Accuracy evaluation in ultrasonic-based
measurement of microscopic change inthickness
H. Kanai, K. Sugimura, Y. Koiwa and Y. Tsukahara
For the diagnosis of atherosclerosis, one acoustical method involves
measuring the change in thickness of the arterial wall during one
cardiac cycle and then noninvasively evaluating the elastic
properties of the arterial wall. An alternative method for obtaining
such a diagnosis is proposed, the phased tracking method, which
can be used to accurately measure the instantaneous displacement
signals on the intima and adventitia of the arterial wall from theskin surface using pulsive ultrasonic waves. A minute change in thethickness of the arterial wall of several micrometres is obtained byintegrating the difference between two instantaneous displacement
signals. Using a rubber plate in a water tank, the accuracy of the
proposed method is evaluated by measuring such microscopicchanges in thickness.

Introduction: During the ejection period, a pressure wave from theheart expands the lumen of the artery, resulting in the arterial wall
becoming slightly thinner. In the literature, the change in diameter ofthe lumen has been measured using ultrasound, and then based on
this, the elasticity has been diagnosed by assuming that the arterial
wall has homogeneous elasticity in the circumferential direction, an
assumption which has been an obstacle to the evaluation of theregional characteristics of atherosclerotic plaque.
Conversely, the measurement of the change in wall thickness
makes it possible to evaluate the elasticity of the arterial wall in each
local area, the locality in the circumferential direction being indispensable to the diagnosis of the inner characteristics of atherosclerotic plaque. To this end, by tracking the instantaneous positions,xA(t) and xB(t), of two points A and B, which are preset at the enddiastole along an ultrasonic beam in the arterial wall, the smallchange in wall thickness, ∆hAB(t), between these two points, A and B,
is obtained during the cardiac cycle by keeping the ultrasonic beam
perpendicular to the wall. From the ratio of the change in thickness,∆hAB(t), to the thickness, h0, between these points preset at the enddiastole, the deformation is obtained. If the deformation is sufficiently small and is in the linear regime, the deformation shows thestrain, and the regional elasticity of the wall is approximately evaluated using the pulse pressure noninvasively measured at the brachialartery [1].
The accuracy required in the simultaneous measurement of theinstantaneous positions xA(t) and xB(t) of points A and B is estimated as follows: the thickness, h0, of the wall is ~1 mm and thechange in thickness, ∆hAB(t), ranges from several micrometres to several tenths of a micrometre during one cardiac cycle for the carotid
artery of normal adults. Therefore, the necessary spatial resolution
in the measurement of the instantaneous position of a waveform is atleast several micrometres.
Although motion (M)-mode echocardiography offers advantages
in critically looking at the motion pattern of the wall, its spatial resolution along the ultrasonic beam is limited to a few wavelengths, i.e.
at most ~1mm for ultrasound at 7MHz.
For the accurate detection of instantaneous displacement on or in
the heart wall or arterial wall, we have developed the following
phased tracking method [2, 3]. In this method, by calculating theconstraint autocorrelation function between the sequentially
received echoes, the phase change caused by the movement of eachpoint (i), (i = A, B) during the pulse repetition period ∆T(= 1/PRF)
is accurately determined and the instantaneous displacement dI(t)during the period is obtained. By adding dI(t) to the previous object
position, xI(t), the next position, xI(t+∆T), is estimated by xI(t) +dI(t) and then the object position is recursively adjusted. Thismethod has been applied to the in vivo detection of small velocity
signals with sufficient reproducibility at points in the interventricular
septum (IVS) of the human heart [2]. The detected instantaneous
displacement shows rapid motion including high frequency components with small amplitudes, which cannot be recognised by Mmode echocardiography.
Moreover, the method has been applied to two points preset along
an ultrasonic beam in the heart/arterial wall so that the instantaneous object positions are obtained for these two points in the IVS[2, 3] and the carotid artery [1]. From the results, by deleting the par-ELECTRONICS LETTERS

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allel component, the thickness change component during one cardiac cycle is detected.
The measurement accuracy of the change in thickness, whichshould be evaluated, is several micrometres during one cardiac cycle.
However, such a microscopic change cannot be easily measured byany other methods in water. Thus, an apparatus with a rubber plate
was assembled and changes in thickness were theoretically derived
by assuming that its Poisson ratio was 0.5. The change in thickness
detected by the phased tracking method was quantitatively compared to the theoretically obtained change and its accuracy evaluated.

