Echocardiographic Assessment of Myocardial Strain

Journal of the American College of Cardiology Vol. 58, No. 14, 2011 © 2011 by the American College of Cardiology Foundation ISSN 0735-1097/$36.00 Published by Elsevier Inc. doi:10.1016/j.jacc.2011.06.038 STATE-OF-THE-ART PAPER Echocardiographic Assessment of Myocardial Strain John Gorcsan III, MD, Hidekazu Tanaka, MD Pittsburgh, Pennsylvania Echocardiographic strain imaging, also known as deformation imaging, has been developed as a means to objectively quantify regional myocardial function. First introduced as post-processing of tissue Doppler imaging velocity converted to strain and strain rate, strain imaging has more recently also been derived from digital speckle tracking analysis. Strain imaging has been used to gain greater understanding into the pathophysiology of cardiac ischemia and infarction, primary diseases of the myocardium, and the effects of valvular disease on myocardial function, and to advance our understanding of diastolic function. Strain imaging has also been used to quantify abnormalities in the timing of mechanical activation for heart failure patients undergoing cardiac resynchronization pacing therapy. Further advances, such as 3-dimensional speckle tracking strain imaging, have emerged to provide even greater insight. Strain imaging has become established as a robust research tool and has great potential to play many roles in routine clinical practice to advance the care of the cardiovascular patient. This perspective reviews the physiology of myocardial strain, the technical features of strain imaging using tissue Doppler imaging and speckle tracking, their strengths and weaknesses, and the state-of-the-art present and potential future clinical applications. (J Am Coll Cardiol 2011;58:1401–13) © 2011 by the American College of Cardiology Foundation Noninvasive assessment of regional myocardial function is Physiology of Left Ventricular Strain important to the field of cardiovascular medicine to diag- nose disease, assess therapeutic interventions, and predict D’Hooge et al. (2), Dandel and Hetzer (6), and Mor-Avi et al. (7) clinical outcomes. Although magnetic resonance imaging published scholarly reviews of the technical features of strain and computed tomography imaging are useful diagnostic imaging in detail previously. Briefly, myocardial regional mechan- alternatives, echocardiography remains advantageous for ics assessed by echocardiographic approaches have been described widespread clinical use because of its portability, low risk, by 4 principal types of strain or deformation: longitudinal, radial, and comparatively high temporal resolution. Echocardio- circumferential, and rotational (Fig. 1). Although myocardial fiber graphic strain imaging, also known as deformation imaging, orientation results in these strain vectors occurring 3 dimensionally is a technological advancement that has been developed as a in an integrated manner, most investigative works have been done means to objectively quantify regional myocardial function using individual strain assessments. The term strain applied to (1–3). First introduced as a post-processing feature of tissue echocardiography in a simplistic sense is to describe lengthening, shortening, or thickening, also known as regional deformation. Doppler imaging (TDI) with velocity data converted to ␧ ϭ⌬ strain and strain rate, strain imaging information has more Strain may be L/L0 understood as an example of an recently also been derived from speckle tracking computer infinitesimally thin bar where the only possible deformation is processing (4,5). Currently, most echocardiography labora- lengthening or shortening. Accordingly linear strain or the tories continue to use the subjective visual assessment of wall amount of deformation can be defined by the change in length divided by the original length expressed by the formula: ␧ ϭ motion for resting and stress imaging for everyday clinical ⌬ ␧ ϭ ⌬ ϭ ϭ use, and strain imaging has been more often regarded as a L/L0, where strain, L change in length, and L0 research tool. Adoption of strain imaging in clinical practice original length. In reality, the myocardial wall as a 3-dimensional appears to have been gaining momentum more recently. (3D) object has strain that may occur along 3 planes (x-, y-, and This paper reviews the physiology of myocardial strain, the z-axes), known as normal strains, and 6 shear strains (2,3,6). technical features of strain imaging using both TDI and Despite the complexities of myocardial wall dynamics, some speckle tracking, their strengths and weaknesses, and the meaningful information has been derived using the simplified potential present and future clinical applications. linear strain or deformation model by echocardiography. Strain by TDI From the University of