Estimates of Primary, Secondary and Tertiary Diffusion-Tensor Eigenvectors in a Short-Axis Section of Rabbit Myocardium

The three-dimensional structure of the diffusion-tensor eigenvectors is explored further in the figure below, which show primary (A), secondary (B) and tertiary (C) eigenvectors in a single equatorial short-axis section. In each panel, line segments are used to display the three-dimensional orientation of each eigenvector. Orientation of the segment with respect to the plane of section is also represented using the indicated color code. Red represents vectors whose percent out-of-plane component approaches unity, whereas blue represents vectors with zero out-of-plane component (i.e., vectors that lie in the plane of section). Primary eigenvectors (A) display a clear transmural rotation. That is, these vectors exhibit a significant vertical component on the epicardial and endocardial surfaces denoted by the yellow and red colors, and rotate to become horizontal (blue) in the mid-wall.

Secondary eigenvectors within the LV and septal myocardium (B) exhibit a transmural orientation, and lie largely within the horizontal plane. This can be seen by the blue and green color coding of many of the segments. Their orientation is more complex within the RV myocardium, with vertical vectors being more common. 

A transmural gradient is also seen for the tertiary eigenvectors (C), especially in the septal and LV myocardium. Since this eigenvector is by definition orthogonal to the plane formed by primary and secondary eigenvectors at each point, tertiary eigenvectors are oriented vertically (red) where the primary and secondary are horizontal (blue). Further, as the secondary eigenvector has a predominant radial orientation, tertiary eigenvectors tend to lie within the circumferential-longitudinal plane. The structure in the RV myocardium is again more complex, with some of the tertiary eigenvectors taking on a radial organization (see the green vectors on the RV free wall endocardium).