Strain rate imaging

Strain rate imaging is a method in echocardiography (medical ultrasound) for measuring regional or global deformation of the myocardium (heart muscle). The term 'deformation' refers to the myocardium changing shape and dimensions during the cardiac cycle. If there is myocardial ischemia, or there has been a myocardial infarction, in part of the heart muscle, this part is weakened and shows reduced and altered systolic function. Also in regional asynchrony, as in bundle branch block, there is regional heterogeneity of systolic function. By strain rate imaging, the simultaneous function of different regions can be displayed and measured. The method was first based on colour tissue Doppler. by using the longitudinal myocardial velocity gradient, already in use transmurally. Later, the regional deformation has also been available by speckle tracking echocardiography, both methods having some, but different methodological weaknesses. Both methods, however, will acquire the same data (measurements may differ somewhat, however, being method dependent), and also can be displayed by the same type of display. Strain rate imaging is a method in echocardiography (medical ultrasound) for measuring regional or global deformation of the myocardium (heart muscle). The term 'deformation' refers to the myocardium changing shape and dimensions during the cardiac cycle. If there is myocardial ischemia, or there has been a myocardial infarction, in part of the heart muscle, this part is weakened and shows reduced and altered systolic function. Also in regional asynchrony, as in bundle branch block, there is regional heterogeneity of systolic function. By strain rate imaging, the simultaneous function of different regions can be displayed and measured. The method was first based on colour tissue Doppler. by using the longitudinal myocardial velocity gradient, already in use transmurally. Later, the regional deformation has also been available by speckle tracking echocardiography, both methods having some, but different methodological weaknesses. Both methods, however, will acquire the same data (measurements may differ somewhat, however, being method dependent), and also can be displayed by the same type of display. The point of deformation imaging, is that a passive segment in the myocardium for instance after an infarct, may move due to the action of an adjacent segment (tethering). Thus the displacement or velocity of a segment do not tell about the function of that segment. Deformation imaging, on the other hand, measures the differences' of motion and velocity within the segment, which is equivalent to the deformation. Strain means Deformation, and is defined as relative change in length. The Lagrangian formula εL = (L-L0)/L0 = ΔL/L0, where L0 is baseline length and L is the resulting length, defines strain in relation to the original length as a dimensionless measure, where shortening will be negative, and lengthening will be positive. It is usually expressed in percent. An alternative definition, Eulerian strain defines the strain in relation to the instantaneous length: εE = ΔL/L. For a change over time, the Lagrangian strain will be: εL = Σ ΔL/L0, and Eulerian Strain εE = Σ (ΔL/L). The term was first used by Mirsky and Parmley in describing regional differences in deformation between normal and ischemic myocardium Strain rate is the rate of deformation. In ultrasound it is usually measured from the velocity gradient SR = (v2 - v1)/L where v2and v1 are the myocardial velocities at two different points, and L is the instantaneous distance between them. This is thus equivalent to the velocity difference per length unit (the spatial derivative of velocity) and has the unit s−1. Strain is then integrated from strain rate. This method, however, yields the Eulerian strain rate and strain. It has become traditional to use the Velocity gradient, but in integrating strain rate it is converted to Lagrangian strain by the formula εL = eεE - 1. Strain in three dimensions: Basically, any object or body is three dimensional, and can be deformed in different directions simultaneously. Strain can be described as a tensor with three principal strains (εx, εy and εz in a Cartesian coordinate system), and six shear strains components. In the heart, it has been customary to describe the three principal strain components as longitudinal (in the direction of the long axis of the ventricles), circumferential (in the direction of the ventricular circumference), and transmural (the deformation across the wall. Transmural deformation has also been called 'radial', but this is unfortunate as in ultrasound in general the term radial describes 'in the direction of the ultrasound beam'). However, as the heart muscle is incompressible, the three principal strain must balance; ((εx+1)(εy+1)(εz+1) = 1). As the ventricle contracts in systole, there is longitudinal shortening (negative strain), circumferential shortening (negative strain) and transmural (wall) thickening (positive strain). Due to this, and the fact that the left ventricle in normal conditions contract with a relatively invariant outer contour, the longitudinal strain contains the main information, while transmural strain (wall thickening) is a function of wall shortening, wall thickness and chamber diameter, while circumferential shortening is mainly a function of wall thickening. It has been shown clinically that longitudinal strain rate and wall thickening are diagnostically equivalent.