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Target Audience:

This activity is designed for all cardiovascular physicians and cardiac sonographers with a primary interest and knowledge base in the field of echocardiography: in addition, residents, researchers, clinicians, intensivists, and other medical professionals with a specific interest in cardiac ultrasound will find this activity beneficial.

Target Audience:

This activity is designed for all cardiovascular physicians and cardiac sonographers with a primary interest and knowledge base in the field of echocardiography: in addition, residents, researchers, clinicians, intensivists, and other medical professionals with a specific interest in cardiac ultrasound will find this activity beneficial.

Objectives:

Upon completing the reading of this article, the participants will better be able to:

• 1.
  

  Define the terms strain and strain rate as they relate to left ventricular (LV) myocardial function.

  
  
• 2.
  

  Describe how speckle tracking echocardiography is used to determine strain and strain rate.

  
  
• 3.
  

  Name the advantages and disadvantages of tissue Doppler-derived strain and strain rate imaging compared to speckle tracking echocardiography (STE) derived strain and strain rate.

  
  
• 4.
  

  Recognize the different directions of normal and shear strain and how shear strain relates to torsion.

  
  
• 5.
  

  Identify the clinical applications of STE for assessment of LV deformation.

Author Disclosure:

The authors of this article reported no actual or potential conflicts of interest in relation to this activity.

Estimated Time to Complete This Activity: 1 hour

Myocardial Strain Imaging

Regional strain is a dimensionless measurement of deformation, expressed as a fractional or percentage change from an object’s original dimension. Strain rate, on the other hand, refers to the speed at which deformation (ie, strain) occurs. As a spatial derivative of velocity, strain rate provides increased spatial resolution for precise localization of diseased segments. However, strain rate needs high temporal resolution (>100 Hz) to avoid underestimation due to undersampling. Therefore, Doppler, because of its high temporal resolution, is superior to speckle tracking for strain rate imaging. However, Doppler-derived strain is angle dependent and highly susceptible to noise arising from the blood pool, aliasing and reverberation. The use of integrated strain helps reduce random noise while maintaining near similar spatial information.

The amount of shortening or stretch in the tissue or fibers describes the normal strain, and the amount of distortion associated with the sliding of plane layers over each other describes the shear strain within a deforming body ( Figure 1 ). There are two methods for assessing deformation on a continuum. One description is made in terms of the material coordinates. This is called “material description” or “Lagrangian description,” which defines motion around a given point in tissue as it traverses through space and time. Similar to tagged magnetic resonance imaging (MRI), speckle-tracking technology analyzes Lagrangian strain, in which the end-diastolic tissue dimension represents the unstressed, initial material length as a fixed reference throughout the cardiac cycle. An alternative way to describe deformation is to consider the relative velocity of motion at a particular location in space as a function of time, referencing the region in terms of the spatial coordinates. This is also called the “spatial description” or “Eulerian description.” DTI analyzes Eulerian strain, which is derived from the temporal integral of the DTI strain rate signal and uses instantaneous lengths for the reference length. In practice, tissue Doppler scanners can convert Eulerian strain into Lagrangian strain. Likewise, by taking the inverse integral of Lagrangian strain, one may also calculate Eulerian strain.

Linking the myofiber architecture and 3-directional deformation of the left ventricle. The left panel shows a schematic representation of the myocardial fiber orientation in the left ventricle that changes continuously from a right-handed helix (R) in the subendocardial region to a left-handed helix (L) in the subepicardial region, as seen over the anterior wall of the left ventricle. The panels in the center and to the left show the normal (ϵ _x , ϵ _y , and ϵ _z ), and 3 components of shear strain (ϵ _xy , ϵ _xz , and ϵ _yz ) in a block of myocardial tissue in which the x -axis is oriented at a tangent to the circumferential direction, the y -axis is oriented longitudinally, and the z -axis corresponds to the radial direction of the left ventricle, with u , v , and w representing displacements in the x , y , and z directions, respectively. Shear strain can be defined exactly the same way as normal strain: the ratio of deformation to original dimensions. In the case of shear strain, however, it is the amount of deformation perpendicular to a given line rather than parallel to it. For example the shear strain ϵ _xy is the average of the shear strain on the x face along the y direction and on the y face along the x direction.

