In vivostudy of myocardial elastography under graded ischemia conditions

Abstract

The capability of currently available echocardiography-based strain estimation techniques to fully map myocardial abnormality at early stages of myocardial ischemia is yet to be investigated. In this study, myocardial elastography (ME), a radio-frequency (RF)-based strain imaging technique that maps the full 2D transmural angle-independent strain tensor in standard echocardiographic views at both high spatial and temporal resolution is presented. The objectives were to (1) evaluate the performance of ME on mapping the onset, extent and progression of myocardial ischemia at graded coronary constriction levels (from partial to complete coronary flow reduction), and (2) validate the accuracy of the strain estimates against sonomicrometry (SM) measurements. A non-survival canine ischemic model (n = 5) was performed by gradually constricting the left anterior descending (LAD) coronary blood flow from 0% (baseline blood flow) to 100% (zero blood flow) at 20% increments. An open-architecture ultrasound system was used to acquire RF echocardiograms in a standard full short-axis view at the frame rate of 211 fps, at least twice higher than what is typically used in conventional echocardiographic systems, using a previously developed, fully automated composite technique. Myocardial deformation was estimated by ME and validated against sonomicrometry. ME estimates and maps transmural (1) 2D displacements using RF cross-correlation and recorrelation; and (2) 2D polar (radial and circumferential) strains, derived from 2D (i.e. both lateral and axial) displacement components, at high accuracy. Full-view strain images were shown and found to reliably depict decreased myocardial function in the region at risk at increased levels of coronary flow reduction. The ME radial strain was deemed to be a more sensitive, quantitative, regional measure of myocardial ischemia as a result of coronary flow reduction when compared to the conventional wall motion score index and ejection fraction. Good agreement (0.22% strain bias, 95% limits of agreement) using Bland–Altman analysis and good correlation (r = 0.84) were found between the ME and SM measurements. These findings demonstrate for the first time that ME could map angle-independent strains to non-invasively detect, localize and characterize the early onset of myocardial ischemia, i.e. at 40%, and possibly as low as 20%, LAD flow reduction, which could be further associated with the severity of coronary stenosis.

Radial Basis Functions for Combining Shape and Speckle Tracking in 4D Echocardiography

“…Performing speckle tracking on the raw RF signal allows for the incorporation of phase information for improved tracking in the axial direction [11] over B-mode tracking. Lee et al have shown that RF-based speckle tracking provides accurate strain measurements when compared to tagged MR in 2D myocardial slices [12], [13]. They went on to show that 2D RF-based speckle tracking could be used to identify regions of abnormal cardiac function [14].…”

Intracardiac myocardial elastography in canines and humans in vivo

“…Therefore, if the axial direction is mainly aligned with the radial direction, positive strain will be obtained whereas negative strain will be obtained if the axial direction is mainly aligned with the circumferential or longitudinal direction. It has to be noted that angle-independency can be achieved by estimating the lateral displacement and strain and therefore derive the angle-independent radial and circumferential strain [29]. However, this goes beyond the scope of this study.…”

“…It has recently been reported that LA strain can predict the success of AF ablation [26-28]. Previous studies have shown that strains could be obtained with ME at a high frame rate and a large field of view of the heart [29]. High frame rate ultrasound imaging can be achieved with techniques such as composite imaging [30] or parallel beamforming using plane wave [31] or diverging beams [32, 33].…”

Evaluation of Coronary Artery Disease Using Myocardial Elastography with Diverging Wave Imaging: Validation against Myocardial Perfusion Imaging and Coronary Angiography

“…Radial strains were derived from the axial and lateral strains with the origin of the polar coordinate system at the centroid of the segmented myocardium. Previous studies have found that end-systolic radial strain was able to differentiate a reperfused from a normal myocardium (Lee, et al 2008) and to characterize early onset of ischemia (Lee, et al 2011). In this study, for each patient, the end-systolic radial strain was averaged in the entire cross-section of the myocardium in the short-axis view.…”

“…This technique has been validated against tagged magnetic resonance imaging and was shown to be able to differentiate normal from reperfused myocardium (Lee, et al 2008). A more recent study has shown that ME was capable of detecting, identifying and characterizing down to 40% blood flow reduction in the left anterior descending artery (LAD) of a canine model in vivo (Lee, et al 2011). In these previous studies, high frame rates were achieved while keeping a high beam density by using ECG-gating to assemble small sectors of RF signals acquired at different heartbeats into a full echocardiographic view.…”