Pittsburgh, Pittsburgh, Pennsylvania. Dr. Gorcsan has received research grant support from GE and Toshiba. Dr. Tanaka has reported that The first description of echocardiographic strain was derived he has no relationships relevant to the contents of this paper to disclose. Manuscript received August 8, 2010; revised manuscript received May 31, 2011, from TDI velocity data using the Doppler equation to accepted June 10, 2011. convert ultrasound frequency shifts to velocity information 1402 Gorcsan III and Tanaka JACC Vol. 58, No. 14, 2011 Myocardial Strain September 27, 2011:1401–13 Abbreviations along the scan lines. Because the lines. The important technical features of TDI acquisition and and Acronyms fundamental data produced by analysis appear in the Online Appendix. TDI were velocity information, cardiac magnetic Strain by Speckle Tracking ؍ CMR resonance strain rate (strain per unit of time) was derived from the ve- cardiac A more recent echocardiographic approach to strain analysis ؍ CRT locity data using the equation: resynchronization therapy is speckle tracking. Speckle tracking is a post-processing ␧ ϭ V Ϫ V /L, where ␧ ϭ strain dobutamine stress 1 2 computer algorithm that uses the routine grayscale digital ؍ DSE echocardiography rate, V ϭ velocity at point 1, 1 images. Although several manufacturers have devised V ϭ velocity at point 2, and L ϭ ؍ EF ejection fraction 2 speckle tracking echocardiographic approaches, the funda- length, usually set at 10 mm. -left bundle branch mental approach is similar (4,12,13). Briefly, routine gray ؍ LBBB block Strain rate and strain data using scale digital images of the myocardium contain unique TDI required the direction of the ؍ LV left ventricular speckle patterns. A user-defined region of interest is placed myocardial wall motion to be tissue Doppler on the myocardial wall. Within this region of interest, the ؍ TDI along the ultrasound scan line imaging image-processing algorithm automatically subdivides re- (1,3). Longitudinal shortening dimensional gions into blocks of pixels tracking stable patterns of-2 ؍ 2D using the apical windows was dimensional speckles. Subsequent frames are then automatically analyzed-3 ؍ 3D often used because of the favor- by searching for the new location of the speckle patterns able Doppler angle of incidence within each of the blocks using correlation criteria and the (3)(Fig. 2). The alternative was from the parasternal views sum of absolute differences (Fig. 4). The location shift of where the relative transmural change in velocity could be these acoustic markers from frame to frame representing calculated as the velocity gradient, similar to strain rate (e.g., tissue movement provides the spatial and temporal data in the posterior wall) (8–10). TDI strain rate data could be used to calculate velocity vectors. Temporal alterations in integrated over time to determine strain (Fig. 3). Because all these stable speckle patterns are identified as moving farther TDI information is affected by the Doppler angle of apart or closer together and create a series of regional strain incidence, Doppler angle correction analysis programs were vectors. Because strain information is not dependent on the developed to determine wall motion for regions where the Doppler angle of incidence like TDI strain, several more motion was not aligned with the Doppler scan line (11). strain analyses are possible, including longitudinal, circum- However, it remained impossible to assess wall motion by ferential, radial, and rotational. Additional technical fea- TDI when the angle of motion was close to 90o. Accord- tures of speckle tracking acquisition and analysis appear in ingly, the majority of the published literature on echocar- the Online Appendix. diographic strain imaging using TDI assessed longitudinal strain from the apical windows with left ventricular (LV) Strain Imaging for shortening and lengthening aligned with the Doppler scan Myocardial Ischemia and Viability Significant insights pertaining to the pathophysiology of ischemic heart disease have resulted from echocardiographic strain imaging over the past decade (14–18). Previously, the clinical assessment of wall motion abnormalities at rest or with stress by exercise or dobutamine has consisted of visual assessment of endocardial excur- sion and wall thickening. The addition of strain imaging has refined the ability of echocardiography to detect and objectively qualify specific patterns of ischemia and infarction. Because ischemic wall motion abnormalities are often associated with passive motion, such as passive expansion and recoil and tethering from adjacent seg- ments, strain imaging has the advantage of differentiating active contraction from passive motion, which often is difficult visually

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