In vivo validation of Myocardial Elastography under graded ischemia conditions

In contrast to other ultrasound-based strain imaging techniques, Myocardial Elastography (ME), developed by our group, is an ultrasound radio-frequency (RF)-based speckle tracking technique that aims at estimating and mapping transmural displacements and angle-independent strains in full echocardiographic views at high precision and high spatial and temporal resolution. ME has previously been evaluated in a theoretical framework and a clinical setting. In this study, ME was validated in canine hearts (n = 5) in vivo against direct measurements of sonomicrometry (SM), regarding its capability of depicting the onset, extent and progression of myocardial ischemia caused by graded reductions in blood flow of the left anterior descending (LAD) coronary artery, from 0% (baseline blood flow) to 100% (zero blood flow) at 20% increments. ME estimates and maps: 1) 2D transmural displacements using RF cross-correlation and recorrelation; and 2) 2D polar (radial and circumferential) transmural strains at high accuracy. A Sonix RP system was used to acquire RF frames in a full, standard short-axis view at the frame rate of 211 fps, higher than in standard ultrasound imaging, using an automated composite technique developed by our group. Not only were abnormal myocardial regions detected and localized on the ME strain images, but the ME strains were also in good agreement (0.22% strain bias, 95% limits of agreement) with SM using the Bland-Altman analysis. These findings demonstrate that Myocardial Elastography could map 2D angle-independent strain to non-invasively detect, localize and characterize early myocardial ischemia starting at its early onset, i.e., at 40% LAD flow reduction.