Yoshiro Koiwa

Transcutaneous measurement and spectrum analysis of heart wall vibrations

For the noninvasive diagnosis of heart disease based on the acoustic and elastic characteristics of the heart muscle, it is necessary to transcutaneously measure small vibration signals, including components with an amplitude of less than 100 /spl mu/m, from various parts of the heart wall continuously for periods of more than several heartbeats in a wide frequency range up to 1 kHz. Such measurement, however, has not been realized by any ultrasonic diagnostic methods or systems to date. By introducing the constraint least-square approach, this paper proposes a new method for accurately tracking the movement of the heart wall based on both the phase and magnitude of the demodulated signal to determine the instantaneous position of the object so that the vibration velocity of the moving object can be accurately estimated. By this method, small vibrations of the heart wall with small amplitudes less than 100 /spl mu/m on the motion resulting from a heartbeat with large amplitude of 10 mm can be successfully detected with sufficient reproducibility in the frequency range up to several hundred Hertz continuously for periods of about 10 heartbeats. The resultant small vibration is analyzed not only in the time domain, but also in the frequency domain. As confirmed by the preliminary experiments herein reported, the new method offers potential for research in acoustical diagnosis of heart disease.

Noninvasive evaluation of local myocardial thickening and its color-coded imaging

For the noninvasive diagnosis of heart disease based on the acoustic characteristics of the heart muscle, we have developed a new method for accurately tracking the movement of the heart wall. By this method, a velocity signal of the heart wall with a small amplitude of less than 10 /spl mu/m on the motion resulting from a heartbeat with large amplitude of 10 mm can be successfully detected with sufficient reproducibility in the frequency range up to several hundred Hertz continuously for periods of about 10 heartbeats. In this paper, the method is applied to multiple points preset in the left ventricular (LV) wall along the ultrasonic beam so that the spatial (depth) distributions of the velocity at these points are simultaneously obtained. The motion of the heart wall is divided into the following two components: parallel global motion of the heart wall and the change in myocardial layer thickening at each depth across the LV wall during myocardial contraction/relaxation. The latter component is superimposed on the M (motion) mode image using a color code to map contraction as red and expansion as blue. By preliminary human studies, the principle of the method proposed in this paper is verified and the frequency band of the components generated by thickening and/or thinning in the myocardium is identified. This new approach offers potential for research on noninvasive acoustical diagnosis of myocardial local motility, that is, the myocardial layer function at each depth in the ventricular wall.

Elasticity Imaging of Atheroma With Transcutaneous Ultrasound Preliminary Study

Background Knowledge of the physical properties of atherosclerotic plaque is essential when evaluating its vulnerability in a clinical setting. Such knowledge, however, is still difficult to obtain with the various approaches developed to date. Methods and Results This article describes a noninvasive method for evaluating the regional elasticity (the elastic modulus in the circumferential direction) of tissue surrounding atherosclerotic plaque in which a novel phased tracking method is applied to measure minute changes in thickness of each of the multiple layers of the arterial wall during one heartbeat. By comparing the pathological findings with the distribution of elasticity, average elasticity of lipid and that of a mixture of smooth muscle and collagen fiber can be determined. On the basis of these reference parameters, each point is statistically categorized as lipid, mixture, or other. Thus, the plaque is electronically stained using transcutaneous ultrasound. By applying the method to the common carotid arteries, the presence of thin collagen fiber was clarified along the arterial axis for normal subjects, whereas soft inclusion of lipid was found for every plaque in subjects with hyperlipidemia. Conclusion This novel method offers potential as a diagnostic technique for detection of plaque vulnerability with high spatial resolution. (Circulation. 2003;107:3018‐3021.)

