Abdominal Computed Tomography Scanning Technique

Updated: Sep 13, 2017
  • Author: Caroline R Taylor, MD; Chief Editor: Mahan Mathur, MD  more...
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Technique

Approach Considerations

Scan protocols are adapted to the specific diagnostic problem. Noncontrast scans are typically used to evaluate renal stones or retroperitoneal hematoma or is used in patients in whom iodinated contrast is contraindicated.

Contrast scans can be classified as single-phase, multiphase, or special. Single-phase scans are typically used to evaluate acute abdomen or suspected abdominal infections, with imaging usually in the portal venous phase. It is usually combined with administration of oral contrast. Oral or intraluminal contrast enhances the evaluation of the bowel. Multiphase scans consist of precontrast and combinations of arterial phase, portal venous phase, and delayed imaging, depending on the organ of interest. [16]

CT cystography uses water-soluble dilute iodinated contrast introduced via a catheter into the bladder, typically to evaluate for bladder rupture or leak. [17] CT enterography combines a negative contrast agent (Volumen) and multiphase scanning to evaluate bowel, arterial supply, and mesenteric and portal venous integrity. [18] CT colonography uses a noncontrast protocol in supine and prone positions after insufflation of the prepared large bowel with gaseous carbon dioxide at a controlled pressure. [19, 20]

Radiation exposure during a CT scan depends on the parameters used for the scan, such as the kilovoltage (kVp) and milliamperage (mA) selected, as well as factors such as speed of tube rotation, speed of table advancement, and the volume of tissue scanned. The dose is modulated to allow a tradeoff of noise (which becomes greater as the dose decreases) versus exposure. Exposure is measured in CTDI and DLP indexes, which can be converted to an estimate of absorbed dose in millisieverts (mSv).

A patient with a large body habitus represents a challenge, as the exposure parameters need to be increased to generate images of adequate diagnostic quality, with a resultant increase in absorbed dose. Exposure parameters for body CT examinations range on average from 3-25 mSv (annual background exposure in the United States estimated at 3.6 mSv), depending on factors such as body habitus, increasing if multiphase examination is needed. Manufacturers are actively developing iterative reconstruction techniques that allow further reduction in radiation exposure.

Contrast reactions are relatively rare, and usually minor, but the healthcare facility must have an emergency plan in place to deal with contrast reactions, including remedial medications (checked and regularly updated), physiologic monitoring apparatus, and staff certified in emergency life-support procedures. If cardiovascular collapse occurs, timely support should be available from emergency services or code teams, while, in the interim, cardiopulmonary resuscitation is administered by the trained CT staff.