Fig. 1 Experimental apparatus using rubber plate which is drawn by stepping motor to achieve minute changes in thickness
Change characteristics theoretically derived using eqn. 3

Experimental apparatus: Fig. 1 shows the experimental apparatus.
The rubber plate with a length l0 of 100.5mm, a thickness h0 of
1.65mm, and a width w0 of 28mm is set in a water tank and bothends are connected to stainless steel wires with a diameter of 0.3mm,
one end being drawn by a stepping motor with a rotation rate of
1.5Hz. At the bottom right of Fig. 1, by applying the cosine theorem
to the triangle POC, the squared distance dPC(t) between point P onpulley A and rotating point C on the cam is given by

where d0 = 285.9mm and r = 0.5mm are, respectively, the distance
between P and Q and the radius of the locus of point C; angle α is49.8 °. The change in length, ∆l(t), of the rubber plate due to therotation of point C is given by ∆l(t) = dPC(t) – d0. From the definitionof Poisson ratio σ,

where ∆hT(t) is the theoretical value of the instantaneous change inthickness between points A and B on both sides of the rubber plate.
By assuming that the Poisson ratio σ of the rubber is 0.5, ∆hT(t) isdescribed by

For ultrasonic measurement, on the other hand, standard ultrasonic diagnostic equipment and an ultrasonic probe operated at5MHz are employed. The pulse repetition interval ∆T is 133 µs. The
received signal is amplified and demodulated. The resultant real andimaginary signals are simultaneously A/D converted with a 12 bit
A/D converter at a sampling rate of 2MHz. Using a computer, thephased tracking method is applied to the digital signals and thechange in thickness, ∆hUS(t), of the rubber plate is obtained.

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Conclusions: We have experimentally evaluated the accuracy of a
new method for measuring small changes in the thickness of thearterial wall using ultrasonic techniques. The experimental results
indicate that the measurement of minute changes in thickness of thearterial wall with an accuracy of < 1mm is achievable for the diagnosis of its acoustic characteristics.
© IEE 1999
Electronics Letters Online No: 19990669
DOI: 10.1049/el:19990669

17 March 1999

H. Kanai and K. Sugimura (Department of Electrical Engineering,Graduate School of Engineering, Tohoku University, Aramaki-aza-Aoba
05, Aoba-ku, Sendai 980-8579, Japan)
Y. Koiwa (First Department of Internal Medicine, Tohoku University
School of Medicine, Seiryou-machi, Aoba-ku, Sendai 980-8574, Japan)

Fig. 2 Experimental resultsa Output of position sensor for detection of origin angle of motorb Change in thickness measured by phased tracking method (∆hUS(t))
and theoretical method (∆hT(t))

Experimental results: Fig. 2 shows the experimental results. Fig. 2ashows the output of the position sensor for the detection of the origin angle of the motor and Fig. 2b shows the experimental results,∆hUS(t), obtained by the phased tracking method and the theoreticalvalue, ∆hT(t), calculated from eqns. 1 and 3. During the rotation of
the cam, the peak-to-peak value of ∆l(t) is equal to 2r = 1mm andthen the theoretical value of the change in thickness, ∆hT(t), ranges
from 0 to 8.2 µm as shown in the Figure. By comparing ∆hUS(t) with∆hT(t), such a minute change in thickness is successfully detected in∆hUS(t). The root of the average squared difference of ∆hUS(t) from∆hT(t) is 0.499µm. From these experimental results, the small changein thickness is successfully detected with submicrometre order accuracy.

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Y. Tsukahara (Department of System Information Sciences, Graduate
School of Information Sciences, Tohoku University, Katahira SKK Bldg.,
Aoba-ku, Sendai 980-8577, Japan)

References
1

HASEGAWA, H., KANAI, H., HOSHIMIYA, N., CHUBACHI, N., and KOIWA, Y.:‘Accuracy evaluation in measurement of a small change in thethickness of arterial walls and the measurement of elasticity of thehuman carotid artery’, Jpn. J. Appl. Phys. 1, 1998, 37, (5B), pp. 3101–3105
2 KANAI, H., SATO, M., KOIWA, Y., and CHUBACHI, N.: ‘Transcutaneous
measurement and spectrum analysis of heart wall vibrations’, IEEE
Trans., 1996, UFFC-43, (5), pp. 791–810
3 KANAI, H., HASEGAWA, H., CHUBACHI, N., KOIWA, Y., and TANAKA, M.:‘Noninvasive evaluation of local myocardial thickening and its colorcoded imaging’, IEEE Trans., 1997, UFFC-44, (4), pp. 752–768

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