CT scanning provides cross-sectional images of the body which can be manipulated and reformatted in various planes. It uses X-rays combined with computer processing to generate 3D images of tissues and organs. CT scanning is more detailed than standard X-rays and can detect abnormalities such as tumors, bleeding, fractures and blockages. It has various medical applications for imaging organs like the brain, lungs, kidneys and blood vessels. While it provides advantages over other imaging methods, it also involves exposure to ionizing radiation.

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1. INTRODUCTION OF TOMOGRAPHY
Tomography refers to imaging by sections or sectioning, through the use of any kind
of penetrating wave. The method is used in radiology, archaeology, biology, atmospheric
science, geophysics, oceanography, plasmaphysics, materialsscience, astrophysics, quantum
information, and other sciences. In most cases it is based on the mathematical procedure
called tomographic reconstruction. Tomography is imaging by sections or sectioning. A
device used in tomography is called a tomography, while the image produced is a tomogram.
The word "tomography" is derived from the Greek tomes (slice) and graphing (to
write).Tomography is a method in which a 3-D structure is reconstructed from a series of 2-D
projections (images) acquired at successive tilts (Radon 1917). First developed for use in
medical imaging (1963, Nobel Prize for Medicine in 1979) using X-rays, ultrasound and
magnetic resonance (e.g. ‘cat-scans’).
1.1. X-RAY SCANNING
X-rays are a form of ionising electromagnetic radiation and have a very high frequency
and a very short wavelength. Their wavelengths range between 0.001 to 10 nm.
An X-ray tube works as follows:
 The heated filament is positively charged and the tungsten target is negative.
 Electrons are emitted from the heated filament towards the tungsten target due to
the very high potential difference between them.
 The tungsten target absorbs the electrons and releases some of the energy in the
form of X-rays.
 This process is very inefficient however and a lot of energy is released in heat. For
this reason the tungsten target has a copper mounting because it conducts heat
and is cooled with by circulating oil through the mount. Spinning the tungsten
target at high speed also helps to stop it overheating.
 Narrower beams of X-rays will produce a sharper image. The tungsten target is
therefore angled so that a wide beam of electrons will produce a narrow beam of
X-rays.
1.1.1 The clinical application of X-rays to form images
Hard and soft X-rays
Hard X-rays are X-rays with a higher frequency and are more penetrating than soft X-
rays. Soft X-rays are usually filtered when doing a scan because they can't penetrate
through a patient's body and add needless risk of radiation damage.

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Attenuation
Attenuation is a measure of how much something absorbs X-rays. The amount of
attenuation increases with atomic density (number of protons in the nuclei). For example,
bones have a higher attenuation than soft tissue and therefore bones produce a dark
shadow when X-rayed where as soft tissue appears much fainter.
Frequency
When a patient has an X-ray, they are usually scanned at a frequency of
approximately 7×108 Hz because body tissues absorb this frequency the best.
1.1.2 Use of X-rays in various parts of the body
X-rays are best suited to imaging bones and have a very high resolution. For imaging soft
tissue however, there is very little contrast and so a contrast medium is needed. Contrast
mediums are substances given to the patient that absorb X-rays and produce an image of the
area under investigation when X-rayed. Usually CAT (Computer Axial Tomography) scans,
in which a series of X-rays are taken from various angles and interpreted by a computer, are
better for imaging soft tissue.
1.2 Computed Tomography overX-ray scanning:
CT is a specialized x-ray machine in which the X-ray source rotates rapidly around the
patient's body as the patient is carried though a large opening in the machine. The speed of
the exam is related to the number of detectors used to read out the X-rays; hence, the 64 -slice
CT scanner is the fastest available today. The CT can take multiple images very rapidly
during a single breath hold. A CT is more detailed than a routine x-ray and is particularly
useful to demonstrate several types of tissues such as the lungs, bone, soft tissue and blood
vessels with great detail.
CT imaging is considered safe. The benefit of a CT scan usually outweighs the risk of
radiation exposure or the injection of contrast during some CT procedures. The physicians at
UDI have worked hard to limit the amount of radiation exposure from every CT procedure to
the minimum necessary for a diagnostic exam... Please inform the technologist if you are
pregnant, have a history of diabetes, asthma, kidney disorders, or allergies to iodine.
The advantages of CT scans over x-rays includes the large amount of data a CT scan
provides, the ability of the physician to manipulate the data into various views without
additional imaging of the patient, and the ability to selectively enhance or remove structures
from the images.
COMPUTED TOMOGRAPHY
A CT scan, also called X-ray computed tomography (X-ray CT) or computerized axial
tomography scan (CAT scan),[1] makes use of computer-processed combinations of many X-
ray images taken from different angles to produce cross-sectional (tomographic) images

