| Computed Tomography (CT) - Sinuses
What is computed tomography of the sinuses? Back to Top
Computed
tomography (CT) scanning, also called CAT scanning, uses special x-ray
equipment to obtain many images of the interior of the body from
different angles and then joins them together to show a cross-section
of body tissues and organs. A physician may order a CT scan of the
facial area to view a patient's sinus cavity.
The paranasal sinuses are hollow, air-filled spaces located within
the bones of the face and surrounding the nasal cavity. The nasal
cavity is a system of air channels connecting the nose with the back of
the throat. There are four pairs of sinuses, each connected to the
nasal cavity by small openings. The paranasal sinuses are lined with a
special tissue called a nasal mucous membrane that secretes mucus to
keep the nose and sinuses moist and thus to humidify the air passing
toward the lungs. Normally, air passes in and out of the sinuses and
mucous drains from the sinuses into the nose.
What are some common uses of the procedure? Back to Top
Computed
tomography (CT) can provide important information about sinus and nasal
structure. CT is able to detect sinuses that are filled with fluid or
sinus membranes that are thickened. It can assist your physician in
diagnosing sinusitis, which can be due to either infection or
inflammation in one or more of the sinuses. This condition may be acute
(sudden and short), chronic (ongoing and long-term) or recurring
(repeated). Among the causes of sinusitis are allergies, upper
respiratory infection and an obstruction in the nose. When the nasal
mucous membrane swells, the opening from the sinuses to the nasal
cavity can become blocked. Fluids that would normally drain into the
nose become trapped in the sinuses and infection can set in. Pressure,
pain, headache, fever and tenderness in the sinus area can result.
CT may also be used to define the anatomy before surgery or to give
further information about certain tumors of the nasal cavity and
sinuses.
How should I prepare for the CAT scan? Back to Top
No
special preparation is needed for a CT scan of the facial region unless
you are to receive contrast material—a substance that highlights
certain tissues and blood vessels making certain abnormalities easier
to see. If your 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. Contrast materials can rarely cause an allergic
reaction. The radiologist also should know if you have asthma, multiple
myeloma, any disorder of the heart, kidneys, or thyroid gland, or if
you have diabetes—and particularly if you are taking the medication
metformin (Glucophage).
Women should always inform their doctor and x-ray technologist if
there is any possibility of pregnancy. In some cases, an alternate
imaging exam will be performed.
You should wear comfortable, loose-fitting clothing for your CT
exam. Anything that contains metal might interfere with imaging of the
head. Items such as earrings, eyeglasses, dentures, dental appliances
or hairpins should be removed.
What does the equipment look like? Back to Top
The CT
scanner is a large, square machine with a hole in the center. You will
lie 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 your body to produce the
images, making clicking and whirring noises as the arm moves.
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.
Numerous x-ray beams will be passed through the facial region at
different angles and special sensors measure the amount of radiation
absorbed by different tissues (and lesions such as a tumor). As you lie
still, the scanner parts revolve around you, 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 facial region. These slices
are called tomograms, hence the name computed tomography.
How is the CAT scan performed? Back to Top
For your CT scan,
you will lie on a table that will be guided into the center of the
scanner. You may be positioned face-down with your chin elevated.
During the procedure, you will be asked to lie very still. You will be
alone in the room during the exam although the technologist will be
able to see and speak to you.
Some patients 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.
The actual CT scan takes less than a minute. The entire process may
take up to 45 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. A CT scan of the sinuses requires a special head
holder that uses soft straps to keep the head and neck in proper
alignment.
CT itself causes no pain, though there may be some discomfort from
having to remain perfectly still for several minutes. 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, for a few hours a patient
will develop itching and hives, which can be relieved by medication. If
you become light-headed or experience difficulty breathing, you should
notify the technologist or nurse, as it may indicate a more severe
allergic reaction. 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, a physician experienced in CT and other imaging
examinations, will analyze the images and provide a report to your
referring physician. You will most often receive these results from
your primary care physician.
What are the benefits vs. risks? Back to Top
Benefits
- A CT scan, one of the safest means of studying the head, can help the physician diagnose the cause of sinus problems.
- CT is the most reliable imaging technique for determining if the sinuses are obstructed.
- CT
of the sinuses is now widely available and is performed in a relatively
short time, especially when compared to magnetic resonance imaging
(MRI).
Risks
- CT involves exposure to radiation in the form of x-rays,
but the benefit of an accurate diagnosis far outweighs the risk. See
the Safety page for more information about radiation dose.
- Women should always inform their doctor and the x-ray technologist if there is any possibility of pregnancy.
- Nursing mothers should wait for 24 hours after contrast material injection before resuming breast-feeding.
- Severe
allergic reactions to the contrast materials now in use are uncommon,
however, CT scanning facilities are well-equipped to deal with them.
What are the limitations of CT of the Sinuses? Back to Top
CT
of the facial region is primarily used to determine the presence of
inflammatory disease and for surgical planning. While CT is
occasionally used to detect the presence of tumors, magnetic resonance
imaging (MRI) is the primary choice for this purpose. Back to Top |
