| Computed Tomography (CT) - Head
What are the limitations of CT Scanning of the Head? Back to Top
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.
What are some common uses of the procedure? Back to Top
- 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 be causing 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.
How should I prepare for the CAT scan? Back to Top
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 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.
What does the equipment look like? Back to Top
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.
How does the procedure work? Back to Top
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."
How is the CAT scan performed? Back to Top
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 two to 20 minutes. When it is completed you will be asked to wait
until the technologist examines the images to determine if more are
needed.
What will I experience during the procedure? Back to Top
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. Usually
the CT scanning takes many seconds or just a few minutes to perform. 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.
Who interprets the results and how do I get them? Back to Top
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.
What are the benefits vs. risks? Back to Top
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 usually used as an initial study 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 the general environment over a period of 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.
What are the limitations of CT Scanning of the Head? Back to Top
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. Back to Top |
