Kazuhiro Sunagawa

Simultaneous measurement of blood flow and arterial wall vibrations in radial and axial directions

The arterial wall expands and contracts during one heartbeat. At the beginning of systole, there is a possibility that vibrations on the arterial wall are caused by wall shear stress due to the rapid increase of blood flow. It is well known that crisis of atherosclerosis and rupture of plaque are basically caused by blood pressure and wall shear stress applied to the arterial wall. In the literature, wall shear stress is estimated by computer simulation. However, measurements of arterial wall vibrations in radial and axial directions as well as their relation to blood flow have not been reported yet. In this paper, by steering ultrasonic beams in two directions, the radial and axial components of arterial wall vibrations and blood flow velocity are simultaneously measured along the two directions. The relationship between the arterial wall vibrations and blood flow near the wall is evaluated based on the time-frequency analysis. In in vivo experiments, the method was applied to the carotid artery of a healthy subject. From experimental results, the radial and axial components of the arterial wall vibrations were measured together with the blood flow near the wall. A clear correlation was observed for each direction component of the vibrations and blood flow velocity. Since arterial wall vibration is caused by change in the blood pressure and shear stress applied to the wall due to the blood flow, the above results might be a clue to estimate the shear stress applied to the arterial wall from measurement of both the wall vibrations and blood flow.

Measurement of shear wave propagation and investigation of estimation of shear viscoelasticity for tissue characterization of the arterial wall.

PurposeThe aim of this study was to find an array of frequency components, ranging from 0 Hz (direct current) to several tens of hertz that comprise the small vibrations on the arterial wall using noninvasive in vivo experiments. These vibrations are caused mainly by blood flow. The viscoelasticity of the arterial wall was estimated from the frequency characteristics of these vibrations propagating from the intima to the adventitia.MethodsPropagation of these frequencies in human tissue displays certain frequency characteristics. Based on the Voigt model, shear viscoelasticity can be estimated from the frequency characteristics of the propagating vibrations. Moreover, we estimated shear viscoelasticity from the measured frequency characteristics of shear wave attenuation.ResultsShear wave propagation from the intima to the adventitia resulting from blood flow was explained theoretically based on the obtained measurements. Shear viscoelasticity was also estimated from the measured frequency characteristics of shear wave attenuation.ConclusionsBased on the proposed method, shear viscoelasticity can be estimated from ultrasonographic measurements. These results have a novel potential for characterizing tissue noninvasively.

Simultaneous measurement of vibrations on arterial wall upstream and downstream of arteriostenosis lesion and their analysis

Acute myocardial infarction and cerebral infarction are generally known to be caused primarily by the rupture of atherosclerotic plaques. It is thus necessary for clinical treatment to predict the rupture of these plaques. Blood-flow velocity around atherosclerotic plaques increases as the arteriostenosis lesion progresses, resulting in turbulence downstream of the lesion. The resulting change in blood pressure produces shear stress, and change in this stress affects the rupture of the atherosclerotic plaques. Cerebral ischemic paroxysm and cerebral infarction have been reported to occur in a high percentage of cases in which inner vessel diameter has decreased to less than 70% of its original diameter as a result of stenosis. This explains the use of standard ultrasonic diagnostic equipment to measure blood flow in the screening of the carotid arteries. On the other hand, the noise signal radiated from an aneurysm as a result of blood flow has been measured using the bruit sensor used to diagnose cerebrovascular diseases. Many unsolved problems with regard to the relationship between noise and turbulence in blood flow remain, however. Here, small vibrations on the arterial wall were measured transcutaneously and analyzed both upstream and downstream of the atherosclerotic plaque of a human carotid artery. Characteristics of the resultant vibrations upstream of the stenosis clearly differed from those downstream of it. These results should prove useful in predicting the rupture of atherosclerotic plaques.