Real-time measurements of local myocardium motion and arterial wall thickening

We have already developed a new method, namely, the phased tracking method, to track the movement of the heart wall and arterial wall accurately based on both the phase and magnitude of the demodulated signals to determine the instantaneous position of an object. This method has been realized by an off-line measurement system, which cannot be applied to transient evaluation of rapid response of the cardiovascular system to physiological stress. In this paper, therefore, a real-time system to measure change in the thickness of the myocardium and the arterial wall is presented. In this system, an analytic signal from standard ultrasonic diagnostic equipment is analogue-to-digital (A/D) converted at a sampling frequency of 1 MHz. By pipelining and parallel processing using four high-speed digital signal processing (DSP) chips, the method described is realized in real time. The tracking results for both sides of the heart and/or arterial wall are superimposed on the M (motion)-mode image in the work station (WS), and the thickness changes of the heart and/or arterial wall are also displayed and digital-to-analogue (D/A) converted in real time. From the regional change in thickness of the heart wall, spatial distribution of myocardial motility and contractility can be evaluated. For the arterial wall, its local elasticity can be evaluated by referring to the blood pressure. In in vivo experiments, the rapid response of the change in wall thickness of the carotid artery to the dose of the nitroglycerine (NTG) is evaluated. This new real-time system offers potential for quantitative diagnosis of myocardial motility, early stage atherosclerosis, and the transient evaluation of the rapid response of the cardiovascular system to physiological stress.

Myocardial rapid velocity distribution.

Myocardial motion exhibits frequency components of up to 100 Hz, as found by a phased tracking method. To simultaneously measure the rapid and minute velocity signals at multiple points along the surface of the left ventricle (LV), in this study, conventional ultrasonic diagnosis equipment was modified to allow 10 scan lines from a sector scanner to be arbitrarily selected in real-time for analysis. By considering the maximum value of the velocity in the heart wall and the maximum depth from the chest surface, the number of transmission directions of the ultrasonic pulses should be carefully confirmed to be 10 to avoid aliasing, which is much less than the number employed in conventional tissue Doppler imaging (TDI). By applying the system, the velocity signals at about 240 points in the heart walls were simultaneously measured for three healthy volunteers. During a short period of 35 ms around end-diastole, the velocity signals varied spatially in the heart wall. At the end of systole, in the wavelets near the base of the interventricular septum (IVS), the slow pulse continued for about 30 ms, just before the radiation timing of the second heart sound. Then, a steep pulse occurred just at the timing of the closure of the aortic valve. The steep pulse at the base preceded that at the apex by several ms. By Fourier transforming each wavelet, the spatial distribution of the phase of the steep pulse components were clearly displayed. By applying the measurement method to two patients with aortic stenosis (AS), irregular vibration signals, which correspond to the murmur of the heart sound, could be directly detected during the ejection period. In conventional TDI, only the large slow movements due to the heartbeat are displayed, but these rapid and minute velocity components cannot be displayed. In this study, moreover, the phase components were detected for the first time from each of the velocity signals simultaneously measured at multiple points along the 10 scan lines. This measurement and method of analysis offer potential for new diagnostic techniques in cardiac dysfunction.

Evaluating the regional elastic modulus of a cylindrical shell with nonuniform wall thickness.

PurposeFor noninvasive diagnosis of atherosclerosis, we attempted to evaluate the elasticity of the arterial wall by measuring small changes in thickness caused by the heartbeat. The elasticity of the arterial wall has been evaluated noninvasively by measuring the change in diameter of the artery or the pulse-wave velocity; however, there is no method for noninvasively evaluating the elasticity of the arterial wall from changes in its thickness.MethodsEmploying the phased tracking method that we developed, changes in thickness of less than 100 µm were measured in each regional area, which corresponded to the diameter of the ultrasonic beam.ResultsThe elasticity of the arterial wall could be evaluated with better spatial resolution from the change in thickness than from the change in diameter of the artery or pulse-wave velocity. We therefore propose a method for evaluating the elastic modulus of an arterial wall of nonuniform wall thickness.ConclusionsIn basic experiments employing silicone rubber tubes with nonuniform wall thickness as arterial models, the elastic moduli of silicone rubber tubes were evaluated by measuring changes in wall thickness. These results confirm the value of the proposed method.

Accuracy Evaluation in the Measurement of a Small Change in the Thickness of Arterial Walls and the Measurement of Elasticity of the Human Carotid Artery

For the diagnosis of the early stages of atherosclerosis, it is important to evaluate the local acoustic characteristics of the arterial wall. For this purpose, it is necessary to increase the spatial resolution in the axial direction to several millimeters, which corresponds to the size of the macular lesion on the surface of the wall. We have proposed a method for measuring small velocity signals on the intima and adventitia of the arterial wall from the skin surface using pulsive ultrasonic waves. The small change in thickness of the arterial wall is obtained by integrating the difference between the two velocity signals on the intima and adventitia. The elastic property of the arterial wall is noninvasively evaluated from the change in thickness and the arterial inner pressure. In this paper, we evaluate the accuracy of the proposed method for measuring the small displacement. Moreover, we applied this method to evaluate the elastic property of the arterial wall of 50 patients and 8 healthy subjects.

