Computed Tomography (CT) - Spine
What is CT Scanning of the Spine?
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.
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What are some common uses of the procedure?
- Perhaps the most frequent use of spinal CT is to detect—or
rule out—spinal 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).
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How should I prepare for the CAT scan?
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.
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What does the equipment look like?
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.
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How does the procedure work?
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, which
contains cerebrospinal fluid, appears dark gray.
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.
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How is the CAT scan performed?
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
surrounding 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.
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What will I experience during the procedure?
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.
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Who interprets the results and how do I get them?
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.
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What are the benefits vs. risks?
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.
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What are the limitations of CT Scanning of the Spine?
- 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.
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