Motion-compensated spin-echo cardiac diffusion tensor imaging in multiple cardiac phases using an ultrahigh gradient strength scanner

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 3 T 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 2.8×2.8x8mm 3 , b=500 s/mm 2 (8 averages) and b=150 s/mm 2 (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 -3 mm 2 /s, p=0.00 2 ). 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.