Quantitative comparison of myocardial fiber structure between mice, rabbit, and sheep using diffusion tensor cardiovascular magnetic resonance

Abstract

Background: Accurate interpretations of cardiac functions require precise structural models of the myocardium, but the latter is not available always and for all species. Although scaling or substitution of myocardial fiber information from alternate species has been used in cardiac functional modeling, the validity of such practice has not been tested.

Methods: Fixed mouse (n = 10), rabbit (n = 6), and sheep (n = 5) hearts underwent diffusion tensor imaging (DTI). The myocardial structures in terms of the left ventricular fiber orientation helix angle index were quantitatively compared between the mouse rabbit and sheep hearts.

Results: The results show that significant fiber structural differences exist between any two of the three species. Specifically, the subepicardial fiber orientation, and the transmural range and linearity of fiber helix angles are significantly different between the mouse and either rabbit or sheep. Additionally, a significant difference was found between the transmural helix angle range between the rabbit and sheep. Across different circumferential regions of the heart, the fiber orientation was not found to be significantly different.

Conclusions: The current study indicates that myocardial structural differences exist between different size hearts. An immediate implication of the present findings for myocardial structural or functional modeling studies is that caution must be exercised when extrapolating myocardial structures from one species to another.

Figure 1

Overview of image processing pipeline for data analysis. Processing pipeline includes (a) realignment of images, (b) determination of fiber helix angle, and (c) sampling of helix angle.

Figure 2

Schematics of the ROIs for myocardial structural analysis. ROIs include (a) mid septal wall, (b) anterior, (c) lateral, and (d) posterior free wall of the left ventricle. The same ROIs are defined for 3 consecutive image slices at the left ventricular equatorial plane, and myocardial structural parameters are sampled transmurally along radial paths separated by 1.5, for a total of 33 trajectories for each circumferential location for each heart.

Figure 3

Visualization of myocardial fiber orientation for representative mouse (a, d), rabbit (b, e) and sheep (c, f) heart specimens. The myocardial fibers from the same short-axis slice are shown either as rendered cylindrical rods (a, b, c) inside semi-opaque volumes of the hearts viewed obliquely from an elevated perspective, or as falsecolor-coded helix angle maps.

Figure 4

Transmural trajectories of the myocardial fiber helix angle. Myocardial fiber helix angles (in degrees) are shown as a function of circumferential ROI from representative mouse, rabbit, and sheep hearts. All individual trajectories are aligned at where the helix angle is zero to better show the slight variability in the ventricular wall thickness and the regional helix angle.