Abstract
Visual assessment of regional and global ventricular function assessment is subjective and leads to significant variability in interpretation. The heart is a mechanical organ and undergoes cyclic deformation in systole and diastole. This deformation can be measured. Noninvasive assessment of deformation is now feasible with magnetic resonance and echocardiography. The early experience in echocardiographic assessment of deformation used tissue Doppler-based methodology to track tissue motion. Further refinement of these techniques enabled two-dimensional (2D) echo-based assessment of regional deformation via determination of strain using speckle tracking. Strain is a measure of tissue deformation. Strain is defined as a change in length of an object relative to its original length. Strain rate is the rate at which this deformation (length change) occurs. Although myocardial deformation is a three-dimensional (3D) phenomenon, echo-based interrogation techniques have generally been limited to interrogating one or more of three imaging planes—longitudinal, circumferential, and radial. More recently, 3D strain has been introduced. Strain and strain rate allow a clinician to determine regional and global myocardial function at the same level as a muscle physiologist by providing parameters similar to shortening fraction and shortening velocity, respectively. Myocardial dysfunction is characterized by reduced systolic strain and systolic strain rate. More recently, interrogation of diastolic strain rate and diastolic strain have enabled novel insights into regional diastolic mechanics. This review presents the fundamental principles underlying strain echocardiography and summarizes potential clinical applications of strain/strain rate imaging in the diagnosis and management of particular cardiac conditions.
Keywords
echocardiography, strain, strain rate, tissue Doppler, speckle tracking, principles, applications
Introduction
Assessment of regional and global ventricular function has long relied on visual assessment. However, this approach is subjective and variable leading to significant interobserver variability in interpretation. The heart is a mechanical organ and undergoes cyclic deformation in systole and diastole. This cyclic deformation can be measured and for decades was restricted to those undergoing open-heart surgery when metal beads were sown onto particular locations on the left ventricle (LV); deformation was then assessed via fluoroscopy. Approximately 20 years ago, magnetic resonance methods were introduced that allowed noninvasive assessment of deformation. Later, tissue Doppler-based methodology was used to track tissue motion by echocardiography. Further refinement of these techniques enabled echo-based assessment of regional deformation via determination of strain.
Principles of Strain Imaging
Strain is a measure of tissue deformation. Strain is defined as a change in length of an object relative to its original length (i.e., reduction to half its original length is 50% strain; Fig. 6.1 ). Strain rate (SR) is the rate at which this deformation (length change) occurs. Although myocardial deformation is a three-dimensional phenomenon, echo-based interrogation techniques have generally been limited to interrogating one or more of three imaging planes—longitudinal, circumferential, and radial ( Fig. 6.2 ). More recently, 3D strain has been introduced.
Strain and SR allow a clinician to determine regional and global myocardial function at the same level as a muscle physiologist by providing parameters similar to shortening fraction and shortening velocity, respectively.
Echo-based techniques measure deformation by two primary methods—tissue Doppler and speckle-based. In tissue Doppler imaging (TDI), SR is the difference in velocity between two points along the myocardial wall (velocity gradient) normalized to the distance between the two points ( Fig. 6.3 ). A similar velocity gradient exists between the endocardium and the epicardium, because the endocardium moves faster. This concept is used to derive myocardial velocity gradient (radial SR). This velocity gradient depicts the rate of change of myocardial wall thickness during systole and diastole. Thus SR measures the rate at which the two points of interest move toward or away from each other. Integration of SR yields strain, the normalized change in length between these two points ( Fig. 6.4 ).
In speckle-tracking methodology, the system tracks unique acoustic patterns within the myocardium termed speckles . These speckles can be tracked over time, and speckle displacement can be used to calculate tissue velocity and strain ( Fig. 6.5 ). This method is relatively angle-independent, because it is not based on the Doppler principle. Because speckle tracking can be automated, this technique lends itself to semiautomated measurements of strain. One such method allows the generation of bull’s-eye plots of longitudinal segmental strain ( Fig. 6.6 and 
). Another similar technique uses arrows to display the direction and amplitude of motion at various points in the heart (velocity vector imaging). Speckle tracking imaging can use preexisting B-mode images; however, it is performed at much lower frame rates (40–90 frames per second) and may not be as accurate in timing mechanical events as Doppler-based imaging (100–250 frames per second).
Commonly measured strain parameters include systolic and diastolic SRs, and systolic strain ( Fig. 6.7 ). Peak systolic SR is the parameter that comes closest to measuring local contractile function in clinical cardiology. It is relatively volume independent and is less pressure independent than strain. In contrast, peak systolic strain is volume dependent and does not reflect contractile function as well as SR.
