Department of Physics, Ryerson University, Toronto, Ontario, M5B 2K3, Canada.
Department of Physics, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada.
Med Phys. 2020 Jan;47(1):152-163. doi: 10.1002/mp.13900. Epub 2019 Nov 4.
To develop a new scintillating fiber-based electronic portal imaging device (EPID) with a high quantum efficiency (QE) while preserving an adequate spatial resolution.
Two prototypes were built: one with a single pixel readout and the other with an active matrix flat-panel imager (AMFPI) for readout. The energy conversion layer of both prototypes was made of scintillating fiber layers interleaved with corrugated lead sheets to form a honeycomb pattern. The scintillating fibers have a diameter of 1 mm and the distance between the centers of neighboring fibers on the same layer is 1.35 mm. The layers have 1.22 mm spacing between them. The energy conversion layer has a thickness of 2 cm. The modulation transfer function (MTF), antiscatter properties and sensitivity of the detector with a single pixel readout were measured using a 6-MV beam on a LINAC machine. In addition, a Monte Carlo simulation was conducted to calculate the zero-frequency detective quantum efficiency (DQE(0)) of the proposed detector with an active matrix flat-panel imager for readout.
The DQE(0) of the proposed detector can be 11.5%, which is about an order of magnitude higher than that of current EPIDs. The frequency of 50% modulation ( ) of the measured MTF is 0.2 mm at 6 MV, which is comparable to that of video-based EPIDs. The scatter to primary ratio (SPR) measured with the detector at 10 cm air gap and 20 × 20 cm field size is approximately 30% lower than that of ionization chamber-based detectors with a comparable QE. The detector noise which includes the x-ray quantum noise and absorption noise is much larger than the electronic noise per pixel of the flat-panel imager at a dose of less than two LINAC pulses. Thus, the proposed detector is quantum noise limited down to very low doses (∼a couple of radiation pulses of the LINAC). A proof-of-concept image has been obtained using a 6-MV beam.
This work indicates that by using scintillating fibers and lead layers it is possible to increase the thickness of the detecting materials, and therefore the QE or the DQE(0) of the detector, while maintaining an adequate spatial resolution for MV x-ray imaging. Due to the use of lead as the spacing material, the new detector also has antiscatter property, which will help improve the signal-to-noise ratio of the images. Further investigation to optimize the design of the detector and achieve a better combination of DQE and spatial resolution is warranted.
开发一种具有高量子效率 (QE) 同时保持适当空间分辨率的新型闪烁纤维型电子射野影像装置 (EPID)。
构建了两个原型：一个带有单像素读出，另一个带有用于读出的有源矩阵平板成像仪 (AMFPI)。两个原型的能量转换层均由闪烁纤维层和波纹铅片交替组成，形成蜂窝状图案。闪烁纤维的直径为 1mm，同一层相邻纤维中心之间的距离为 1.35mm。层与层之间的间隔为 1.22mm。能量转换层的厚度为 2cm。使用 LINAC 机器上的 6-MV 光束测量具有单像素读出的探测器的调制传递函数 (MTF)、抗散射特性和灵敏度。此外，进行了蒙特卡罗模拟以计算具有有源矩阵平板成像仪用于读出的探测器的零频率探测量子效率 (DQE(0))。
所提出的探测器的 DQE(0) 可以达到 11.5%，比当前的 EPID 高一个数量级。在 6MV 下，测量 MTF 的 50%调制频率 ( ) 为 0.2mm ，与基于视频的 EPID 相当。在 10cm 空气间隙和 20×20cm 射野尺寸下，探测器测量的散射与初级比 (SPR) 比具有可比 QE 的电离室探测器低约 30%。在剂量小于两个 LINAC 脉冲的情况下，探测器噪声（包括 X 射线量子噪声和吸收噪声）远大于平板成像仪每个像素的电子噪声。因此，在所提出的探测器在非常低的剂量（约为 LINAC 的几个辐射脉冲）下是量子噪声限制的。已经使用 6MV 光束获得了概念验证图像。
这项工作表明，通过使用闪烁纤维和铅层，可以增加检测材料的厚度，从而提高探测器的 QE 或 DQE(0)，同时保持 MV X 射线成像的适当空间分辨率。由于使用铅作为间隔材料，新探测器还具有抗散射特性，这将有助于提高图像的信噪比。需要进一步研究以优化探测器的设计并实现 DQE 和空间分辨率的更好结合。
