モーション補償スピンエコー心臓拡散テンソルイメージングにおける超高勾配強度スキャナを用いた複数心周期撮像

Cardiac diffusion tensor imaging (cDTI) has traditionally relied on inefficient stimulated echo techniques to robustly assess microstructural changes over the cardiac cycle. Ultrahigh gradient strength systems (>80mT/m) allow shorter motion compensated diffusion encoding. This study compares the ability of high and ultrahigh strength gradient systems to provide systolic and diastolic motion compensated spin echo (MCSE) cDTI.

Second order MCSE sequences were developed for a research-only Siemens 3T Connectom (300mT/m maximum gradient amplitude per axis) and breath hold cDTI was acquired at peak systole and end diastole. Acquisitions used the maximum achievable gradient strength (G_UH, 116mT/m) and also limited to typical high gradient strengths (G_H, 66mT/m based on 80mT/m maximum allowable), giving TE=48ms and 58ms respectively. Data were acquired at 2.8×2.8x8mm³, b=500s/mm² (8 averages) and b=150s/mm² (2 averages) in 6 encoding directions.

22 healthy subjects were recruited. 20/21 and 21/22 systolic acquisitions at G_UH and G_H respectively met the >50% criteria of the circumferential myocardium showing the expected transmural variation in helix angle. For G_UH and G_H (16/20) 80% and (16/22) 73% of diastolic acquisitions were successful respectively. SNR was increased using G_UH compared to G_H (median [IQR]: 112.9 [3.8] vs. 9.6 [2.9], p=0.0002 diastole, 15.6 [5.9] vs. 12.5 [6.7], p=0.006 systole). Using G_UH fractional anisotropy was lower in systole (0.349 [0.040] vs. 0.373 [0.019], p=0.002) and G_UH transmural helix angle gradient (HAG) was steeper in diastole (-0.70 [0.17] vs. -0.55 [0.12] ˚/%, p=0.04). At both G_UH and G_H, sheetlet angle (|E2A|) was higher in systole than in diastole (30.7 [7.3] vs. 21.3 [6.7]˚ p=10^-4 and 32.6 [10.9] vs. 26.0 [7.4]˚, p=0.03 respectively). Differences in HAG between phases were only apparent with G_H (-0.88 [0.23] vs. -0.55 [0.15], p=10^-4) and differences in the mean diffusivity only with G_UH (1.64 [0.11] vs. 1.52 [0.24] x10^-3mm²/s, p=0.00²).

Ultrahigh strength gradient systems deliver higher SNR for MCSE and more robust imaging in diastole. While further work is required to further improve the reliability in diastole, at ultrahigh gradient strengths, cDTI using MCSE can identify dynamic changes in the cardiac microstructure. These findings will lead to more widespread use of multiphase MCSE in cDTI clinical research.