Accurate ultrasonic measurement of myocardial regional strain rate at high temporal and spatial resolutions

Methods for imaging of strain rate in the heart wall are useful techniques for the quantitative evaluation of regional myocardial function. However, a mechanism of the transitions between myocardial contraction and relaxation is unclear. Except for a method based on ECG triggering, a required high temporal resolution was realized by scanning the heart wall sparsely at the expense of the lateral spatial resolution. Therefore, the spatial resolution in measurement of the transition of myocardial contraction / relaxation in the lateral direction have been limited. In this study, the RF data was acquired in a typical cross-sectional view (interventricular septum (IVS) longitudinal-axis view) based on parallel beam forming (PBF). A wide transmitted beam scanned 7 different directions sparsely and 16 receiving beams were created in each transmit. The typical cross-sectional image was realized to obtain with high spatial (the angle between neighbor beams was 0.375 degree) and temporal (the frame rate (FR) was 1020 Hz) resolution. In addition, the strain rate was obtained by spatial differentiation of the velocity distribution along the ultrasonic beam using the phased tracking method applied to multiple points in the heart wall. Slight spatial transition of contraction / relaxation in the axial and lateral directions during a very short period less than 10 ms was able to visualized by PBF. On the other hand, the transition in only the axial direction was visualized by sparse scan. Measurement of myocardial strain rate at high temporal and spatial resolutions was achieved using PBF. In vivo experimental results show a possibility of this method for elucidation of the transition of myocardial contraction and relaxation in two dimensions. It is supposed that such transition corresponds to the propagation of myocardial excitation along the conduction system of the heart (from sinoatrial node to Purkinje fibers).