Improved Outcomes Requires Better Detection: Insights into Myocardial Structure and Measuring Ventricular Function by Strain Imaging
Cardiac contraction is more complex than can be assessed by LVEF. The cardiac structure has been an area of interest since the beginning of the sixteenth century. It was not until 1942 that Robb et al. dissected and detailed the macroscopic anatomy of the heart and confirmed that the ventricles were made of discrete myocardial bands83. These findings were confirmed by multiple investigators who further detailed the direction of the myocardial fiber bands using various methods including the use of diffusion tensor imaging84, 85. The right and left ventricular myocardium has been shown to formed from a continues fiber sheet with the predominant oblique (circumferential) fibers in the bulk of the left ventricular myocardium and longitudinal fiber orientation found only in the subendocardium and subepicardium74, 83, 84. Multiple studies have shown that the myocardial fiber structure and orientation dictates its mechanical property including principal strain direction 86-89.
Myocardial strain (ε) by CMR provides direct quantitative assessment of regional and global myocardial contractility beyond what LVEF can provide and has been shown to be a sensitive marker of cardiac function in both acquired and congenital heart disease32, 53, 55, 90-96. The concept of strain is deformation of an object normalized to its original shape or the fractional change in the length of a small segment of myocardium and is fundamentally important in the assessment of regional ventricular function. Knowledge of the strain tensor at a specific point in space allows precise determination of the fractional change in length of an infinitesimal line segment oriented in any direction. In practice, myocardial strain can be measured along the three principal axes of cardiac contraction – longitudinal, radial, and circumferential77, 91, 97. For characterization of regional cardiac systolic contractility, we typically consider the normal strains (wall thickening (radial strain εrr), longitudinal compression/shortening (longitudinal strain εll) and circumferential shortening (ringing of the heart tangentially εcc) as the major axes of contraction in the left ventricle (LV) (Fig 5). Of these, the circumferential strain is the major component of systolic contractility in the left ventricle and has been used by several investigators as the primary strain direction32, 88, 89, 94, 98-101.
CMR myocardial tagging, described > 20 years ago, allowed intramyocardial, subendocardial, and subepicardial strain measurements in all three principal directions (circumferential, radial, and longitudinal) (Fig. 1g-h and Fig. 5)77, 91, 102-105. Using complex analytic techniques, it has been shown that two-dimensional strain analysis is more accurate in describing regional function than wall thickening analysis (with a sensitivity of 92% and 69% and specificity of 99% and 92%, respectively) in discriminating dysfunctional myocardium from remote functional myocardium106. Despite its accuracy, CMR-tagging techniques have not been routinely used in clinical practice due to the perceived time-consuming and labor-intensive post-processing required. Recently, post-processing software such as harmonic phase (HARP) technique work by filtering of the harmonic peak of the Fourier transform of the tagged image has the made analysis easier. The entire processing time required for obtaining strain data on any region of myocardium is < 10 minutes. Analysis of myocardial strains from tagged CMR images based on this technique has been demonstrated to be fast and accurate54, 102, 52, 73.