Introduction
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 specific areas.
The Basic Idea
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 human body.
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.
Scanning Procedure
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.
Photo courtesy Department of Defense
Doctors usually operate CT scan machines from a
separate room so they aren't repeatedly exposed to radiation.In this 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.
How is the CT scan performed?
A scanned liver slice
Photo courtesy NASA
The 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.
A CT 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 the exam.
A CT 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 are needed.
What 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 study.
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.
Commonly, 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.
You will 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 exposure.
Benefits
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 clinical problems.
Risks
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 pregnant.
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.
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