van der Kouwe André J W, Benner Thomas, Dale Anders M
Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, Massachusetts 02129, USA.
Magn Reson Med. 2006 Nov;56(5):1019-32. doi: 10.1002/mrm.21038.
Subject motion during scanning can greatly reduce MRI image quality and is a major reason for discarding data in both clinical and research scanning. The quality of the high-resolution structural data used for morphometric analysis is especially compromised by subject movement because high-resolution scans are of longer duration. A method is presented that measures and corrects rigid body motion and associated first-order shim changes in real time, using a pulse sequence with embedded cloverleaf navigators and a feedback control mechanism. The procedure requires a 12-s preliminary mapping scan. A single-path, 4.2-ms cloverleaf navigator is inserted every repetition time (TR) after the readout of a 3D fast low-angle shot (FLASH) sequence, requiring no additional RF pulses and minimally impacting scan duration. Every TR, a rigid body motion estimate is made and a correction is fed back to adjust the gradients and shim offsets. Images are corrected and reconstructed on the scanner computer for immediate access. Correction for between-scan motion can be accomplished by using the same reference map for each scan repetition. Human and phantom tests demonstrated a consistent improvement in image quality if motion occurred during the acquisition.
扫描过程中的受试者运动可极大降低磁共振成像（MRI）图像质量，这是临床和研究扫描中数据被丢弃的主要原因。用于形态学分析的高分辨率结构数据的质量尤其会因受试者运动而受损，因为高分辨率扫描持续时间更长。本文提出了一种方法，该方法使用带有嵌入式苜蓿叶导航器的脉冲序列和反馈控制机制，实时测量并校正刚体运动及相关的一阶匀场变化。该过程需要进行一次12秒的初步映射扫描。在三维快速低角度激发（FLASH）序列读出后，每隔一个重复时间（TR）插入一个单路径、4.2毫秒的苜蓿叶导航器，无需额外的射频脉冲，且对扫描持续时间的影响最小。每个TR都会进行一次刚体运动估计，并反馈校正以调整梯度和匀场偏移。图像在扫描仪计算机上进行校正和重建以便立即查看。对于扫描间的运动校正，可以通过在每次扫描重复时使用相同的参考图来实现。人体和模型测试表明，如果在采集过程中发生运动，图像质量会持续改善。