3. 3
(virtual 'slices') of specific areas of a scanned object, allowing the user to see inside the object
without cutting.
Digital geometry processing is used to generate a three-dimensional image of the inside of the
object from a large series of two-dimensional radiographic images taken around a single axis
of rotation.[2] 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.[3] The rest of this article discusses medical-imaging X-ray CT; industrial
applications of X-ray CT are discussed at industrial computed tomography scanning.
As X-ray CT is the most common form of CT in medicine and various other contexts, the
term computed tomography alone (or CT) is often used to refer to X-ray CT, although other
types exist (such as positron emission tomography [PET] and single-photon emission
computed tomography [SPECT]). Older and less preferred terms that also refer to X-ray CT
are computed axial tomography (CAT scan) and computer-aided/assisted tomography. X-ray
CT is a form of radiography, although the word "radiography" used alone usually refers, by
wide convention, to non-tomographic radiography.
CT produces a volume of data that can be manipulated in order to demonstrate various bodily
structures based on their ability to block 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 non destructive 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.
Usage 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 usage; however,
this estimate is disputed, as there is not a consensus about the existence of damage from low
levels of radiation. Kidney problems may occasionally occur following intravenous contrast
agents used in some types of studies.

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2. PRINCIPLE
In CT scanning, the image is reconstructed from a large number of absorption profiles
taken at regular angular intervals around a slice, each profile being made up from a parallel
set of absorption values through the object. i.e., CT also passes X-rays through the body of
the patient but the detection method is usually electronic in nature, and the data is converted
from analog signal to digital impulses in an analog to digital (ad) converter. This digital
representation of the X-ray intensity is fed into a computer, which then reconstructs an image.
The method of doing of tomography uses an x-ray detector which translates linearly on a
track across the X-ray beam, and when the end of the scan is reached the X-ray tube and
the detector are rotated to a new angle and the linear motion is repeated. The latest generation
of CT machines use ‘fan-beam’ geometry with an array of detectors which simultaneously
detect X-rays on a number of different paths through the patient.
CT scanner is a large square machine with a hole in the centre, something like
a doughnut. The patient lies still on a table that can move up/down and slide into and out
from the centre of hole. Within the machine an X-ray tube on a rotating gantry moves around
the patient's body.
Fig1: block diagram of Computed Tomography (CT)

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2.1 PROCEDURE
In many ways CT scanning works very much like other X-ray examinations. Different
body parts absorb the X-rays in varying degrees. It is this crucial difference in absorption that
allows the body parts to be distinguished from one another on an X-ray film or CT electronic
image.
In a conventional X-ray exam, a small amount of radiation is aimed at and passes through
the part of the body being examined, recording an image on a special electronic image
recording plate. Bones appear white on the X-ray; soft tissue, such as organs like the heart or
liver, shows up in shades of gray, and air appears black.
With CT scanning, numerous X-ray beams and a set of electronic X-ray detectors rotate
around you, measuring the amount of radiation being absorbed throughout your body.
Sometimes, the examination table will move during the scan, so that the X-ray beam follows
a spiral path. A special computer program processes this large volume of data to create two-
dimensional cross-sectional images of your body, which are then displayed on a monitor. CT
imaging is sometimes compared to looking into a loaf of bread by cutting the loaf into thin
slices. When the image slices are reassembled by computer software, the result is a very
detailed multidimensional view of the body's interior.
Refinements in detector technology allow nearly all CT scanners to obtain multiple slices
in a single rotation. These scanners, called multiline CT or multidetector CT, allow thinner
slices to be obtained in a shorter period of time, resulting in more detail and additional view
capabilities.
Modern CT scanners are so fast that they can scan through large sections of the body in
just a few seconds, and even faster in small children. Such speed is beneficial for all patients
but especially children, the elderly and critically ill, all of whom may have difficulty in
remaining still, even for the brief time necessary to obtain images.
For children, the CT scanner technique will be adjusted to their size and the area of
interest to reduce the radiation dose.
For some CT exams, a contrast material is used to enhance visibility in the area of the
body being studied.

