Department of Radiation Oncology, Dana, Farber/Brigham and Women's Cancer Center, Harvard Medical School, Boston, MA, 02115, USA.
Varian Medical Systems, Baden, CH-5405, Switzerland.
Med Phys. 2020 Apr;47(4):1827-1835. doi: 10.1002/mp.14017. Epub 2020 Jan 28.
The feasibility of low-dose megavoltage cone-beam acquisition (MVCBCT) using a novel, high detective quantum efficiency (DQE) multi-layer imager (MLI) was investigated. The aim of this work was to reconstruct MVCBCT images using the MLI at different total dose levels, and assess Hounsfield Unit (HU) accuracy, noise and contrast-to-noise ratio (CNR) for low-dose megavoltage cone-beam acquisition.
The MLI has four stacked layers; each layer contains a combination of copper filter/converter, gadolinium oxysulfide (GOS) scintillator and a-Si detector array. In total, 720 projections of a CATPHAN phantom were acquired over 360° at 2.5, 6, and 6 MV flattening filter free (FFF) beam energies on a Varian TrueBeam LINAC. The dose per projection was 0.01, 0.0167, and 0.05 MU for 2.5, 6, and 6 MV FFF, respectively. MVCBCT images were reconstructed with varying numbers of projections to provide a range of doses for evaluation. Hounsfield Unit uniformity, accuracy, noise and CNR were estimated. Improvements were quantified relative to the standard AS1200 single-layer imager.
Average HU uniformity for the MLI reconstructions was within a range of 95%-99% for all of the energies studied. Relative electron density estimation from HU values was within 0.4% ± 1.8% from nominal values. The CNR for MVCBCT based on MLI projections was 2-4× greater than from AS1200 projections. The 2.5 MV beam acquisition with the MLI exhibited the lowest noise and the best balance between CNR and dose for low-dose reconstructions.
Megavoltage cone-beam acquisition imaging with a novel MLI prototype mounted on a clinical linear accelerator demonstrated substantial improvement over the standard AS1200 EPID. Further optimization of MVCBCT reconstruction, particularly for 2.5 MV acquisitions, will improve image metrics. Overall, the MLI improves CNR at substantially lower doses than currently required by conventional detectors. This new high DQE detector could provide high-quality MVCBCT at clinically acceptable doses.
研究使用新型高探测量子效率（DQE）多层成像仪（MLI）进行低剂量兆伏锥形束采集（MVCBCT）的可行性。本工作旨在使用 MLI 在不同总剂量水平下重建 MVCBCT 图像，并评估低剂量兆伏锥形束采集的体素值（HU）准确性、噪声和对比噪声比（CNR）。
MLI 有四个堆叠层；每个层包含铜滤光片/转换器、氧化钆（GOS）闪烁体和 a-Si 探测器阵列的组合。在瓦里安 TrueBeam LINAC 上，用 2.5、6 和 6 MV 无均整滤波器（FFF）射束能量共采集 720 个 CATPHAN 体模的投影，角度为 360°。每个投影的剂量分别为 2.5、6 和 6 MV FFF 的 0.01、0.0167 和 0.05 MU。使用不同数量的投影重建 MVCBCT 图像，以提供不同剂量的评估。估计了体素值均匀性、准确性、噪声和 CNR。与标准 AS1200 单层成像仪相比，对改进进行了量化。
对于所有研究的能量，MLI 重建的平均体素值均匀性在 95%-99%的范围内。根据 HU 值估算的相对电子密度与标称值相差在 0.4%±1.8%以内。基于 MLI 投影的 MVCBCT 的 CNR 比基于 AS1200 投影的 CNR 高 2-4 倍。在 2.5 MV 射束采集时，MLI 表现出最低的噪声和低剂量重建中 CNR 和剂量之间的最佳平衡。
安装在临床直线加速器上的新型 MLI 原型进行的兆伏锥形束采集成像与标准 AS1200 EPID 相比有显著提高。进一步优化 MVCBCT 重建，特别是对于 2.5 MV 采集，将改善图像指标。总体而言，与传统探测器相比，新型高 DQE 探测器可在临床可接受剂量下提供高质量的 MVCBCT。
