CT - Computed Tomography
How the test is
How to prepare
for the test
How the test
What the risks
Why the test
Tomography is a method of body imaging in which the
X-ray source and/or detection device (e.g., film) rotate
around the patient. In computed tomography (CT), a thin
X-ray beam rotates as small detectors measure the amount
of X-rays which make it through the patient or particular
area of interest.
By use of a complex algorithm, a computer analyzes
the data to construct a cross-sectional (axial) image.
These images can be stored, viewed on a monitor, or
printed on film. In addition, three-dimensional models
of organs can be created by stacking the individual
images, or "slices."
CAT scan; Computed axial tomography (CAT) scan; Computerised
How the test is performed
The patient will be asked to lie on a narrow table
(gantry) that slides into the center of the scanner.
Depending on the study being performed, the patient
may need to lie on his/her stomach, back, or side. If
contrast media (dye) is to be administered, an IV will
be placed in a small vein of a hand or arm.
Much like standard photographic cameras, subject motion
causes blurred images in CT. Therefore, the technologist
operating the scanner and supervising the patient will
give instructions through an intercom when to hold one's
breath and not move.
As the exam takes place, the gantry will advance small
intervals through the scanner. Modern "spiral"
scanners can perform the examination in one continuous
motion of the gantry. Generally, complete scans will
only take a few minutes, however, additional contrast-enhanced
or higher-resolution scans will add to the scan time.
The newest multidetector scanners can image the entire
body, head to toe, in less than 30 seconds.
How to prepare for the test
The patient may be asked to drink oral contrast either
immediately prior to, or 4 to 6 hours before, the CT
scan. The contrast may be composed of non-reactive (inert)
chalky-tasting barium sulfate, which will eventually
pass in the stools, or absorbable clear Gastrografin
solution. The health care provider may also advise fasting
(no solids or liquids) for 4 to 6 hours if contrast
dye is to be used.
The CT scanner has a weight limit to prevent damage
to the mechanized gantry. Have the health care provider
contact the scanner operator if you weigh more than
Since metal is very, very dense, the X-ray beam has
difficulty passing through it and results in errors
in the involved constructed slices (artifact). Therefore,
the patient will be asked to remove jewelry and wear
a hospital gown during the study.
How the test will feel
The X-rays are painless. The primary discomfort may
be from the need to lie still on the table.
If intravenous contrast dye is given, the patient may
initially feel a slight burning sensation within the
injected arm, a metallic taste in the mouth, and a warm
flushing of the body. These sensations are normal and
usually reside within a few seconds.
What the risks are
CT scans and other X-rays are monitored and regulated
to provide the minimum amount of radiation exposure
needed to produce the image. CT scans provide low levels
of ionizing radiation which has the potential to cause
cancer and heritable defects. The risk associated with
any individual scan is small; however, the risk increases
as numerous additional studies are performed.
During pregnancy, an abdominal CT scan is usually not
recommended, due to risk to the exposed fetus, including
developmental malformations and childhood cancers. Patients
who are or may be pregnant should speak with their health
care provider in order to first take a pregnancy test
or choose an appropriate alternative imaging modality
without risk to the fetus, such as ultrasound.
The most common intravenous contrast dye is iodine based.
A person who is allergic to iodine (such as those with
seafood allergies) may experience nausea, sneezing,
vomiting, itching, or hives. If contrast administration
is essential for a patient with any of the prior reactions,
the health care provider may choose to pre-medicate
the patient before the scan with a short course of immune-suppressing
steroids and/or Benedryl. Alternatively, other contrast
media or other imaging modalities (such as ultrasound
or MR) may be used.
Rarely, the dye may cause anaphylaxis (a life-threatening
allergic response), usually manifested by swelling in
the airway. The patient is instructed prior to the scan
to notify the technologist via the intercom if he/she
has difficulty breathing. If such a rare reaction does
take place, the exam will be stopped, and the patient
will be rapidly treated with special medication and
closely monitored by a physician.
Iodine based contrast is primarily filtered out of
the bloodstream by the kidneys, and thus patients with
diabetes and/or renal disease will require continuous
hydration and close monitoring of kidney function. Diabetics
on certain a glucose-lowering medication (glucophage/metformin)
and renal dialysis patients should speak with their
physician regarding stopping the medication, and the
proper scheduling of the scan in conjunction with dialysis,
respectively. Consent from the patient or designated
guardian must be obtained prior to the use of intravenous
Why the test is performed
CT provides rapid, detailed cross-sectional imaging
of the patient which can then be reconstructed into
three-dimensional models, as needed. Intravenous contrast
enhanced scans allow for evaluation of vascular structures
and further evaluation of masses and tumors.
CT is often utilized in the trauma setting to evaluate
the brain, chest, and abdomen. As well, CT can be used
to guide interventional procedures, such as biopsies
and placement of drainage tubes.
Text courtesy of Yahoo! Health Encyclopedia.
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