| Computed Tomography (CT) - Spine
What is CT Scanning of the Spine? Back to Top
Computed
tomography (better known as CT or CAT scan) of the spine, is a type of
x-ray examination that uses a machine called a scanner to obtain
multiple images of the spinal column, as well as three-dimensional
images if needed. Modern CT scanners employ a method called spiral (or
helical) CT, which produces images of the spine and, with the aid of a
computer, processes the images to create cross-sectional "slices" of
the area of interest. These images may be examined on a computer
monitor or printed out and viewed like conventional x-rays. CT images
are far more detailed than those obtained by a conventional x-ray unit.
In addition, CT is a very useful diagnostic method because it can
display and distinguish many different types of tissue in the same
region, including bone, muscle, soft tissue and blood vessels.
Compared to regular x-rays, CT scanning uses a relatively low dose
of radiation. It is not an invasive procedure, although contrast
material sometimes is injected into a vein before scanning to show fine
structural details and highlight any abnormalities. The bony structure
of the spinal vertebrae is clearly and accurately shown by CT scanning,
as are the intervertebral disks and, to some degree, the spinal cord.
What are some common uses of the procedure? Back to Top
Perhaps the most frequent use of spinal CT is to detect—or rule out—spinal column damage in patients who have been injured.
CT is a very helpful means of evaluating the spine before and after surgery.
CT scanning is able to detect various types of tumor in the
vertebral column, including those that have spread there from another
area of the body. Some tumors that arise elsewhere are first identified
by finding deposits of malignant cells (metastases) in the vertebrae;
prostate cancer is an example.
In patients with narrowing of the spinal canal, vertebral fracture,
infection or degenerative disease such as arthritis, CT of the spine
may provide important information when carried out by itself or in
addition to magnetic resonance imaging (MRI). One of the most common
causes of spinal pain that may be diagnosed by CT is a herniated
intervertebral disk.
When a patient is at risk of osteoporosis, CT can accurately measure
bone density in the spine and predict whether vertebral fractures are
likely to occur.
CT is a valuable means of guiding certain diagnostic procedures such
as the biopsy of a suspicious area to detect cancer, or the removal of
fluid from a localized infection (abscess).
How should I prepare for the CAT scan? Back to Top
You
should wear comfortable, loose-fitting clothing to the imaging center,
where you will be given a hospital gown to put on. Metal objects
including jewelry, eyeglasses, dentures, hairpins and the like may
affect the CT images and should be removed. You may be asked not to eat
or drink anything for several hours beforehand, especially if you are
to receive contrast material by mouth or by injection. You should tell
the physician and radiology staff if you have a history of allergic
reactions—especially to iodine, which is often present in contrast
material. They also should know if you have a history of heart disease,
asthma, diabetes or thyroid problems. Any of these conditions may
increase the risk of an allergic reaction. You will be asked to sign a
consent form before the examination.
If your infant or young child is to have spinal CT, there are
measures that can be taken to ensure that the test will go smoothly and
will not be a cause of anxiety for either the child or parent. A woman
of reproductive age should inform her physician or the x-ray
technologist if there is any possibility that she is pregnant.
What does the equipment look like? Back to Top
The CT
scanner is a large unit with a hole, or tunnel, running directly
through its center. The patient lies on a table that can be moved up or
down and that slides into and out of the center of the tunnel. The
technologist will be in an adjoining room to watch you through an
observation window and by video camera throughout the procedure. In
addition, there is an intercom system through which you can communicate
with the technologist.
How does the procedure work? Back to Top
When x-rays are
beamed through the body, some are absorbed while the rest pass through
to produce an image. For conventional x-rays, a film absorbs those
x-rays that have penetrated the patient's body. In CT scanning, a
source of x-rays and a set of electronic x-ray detectors rotate around
the patient. The detectors absorb penetrated x-rays and measure their
amount. Because the x-ray source rotates about the patient and, at the
same time, the examination table advances through the scanner, the
x-ray beam follows a spiral path, giving rise to the term "spiral" (or
"helical") CT. The information collected by the detectors is sent to a
computer system that processes it and reconstructs two-dimensional
cross-sectional images (the "slices") that depict the interior of the
body. A single slice is recorded in only a few seconds. The CT images
represent the density of different tissues. More dense tissues such as
bone appear white, whereas less dense tissues such as the spinal cord
appear in shades of gray. The spinal canal contains the gray spinal
cord and the very dark cerebrospinal fluid.