Twist and Torsion
Myocardium are three-dimensional continuous fibers that change direction from subendocardial right-handed helix to a subepicardial left-handed helix. The fibers arranged in counter-direction generate sliding or shear deformation during contraction. When viewed from apex to base, the apex rotates counterclockwise during systole, while base rotates in clockwise ( Fig. 6.8 ). Twist is the apex-to-base difference in rotation, which is expressed in degrees. Torsion refers to the normalized twist, where the twist angle is divided by the distance between LV base and apex, which is expressed in degrees per centimeter. The systolic twist and diastolic untwist can be influenced by age, change of preload or afterload, diastolic dysfunction, cardiomyopathy, and valvular heart disease.
Regional and Global Function
Left Ventricle
Circumferential and radial strain values are obtained in standard parasternal short axis view at mitral valve, papillary muscle, and apex level. Longitudinal strain is calculated from apical two-, three-, and four-chamber views, with basal, mid, and apical segments in each of the six walls. Timing of aortic valve closure (AVC) is used to define end-systole in a cardiac cycle. To avoid underestimation, it is important to get a circular LV images when performing circumferential and radial strain analyses, and avoid foreshortened apical chamber views in longitudinal strain analysis. Global longitudinal strain (GLS), calculated as the average from all segments, is commonly used as a measure of global LV function. Fig. 6.9 shows typical segmental speckle-tracking strain in a healthy normal heart. Timing of end systole needs to be defined clearly to identify postsystolic shortening from systolic shortening. Normal GLS value is reported between 18% and 25% in healthy participants. However, there is intervendor variability.
Right Ventricle
The RV wall is thinner than the LV myocardium, and these two ventricles have different shapes. DTI has been validated in quantification of RV myocardial deformation in healthy individuals. RV longitudinal velocities demonstrate a typical base-to-apex gradient with higher velocities at the base ( Fig. 6.10A ). The deformation properties within the LV are more homogeneous. Conversely, the SR and strain values are less homogeneously distributed in the right ventricle and show a reverse base-to-apex gradient, with the highest values in the apical segments and outflow tract (see Fig. 6.10B ). This reverse pattern can be explained by the complex geometry of the thin-walled, crescent-shaped right ventricle, and the less homogeneous distribution of regional wall stress. DTI-derived and speckle tracking-derived strain and SR can be used to evaluate RV dynamics, and both were found to be feasible and generally comparable. Strain and SR correlate well with radionuclide RV EF. Systolic velocity and strain best correlated with invasively determined right ventricular stroke volume and change in right ventricular function after vasodilator infusion ( Fig. 6.11 ). SRs and strain quantitate regional right ventricular systolic function in children and adults with various conditions.
Left Atrium
Under normal conditions, the left atrium (LA) is a low-pressure, highly expandable chamber, but in the presence of acute and chronic injury, the left atrial wall stretches and stiffens. LA volume is not a specific marker for LA function, as it reflects the chronic effect of LV filling pressure but may also be increased in patients with atrial arrhythmias or in athletes whose LV filling pressure is actually normal.
Assessment of LA strain with two-dimensional (2D) speckle tracking and Doppler-based strain provide additional information on LA mechanics. Components of atrial function include reservoir, conduit, and active booster contraction. The LA strain and SR demonstrate atrial physiology and closely follow LV dynamics during the cardiac cycle. LA reservoir function is displayed by “total” strain, and contractile function is presented by the negative strain following the beginning of the “P” wave. There are two different ways to define the reference (zero) point: One is to define the onset of P wave as baseline, and then the first negative peak strain corresponds to atrial contractile function, peak positive strain as conduit function, and the total sum represents reservoir function. The other is to set the peak of QRS complex as baseline, and then peak positive longitudinal strain represents atrial reservoir function, and strains during early and late ventricular diastole equal to conduit and atrial contractile function ( Fig. 6.12 ). Global strain and SR are calculated by averaging values observed in all LA segments, either with a 15-segment model (six segments in the apical four-chamber view, six in the two-chamber view, and three in the three-chambers views) or a 12-segment (six segments in the four-chamber view and six more in the two-chamber views) model. LA deformation assessment has an established role in assessing LA performance and LV diastolic function. For patients with atrial fibrillation, those who have higher atrial strain and SR appear to have a greater likelihood of staying in sinus rhythm after cardioversion ( Fig. 6.13 ).
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