Brookfield Hannah, Manning-Stanley Anthony, England Andrew
Radiol Technol. 2015 Mar-Apr;86(4):379-91.
To investigate the range of collimation errors in x-ray rooms and to calculate their possible effects on the radiation dose for anteroposterior pelvic examinations.
A collimator test tool was suspended at 3 heights (14, 21, and 28 cm) above the table Bucky in 9 x-ray rooms. Heights corresponded to the typical patient thickness (mean, ± 2 SD) of 67 patients undergoing anteroposterior pelvic radiography. The x-ray beam was visually collimated to the inner boundary of the test tool and exposed to radiation. Differences between the visualized field size and the resultant x-ray field size (corrected for magnification) indicated a collimation error. Next, using a pelvic phantom, minimum textbook collimation was set and then changed and verified to simulate a range of possible collimation errors. Phantom examinations used a standard anteroposterior technique with exposure termination using outer automatic exposure control chambers. Dose area product (DAP) was recorded.
All but 1 of the 9 x-ray machines had a smaller irradiated area than was visually set. Errors ranged from a 16% reduction in irradiated field size to a slight overirradiation by 0.4%. Assuming that these errors could be larger in other institutions, additional errors with a range of -27% to 18% were simulated. Increases in field size by 1 cm (superiorly/inferiorly) increased the DAP by 5%. Laterally, a 1-cm increase caused a 4% rise in DAP. Increases of 1 cm in both planes raised DAP by 4%.
Within a single clinical department, minimal collimation errors were demonstrated. Further evidence from multiple centers would be beneficial; however, such low incidences might reflect strict legislative requirements governing the use of ionizing radiation. Understanding the magnitude of any error is important, but it is also important to ascertain an error's influence on the effective radiation dose for any given examination.
Overall, collimation errors were minimal and favored underirradiation. Small collimation errors can affect DAP and are more dose significant in the superior/inferior plane.
研究X线检查室中准直误差的范围，并计算其对骨盆前后位检查辐射剂量的可能影响。
在9个X线检查室中，将一个准直器测试工具悬挂在检查床滤线栅上方3个高度（14、21和28厘米）处。这些高度对应于67例行骨盆前后位X线摄影患者的典型患者厚度（平均值±2标准差）。将X线束在视觉上对准测试工具的内边界并进行辐射曝光。视觉视野大小与最终X线视野大小（校正放大率后）之间的差异表明存在准直误差。接下来，使用骨盆模体，设置最小教科书式准直，然后改变并验证以模拟一系列可能的准直误差。模体检查采用标准的前后位技术，使用外部自动曝光控制腔室进行曝光终止。记录剂量面积乘积（DAP）。
9台X线机中除1台外，所有机器的照射面积均小于视觉设定值。误差范围从照射野大小减少16%到轻微过度照射0.4%。假设这些误差在其他机构可能更大，模拟了范围为-27%至18%的额外误差。视野大小在上下方向增加1厘米会使DAP增加5%。在左右方向，增加1厘米会使DAP上升4%。在两个平面上都增加1厘米会使DAP增加4%。
在单个临床科室中，准直误差最小。来自多个中心的进一步证据将是有益的；然而，如此低的发生率可能反映了关于电离辐射使用的严格立法要求。了解任何误差的大小很重要，但确定误差对任何给定检查的有效辐射剂量的影响也很重要。
总体而言，准直误差最小，且倾向于照射不足。小的准直误差会影响DAP，并且在上下平面中对剂量的影响更大。
