Computed Tomography or
Computerized tomography (CT) is a type of medical examination that makes use of
X-rays and computer processing to create cross sectional images of the body. CT
scan has also been known as Computerized Axial Tomography scan (CAT scan). The
word Tomography is derived from the Greek words tomos,
or “section” and graphia,
meaning “to describe” or “to write”. 3-D
imaging and additional visualization can be performed with multiple tomographic
images. CT procedure can scan nearly every part of the body and provide much
more efficient results when compared to X-rays.
How does CT scanner operate?
A motorized X-ray
rotates around the circular opening called the gantry, of a donut-shaped
structure in the Computed Tomography scanner. During a CT scan, the subject
lies on a surface that slowly moves through the gantry while the X-ray source
rotates around in the structure, shooting narrow beams of X-rays through a
section of the subject’s body. Digital X-ray detectors are used in the CT
scanners, which are located directly opposite to the X-ray source. The
detectors pick up the X-rays on their way out of the subject. Many images taken
at different angles are collected during one complete rotation and are
transmitted to a computer. For each rotation, the CT computer reconstructs the
image data collected into one or multiple cross-sectional images of the
internal organs or tissues using complex mathematical formulae.
Types of CT scan
Computed Tomographic scans can be
differentiated based on the part of the body being examined. These scans often
use a contrast material to enhance the visibility of certain tissues. The
different types are Abdominal CT, Head CT, CT
, Chest CT, Spinal cord CT, Urinal tract CT etc.
Types of CT scanners
The CT scanners can be sorted
into types based on the technology used in the machine. They include:
- X-ray Computed Tomography: This is the
conventional CT scanner that scans a particular section of the subject at
a time and sends it across to a computer for further processing
- Spiral or Helical Computed Tomography: These
provide accurate information on the internal organs. In spiral CT, the
X-ray beam is emitted on a continuous basis and rotates around the
subject, as the subject is moved through.
- Micro Computed Tomography: In micro CT, the
pixel size of the images is in micrometer. It is used in cases involving
small animals, biomedical samples and other studies where minute detailing
- Cone Beam Computed Tomography: The Cone Beam
CT is a recent addition, where the X-ray source is cone shaped
and the resultant image is in 3-D. It has major uses in the field of implant
dentistry, interventional radiology etc.
Uses and Advantages
Uses of CT scanning
- Organs such as stomach, gall bladder, liver,
spleen, pancreas, kidneys, lower gastrointestinal (GI) tract, the colon
and rectum can be visualized with great clarity using CT imaging of the
abdomen. They are used for the diagnosis of appendicitis, stage of cancer,
tumours and gangrene.
- Head CT scan is used for the diagnosis of Alzheimer’s disease, brain
tumours, bleeds, injuries to the brain and other major brain diseases. It
is also used for CT guided stereotactic surgery and radiosurgery for
treatment of intracranial tumors, arteriovenous malformations etc.
- Computed Tomography Angiography helps in the
visualization of blood flow in the arteries throughout the body. It is
used in the diagnosis of aneurysms (bulging), stenosis (narrowing) of the
arteries, dissection of the aorta etc.
- Chest CT scan uses special equipment to visualize
organs and tissues of the chest which provides images of multiple tissues
such as lungs, heart, bones, muscles, blood vessels, soft tissues etc. These
images are used to detect acute and chronic changes in lung parenchyma,
diagnose tumours, emphysema, inflammations etc.
- Micro CT scans are in the micrometres range and
have a variety of uses in the biomedical field, electronics, microdevices,
food technology, geology, microfossils etc.
- Industrial Computed Tomography is mainly used for
internal inspection of components in industries. Its varied uses include
flaw detection, failure analysis, metrology, assembly analysis and reverse
- Recently, CT scanners have been superimposed with a
Nuclear Medicine procedure such as Positron Emission Tomography (PET scan).
This produces much more accurate 3-D images of organs and tissues.
Advantages of CT scanning
- CT scanning is often used in the final stage of
diagnosis and additional work up will not be necessary. It helps to determine
if and when surgery is required
- Usage of CT scans reduces the need for painful
- The inherent high-contrast resolution of CT helps
in differentiating between tissues that differ in physical density by less
- It has the ability to eliminate the superimposition
of images of structures outside the area of interest
- CT scans greatly improve the process of cancer
diagnosis and detection of the stage of cancer
- Using CT scanning helps in placing the patient into
appropriate areas of hospital care
- In the case of CT angiography or CT colonography,
invasive insertions into the patient’s body can be avoided
Complications involved in CT scan
CT scan involves exposure to radiation, which has its own
added risks stated as follows:
- Ionizing radiation
is produced by X-rays, which may cause biological effects like increased
lifetime risk of cancer.
- X-rays can cause
adverse health effects such as skin redness, skin tissue injury, hair
loss, cataracts or birth defects in case of prolonged high dose exposure.
Hence, women are asked to inform the CT scan technician if they are
pregnant before the CT examination.
- Allergic reaction to
the contrast material being injected may also occur. In such cases, the
patient is advised to take special medication 24 hours before the exam. It
can also harm kidney function.
- Since the radiation
exposure is much larger for a CT scan than a traditional X-ray, the
American College of Radiology advises that CT exams should be limited
only to those cases where the benefit to be gained greatly outweighs the
As children are more sensitive to ionizing radiation
and the amount of radiation received increases due to their smaller size, they
have a relatively higher risk for developing cancer. Hence, the scanners are to
be adjusted to reduce the level of radiation.