Overranging in multisection CT: quantification and relative contribution to dose--comparison of four 16-section CT scanners

Abstract

Purpose: To quantify the number of overrange rotations and to assess their relative contribution to organ and effective doses at 16-section body computed tomography (CT).

Materials and methods: Overranging was quantified for four 16-section scanners by means of free-in-air dose measurements at different scan lengths. Overrange rotations and lengths at a certain section width were derived for all collimations and clinically used pitches by extrapolation. The effect of reconstructed section width on overranging was analyzed separately. Results were applied to clinical protocols for the chest and abdomen. Thyroid and testicular dose and effective dose were established, and relative dose contributions from overranging were calculated. Statistical analysis was performed by using Pearson correlation and paired t tests. P<.05 indicated a significant difference.

Results: The number of overrange rotations showed considerable differences between scanners, with a range of 1.99-4.04 at the lowest and 0.93-2.59 at the highest pitch. Number of rotations correlated negatively with pitch, while overrange length correlated positively with collimation and pitch. The effect of section width was variable. In the protocols, overrange length ranged from 3.2 to 5.8 cm for chest and from 3.2 to 5.2 cm for abdominal CT. When the contribution of overranging was not taken into account, significantly lower values for thyroid (P=.012) and testicular (P=.025) doses and effective doses for chest (P=.005) and abdominal (P=.011) CT resulted.

Conclusion: Overranging is reconstruction-algorithm specific, and its length generally increases with collimation and pitch, while the effect of section width is variable. Overranging may lead to substantial but unnoticed exposure to radiosensitive organs.