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Principles and Practical Aspects of Strain Echocardiography


Introduction

Assessment of regional and global ventricular function has long relied on visual assessment. However, this approach is subjective and variable leading to significant interobserver variability in interpretation. The heart is a mechanical organ and undergoes cyclic deformation in systole and diastole. This cyclic deformation can be measured and for decades was restricted to those undergoing open-heart surgery when metal beads were sown onto particular locations on the left ventricle (LV); deformation was then assessed via fluoroscopy. Approximately 20 years ago, magnetic resonance methods were introduced that allowed noninvasive assessment of deformation. Later, tissue Doppler-based methodology was used to track tissue motion by echocardiography. Further refinement of these techniques enabled echo-based assessment of regional deformation via determination of strain.

Principles of Strain Imaging

Strain is a measure of tissue deformation. Strain is defined as a change in length of an object relative to its original length (i.e., reduction to half its original length is 50% strain; Fig. 6.1 ). Strain rate (SR) is the rate at which this deformation (length change) occurs. Although myocardial deformation is a three-dimensional phenomenon, echo-based interrogation techniques have generally been limited to interrogating one or more of three imaging planes—longitudinal, circumferential, and radial ( Fig. 6.2 ). More recently, 3D strain has been introduced.

FIG. 6.1, Strain is a dimensionless index defined as change in length normalized to the original length. This reduction in length of the myocardial segment by 20% would indicate a strain of −20%. Conversely, a lengthening of a myocardial segment by 20% would yield a strain of +20%. Strain rate is the rate at which these length changes occur.

FIG. 6.2, Myocardial deformation is a three-dimensional phenomenon. However, echocardiographic interrogation of strain occurs along three primary directions—longitudinal (apex to base), circumferential (along the short axis curvature), and radial (endocardial to epicardial). There is systolic shortening and diastolic lengthening in the longitudinal and circumferential directions. There is systolic thickening and diastolic thinning in the radial direction.

Strain and SR allow a clinician to determine regional and global myocardial function at the same level as a muscle physiologist by providing parameters similar to shortening fraction and shortening velocity, respectively.

Echo-based techniques measure deformation by two primary methods—tissue Doppler and speckle-based. In tissue Doppler imaging (TDI), SR is the difference in velocity between two points along the myocardial wall (velocity gradient) normalized to the distance between the two points ( Fig. 6.3 ). A similar velocity gradient exists between the endocardium and the epicardium, because the endocardium moves faster. This concept is used to derive myocardial velocity gradient (radial SR). This velocity gradient depicts the rate of change of myocardial wall thickness during systole and diastole. Thus SR measures the rate at which the two points of interest move toward or away from each other. Integration of SR yields strain, the normalized change in length between these two points ( Fig. 6.4 ).

FIG. 6.3
Tissue velocity–based strain imaging measures tissue velocities along to locations in the long axis direction. The difference in peak velocities normalized to the distance between them yields myocardial velocity gradient or strain rate.

FIG. 6.4
In tissue velocity-based strain imaging, a region of interest is placed in a particular location on the myocardium. This measures strain rate at that location. Integration of strain rate deals strain.

In speckle-tracking methodology, the system tracks unique acoustic patterns within the myocardium termed speckles . These speckles can be tracked over time, and speckle displacement can be used to calculate tissue velocity and strain ( Fig. 6.5 ). This method is relatively angle-independent, because it is not based on the Doppler principle. Because speckle tracking can be automated, this technique lends itself to semiautomated measurements of strain. One such method allows the generation of bull’s-eye plots of longitudinal segmental strain ( Fig. 6.6 and ). Another similar technique uses arrows to display the direction and amplitude of motion at various points in the heart (velocity vector imaging). Speckle tracking imaging can use preexisting B-mode images; however, it is performed at much lower frame rates (40–90 frames per second) and may not be as accurate in timing mechanical events as Doppler-based imaging (100–250 frames per second).

FIG. 6.5
Two-dimensional strain imaging uses speckle tracking methodology. A speckle is a particular acoustic pattern that can be computationally identified within the myocardium. For strain estimation, a speckle is identified at end diastole (yellow box) and tracked until end systole (blue box) . The distance traveled by the speckle is displacement, which is used to calculate strain, the temporal derivative of which in turn yields strain rate.

FIG. 6.6
A representative example of two-dimensional strain output. Segmental strain rate tracings are provided for each apical view: four-chamber (upper left) , two-chamber (upper right) , and apical long (lower left) . Peak strain values are converted into a color code depicted as a bull’s-eye plot (lower right) .

Commonly measured strain parameters include systolic and diastolic SRs, and systolic strain ( Fig. 6.7 ). Peak systolic SR is the parameter that comes closest to measuring local contractile function in clinical cardiology. It is relatively volume independent and is less pressure independent than strain. In contrast, peak systolic strain is volume dependent and does not reflect contractile function as well as SR.

FIG. 6.7
Representative tracings of strain rate (left panel) and strain (right panel) . Commonly measured parameters include peak systolic strain rate (SRs), early and late diastolic strain rates (SRe and SRa, respectively), and peak systolic strain (S).

Twist and Torsion

Myocardium are three-dimensional continuous fibers that change direction from subendocardial right-handed helix to a subepicardial left-handed helix. The fibers arranged in counter-direction generate sliding or shear deformation during contraction. When viewed from apex to base, the apex rotates counterclockwise during systole, while base rotates in clockwise ( Fig. 6.8 ). Twist is the apex-to-base difference in rotation, which is expressed in degrees. Torsion refers to the normalized twist, where the twist angle is divided by the distance between LV base and apex, which is expressed in degrees per centimeter. The systolic twist and diastolic untwist can be influenced by age, change of preload or afterload, diastolic dysfunction, cardiomyopathy, and valvular heart disease.

FIG. 6.8, A, Subepicardial fibers wrap around the left ventricle (LV) in a left-handed helix (yellow arrows) , and subendocardial fibers wrap around the LV in a right-handed helix (green arrows) . B, The outer epicardial layer (red arrows) in LV base rotates in a clockwise direction, whereas the inner endocardium (blue arrows) rotates in an opposite direction. For the apex, the epicardial layer rotates in a counterclockwise direction, and the endocardium rotates in clockwise rotation. The overall LV rotational direction is dominated by the epicardial rotation because the epicardial layer has a larger radius.

Regional and Global Function

Left Ventricle

Circumferential and radial strain values are obtained in standard parasternal short axis view at mitral valve, papillary muscle, and apex level. Longitudinal strain is calculated from apical two-, three-, and four-chamber views, with basal, mid, and apical segments in each of the six walls. Timing of aortic valve closure (AVC) is used to define end-systole in a cardiac cycle. To avoid underestimation, it is important to get a circular LV images when performing circumferential and radial strain analyses, and avoid foreshortened apical chamber views in longitudinal strain analysis. Global longitudinal strain (GLS), calculated as the average from all segments, is commonly used as a measure of global LV function. Fig. 6.9 shows typical segmental speckle-tracking strain in a healthy normal heart. Timing of end systole needs to be defined clearly to identify postsystolic shortening from systolic shortening. Normal GLS value is reported between 18% and 25% in healthy participants. However, there is intervendor variability.

FIG. 6.9, Representative strain tracings from three apical views—four-chamber (upper left) , two-chamber (upper right) , and apical long-axis (lower left) , and the resulting “bull’s-eye” plot (lower right) . In this example, segmental strain values are all normal and represented by shades of red in the bull’s-eye plot. Global longitudinal strain was −20% (normal range).

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