Ultrasonics 38 (2000) 381–386
www.elsevier.nl/locate/ultras

Real-time velocimetry for evaluation of change inthickness of arterial wall
Hiroshi Kanai a, *, Yoshiro Koiwa b
a Department of Electrical Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japanb First Department of Internal Medicine, Tohoku University School of Medicine, Seiryou-machi, Aoba-ku, Sendai 980-8574, Japan

Abstract
We previously developed a new method, namely, the phased tracking method, for accurately tracking the movement of theheart wall and arterial wall based on both the phase and magnitude of the demodulated signals to determine the instantaneous
position of an object. By this method, the local change in wall thickness during one heartbeat can be determined. We have now
developed a real-time system for measuring change in thickness of the myocardium and arterial wall. In this system, four highspeed digital signal processing (DSP) chips are employed for obtaining the initially developed method in real time. The tracking
results for both sides of the wall are superimposed on the M (motion)-mode image in the workstation, and the thickness changesof the arterial wall are displayed in real time. Using this system, reported herein, velocity signals of the arterial wall withamplitudes less than several micrometers can be successfully detected in real time with sufficient reproducibility. The elasticity of
the arterial wall is 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 nitroglycerine (NTG) is evaluated for a young healthy subject and a young smoker.
This new real-time system offers potential for quantitative diagnosis of early-stage atherosclerosis by the transient evaluation of
the rapid response of the cardiovascular system to physiological stress. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Atherosclerosis; Elasticity; Local change in thickness; Nitroglycerine; Phased tracking method; Real-time system; Stress test;
Transient response

1. Introduction
At the beginning of the ejection period, the arterial
wall is affected by the pressure wave, which comes fromthe left ventricle (LV ) and expands the lumen.
Consequently, the arterial wall becomes slightly thin
during the ejection period. In the literature, measurements of the change in diameter, Dd (t), of the lumenBA∞
in Fig. 1 have been reported. From Dh (t), the elasticity
ABof the arterial wall can be determined, but it is assumed
that the arterial wall is homogeneously changed in thecircumferential direction due to atherosclerosis.
However, measurement of the change in thickness of
the wall makes it possible to evaluate the elasticity of
the arterial wall in each local area, the locality in thecircumferential direction being indispensable to diagnosis of the inner characteristics of atherosclerotic plaque.
For this purpose, by tracking the instantaneous posi* Corresponding author. Fax: +81-22-263-9444.
E-mail address: hkanai@ecei.tohoku.ac.jp (H. Kanai)tions, x (t) and x (t), which are respectively preset at
AB
x (t ) and x (t ) at the end-diastole along the ultrasonic
A 0
B 0
beam in the arterial wall, as illustrated in Fig. 1(b), thesmall change in wall thickness, Dh (t), between theseAB
two points, A and B, is obtained [1], as illustrated inFig. 1(c) when the ultrasonic beam is almost perpendicular to the wall. From the ratio of the change in thickness,
Dh (t), to the thickness, h (t ), between these pointsAB
AB 0preset at the end-diastole, the deformation is obtained.
If the deformation is sufficiently small and is in thelinear regime, it shows the strain, and thus the regionalelasticity of the wall can be approximately evaluatedusing the pulse pressure measured at the brachial artery.
Therefore, the transcutaneous measurement of thechange in thickness of the regional area of the arterial
wall during each cardiac cycle provides the essential
tools for diagnosis of atherosclerosis.
The accuracy required for the simultaneous measurement of the instantaneous positions x (t) and x (t) ofAB
points A and B is estimated as follows. For the carotid
artery, the thickness of the wall is about 1 mm, and the0041-624X/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0 0 4 1 -6 2 4 X ( 9 9 ) 0 0 11 8 - 3

