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.