Japanese Journal of Applied Physics
Vol. 45, No. 5B, 2006, pp. 4722–4726#2006 The Japan Society of Applied Physics

Designing Beam Steering for Accurate Measurement of Intima-Media Thickness
at Carotid Sinus
Takashi M ASHIYAMA, Hideyuki HASEGAWA and Hiroshi K ANAI
Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan(Received November 30, 2005; accepted February 22, 2006; published online May 25, 2006)

Recently, cardiovascular disease has become the second most common cause of death in Japan following malignant neoplasm
formation. Therefore, it is necessary to diagnose atherosclerosis during its early stages because atherosclerosis is one of themain causes of cardiovascular diseases. The carotid sinus is a site that is easily aﬀected by atherosclerosis [C. K. Zarins et al.:Circ. Res. 53 (1983) 502]; therefore, the diagnosis of this disease at this site is important [S. C. Nicholls et al.: Stroke 20(1989) 175]. However, it is diﬃcult to accurately diagnose atherosclerosis in the carotid sinus in the long-axis plane, which isparallel to the axis of the vessel, using conventional linear scanning because the carotid sinus is not ﬂat along the axis of thevessel, and the ultrasonic beams used in linear scanning are perpendicular to the arterial wall in a limited region. Echoes fromregions that are not perpendicular to the ultrasonic beams are very weak and the arterial wall in such regions is hardly
recognized in a B-mode image. In this study, the position of the arterial wall was predetermined on the basis of the B-mode
image obtained by conventional linear scanning, then ultrasonic beams were transmitted again so that all beams were almostperpendicular to the arterial wall. In basic experiments, a nonﬂat object made of silicone rubber was measured and it was
shown that it is possible to image a nonﬂat object over the entire scanned area using the proposed beam steering method.
Furthermore, in in vivo experiments, the intima–media complex was imaged over the entire scanned area at the carotidsinus. [DOI: 10.1143/JJAP.45.4722]
KEYWORDS: ultrasonic beam steering, intima–media thickness, carotid sinus, atherosclerosis

1.

Introduction

Cardiovascular disease is now the second most commonform of death in Japan following malignant neoplasm
formation. The diagnosis of early-stage atherosclerosis has
become increasingly important because atherosclerosis is
one of the main causes of cardiovascular disease. The double
line pattern of the arterial wall, which is shown in an
ultrasound B-mode image, represents echoes from thelumen–intima and media–adventitia boundaries.1) The area
between these two lines corresponds to the intima–mediacomplex. The intima–media thickness (IMT) is a usefulmarker for the diagnosis of atherosclerosis.1,2) Cross sectional images of arteries are commonly obtained in the longaxis and short-axis planes, which are parallel and perpendicular to the axis of the artery, respectively. However, theintima–media complex is imaged in the short-axis plane in alimited region because ultrasonic beams are perpendicular to
the wall in the limited region. Therefore, an alternative beamscanning method was developed so that the ultrasonic beams
were always perpendicular to the arterial wall in the shortaxis plane by assuming that the cross section of the arterylumen is a circle.3) Using this method, the IMT of the artery
could be measured accurately in the short-axis plane, andthis method was combined with the phased tracking method
proposed by our group for measuring the regional strain andviscoelasticity of the arterial wall.4–14)
The carotid sinus is a site that is easily aﬀected byatherosclerosis; therefore, the diagnosis of this disease at thissite is important.15,16) However, it is diﬃcult to diagnose
atherosclerosis in this site in the long-axis plane accurately
using conventional linear scanning because the carotid sinus
is not ﬂat along the axis of the vessel, and the ultrasonicbeams used in linear scanning are perpendicular to thearterial wall in a limited region. Echoes from regions that are

not perpendicular to the ultrasonic beams are very weak andthe arterial wall in such regions is hardly recognized in a Bmode image. To improve the quality of B-mode images,
multi angle compound imaging17–19) was developed. In thistechnique, images are recorded from multiple angles(typically 3 to 9), and these single-angle images are then
combined to form the compound image. In most investigations on this technique so far, the individual single-angle
images have been simply averaged to form the compound
image.
In the method proposed in this study, the position of thearterial wall is predetermined on the basis of the B-mode
image obtained by conventional linear scanning, thenultrasonic beams are transmitted again so that all beams
are almost perpendicular to the arterial wall.
2.

Principles

Figure 1 shows a schematic diagram of the beam steering.
The ultrasonic beam scans M positions along the x-axis byconventional linear scanning. The depth of the arterial wall,∆xprobex1...xi −1 xi xi+1xy...

xMbi

θiy 1 ... yi −1yiyi+1
aiaw,iskin

...

yM

arterial wall(posterior)Fig. 1. Determination of optimum beam position and angle (N ¼ 1).