382

H. Kanai, Y. Koiwa / Ultrasonics 38 (2000) 381–386

Fig. 1. Illustration explaining the procedure of measuring the change
in thickness, Dh (t), of the arterial wall, and the change in diameter,AB
Dd (t), of the lumen by tracking the movement x (t) and x (t) ofBA∞
AB
points A and B preset in the arterial wall along the ultrasonic beam
[1]. (a) Cross-sectional image. (b) Displacement x (t) and x (t). (c)A
B
Change in thickness, Dh (t) of the arterial wall and change in diameABter, Dd (t).
BA∞change in thickness, Dh (t), is less than 100 mm during
AB
one cardiac cycle in normal adults [1]. Therefore, thenecessary spatial resolution in the measurement of
instantaneous position as a waveform is at least 10 mm.
If the velocity signals, v (t) and v (t), of these twoABpoints, A and B, are detected based on the pulse Doppler
method, the necessary accuracy of the velocity measurement is about 10 mm/200 ms=0.05 m/s when the pulse
repetition frequency (PRF ) of the transmission-pulse
train is 5 kHz (=1/200 ms). If the equivalent sampling
period of the velocity signal, v (t), is longer than 200 msi
as in the FFT (fast Fourier transform)-based standard
Doppler system, for example, a more accurate velocity
measurement is required.
Though M-mode echocardiography offers an advantage in critically examining the motion pattern of thewall, its spatial resolution along the ultrasonic beam islimited to a few wavelengths, namely, at most about
several hundred micrometers for ultrasound of 7 MHz
because an M-mode image is displayed based on theamplitude of the reflected ultrasound. However, numerous elaborate techniques have been proposed for noninvasive measurement of the velocity of the blood flowin the arteries based on the Doppler effect. Moreover,
several methods, including the phase-locked-loop (PLL)

techniques, have been proposed to measure roughchanges in the diameter of the arterial walls by tracking
arterial wall displacement in real time.
For the accurate detection of the velocity signal, thatis, the instantaneous movement, on or in the heart wallor arterial wall, we have developed the following phased
tracking method [1]. In this method, by calculating theauto-correlation function with the constraint least-meansquares technique between the sequentially received
echoes, the phase change caused by movement of thepreset point (i) during the pulse repetition period DT(=1/PRF ) can be accurately determined, and theaverage velocity, v (t), during the period can be obtained.
i
By adding the product of v (t) and DT to the previousiobject position x (t), the next position, x (t+DT ), isiiestimated as x (t)+v (t)×DT. This method has beeniiconfirmed by experiments using a water tank and has
been applied to the in vivo detection of low-velocity
signals, with sufficient reproducibility, at points in theinterventricular septum (IVS ) of the human heart [1].
The detected velocity signal shows rapid motion, including high-frequency components with small amplitudes,
which cannot be recognized by M-mode echocardiography.
Moreover, the method has been applied to multiple
points preset along an ultrasonic beam in the LV wall
so that the instantaneous object positions {x (t)} andithe velocity signals {v (t)} are obtained for these multipleipoints {i} in the LV wall [2]. The wall changes position
and thickness with time. By making the location of theRV side of the IVS the reference point, the thickness
change during myocardial contraction/relaxation can bedetected. Then, their spatial distribution can be obtained
and can be superimposed on the M-mode image using
color coding.
The method of measurement herein described wasdeveloped by batch processing on an off-line system in
our laboratory. However, the quantity of A/D converted
data is considerable, and it is time-consuming to transfer
the data from the A/D converter to a computer in theoff-line system. Thus, real-time processing is indispensable in obtaining new clinical examination tools for an
evaluation of transient change in the tolerance test.
The development and application of real-time processing significantly facilitate its use in clinical diagnosis
and will offer a new tool for clinical examination. For
example, nitroglycerine (NTG) is used medically as avasodilator for rapid treatment of angina pectoris [3].
Real-time measurement of the transient change in thickness of the heart wall or the arterial wall just after the
sublingual administration of NTG will be of use in a
direct evaluation of the response of the heart wall or
the arterial wall to the antianginal drug. At the sametime, real-time monitoring during clinical intervention
is essential for avoiding hyper-responses such as serious
hypotension and/or arrhythmia.

H. Kanai, Y. Koiwa / Ultrasonics 38 (2000) 381–386

383

Fig. 2. In vivo experimental results of the vibration at points (A) on the adventitia side and (B) on the intima side of the anterior wall of the
carotid artery [1]. (a) Tracking results {x/ (t)} and {x/ (t)} of points A and B, which are superimposed on the M-mode image. (b) and (c) EstimatesABof the vibration velocity signals {v/ (t)} and {v/ (t)} of points A and B. (d) ECG and the PCG. (e) Change in thickness, D{h3 (t)}, of the anterior
A
B
AB
wall of the artery.

