Speckle-Tracking and Strain Measurements: Principles, Techniques, and Limitations

General Concepts

The word “strain,” which in everyday language means “stretching,” in echocardiography indicates a measure of tissue deformation; the “strain rate” is the rate at which the deformation occurs. Considering a given one-dimensional object under either lengthening or shortening deformation, the initial length could be indicated as L ₀ and its length at a given time as L ( t ). The normalized deformation, strain ε, can be mathematically represented by the following equation:

This is called Lagrangian strain, which occurs when the initial length is known. However, whenever the original length is unknown, strain can be assessed considering its small temporal variations d ε _N (t) during an infinitesimal time increment dt , as mathematically translated by the equation:

This is the natural strain and represents variations during the total process of shortening or lengthening. Regarding small changes, Lagrangian and natural strain share almost the same values. Nevertheless, considering the large cardiac deformations that occur during systole and diastole, natural strain seems to be more appropriate to use.

Left ventricular (LV) muscular wall has been commonly considered to be shaped by three different layers, called the endocardium, myocardium, and epicardium. In the past years, several studies have explored the three-dimensional (3D) deformation of the ventricular tissue, describing myocyte arrangements as a continuum of two helical fiber geometries and not as three separated layers. These studies, performed using cardiac magnetic resonance imaging (MRI), demonstrated that the subendocardial region shows a right-handed helical myofiber geometry, which changes gradually into a left-handed helical geometry in the subepicardium. This position of fibers allows a better pump function of the ventricle. Sallin in 1969 reported that the degree of shortening of the left ventricle would be 15% if fibers were disposed only longitudinally, 30% with circumferential fibers, and 60% or more with fibers oriented in an oblique spiral direction, as really occurs. This myocardial structure broadly determines the components of myocardial deformation. We can also distinguish longitudinal, radial, and circumferential strains. Longitudinal and circumferential strains are shown as negative curves because they are the expression of myocardial shortening, whereas the radial strain produces a positive curve because it is the representation of myocardial lengthening. In addition, the helical nature of the heart muscle determines a wringing motion during the cardiac cycle, with counterclockwise rotation of the apex and clockwise rotation of the base around the LV long axis, when observed from the apical perspective. This deformation is called twisting . The subsequent recoil of twisting is defined untwisting and is associated with the release of restoring forces that contribute to diastolic suction and facilitate early LV filling. In general, longitudinal LV strain is the most sensitive to the presence of myocardial disease, and it explains why longitudinal strain is considered the most sensitive parameter for the assessment of subtle systolic dysfunction and why it can already be reduced in the early phases of cardiac diseases when LV ejection fraction (EF), the most robust parameter for the evaluation of systolic dysfunction, is still normal. Indeed, when endocardial function is impaired but midmyocardial and epicardial function are unaffected, it may result in normal or nearly normal circumferential and twist mechanics with relatively preserved LV pump function and EF. On the other hand, an acute transmural insult or progression of disease results in simultaneous midmyocardial and subepicardial dysfunction, leading to a reduction in LV circumferential and twist mechanics and a reduction in EF.