Modern spiral CT units produce high-quality images in a short time,
making it a convenient study for children and patients who are
critically ill. It now is possible to image large areas of the body
such as the spinal column in just minutes. It also is possible with
modern equipment to combine multiple CT images so as to produce a
three-dimensional display.
How is the CAT scan performed? Back to Top
Spinal CT
scanning is carried out with the patient lying on his or her back. The
technologist will make sure that you are properly positioned and may
use pillows to help you maintain a correct posture during the study. If
indicated, a contrast material will be injected into an arm vein during
the procedure so as to sharpen the images of various tissues. A scan of
the lower spine may also be done after injecting contrast material into
the spinal canal (usually well below the bottom of the spinal cord)
during a lumbar puncture. This will help to detect tumors or locate
areas of inflammation or nerve compression. Initially the table will
move rapidly through the scanner to determine the correct starting
position. Further scans then are made as the table moves more slowly
through the tunnel in the scanner.
The actual imaging takes only a very short time and a complete exam,
including set-up time, takes from five to 30 minutes. When the exam is
completed, you may have to wait a short time while the radiologist, a
physician specially trained to obtain and interpret medical images,
checks the images to be sure that they are of high enough quality to be
correctly interpreted. If necessary, a few additional scans will be
obtained. Less patient movement during the procedure produces clearer
CT images.
What will I experience during the procedure? Back to Top
Spinal
CT scanning is a painless procedure, apart from a needle stick if an
intravenous injection is needed. Discomfort comes mainly from having to
lie still on the table for some time. Injection of contrast material
may cause a slight burning feeling in the arm, a metallic taste and
warm flushing of the entire body. These all are normal reactions and
usually end within a few seconds. Patients who have a hard time
remaining still or who are claustrophobic may find CT to be stressful.
The same may be the case for those who have chronic pain. If you are
one of these patients, the technologist may give you a mild sedative to
help get you through the exam.
Who interprets the results and how do I get them? Back to Top
A
radiologist trained to interpret CT images will review the findings and
make a detailed report to your primary care physician. Your doctor's
office will let you know when the results are in and how to obtain
them. Having had a spinal CT study, you and your doctor will be better
able to decide on further diagnostic procedures or treatment.
What are the benefits vs. risks? Back to Top
Benefits
- Spinal CT scanning is a rapid procedure and offers an
accurate evaluation of bone and most soft tissues. Using the latest
equipment, the spine may be displayed in multiple planes and
three-dimensional imaging is an option.
- CT is able to depict internal bleeding and fractures in trauma victims shortly after they arrive at the hospital.
- CT scans of the spine are non-invasive and cause no pain.
- CT
is less expensive and more cost-effective than MRI. In addition, it is
less sensitive to patient movement. Unlike MRI, CT may be carried out
in patients who have an implanted device of any kind.
- CT
imaging can help guide a biopsy needle when taking a tissue sample and
can aid the removal of fluid or drainage of an abscess.
Risks
- Like all x-ray procedures, CT scanning involves exposure
to potentially harmful radiation. Radiation doses are lower than those
used in some general x-ray exams but higher than in others. Damaging
effects of radiation may be more of a risk when multiple CT studies are
carried out over a period of time.
- Women who are
pregnant, especially those in the first trimester, should consult their
physician about possible risk to the fetus. Children should have a CT
study only if it is essential for making a diagnosis. In general, the
benefits of CT scanning outweigh the potential harm from radiation.
- Iodine-containing
contrast material may cause a brief allergic reaction such as itching,
hives, nausea or rapid breathing, which is easily treated. Severe
reactions including difficulty breathing are quite rare but do occur.
Kidney failure is another very rare occurrence; it is likeliest to
develop in patients whose kidney function already is impaired.
- Nursing mothers should avoid breast-feeding their infants for 24 hours after receiving an injection of contrast medium.
What are the limitations of CT Scanning of the Spine? Back to Top
Spinal
CT does not consistently show enough detail to properly assess the
spinal cord. MRI is also more suitable than CT for demonstrating
injured ligaments, the status of the intervertebral disks and hematomas
in the area of the spine.
CT scanning fails to identify some vertebral fractures that are not displaced.
Contrast material must sometimes be injected to obtain clear images of blood vessels, tumor tissue, muscle and fluid.
CT scanning is, in general, not recommended for pregnant women.
Young children should not have repeated CT studies unless absolutely
necessary. Back to Top |
