Patient Education - Endocrine Encyclopedia
Endocrine Surgery Encyclopedia
 Definition:
Computed tomography (CT) a method of body imaging in which a thin x-ray beam rotates around
the patient. Small detectors measure the amount of x-rays that make it through the patient
or particular area of interest.
A computer analyzes the data to construct a cross-sectional 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."
Alternative Names: CAT scan; Computed axial tomography (CAT) scan
How the test is performed:
You will be asked to lie on a narrow table that slides into the center of the scanner. Depending
on the study being performed, you may need to lie on your stomach, back, or side. If contrast
dye is to be administered, an IV will be placed in a small vein of a hand or arm.
Much like standard photographic cameras, any motion you make causes blurred images in
CT. Therefore, the operator will give you instructions through an intercom on when to hold
your breath and not move.
As the exam takes place, the table will advance small intervals through the scanner. Modern "spiral" scanners
can perform the exam in one continuous motion. 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 your entire body, head to toe,
in less than 30 seconds.
How to prepare for the test:
You may be asked to drink contrast immediately prior to the CT scan, or 4 to 6 hours beforehand.
The contrast may be non-reactive, chalky-tasting barium sulfate, which will eventually pass
in the stools, or absorbable clear Gastrografin solution. You may also be asked to fast (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 its internal mechanisms. Have the
health care provider contact the scanner operator if you weigh more than 300 pounds.
Since x-rays have difficulty passing through metal, 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.
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.
Normal Values:
What abnormal results mean:
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 Benadryl. 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 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
contrast.
For additional information regarding why the test is performed and normal and abnormal
results, please see the specific CT topics:
- orbit CT scan
- cranial CT scan
- lumbosacral spine CT scan
- thoracic CT scan
Special considerations:
Review Date: 11/3/2004
Reviewed By: Jeffrey J. Brown, MD FACR, Mallinckrodt Institute of Radiology, Washington University
School of Medicine, St. Louis, MO. Review provided by VeriMed Healthcare Network.
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