Though myocardial viscoelasticity is essential in the evaluation of heart diastolic properties, it has never been noninvasively measured in vivo. By the ultrasonic measurement of the myocardial motion, we have already found that some pulsive waves are spontaneously excited by aortic-valve closure (AVC) at end-systole (T0) (IEEE UFFC-43(1996)791-810). Using a sparse sector scan, in which the beam directions are restricted to about 16, the pulsive waves were measured almost simultaneously at about 160 points set along the heart wall at a sufficiently high frame rate (UMB 27(2001)752-768). The consecutive spatial phase distributions clearly revealed wave propagation along the heart wall for the first time (IEEE UFFC51(2005)1931-1942). The propagation time of the wave along the heart wall is very small and cannot be measured by conventional equipment. Based on this phenomenon, we developed a means to measure the myocardial viscoelasticity in vivo. The phase velocity of the wave is determined for each frequency component. By comparing the dispersion of the phase velocity with the theoretical one of the Lamb wave, which propagates along the viscoelastic plate (heart wall) immersed in blood, the instantaneous viscoelasticity is determined noninvasively (IEEE UFFC-51(2005)1931-1942). In this study, the phase distribution obtained by the sparse scan is interpolated and extrapolated, and then the spatial distribution of the instantaneous phase velocity of the wave components propagating from the base side to the apical side of the heart wall is obtained for the longitudinal cross-sectional image.