J Med Ultrasonics (2007) 34:23–27
DOI 10.1007/s10396-006-0125-8© The Japan Society of Ultrasonics in Medicine 2007

ORIGINAL ARTICLE
Kazuki KUDO · Hideyuki HASEGAWA
Hiroshi KANAI

Improved imaging of the carotid artery in the short-axis plane by amechanical scanning ultrasonic probe

Received: June 2, 2006 / Accepted: August 7, 2006

Abstract To image the intima–media complex of thecarotid artery in a wider region, a method for measuringcross-sectional images in the arterial short-axis plane is presented. Using the proposed mechanical scanning system foran ultrasonic probe, cross-sectional images of a silicon rubber tube and a human carotid artery are measured in basic
experiments and in in vivo experiments, respectively. Theseexperiments show that this method successfully images theshort-axis cross sections. Using the method proposed in thisarticle, B-mode images in the short-axis plane can be accurately measured in a wider region than is possible withconventional methods.
Keywords B-mode image of artery · short-axis plane · intima–media thickness · ultrasoundIntroduction
Recently, the increase in the number of patients sufferingfrom cardiovascular or cerebrovascular diseases has become a serious social problem. Because such circulatory
diseases are mainly caused by atherosclerosis, it is important to diagnose this disease in an early stage. Intima–media
thickness (IMT) of the carotid artery noninvasively measured with conventional ultrasonic diagnostic equipment
is a useful marker for diagnosis of atherosclerosis.1–3 Botset al. showed that IMT increases inhomogeneously, developing atherosclerosis plaque.1 Therefore, the intima–media
complex should be imaged omnidirectionally. However,when an artery is imaged in the short-axis plane, which isthe plane perpendicular to the axis of the artery, with conventional linear scanning, ultrasonic beams that do not passthrough the center of the artery are not perpendicular toK. Kudo (*) · H. Hasegawa · H. Kanai
Department of Electronic Engineering, Graduate School ofEngineering, Tohoku University, 6-6-05 Aoba, Aramaki, Aoba,Sendai, Miyagi 980-8579, Japan
Tel. +81-22-795-7078; Fax +81-22-263-9444
e-mail: hkanai@ecei.tohoku.ac.jp

the arterial wall. To image an object clearly, it is importantthat ultrasonic beams are perpendicular to an object. Wilhjelm et al. measured the power of the received echo, E (θ),
while varying the angle of incidence, θ.4 In their report, inthe case of a smooth interface placed at the focal point, themaximum power was measured at θ = 90°, and the width at
half-maximum was determined to be ± 4° using a 5-MHz
concave transducer at a 25.4-mm diameter. From thesefacts, IMT cannot be measured accurately when ultrasonicbeams are not perpendicular to the wall because the decrease in the echo amplitude makes the B-mode image unclear. Therefore, it is very important that ultrasonic beams
are perpendicular to the wall for measurement of IMT.
The cross-sectional image of the arterial wall in theshort-axis plane is measured by intravascular ultrasonography (IVUS). Nissen et al. reported that the size and shapeof the atherosclerotic plaque on the arterial wall could beobserved in B-mode images acquired by IVUS.5 de Korte
et al. developed a method using IVUS for measuring thestrain of the arterial wall caused by the change in innerpressure.6 Furthermore, Mita et al. measured the strain during an entire heart cycle by compensating for the change inthe positions of the IVUS probe.7 However, IVUS cannot
be applied to noninvasive measurement of the carotid
artery, which is the most suitable region for measurementof IMT.3 Therefore, a method for noninvasive measurementof IMT in the short-axis plane should be developed. To
make all the ultrasonic beams perpendicular to the wall,
Nakagawa et al. proposed a new scanning method in which
ultrasonic beams are applied to a plane perpendicular to theaxis of the artery (short-axis plane).8 Although this method
extends the region that can be imaged, the region is still
limited to 45° because the increase in the grating lobe leads
to a loss of precision.
In this study, we constructed a system with a mechanicalscanning ultrasonic probe to image the intima–media complex in a wider region in combination with the method
proposed by Nakagawa et al.8 Using this mechanical scanning system, we measured cross-sectional (B-mode) imagesof a silicon rubber tube in basic experiments and a humancommon carotid artery in in vivo experiments. The results

24Fig. 1. Schematic diagram of the mechanical probe shows round bars(a), rails (b), stepping motor (c), and device to hold a linear-type probe(d)Fig. 2. Schematic diagram of mechanical scanning with an ultrasonicprobe

showed that the intima–media complex can be imaged in awider region in the short-axis plane.

