Varian Medical Systems, Memorial Sloan Kettering Cancer Center, USA.
Radiother Oncol. 2011 Jul;100(1):145-9. doi: 10.1016/j.radonc.2011.07.010. Epub 2011 Aug 5.
To perform kilovoltage (kV) cone beam computed tomography (CBCT) imaging concomitant with the delivery of megavoltage (MV) RapidArc treatment, and demonstrate the feasibility of obtaining MV-scatter-free kV CBCT images.
RapidArc/CBCT treatment and imaging plans are designed, and delivered on the Varian TrueBeam, using its Developer Mode. The plan contains 250 control points for MV-radiation delivery, each over an arc of 0.4-0.7(o). Interlaced between successive MV delivery control points are imaging control points, each over an arc of 0.7-1.1(o). During the 360(o) gantry rotation for the RapidArc delivery, CBCT projections of a phantom are acquired at 11 frames per second. The kV projections with minimal MV-scatter are selected, based on gantry angle, and the CBCT(s) image reconstructed. For comparison, a reference CBCT(r) image is acquired in the normal way. In addition, to examine the effect of MV-scatter we acquire CBCT(c) using the same treatment plan without the imaging control points, i.e. with continuous MV delivery during the 360(o) rotation. Quantitative evaluation of image qualities is performed based on the concepts of CNR (contrast-to-noise ratio) and NSTD (normalized standard deviation).
The different types of CBCT images were reconstructed, evaluated, and compared. Visual comparison indicates that the image quality of CBCT(s) is similar to that of the reference CBCT(r), and that the quality of CBCT(c) is significantly degraded by the MV-scatter. Quantitative evaluation of the image quality indicates that MV-scatter significantly decreases the CNR of CBCT (from ∼7 to ∼3.5 in one comparison). Similarly, MV-scatter significantly increases the inhomogeneity of image intensity, e.g. from ∼0.03 to ∼0.06 in one comparison.
We have developed a method to acquire MV-scatter-free kV CBCT images concomitant with the delivery of RapidArc treatment. Engineering development is necessary to improve the process, e.g. by synchronization of the MV and kV beams.
在实施兆伏级 RapidArc 治疗的同时进行千伏锥形束 CT（CBCT）成像，并验证获得兆伏级散射线-free 千伏 CBCT 图像的可行性。
利用瓦里安 TrueBeam 的开发者模式设计 RapidArc/CBCT 治疗和成像计划。该计划包含 250 个用于 MV 辐射输送的控制点，每个控制点在 0.4-0.7(o)的弧形上。在连续的 MV 输送控制点之间交错设置成像控制点，每个控制点在 0.7-1.1(o)的弧形上。在 RapidArc 输送的 360(o)机架旋转过程中，以每秒 11 帧的速度采集体模的千伏投影。根据机架角度选择具有最小兆伏散射的千伏投影，并重建 CBCT(s)图像。作为比较，以正常方式获取参考 CBCT(r)图像。此外，为了检查兆伏散射的影响，我们在没有成像控制点的情况下使用相同的治疗计划获取 CBCT(c)，即在 360(o)旋转过程中连续输送 MV。基于对比噪声比（CNR）和归一化标准差（NSTD）的概念，对图像质量进行定量评估。
重建、评估和比较了不同类型的 CBCT 图像。视觉比较表明，CBCT(s)的图像质量与参考 CBCT(r)相似，而 CBCT(c)的质量因兆伏散射而显著降低。图像质量的定量评估表明，兆伏散射显著降低了 CBCT 的 CNR（在一次比较中从约 7 降至约 3.5）。同样，兆伏散射显著增加了图像强度的不均匀性，例如在一次比较中从约 0.03 增至约 0.06。
我们已经开发了一种在实施 RapidArc 治疗的同时获取兆伏级散射线-free 千伏 CBCT 图像的方法。需要进行工程开发来改进该过程，例如通过同步 MV 和千伏束。
