Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan 48019, USA.
Med Phys. 2010 Jun;37(6):2738-48. doi: 10.1118/1.3416924.
Electronic portal imaging devices based on megavoltage (MV), active matrix, flat-panel imagers (AMFPIs) are presently regarded as the gold standard in portal imaging for external beam radiation therapy. These devices, employing indirect detection of incident radiation by means of a metal plate plus phosphor screen combination, offer a quantum efficiency of only approximately 2% at 6 MV, leading to a detective quantum efficiency (DQE) of only approximately 1%. In order to significantly improve the DQE performance of MV AMFPIs, a strategy based on the development of direct detection imagers incorporating thick films of polycrystalline mercuric iodide (HgI2) photoconductor was undertaken and is reported.
Two MV AMFPI prototypes, one incorporating an approximately 300 microm thick HgI2 layer created through physical vapor deposition (PVD) and a second incorporating an approximately 460 microm thick HgI2 layer created through screen-printing of particle-in-binder (PIB) material, were quantitatively evaluated using a 6 MV photon beam. The reported measurements include empirical determination of x-ray sensitivity, lag, modulation transfer function (MTF), noise power spectrum, and DQE.
For both prototypes, MTF and DQE results were found to be consistent with theoretical expectations and the MTFs were also found to be higher than that measured from a conventional MV AMFPI. In addition, the DQE results exhibit input-quantum-limited behavior, even at extremely low doses. Compared to PVD, the PIB prototype exhibits much lower dark current, slightly higher lag, and similar DQE. Finally, the challenges associated with this approach, as well as strategies for achieving considerably higher DQE through thicker HgI2 layers, are discussed.
The DQE of each of the prototypes is found to be comparable to that of conventional MV AMFPIs, commensurate with the modest photoconductor thicknesses of these early samples. It is anticipated that thicker layers of HgI2 based on PIB deposition can provide higher DQE while maintaining good material properties.
基于兆伏级（MV）、有源矩阵、平板成像仪（AMFPIs）的电子门户成像设备目前被认为是外部束放射治疗门户成像的金标准。这些设备采用间接检测方法，通过金属板加磷光屏组合来检测入射辐射，在 6 MV 时量子效率仅约为 2%，导致检测量子效率（DQE）仅约为 1%。为了显著提高 MV AMFPI 的 DQE 性能，我们采用了一种基于开发直接检测成像仪的策略，该成像仪采用厚膜多晶碘化汞（HgI2）光电导体制成，并进行了报道。
使用 6 MV 光子束对两种 MV AMFPI 原型进行了定量评估，一种原型包含通过物理气相沉积（PVD）制成的约 300 微米厚的 HgI2 层，另一种原型包含通过粒子-粘合剂（PIB）材料的丝网印刷制成的约 460 微米厚的 HgI2 层。报告的测量包括对 X 射线灵敏度、滞后、调制传递函数（MTF）、噪声功率谱和 DQE 的经验确定。
对于两种原型，MTF 和 DQE 结果均与理论预期一致，并且 MTF 也高于从传统 MV AMFPI 测量的值。此外，即使在极低剂量下，DQE 结果也表现出输入量子限制行为。与 PVD 相比，PIB 原型的暗电流低得多，滞后略高，而 DQE 相似。最后，讨论了这种方法所面临的挑战以及通过更厚的 HgI2 层实现更高 DQE 的策略。
每个原型的 DQE 都被发现与传统 MV AMFPI 相当，与这些早期样品的光电导器适度厚度相符。预计基于 PIB 沉积的更厚 HgI2 层可以提供更高的 DQE，同时保持良好的材料性能。