The general state of strain at a point in a body is composed of 3 components of normal strain (ϵ _x , ϵ _y , and ϵ _z ), and 3 components of shear strain (ϵ _xy , ϵ _xz , and ϵ _yz ). Therefore, for the left ventricle, 3 normal strains (longitudinal, circumferential, and radial) and 3 shear strains (circumferential-longitudinal, circumferential-radial, and longitudinal-radial) are used to describe left ventricular (LV) deformation in 3 dimensions ( Figure 1 ). One of the principal purposes of LV shearing deformation lies in amplifying the 15% shortening of myocytes into 40% radial LV wall thickening, which in turn results in a >60% change in LV ejection fraction in a normal heart. Because the degree of shearing increases toward the subendocardium, higher strains are seen at the subendocardium resulting in a subepicardial-to-subendocardial thickening strain gradient.

Myocardial shear in the circumferential-longitudinal plane results in twist or torsional deformation of the LV during ejection such that, when viewed from the apex, the LV apex rotates in a clockwise direction and the base rotates in a counterclockwise direction. Terms such as rotation , twist , and torsion are often used interchangeably for explaining the circumferential-longitudinal shear deformation of the left ventricle. For a uniform description, we emphasize that the term rotation should refer to the rotation of short-axis sections of the left ventricle as viewed from the apical end and defined as the angle (in degrees or radians) between radial lines connecting the center of mass of that specific cross-sectional plane to a specific point in the myocardial wall at end-diastole and at any other time during systole. The term torsion should be used for defining the base-to-apex gradient in the rotation angle along the longitudinal axis of the left ventricle, expressed in degrees per centimeter or radians per meter. The absolute apex-to-base difference in LV rotation (also in degrees or radians) is stated as the net LV twist angle or the net LV torsion angle. It must be emphasized that LV length and diameter change dynamically during a cardiac cycle, and therefore these normalization schemes permit comparison of only the peak magnitude of torsion for different sizes of the left ventricle. LV torsion during ejection results in storage of potential energy into the deformed myofibers and myocardial matrix. With the onset of relaxation, the stored energy is released back, like a spring uncoiling, generating suction and forces for rapid early diastolic restoration. Myocardial deformation is thus of functional interest during both systole and diastole.

Myocardial deformation during ejection demonstrates extensive transmural tethering such that subendocardial and subepicardial regions undergo simultaneous shortening along the fiber and cross-fiber direction during ejection. Subendocardial strains are higher in magnitude than subepicardial strains. Within the subendocardium, the magnitude of circumferential strains during ejection exceeds that of longitudinal strains.

Validation of Speckle-Tracking Echocardiography

Speckle tracking requires a thorough understanding of echocardiographic imaging technique for both image acquisition and myocardial border tracing. In addition, images must be of high-resolution quality to track regions of interest accurately. Myocardial strain derived from STE has been validated using sonomicrometry and tagged MRI. Speckle-tracking strain results correlate significantly with tissue Doppler–derived measurements. Tissue Doppler technology is dependent on achieving a parallel orientation between the ultrasound beam and the direction of motion and therefore is applied mostly in apical views for recording longitudinal strains and from midanterior and midinferior segments of the left ventricle in short-axis views for recording radial strains. STE, in contrast, can analyze the longitudinal and radial deformation of all LV segments from apical views and radial and circumferential strain of all LV segments from the short-axis views. In comparison with DTI, receiver operating characteristic curve analysis has shown that longitudinal and radial strain measured using STE has a significantly greater area under the curve than DTI strain in differentiating normal and dysfunctional segments.

Speckle tracking–derived strain and strain rates do not require scaling for any index of LV morphology. Overall, speckle tracking appears to be highly reproducible and minimally affected by intraobserver and interobserver variability. However, some studies have suggested underestimation of longitudinal strain with STE. Variations between MRI tagging and STE may be secondary to misaligned image planes and out-of-plane motion, which may not be accounted for by STE. Furthermore, initial studies using contrast echocardiography have shown wide interindividual variability in the precision of strain quantification, suggesting that additional investigations are needed to understand thoroughly the use of strain techniques concomitant with contrast echocardiography.

Application of STE has also been extended for studying regional and global function of other cardiac chambers including the right ventricle and the left atrium. The complex geometry and thin walls of the right ventricle and left atrium may present considerable challenges in optimal positioning of the region of interest. Investigations with STE thus far have primarily attempted to define the longitudinal deformation of the right ventricle, with some preliminary data suggesting a potential role in measuring the circumferential and rotational deformation of the right ventricle. For the left atrium, DTI-derived deformation analyses have generally been restricted to the annular region and the midsegments of the left atrium. STE may have potential advantages in assessing deformation from all segments of the left atrium. Further studies are needed to understand the incremental value and potential impact of these applications in clinical settings.

