Siemens Healthcare GmbH, Siemensstr. 3, 91301, Forchheim, Germany.
Department of Medical Physics, Faculty of Physics, Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748, Garching, Germany.
Med Phys. 2019 Dec;46(12):5528-5537. doi: 10.1002/mp.13690. Epub 2019 Jul 26.
It has been a long-standing wish in computed tomography (CT) to compensate the emitted x-ray beam intensity for the patient's changing attenuation during the rotation of a CT data acquisition. The patient attenuation changes both spatially, along the fan beam angle, and temporally, between different projections. By modifying the pre-patient x-ray intensity profile according to the attenuation properties of the given object, image noise can be homogenized and dose can be delivered where it is really needed. Current state-of-the-art bowtie filters are not capable of changing attenuation profiles during the CT data acquisition. In our work, we present the sheet-based dynamic beam attenuator (sbDBA), a novel technical concept enabling dynamic shaping of the transmission profile.
The sbDBA consists of an array of closely spaced, highly attenuating metal sheets, focused toward the focal spot. Intensity modulation can be achieved by controlled defocusing of the array such that the attenuation of the x-ray fan beam depends on the fan angle. The sbDBA concept was evaluated in Monte-Carlo (MC) simulations regarding its spectral and scattering properties. A prototype of the sbDBA was installed in a clinical CT scanner and measurements evaluating the feasibility and the performance of the sbDBA concept were carried out.
Experimental measurements on a CT scanner demonstrate the ability of the sbDBA to produce an attenuation profile that can be changed in width and location. Furthermore, the sbDBA shows constant transmission properties at various tube voltages. A small effect of the flying focal spot (FFS) position on the transmission profile can be observed. MC simulations confirm the essential properties of the sbDBA: In contrast to conventional bowtie filters, the sbDBA has almost no impact on the energy spectrum of the beam and there is negligible scatter emission toward the patient.
A new concept for dynamic beam attenuation has been presented and its ability to dynamically shape the transmission profile has successfully been demonstrated. Advantages compared to regular bowtie filters including the lack of filter-induced beam hardening and scatter have been confirmed. The novel concept of a DBA paves the way toward region of interest (ROI) imaging and further reductions in patient dose.
在计算机断层扫描（CT）中，补偿患者在 CT 数据采集旋转过程中衰减变化的发射 X 射线束强度一直是人们的长期愿望。患者的衰减变化既沿着扇形束角度空间变化，也在不同投影之间随时间变化。通过根据给定物体的衰减特性修改患者前的 X 射线强度分布，可以使图像噪声均匀化，并在真正需要的地方提供剂量。当前最先进的 Bowtie 滤波器在 CT 数据采集过程中无法改变衰减分布。在我们的工作中，我们提出了基于薄片的动态光束衰减器（sbDBA），这是一种新的技术概念，可以实现传输轮廓的动态成型。
sbDBA 由一组紧密间隔的、高度衰减的金属薄片组成，这些薄片聚焦在焦点处。通过控制阵列的离焦，可以实现强度调制，使得 X 射线扇形束的衰减取决于扇形角度。在 Monte-Carlo（MC）模拟中评估了 sbDBA 的光谱和散射特性。在临床 CT 扫描仪中安装了 sbDBA 的原型，并进行了评估 sbDBA 概念的可行性和性能的测量。
在 CT 扫描仪上的实验测量表明，sbDBA 能够产生可改变宽度和位置的衰减分布。此外，sbDBA 在各种管电压下具有恒定的传输特性。可以观察到飞行焦点（FFS）位置对传输分布的微小影响。MC 模拟证实了 sbDBA 的基本特性：与常规 Bowtie 滤波器相比，sbDBA 对光束的能谱几乎没有影响，并且向患者方向的散射发射可以忽略不计。
提出了一种新的动态光束衰减概念，并成功地演示了其动态成型传输分布的能力。与常规 Bowtie 滤波器相比，它具有无需滤波器引起的束硬化和散射的优势。这种新型 DBA 概念为感兴趣区域（ROI）成像和进一步降低患者剂量铺平了道路。
