Department of Medical Physics, University of Wisconsin-Madison, Madison, WI 53705, USA.
Med Phys. 2013 Feb;40(2):021906. doi: 10.1118/1.4773880.
The purpose of this work is to present a performance study of the digital beam attenuator (DBA) for implementing fluence field modulated CT (FFMCT) using a simulation framework developed to model the incorporation of the DBA into an existing CT system. Additionally, initial results will be presented using a prototype DBA and the realization of the prototype will be described. To our knowledge, this study represents the first experimental use of a device capable of modulating x-ray fluence as a function of fan angle using a CT geometry.
To realize FFMCT, the authors propose to use a wedge design in which one wedge is held stationary and another wedge is moved over the stationary wedge. Due to the wedge shape, the composite thickness of the two wedges changes as a function of the amount of overlap between the wedges. This design allows for the wedges to modulate the photon fluence incident onto a patient. Using a simulation environment, the effect of changing the number of wedges has on dose, scatter, detector dynamic range, and noise uniformity is explored. Experimental results are presented using a prototype DBA having ten Fe wedges and a c-arm CT system geometry. The experimental DBA results are compared to non-DBA scans using scatter and detector dynamic range as metrics. Both flat field and bowtie filtered CT acquisitions were simulated for comparison with the DBA.
Numerical results suggest that substantial gains in noise uniformity and scatter-to-primary ratio (SPR) can be obtained using only seven wedges. After seven wedges, the decrease in noise ununiformity and SPR falls off at a lower rate. Simulations comparing CT acquisitions between flat field, bowtie enabled, and DBA CT acquisitions suggest DBA-FFMCT can reduce dose relative to flat field CT by ≈3 times. A bowtie filter under the same imaging conditions was shown to only allow a dose reduction of 1.65 times. Experimentally, a 10 wedge DBA prototype result showed a SPR reduction of ≈4 times relative to flat field CT. The dynamic range for the DBA prototype was 3.7 compared to 84.2 for the flat field scan.
Based on the results presented in this paper and the companion paper [T. Szczykutowicz and C. Mistretta, "Design of a digital beam attenuation system for computed tomography. Part I. System design and simulation framework," Med. Phys. 40, 021905 (2013)], FFMCT implemented via the DBA device seems feasible and should result in both a dose reduction and an improvement in image quality as judged by noise uniformity and scatter reduction. In addition, the dynamic range reduction achievable using the DBA may allow photon counting imaging to become a clinical reality. This study may allow for yet another step to be taken in the field of patient specific dose modulation.
本工作旨在介绍一种数字射线衰减器（DBA）的性能研究，该 DBA 用于通过开发的模拟框架实现用于实施调制射线剂量场的 CT（FFMCT），以对 DBA 集成到现有 CT 系统中进行建模。此外，还将展示使用原型 DBA 的初步结果，并描述原型的实现。据我们所知，这项研究代表了首次使用能够在 CT 几何形状下作为扇形角函数调制 X 射线剂量率的设备进行的实验。
为了实现 FFMCT，作者提出使用楔形设计，其中一个楔形保持固定，另一个楔形在固定楔形上移动。由于楔形的形状，两个楔形的复合厚度随楔形之间的重叠量而变化。该设计允许楔形调制入射到患者上的光子剂量率。通过模拟环境，研究了改变楔形数量对剂量、散射、探测器动态范围和噪声均匀性的影响。使用具有十个 Fe 楔形和 C 臂 CT 系统几何形状的原型 DBA 呈现实验结果。使用散射和探测器动态范围作为指标，将实验 DBA 结果与非 DBA 扫描进行比较。比较了平场和蝴蝶结滤波 CT 采集，以与 DBA 进行比较。
数值结果表明，使用仅七个楔形就可以获得显著的噪声均匀性和散射到主射线比（SPR）增益。使用七个楔形之后，噪声均匀性和 SPR 的降低速度会以较低的速度下降。比较平场、蝴蝶结启用和 DBA CT 采集之间的 CT 采集的模拟表明，DBA-FFMCT 可以将剂量相对于平场 CT 降低约 3 倍。在相同的成像条件下使用蝴蝶结滤波器只能将剂量降低 1.65 倍。实验上，10 楔形 DBA 原型结果显示与平场 CT 相比，SPR 降低约 4 倍。DBA 原型的动态范围为 3.7，而平场扫描的动态范围为 84.2。
根据本文和配套论文 [T. Szczykutowicz 和 C. Mistretta，“用于计算机断层扫描的数字射线衰减系统的设计。第一部分：系统设计和模拟框架”，医学物理 40, 021905（2013）] 中呈现的结果，通过 DBA 设备实施的 FFMCT 似乎是可行的，并且应该会导致剂量降低和图像质量提高，这可以通过噪声均匀性和散射减少来判断。此外，使用 DBA 可实现的动态范围降低可能允许光子计数成像成为临床现实。这项研究可能会使患者特定剂量调制领域又迈出一步。
