Transmural gradients of preferential diffusion motility in the normal rat myocardium characterized by diffusion tensor imaging
Journal of Cardiovascular Magnetic Resonance volume 17, Article number: Q117 (2015)
In recent years, diffusion tensor imaging (DTI) has emerged as a powerful noninvasive tool that infers the cardiac tissue organization through measuring water molecule motility. Previous DTI studies  on normal hearts largely assumed that the measured diffusion magnitude along the 3 preferred directions and the measured degree of diffusion anisotropy remain the same over the endo-, meso-, and epi-cardial layers of the left ventricular (LV) wall. Our goal is to test the validity of this assumption.
DTI of 6 excised Wistar Kyoto rat hearts was performed on a 7T Bruker BioSpec using a 3D spin echo sequence with 12 optimized  diffusion directions at b=1000smm-2 (+b0) with 160μm isotropic resolution.
Tensor data was fitted  and diagonalized on a pixel-by-pixel basis. Maps of fractional anisotropy (FA), and longitudinal (λL), transverse (λT) and mean (MD) diffusivities were computed. The equatorial short-axis slice with the largest area was analysed. 3 transmural sectors were defined evenly spanning the LV wall (Fig 1a). Zonal averages fοr all parameters were computed. To test the statistical significance of the differences among the zonal averages, one-way ANOVA was employed.
Distinct transmural gradients were observed (Fig 1b) in all measurements. Even though the exact mechanisms are still not perfectly understood, the measured extracellular diffusivity properties are, intuitively, sensitive to local cellular characteristics. Since the primary eigenvector is aligned to the cell long-axis, the revealed increase trend (p<0.05) of λL from the endo- to the epi-cardium is consistent with previous studies  on isolated LV myocytes that showed a similar trend in the cell length. The rises in measured extracellular λΤ (p<0.05) and MD (p<0.05) from the inner to the outer wall layers may be explained by the documented  reverse trend in the inversely proportional cell cross-sectional area. On similar grounds, the decrease in FA (p<0.05) from endo- to epi-cardium is in agreement with the manifested5 reverse trend in the cell length/width ratio.
Significant transmural heterogeneity of preferential diffusion properties was observed in the normal rat myocardium. The results corroborate previous labor-intensive cellular morphometry studies on the same animal model. They may be used to improve our understanding of the heart functioning through dissecting critical underlying mechanisms.
NIHR Cardiovascular BRU, Royal Brompton & Harefield NHS Trust and Imperial College London. NIH:R01 EB007219, the Director, SC, BER, Medical Sciences Division, U.S. DOE under Contract DE-AC02-05CH11231.
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Giannakidis, A., Ferreira, P., Gullberg, G.T. et al. Transmural gradients of preferential diffusion motility in the normal rat myocardium characterized by diffusion tensor imaging. J Cardiovasc Magn Reson 17 (Suppl 1), Q117 (2015). https://doi.org/10.1186/1532-429X-17-S1-Q117