Clinical Applications of Speckle-Tracking Echocardiography for Assessment of Left Ventricular Deformation

Table 1 presents a general classification scheme that may be helpful for the application of STE-derived multidirectional strains in clinical practice. In general, longitudinal LV mechanics, which are predominantly governed by the subendocardial region, are the most vulnerable component of LV mechanics and therefore most sensitive to the presence of myocardial disease. The midmyocardial and epicardial function may remain relatively unaffected initially, and therefore circumferential strain and twist may remain normal or show exaggerated compensation for preserving LV systolic performance. Increase in cardiac muscle stiffness, however, may cause progressive delay in LV untwisting. Loss of early diastolic longitudinal relaxation and delayed untwisting attenuates LV diastolic performance, producing elevation in LV filling pressures and a phase of predominant diastolic dysfunction, although the LV ejection fraction may remain normal. On the other hand, an acute transmural insult or progression of disease results in concomitant midmyocardial and subepicardial dysfunction, leading to a reduction in LV circumferential and twist mechanics and a reduction in LV ejection fraction. Assessment of myocardial mechanics, therefore, can be tailored per the clinical goals. The detection of altered longitudinal mechanics alone may suffice if the overall goal of analysis is to detect the presence of early myocardial disease. Further characterization of radial strains, circumferential strains, and torsional mechanics provides assessment of the transmural disease burden and provides pathophysiologic insight into the mechanism of LV dysfunction. For example, pericardial diseases, such as constrictive pericarditis, cause subepicardial tethering and predominantly affect LV circumferential and torsional mechanics. The presence of attenuated longitudinal mechanics in constrictive pericarditis may signify the presence of transmural dysfunction. As another example, a pathophysiologic process such as radiation that affects both the pericardium and the subendocardial region may produce attenuation of both longitudinal and circumferential LV function.

The following sections comprehensively overview the application of STE in common cardiovascular diseases affecting LV function. For this, we performed a search of the Ovid Medline database and identified English-language articles relevant to strain imaging and cardiac function in human subjects (see the Appendix for details of the search strategy). The primary outcomes of each study for the following sections are reported in written and table format.

Coronary Artery Disease

The subendocardium is the area of the left ventricle most vulnerable to the effects of hypoperfusion and ischemia. LV longitudinal mechanics at rest may therefore be attenuated in patients with coronary artery disease ( Table 2 ). For example, Liang et al found that a peak longitudinal strain rate of −0.83 s ⁻¹ and an early diastolic strain rate of 0.96 s ⁻¹ obtained from resting echocardiography could predict >70% coronary stenosis with sensitivity of 85% and specificity of 64%. Speckle tracking–derived longitudinal strain is also useful in predicting the extent of coronary artery disease. Choi et al reported that a segmental mid and basal peak longitudinal strain cutoff value of −17.9% was capable of discriminating severe 3-vessel or left main coronary artery disease from disease with lesser severity with sensitivity of 78.9% and specificity of 79.3%.

Myocardial Infarction

Consistent with DTI, longitudinal strains are significantly reduced in patients with myocardial infarctions, proportionately within the area of infarction, and correlate closely with peak infarct mass and ejection fraction ( Table 2 ). Patients with smaller infarcts and preserved global LV ejection fractions show reduced radial and longitudinal strain, although LV circumferential strains and twist mechanics remain relatively preserved. In contrast, a larger transmural infarction is associated with additional reduction of circumferential strains ( Figure 2 ). In addition to normal strains, systolic twist and diastolic untwist are also reduced and correlate with the reduction in the LV ejection fraction. Bertini et al found that both the peak LV twist and untwisting rate are reduced in acute myocardial infarctions and correlate with the grade of diastolic and systolic LV dysfunction.

Circumferential strains in a 37-year-old female patient with transmural anteroseptal myocardial infarction. Panels A and B show the cross-sectional view of the left ventricle obtained on 2D echocardiography and MRI, respectively. Note the transmural enhancement seen on delayed enhancement MRI (gray and green arrows) . Panel C shows the circumferential strain (2D Cardiac Performance Analysis; TomTec, Munich, Germany) obtained from 6 LV segments. Dyskinetic circumferential strain is recorded from anterior and anteroseptal segments (grey and green arrows) , where there is evidence of transmural scarring.

Speckle-tracking strains have increased sensitivity and specificity in comparison with tissue Doppler for determining the transmural extent of a myocardial infarction. Using a longitudinal strain cutoff value of −15%, Gjesdal et al reported that infarcted segments could be detected with sensitivity of 76% and specificity of 95% at the segmental level and 83% and 93%, respectively, at the global level. Becker et al compared radial and circumferential strains by speckle tracking with the extent of infarction as delineated by contrast-enhanced MRI ( Table 2 ). Using a segmental radial strain cutoff value of 16.5%, nontransmural infarcts could be distinguished from transmural infarcts with sensitivity of 70.0% and specificity of 71.2%. On the other hand, a circumferential strain value < −11.10% distinguished nontransmural infarction from transmural infarction with sensitivity of 70.4% and specificity of 71.2%. Roes et al identified that a regional longitudinal strain cutoff value of −4.5% could distinguish a nontransmural infarct from a transmural infarct with sensitivity of 81.2% and specificity of 81.6% ( Table 2 ).

