There are few studies of right ventricular (RV) myofiber architecture and no established convention for describing and reporting RV myofiber anatomy.¹ This study aims to define the detailed myofiber pattern of the right ventricle using novel MR diffusion compartment imaging protocols and analysis methods, and to propose a convention for describing the myofiber anatomy.

A juvenile swine heart was fixed (5% formalin and 50% ethanol) under mild distending pressure (~10cm H2O), immersed in Galden®, and imaged using a 3T Siemens Skyra. Diffusion-weighted MRI was acquired with a pulsed gradient spin echo sequence, single shot EPI readout and the following parameters: FOV=96x96mm², matrix=96x96, 84 slices, 1x1x1mm³ spatial resolution, TE=109ms, TR=19s. Our gradient scheme contained 4 b=0s/mm² images and 3 shells (b=1000, 1500, 2000s/mm²) of 30 gradients each. Eight averages were acquired with both LR and RL phase encoding direction. The total acquisition time was lower than 8h.

DW images were compensated for distortion using the RL/LR images and Topup.² Diffusion compartment imaging (DCI)³ was computed by considering one isotropic free diffusive component and one anisotropic compartment per voxel. The diffusion data displayed in glyph format were used for qualitative description of myofibers. Helix angle was measured using the short axis of the RV as a reference plane (Figure 1). The compartment mean diffusivity (cMD) and fractional anisotropy (cFA) of the anisotropic compartment were measured and displayed as 2D sectional maps.

Blocks of ventricular myocardium were removed after imaging, sectioned parallel to the epicardial surface, photographed, and stored digitally for estimation of fiber angle. A CT scan of the heart was put in registration with the diffusion images allowing precise correlation between diffusion-derived fiber maps and histological sections.

Myofiber arrangement – The RV free wall and septal wall, both convex anteriorly, form a crescent in cross section (Figure 2). At the apex the RV cones down to a small whorl and at the base it is divided by the parietal band (PB) into inflow and outflow. A single layer of free wall myofibers run circumferentially from the inferior to the superior junction with the septal wall (Figure 1, 2). A second oblique longitudinal layer is added to the endocardial side of the free wall from the moderator band to the base (Figure 2). This layer is composed of trabeculae and papillary muscles. Septal wall endocardial fibers run obliquely longitudinal while deeper fibers are circumferential (Figure 1,2). Myofibers within papillary muscles and other trabeculations run approximately parallel to the long axis of the muscle bundle and smoothly join endocardial fibers where trabeculations join the RV wall (Figure 3). Exceptionally, fibers in the parietal and moderator bands run obliquely (~45ᵒ to long axis) and moderator band fibers intersect those of the septal band at nearly right angles (Figure 2C).

RV reference system – We have used the short-axis plane of the RV as the reference for estimating the helix angle (Figure 1). Fibers in the short-axis plane are assigned a helix angle of 0ᵒ, those directed from inferior to superior toward the base (viewed from outside the RV free wall) a negative angle, and those directed from inferior to superior toward the apex a positive angle.

Diffusion characteristics – cMD is higher in the RV myocardium compared with the left ventricle (Figure 4) consistent with the higher percentage of fibrosis in the RV.

Histology – Fiber orientation by DCI closely matched histology (Figure 5).

The RV walls are composed of an external circumferential layer (considering the mid septal wall as an external layer) and an internal oblique longitudinal layer for the most part. The portion of the RV free wall between the moderator band and the apex has only the circumferential layer. The longitudinal fibers are mostly contained in trabeculations, especially on the free wall, and run parallel to the trabeculations. This is consistent with the contraction pattern of apex-to-base shortening produced by the longitudinal fibers coupled with a bellows effect of the free wall against the septal wall produced by shortening of the circumferential fibers.

Muscle bands that cross the cavity from free wall to septum (moderator and parietal bands) have obliquely running fibers. This fiber orientation plus the right-angle junction of the parietal and septal bands likely reflect their embryological development⁴.

Previous reference frames for fiber orientation have been based solely on the left ventricle. We have proposed a separate reference frame for the RV because it is often the only ventricle present in congenital heart defects.