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Fig2: Basic CT Scanner

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7. EXAMPLES OF CT
7.1 CT SCAN FOR HEAD
Computed tomography, more commonly known as a CT or CAT scan, is a diagnostic
medical test that, like traditional X-rays, produces multiple images or pictures of the inside of
the body.
The cross-sectional images generated during a CT scan can be reformatted in multiple
planes, and can even generate three-dimensional images. These images can be viewed on a
computer monitor, printed on film or transferred to a Compact Disc (CD)or DVD.
CT images of internal organs, bones, soft tissue and blood vessels typically provide
greater detail than traditional X-rays, particularly of soft tissues and blood vessels.
CT scanning provides more detailed information on head injuries, stroke, brain tumours
and other brain diseases than regular radiographs (X-rays).
CT scanning of the head is typically used to detect:
 Bleeding, brain injury and skull fractures in patients with head injuries.
 Bleeding caused by a ruptured or leaking aneurysm in a patient with a sudden severe
headache.
 A blood clot or bleeding within the brain shortly after a patient exhibits symptoms of
a stroke.
 A stroke, especially with a new technique called Perfusion CT.
 Brain tumours.
 Enlarged brain cavities (ventricles) in patients with hydrocephalus.
 Diseases or malformations of the skull.
7.2 CT SCAN FOR KIDNEY
CT scans of the kidneys can give more detailed information about the kidneys than
standard X-rays. This can provide more information related to injuries and/or diseases of the
kidneys. CT scans of the kidneys can help your health care provider find problems such as
tumours or other lesions, obstructive conditions, such as kidney stones, congenital anomalies,
polycystic kidney disease, accumulation of fluid around the kidneys, and the location of
abscesses.
A CT scan of the kidney may be done to check the kidneys for:
 Tumors or other lesions
 Obstructions such as kidney stones
 Abscesses
 Polycystic kidney disease
 Abnormalities you were born with

8. 8
A CT scan is also useful when another type of exam, such as X-ray or physical exam, is
not conclusive. CT scans of the kidney may be used to evaluate the retroperitoneum. This is
located in the back part of the abdomen. CT scans of the kidney may be used to help guide
the needle placement in kidney biopsies.
After the removal of a kidney, CT scans may be used to locate abnormal masses in the
empty space where the kidney once was. CT scans of the kidneys may be done after kidney
transplants to look at the size and location of the new kidney in relation to the bladder.

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8. ADVANTAGES OF CT SCANNING
 CT scanning is painless, non-invasive and accurate.
 A major advantage of CT is its ability to image bone, soft tissue and blood vessels all
at the same time.
 Unlike conventional X-rays, CT scanning provides very detailed images of many
types of tissue as well as the lungs, bones, and blood vessels.
 CT examinations are fast and simple; in emergency cases, they can reveal internal
injuries and bleeding quickly enough to help save lives.
 CT has been shown to be a cost-effective imaging tool for a wide range of clinical
problems.
 CT is less sensitive to patient movement than Magnetic Resonance Imaging (MRI).
 CT can be performed if you have an implanted medical device of any kind, unlike
MRI.
 CT imaging provides real-time imaging, making it a good tool for guiding minimally
invasive procedures such as needle biopsies and needle aspirations of many areas of
the body, particularly the lungs, abdomen, pelvis and bones.
 A diagnosis determined by CT scanning may eliminate the need for exploratory
surgery and surgical biopsy.

