Real-time B0 compensation during gantry rotation in a 0.35-T MRI-Linac.
Real-time B0 compensation during gantry rotation in a 0.35-T MRI-Linac.
复制标题
0.35-T MRI-Linac 中机架旋转期间的实时 B0 补偿。
DOI：
10.1002/mp.15892
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发表时间：
2022
期刊：
影响因子：
3.8
通讯作者：
Gach,HMichael
中科院分区：
文献类型：
--
作者：
Curcuru,AustenN;Kim,Taeho;Yang,Deshan;Gach,HMichael
低磁场mri直线加速器的铁磁门架的旋转先前被证明会导致±400 Hz的大中心频率偏移。bo0off共振导致图像伪影和成像等中心偏移，这将妨碍MRI引导的电弧治疗。本研究的目的是使用自由感应衰减（FID）导航仪在0.35 - T mri直线加速器上实时测量和补偿龙门旋转期间的中心频率偏移。方法在0.35 - T MRI-Linac上，在每次二维平衡稳态自由进动影像采集前，添加非选择性FID导航仪。以每秒7.3帧的速度获取图像。初始FID导航仪的相位数据（当龙门架静止时）用作参考。每个后续FID导航器的相位数据用于计算实时bo0off谐振。调整发射器/接收器相位和相邻图像采集上的相位，以纠正中心频率偏移。在龙门顺时针和逆时针旋转之前和同时，使用MRI-Linac动态模体进行测量。使用静止时获得的参考图像，比较未校正和b0校正mri的图像质量和信噪比（SNR）。在第一帧图像上对幻影中的四个目标进行手动轮廓，并在随后的每一帧上回顾性地使用主动轮廓算法来评估图像变化并计算Dice系数。此外，三名健康志愿者在龙门旋转过程中使用相同的脉冲序列进行成像，有和没有实时b0补偿。计算了幻影和活体的归一化均方根误差（nrmse），以评估b0补偿对图像质量的影响。还比较了志愿者和MRI动态幻影导航仪数据的测量中心频率偏移。基于龙门布局和龙门旋转产生的长时间恒定涡流，对中心频率偏移的正弦特性进行了建模。结果FID的导航和处理时间为4.5 ms。FID导航导致幻体和活体（肝脏）的信噪比下降≤11%。在b0补偿下，MRI引导放射治疗（MR - IGRT）幻影轮廓测量的骰子系数保持在0.8以上。在没有b0补偿的情况下，根据轮廓，Dice系数下降到0.8以下的时间高达21%。实时b0补偿导致MR - IGRT模型和体内nRMSE分别平均降低51%和16%。峰对峰中心频率偏移量在假体中为757 ~ 773 Hz，在活体中为760 ~ 871 Hz。结论动态实时b0补偿可显著提高图像质量，减少假影。然而，FID导航仪导致成像占空比和信噪比略有下降。
BackgroundRotation of the ferromagnetic gantry of a low magnetic field MRI–Linac was previously demonstrated to cause large center frequency offsets of ±400 Hz. TheB0off‐resonances cause image artifacts and imaging isocenter shifts that would preclude MRI‐guided arc therapy.PurposeThe purpose of this study was to measure and compensate for center frequency offsets in real time during gantry rotation on a 0.35‐T MRI–Linac using a free induction decay (FID) navigator.MethodsA nonselective FID navigator was added before each 2D balanced steady‐state free precession cine image acquisition on a 0.35‐T MRI–Linac. Images were acquired at 7.3 frames per second. Phase data from the initial FID navigator (while the gantry was stationary) was used as a reference. The phase data from each subsequent FID navigator was used to calculate the real‐timeB0off‐resonance. The transmitter/receiver phase and the phase accrual over the adjacent image acquisition were adjusted to correct for the center frequency offset. Measurements were performed using an MRI–Linac dynamic phantom prior to and while the gantry rotated clockwise and counterclockwise. Image quality and signal‐to‐noise ratio (SNR) were compared between uncorrected andB0‐corrected MRIs using a reference image acquired while the gantry was stationary. Four targets in the phantom were manually contoured on the first image frame, and an active contouring algorithm was used retrospectively on each subsequent frame to assess image variations and calculate Dice coefficients. Additionally, three healthy volunteers were imaged using the same pulse sequences with and without real‐timeB0compensation during gantry rotation. Normalized root mean square errors (nRMSEs) were calculated for the phantom and in vivo to assess the efficacy of theB0compensation on image quality. The measured center frequency offsets from the volunteer and MRI dynamic phantom navigator data were also compared. The sinusoidal behavior of the center frequency offsets was modeled based on the gantry layout and long‐time constant eddy currents resulting from gantry rotation.ResultsThe duration of the FID navigator and processing was 4.5 ms. The FID navigator resulted in a ≤11% drop in SNR in the phantom and in vivo (liver). Dice coefficients from the MRI‐guided radiation therapy (MR‐IGRT) phantom contour measurements remained above 0.8 withB0compensation. WithoutB0compensation, the Dice coefficients dropped below 0.8 for up to 21% of the time depending on the contour. Real‐timeB0compensation resulted in mean reductions in nRMSE of 51% and 16% for the MR‐IGRT phantom and in vivo, respectively. Peak‐to‐peak center frequency offsets ranged from 757 to 773 Hz in the phantom and 760 to 871 Hz in vivo.ConclusionDynamic real‐timeB0compensation significantly improved image quality and reduced artifacts during gantry rotation in the phantom and in vivo. However, the FID navigator resulted in a small drop in the imaging duty cycle and SNR.