低场磁共振直线加速器中大视场失真的评估。
Large field of view distortion assessment in a low-field MR-linac.
机构信息
Department of Radiation Oncology, Henry Ford Cancer Institute, 2799 West Grand Blvd., Detroit, MI, 48202, USA.
出版信息
Med Phys. 2019 May;46(5):2347-2355. doi: 10.1002/mp.13467. Epub 2019 Mar 23.
PURPOSE
MR-guided radiation therapy (RT) offers unparalleled soft tissue contrast for localization and target tracking. However, MRI distortions may be detrimental to high precision RT. This work characterizes the gradient nonlinearity (GNL) and total distortions over the first year of clinical operation of a 0.35T MR-linac.
METHODS
For GNL characterization, an in-house large field of view (FOV) phantom (60 × 42.5 × 55 cm , >6000 spherical landmarks) was configured and scanned at four timepoints with forward/reverse read polarities (Gradient Echo sequence, FA/TR/TE = 28°/30 ms/6 ms). GNL was measured in Anterior-Posterior (AP), Left-Right (LR), and Superior-Inferior (SI) frequency-encoding directions based on deviation of the auto-segmented landmark centroids between rigidly registered MR and CT images and assessed based on radial distance from magnet isocenter. Total distortion was assessed using a 30 × 30 cm grid phantom oriented along the cardinal axes over >1 year of operation.
RESULTS
The scanner's spatial integrity within the first ~10 months was stable (maximum total distortion variation = 10/6/8%, maximum distortion = 1.41/0.99/1.56 mm in Axial/Coronal/Sagittal planes, respectively). GNL distortions measured during this time period <10 cm from isocenter were (-0.74, 0.45), (-0.67, 0.53), and (-0.86, 0.70) mm in AP/LR/SI directions. In the 10-20 cm range, <1.5% of the distortions exceeded 2 mm in the AP and LR axes while <4% of the distortions exceeded 2 mm for SI. After major repairs and magnet re-shim, detectable changes were observed in total and GNL distortions (20% reduction in AP and 36% increase in SI direction in the 20-25 cm range). Across all timepoints and axes, 38-53% of landmarks in the 20-25 cm range were displaced by >1 mm.
CONCLUSIONS
GNL distortions were negligible within a 10 cm radius from isocenter. However, in the periphery, non-negligible distortions of up to ~7 mm were observed, which may necessitate GNL corrections for MR-IGRT for treatment sites distant from magnet isocenter.
目的
MR 引导的放射治疗（RT）提供了无与伦比的软组织对比，用于定位和目标跟踪。然而，MRI 失真可能对高精度 RT 有害。本研究描述了在 0.35T 磁共振直线加速器临床运行的第一年中，梯度非线性（GNL）和总失真的特征。
方法
为了进行 GNL 特征描述，使用内部大视野（FOV）体模（60×42.5×55cm，>6000 个球形标志点），在四个时间点以正向/反向读极性（梯度回波序列，FA/TR/TE=28°/30ms/6ms）进行扫描。基于刚性配准的 MR 和 CT 图像之间自动分割标志点质心的偏差，在前后（AP）、左右（LR）和上下（SI）频率编码方向测量 GNL，并根据距磁体等中心的径向距离进行评估。在>1 年的运行期间，使用沿主要轴的 30×30cm 网格体模评估总失真。
结果
在前~10 个月内，扫描仪的空间完整性稳定（最大总失真变化=10/6/8%，轴向/冠状/矢状平面的最大失真分别为 1.41/0.99/1.56mm）。在此期间测量的小于等中心 10cm 处的 GNL 失真值为（-0.74，0.45）、（-0.67，0.53）和（-0.86，0.70）mm，在 AP/LR/SI 方向上。在 10-20cm 范围内，AP 和 LR 轴上超过 2mm 的失真<1.5%，而 SI 轴上超过 2mm 的失真<4%。在进行重大维修和磁体重新调谐后，在总失真和 GNL 失真方面观察到可检测的变化（AP 方向减少 20%，SI 方向增加 36%，在 20-25cm 范围内）。在所有时间点和轴上，20-25cm 范围内的 38-53%的标志点位移超过 1mm。
结论
在等中心 10cm 半径内，GNL 失真可以忽略不计。然而，在周边区域，观察到高达~7mm 的不可忽略的失真，这可能需要对远离磁体等中心的治疗部位进行 GNL 校正，以实现 MR-IGRT。
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