CT Scan
CT stands for computerized tomography. In this procedure, a thin
X-ray beam is rotated around the area of the body to be visualized. Using very complicated
mathematical processes called algorithms, the computer is able to generate a 3-D image of
a section through the body. CT scans are very detailed and provide excellent information
for the physician.
How is a CT Scan 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 a CT Scan
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 will a CT Scan 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 is a CT Scan perfprmed
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.
What the risks when having a CT Scan
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.
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