Department of Physics, University of Alberta, Edmonton, Alberta, Canada.
Med Phys. 2012 Oct;39(10):6139-47. doi: 10.1118/1.4752422.
In integrated linac-MRI systems, the RF coils are exposed to the linac's pulsed radiation, leading to a measurable radiation induced current (RIC). This work (1) visualizes the RIC in MRI raw data and determines its effect on the MR image signal-to-noise ratio (SNR) (b) examines the effect of linac dose rate on SNR degradations, (c) examines the RIC effect on different MRI sequences, (d) examines the effect of altering the MRI sequence timing on the RIC, and (e) uses a postprocessing method to reduce the RIC signal from the MR raw data.
MR images were acquired on the linac-MR prototype system using various imaging sequences (gradient echo, spin echo, and bSSFP), dose rates (0, 50, 100, 150, 200, and 250 MU∕min) and repetition times (TR) with the gradient echo sequence. The images were acquired with the radiation beam either directly incident or blocked from the RF coils. The SNR was calculated for each of these scenarios, showing a loss in SNR due to RIC. Finally, a postprocessing method was applied to the image k-space data in order to remove partially the RIC signal and recover some of the lost SNR.
The RIC produces visible spikes in the k-space data acquired with the linac's radiation incident on the RF coils. This RIC leads to a loss in imaging SNR that increases with increasing linac dose rate (15%-18% loss at 250 MU∕min). The SNR loss seen with increasing linac dose rate appears to be largely independent of the MR sequence used. Changing the imaging TR had interesting visual effects on the appearance of RIC in k-space due to the timing between the linac's pulsing and the MR sequence, but did not change the SNR loss for a given linac dose rate. The use of a postprocessing algorithm was able to remove much of the RIC noise spikes from the MR image k-space data, resulting in the recovery of a significant portion, up to 81% (Table II), of the lost image SNR.
The presence of RIC in MR RF coils leads to a loss of SNR which is directly related to the linac dose rate. The RIC related loss in SNR is likely to increase for systems that are able to provide larger than 250 MU∕min dose. Some of this SNR loss can be recovered through the use of a postprocessing algorithm, which removes the RIC artefact from the image k-space.
在集成直线加速器-MRI 系统中，射频线圈会受到直线加速器脉冲辐射的影响，导致可测量的辐射感应电流（RIC）。本研究（1）在 MRI 原始数据中可视化 RIC，并确定其对磁共振图像信噪比（SNR）的影响；（2）检查直线加速器剂量率对 SNR 降低的影响；（3）检查 RIC 对不同 MRI 序列的影响；（4）检查改变 MRI 序列定时对 RIC 的影响；（5）使用后处理方法从 MR 原始数据中减少 RIC 信号。
在直线加速器-MR 原型系统上使用各种成像序列（梯度回波、自旋回波和 bSSFP）、剂量率（0、50、100、150、200 和 250 MU/min）和重复时间（TR）采集图像，其中梯度回波序列。当辐射束直接入射或被 RF 线圈阻挡时，采集图像。对于这些情况，计算 SNR，显示由于 RIC 导致 SNR 损失。最后，将后处理方法应用于图像 k 空间数据，以部分去除 RIC 信号并恢复部分丢失的 SNR。
当直线加速器的辐射入射到 RF 线圈时，在采集的 k 空间数据中会产生可见的 RIC 尖峰。这种 RIC 导致成像 SNR 损失增加，随着直线加速器剂量率的增加（250 MU/min 时损失 15%-18%）而增加。随着直线加速器剂量率的增加而出现的 SNR 损失似乎在很大程度上与所使用的 MR 序列无关。由于直线加速器的脉冲和 MR 序列之间的定时，改变成像 TR 对 k 空间中 RIC 的出现有有趣的视觉影响，但对于给定的直线加速器剂量率，不会改变 SNR 损失。使用后处理算法能够从 MR 图像 k 空间数据中去除大部分 RIC 噪声尖峰，从而恢复高达 81%（表二）的丢失图像 SNR。
MR RF 线圈中存在 RIC 会导致 SNR 损失，这与直线加速器的剂量率直接相关。随着能够提供大于 250 MU/min 剂量的系统的出现，这种与 RIC 相关的 SNR 损失可能会增加。通过使用后处理算法，可以从图像 k 空间中去除 RIC 伪影，从而恢复部分 SNR。
