3D elasticity imaging on an open-chest dog heart

Myocardial ischemia and infarction alter myocardial viability and contractility. We have hypothesized that contractility changes can be detected by ultrasound strain imaging. Current ultrasound strain imaging methods are mainly 1D and 2D. However, heart motion is complex and 3D. Previous studies showed that 3D speckle tracking on a left ventricular (LV) phantom and a 3D LV simulation reduced low dimensional tracking error. Before applying this method clinically, 3D speckle tracking was tested using 3D RF data acquired with a commercial 3D scanner (iE33, Philips, Andover, MA) on an openchest dog heart. Image data were recorded before and after occlusion of the left anterior descending (LAD) coronary artery to detect acute ischemia induced by this occlusion. Following a local animal protocol, 3D RF volume data were acquired on an open-chest dog heart using a commercial 2D phased array (X7-2, Philips, Andover, MA) placed in front of the anterior wall of the left-ventricle with a small water stand-off. The LAD artery was occluded to produce acute ischemia. Data acquisition before and after occlusion includes 52 volumes/cycle and 46 volumes/cycle (frame rate = 77 Hz), respectively. Each data volume covered 77.5 degrees in the azimuthal direction, 70 degrees in the zenithal direction, and 6.5 cm in depth (anterior wall) at a transmit frequency of 3.8 MHz. 3D speckle tracking was applied to estimate strain with tracking resolution of 1 mm in the axial direction and 7 mm in azimuthal and zenithal directions. Frame-to-frame tracking results were accumulated referenced to the heart geometry at the end of diastole. 3D speckle tracking successfully estimated the displacement in three directions. Radial strains at the end of systole derived from accumulated 3D displacements referenced to the end of diastole in the zoomed region detected abnormal wall thinning due to LAD occlusion. Unlike 1D or 2D methods, 3D speckle tracking can estimate 3D motion and reduce low dimensional tracking error for 3D complex cardiac motion, as suggested in our previous studies. By accounting for out-of-plane motion, strain imaging using 3D tracking may allow more accurate detection of abnormal myocardial deformation associated with myocardial ischemia or infarction.