The MR-Linac is a combination of an MR-scanner and radiotherapy linear accelerator (Linac) which holds the promise to increase the precision of radiotherapy treatments with MR-guided radiotherapy by monitoring motion during radiotherapy with MRI, and adjusting the radiotherapy plan accordingly. Optimal MR-guidance for respiratory motion during radiotherapy requires MR-based 3D motion estimation with a latency of 200–500 ms. Currently this is still challenging since typical methods rely on MR-images, and are therefore limited by the 3D MR-imaging latency. In this work, we present a method to perform non-rigid 3D respiratory motion estimation with 170 ms latency, including both acquisition and reconstruction. The proposed method called real-time low-rank MR-MOTUS reconstructs motion-fields directly from ${k}$ -space data, and leverages an explicit low-rank decomposition of motion-fields to split the large scale 3D+t motion-field reconstruction problem posed in our previous work into two parts: (I) a medium-scale offline preparation phase and (II) a small-scale online inference phase which exploits the results of the offline phase for real-time computations. The method was validated on free-breathing data of five volunteers, acquired with a 1.5T Elekta Unity MR-Linac. Results show that the reconstructed 3D motion-field are anatomically plausible, highly correlated with a self-navigation motion surrogate ( ${R}={0.975} \pm {0.0110}$ ), and can be reconstructed with a total latency of 170 ms that is sufficient for real-time MR-guided abdominal radiotherapy.

中文翻译：

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使用 MR-MOTUS 进行 MR 引导放射治疗的实时非刚性 3D 呼吸运动估计

MR-Linac 是 MR 扫描仪和放射治疗直线加速器 (Linac) 的组合，它有望通过 MRI 监测放射治疗期间的运动并相应地调整放射治疗计划来提高 MR 引导放射治疗的放射治疗精度。放射治疗期间呼吸运动的最佳 MR 引导需要基于 MR 的 3D 运动估计，延迟为 200-500 ms。目前这仍然具有挑战性，因为典型的方法依赖于 MR 图像，因此受到 3D MR 成像延迟的限制。在这项工作中，我们提出了一种以 170 毫秒延迟执行非刚性 3D 呼吸运动估计的方法，包括采集和重建。所提出的称为实时低秩 MR-MOTUS 的方法直接从 ${k}$ -空间数据，并利用运动场的显式低秩分解将我们之前工作中提出的大规模 3D+t 运动场重建问题分为两部分：（I）中规模离线准备阶段和（ II) 小规模在线推理阶段，利用离线阶段的结果进行实时计算。该方法通过 1.5T Elekta Unity MR-Linac 获得的五名志愿者的自由呼吸数据进行了验证。结果表明，重建的 3D 运动场在解剖学上是合理的，与自导航运动代理高度相关。 ${R}={0.975} \pm {0.0110}$ )，并且可以以 170 毫秒的总延迟进行重建，这足以进行实时 MR 引导的腹部放射治疗。