Methods
Experimental system
Figure 1 shows a schematic diagram of the mechanical scanning system, which mainly consists of round bars, rails, a
stepping motor, and a device to hold a linear-type ultrasonicprobe. By rotating the stepping motor (RK544AA-N10;
Oriental Motor, Tokyo, Japan), the round bars (Fig. 1, a)move along the rails (Fig. 1, b). These rails are in the formof concentric circular arcs. In preliminary measurements, wemeasured the carotid artery in four healthy men. As the distance from the ultrasonic probe to the center of the arterywas 9.0–13.1 mm in these subjects, we designed this mechanical probe to enable measurement of the IMT when thedistance from the ultrasonic probe to the center of the arteryis 8.0–15.0 mm. As shown in Fig. 1, we can adjust the position
where the probe is fixed to match the distance from the skinsurface to the artery. This adjustment of the probe position
enables the ultrasonic probe to move along a circular arc
whose center coincides with the center of the artery. At eachmechanical rotation angle, θ (Fig. 2), the ultrasonic beamelectronically scans the region of ± 22.5° so as to be perpendicular to the arterial wall. Using this mechanical scanning

Fig. 3. Appearance of in vivo experiment

system, ultrasonic beams become perpendicular to the wallin the region of 65° when the ultrasonic probe moves at themaximum angle. The device is small and light enough to beheld in one hand (Fig. 3). Various ultrasonic probes can be
fixed to the mechanical scanning system.
In basic experiments and in vivo experiments, a conventional linear-type ultrasonic probe was used (center25Fig. 5. Schematic diagram of the region of radiofrequency (RF) dataacquisition

Fig. 4. Schematic diagram of mechanical scanning systemfrequency, f0 = 10 MHz; interval of centers of apertures fortransmitted beams, L = 0.4 mm; element pitch, d = 0.2 mm).
The number of electronically scanned beams is 30 perframe. Ultrasonic radiofrequency (RF) echoes are sampled
at 40 MHz with 16-bit resolution.
RF data acquisition and image reconstruction
Figure 4 shows a schematic diagram of the mechanical scanning system. The position of an ultrasonic probe can bechanged in the depth direction (vertical direction in Fig. 4)
so that the center of rotation of the ultrasonic probe coincides with the center of the carotid artery. Using the depthof the carotid artery, dc, the position, da, where the ultrasonic probe is fixed to the mechanical scanning system, becomes 75.0 − dc (mm). Thus, it is necessary that the depthof the carotid artery be measured. Therefore, the depth ofthe carotid artery is measured beforehand by the conventional linear scanning method, and the ultrasonic probe isattached to the position where da = 75.0 − dc (mm).
After attaching the probe, RF echoes from an object areacquired at a rotation angle of θr = 0°, using the method
proposed by Nakagawa et al.8 Then, the ultrasonic probe ismoved at a speed of 10°/s for t s, resulting in the rotation
angle of θr = 10 t. After moving the probe, reflected RFechoes from the object are acquired by the same method.
From these procedures, two B-mode images are acquired
before and after moving the ultrasonic probe. Figure 5
shows the regions of RF data acquisition; there is a region
where two B-mode images overlap because the rotation

angle of the ultrasonic probe, θr, is smaller than the maximum beam angle, θmax ( = 22.5°). To reconstruct a B-mode
image using RF data obtained by a couple of acquisitions,
RF echoes obtained by beams with smaller steering angles
are used in the region where B-mode images overlap because a smaller steering angle achieves a smaller gratinglobe. Therefore (see Fig. 5), a B-mode image is constructedby RF echoes acquired in the region between −θmax° and +θ
r/2° before moving the ultrasonic probe and between −θr/2°
and +θmax° after moving the probe. In this article, the rotation angles of the ultrasonic probe, θr, in basic experiments
and in vivo experiments are 20° and 10°, respectively.

Results
Results of basic experiments using a silicon rubber tube
Figure 6a shows a B-mode image of a silicon rubber tube
obtained by conventional linear scanning; echoes from thesilicon rubber tube were detected only in the region of 30°.
In this region, the angle of the beam to the wall changes byless than 30°/2 = 15° from the angle in the case of perpendicular insonification. In other regions, the wall was notdetected.
Figure 6b shows a B-mode image of the same tube obtained by mechanical scanning with an ultrasonic probe, asproposed in Methods. The B-mode image shown in Fig. 6b
was obtained by combining B-mode images at rotation angles of θ = −10° and 10°. Using the proposed method, echoesfrom the silicon rubber tube were clearly detected in theregion of 65° because these ultrasonic beams become perpendicular to the wall of the silicon rubber tube. Therefore,
using the scanning system proposed in this article, the region where the silicon rubber tube was imaged in the short-26Fig. 6. B-mode images of asilicon rubber tube obtained byconventional linear scanning (a)and by the proposed method(b). B-mode images of a carotid
artery of a 22-year-old healthy
man were obtained by conventional linear scanning (c) and by
the proposed method (d)axis plane became 35° and 20° wider than those obtainedby the conventional method and the method previously
proposed by Nakagawa et al.,8 respectively.

