undersampling, photon starvation, motion, beam hardening, and Compton
scatter. This type of artifact commonly occurs in the posterior fossa
of the brain, or if there are metal implants. The streaks can be
reduced using newer reconstruction techniques or approaches
such as metal artifact reduction (MAR). MAR techniques include
spectral imaging, where CT images are taken with photons of different
energy levels, and then synthesized into monochromatic images with
special software such as GSI (Gemstone Spectral Imaging).
Partial volume effect
This appears as "blurring" of edges. It is due to the scanner being
unable to differentiate between a small amount of high-density
material (e.g., bone) and a larger amount of lower density (e.g.,
cartilage). The reconstruction assumes that the
within each voxel is homogenous; this may not be the case at sharp
edges. This is most commonly seen in the z-direction, due to the
conventional use of highly anisotropic voxels, which have a much lower
out-of-plane resolution, than in-plane resolution. This can be
partially overcome by scanning using thinner slices, or an isotropic
acquisition on a modern scanner.
Probably the most common mechanical artifact, the image of one or many
"rings" appears within an image. They are usually caused by the
variations in the response from individual elements in a two
X-ray detector due to defect or miscalibration. Ring
artefacts can largely be reduced by intensity normalization, also
referred to as flat field correction. Remaining rings can be
suppressed by a transformation to polar space, where they become
linear stripes. A comparative evaluation of ring artefact
X-ray tomography images showed that the method of Sijbers
and Postnov  can effectively suppress ring artefacts.
This appears as grain on the image and is caused by a low signal to
noise ratio. This occurs more commonly when a thin slice thickness is
used. It can also occur when the power supplied to the
X-ray tube is
insufficient to penetrate the anatomy.
This is seen as blurring and/or streaking, which is caused by movement
of the object being imaged. Motion blurring might be reduced using a
new technique called IFT (incompressible flow tomography).
Streaking appearances can occur when the detectors intersect the
reconstruction plane. This can be reduced with filters or a reduction
This can give a "cupped appearance" when grayscale is visualized as
height. It occurs because conventional sources, like
X-ray tubes emit
a polychromatic spectrum.
Photons of higher photon energy levels are
typically attenuated less. Because of this, the mean energy of the
spectrum increases when passing the object, often described as getting
"harder". This leads to an effect increasingly underestimating
material thickness, if not corrected. Many algorithms exist to correct
for this artifact. They can be divided in mono- and multi-material
Dose versus image quality
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, 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 (See the Scan dose section). Several methods that can
reduce the exposure to ionizing radiation during a
CT scan exist.
New software technology can significantly reduce the required
Individualize the examination and adjust the radiation dose to the
body type and body organ examined. Different body types and organs
require different amounts of radiation.
Prior to every CT examination, evaluate the appropriateness of the
exam whether it is motivated or if another type of examination is more
suitable. Higher resolution is not always suitable for any given
scenario, such as detection of small pulmonary masses.
Industrial CT Scanning
Industrial CT Scanning (industrial computed tomography) is a process
X-ray equipment to produce 3D representations of
components both externally and internally. Industrial CT scanning has
been utilized 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 and reverse engineering applications. CT
scanning is also employed in the imaging and conservation of museum
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) 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).
Main article: History of computed tomography
The history of
X-ray computed tomography goes back to at least 1917
with the mathematical theory of the Radon transform. In
October 1963, Oldendorf received a U.S. patent for a "radiant energy
apparatus for investigating selected areas of interior objects
obscured by dense material". The first commercially viable CT
scanner was invented by Sir
Godfrey Hounsfield in 1967.
The word "tomography" is derived from the Greek tome (slice) and
graphein (to write).
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. It was later known as computed axial tomography (CAT or CT
scan) and body section röntgenography.
Although the term "computed tomography" could be used to describe
positron emission tomography or single photon emission computed
tomography (SPECT), in practice it usually refers to the computation
of tomography from
X-ray images, especially in older medical
literature and smaller medical facilities.
In MeSH, "computed axial tomography" was used from 1977 to 1979, but
the current indexing explicitly includes "X-ray" in the title.
The term sinogram was introduced by Paul Edholm and Bertil Jacobson in
Types of machines
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 lower cost of production and purchase. The main limitation of
this type is the bulk and inertia of the equipment (
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.
Electron beam tomography
Electron beam tomography (EBT) is a specific form of CT in which a
X-ray tube is constructed so that only the path of the
electrons, travelling between the cathode and anode of the
are spun using deflection coils. 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. 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. Only
one manufacturer (Imatron, later acquired by General Electric) ever
produced scanners of this design. Production ceased in early
In multislice computed tomography (MSCT), a higher number of
tomographic slices allow for higher-resolution imaging.
Photon counting computed tomography is a CT technique currently under
development. 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. 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. 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. Some early research has found the dose reduction potential
of photon counting CT for breast imaging to be very promising.
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