CT - Computed Tomography Overview

Definition
How the test is performed
How to prepare for the test
How the test will feel
What the risks are
Why the test is performed

Definition
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."

Alternative Names
CAT scan; Computed axial tomography (CAT) scan; Computerised Tomography

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 300 pounds.

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 contrast.

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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