Kontson Kimberly, Jennings Robert J
Department of Bioengineering, University of Maryland, College Park, Maryland 20742 and Division of Imaging and Applied Mathematics, Office of Science and Engineering Laboratories, Center for Devices and Radiological Health, U.S. Food and Drug Administration, 10903 New Hampshire Avenue, Silver Spring, Maryland 20993.
Med Phys. 2015 Sep;42(9):5270-7. doi: 10.1118/1.4928476.
For a given bowtie filter design, both the selection of material and the physical design control the energy fluence, and consequently the dose distribution, in the object. Using three previously described bowtie filter designs, the goal of this work is to demonstrate the effect that different materials have on the bowtie filter performance measures.
Three bowtie filter designs that compensate for one or more aspects of the beam-modifying effects due to the differences in path length in a projection have been designed. The nature of the designs allows for their realization using a variety of materials. The designs were based on a phantom, 14 cm in diameter, composed of 40% fibroglandular and 60% adipose tissue. Bowtie design #1 is based on single material spectral matching and produces nearly uniform spectral shape for radiation incident upon the detector. Bowtie design #2 uses the idea of basis-material decomposition to produce the same spectral shape and intensity at the detector, using two different materials. With bowtie design #3, it is possible to eliminate the beam hardening effect in the reconstructed image by adjusting the bowtie filter thickness so that the effective attenuation coefficient for every ray is the same. Seven different materials were chosen to represent a range of chemical compositions and densities. After calculation of construction parameters for each bowtie filter design, a bowtie filter was created using each of these materials (assuming reasonable construction parameters were obtained), resulting in a total of 26 bowtie filters modeled analytically and in the penelope Monte Carlo simulation environment. Using the analytical model of each bowtie filter, design profiles were obtained and energy fluence as a function of fan-angle was calculated. Projection images with and without each bowtie filter design were also generated using penelope and reconstructed using FBP. Parameters such as dose distribution, noise uniformity, and scatter were investigated.
Analytical calculations with and without each bowtie filter show that some materials for a given design produce bowtie filters that are too large for implementation in breast CT scanners or too small to accurately manufacture. Results also demonstrate the ability to manipulate the energy fluence distribution (dynamic range) by using different materials, or different combinations of materials, for a given bowtie filter design. This feature is especially advantageous when using photon counting detector technology. Monte Carlo simulation results from penelope show that all studied material choices for bowtie design #2 achieve nearly uniform dose distribution, noise uniformity index less than 5%, and nearly uniform scatter-to-primary ratio. These same features can also be obtained using certain materials with bowtie designs #1 and #3.
With the three bowtie filter designs used in this work, the selection of material is an important design consideration. An appropriate material choice can improve image quality, dose uniformity, and dynamic range.
对于给定的蝴蝶结滤波器设计，材料的选择和物理设计均会控制物体中的能量注量，进而控制剂量分布。本研究使用三种先前描述的蝴蝶结滤波器设计，旨在证明不同材料对蝴蝶结滤波器性能指标的影响。
设计了三种蝴蝶结滤波器，用于补偿由于投影中路径长度差异导致的光束修正效应的一个或多个方面。这些设计的性质允许使用多种材料来实现。设计基于一个直径为14 cm的体模，该体模由40%的纤维腺体组织和60%的脂肪组织组成。蝴蝶结设计#1基于单一材料光谱匹配，可使入射到探测器上的辐射产生近乎均匀的光谱形状。蝴蝶结设计#2利用基础材料分解的理念，使用两种不同材料在探测器处产生相同的光谱形状和强度。对于蝴蝶结设计#3，通过调整蝴蝶结滤波器厚度，使每条射线的有效衰减系数相同，从而有可能消除重建图像中的光束硬化效应。选择了七种不同材料来代表一系列化学成分和密度。在计算出每种蝴蝶结滤波器设计的结构参数后，使用这些材料中的每一种制作一个蝴蝶结滤波器（假设获得了合理的结构参数），从而在解析模型和penelope蒙特卡罗模拟环境中总共模拟了26个蝴蝶结滤波器。利用每个蝴蝶结滤波器的解析模型，获得设计轮廓并计算能量注量随扇角的变化。还使用penelope生成了有无每种蝴蝶结滤波器设计的投影图像，并使用FBP进行重建。研究了剂量分布、噪声均匀性和散射等参数。
对有无每种蝴蝶结滤波器的解析计算表明，对于给定设计，某些材料制成的蝴蝶结滤波器对于乳腺CT扫描仪来说太大而无法实现，或者太小而难以精确制造。结果还表明，对于给定的蝴蝶结滤波器设计，通过使用不同材料或不同材料组合，可以操纵能量注量分布（动态范围）。在使用光子计数探测器技术时，这一特性尤为有利。penelope的蒙特卡罗模拟结果表明，蝴蝶结设计#2的所有研究材料选择均实现了近乎均匀的剂量分布、小于5%的噪声均匀性指数以及近乎均匀的散射与原发射线比值。使用某些材料与蝴蝶结设计#1和#3也可获得这些相同的特性。
在本研究使用的三种蝴蝶结滤波器设计中，材料的选择是一个重要的设计考虑因素。选择合适的材料可以提高图像质量、剂量均匀性和动态范围。
