Technical Note: Effects of rotating gantry on magnetic field and eddy currents in 0.35 T MRI-guided radiotherapy (MR-IGRT) system.
Technical Note: Effects of rotating gantry on magnetic field and eddy currents in 0.35 T MRI-guided radiotherapy (MR-IGRT) system.
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本研究的目的是通过比较台架运动和静止时的涡流、中心频率和场不均匀性测量值，确定0.35 T MRI直线加速器中台架旋转期间发生严重图像伪影的原因。梯度和B 0涡电流计算自由感应衰减（FID）产生的选择性激发在200 ms/测量的时间分辨率。B 0涡电流也计算从FID获得的非选择性激励在100 ms/测量的时间分辨率。中心频率和B 0的不均匀性进行了测量，通过采集FID的重复时间（TR）为290 ms。笛卡尔和径向二维真快速成像与稳态进动（TrueFISP）脉冲序列中使用的实时MRI引导放射治疗（MR-IGRT）被收购。为了评估伪影严重程度，计算了参考MRI（静态机架）与机架旋转期间采集的MRI之间的归一化均方根误差（nRMSE）。图像伪影定性分级为标称、轻微或严重。在机架顺时针和逆时针旋转整个范围时进行测量。将机架旋转期间的测量值与固定机架（每30°）的测量值进行比较。当机架旋转时，22-35%的时间观察到严重的图像伪影。短时间恒定涡流不受机架旋转的影响。对于旋转机架与固定机架，峰到峰中心频率和FWHM分别上升了13.2-14.5和1.1-1.6倍。中心频率偏移和场不均匀性的大小取决于机架旋转的方向。机架旋转期间的图像伪影主要由中心频率变化和场不均匀性引起。因此，动态B 0补偿技术应该能够减少机架旋转期间的伪影。
The purpose of this study was to identify the cause of severe image artifacts that occurred during gantry rotation in a 0.35 T MRI-Linac by comparing measurements of eddy currents, center frequency, and field inhomogeneities made with the gantry in motion and stationary. Gradient and B0 eddy currents were calculated from the free induction decays (FIDs) resulting from selective excitation at a temporal resolution of 200 ms/measurement. B0 eddy currents were also calculated from FIDs acquired with nonselective excitation at a temporal resolution of 100 ms/measurement. Center frequencies and B0 inhomogeneities were measured by acquiring FIDs with a repetition time (TR) of 290 ms. Cartesian and radial 2D true fast imaging with steady-state precession (TrueFISP) pulse sequences used in real-time MRI-guided radiation therapy (MR-IGRT) were acquired. To assess artifact severity, the normalized root mean square error (nRMSE) was calculated between a reference MRI (static gantry) and MRIs acquired during gantry rotation for each serial acquisition. Image artifacts were qualitatively graded as nominal, minor, or severe. Measurements were conducted while the gantry was rotated through its entire range for both clockwise and counterclockwise. Measurements during gantry rotation were compared to measurements with a stationary gantry (every 30°). Severe image artifacts were observed 22–35% of the time while the gantry was rotating. Short time constant eddy currents were not affected by gantry rotation. The peak to peak center frequency and FWHM rose by factors of 13.2–14.5 and 1.1–1.6, respectively, for the rotating versus stationary gantry. The magnitude of the center frequency offset and field inhomogeneities depended on the direction of the gantry rotation. Image artifacts during gantry rotation were primarily caused by center frequency variations and field inhomogeneities. Therefore, dynamic B0 compensation techniques should be able to reduce artifacts during gantry rotation.