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9. RISKS OF CT SCANNING
 There is no conclusive evidence that radiation at small amounts delivered by a CT
scan causes cancer. Large population studies have shown a slight increase in cancer
from much larger amounts of radiation, such as from radiation therapy.
 Thus, there is always concern that this risk may also apply to the lower amounts of
radiation delivered by a CT exam.
 When a CT scan is recommended by your doctor, the expected benefit of this test
outweighs the potential risk from radiation. You are encouraged to discuss the risks
versus the benefits of your CT scan with your doctor, and to explore whether
alternative imaging tests may be available to diagnose your condition.
 The effective radiation dose for this procedure varies.
 Women should always inform their physician and X-ray or CT technologist if there is
any possibility that they are pregnant.. CT scanning is, in general, not recommended
for pregnant women unless medically necessary because of potential risk to the baby
in the womb.
 Manufacturers of intravenous contrast indicate mothers should not breastfeed their
babies for 24-48 hours after contrast medium is given..
 The risk of serious allergic reaction to contrast materials that contain iodine is
extremely rare, and radiology departments are well-equipped to deal with them.
 Because children are more sensitive to radiation, they should have a CT exam only if
it is essential for making a diagnosis and should not have repeated CT exams unless
absolutely necessary.

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10. APPLICATIONS & IT’S CLINICAL BENFITS
Unlike other medical imaging techniques, such as conventional x-ray imaging
(radiography), CT enables direct imaging and differentiation of soft tissue structures, such
as liver, lung tissue, and fat. CT is especially useful in searching for large space occupying
lesions, tumours and metastasis and can not only reveal their presence, but also the size,
spatial location and extent of a tumor.
fig3: ct image of chest
High resolution axial CT image of the chest showing the vessels of the heart ( (center
of image) and pneumonia in the both lungs.
CT imaging of the head and brain can detect tumours, show blood clots and blood
vessel defects, show enlarged ventricles (caused by a build up of cerebrospinal fluid) and
image other abnormalities such as those of the nerves or muscles of the eye.
Due to the short scan times of 500 milliseconds to a few seconds, CT can be used for
all anatomic regions, including those susceptible to patient motion and breathing. For
example, in the thorax CT can be used for visualization of nodular structures, infiltrations
of fluid, fibrosis (for example from asbestos fibers), and effusions (filling of an air space
with fluid).
CT has been the basis for interventional work like CT guided biopsy and minimally
invasive therapy. CT images are also used as basis for planning radiotherapycancer
treatment. CT is also often used to follow the course of cancer treatment to determine how
the tumor is responding to treatment.

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CT imaging provides both good soft tissue resolution (contrast) as well as high spatial
resolution. This enables the use of CT in orthopedic medicine and imaging of bony
structures including prolapses (protrusion) of vertebral discs, imaging of complex joints
like the shoulder or hip as a functional unit and fractures, especially those affecting the
spine. The image postprocessing capabilities of CT - like multiplanar reconstructions and
3-dimensional display (3D) - further enhance the value of CT imaging for surgeons.
For instance, 3-D CT is an invaluable tool for surgical reconstruction following facial
trauma.
CT IMAGE OF DISK OF SPINE
Axial CT image of the lumbar spine showing a slight prolapse of the disk impinging on
the spinal cord.
CT is becoming the method of choice for imaging trauma patients. CT exams are fast
and simple and enable a quick overview of possibly life-threatening pathology and rapidly
enables a dedicated surgical treatment.
With the advent of spiral CT, the continuous acquisition of complete CT volumes can
be used for the diagnosis of blood vessels with CT Angiography. For instance, abdominal
aortic aneurysms, the renal arteries, the carotids vessels and the Circle of Willis can all now
be quickly imaged with CT with minimal intervention.
Due to the short total acquisition time of spiral CT, imaging of the liver is now possible
in different contrast enhancement phases. These so-called "multi-phase" studies offer a step
towards differential diagnosis of lesions in the liver. In other words, doctors can use
differential diagnosis to determine "what kind of abnormality is this?" For example, the
three-phase liver study below shows tumor enhancement on the arterial-phase and venous-
phase images that is useful in diagnosing the disease.

