Viscoelasticity Measurement of Heart Wall in in vivo Hiroshi Kanai Department of Electronic Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan. Email: hkanai@ecei.tohoku.ac.jp Abstract— By measuring spatial distribution of the minute vibrations in the heart wall from the chest wall using ultrasound, we find that some impulses propagate along the heart wall in healthy human subjects just after closure of the aortic valve for the first time. Their amplitude is found to be on the order of several tens of micrometers, and up to 100 Hz. Their propagation speed shows frequency dispersion, which agrees with the theoretical characteristics of the Lamb wave. The instantaneous viscoelasticity of the wall is then noninvasively determined. These findings have a novel potential for myocardial tissue characterization in clinical diagnosis. I. I NTRODUCTION Magnetic resonant imaging [1], [2], computer tomogra- phy [3], and conventional ultrasonography enable clinical visualization of cross-sectional images of the human heart, but their imaging is restricted to large motion (> 1 mm) and low frequency components (< 30 Hz). Analysis of the motion-mode (M-mode) image [4], [5]—the magnitude of the sequentially obtained radio frequency (RF) data acquired in conventional echocardiography—is a candidate for transtho- racic measurement of the heart-wall motion. However, the detectable amplitude is still greater than the wavelength, which is equal to 410 μm for ultrasound with a frequency of 3.75 MHz. The tissue Doppler imaging technique [6]– [10]—modified ultrasound two-dimensional (2-D) color flow mapping—enables us to acquire motion distribution of the my- ocardium. Even in this measurement, however, the sampling frequency of the displayed motion of the heart wall is low (at most 60 Hz). All these previous clinical methodologies, therefore, are employed to measure only large (> 0.4 mm) and slow (< 30 Hz) motion due to the heartbeat which can be recognized by medical doctors with the naked eye. By artificially actuating shear waves in tissues or phantoms, their propagation speed and/or viscoelasticity can be deter- mined for tissue characterization [11]–[17]. Yet spontaneously actuated vibrations propagating in the heart wall, which differ from electrically excited waves [18], [19], have not been recognized at all. In this paper, using a newly developed method to measure spatial distribution of the minute vibrations in the heart wall from the chest wall using ultrasound [20], we find that some impulses propagate along the heart wall in healthy human subjects just after closure of the aortic valve for the first time and their propagation speed shows frequency dispersion, which agrees with the theoretical characteristics of the Lamb wave [21], [22]. base side A aortic valve (open) root of aortic valve left ventricle apex side IVS PCG right ventricle 1 cm ECG 6 16 10 12 15 11 13 14 number beam 2 5 1 7 9 8 3 4 LV RV IVS LA US probe chest Ao valve RA Ao scan area 0 0 0 0 200 400 600 1000 1200 time from R-wave [ms] 800 0.05 m/s diastole systole T0 heart sound second isovolumic relaxation period PCG ECG 0.05 m/s velocity on RV-side of IVS velocity on LV-side of IVS B Fig. 1. (A) A cross-sectional image measured by a conventional ultrasound diagnosis system for a young healthy male. The upper-right illustration shows the scanning range of the ultrasonic beams in this imaging. (B) In vivo measurement results for the healthy man in Fig. A at two points set along the 13th ultrasonic beam. Each waveform for six consecutive cardiac cycles was overlaid. The timing of the aortic-valve closure is denoted by T 0 . (LV: left ventricle, LA: left atrium, RV: right ventricle, RA: right atrium, US probe: ultrasonic probe, IVS: interventricular septum, Ao: aorta, ECG: electrocardiogram, PCG: phonocardiogram (heart sound).) 482 0-7803-8412-1/04/$20.00 (c)2004 IEEE. 2004 IEEE International Ultrasonics, Ferroelectrics, and Frequency Control Joint 50th Anniversary Conference 2004 IEEE Ultrasonics Symposium