In the process of tracking the instantaneous position,
the object position is moved along the ultrasonic beam
based on the previous results from the complex autocorrelator. For flexible processing, we have developed areal-time system based on high-speed floating-point digital signal processing (DSP) chips and a workstation( WS) [4].
In the hardware system employed, the received echo
is quadrature-demodulated, and then the resultant
in-phase and quadrature signals are simultaneously
A/D-converted at a sampling frequency of 1 MHz witha 12-bit accuracy. For these large-scale data, it is notpossible to transfer them from the A/D converter to the
DSP boards using the VME-bus (versa module
European bus, the IEEE1024 standard ) because its
transfer speed is not so high. Much time-consuming
processing is necessary from the A/D conversion to areal-time display of the M-mode image, the resultant
waveforms, and the tracking results on the CRT(cathode-ray tube) of the WS. Thus, we employ four
DSP chips. The A/D converter boards are directly
connected to the DSP board. All of the processing in

384

H. Kanai, Y. Koiwa / Ultrasonics 38 (2000) 381–386

Fig. 3. Transient response of a 25 year old healthy male subject to the administration of nitroglycerine (NTG). (a) ECG for each heartbeat. (b)Pulse rate detected from the interval of the R-waves in the ECG. (c) Blood pressure continuously measured at the radial artery by the manometer.
(d) Pulse pressure (bold line) and the delay time of peak pressure from the R-wave (thin line). (e) Change in thickness of the carotid arterial wall.
(f ) Maximum value of the change in thickness, Dh (t), of the CCA detected for each heartbeat.
AB

the four DSPs should be synchronized with the transmission timing of the ultrasonic pulse. By making free useof four DSP chips, pipelining and parallel processing
are employed to increase the throughput. In this pipelining, each of the resultant tasks is completed within thepulse repetition period, DT, which carries out the realtime processing, though these tasks cannot be performedin real time with a single DSP chip.
In this study, the real-time system was applied to invivo experiments on the measurement of thickening andthinning in the human carotid arterial wall. Finally, thesystem was applied to the transient evaluation of therapid response of the change in wall thickness of theartery to the dose of NTG.

2. In vivo results for the carotid artery

Fig. 4. Relationships among the maximum change in wall thickness in
Fig. 3(f ), the pulse pressure in Fig. 3(d ), and the pulse rate in
Fig. 3(b).

The developed real-time system in [4] is applied tothe human common carotid artery (CCA) of a presumedly healthy 39 year old male volunteer. The ultrasonicfrequency is 7 MHz. Points (A) and (B) are set on theintima and adventitia of the anterior wall of the same

H. Kanai, Y. Koiwa / Ultrasonics 38 (2000) 381–386

carotid artery, respectively. The ultrasonic beam passing
through these two points is perpendicular to the wall
during the measurements. Fig. 2(b) and (c) show thevelocity signals, v/ (t) and v/ (t), respectively. TheA
Bchange in thickness, Dh3 (t), of the anterior wall isABestimated as shown in Fig. 2(e). A minute change inthickness of about 80 mm is measured with sufficient
reproducibility. In the waveforms of Fig. 2(b), (c) and(e), the dicrotic notch is obviously observed at theradiation timing of the second heart sound (II ).

3. Transient evaluation of arterial response tonitroglycerine
As described in Section 1, the real-time system tomeasure the change in thickness of the arterial wall willoffer new tools for clinical examinations. In this study,
the developed system is applied to evaluation of transient
response of the change in thickness of the carotid arteryfor about the first 160 s after the sublingual administration of spray-type NTG.
The first subject is a 25 year old healthy male volunteer. The change in thickness, Dh (t), of the posteriorABwall of the CCA and the ECG are continuously measured by the developed real-time system. The blood385

pressure, p(t), is also continuously and non-invasively
measured at the radial artery using a blood-pressure
manometer (Japan Colin, Jentow-7700).
Fig. 3 shows the transient response to the sublingual
administration of NTG. For each heartbeat, the waveform of the change in thickness, Dh (t), is measured
AB
as shown in Fig. 3(e), and the maximum of |Dh (t)|ABduring each heartbeat is detected and is shown in
Fig. 3(f ). From about 40 s after the administration of
NTG, the pulse rate shown in Fig. 3(b), which isdetermined from the R–R interval of the ECG in
Fig. 3(a), gradually increases, and the pulse pressure
shown in Fig. 3(d ) gradually decreases, where the pulsepressure is the difference between the maximal bloodpressure and the diastolic pressure. At the same time,the maximum change in wall thickness increases. Thus,
the arterial wall becomes compliant about 40 s after thedose of NTG. By assuming that the change in arterial
compliance is one of the factors influencing the prolongation of the arrival time of the peak pressure at theradial artery, this observation is supported by thegradual increase of the delay — shown in Fig. 3(d) —between the peak blood pressure and the peak of
the R-wave.
By contrast, the maximum change in wall thickness
varies widely, especially from about 40 s after the NTG

Fig. 5. Transient response of a 27 year old male smoker to the administration of nitroglycerine (NTG). For details, see Fig. 3.

