Extra Z-axis coverage at CT imaging resulting in excess radiation dose: frequency, degree, and contributory factors.

Objective: To assess the degree of extra scanning beyond the prescribed anatomic boundaries for thoracic and body computed tomographic (CT) scans and to identify associated factors.

Methods: For 442 consecutive chest, abdomen, and/or pelvis CT examinations, the length of extra scanning beyond the prescribed anatomic boundaries was determined. Examinations were grouped according to the locations/types of the prescribed boundaries and compared with regard to length of extra scanning.

Results: Of 442 CT examinations, 438 (99%) included extraneous imaging, showing a mean excess scanning length of 43.2 mm per examination (range, 0-180 mm). Significantly more extraneous imaging was performed when soft tissue or vascular structures defined anatomic boundaries compared to when osseous (P < 0.001) or air/soft tissue interfaces (P < 0.0001) defined the boundaries. The average percent of total scan dose attributable to extra imaging was 8.64% to 10.38%.

Conclusions: Computed tomographic scanning beyond the prescribed anatomic boundaries occurs commonly, resulting in moderate extra radiation dose.