Simultaneous measurement of vibrations on the arterial wall downstream and upstream from an atherosclerotic lesions

In the literature, it has been reported that cerebral ischemia paroxysm and cerebral infarctions occurs with high percentages when the inner diameter is decreased less than 70% due to a stenosis. The most primary reason of these events is rupture of atherosclerotic plaques of the carotid artery. However, it is difficult to noninvasively predict the rupture. For solving this problem, the authors have investigated the influence of blood flow on the atherosclerotic plaque by transcutaneously measuring small vibrations caused by the pulsatile flow on the arterial wall. In this paper, small vibrations on the carotid arterial wall are measured and analyzed for patients with atherosclerosis and for healthy subjects. From in vivo experimental results, high frequency components were present in the resultant small vibrations on the wall of downstream from the atherosclerotic plaque. Moreover, from experiments using a silicone tube with small pressure sensors, it is found that there is close relationship between the vibrations and inner pressure for both cases with and without an artificial stenosis. These results might open the possibility to predict rupture of atherosclerotic plaques in future by measuring the small vibrations of the arterial wall.

Measurement of Shear Wave Propagation and Investigation of Estimation of Shear Viscoelasticity for Tissue Characterization of the Arterial Wall

はじめに : 超音波で動脈壁の微小振動を高精度に計測する位相差トラッキング法を用いて, 経皮的にヒト頸動脈の壁振動を計測した結果, 血流が主因と考えられる直流から数十Hzの周波数成分を含んでいることが分かった. これら動脈壁振動の内膜から外膜への伝搬周波数特性から, 動脈壁の粘弾性特性の推定を試みた. 方法 : 動脈壁組織をHookeの法則が成り立つVoigtモデルと仮定することによって, 求めた振動伝搬減衰の周波数特性から組織の粘弾性定数を推定する手法を提案し, 動脈壁の内膜と外膜の振動速度を超音波で同時計測した結果から, 動脈壁振動の伝搬減衰の周波数特性を求め, 動脈壁組織のずり粘弾性定数の推定を行った. 結果と考察 : 内膜側と外膜側の壁振動速度波形の周波数ごとの関連性を評価することにより, 血流により動脈壁内表面に直流から数十Hzまでの周波数帯域を持った微小振動が発生し, 内膜側から外膜側に伝搬していることが分かった. また, このずり弾性波の伝搬減衰の周波数特性から, 健常者の総頸動脈壁のずり弾性定数, ずり粘性定数の推定を試みた. 結語 : 本手法は, 他の加振源や応力計測の手段を必要とせず, 超音波で経皮的に計測した動脈壁振動の伝搬特性から組織の分別・同定を実現できる可能性を示唆している.

Time-frequency analysis of vibration propagation from intima to adventitia of arterial wall

Based on the phased tracking method, it is newly found that there are wide frequency components from d.c. to one hundred several tens Hz in the vibrations on the arterial wall non-invasively measured in in vivo experiments. These vibrations are mainly caused by the blood flow, and the wall vibration propagates from the intima to the adventitia. Moreover, the characteristics of the arterial wall vibration highly depend on the viscoelasticity of tissue components. In this paper, therefore, the propagation of the vibration in the regional area of the arterial wall is analyzed during one cardiac cycle in the frequency domain. From the measurement of the frequency characteristics in the vibration propagation, the tissue viscoelasticity is estimated. In the low frequency range, both the attenuation and the phase of the vibration are minute values, therefore, it is so difficult to measurement of these precisely. For solving this problem, the viscoelasticity of the arterial wall is estimated by the frequency characteristics in the attenuation in the wide frequency range up to one hundred several tens Hz. The proposed method was applied to human common carotid arteries of two patients with atherosclerotic plaques, a smoking subject and a healthy subject. From in vivo experimental results, clear differences are found in the the frequency characteristics in the attenuation and the estimated viscoelasticity of the arterial wall between atherosclerotic plaque of the patient and healthy subject. These results has novel potential for the tissue characterization.