Speckle tracking echocardiography

In the fields of cardiology and medical imaging, Speckle Tracking Echocardiography (STE) is an echocardiographic imaging technique that analyzes the motion of tissues in the heart by using ultrasonic sound waves to generate interference patterns and natural acoustic reflections.^[1] These reflections, also described as ‘‘speckles’’, ‘‘markers’’, ‘‘patterns’’, ‘‘features’’, or ‘‘fingerprints’’ are tracked consecutively frame to frame and ultimately resolved into angle-independent two-dimensional (2D) and three-dimensional strain-based sequences (3D) ^[2][3][4] These sequences provide both quantitative and qualitative information regarding tissue deformation and motion.

Strain

Strain is defined as the fractional or percentage change in an objects dimension in comparison to the object’s original dimension.^[5] Similarly, strain rate can be defined as the speed at which deformation occurs. Mathematically, three components of normal strain (εx, εy, and εz) and three components of shear strain (εxy, εxz, and εyz) are recognized. Congruently, when applied to the left ventricle, left ventricular deformation is defined by the three normal strains (longitudinal, circumferential, and radial) and three shear strains (circumferential-longitudinal, circumferential-radial, and longitudinal-radial). The principal benefit of LV shear strains is amplification of the 15% shortening of myocytes into 40% radial LV wall thickening, which ultimately translates into a >60% change in LV ejection fraction. Left ventricular shearing increases towards the subendocardium, resulting in a subepicardial to subendocardial thickening strain gradient. Similar to MRI, STE utilizes “Lagrangian strain” which defines motion around a particular point in tissue as it revolves through time and space.^[6] Throughout the cardiac cycle, the end-diastolic tissue dimension represents the unstressed initial material length.

Twist or torsional deformation define the base-to-apex gradient and is the result of myocardial shearing in the circumferential-longitudinal planes such that, when viewed from the apex, the base rotates in a counterclockwise direction. Likewise the LV apex concomitantly rotates in a clockwise direction. During ejection, LV torsion results in the storage of potential energy into the deformed myofibers. This stored energy is released with the onset of relaxation similar to a spring uncoiling and results in suction forces. These forces are then used for rapid early diastolic restoration.

Applications and Limitations

The utilities of STE are increasingly recognized. Strain results derived from STE have been validated using sonomicrometry and tagged MRI and results correlate significantly with tissue Doppler–derived measurements.^[7][8][9] Tissue Doppler technology, the predecessor of speckle tracking technology, requires achieveing parallel orientation between the direction of motion and the ultrasound beam. Its use has remained limited due to angle dependency, substantial intraobserver and interobserver variability and noise interference. Speckle tracking technology has overcome these limitations. Studies have proven greater both longitudinal and radial strain measurements with STE to have significantly greater area under the curve than DTI on receiver operating characteristic curve analysis in differentiating dysfunctional from normal segments.^[10]

References

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