Strain imaging is a cardiac imaging technique that detects ventricular deformation patterns and functional abnormalities before they become obvious as regional wall motion abnormalities on conventional cine imaging or echo. It has become more popular lately due to several technological improvements 1-4.
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Clinical applications
Myocardial strain imaging is a way to quantify cardiac function with different parameters. Global longitudinal strain (GLS) is the best-evaluated strain parameter so far and is considered more sensitive for the assessment of systolic function than left ventricular ejection fraction 1,5.
Strain imaging has potential use in the following clinical conditions 1-4, 6-9:
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detection of systolic dysfunction
in patients with preserved or normal left ventricular ejection fraction
evaluation in suspicion of cardiomyopathy
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longitudinal strain reduced early in the disease, whereas ejection fraction is normal
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differentiation towards hypertrophic cardiomyopathy
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chemotherapy-induced cardiotoxicity
detection of subclinical left ventricular dysfunction
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supplementary tool in unclear cases
myocardial strain is inversely related to the area at risk and infarct size
reduction in peak systolic strain, systolic lengthening and post-systolic shortening
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circumferential strain is 1/3 of that in normal individuals
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potential identification of patients at risk for arrhythmia
potential to guide lead placement for cardiac resynchronization therapy (CRT)
acute transplant rejection
Technique
Physical principles
Myocardial strain describes the deformation of the myocardium from an unwound to a tense or contracted condition and can be expressed by a mathematical principle with the following formula 1-3:
ɛ = (L-L0)/L0
ɛ is strain, L0 is the baseline length and L is the length in systole
In strain imaging left ventricular contraction can be divided into three basic spatial orientations or directions 1-3:
longitudinal strain
radial strain
circumferential strain
Longitudinal strain is the deformation of the left ventricle in the z-direction that is from the base or annular ring to the apex usually resulting in a negative strain since the left ventricular chamber length (L) in systole is shorter than the respective length (L0) at baseline in the normal heart 2,3.
Radial strain refers to the radial contraction or thickening of the left ventricular wall, leading to a positive radial strain value in a normal individual since the diameter of the left ventricular wall L0 increases with contraction in systole 2,3.
Circumferential strain represents the myocardial contraction along the circular outline in the short axis. It is negative in normal individuals since the circumference of the left ventricle in a relaxed state Lo decreases in systole 2,3.
Strain rate is the amount of strain within a certain time.
Left ventricular torsion is measured in degrees and is related to the clockwise twirl from apex to base and an counterclockwise rotation from the base to the apex as well as to the circumferential-longitudinal shear angle 3.
Image acquisition
Strain imaging can be conducted with echocardiography and cardiac magnetic resonance with the common principle that specific features or patterns in an image are detected and followed over a certain time course and re-identified in the subsequent images 4.
Different image processing algorithms are available for echocardiography and MRI, which include:
Tissue Doppler imaging has been used to estimate the 'strain rate' with the velocity gradient and to calculate strain as a temporal integral of that information.
Speckle tracking echocardiography (STE) is a more recently used technique, which can be applied to conventional 2D B-mode images of sufficient quality or 3D echocardiographic images. Left ventricular deformation analysis occurs mainly from different image intensities, including cardiac contours and the speckled image texture of the myocardium 2,4. Advantages of speckle tracking echocardiography include high spatial and temporal resolution. Disadvantages are that speckle motion is more measured at the endocardial border and less in the myocardium. The difference between 2D and 3D speckle tracking echocardiography is that in 2D only two directions can be measured at a time, which is circumferential and radial in a short-axis view and longitudinal and radial in left ventricular long-axis views, whereas in 3D speckle tracking echocardiography, the strain of all three directions can be obtained at once 1.
MR tagging has been used for quite some time with the main advantage that deformation is directly measured by myocardial tissue properties, but with the offset of low temporal resolution, extra acquisition time, time-consuming post-processing etc. 4.
MR feature tracking is a post-processing technique, which can be used on normal cine SFFP sequences, without additional acquisition time or complicated post-processing software. The main advantage is the ease of use. The downside is low spatial and temporal resolution and the tracking algorithm is only based on contours.
Displacement encoding with stimulated echoes (DENSE) and strain-encoded (SENC) are cardiac magnetic resonance techniques mainly used for research.
Post-processing
The workflow for both cardiac MRI and echocardiography is similar and includes the following steps 4:
identification of end-diastole and end-systole
definition of the myocardial region of interest and dimension to be examined
definition of the points or features to be tracked (segmentation)
tissue tracking and computing of the respective strain curves
The region of interest defines the following 6:
endocardial: inner contour of the cardiac wall
epicardial: outer contour of the cardiac wall
myocardial: refers to the middle between inner and outer contours
Interpretation
Normal values
Normal values still differ substantially between imaging modalities, acquisition methods and software algorithms as well as other potential influences such as patient age and gender. Due to those difficulties and influences, software-specific cut-off values were recommended for use 3,4.
It has been suggested that a global longitudinal strain <12% indicates severe systolic dysfunction and a value <15-16% seems to pose a risk in patients with preserved ejection fraction 1.
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