E-mail address: kanai@ecei.tohoku.ac.jp4722

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T. M ASHIYAMA et al.

yi ði ¼ 1; 2; . . . ; MÞ, at each beam position, xi ði ¼ 1; 2; . . . ;MÞ, is manually predetermined in the B-mode image
obtained by conventional linear scanning. The intervalbetween the neighboring ultrasonic beams is deﬁned byx ¼ xiþ1  xi . With respect to each position, ðxi ; yi Þ, of thei-th ultrasonic beam on the wall, a regional slope, aw;i , of thearterial wall to the surface of the ultrasonic probe isestimated by the least-squares method using positions,fðxi ; yi Þg, of neighboring N ultrasonic beams on the arterial
wall as follows:!
!
NN
N
X
X
X
ð2N þ 1Þxi  yi xiyii¼Nð2N þ 1ÞNXx2i i¼NNXi¼N!2:y...
steered beam No.: 12beam angle θ [deg]: θ’1K
3(b)

b’i

transmit position : b’1 ... b’j−2 b’j−1 b’j b’j+1 b’j+2 ... b’M
transmit position No.:1 ... j−2 j−1 j j+1 j+2 ... M
xxibiðradÞ:

ð2:3Þ

ðmmÞ:

(x i , y i )ð2:4Þ

2.5 mm15 mm(a)silicone(b)

Basic Experiment

Figure 3 shows (a) the size and (b) cross section of aphantom made of silicone rubber. The shape of the phantom
simulates the arterial wall at the carotid sinus and thephantom has a hollow. The width of the hollow is 15 mm,
and the depth is 2.5 mm. Figure 4 shows the experimental
system. The phantom was ﬁxed in a water bath and it wasmeasured using diagnostic equipment (Aloka SSD-6500)
with a conventional linear-type ultrasonic probe at 10 MHz.
The distance from the surface of the ultrasonic probe to thesurface of the phantom was 13 mm. The focal distance was
set to be 18 mm from the surface of the ultrasonic probe
because the ultrasonic beam became wider and the measurement became more robust by setting the focal depth deeper( x’i , y’i )Fig. 2. (a) Determination of i0 (dashed line: calculated beam with i ).
(b) Determination of b0i .

By determining i and bi (mm) for all positions, fðxi ; yi Þg, on
the wall, all beams are designed to be perpendicular to thearterial wall.
In this study, beam angles must be assigned by discretevalues. Therefore, the discrete beam angle, i0 , that is nearest
to i is selected from preassigned angles of K beams[Fig. 2(a)]. The bi of the aperture is calculated again usingi0 . The center of the aperture should be also assigned by adiscrete value that depends on the element pitch of theultrasonic probe. As shown in Fig. 2(b), the discrete transmit
position, b0i , of the aperture, which is nearest to bi , isselected, as well as i0 . In this way, the ultrasonic beam istransmitted to the position, ðx0i ; y0i Þ, on the wall as perpendicular as possible. Although the designed ultrasonic beam
does not exactly pass through the ðxi ; yi Þ because of thediscrete values of b0i and i0 , the diﬀerence, jx0i  xi j, is lessthan half the element pitch.
3.

θi’yi¼Nyiyi¼ xi 
tan iai.....
.

...
...

θ’K

θ’i

ð2:1Þ

From the determined i (rad) and the ðxi ; yi Þ of the arterial
wall, the center, bi , of the aperture that transmits anultrasonic beam to ðxi ; yi Þ is determined as follows:bi ¼ x i .

..
θ’2
θ’3 .. k.
θ’k

From the estimated aw;i , the slope, ai , and the angle, i , ofthe beam that is perpendicular to the arterial wall at ðxi ; yi Þare determined as follows:1ai ¼ ;ð2:2Þaw;i
i ¼ tan1 ai

dashed arrow:designed beamwith angle θ i

θi=

i¼N

x=aw;i ¼probe(a)Fig. 3. (a) Size of phantom. (b) Cross sectional picture.

than the surface of the phantom.20,21) As schematicallyshown in Fig. 5, 105 ultrasonic beams with intervals, x, of0.2 mm are transmitted in a frame, and the beam angle was
0changed by 2 frame by frame from 10 ¼ 70 to 21¼ 110 .
Therefore, the elapsed time for acquiring the RF data of 21
frames was 165 ms in the case of a frame rate of 127 Hz inthis experiment. Ultrasonic RF echoes were acquired at asampling frequency of 40 MHz.
Figure 6(a) shows a B-mode image of the object obtained

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T. M ASHIYAMA et al.

distance [mm]100

distance [mm]100distance [mm]10200xdepth [mm]probe

y13 mmultrasounddiagnostic
equipment

0phantomstage

water bath1020(a)Fig. 4. Experimental system.

