Study on Ultrasonic Elasticity Imaging forArterial Tissue Characterization
Hiroshi Kanai*1,2)and Hideyuki Hasegawa2,1)
1) Department of Electronic Engineering, Graduate School of Engineering2) Department of Biomedical Engineering, Graduate School of Biomedical Engineering
E-mail: hkanai@ecei.tohoku.ac.jp

Abstract
Knowledge of the physical properties ofatherosclerotic 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. We have developed anoninvasive method for evaluating the regionalelasticity (the elastic modulus in the circumferentialdirection) of tissue surrounding atherosclerotic plaquein which a novel approach is applied to measure minutechanges in thickness of each of the multiple layers of
the arterial wall during one heartbeat. By comparing
the pathological findings with the distribution ofelasticity, average elasticity of lipid and that of a
mixture of smooth muscle and collagen fiber can bedetermined. Based on these reference parameters, eachpoint is statistically categorized as lipid, mixture, orother. Thus, the plaque is electronically stained usingtranscutaneous ultrasound. This novel method offers
potential as a diagnostic technique for detection ofplaque vulnerability with high spatial resolution.

atherosclerosis, is becoming a serious problem.
Therefore, it is important to diagnose atherosclerosis in
the early stage. Computed tomography (CT), magnetic
resonance imaging (MRI) and ultrasonic diagnostic
equipment are employed for the diagnosis ofatherosclerosis.
However, they provide information only on the
shape of the artery such as the diameter of the lumen.
In addition, CT and MRI are not suitable for repetitivediagnosis due to the exposure to radiation and the highexpense. However, the diameter of the lumen, which
can be measured by these methods, is not changed byearly-stage atherosclerosis. In addition, it is essential toevaluate the mechanical properties and tissue
composition of atherosclerotic plaque for diagnosing
its vulnerability. Therefore, it is necessary to develop amethod for measuring the elastic property of thearterial wall.

2. Conventional Measurements of Elasticity
To measure the elasticity of the arterial wall, theFig. 1. Tissue classification of arterial wall by noninvasive elasticity measurement.

1. Introduction
The steady increase in the number of patients withmyocardial infarction or cerebral infarction, both ofwhich are considered to be mainly caused bypulse wave velocity (PWV) method was developed as atechnique for the noninvasive diagnosis ofatherosclerosis [1]. In this method, the elasticity of thearterial wall is evaluated by measuring the velocity ofthe pressure wave propagating from the heart to thefemoral artery. Though it is useful in terms of
noninvasive evaluation of elasticity, regional propertiescannot be evaluated due to a low spatial resolution oftens of centimeters, which almost corresponds to thedistance from the heart to the femoral artery.
Methods for measurement of the change in arterydiameter have been proposed. in order to obtain thecircumferential distensibility of the arterial wall in theplane which is perpendicular to the axial direction of
the artery. By assuming the artery to be a cylindricalshell, the average elasticity of the entire circumferenceof the plane can be evaluated [2,3]. However, theregional elasticity of atherosclerotic plaque cannot be
obtained by these methods, because an artery with suchplaque cannot be assumed to be a cylindrical shell withuniform wall thickness.

3. Exploration of High Resolution UltrasoundElasticity Imaging
In our series of studies, we have been attempting toevaluate the elasticity of a local region, even in the caseof atherosclerotic plaque using transcutaneous
ultrasound (Fig. 1) [4-6]. Such a technique fornoninvasive measurement of the spatial distribution of
the regional elasticity would be useful for diagnosis of
the vulnerability of such plaque as well as for thediagnosis of early-stage atherosclerosis. In our method,
the small displacement gradient (change in thickness)of the arterial wall due to the heartbeat was accurately
measured in each local region which corresponds to the
focal area of the ultrasonic beam. From the resultant
displacement gradient, the regional strain and theelasticity of the arterial wall were noninvasivelyevaluated. Recently many studies were conducted tomeasure the elasticity of the arterial wall withultrasound [7-9].
In addition to diagnosis of developed atherosclerosis,
it is also important to diagnose atherosclerosis in its
early stage. Ross hypothesized that endothelium cells,
which covers the luminal surface of the arterial wall(Fig. 2), are injured in the early-stage atherosclerosis[10]. The endothelial function can be evaluated bymeasuring the change in elasticity of the arterial wall
due to the endothelial-dependent dilation after
avascularization because the endothelium cells
produces nitric oxide (NO) in response to the increased
shear stress due to recirculation and NO produced byendothelium cells relaxes smooth muscle in the media.
As described above, ultrasound elasticity imaging of
the arterial wall has various possibilities for accurate
diagnosis of atherosclerosis.

4. Conclusion
High-resolution images of arterial walls can beobtained by recent ultrasonic diagnostic equipment.
However, morphological information is mainly derived
from such cross-sectional ultrasound images whereas
the mechanical properties and tissue composition in thearterial wall are more important for diagnosis ofatherosclerosis. Ultrasound elasticity imaging based on
the detailed analysis of ultrasound signal provides
additional useful information for accurate diagnosis ofboth developed and early-stage atherosclerosis.

Fig.2. Structure of arterial wall [10].

References[1] Hallock P. Arterial elasticity in man in relation toage as evaluated by the pulse wave velocity method.
Arch Int Med 54, 770-798, 1934.
[2] Hoeks APG, Ruissen CJ, Hick P, and Reneman RS.
Transcutaneous detection of relative changes in arterydiameter. Ultrasound Med Biol 11, 51-59, 1985.
[3] Hayashi K, Handa H, Nagasawa S, Okumura A,and Moritake K. Stiffness and elastic behavior ofhuman intracranial and extracranial arteries. J Biomech13, 175-184, 1980.
[4] Kanai H, Sato M, Chubachi N, and Koiwa Y.
Transcutaneous measurement and spectrum analysis ofheart wall vibrations. IEEE Trans Ultrason Ferroelectr
Freq Control 43, 791-810, 1996.
[5] Hasegawa H, Kanai H, Hoshimiya N, and Koiwa
Y. Evaluating the regional elastic modulus of acylindrical shell with nonuniform wall thickness. JMed Ultrason 31, 81-90, 2004.
[6] Kanai H, Hasegawa H, Ichiki M, Tezuka F, andKoiwa Y. Elasticity imaging of atheroma withtranscutaneous ultrasound -preliminary study-.
Circulation 107, 3018-3021, 2003.
[7] de Korte CL, Céspedes, van der Steen AFW, andLancée. Intravascular elasticity imaging using
ultrasound: Feasibility studies in phantoms. Ultrasound
Med Biol 23, 735-746, 1997.
[8] Rabben SI, Bjærum S, Sørhus, and Torp H.
Ultrasound-based vessel wall tracking: An autocorrelation technique with RF center frequency
estimation. Ultrasound Med Biol 28, 507-517, 2002.
[9] Maurice RL, Ohayon J, Frétigny Y, Bertrand M,Soulez G, and Cloutier G. Noninvasive vascularelastography: Theoretical framework. IEEE Trans Med
Imaging 23, 164-180, 2004.
[10] Ross R. Atherosclerosis -an inflammatory disease-.
New Engl J Med 340, 115-126, 1999.