As with plain radiographs, computed tomography (CT) exploits the fact that X-rays penetrate different tissues in the body by different amounts depending on their atomic number and density. A narrow fan of X-rays rotates around the patient and the emergent X-rays are detected by a ring of detectors. A technique called filtered back projection is used to reconstruct cross-sectional slices through the body.

Images are viewed on different window settings to maximize the detail seen within the tissues of interest.

Recent advances in CT, in particular multislice imaging, have expanded the role of this modality. Multislice CT is fast and provides high-resolution images, which can be reconstructed in any projection to provide a great deal of anatomical information. The speed of the examination is an advantage in many situations involving sick, confused or very young patients.

The main disadvantage with CT is the relatively high radiation dose and the need to administer intravenous contrast for the majority of studies.

A radiologist usually decides the scanning protocol and adequate clinical information is required for the correct protocol to be chosen.

Perforation of the gastrointestinal tract

The objective of a CT scan of the abdomen and pelvis in cases of suspected perforation is three fold: to detect the presence and level of the perforation with greater sensitivity than plain films and contrast studies, assess the underlying cause, and identify complications. The scan should include the entire abdomen and pelvis; intravenous and water-soluble oral/rectal contrast should be administered in the absence of contraindications. The images should be viewed on a wide window setting.