Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada.
Med Phys. 2012 Jul;39(7):4209-18. doi: 10.1118/1.4728977.
This is a proof-of-concept study addressing volume of interest (VOI) cone beam CT (CBCT) imaging using an x-ray beam produced by 2.35 MeV electrons incident on a carbon linear accelerator target. Methodology is presented relevant to VOI CBCT image acquisition and reconstruction. Sample image data are given to demonstrate and compare two approaches to minimizing artifacts arising from reconstruction with truncated projections. Dosimetric measurements quantify the potential dose reduction of VOI acquisition relative to full-field CBCT. The dependence of contrast-to-noise ratio (CNR) on VOI dimension is investigated.
A paradigm is presented linking the treatment planning process with the imaging technique, allowing definition of an imaging VOI to be tailored to the geometry of the patient. Missing data in truncated projection images are completed using a priori information in the form of digitally reconstructed radiographs (DRRs) generated from the planning CT set. This method is compared to a simpler technique of extrapolating truncated projection data prior to reconstruction. The utility of these approaches is shown through imaging of a geometric phantom and the head-and-neck section of a lamb. The total scatter factor of the 2.35 MV∕carbon beam on field size is measured and compared to a standard therapeutic beam to estimate the comparative dose reduction inside the VOI. Thermoluminescent dosimeters and Gafchromic film measurements are used to compare the imaging dose distributions for the 2.35 MV∕carbon beam between VOI and full-field techniques. The dependence of CNR on VOI dimension is measured for VOIs ranging from 4 to 15 cm diameter.
Without compensating for missing data outside of truncated projections prior to reconstruction, pronounced boundary artifacts are present, in three dimensions, within 2-3 cm of the edges of the VOI. These artifacts, as well as cupping inside the VOI, can be reduced substantially using either the DRR filling or extrapolation techniques presented. Compared to 6 MV, the 2.35 MV∕carbon beam shows a substantially greater dependence of total scatter factor on field size, indicating a comparative advantage of the VOI approach when combined with the low-Z target beam. Dosimetric measurements in the anthropomorphic head phantom demonstrate a dose reduction by up to 15% and 75% inside and outside of the VOI, respectively, compared to full-field imaging. For the 2.35 MV∕carbon beam, CNR was shown to be approximately invariant with VOI dimension for bone and lung objects.
The low-Z target, VOI CBCT technique appears to be feasible and combines the desirable characteristics of the low-Z target beam with regard to CNR, with the capacity to localize the imaging dose to the anatomy relevant to the image guidance task.
本研究旨在探讨利用 2.35MeV 电子束撞击碳直线加速器靶产生的 X 射线束进行感兴趣区（VOI）锥形束 CT（CBCT）成像。介绍了与 VOI CBCT 图像采集和重建相关的方法。给出了样本图像数据，以演示和比较两种方法，以最小化由于截断投影重建而产生的伪影。剂量测量量化了相对于全野 CBCT 采集的潜在剂量减少。研究了对比度噪声比（CNR）随 VOI 尺寸的变化。
提出了一种将治疗计划过程与成像技术联系起来的范例，允许根据患者的几何形状定制成像 VOI。使用从计划 CT 集生成的数字重建射线照片（DRR）的先验信息来完成截断投影图像中的缺失数据。将这种方法与在重建之前外推截断投影数据的更简单方法进行了比较。通过对几何体模和羊头颈部进行成像，展示了这些方法的实用性。测量了 2.35MV/碳束在射野大小上的总散射因子，并与标准治疗束进行比较，以估计 VOI 内的相对剂量减少。使用热释光剂量计和 Gafchromic 胶片测量来比较 2.35MV/碳束在 VOI 和全野技术之间的成像剂量分布。测量了 CNR 随 VOI 尺寸的变化，VOI 直径范围为 4 至 15cm。
在重建之前不补偿截断投影外的缺失数据，在 VOI 边缘 2-3cm 范围内，三维空间内存在明显的边界伪影。通过使用本文提出的 DRR 填充或外推技术，可以大大减少这些伪影以及 VOI 内的杯状伪影。与 6MV 相比，2.35MV/碳束的总散射因子对射野大小的依赖性明显更大，这表明在与低 Z 靶束结合使用时，VOI 方法具有比较优势。在人体头部模拟体模中的剂量测量表明，与全野成像相比，VOI 内和 VOI 外的剂量分别降低了高达 15%和 75%。对于 2.35MV/碳束，CNR 显示为对于骨和肺物体，随 VOI 尺寸大致不变。
低 Z 靶 VOI CBCT 技术似乎是可行的，它结合了低 Z 靶束在 CNR 方面的理想特性，以及将成像剂量定位到与图像引导任务相关的解剖结构的能力。
