Electromechanical imaging of the myocardium at normal and pathological states (2006)

The motion of the myocardium is mainly due to the contraction and the relaxation of the cardiac muscle. However besides this slow and large motion component, more rapid displacements occur at several periods of the cardiac cycle. Indeed, the contraction of the myocardium is induced by electrical waves that propagate very fast (about 1 m/s) in the cardiac tissue. Due to the electrical excitation, the fibers' contraction results in a strong mechanical wave that propagates in the myocardium. We propose here a method for measuring the electromechanical coupling properties in the myocardium. Our method is based on imaging and analyzing the delay in small tissue displacements resulting from the propagation of this contraction wave. In-vivo experiments are performed in anesthetized normal and ischemic animals. The contraction wave is first observed in dogs using a commercial clinical scanner. The displacement maps are estimated using a 1D cross-correlation- based technique. The displacement maps clearly show the propagation of a strong mechanical wave along the circumference of the myocardium. The speed of the wave is found to be lower in the ischemic region. Then, thanks to the high frame rate capability of an ultrasound scanner for small animals (Vevo 770, visualsonics), some sequences of approximately three cardiac cycles are acquired at a frame rate of 4000 images/s in mice. The wave velocity is found to be approximately 0.87 m/s in the posterior wall. Temporary regional ischemia is then induced by coronary artery ligation. The velocity of the electromechanical wave is found to decrease to approximately 0.66 m/s in the ischemic region. myocardial viability. Using these techniques, the evaluation of the heart function is based on the mechanical interpretation of the global heart contraction (systole) and relaxation (diastole). However, the contraction of the cardiac muscle is an active process that results from electrical excitations. During the cardiac cycle, electrical waves propagate in the myocardium in order to initiate the contraction and relaxation of the myocardium. The fiberscontraction induces a strong mechanical wave that propagates in the myocardium. Since this wave results from coupling of the electrical excitation and the mechanical response of the tissue, it is hereby named 'electromechanical wave'. This electrical activation occurs in a very short time compared to the following contraction or relaxation of the muscle and therefore cannot be detected in most of the conventional imaging devices. Moreover, some evidences of mechanical vibrations have been shown by Kanai et al. (5),(6), in human patients. They demonstrated that several pulsive low frequency mechanical vibrations were obtained around end-systole and end-diastole in the frequency range of 25 to 100 Hz. These waves propagate in the myocardium with high velocities up to 5m/s. We propose here a new approach to image the local mechanical properties of the myocardium. Our method is based on imaging small displacements of the myocardium at high frame rate. This method allows imaging the propagation of the mechanical waves induced both by electrical excitations and mechanical excitation. The wave speed is a function of both electrical and mechanical properties of the myocardium, i.e., the electrical conductivity and the shear modulus, this method could potentially be used for early, noninvasive and simultaneous detection of electrical and mechanical dysfunctions in the heart. The goal of this paper is to present preliminary results demonstrating the feasibility of imaging in vivo the electromechanical coupling mechanisms in the heart.