Results of in vivo measurements of a human
carotid artery
Figure 6c shows a B-mode image of a human common
carotid artery (CCA) of a 22-year-old healthy man obtainedby conventional linear scanning. Using conventional linear
scanning for in vivo experiments, the intima–media complex could not be measured in the region where the angleof ultrasonic beams changed by more than ± 10° (= 20°/2)
from the angle in the case of perpendicular insonification
to the arterial wall. In an in vivo experiment, the amplitudesof echoes from the lumen–intima interface were lower thanthose from the silicon rubber tube in basic experiments because there is not much difference in acoustic impedance
between the blood and the intima at the interface. There-abcdfore, the region where echoes were clearly detected in invivo experiments was 10° narrower than the region in thebasic experiments already presented.
Figure 6d shows a B-mode image of the same CCA obtained by mechanical scanning with an ultrasonic probe.
This image was acquired by combining two B-mode images
obtained at rotation angles of −5° and 5°. For in vivo measurement, the distance from the ultrasonic probe to the artery is slightly changed during mechanical scanning because
the center of the neck does not coincide with that of an artery. Therefore, this change in distance was corrected basedon the spatial cross-correlation between the RF echo lines
along the axis shown by the white solid line in Fig. 6d, whichwere obtained at two different rotation angles, θ, of −5° and5°. Using the mechanical probe, the intima–media complex
can be accurately measured in a wider region, as was possible in the basic experiments. The IMT measured in thisexperiment was 0.6 mm, the same as with the conventional
linear scanning method. As shown in Fig. 6d, the IMT canbe measured in a region of 55°, which is 35° wider than that

27

possible with the conventional linear scanning method, and
it is 10° wider than that achieved by the method previously
proposed by Nakagawa et al.8

Table 1. Comparison of regions scanned by conventional method,method proposed by Nakagawa et al.,8 and method proposed in thisarticleMethodDiscussion
A method for steering the ultrasonic beam is used in thisarticle. Thus, grating lobes are generated and may cause
artifacts in a reconstructed image. Using the element pitch,d, width of element, a, wavelength, λ, and beam angle ofmain lobe, θ0, the directional characteristic of an ultrasonicbeam is defined as follows9:
R(θ) = Re (θ)Ra (θ)(1)

whereπasin  sin θ λ
Re (θ) =,πasin θ
λN πdsin(sin θ − sin θ0 )λRa (θ) =,πdN sin(sin θ − sin θ0 )
λ{{}}

(2)

(3)

and N is the number of elements. From the above equations, an amplitude of a grating lobe is equivalent to that of
the main lobe under the condition of sin θ0 = λ/2d. In thisstudy, the angle when an amplitude of the grating lobe isequivalent to that of the main lobe, θs, is ±22.5°, using theelement pitch of d = 0.2 mm and wavelength of λ. = 150 µm.
Therefore, in this study, the steering angle, θ0, was restricted
to be less than ±22.5°.
To suppress the grating lobe, the element pitch must besufficiently narrower than the wavelength. However, formeasurement of IMT, the spatial resolution of a B-mode
image should be of submillimeter order.1 Spatial resolution
in the depth direction depends on the wavelength and is 75(= 150/2) µm when the center frequency is 10 MHz. Thus, alarger wavelength is not desirable in terms of the spatialresolution. Therefore, reduction of the element pitch is abetter solution for suppressing the grating lobe. Using current technology, the element pitch of a 10-MHz ultrasonicprobe can be reduced to 0.125 mm.10 In this case, θs becomes
36.9°. However, it is difficult to fabricate a probe with sucha narrow element pitch, and it is not common at this moment. Therefore, the proposed mechanical scanning systemis effective for imaging the carotid artery in the short-axis
plane.
Table 1 shows the regions that can be accurately measured in the short-axis plane with the conventional linear
scanning method, the method proposed by Nakagawa etal.,8 and the method proposed here. Table 1 indicates thatthe method proposed in this article is an effective method
for measuring IMT in a wider region in the short-axis
plane.

Conventional method
Method proposed byNakagawa et al.
Method proposed inthis article

Scanned region (degrees)
Basic experiment

In vivo experiment

∼30
∼45

∼15
∼45

∼65∼55Conclusions
In this study, we proposed a system for mechanically scanning with an ultrasonic probe to image the intima–media
complex in a wider region in the short-axis plane in combination with the method previously proposed by Nakagawa
et al.8 The proposed method was evaluated by basic experiments using a silicon rubber tube and then applied to in vivo
measurement of the human carotid artery. Using the proposed scanning method, B-mode images in the short-axis
plane could be accurately measured in a wider region thanis possible with the conventional linear scanning method
and the previous method proposed by Nakagawa et al.

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