Speckle tracking echocardiography

Speckle tracking echocardiography describes the semi-automated analysis of grey scale (B-mode) echocardiographic studies in which the spatial translocation of derived functional units (speckles) allows quantitative analysis of myocardial function.

Speckle tracking constitutes a subset of deformation imaging, which is utilized in the assessment of left ventricular function in the setting of such pathologies as diastolic dysfunction, cardiomyopathies and ischemic heart disease.

Terminology

Discrete foci where ultrasound waves and myocardium interact are defined and mapped as "speckles" and form the basic functional unit of speckle tracking. The displacement of individual speckles from their original position with the cardiac cycle is recorded as a composite of longitudinal, radial, and circumferential excursion. Spatial translocation of speckles pertaining to a region of myocardium, therefore, represent the motion of that myocardial segment during the cardiac cycle, allowing the calculation of deformation, displacement, and the rate at which the aforementioned proceed. This data is interpreted within the following parameters ⁷:

• strain 
  • fractional change in dimensions of a segment of interest, calculated with the strain equation and reported as a percentage
  • strain (ε) = (final length - initial length) / (initial length)
  • the product of strain over time (in seconds) yields a strain rate
  • this deformation is recorded in three cardinal directions
    • basal-apical deformation (longitudinal strain)
    • central deformation, toward the left ventricular cavity (radial strain)
    • peripheral deformation around the short axis of the LV (circumferential strain)
• torsion and twisting
  • quantification of the relative regional deformation of the left ventricle during systole
    • the clockwise rotation of the base opposes the counterclockwise rotation of the apex during systole
    • relative apico-basal deformation defines "twisting" and is reported in degrees
      • adjustment based on apico-basal distance defines "torsion"
    • subsequent relaxation results in the "untwisting" of the left ventricle
      • contributes to generation of LA-LV pressure gradient during early diastolic filling

Radiographic features

Echocardiography

The addition of speckle tracking to transthoracic echocardiography allows quantitative of the following parameters:

• left ventricular function ⁸
  • the left ventricular global longitudinal strain (GLS) is the most clinically validated surrogate for left ventricular systolic function
    • normal values of GLS exceed -17%
    • severely depressed LV systolic function correlates with GLS values less than -15%
      • corresponding ejection fraction may be roughly estimated as (3 x |GLS|) + 8
  • global circumferential strain < -20.9% also consistent with decreased systolic function
  • global radial strain values typically range between 35 - 59% ⁷

Differential diagnosis

• hypertrophic cardiomyopathy (HCM)
  • supra-normal radial and circumferential strain values, with a reduction in longitudinal strain and strain rate
• dilated cardiomyopathy (DCM)
  • global reduction in circumferential, radial and longitudinal left ventricular peak strain values
  • reduction in peak left ventricular twist
    • normal LV twist exceeds 2.2 degrees in males and 1.9 degrees in females