1Graduate School of Systems and Information Engineering, University of Tsukuba, 2Instituute for Human Science and Biomedical Engineering, National Institute of Advanced Industrial Science and Technology, 3Faculty of Medicine, Kagawa University, 4Sakaide City Hospital
Decreases in myocardial motion caused by changes in tissue stiffness often appear in the early stage of ischemic heart disease. Since the myocardium exhibits complex 3–D motion, 3–D assessment of the stiffness distribution is required for accurate diagnosis. However, conventional tissue Doppler and strain-rate imaging techniques cannot meet the above requirement completely because they are angle-dependent, use in-plane (2–D) processing, and suffer from aliasing in the case of large myocardial motion. In order to overcome these problems, we propose novel methods to track the 3–D motion by using a 2–D phased array with
a small aperture and to assess myocardial malfunction based on full strain tensors obtained by 3–D motion analysis. As a new method of 3–Dmyocardial motion tracking, we incorporated our phase-gradient method, which is capable of real-time 3–D displacement vector measurement using a 2–D phased array, and our combined autocorrelation method, which accurately measures large phase shifts at each element without aliasing. As a new method of assessing a myocardial ischemic region, the full strain tensor invariant obtained by the measured 3–D vectors is visualized as a 3–D myocardial strain image. We evaluated the feasibility of the proposed methods by numerically simulating left ventricle short-axis imaging of a 3–D elliptic myocardial model including infarction. RF signals received at each element on the 2–D array probe were faithfully simulated. When the simulated echoes were processed by the proposed methods, the invariant image obtained by the full strain tensor clearly depicted
the hard infarction area where conventional imaging could not. The proposed methods were also modified for systems
with an ordinary 1–D array probe. Finally, a phantom experiment was conducted by using the basic system based on the 1–D array probe. These results validated the easibility of the proposed methods.