van der Molen Aart J, Geleijns Jacob
Department of Radiology C-2S, Leiden University Medical Center, Albinusdreef 2, NL-2333 ZA Leiden, the Netherlands.
Radiology. 2007 Jan;242(1):208-16. doi: 10.1148/radiol.2421051350. Epub 2006 Nov 7.
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).
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.
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.
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.
量化16层螺旋CT扫描时超出范围的旋转次数，并评估其对器官剂量和有效剂量的相对贡献。
通过在不同扫描长度下进行空气剂量测量，对四台16层螺旋CT扫描仪的超出范围情况进行量化。通过外推法得出所有准直器和临床常用螺距下特定层厚时的超出范围旋转次数和长度。分别分析重建层厚对超出范围的影响。将结果应用于胸部和腹部的临床扫描方案。确定甲状腺和睾丸剂量以及有效剂量，并计算超出范围的相对剂量贡献。采用Pearson相关性分析和配对t检验进行统计学分析。P<0.05表示差异有统计学意义。
不同扫描仪之间超出范围的旋转次数存在显著差异，最低螺距时范围为1.99 - 4.04，最高螺距时为0.93 - 2.59。旋转次数与螺距呈负相关，而超出范围长度与准直和螺距呈正相关。层厚的影响各不相同。在扫描方案中，胸部CT超出范围长度为3.2至5.8 cm，腹部CT为3.2至5.2 cm。若不考虑超出范围的影响，胸部（P = 0.012）和腹部（P = 0.011）CT的甲状腺（P = 0.025）和睾丸剂量以及有效剂量的计算结果会显著降低。
超出范围情况因重建算法而异，其长度通常随准直和螺距增加，而层厚的影响各不相同。超出范围可能导致对放射敏感器官的大量但未被注意到的辐射暴露。