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CT image of the liver and abdomen with no contrast enhancement
CT image of the liver and abdomen with arterial phase contrast enhancement
CT image of the liver and abdomen with venous phase contrast enhancement
Fig 4: CT image of liver and abdomen

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11.FUTURE USE OF CT
Medical imaging has advanced rapidly in the early years of the twenty-first century.
Doctors can now observe events at the molecular level, examine the characteristics of
individual heartbeats, and study processes in the brain in minute detail — tasks all but
impossible a decade ago. “We are entering the age of precision medicine,” says Roderic
Pettigrew, director of the US National Institute of Biomedical Imaging and Bioengineering in
Bethesda, Maryland. “We try to be precise in diagnosing, fashioning treatments, targeting
treatments, delivering treatments, and monitoring the effects of treatments.”
Much of the progress has stemmed from improvements in existing technologies, such as
computed tomography (CT), ultrasound and magnetic resonance imaging (MRI). “MRI now
allows you to track the diffusion of water molecules in the brain with such precision that you
can compute their trajectories along fibre pathways,” Pettigrew says.
The latest imaging methods emerging from the laboratory promise to complement these
advances. They can detect and monitor cancers, for example, locate individual cells for the
delivery of drugs, and provide unprecedented accuracy for surgically treating heart disease
and other conditions. Several technologies have yet to reach the preclinical stage, but others
have begun the journey to clinical trials. In most cases, they perform tasks currently carried
out by conventional systems, but do so faster, more precisely and more safely.
A spoonful of sugar
One such advance in safety relates to a way of identifying tumours. Tumours are avid
consumers of glucose, so patients are usually given radioactively labelled analogues of
glucose, which congregate in the tumours and are detected by positron emission tomography
(PET). But the danger of radioactive exposure prevents this method being used in certain
individuals, such as young children and pregnant women, and limits the number of doses for
other patients. A team headed by Simon Walker-Samuel at University College London has
developed a method that avoids this problem by labelling glucose magnetically with bursts of
radio waves instead, so it can be detected by standard MRI. This non-invasive approach is
safer - patients merely need to take a sugary drink, rather than a radioactive isotope. It also
enables medical teams to differentiate among various types of tumour, allowing them both to
determine the appropriate therapy more effectively and to assess its effect.
The technique-dubbed glucocest , for glucose chemical exchange saturation transfer -
measures the exchange of protons between the hydroxyl groups in glucose molecules and
water molecules in biological tissue. The pulses of radio waves alter the magnetic character
of the protons in the hydroxyl groups, masking the signal from water molecules detected by

15. 15
MRI. “The effect is small, but can be measured if we repeat the experiment a large number of
times,” says team member Xavier Go lay.
The researchers applied the technique to two types of human colorectal tumour
transplanted into mice. Studying MRI images taken before they injected glucose into the
tumours and one hour after injection clearly revealed differences between the tumour types,
as different tumours consistently take up different amounts of glucose. The researchers are
now starting human studies: they are recruiting patients with tumours in their neck, and have
already scanned about a dozen. For precision, the team injected the glucose into the mouse
tumours, but the human patients receive it in the form of a drink. In future, the technology
may not be limited to tumours. “One could imagine using it for assessing any organ with a
high glucose consumption- for example, the heart or the brain,” Go lay says.
Other researchers are expressing cautious optimism about the technique. “We feel it is
feasible but will require some more MRI development, because the human studies will have
to be done at magnetic fields much lower than for the animals,” says radiologist Peter van
Zeal of Johns Hopkins University in Baltimore, Maryland, whose team is performing its own
human studies of glucoCEST technology.

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CONCLUSION
CT scanning has a role in the management of patients in critical care and is a
financially viable means for improving patient care. Volumetric cone beam technology is
currently not feasible for brain imaging but continuing advances will likely see it used in a
broader range of applications. Head CT images acquired with the Cerrito portable scanner
are satisfactory for clinical use at an effective dose of 1.7 move per scan and were found to
be diagnostically accurate in all cases.

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