Yang Ching-Ching, Yu Pei-Chieh, Ruan Jau-Ming, Chen Yu-Cheng
Department of Medical Imaging and Radiological Sciences, Tzu-Chi University of Science and Technology, Hualien, Taiwan.
Department of Radiation Oncology, Cathay General Hospital, Taipei, Taiwan.
J Appl Clin Med Phys. 2018 May;19(3):310-317. doi: 10.1002/acm2.12306. Epub 2018 Mar 8.
The target detectability of cone beam computed tomography (CBCT) performed in image-guided radiation therapy (IGRT) was investigated to achieve sufficient image quality for patient positioning over a course of treatment session while maintaining radiation exposure from CBCT imaging as low as reasonably achievable (ALARA).
Body CBCT scans operated in half-fan mode were acquired with three different protocols: CBCT , CBCT , and CBCT , which resulted in weighted CT dose index (CTDI ) of 0.36, 1.43, and 2.78 cGy, respectively. An electron density phantom that is 18 cm in diameter was covered by four layers of 2.5-cm-thick bolus to simulate patients of different body sizes. Multivariate analysis was used to examine the impact of body size, radiation exposure, and tissue type on the target detectability of CBCT imaging, quantified as contrast-to-noise ratio (CNR).
CBCT allows sufficient target detection of adipose, breast, muscle, liver in a background of water for normal-weight adults with cross-sectional diameter less than 28 cm, while CBCT is suitable for adult patients with larger body sizes or body mass index over 25 kg/m . Once the cross-sectional diameter of adult patients is larger than 35 cm, the CTDI of CBCT scans should be higher than 2.78 cGy to achieve required CNR. As for pediatric and adolescent patients with cross-sectional diameter less than 25 cm, CBCT is able to produce images with sufficient target detection.
The target detectability of soft tissues in default CBCT scans may not be sufficient for overweight or obese adults. Contrary, pediatric and adolescent patients would receive unnecessarily high radiation exposure from default CBCT scans. Therefore, the selection of acquisition parameters for CBCT scans optimized according to patient body size was proposed to ensure sufficient image quality for daily patient positioning in radiation therapy while achieving the ALARA principle.
研究在图像引导放射治疗（IGRT）中进行的锥形束计算机断层扫描（CBCT）的目标可检测性，以在整个治疗过程中实现足够的图像质量用于患者定位，同时将CBCT成像的辐射暴露保持在合理可达到的尽可能低的水平（ALARA）。
采用三种不同方案获取以半扇形模式运行的体部CBCT扫描：CBCT 、CBCT 和CBCT ，其加权CT剂量指数（CTDI ）分别为0.36、1.43和2.78 cGy。一个直径18 cm的电子密度体模被四层2.5 cm厚的 bolus覆盖，以模拟不同体型的患者。采用多变量分析来检查体型、辐射暴露和组织类型对CBCT成像目标可检测性的影响，以对比度噪声比（CNR）进行量化。
对于正常体重、横截面直径小于28 cm的成年人，CBCT能够在水背景中对脂肪、乳腺、肌肉、肝脏进行充分的目标检测，而CBCT适用于体型较大或体重指数超过25 kg/m 的成年患者。一旦成年患者的横截面直径大于35 cm，CBCT扫描的CTDI应高于2.78 cGy才能达到所需的CNR。对于横截面直径小于25 cm的儿童和青少年患者，CBCT能够产生具有足够目标检测的图像。
默认CBCT扫描中软组织的目标可检测性对于超重或肥胖成年人可能不足。相反，儿童和青少年患者会从默认CBCT扫描中接受不必要的高辐射暴露。因此，建议根据患者体型优化CBCT扫描的采集参数选择，以确保在放射治疗中每日患者定位有足够的图像质量，同时实现ALARA原则。
