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.
CT scanning provides more detailed information on head
injuries, stroke, brain tumors and other brain diseases
than regular radiographs (x-rays).
CT scanning of the head is typically used to detect:
- bleeding, brain damage and skull fractures in patients
with head injuries
- bleeding caused by a ruptured or leaking aneurysm
in a patient with a sudden severe headache
- a blood clot or bleeding within the brain shortly
after a patient exhibits symptoms of a stroke
- a stroke, especially with a new technique called
Perfusion CT
- brain tumors
- enlarged brain cavities (ventricles) in patients
with hydrocephalus
- diseases or malformations of the skull
CT scanning is also performed to:
- evaluate the extent of bone and soft tissue damage
in patients with facial trauma, and planning surgical
reconstruction
- diagnose diseases of the temporal bone on the side
of the skull, which may be causing hearing problems
- determine whether inflammation or other changes are
present in the paranasal sinuses
- plan radiation therapy for cancer of the brain or
other tissues
- guide the passage of a needle used to obtain a tissue
sample (biopsy) from the brain
- assess aneurysms or arteriovenous malformations through
a technique called CT angiography.
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.
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.
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 you on the CT
examination table, usually lying flat on your back or
possibly on your side or on your stomach. Straps and
pillows may be used to help you maintain the correct
position and to hold still during the exam.
If a contrast material is used, it will be swallowed,
injected through an intravenous line (IV) or administered
by enema, depending on the type of examination.
N
ext, 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.
A CT scan of the head is usually completed within 20
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.
If the contrast material is swallowed, you may find the
taste mildly unpleasant; however, most patients can easily
tolerate it. You can expect to experience a sense of
abdominal fullness and an increasing need to expel the
liquid if your contrast material is given by enema. In
this case, be patient, as the mild discomfort will not
last long.
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
- 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 2 mSv, which is about the same as the average
person receives from background radiation in eight
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.
A person who is very obese may not fit into the opening
of a conventional CT unit.
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.
Reference:
http://www.radiologyinfo.org/
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Diagnostics 
