Spin echo based cardiac diffusion imaging at 7T: An ex vivo study of the porcine heart at 7T and 3T

Purpose of this work was to assess feasibility of cardiac diffusion tensor imaging (cDTI) at 7 T in a set of healthy, unfixed, porcine hearts using various parallel imaging acceleration factors and to compare SNR and derived cDTI metrics to a reference measured at 3 T. Magnetic resonance imaging was performed on 7T and 3T whole body systems using a spin echo diffusion encoding sequence with echo planar imaging readout. Five reference (b = 0 s/mm2) images and 30 diffusion directions (b = 700 s/mm2) were acquired at both 7 T and 3 T using a GRAPPA acceleration factor R = 1. Scans at 7 T were repeated using R = 2, R = 3, and R = 4. SNR evaluation was based on 30 reference (b = 0 s/mm2) images of 30 slices of the left ventricle and cardiac DTI metrics were compared within AHA segmentation. The number of hearts scanned at 7 T and 3 T was n = 11. No statistically significant differences were found for evaluated helix angle, secondary eigenvector angle, fractional anisotropy and apparent diffusion coefficient at the different field strengths, given sufficiently high SNR and geometrically undistorted images. R≥3 was needed to reduce susceptibility induced geometric distortions to an acceptable amount. On average SNR in myocardium of the left ventricle was increased from 29±3 to 44±6 in the reference image (b = 0 s/mm2) when switching from 3 T to 7 T. Our study demonstrates that high resolution, ex vivo cDTI is feasible at 7 T using commercial hardware.

本研究旨在评估7T场强下，采用不同并行成像加速因子对11例健康未固定的离体猪心脏开展心脏弥散张量成像（cardiac diffusion tensor imaging, cDTI）的可行性，并将信噪比（signal-to-noise ratio, SNR）及衍生的cDTI指标与3T场强下的参考测量结果进行对比。磁共振成像分别在7T和3T全身磁共振系统上完成，采用自旋回波弥散编码序列结合回波平面成像（echo planar imaging, EPI）读出模式。在7T与3T场强下，均使用广义自动校准部分并行采集（GRAPPA）加速因子R=1，采集了5幅b值为0 s/mm²的参考图像以及30个弥散方向（b值为700 s/mm²）的图像。7T场强下的扫描分别使用R=2、R=3及R=4的加速因子重复进行。信噪比评估基于左心室30层的30幅b=0 s/mm²参考图像，心脏DTI指标的对比采用美国心脏协会（American Heart Association, AHA）分段法。本次研究共扫描11例猪心脏，7T与3T场强下均完成对应扫描。在信噪比足够高且图像几何无畸变的前提下，不同场强下所评估的螺旋角、第二特征向量角、各向异性分数（fractional anisotropy, FA）以及表观弥散系数（apparent diffusion coefficient, ADC）均未出现统计学显著差异。当加速因子R≥3时，方可将磁敏感诱导的几何畸变降低至可接受水平。从3T切换至7T场强时，左心室心肌在b=0 s/mm²参考图像中的平均信噪比从29±3提升至44±6。本研究证实，使用商用硬件即可在7T场强下开展高分辨率离体心脏弥散张量成像。