Diffusion MRI tractography of the human heart In Vivo at end-diastole and end-systole
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Diffusion MRI tractography of the human heart In Vivo at end-diastole and end-systole Choukri Mekkaoui1,2*, Sonia Nielles-Vallespin3, Peter D Gatehouse3, Marcel P Jackowski4, David N Firmin3, David E Sosnovik1,5 From 15th Annual SCMR Scientific Sessions Orlando, FL, USA. 2-5 February 2012 Summary Diffusion Tensor MRI (DTI) of the human heart in vivo has to date been performed in 2D and at a single phase of the cardiac cycle. Here we perform 3D tractography of the human heart in vivo at both end diastole and end systole. We show that fiber orientation in the subepicardium becomes more oblique during systole, and that scalar indices of diffusion (mean diffusivity and fractional anisotropy) decrease during systole. Our data suggest that myocardial fiber architecture is dynamic and is a function of both chamber geometry and myocardial contraction. Background Diffusion Tensor Imaging (DTI) of the human heart in vivo has been described [1,2] but has only been performed in 2D and at a single phase of the cardiac cycle. The impact of myocardial contraction on 3D fiber architecture in vivo thus remains poorly defined. Here we use a recently developed diffusion-weighted stimulated echo single shot EPI sequence [3], to address this question with a 3D tractographic approach [4]. The purpose of this study was thus to perform in vivo DTI of normal human hearts at end-diastole and end-systole and quantify changes in myofiber organization as the myocardium contracts and relaxes. Methods DTI of three normal volunteers was performed on a 3T scanner (Skyra, Siemens) with the following parameters: 6 diffusion-encoding directions, b=350s/mm 2 , TR/ TE=1100/23ms, BW=2442Hz/pixel, spatial resolution=2.7x2.7x8mm 3 , 3 slices, 6-8 averages, multiple 1 Harvard Medical School-MGH-Athinoula A. Martinos Center for Biomedical Imaging, Charlestown, MA, USA Full list of author information is available at the end of the article
breathholds. Fiber tracts were constructed by integrating the primary eigenvector field from the dyadic tensor into streamlines using a 4th order Runge-Kutta approach. Myofiber tracts were color-coded by their median helix angle [4]. Mean diffusivity (MD), fractional anisotropy (FA) and individual eigenvalue maps were averaged in 12 sectors of the anterior, lateral, inferior and septal walls of the left ventricle (LV).
Results Tractography showed that myofibers in the subepicardium of the LV assumed a more oblique orientation at end-systole versus end-diastole (Figure 1). MD and FA were significantly (p
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