Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5G 2M9, Canada.
Med Phys. 2011 Feb;38(2):897-914. doi: 10.1118/1.3539575.
X-ray scatter contributes significantly to image degradation in cone-beam CT (CBCT) reconstructed images in the form of CT number inaccuracy image artifacts and loss of contrast. The need for an understanding of the relationship between the scatter distribution and common imaging parameters (cone angle, air gap, filtration, object size) is an essential element to developing methods for efficiently correcting for the effects of scatter in CBCT. The first objective of this study is to validate the scatter distributions calculated using a CBCT Monte Carlo (MC) model against measured scatter estimates. The second objective is to use the CBCT MC model to investigate the effects of common imaging parameters and bowtie compensators on the resulting scatter distribution.
This investigation employs the use of a CBCT MC model, developed using the EGSnrc code, to simulate the primary and scatter fluence arriving at the detector. The simulation is validated against projection images, scatter-to-open field ratio (SOR), and scatter-to-primary ratio (SPR) measurements taken using a bench-top CBCT system. The CBCT MC model is used to simulate the scatter distribution arriving at the detector for different cone angles {1.4 degrees, 2.8 degrees, 5.7 degrees, and 11.3 degrees}, source-to-axis distances (SADs) {50, 75 and 100 cm}, and axis-to-detector distances (ADDs) {9, 18, 30, 44, 56 cm} for both a 16.4 and 30.6 cm diameter water cylinder. The effects of different bowtie filters are also simulated using the CBCT MC model for the aforementioned cylinder sizes.
Profiles of the simulated and measured projection images agree within 6%. The measurements of the SOR and SPR, taken using beam stop techniques, show good agreement with the simulated results. Limitations of current beam stop techniques in estimating scatter profiles of objects with varying thickness are also demonstrated. A functional relationship between scatter (SOR, SPR) and air gap, cone angle is reported. The bowtie filter was found to have the beneficial effect of decreasing the magnitude of scatter in the projection images (SPR decreased by 56%) as well as altering the spatial distribution of the scatter.
The CBCT MC model accurately simulates scatter and primary fluences in the CBCT imaging components and geometry. The CBCT MC model provides a useful tool in investigating the effects of varying CBCT imaging parameters on the scatter distribution. Increasing the air gap, decreasing the cone angle, and the use of bowtie filtration were all found to be effective ways to minimize scatter in CBCT. The bowtie filter was particularly effective in both minimizing the magnitude and modifying the spatial distribution of the scattered photons. This observation directs further research in optimizing bowtie filter design in an effort to minimize scatter induced artifacts.
在锥形束 CT（CBCT）重建图像中，X 射线散射以 CT 数不准确、图像伪影和对比度损失的形式对图像退化有显著影响。了解散射分布与常见成像参数（锥角、气隙、滤过、物体大小）之间的关系是开发有效校正 CBCT 散射影响的方法的必要因素。本研究的第一个目标是验证使用 CBCT 蒙特卡罗（MC）模型计算的散射分布与测量的散射估计值之间的一致性。第二个目标是使用 CBCT MC 模型研究常见成像参数和 Bowtie 补偿器对散射分布的影响。
本研究采用 EGSnrc 代码开发的 CBCT MC 模型来模拟到达探测器的初级和散射辐射。该模拟通过使用桌面 CBCT 系统进行的投影图像、散射-开放场比（SOR）和散射-初级比（SPR）测量进行验证。使用 CBCT MC 模型模拟不同锥角（1.4 度、2.8 度、5.7 度和 11.3 度）、源轴距离（SAD）（50、75 和 100 cm）和轴探测器距离（ADD）（9、18、30、44 和 56 cm）下，直径为 16.4 和 30.6 cm 的水圆柱体到达探测器的散射分布。还使用 CBCT MC 模型模拟了不同 Bowtie 滤过器对上述圆柱体尺寸的影响。
模拟和测量的投影图像的轮廓在 6%以内一致。使用束流截止技术测量的 SOR 和 SPR 与模拟结果吻合良好。还证明了当前束流截止技术在估计具有不同厚度的物体散射轮廓方面的局限性。报告了散射（SOR、SPR）与气隙、锥角之间的函数关系。发现 Bowtie 滤过器具有降低投影图像中散射幅度（SPR 降低 56%）以及改变散射空间分布的有益效果。
CBCT MC 模型准确模拟了 CBCT 成像组件和几何结构中的散射和初级辐射。CBCT MC 模型为研究不同 CBCT 成像参数对散射分布的影响提供了有用的工具。增加气隙、减小锥角和使用 Bowtie 滤过都被发现是减少 CBCT 散射的有效方法。Bowtie 滤过器在最小化散射光子的幅度和改变其空间分布方面特别有效。这一观察结果促使进一步研究优化 Bowtie 滤过器设计，以尽量减少散射引起的伪影。
