Computed Tomography (CT) - Head
What is CT Scanning of the Head?
Computed tomography (CT), sometimes called CAT scan, uses special x-ray
equipment to obtain many images from different angles, and then join them
together to show a cross-section of body tissues and organs. CT scanning
provides more detailed information on head injuries, stroke, brain tumors,
and other brain diseases than do regular radiographs (plain x-ray films).
It also can show bone, soft tissues and blood vessels in the same images.
CT of the head and brain is a patient-friendly exam that involves radiation
exposure.
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What are some common uses of the procedure?
- Detection of bleeding, brain damage and skull fractures in patients
with head injuries.
- Detecting a blood clot or bleeding within the brain shortly after
a patient exhibits symptoms of a stroke.
- Detection of stroke, especially with a new technique called Perfusion
CT.
- Evaluation of the extent of bone and soft tissue damage in patients
with facial trauma, and planning surgical reconstruction.
- Detection of bleeding in a patient with a sudden severe headache
who may have a ruptured or leaking aneurysm.
- Detection of most brain tumors.
- Diagnosing diseases of the temporal bone on the side of the skull,
which may cause hearing problems.
- Detection of enlarged brain cavities (ventricles) in patients
with hydrocephalus.
- Determining whether inflammation or other changes are present
in the paranasal sinuses.
- Planning radiation therapy for cancer of the brain or other
tissues.
- Guiding the passage of a needle used to obtain a tissue
sample (biopsy) from the brain.
- Non-invasive assessment of aneurysms or arteriovenous malformations
through a technique called CT angiography.
- Detecting diseases or malformations of the skull.
- Three-dimensional imaging of the skull and brain structures.
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How should I prepare for the CAT scan?
You should wear comfortable, loose-fitting clothing for your CT exam. Anything
that might interfere with imaging of the head—such as earrings, eyeglasses,
dentures, dental implants or hairpins—should be removed.
No special preparation is needed for a CT scan of the head unless you are
to receive a contrast material—a substance that highlights the brain
and its blood vessels and makes abnormalities easier to see. If the radiologist
believes that an intravenous (IV) injection of a contrast material will be
helpful, you will be asked in advance whether you have had allergies in the
past or have ever had a serious reaction to medication. CT scan contrast
materials contain iodine, which can cause such a reaction in persons who
are allergic. If you have known allergies to other medications it may raise
the possibility that you might have an allergic reaction to the contrast
material. The radiologist also should know if you have asthma, multiple myeloma
or any disorder of the heart, kidneys or thyroid gland, or if you have diabetes—particularly
if you are taking Glucophage. Typically you will be asked to sign an informed
consent form before having CT with injection of a contrast material.
Women should always inform their doctor or x-ray technologist if there is
any possibility that they are pregnant. In some cases an alternate study
will be performed to reduce or eliminate the radiation exposure to the fetus.
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What does the equipment look like?
The CT scanner is a large, square machine with a hole in the center, something
like a doughnut. The patient lies still on a table that can move up or down
and slide into and out of the center of the hole. Within the machine, an
x-ray tube on a rotating gantry (or frame) moves around the patient's body
to produce the images, making clicking and whirring noises as the arm moves.
Though the technologist will be able to see and speak to you, you will be
alone in the room during the exam.
An example of the radiography equipment that may be used is shown above.
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How does the procedure work?
Normal head CAT scan with intravenous contrast.Unlike conventional x-rays,
which produce pictures of the shadows cast by body structures of different
density, CT scanning uses x-rays in a much different way. In CT of the head,
numerous x-ray beams are passed through the skull and brain at different
angles, and special sensors measure the amount of radiation absorbed by different
tissues (and lesions such as a bleeding tumor). As you lie still, the scanner
parts revolve around you (although you cannot see this happen), emitting
and recording x-ray beams from as many as a thousand points on the circle.
A special computer program then uses the differences in x-ray absorption
to form cross-sectional images, or "slices" of the head and brain.
These slices are called tomograms, hence the name "computed tomography."
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How is the CAT scan performed?
CT scanning of the head may be performed in the hospital or at an outpatient
radiology center, but in either case your doctor must give you a written
referral with the reason why the study should be performed. You will lie
on a table that is guided into the center of the scanner and you will be
asked to lie very still.
As stated earlier, some patients will require an injection of a contrast
material to enhance the visibility of certain tissues or blood vessels. A
small needle connected to an intravenous line is placed in an arm or hand
vein. The contrast material will be injected through this line.
Depending on the number of images needed, a CT exam of the head and brain
can take between two and 20 minutes. When it is completed you will be asked
to wait until the technologist examines the images to determine if more are
needed.
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What will I experience during the procedure?
When you enter the scanner, special lights may be turned on to ensure correct
positioning. Some types of exam (such as a scan of the sinuses) call for
a special head holder that uses soft straps to keep the head and neck in
proper alignment. In some cases you will lie on your stomach; in others on
your back. The patient and technologist can talk at any time via an intercom.
CT itself causes no pain, though there may be some discomfort from the need
to remain still. If contrast material is injected you may have a warm, flushed
sensation during the injection. You may also experience a metallic taste
in your mouth that lasts for about two minutes. Occasionally a patient will
develop itching and hives for up to a few hours after the injection; this
can be relieved by medication. If you develop light-headedness or difficulty
breathing, it may indicate a more severe allergic reaction—a physician
or nurse will be present nearby to assist you.
Because CT uses x-rays, you may not have a relative or friend in the CT
room during the exam.
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Who interprets the results and how do I get them?
A radiologist, who is a physician experienced in CT and other radiology
examinations, will analyze the images and provide a signed report with his
or her interpretation to the patient's referring physician. The patient receives
the results from the referring physician who ordered the test. New technology
also allows for distribution of diagnostic reports and referral images over
the Internet at many facilities.
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What are the benefits vs. risks?
Benefits
- CT of the head is now widely available and is performed in a relatively
short time, at a reasonable cost—especially when compared to MR
imaging.
- The exam shows some changes in bone better than any other imaging method.
- It readily detects bleeding.
- The exam is used for stroke detection.
- It provides detailed images of bone, soft tissue and blood vessels.
- CT is the method of choice for rapidly screening trauma victims to detect
internal bleeding or other life-threatening conditions.
- CT Angiography depicts brain blood vessels, revealing aneurysms and
occlusion.
Risks
- CT does involve exposure to radiation in the form of x-rays, but the
benefit of an accurate diagnosis far outweighs the risk. The effective
radiation dose from this procedure is about 2 mSv, which is about the same
as the average person receives from background radiation in eight months.
See the Safety page for more information about radiation dose.
- 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 24 hours after contrast injection before
resuming breast feeding.
- The risk of serious allergic reaction to iodine-containing contrast
material is rare and personnel working at CT units are well equipped to
deal with them.
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What are the limitations of CT Scanning of the Head?
Compared to MR imaging, the precise details of soft tissue (particularly
the brain) are less visible on CT scans. CT is not sensitive in detecting
inflammation of the meninges—the membranes covering the brain.
Compared to conventional angiography, computed tomography angiography (CTA)
may, in some cases, not be as sensitive in the detection of aneurysms and
arteriovenous malformations of the brain.
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