El-Mohri Y, Choroszucha R, Antonuk L, Zhao Q, Jiang H
University of Michigan, Ann Arbor, MI.
Med Phys. 2012 Jun;39(6Part6):3654. doi: 10.1118/1.4734842.
Thick segmented scintillators, incorporating a 2-dimensional matrix of optically-isolated scintillator elements, have shown considerable potential for improving the performance of megavoltage active matrix, flat-panel imagers (MV AMFPIs). While over a factor of 20 improvement in DQE at zero spatial-frequency has been demonstrated for prototypes incorporating CsI(Tl) and BGO scintillators, less-than-optimal element-to-element alignment (misalignment) as well as mis-registration to the underlying AMFPI array pixels can result in spatial resolution loss, reducing DQE improvement at higher spatial frequencies. In this presentation, a method to restore spatial resolution and DQE, based on the use of a high resolution AMFPI array along with special binning techniques, is investigated.
The effect of misalignment and mis-registration of segmented scintillators on imaging performance was investigated theoretically and empirically through determination of the modulation transfer function (MTF) and DQE, as well as through realization of reconstructed images of a phantom in a cone-beam CT geometry. The empirical investigation, which was conducted using a 6 MV photon beam, employed a prototype BGO segmented scintillator consisting of 120×60 elements separated by 50 μm-thick septal walls and an element-to-element pitch of 1016 μm. The scintillator was coupled to a higher resolution 127-μm-pitch AMFPI array.
Misalignment and mis-registration result in significant degradation of spatial resolution, leading to DQE reduction at non-zero spatial frequencies. While mis-registration for a well-aligned scintillator can be overcome through 8×8 binning of the array pixels to match the scintillator elements, any misalignment will affect such binning and lead to spatial resolution loss. However, the use of 'selective' binning, consisting of the selection of those pixels corresponding to the interior locations of each element, improves resolution while preserving DQE.
The use of high-resolution AMFPI arrays combined with selective binning allows prototype AMFPIs incorporating thick, segmented scintillators to achieve imager performance limited only by scintillator performance. Work Supported by NIH grant R01-CA051397.
厚分段闪烁体包含二维光学隔离闪烁体元件矩阵，已显示出在改善兆伏有源矩阵平板成像器（MV AMFPI）性能方面具有相当大的潜力。虽然对于包含CsI(Tl)和BGO闪烁体的原型，在零空间频率下DQE有超过20倍的提升，但元件间对准不理想（未对准）以及与底层AMFPI阵列像素的配准错误会导致空间分辨率损失，降低在较高空间频率下的DQE提升。在本报告中，研究了一种基于使用高分辨率AMFPI阵列以及特殊合并技术来恢复空间分辨率和DQE的方法。
通过确定调制传递函数（MTF）和DQE，以及通过在锥束CT几何结构中实现体模的重建图像，从理论和实验上研究了分段闪烁体的未对准和配准错误对成像性能 的影响。实验研究使用6 MV光子束，采用一个由120×60个元件组成的原型BGO分段闪烁体，元件由50μm厚的间隔壁隔开，元件间距为1016μm。闪烁体与更高分辨率的127μm间距的AMFPI阵列耦合。
未对准和配准错误会导致空间分辨率显著下降，从而导致在非零空间频率下DQE降低。虽然对于对准良好的闪烁体，通过对阵列像素进行8×8合并以匹配闪烁体元件可以克服配准错误，但任何未对准都会影响这种合并并导致空间分辨率损失。然而，使用“选择性”合并，即选择与每个元件内部位置对应的那些像素，可以在保持DQE的同时提高分辨率。
使用高分辨率AMFPI阵列结合选择性合并，可使包含厚分段闪烁体的原型AMFPI实现仅受闪烁体性能限制的成像器性能。由美国国立卫生研究院R01-CA051397资助。
