Strain rate imaging is a method in echocardiography for measuring regional or global deformation of the myocardium.
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.
There is regional asynchrony, as in bundle branch block.
Strain rate imaging can measure different regions simultaneously.
Strain rate imaging uses Doppler or echocardiography processes.
Deformation imaging, indicates that a passive segment in the myocardium following an infarct, may move due to the action of an adjacent segment.
The displacement or velocity of a segment does not tell about the function of that segment.
A deformation imaging, 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.
Strain rate is the rate of deformation.
The heart has three principal strain components as longitudinal in the long axis of the ventricles, circumferential in the direction of the ventricular circumference, and transmural the deformation is across the wall.
As the ventricle contracts in systole, there is longitudinal shortening with negative strain, circumferential shortening with negative strain and transmural wall thickening a positive strain.
The longitudinal strain contains the main information.
Transmural strain or wall thickening, is a function of wall shortening, wall thickness and chamber diameter, while circumferential shortening is mainly a function of wall thickening.
Longitudinal strain rate and wall thickening are diagnostically equivalent.
Strain rate imaging can be done by two principally different methods:
Tissue Doppler echocardiography
Speckle tracking echocardiography
The Tissue Doppler method is based on the color Doppler, giving a velocity field with velocity vectors, and measures the velocity gradient between two points along the ultrasound beam with a set distance.
The Tissue Doppler method is limited to one direction; along the ultrasound beam, can thus mainly be used from the apical window, and for longitudinal strain and strain rate measurements only.
Speckle tracking echocardiography is based on grey scale echocardiography (B-mode), and the fact that the reflected echo from the myocardium shows a speckle pattern that is a mixture of small scatters and interference patterns.
Strain curves shows the gradual decrease in length during systole, and then the gradual lengthening during diastole, but strain rate fremans negative during the whole heart cycle, as the ventricular length is shorter than at end systole.
Strain rate is the rate of deformation, and is negative during systole, when the ventricle shortens.
Strain rate, becomes positive when the ventricle lengthens.
Displaying curves of the strain and strain rate, are typically done during one heart cycle.
Each curve represents the deformation in one region of the myocardium, and acquisition of a full sector allows display of multiple curves simultaneously in the same image for comparison.
Strain rate imaging is part of an integrated echocardiographic examination.
Deformation measurements have limited accuracy, and should be considered together with the rest of findings.
Deformation imaging in myocardial infarction, shows a limited region of the heart muscle that has reduced or totally absent function.
Deformation imaging is at least as accurate as B-mode echocardiography in analyzing myocardial damage with myocardial infarction.
Deformation imaging is useful in following recovery of an infarcted myocardial area, to ascertain the amount of myocardial stunning vs. necrosis.
In Cardiac stress testing, the regional dysfunction due to ischemia will become evident when the myocardial oxygen demand surpasses the Coronary flow reserve of a stenosed coronary artery.
Strain rate imaging during stress gives incremental value over ordinary echocardiography in diagnostic and prognostic information.
In Left bundle branch block (LBBB), the asynchronous activation of the left ventricle gives asynchronous contraction that can be visualized by echocardiography.
Strain rate imaging can demonstrate the distribution of the asynchrony, and the demonstration of the amount of inefficient work done by the asynchronous ventricle.
Speckle tracking has an advantage over Ejection fraction (EF), it shows reduced cardiac function also in hypertrophic hearts with small ventricles and normal ejection fraction, which is often seen in hypertensive heart disease, hypertrophic cardiomyopathy and aortic stenosis.
The Tissue Doppler method is based on the colour Doppler, giving a velocity field with velocity vectors along the ultrasound beam measuring the velocity gradient between two points along the ultrasound beam with a set distance.
Strain rate imaging is just a part of an integrated echocardiographic examination, it has limited accuracy, and should be considered together with the rest of findings.
In myocardial infarction, a limited region of the heart muscle has reduced or totally absent function, and cardiac strain studies are at least as accurate as B-mode echocardiography.
Deformation imaging following recovery of an infarcted myocardial area, helps ascertain the amount of myocardial stunning vs. necrosis.
Left bundle branch block (LBBB), the asynchronous activation of the left ventricle gives asynchronous contraction.
Strain rate imaging demonstrates the distribution of the asynchrony, and the demonstration of the amount of inefficient work done by the asynchronous ventricle.