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.

By strain rate imaging, the simultaneous function of different regions can be displayed and measured.

[2] Later, the regional deformation has also been available by speckle tracking echocardiography,[3][4] both methods having some, but different methodological weaknesses.

[6] Strain in three dimensions: Basically, any object or body is three dimensional, and can be deformed in different directions simultaneously.

It has been shown clinically that longitudinal strain rate and wall thickening are diagnostically equivalent.

[6] This method has been validated experimentally in a mechanical model,[11] in an animal model,[12] and in patients against echocardiography,[13] coronary angiography [10] and MR [14][15] The 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.

[citation needed] 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.

Also, instead of angle independency, the resulting strain values are dependent on the ROI (Region Of Interest) size and shape.

Finally, in order to achieve tracking quality, the values are in most commercial applications smoothed by a spline smoothing function along the ROI, so the regional measurements are not pure regional, but rather to a degree, spline functions of the global average.

[citation needed] Both methods measures the same physiological phenomena (deformation), and results can in principle be displayed the same way.

The strain rate values are reduced to semi quantitative visual display, but this mode allows measurement of timing, as well as depth, and is best suited to space-time relation measurement[citation needed] It is a major point that strain rate imaging is just a part of an integrated echocardiographic examination.

However, the methods offer unique ways of imaging regional dysfunction, that may strengthen the conclusion.

[23] In myocardial infarction, a limited region of the heart muscle has reduced or totally absent function.

Strain rate imaging during stress has been shown to give incremental value over ordinary echocardiography, both diagnostic[28][29] and prognostic.

Disappointingly, large scale studies have not been able to establish additional echo criteria for selection of Heart failure patients with LBBB who may respond to Cardiac resynchronization therapy,[32] although smaller studies are promising[33] In later years, Global strain by speckle tracking has achieved popularity as the global functional measure.

It has an advantage over Ejection fraction (EF), it shows reduced cardiac function also in hypertrophic hearts with small ventricles and normal ejection fraction (HFNEF), which is often seen in Hypertensive heart disease, Hypertrophic cardiomyopathy and Aortic stenosis.

Colour-coded left atrial strain assessed by speckle tracking during a representative cardiac cycle.
Systemic sclerosis (SSc) patient: 2D longitudinal strain of the right ventricle. The values of the basal, mid and apical segment of the free lateral wall were taken for further analysis.
Longitudinal strain rate and strain: multiple simultaneous traces from three different regions in the septum. Left: strain rate, Right: Strain. As thelongitudinal systolic deformation is shortening, the systolic strain and strain rate is negative. The 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, however, becomes positive when the ventricle lengthens . Thus the more rapid phase shifts show details of the lengthening, displaying that it is not homogeneous.
Strain rate colour curved anatomical M-mode . The yellow line is drawn from the base of the septum, through the apex and down the lateral wall as can be seen on the small images on the left. The rectangular area then represents the colour M-mode, where the vertical axis is distance along the line, while the horizontal axis is time. The image represents one heart cycle. Strain rate values are yellow to red for negative strain rate (shortening) and cyan to blue for positive strain rate (lengthening). Green represents area and time of no deformation.