Stress Echocardiography

Tissue Doppler technology has shown that patients with newly developed myocardial ischemia have reduced peak longitudinal, circumferential, and radial systolic strains during dobutamine infusion, with the greatest deterioration of myocardial shortening occurring in the circumferential direction. In comparison with tissue velocity–derived strains, speckle tracking longitudinal strains during dobutamine stress echocardiography have similar accuracies for detecting ischemia in the left anterior descending coronary artery territory but reduced accuracy for the left circumflex and right coronary artery territories ( Table 2 ). This may be due to the dependency of 2D strain on grayscale image quality.

The crossover sequence from systole to diastole is the most energy demanding phase of the cardiac cycle. In the ischemic cascade, diastolic dysfunction therefore develops earlier than the appearance of regional systolic wall motion abnormalities. Speckle tracking–derived strain values measured in diastole may have superior sensitivity for identifying ischemic regions. For example, Ishii et al measured regional LV radial strain obtained from apical long-axis views (also referred to as transverse strains) during the first third of diastole (strain imaging diastolic index) and at baseline and 5 and 10 minutes after exercise. A strain imaging diastolic index ratio of 0.74 detected significant coronary artery disease (>50% stenosis of ≥1 large coronary vessel) with sensitivity of 97% and specificity of 93%. More investigations are required to further clarify the diagnostic utility of measuring diastolic strain indices in patients with coronary artery disease.

Speckle-tracking strains have also been used in conjunction with low-dose dobutamine for detecting viability and myocardial contractile reserve. Myocardial segments with transmural scars show reduced contractile reserve with dobutamine infusion. Circumferential strains are particularly useful in differentiating transmural from nontransmural infarctions. Torsion is reduced in infarcted segments but not within ischemic regions.

Revascularization

The effects of balloon occlusion and time to reperfusion on regional myocardial function have been evaluated using STE. Balloon occlusion during catheterization of the coronary arteries results in a transient reduction in systolic and diastolic strain at the proximal and distal at-risk segments, which return to normal following reperfusion. Shorter symptom-to-balloon times in patients with acute coronary syndromes typically result in lower impairment of systolic longitudinal strain, which relates closely to peak levels of cardiac troponin T. Speckle-tracking strains are useful in predicting myocardial segments with resting dysfunction following revascularization that will likely improve on follow-up ( Table 2 ). Park et al reported that longitudinal strain < −10.2% following reperfusion therapy in patients with acute myocardial infarction predicted nonviable myocardium in a remodeled left ventricle with sensitivity of 90.9% and specificity of 85.7%. In addition, longitudinal strain < −6.4% predicted the development of heart failure or death with sensitivity of 81.8% and specificity of 84.6%. STE-derived peak radial strain correlates with the extent of hyperenhancement on delayed contrast-enhanced MRI. Becker et al found that by using a cutoff of 17.2% for peak systolic radial strain, one could predict functional recovery with sensitivity of 70.2% and specificity of 85.1%, similar to results using contrast-enhanced MRI (sensitivity, 71.6%; specificity, 92.1%). Another study combined delayed hyperenhancement seen on contrast-enhanced MRI incrementally with radial deformation patterns observed on STE and showed sensitivity of 82.2% and specificity of 78.3% for predicting segmental functional recovery.

Valvular Disease

Because of adaptive remodeling of the left ventricle, patients can remain asymptomatic or minimally symptomatic for prolonged periods, even in the presence of severe valvular disease. STE improves the yield of routine 2D echocardiography in valvular heart diseases by providing insights into the pattern of adaptive remodeling and detecting the presence of subclinical cardiac dysfunction.

Aortic Stenosis

Aortic stenosis results in progressive LV hypertrophy due to increased afterload. The LV ejection fraction, however, remains preserved. Previous studies with DTI established that LV systolic longitudinal strain and strain rate are significantly attenuated in patients with aortic stenosis and improve immediately following aortic valve replacement. Similarly, speckle tracking–derived longitudinal strains have also been shown to be reduced in severe aortic stenosis. However, radial and circumferential strains and LV twist mechanics remain relatively preserved ( Table 3 , Figure 3 ). In addition, LV deformation shows improvement in all the 3 normal directions following aortic valve replacement. Tzemos et al reported an increase in LV twist in pregnant patients with aortic stenosis.

