CT scanning—sometimes called CAT scanning—is a noninvasive,
painless medical test that helps physicians diagnose and
treat medical conditions.
CT imaging uses special x-ray equipment to produce multiple
images or pictures of the inside of the body and a computer
to join them together in cross-sectional views of the area
being studied. The images can then be examined on a computer
monitor or printed.
CT scans of internal organs, bone, soft tissue and blood
vessels provide greater clarity than conventional x-ray
exams.
A CT scan of the facial area produces images of 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, 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.
CT of the sinuses primarily is used to:
- detect the presence of inflammatory diseases
- plan for surgery by defining anatomy or giving further
information about tumors of the nasal cavity and sinuses
- evaluate sinuses that are filled with fluid or thickened
sinus membranes
- help diagnose sinusitis
You should wear comfortable, loose-fitting clothing to
your exam. You may be given a gown to wear during the procedure.
Metal objects including jewelry, eyeglasses, dentures and
hairpins may affect the CT images and should be left at
home or removed prior to your exam. You may also be asked
to remove hearing aids and removable dental work.
You may be asked not to eat or drink anything for several
hours beforehand, especially if a contrast material will
be used in your exam. You should inform your physician
of any medications you are taking and if you have any allergies,
especially to contrast materials.
Also inform your doctor of any recent illnesses or other
medical conditions, and if you have a history of heart
disease, asthma, diabetes, kidney disease or thyroid problems.
Any of these conditions may increase the risk of an unusual
adverse effect.
Women should always inform their physician or technologist
if there is any possibility that they are pregnant.
The CT scanner is typically a large machine with a hole,
or tunnel, in the center. A moveable examination table
slides into and out of this tunnel. In the center of the
machine, the x-ray tube and electronic x-ray detectors
are located opposite each other on a ring, called a gantry,
which rotates around you. The computer that processes the
imaging information and monitor are located in a separate
room.
In many ways CT scanning works very much like other x-ray
examinations. X-rays are a form of radiation—like light
or radio waves—that can be directed at the body. Different
body parts absorb the x-rays in varying degrees.
In a conventional x-ray exam, a small burst of radiation
is aimed at and passes through the body, recording an image
on photographic film or a special image recording plate.
Bones appear white on the x-ray; soft tissue shows up in
shades of gray and air appears black.
With CT scanning, numerous x-ray beams and a set of electronic
x-ray detectors rotate around you, measuring the amount
of radiation being absorbed throughout your body. At the
same time, the examination table is moving through the
scanner, so that the x-ray beam follows a spiral path.
A special computer program processes this series of pictures,
or slices of your body, to create two-dimensional cross-sectional
images, which are then displayed on a monitor.
CT imaging is sometimes compared to looking into a loaf
of bread by cutting the loaf into thin slices. When the
image slices are reassembled by computer software, the
result is a very detailed multidimensional view of the
body's interior.
Refinements in detector technology allow new CT scanners
to obtain multiple slices in a single rotation. These scanners,
called "multislice CT" or "multidetector
CT," allow thinner slices to be obtained in a shorter
period of time, resulting in more detail and additional
view capability.
Modern CT scanners are so fast that they can scan through
large sections of the body in just a few seconds. Such
speed is beneficial for all patients but especially children,
the elderly and critically ill.
For some CT exams, a contrast material is used to enhance
visibility in the area of the body being studied.
The technologist begins by positioning patients on the
CT examination table.
For a CT scan of the sinuses, the patient is positioned
face-down with the chin elevated.
Straps and pillows may be used to help the patient maintain
the correct position and to hold still during the exam.
Some patients require an injection of a contrast material
to enhance the visibility of certain tissues or blood vessels.
A nurse or technologist will insert an intravenous (IV)
line into a small vein in the patient's hand or arm. The
contrast material will be injected through this line.
Next, the table will move quickly through the scanner to
determine the correct starting position for the scans.
Then, the table will move slowly through the machine as
the actual CT scanning is performed.
You may be asked to hold your breath during the scanning.
When the examination is completed, you will be asked to
wait until the technologist determines that the images
are of high enough quality for the radiologist to read.
The actual CT scan takes less than a minute and the entire
process is usually completed within 45 minutes.
Most CT exams are painless, fast and easy. With spiral
CT, the amount of time that the patient needs to lie still
is reduced.
Though the scanning itself causes no pain, there may be
some discomfort from having to remain still for several
minutes. If you have a hard time staying still, are claustrophobic
or have chronic pain, you may find a CT exam to be stressful.
The technologist or nurse may offer you a mild sedative
to help.
If an intravenous contrast material is used, you will feel
a slight pin prick when the needle is inserted into your
vein. You may have a warm, flushed sensation during the
injection of the contrast materials and a metallic taste
in your mouth that lasts for a few minutes. Occasionally,
a patient will develop itching and hives, which can be
relieved with 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.
When you enter the CT scanner, special lights may be used
to ensure that you are properly positioned. With modern
CT scanners, you will hear only slight buzzing, clicking
and whirring sounds as the CT scanner revolves around you
during the imaging process.
You will be alone in the exam room during the CT scan,
however, the technologist will be able to see, hear and
speak with you at all times.
With pediatric patients, a parent may be allowed in the
room but will be required to wear a lead apron to prevent
radiation exposure.
After a CT exam, you can return to your normal activities.
If you received a contrast material, you may be given special
instructions.
A radiologist, a physician specifically trained to supervise
and interpret radiology examinations, will analyze the
images and send a signed report to your primary care or
referring physician, who will share the results with you.
Benefits
- A CT scan is one of the safest means of studying the
head.
- 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).
- CT scanning is painless, noninvasive and accurate.
- A major advantage of CT is that it is able to image
bone, soft tissue and blood vessels all at the same time.
- Unlike conventional x-rays, CT scanning provides very
detailed images of many types of tissue as well as the
lungs, bones, and blood vessels.
- CT examinations are fast and simple; in emergency cases,
they can reveal internal injuries and bleeding quickly
enough to help save lives.
- CT has been shown to be a cost-effective imaging tool
for a wide range of clinical problems.
- CT may be less expensive than MRI. In addition, it
is less sensitive to patient movement.
- CT can be performed if you have an implanted medical
device of any kind, unlike MRI.
- CT imaging provides real-time imaging, making it a
good tool for guiding minimally invasive procedures such
as needle biopsies and needle aspirations of many areas
of the body, particularly the lungs, abdomen, pelvis
and bones.
- A diagnosis determined by CT scanning may eliminate
the need for exploratory surgery and surgical biopsy.
- No radiation remains in a patient's body after a CT
examination.
- X-rays used in CT scans usually have no side effects.
Risks
- There is always a slight chance of cancer from radiation.
However, the benefit of an accurate diagnosis far outweighs
the risk.
- The effective radiation dose from this procedure is
about 0.6 mSv, which is about the same as the average
person receives from background radiation in two months.
- Women should always inform their physician or x-ray
technologist if there is any possibility that they are
pregnant.
- CT scanning is, in general, not recommended for pregnant
women because of potential risk to the baby.
- Nursing mothers should wait for 24 hours after contrast
material injection before resuming breast-feeding.
- The risk of serious allergic reaction to contrast materials
that contain iodine is rare, and radiology departments
are well-equipped to deal with them.
- Children should have a CT study only if it is essential
for making a diagnosis and should not have repeated CT
studies unless absolutely necessary.
Reference:
http://www.radiologyinfo.org/
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