200y

θ1’probexyθk’probexy

θK’

depth [mm]probe
x10Fig. 5. Schematic diagram of beam scanning.

20

4.

In Vivo Experiment

A human common carotid sinus of a 25-year-old male wasmeasured in the long-axis plane. To reduce the elapsed time
for acquiring RF data, ultrasonic beams were transmitted
every 5 from 10 ¼ 70 to 90 ¼ 110 and discrete steered
angles, fk0 g, were changed frame by frame. Ultrasonic RFdata were acquired during nine frames (82 ms at a frame rateof 110 Hz) just before the time of the R-wave of theelectrocardiogram. Figure 7(a) shows a B-mode image
obtained by conventional linear scanning, composed of105 ultrasonic beams with intervals, x, of 0.2 mm(50 ¼ 90 ). The white dotted line in Fig. 7(b) shows the(b)200depth [mm]by conventional linear scanning using 105 ultrasonic beams0(11¼ 90 ). The surface of the phantom was manuallyassigned at 105 positions fðxi ; yi Þg ði ¼ 1; 2; . . . ; 105Þ on theB-mode image [white dotted line in Fig. 6(a)]. On the basisof the principle in §2, the beam, with angle i and transmit
position bi , that is perpendicular to the surface of thephantom was determined using fðxi ; yi Þg. The white lines inFig. 6(b) show the calculated optimum beams. Then, thebeam with i0 and b0i was selected as shown in Fig. 2.
Figure 6(c) shows a B-mode image constructed from theultrasonic RF echoes obtained using the calculated beamswith fði0 ; b0i Þg.
As shown in the areas surrounded by circles in Fig. 6(a),the surface of the phantom could not be imaged clearly inthe areas where the ultrasonic beams incline from a verticaldirection by more than 20 as shown in the B-mode image
obtained by conventional linear scanning. On the other hand,strong reﬂected echoes were obtained in such areas using theproposed method, and the phantom was imaged over theentire scanned area, as shown in Fig. 6(c).

1020(c)Fig. 6. (a) B-mode image obtained by conventional linear scanning (white
dotted line: assigned silicone surface). (b) Calculated beams (white lines)
superimposed on B-mode image. (c) B-mode image obtained by proposedmethod.

assigned (xi ; yi ) of the posterior wall, and the white lines inFig. 8(a) show the calculated optimum beams. Figure 8(b)shows a B-mode image constructed from the ultrasonic RF
echoes obtained using the calculated beams with fði0 ; b0i Þg.
As shown by the area surrounded by a circle in Fig. 7(a), theintima–media complex could not be imaged clearly by
conventional linear scanning in the area where the ultrasonicbeams incline from the perpendicular direction relative tothe wall by more than 2.5 . However, suﬃcient reﬂectedechoes were obtained from such an area using the proposed
method, and the intima–media complex was almost imaged
over the entire scanned area as shown in Fig. 8(b).

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distance [mm]10200001010

depth [mm]depth [mm]0T. M ASHIYAMA et al.

20

distance [mm]102020(a)(b)

Fig. 7. (a) B-mode image obtained by conventional linear scanning. (b) Assigned posterior wall surface (white dashed line)
superimposed on B-mode image of (a).

distance [mm]10200001010depth [mm]depth [mm]020

distance [mm]102020(a)(b)

Fig. 8. (a) Calculated beams (white lines) superimposed on B-mode image of Fig. 7(a). (b) B-mode image obtained by proposedmethod.

5.

Discussion

To realize the proposed method in a clinical situation, thefollowing procedure is considered suitable. A B-mode image
is obtained by conventional linear scanning, and positions on
the arterial wall are detected automatically during a frameinterval. Then, the ultrasonic beam that is perpendicular to
the arterial wall is designed with respect to each detected

position. In this case, frame rate does not decrease because
one ultrasonic beam must be transmitted per assigned
position on the wall. However, such a method for automatically detecting the position on the wall during a frameinterval was not realized in this study. Therefore, RF data
were acquired at various ultrasonic beam angles beforehand.
In in vivo experiments, ultrasonic RF data were acquired
during nine frames, which corresponds to 82 ms at a frame4725

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0T. M ASHIYAMA et al.

distance [mm]106.

In this paper, a method of beam steering that makes
ultrasonic beams perpendicular to the arterial wall at thecarotid sinus in the long-axis plane was proposed. In basic
experiments using a phantom made of silicone rubber, it wasshown that it is possible to image a nonﬂat object over theentire scanned area using the proposed beam steering
method. Furthermore, in in vivo experiments at a carotidsinus, it was shown that it is possible to image the intima–
media complex over the entire scanned area using theproposed beam steering method.

