Faculty of Medical Radiation Engineering, Shahid Beheshti University, Tehran, Iran.
Phys Med. 2012 Jul;28(3):191-9. doi: 10.1016/j.ejmp.2011.06.002. Epub 2011 Jul 8.
The operation of the bowtie filter in x-ray CT is correct if the object being scanned is properly centered in the scanner's field-of-view. Otherwise, the dose delivered to the patient and image noise will deviate from optimal setting. We investigate the effect of miscentering on image noise and surface dose on three commercial CT scanners. Six cylindrical phantoms with different size and material were scanned on each scanner. The phantoms were positioned at 0, 2, 4 and 6 cm below the isocenter of the scanner's field-of-view. Regression models of surface dose and noise were produced as a function of miscentering magnitude and phantom's size. 480 patients were assessed using the calculated regression models to estimate the influence of patient miscentering on image noise and patient surface dose in seven imaging centers. For the 64-slice CT scanner, the maximum increase of surface dose using the CTDI-32 phantom was 13.5%, 33.3% and 51.1% for miscenterings of 2, 4 and 6 cm, respectively. The analysis of patients' scout scans showed miscentering of 2.2 cm in average below the isocenter. An average increase of 23% and 7% was observed for patient dose and image noise, respectively. The maximum variation in patient miscentering derived from the comparison of imaging centers using the same scanner was 1.6 cm. Patient miscentering may substantially increase surface dose and image noise. Therefore, technologists are strongly encouraged to pay greater attention to patient centering.
如果被扫描的物体在扫描仪的视场中正确居中，那么蝶形滤光片在 X 射线 CT 中的运行是正确的。否则，向患者提供的剂量和图像噪声将偏离最佳设置。我们研究了在三个商业 CT 扫描仪上的偏中心对图像噪声和表面剂量的影响。在每个扫描仪上扫描了六个具有不同大小和材料的圆柱形体模。将体模定位在距扫描仪视场等中心下方 0、2、4 和 6cm 处。制作了表面剂量和噪声与偏中心量和体模大小的回归模型。使用计算出的回归模型评估了 480 名患者，以估计患者在七个成像中心的偏中心对图像噪声和患者表面剂量的影响。对于 64 层 CT 扫描仪，使用 CTDI-32 体模的表面剂量最大增加量分别为 2cm、4cm 和 6cm 时的 13.5%、33.3%和 51.1%。对患者的扫描图像进行分析显示，平均有 2.2cm 的偏中心位于等中心下方。患者剂量和图像噪声分别观察到平均增加 23%和 7%。使用相同的扫描仪比较成像中心时，患者偏中心的最大变化为 1.6cm。患者偏中心可能会大大增加表面剂量和图像噪声。因此，强烈鼓励技术人员更加关注患者的中心位置。
