Department of Radiation Oncology, Brigham and Women's Hospital, Dana Farber Cancer Institute and Harvard Medical School, Boston, MA, United States of America.
Department of Physics and Applied Physics, University of Massachusetts Lowell, Lowell, MA, United States of America.
Phys Med Biol. 2021 Apr 16;66(8). doi: 10.1088/1361-6560/abe051.
Multi-layer imaging (MLI) devices improve the detective quantum efficiency (DQE) while maintaining the spatial resolution of conventional mega-voltage (MV) x-ray detectors for applications in radiotherapy. To date, only MLIs with identical detector layers have been explored. However, it may be possible to instead use different scintillation materials in each layer to improve the final image quality. To this end, we developed and validated a method for optimally combining the individual images from each layer of MLI devices that are built with heterogeneous layers. Two configurations were modeled within the GATE Monte Carlo package by stacking different layers of a terbium doped gadolinium oxysulfide Gd2O2S:Tb (GOS) phosphor and a LKH-5 glass scintillator. Detector response was characterized in terms of the modulation transfer function (MTF), normalized noise power spectrum (NNPS) and DQE. Spatial frequency-dependent weighting factors were then analytically derived for each layer such that the total DQE of the summed combination image would be maximized across all spatial modes. The final image is obtained as the weighted sum of the sub-images from each layer. Optimal weighting factors that maximize the DQE were found to be the quotient of MTF and NNPS of each layer in the heterogeneous MLI detector. Results validated the improvement of the DQE across the entire frequency domain. For the LKH-5 slab configuration, DQE(0) increases between 2%-3% (absolute), while the corresponding improvement for the LKH-5 pixelated configuration was 7%. The performance of the weighting method was quantitatively evaluated with respect to spatial resolution, contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR) of simulated planar images of phantoms at 2.5 and 6 MV. The line pair phantom acquisition exhibited a twofold increase in CNR and SNR, however MTF was degraded at spatial frequencies greater than 0.2 lp mm. For the Las Vegas phantom, the weighting improved the CNR by around 30% depending on the contrast region while the SNR values are higher by a factor of 2.5. These results indicate that the imaging performance of MLI systems can be enhanced using the proposed frequency-dependent weighting scheme. The CNR and SNR of the weighted combined image are improved across all spatial scales independent of the detector combination or photon beam energy.
多层成像（MLI）设备提高了探测量子效率（DQE），同时保持了传统兆伏（MV）X 射线探测器的空间分辨率，适用于放射治疗。迄今为止，仅探索了具有相同探测器层的 MLI。然而，在每个层中使用不同的闪烁材料来改善最终图像质量可能是可能的。为此，我们开发并验证了一种优化 MLI 设备各层图像组合的方法，这些设备采用的是具有异质层的 MLI 设备。通过堆叠不同的 LKH-5 玻璃闪烁体和掺铽的钆氧硫化物 Gd2O2S:Tb（GOS）荧光粉的层，在 GATE 蒙特卡罗包内对两种配置进行建模。探测器响应以调制传递函数（MTF）、归一化噪声功率谱（NNPS）和 DQE 来描述。然后分析导出了每个层的空间频率相关加权因子，使得所有空间模式下组合图像的总 DQE 最大化。最终的图像是通过对每个层的子图像进行加权求和得到的。发现优化加权因子是异质 MLI 探测器中每个层的 MTF 和 NNPS 的商，可最大化 DQE。结果验证了整个频域 DQE 的提高。对于 LKH-5 平板配置，DQE(0)增加了 2%-3%（绝对值），而对于 LKH-5 像素化配置，相应的提高了 7%。通过对 2.5 和 6 MV 的模拟平面图像的幻影进行空间分辨率、对比噪声比（CNR）和信噪比（SNR）的定量评估，对加权方法的性能进行了评估。线对幻影采集的 CNR 和 SNR 增加了一倍，但在大于 0.2 lp mm 的空间频率下，MTF 会降低。对于拉斯维加斯幻影，根据对比度区域，加权提高了大约 30%的 CNR，而 SNR 值提高了 2.5 倍。这些结果表明，使用所提出的频率相关加权方案可以增强 MLI 系统的成像性能。加权组合图像的 CNR 和 SNR 在所有空间尺度上都得到了提高，与探测器组合或光子束能量无关。
