Ultrafast myocardial elastography using coherent compounding of diverging waves during simulated stress tests: An in vitro study

Objective myocardial deformation assessment during stress tests could help clinicians to better diagnose myocardial ischemia. However, the use of conventional focused echocardiography is compromised at increased heart rates due to its limited lateral field of view and frame rate. Ultrafast echocardiography using coherent compounding of diverging waves improves temporal resolution while maintaining a large field of view and could be a valuable alternative during stress tests. This study aimed to illustrate the feasibility of estimating myocardial strain using ultrafast echocardiography combined with a Lagrangian speckle model estimator (LSME) at increased heart rates. Myocardial strain assessment was tested on a dynamic cardiac phantom at heart rates ranging from 60 to 180 beats-per-minute (bpm). Ultrafast echocardiography was obtained with a Verasonics platform equipped with a 2.5 MHz phased array transducer (PRF: 4500 Hz). Negative effects of side lobes and phase delays during the large tilted transmission and compounding of diverging waves were suppressed through a triangle transmit sequence and motion compensation strategy. The robustness and accuracy of affine strain estimation were then enhanced using radiofrequency least-squares-based LSME combined with a coarse-to-fine strain estimation and a time-ensemble estimation strategy. 2D myocardial strain images at systole and early-diastole as well as regional strain curves were estimated. Myocardial strains with high contrast-to-noise and signal-to-noise ratios were obtained at all simulated heart rates. Regional strain curves were accurately estimated and periods matched those of the phantom pump cycles. These preliminary results suggest that the use of ultrafast echocardiography combined with the modified LSME could be useful clinically to provide an accurate and objective method of myocardial strain assessment at high heart rates.