Detection of lumen-intima interface of posterior wall for measurement of elasticity of the human carotid artery

In our series of studies on noninvasive assessment of the regional elasticity of the arterial wall, the displacement gradient (change in thickness) of the arterial wall caused by the heartbeat was measured by the phased tracking method. Because the displacement gradient corresponds to the strain due to the change in blood pressure, the elasticity can be evaluated from the displacement gradient of the arterial wall and the blood pressure, which are noninvasively measured at the upper arm. In the measurement of the elasticity of the arterial wall by our method, the region in which the elastic modulus is estimated must be assigned beforehand; currently, the lumen-intima boundary of the arterial wall is manually determined by the operator. For the real-time measurement of the elasticity of the arterial wall, a fast, automated method is necessary for detection of the boundary. In this paper, a cost function is proposed for differentiation of the arterial wall from the lumen. The proposed cost function was applied to ultrasound data, which were noninvasively obtained for five human carotid arteries. In comparison with the case of detection using only the amplitude of the echo, the root mean square error between the automatically detected lumen-intima boundary and the manually assigned boundary was significantly improved by using the proposed cost function. Furthermore, the lumen- intima boundary was automatically detected in a short period. Such a method is required for real-time measurement of the elasticity of the arterial wall, though detection of the outer boundary of the adventitia, which is not described in this paper, is also necessary to realize real-time elasticity measurement by our method.

Modified Phased Tracking Method for Measurement of Change in Thickness of Arterial Wall

In this study, the change in thickness of the arterial wall caused by the heartbeat was measured by the phased tracking method [IEEE Trans. UFFC. 43 (1996) 791] for noninvasive assessment of the regional elasticity of the arterial wall. In the phased tracking method, the change in thickness of the arterial wall is obtained from the difference between displacements of two points set along an ultrasonic beam. The displacement during the pulse repetition interval is determined by the phase of the complex correlation between the quadrature modulated ultrasonic waves. For suppressing noise components, the complex correlation function is spatially averaged in the region, which corresponds to the ultrasonic wavelength. However, spatial averaging of displacements is not desirable for measurement of the change in thickness, because the change in thickness is caused by the spatial inhomogeneity of displacements. In this paper, the phased tracking method was modified for direct estimation of the change in thickness without spatial averaging of displacements.

Modification of Human Left Ventricular Relaxation by Small-Amplitude, Phase-Controlled Mechanical Vibration on the Chest Wall

BACKGROUND Direct clinical manipulation to improve an impairment of left ventricular (LV) relaxation has not been reported. We investigated whether the LV relaxation rate in humans could be modulated by phase-controlled mechanical vibration applied to the patients anterior chest wall and whether there are some quantitative differences in the responses of normal (N), hypertrophied (H), and failing (F) ventricle. METHODS AND RESULTS In 46 patients (N, 10; H, 18 [hypertrophic cardiomyopathy]; F, 18 [heart failure]), the vibrator was attached to the precordium and a 50-Hz, 2-mm sinusoidal mechanical vibration was applied, with the timing restricted from the onset of isovolumic relaxation to end-diastole during cardiac catheterization. Heart rate and peak LV pressure showed no difference with vibration. However, in all patients, precordial vibration caused an acceleration of the LV pressure fall. The magnitude of the induced reduction of the time constant of LV pressure decay (delta T) was larger (P < .01) in H and F than in N (4.6 +/- 2.3, 4.0 +/- 1.6, and 0.6 +/- 1.5 ms for H, F, and N, respectively). Delta T correlated strongly with the magnitude of impaired relaxation and the magnitude of transmitted vibration to the ventricle. CONCLUSIONS Phase-controlled, small-amplitude vibration on the chest wall can directly modulate LV relaxation rate, especially in those with hypertrophy or failing ventricle.