386

H. Kanai, Y. Koiwa / Ultrasonics 38 (2000) 381–386

administration [see the fluctuations in the trend of thechange in thickness of Fig. 3(f )]. As shown in Fig. 4(b)and (c), however, there is a correlation between the
maximum change in wall thickness of Fig. 3(f ) and thepulse pressure of Fig. 3(c) or the pulse rate of Fig. 3(b).
Thus, the scattering in the maximum change in wall
thickness shown in Fig. 3(f ) originates in the scatteringin the pulse pressure shown in Fig. 3(d) or the pulse
rate of Fig. 3(b).
Next, the same measurement was applied to theposterior wall of the CCA of a 27 year old male smoker
consuming about 20 cigarettes per day for 7 years, andthe results are shown in Fig. 5. The wall thickness isabout 1.3 mm, which is almost same as the first subject
in Fig. 3.
Even though there are fluctuations in Fig. 5, bycomparing Fig. 5 with Fig. 3, the following observations
were found:
1. The change in thickness during one heartbeat inFig. 5(e) and (f ) is relatively small for the smoker.
2. The changes in the pulse rate of Fig. 5(b), the pulsepressure of the bold line in Fig. 5(d ), and the maximum change in thickness of Fig. 5(f ) due to the
response to the dose of NTG are also relatively small
for the smoker.
3. These changes start about 60 s after the dose of NTG
for the smoker in Fig. 5, while for the healthy subject
in Fig. 3, the changes start after about 40 s. Similar
results are obtained for other several healthy subjects.
Thus, the response to the dose of NTG is delayedand smaller for the smoker, compared to the young
healthy volunteers.

local change in thickness of the arterial wall was evaluated in real time. In the preliminary clinical study, theresponse of the arterial wall to NTG is directly evaluatedby the change in wall thickness for the first time. In
such a measurement, the real-time measurement system
is indispensable.
Further investigations are necessary to fully support
the evaluation method of the response of the arterial
wall to administration of NTG since the administration
carries various side-effects such as the reduction of the
preload for the heart. Detailed hemodynamic studies of
the cardiac output flow, the ventricular stroke volume
and associated pressure, as well as pressure build-up
during the ventricular systole are also necessary for amore accurate evaluation of discriminatory effects of
NTG on the arterial wall. Application of the measurement and evaluation to the response to NTG for thepatients with arterial diseases also needs to beinvestigated.

Acknowledgements
The authors are grateful to Prof. Emeritus Noriyoshi
Chubachi and Prof. Emeritus Motonao Tanaka of
Tohoku University, Prof. Floyd Dunn of the
Bioacoustics Research Laboratory, the University of
Illinois and Prof. Kunio Shirato, Prof. Takeshi Sasaki,
and Dr. Yasuhiko Munakata of Tohoku University
School of Medicine for discussion.

References
4. Conclusions
In this paper, we have demonstrated a novel realtime system for simultaneous measurement of velocity
signals at two points preset on an ultrasonic beam bytracking the large movement of the object during the
cardiac cycle. From the resultant velocity signals, the[1] H. Kanai, M. Sato, Y. Koiwa, N. Chubachi, IEEE Trans. Ultrason.
Ferroelect. Freq. Cont. 43 (5) (1996) 791–810.
[2] H. Kanai, H. Hasegawa, N. Chubachi, Y. Koiwa, M. Tanaka,
IEEE Trans. Ultrason. Ferroelect. Freq. Cont. 44 (4) (1997)
752–768.
[3] Churchill’s Illustrated Medical Dictionary, Churchill Livingstone,
New York, 1989.
[4] H. Kanai, Y. Koiwa, J. Zhang, IEEE Trans. Ultrason. Ferroelect.
Freq. Cont. 46 (5) (1999) 1229–1241.