Phys Med Biol. 2014 Feb 21;59(4):797-818. doi: 10.1088/0031-9155/59/4/797. Epub 2014 Feb 3.
Thick, segmented crystalline scintillators have shown increasing promise as replacement x-ray converters for the phosphor screens currently used in active matrix flat-panel imagers (AMFPIs) in radiotherapy, by virtue of providing over an order of magnitude improvement in the detective quantum efficiency (DQE). However, element-to-element misalignment in current segmented scintillator prototypes creates a challenge for optimal registration with underlying AMFPI arrays, resulting in degradation of spatial resolution. To overcome this challenge, a methodology involving the use of a relatively high resolution AMFPI array in combination with novel binning techniques is presented. The array, which has a pixel pitch of 0.127 mm, was coupled to prototype segmented scintillators based on BGO, LYSO and CsI:Tl materials, each having a nominal element-to-element pitch of 1.016 mm and thickness of ∼ 1 cm. The AMFPI systems incorporating these prototypes were characterized at a radiotherapy energy of 6 MV in terms of modulation transfer function, noise power spectrum, DQE, and reconstructed images of a resolution phantom acquired using a cone-beam CT geometry. For each prototype, the application of 8 × 8 pixel binning to achieve a sampling pitch of 1.016 mm was optimized through use of an alignment metric which minimized misregistration and thereby improved spatial resolution. In addition, the application of alternative binning techniques that exclude the collection of signal near septal walls resulted in further significant improvement in spatial resolution for the BGO and LYSO prototypes, though not for the CsI:Tl prototype due to the large amount of optical cross-talk resulting from significant light spread between scintillator elements in that device. The efficacy of these techniques for improving spatial resolution appears to be enhanced for scintillator materials that exhibit mechanical hardness, high density and high refractive index, such as BGO. Moreover, materials that exhibit these properties as well as offer significantly higher light output than BGO, such as CdWO4, should provide the additional benefit of preserving DQE performance.
厚的、分段的结晶闪烁体作为目前用于放射治疗中的有源矩阵平板成像器 (AMFPI) 中的磷光屏的替代 X 射线转换器，由于提供了超过一个数量级的探测量子效率 (DQE) 改进，因此越来越有前途。然而，当前分段闪烁体原型中的元素间失准给与底层 AMFPI 阵列的最佳配准带来了挑战，导致空间分辨率下降。为了克服这一挑战，提出了一种涉及使用相对高分辨率的 AMFPI 阵列与新颖的 binning 技术相结合的方法。该阵列的像素间距为 0.127mm，与基于 BGO、LYSO 和 CsI:Tl 材料的原型分段闪烁体耦合，每个材料的标称元素间间距为 1.016mm，厚度约为 1cm。将这些原型纳入的 AMFPI 系统在放射治疗能量为 6MV 的情况下进行了表征，评估了调制传递函数、噪声功率谱、DQE 以及使用锥束 CT 几何形状获取的分辨率体模的重建图像。对于每个原型，通过使用最小化失准的对准度量标准来优化应用 8×8 像素 binning 以实现 1.016mm 的采样间距，从而提高了空间分辨率。此外，应用排除在隔板壁附近收集信号的替代 binning 技术，进一步显著提高了 BGO 和 LYSO 原型的空间分辨率，而对于 CsI:Tl 原型则没有，因为在该设备中，由于光在闪烁体元件之间的大量扩散，导致光学串扰很大。对于表现出机械硬度、高密度和高折射率的闪烁体材料，如 BGO，这些技术提高空间分辨率的效果似乎更强。此外，表现出这些特性以及比 BGO 提供更高光输出的材料，如 CdWO4，应该提供保持 DQE 性能的额外好处。
