Computed Tomography (CT or
CAT) scans take the idea of conventional X-ray imaging to
a new level. Instead of finding the outline of bones and
organs, a CT machine forms a three-dimensional computer
model of patients' insides. Doctors can use CT scans to
examine the body one narrow slice at a time to pinpoint
CT scan machines produce X-rays, a powerful form of
electromagnetic energy. X-ray photons are basically the
same thing as visible light photons, but they have much
more energy. This higher energy level allows X-ray beams
to pass straight through most of the soft material in the
A conventional X-ray image is basically a shadow: You
shine a light on one side of the body, and a piece of film
on the other side registers the silhouette of the bones.
In a CT scan machine, the X-ray beam moves all around the
patient, scanning from hundreds of different angles. The
computer takes all this information and puts together a
3-D image of the body.
The CT machine looks like a giant donut tipped on its
side. The patient lies down on a platform, which slowly
moves through the hole in the machine. The X-ray tube is
mounted on a movable ring around the edges of the hole.
The ring also supports an array of X-ray detectors
directly opposite the X-ray tube. A motor turns the ring
so that the X-ray tube and the X-ray detectors revolve
around the body (in an alternative design, the tube
remains stationary and the X-ray beam is bounced off a
revolving reflector). Each full revolution scans a narrow,
horizontal "slice" of the body. The control
system moves the platform farther into the hole so the
tube and detectors can scan the next slice.
courtesy Department of Defense
Doctors usually operate CT scan machines from a
separate room so they aren't repeatedly exposed to
way, the machine records X-ray slices across the body in a
spiral motion. The computer varies the intensity of the
X-rays in order to scan each type of tissue with the
optimum power. After the patient passes through the
machine, the computer combines all the information from
each scan to form a detailed image of the body. It's not
usually necessary to scan the entire body, of course. More
often, doctors will scan only a small section.
Since they examine the body slice by slice from all
angles, CT scans are much more comprehensive than
conventional X-rays. Today, doctors use CT scans to
diagnose and treat a wide variety of ailments, including
head trauma, cancer and osteoporosis. They are an
invaluable tool in modern medicine.
What are some common uses of the procedure?
Because it provides detailed, cross-sectional
views of all types of tissue, CT is one of the best tools
for studying the chest and abdomen. It is often the
preferred method for diagnosing many different cancers,
including lung, liver and pancreatic cancer, since the
image allows a physician to confirm the presence of a
tumor and measure its size, precise location and the
extent of the tumor's involvement with other nearby
tissue. CT examinations are often used to plan and
properly administer radiation treatments for tumors, to
guide biopsies and other minimally invasive procedures and
to plan surgery. CT can clearly show even very small
bones, as well as surrounding tissues such as muscle and
blood vessels. This makes it invaluable in diagnosing and
treating spinal problems and injuries to the hands, feet
and other skeletal structures. CT images can also be used
to measure bone mineral density for the detection of
osteoporosis. In cases of trauma, CT can quickly identify
injuries to the liver, spleen, kidneys or other internal
organs. Many dedicated shock-trauma centers have a CT
scanner in the emergency room. CT can also play a
significant role in the detection, diagnosis and treatment
of vascular diseases that can lead to stroke, kidney
failure, or even death.
is the CT scan performed?
scanned liver slice
Photo courtesy NASA
technologist begins by positioning the patient on the CT
table. The patient's body may be supported by pillows to
help hold it still and in the proper position during the
scan. As the study proceeds, the table will move slowly
into the CT scanner "doughnut." Depending on the
area of the body being examined, the increments of
movement may be so small that they are almost
undetectable, or large enough that the patient feels the
sensation of motion.
examination often requires the use of different contrast
materials to enhance the visibility of certain tissues or
blood vessels. The contrast material may be injected
through an IV directly into the blood stream, swallowed or
administered by enema, depending on the type of
examination. Before administering the contrast material,
the radiologist or technologist will ask whether the
patient has any allergies, especially to medications or
iodine, and whether the patient has a history of diabetes,
asthma, a heart condition, kidney problems or thyroid
conditions. These conditions may indicate a higher risk of
reaction to the contrast material or potential problems
eliminating the material from the patient's system after
examination usually takes five minutes to half an hour.
When the exam is over, the patient may be asked to wait
until the images are examined to determine if more images
will I experience during the procedure?
CT scanning causes no pain, and with spiral CT,
the need to lie still for any length of time is reduced.
For different parts of the body, the patient preparation
will be different. You may be asked to swallow either
water or a positive contrast material, a liquid that
allows the radiologist to better see the stomach, small
bowel and colon. Some patients find the taste of the
contrast material mildly unpleasant, but most can easily
tolerate it. Your exam may require the administration of
the material by enema if the colon is the focus of the
You will experience a
sense of abdominal fullness and may feel an increasing
need to expel the liquid. Be patient; the mild discomfort
will not last long.
a contrast material is injected into a vein to better
define the blood vessels and kidneys, and to accentuate
the appearance between normal and abnormal tissue in
organs like the liver and spleen. Some people report
feeling a flush of heat and sometimes a metallic taste in
the back of the mouth. These sensations usually disappear
within a minute or two. Some people experience a mild
itching sensation. If it persists or is accompanied by
hives (small bumps on the skin), the itch can be treated
easily with medication. In very rare cases, a patient may
become short of breath or experience swelling in the
throat or other parts of the body. These can be
indications of a more serious reaction to the contrast
material that should be treated promptly, so tell the
technologist immediately if you experience these symptoms.
Fortunately, with the safety of the newest contrast
materials, these adverse effects are very rare.
be alone in the room during the scan; however, the
technologist can see, hear and speak with you at all
times. In pediatric patients, a parent may be allowed in
the room with the patient to alleviate fear, but will be
required to wear a lead apron to prevent radiation
Unlike other imaging methods, CT scanning offers detailed
views of many types of tissue, including the lungs, bones,
soft tissues and blood vessels. CT scanning is painless,
noninvasive and accurate.
- CT examinations are
fast and simple. For example, in trauma cases, they
can reveal internal injuries and bleeding quickly
enough to help save lives.
- Diagnosis made with the
assistance of CT can eliminate the need for invasive
exploratory surgery and surgical biopsy.
- CT scanning can
identify both normal and abnormal structures, making
it a useful tool to guide radiotherapy, needle
biopsies and other minimally invasive procedures.
- CT has been shown to be
a cost-effective imaging tool for a wide range of
CT does involve exposure to radiation in the form of
x-rays, but the benefit of an accurate diagnosis far
outweighs the risk.
Special care is taken during x-ray examinations to ensure
maximum safety for the patient by shielding the abdomen
and pelvis with a lead apron, with the exception of those
examinations in which the abdomen and pelvis are being
imaged. Women should always inform their doctor or x-ray
technologist if there is any possibility that they are
Nursing mothers should wait for 24 hours after contrast
injection before resuming breast feeding. The risk of
serious allergic reaction to iodine-containing contrast
material is rare, and radiology departments are well
equipped to deal with them.