0depth [mm]Conclusions

10520Fig. 9. B-mode image in short-axis plane obtained by conventional linearscanning.

rate of 110 Hz, and the increment step of the beam angle wasincreased to 5 in in vivo experiments to reduce the period
for acquiring RF data to avoid the inﬂuence of movements ofthe arterial wall caused by the heartbeat. The increment stepof 5 was determined as follows: Figure 9 shows a B-mode
image of the carotid artery shown in Figs. 7 and 8 in theshort-axis plane that is perpendicular to the axis of theartery. In Fig. 9, the intima–media complex of the posteriorwall is imaged in the region where ultrasonic beams do notincline from the perpendicular insoniﬁcation by more than
2.5 . Therefore, the increment step should be set to be at
most 5 .
The period for acquiring RF data in this study is longer
than a frame interval of 9 ms, and movements of the arterial
wall caused by the heartbeat during this acquisition period
may change the insoniﬁcation angles of calculated beams
from those expected by the proposed method. However, theintima–media complex was imaged as shown in Fig. 8(b) bysetting the period for acquiring RF data to be just before thetime of the R-wave of the electrocardiogram because themovement of the arterial wall is small in this phase of theheart cycle.

1) A. D. M. van Swijndregt, S. H. K. The, E. J. Gussenhoven, C. T.
Lancee, H. Rijsterborgh, E. de Groot, A. F. W. van der Steen, N. Bom
and R. G. A. Ackerstaﬀ: Ultrasound Med. Biol. 22 (1996) 1007.
2) A. D. M. van Swijndregt, E. E. de Lange, E. de Groot and R. G. A.
Ackerstaﬀ: Ultrasound Med. Biol. 25 (1999) 323.
3) N. Nakagawa, H. Hasegawa and H. Kanai: Jpn. J. Appl. Phys. 43(2004) 3220.
4) H. Kanai, M. Sato, Y. Koiwa and N. Chubachi: IEEE Trans. Ultrason.
Ferroelectr. Freq. Control 43 (1996) 791.
5) H. Kanai, Y. Koiwa and J. Zhang: IEEE Trans. Ultrason. Ferroelectr.
Freq. Control 46 (1999) 1229.
6) H. Hasegawa, H. Kanai, N. Hoshimiya and Y. Koiwa: Jpn. J. Appl.
Phys. 39 (2000) 3257.
7) H. Hasegawa, H. Kanai and Y. Koiwa: Jpn. J. Appl. Phys. 41 (2002)
3563.
8) H. Kanai, H. Hasegawa, M. Ichiki, F. Tezuka and Y. Koiwa:
Circulation 107 (2003) 3018.
9) H. Hasegawa, H. Kanai, Y. Koiwa and J. P. Butler: Jpn. J. Appl. Phys.
42 (2003) 3255.
10) H. Hasegawa and H. Kanai: Jpn. J. Appl. Phys. 43 (2004) 3197.
11) J. Tang, H. Hasegawa and H. Kanai: Jpn. J. Appl. Phys. 44 (2005)
4588.
12) J. Inagaki, H. Hasegawa, H. Kanai, M. Ichiki and F. Tezuka: Jpn. J.
Appl. Phys. 44 (2005) 4593.
13) H. Hasegawa and H. Kanai: Jpn. J. Appl. Phys. 44 (2005) 4609.
14) M. Sugimoto, H. Hasegawa and H. Kanai: Jpn. J. Appl. Phys. 44(2005) 6297.
15) C. K. Zarins, D. P. Giddens, B. K. Bharadvaj, V. S. Sottiurai, R. F.
Mabon and S. Glagov: Circ. Res. 53 (1983) 502.
16) S. C. Nicholls, D. J. Phillips, J. F. Primozich, R. L. Lawrence, T. R.
Kohler, T. G. Rudd and D. E. Strandness: Stroke 20 (1989) 175.
17) A. Hernandez, O. Basset, P. Chirossel and G. Gimenez: UltrasoundMed. Biol. 22 (1996) 229.
18) S. K. Jespersen, J. E. Wilhjelm and H. Sillesen: Ultrasound Med. Biol.
26 (2000) 1357.
19) J. E. Wilhjelm, M. S. Jensen, S. K. Jespersen, B. Sahl and E. Falk:IEEE Trans. Med. Imaging 23 (2004) 181.
20) M. Watanabe and H. Kanai: Jpn. J. Appl. Phys. 40 (2001) 3918.
21) M. Watanabe, H. Hasegawa and H. Kanai: Jpn. J. Appl. Phys. 41(2002) 3613.

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