Table 3

Studies evaluating myocardial strain in valvular disease

Study	Subjects (n)	Purpose	Strain/twist
AS
Becker et al (2007)	AS (22)	RS and CS in AVR	Improved RS and CS after AVR
Tzemos et al (2008)	Pregnancy with AS (10); pregnancy, no AS (10); controls (10)	RS in AS with pregnancy	No change in RS prepartum or postpartum; higher twist in pregnancy with AS
Lafitte et al (2009)	AS (65), controls (60)	Exercise stress test in severe AS	Reduced LS with stress testing with preserved CS and RS
AI
Becker et al (2007)	AI (18)	Strain before and after AVR	Reduction in CS and RS after AVR
Gabriel et al (2008)	AI (39), controls (10)	GLS in AI with stress echocardiography and BNP	No association between GLS and plasma BNP levels
Stefani et al (2009)	AI (20), no AI (40)	Strain in athletes with AI	Reduced LS in basal regions with basal-to-apical gradient
MR
Borg et al (2008)	Patients (38), controls (30)	LV torsion in MR	Delayed and reduced rate of LV untwisting
Kim et al (2009)	Preserved contractility (30), reduced contractility (29), controls (34)	Strain in MR and contractility	Reduced LS, RS, and CS in reduced contractile function
Lancellotti et al (2008)	Patients (71), controls (23)	Strain at rest and after exercise stress echocardiography	Reduced LS in patients with blunted response to exercise

AI , Aortic insufficiency; AS , aortic stenosis; AVR , aortic valve replacement; BNP , brain natriuretic peptide; CS , circumferential strain; GLS , global longitudinal strain; LS , longitudinal strain; LV , left ventricle; MR , mitral regurgitation; RS , radial strain.

LV mechanics in aortic stenosis. Continuous-wave Doppler signal across the stenotic aortic valve in panel A shows peak and mean gradients of 80 and 44 mm Hg, respectively. Pulsed-wave tissue Doppler from the septal corner of the mitral valve annulus in panel B shows a reduced peak early diastolic longitudinal relaxation velocity (5 cm/s). Longitudinal strain obtained by speckle tracking (2D strain; GE Healthcare, Milwaukee, WI) shows attenuated peak longitudinal strain in panels C and D from apical (green curve) , mid (yellow curve) , and basal (blue curve) of the lateral wall of the left ventricle (peak strain values < 10%). The dotted white line in panel D also shows a reduced global longitudinal strain averaged from the septum and lateral wall of the left ventricle (global strain = 12%). Peak counterclockwise rotation from the apex (E) and clockwise rotation from the base (F) are obtained by speckle-tracking imaging. The difference of the two rotational values provides the peak net twist angle. This example illustrates the presence of exaggerated LV rotation, particularly near the LV base with a relatively high net LV twist angle value. A _a , peak late diastolic annular velocity; AVC , aortic valve closure; E _a , peak early diastolic annular velocity; S _a , peak systolic velocity during ejection. Reproduced with permission from J Am Coll Cardiol .

Left Ventricular Hypertrophy

STE has been used in detecting subclinical myocardial changes in LV hypertrophy, as well as in distinguishing the different causes of LV hypertrophy.

Physiologic Hypertrophy

Several speckle-tracking echocardiographic studies have attempted to decipher the complex adaptive changes in LV mechanics seen with exercise. Most studies identified a significant increase in strains and the development of a higher regional function reserve during high-intensity training. Endurance training, however, results in reductions in peak longitudinal, circumferential, and radial strains. Nottin et al reported reduced peak radial strain at the LV apical level in cyclists in comparison with controls, despite normal peak circumferential shortening ( Table 4 ). Endurance training in rowers has been reported to result in increase radial strain equally in all segments, decrease in longitudinal strain increases from the base toward the apex, and an increase in circumferential strain in the LV free wall with reductions in the septum due to changes in RV structure. LV twist may be reduced with endurance training. For example, Zocalo et al reported reductions in LV twist in soccer players that occurred conjunctly with the development of higher ejection fractions. Similarly, cyclists have been reported to experience reductions in LV twist. Surprisingly, reduced twist has not been identified by all authors. Neilan et al found increased twist following exercise in athletes. In addition, exercise may result in a delay in age-related reduction in LV longitudinal function. For example, elderly marathon runners showed no evidence of LV systolic dysfunction following a marathon with preserved longitudinal strain and fractional shortening. The underlying physiologic mechanisms for explaining the variability of these data remain yet to be established.

Table 4

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