Diffusion‐weighted imaging (DWI) provides a valuable diagnostic tool for tumor evaluation. Yet, it is difficult to acquire daily MRI data sets in the traditional radiotherapy clinical setting due to patient burden and limited resources. However, integrated MRI radiotherapy treatment systems facilitate daily functional MRI acquisitions like DWI during treatment exams. Before ADC values from MR‐RT systems can be used clinically their reproducibility and accuracy must be quantified. This study used a NIST traceable DWI phantom to verify ADC values acquired on a 0.35 T MR‐LINAC system at multiple gantry angles. A diffusion‐weighted echo planar imaging sequence was used for all image acquisitions, with b‐values of 0, 500, 900, 2000 s/mm² for the 1.5 T and 3.0 T systems and 0, 200, 500, 800 s/mm² for the 0.35 T system. Images were acquired at multiple gantry angles on the MR‐LINAC system from 0° to 330° in 30° increments to assess the impact of gantry angle on geometric distortion and ADC values. CT images, and three fiducial markers were used as ground truth for geometric distortion measurements. The distance between fiducial markers increased by as much as 7.2 mm on the MR‐LINAC at gantry angle 60°. ADC values of deionized water vials from the 1.5 T and 3.0 T systems were 8.30 × 10^‐6 mm²/s and −0.85 × 10^‐6 mm²/s off, respectively, from the expected value of 1127 × 10^‐6 mm²/s. The MR‐LINAC system provided an ADC value of the pure water vials that was −116.63 × 10^‐6 mm²/s off from the expected value of 1127 × 10^‐6 mm²/s. The MR‐LINAC also showed a variation in ADC across all gantry angles of 33.72 × 10^‐6 mm²/s and 20.41 × 10^‐6 mm²/s for the vials with expected values of 1127 × 10^‐6 mm²/s and 248 × 10^‐6 mm²/s, respectively. This study showed that variation of the ADC values and geometric information on the 0.35 T MR‐LINAC system was dependent on the gantry angle at acquisition.

中文翻译：

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0.35T MR-LINAC 多机架角度扩散加权 MRI 和几何畸变的评估

弥散加权成像 (DWI) 为肿瘤评估提供了有价值的诊断工具。然而，由于患者负担和资源有限，很难在传统放射治疗临床环境中获取每日 MRI 数据集。然而，集成的 MRI 放射治疗系统有助于在治疗检查期间进行 DWI 等日常功能性 MRI 采集。在将 MR-RT 系统的 ADC 值用于临床之前，必须对其再现性和准确性进行量化。本研究使用 NIST 可追溯 DWI 体模来验证在 0.35 T MR-LINAC 系统上以多个机架角度采集的 ADC 值。扩散加权回波平面成像序列用于所有图像采集，1.5 T 和 3.0 T 系统的 b 值为 0、500、900、2000 s/mm ²和 0、200、500、800 s/mm ^2个对于 0.35 T 系统。在 MR-LINAC 系统上以 0° 到 330° 的多个机架角度以 30° 的增量获取图像，以评估机架角度对几何失真和 ADC 值的影响。CT 图像和三个基准标记被用作几何失真测量的地面实况。在机架角度为 60° 的 MR-LINAC 上，基准标记之间的距离增加了 7.2 mm。来自 1.5 T 和 3.0 T 系统的去离子水瓶的 ADC 值分别为 8.30 × 10 -6 ^mm  2 ^/ s 和 −0.85 × 10 ^-6  mm ² /s，与预期值 1127 × 10 ^-6  mm相差² /秒。MR-LINAC 系统提供的纯水瓶的 ADC 值为 -116.63 × 10与预期值 1127 × 10 ^-6  mm ^{2 /s 相差‐6}  mm ² /s 。MR-LINAC 还显示 ADC 在所有机架角度上的变化为 33.72 × 10 -6 ^mm  2 ^/ s 和 20.41 × 10 ^-6  mm ^{2 /s，预期值为 1127 × 10 -6}  mm ² /s的小瓶和 248 × 10 ^-6  mm ² /s。本研究表明，0.35 T MR-LINAC 系统上 ADC 值和几何信息的变化取决于采集时的机架角度。