Commentary: Peeking under the surface with multilayer strain in cardiac MRI

The structure of the myocardium is as elegant as it is efficient. It is composed of three layers: the endocardium, mid-myocardium, and epicardium, each of which has distinct orientation of their fibers. The fibers of the myocardium are oriented in a leftward helix in the subepicardium and this shifts to a rightward helix in the subendocardium, with the fibers of the mid-myocardium serving as a transition point. As a result, the rotations of the subepicardium and subendocardium are in opposite directions and additionally rotate in opposite directions at the base versus the apex. Therefore, the subepicardial myofiber contraction rotates the base in the clockwise direction and the apex in the counterclockwise direction. The subendocardial fibers have the opposite effect. This results in storage of potential energy at end-systole which aids in undoing this twisting and creating diastolic suction. The net effect is that myofiber shortening of approximately 20% results in an ejection fraction of approximately 60%. …