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Japanese Journal of Applied Physics 56, 07JF01 (2017)https://doi.org/10.7567/JJAP.56.07JF01

Imaging of human vertebral surface using ultrasound RF data received
at each element of probe for thoracic anesthesia
Kazuki Takahashi1*, Hirofumi Taki1,2, Eiko Onishi3, Masanori Yamauchi3, and Hiroshi Kanai2,1
1

Graduate School of Biomedical Engineering, Tohoku University, Sendai 980-8579, Japan
Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan3
Department of Anesthesiology and Perioperative Medicine, Tohoku University School of Medicine, Sendai 980-8575, Japan2*E-mail: kanai@ecei.tohoku.ac.jp
Received November 25, 2016; accepted February 6, 2017; published online May 23, 2017
Epidural anesthesia is a common technique for perioperative analgesia and chronic pain treatment. Since ultrasonography is insuﬃcient for
depicting the human vertebral surface, most examiners apply epidural puncture by body surface landmarks on the back such as the spinous
process and scapulae without any imaging, including ultrasonography. The puncture route to the epidural space at thoracic vertebrae is muchnarrower than that at lumber vertebrae, and therefore, epidural anesthesia at thoracic vertebrae is diﬃcult, especially for a beginner. Herein, anovel imaging method is proposed based on a bi-static imaging technique by making use of the transmit beam width and direction. In an in vivo
experimental study on human thoracic vertebrae, the proposed method succeeded in depicting the vertebral surface clearly as compared withconventional B-mode imaging and the conventional envelope method. This indicates the potential of the proposed method in visualizing thevertebral surface for the proper and safe execution of epidural anesthesia. © 2017 The Japan Society of Applied Physics

1.

Introduction

Epidural anesthesia is a type of regional anesthesia without
loss of consciousness, used to produce anesthetic eﬀects inan optional range below the neck. Especially, it has beenreported that 60% of American women in 27 states who
experienced a single birth received epidural or spinalanesthesia,1) and it is reported that epidural anesthesia wasemployed for lower limb surgery longer than 2 h in 57 of148 hospitals (39%) in Japan.2) Epidural anesthesia has thefollowing advantages compared with general anesthesia oropioid analgesics: superior eﬀects against pain with movement, gastrointestinal peristalsis and blood circulation, andfewer systemic side eﬀects.3) Respiratory complications suchas atelectasis and pneumonia are less than those with opioiduse, and the function of the gastrointestinal tract, blood
circulation and wound healing can recover faster than without
epidural anesthesia due to the sympathetic nerve blocking
eﬀects. Consequently, continuous epidural anesthesia using acatheter is a common technique for perioperative analgesia
and an eﬀective treatment for early postoperative recovery.
In epidural anesthesia, as shown Fig. 1(a), an examineradvances an anesthetic needle to the epidural space located at
the outer layer of the dura mater. The needle tip does notalways straightly obtain the epidural space, and dorsal bone
structures, namely, the spinous process and the vertebral
arch, sometimes block the needle. Moreover, measurementsof the width of the epidural space and observation of epidural
ﬂuid or abscess is needed to perform safe epidural puncture
or prevent post-puncture complications, such as paresis or
post-dura puncture headache due to hematoma or outﬂux ofthe cerebrospinal ﬂuid, respectively.
For imaging the internal human body, computed tomography (CT) and magnetic resonance imaging (MRI) are
employed. However, CT has a risk of radiation exposure, andMRI has disadvantages in that the scale of the equipment islarge and measurement time is long. Since non-invasive,
iterative diagnosis and short-time measurement are possible,
ultrasound measurement is one of the main methods ofin vivo imaging, and some studies have suggested itsusefulness.4–7)

Although an examiner often conﬁrms an ultrasound image
before the application of epidural puncture,8) it is still diﬃcult
to advance an epidural needle with real-time ultrasoundguidance because the structure of vertebrae is complex.
Previously, imaging of the human radius surface using phasesymmetry9) and an automatic classiﬁcation of epidural space
and the vertebral arch have been studied.10–12) However,ultrasound imaging is diﬃcult when the target, such as thethoracic surface, has a complex shape. Especially, in thethoracic area, as shown in Fig. 1(b), spinous processes andthe vertebral arch are extended caudally, but the thoracic
window to access the epidural space is much narrower thanthat in the cervical or lumbar region.13) The narrow acoustic
window to observe the epidural space in the thoracic area
limits the ultrasound depiction of this space.14) Since spinous
processes and the vertebral arch prevent easy advancement of
the puncture to the epidural space, the task of anesthesiologists would be facilitated by auxiliary imaging with easy
and noninvasive techniques.
Conventional B-mode imaging is an easy and noninvasive
technique to depict the spinal surface. In conventional Bmode imaging, since the direction of the receive beam andthat of the transmit beam are the same, the surface shouldbe perpendicular to the ultrasound beam. However, thebone surface is smooth and the received signal is severely
decreased when the ultrasound beam is not perpendicular tothe surface. Therefore, ultrasonography is insuﬃcient for
depicting the vertebral surface, and successful administrationof epidural anesthesia depends on the examiner’s experienceand ability of locating a gap between vertebrae by touch.
The failure rate of epidural anesthesia has been reported tobe 6–25%,15–17) and back pain (22%) and emotional distress(14%)18) may be experienced.
To accurately depict a target surface that has curves, theenvelope method19) and the range point migration (RPM)
method20,21) have been proposed. These methods use a bistatic imaging technique, where the receive position is set
o be diﬀerent from the transmit position, and they transmit
ultrasound or electromagnetic waves of hemisphericalshapes. They use an ellipse, which is determined by the path
length and the transmitted and received positions, to show a07JF01-1© 2017 The Japan Society of Applied Physics

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K. Takahashi et al.

(a)

(b)

Fig. 2. (Color online) Schema of conventional B-mode imaging method.

ratio is improved in comparison with the conventional
envelope method by employing a narrow transmit beam toreduce the number of reﬂected echoes from plural targets.
The proposed method depicts the target surface even whentargets are close to each other and has potential to be ofassistance to an inexperienced examiner in administering
epidural anesthesia in the thoracic area.
2.
(c)Fig. 1. (Color online) Illustrations for depicting the surface of thoracic
vertebrae by ultrasound. (a) Schema of epidural anesthesia. (b) Longitudinalcross section. (c) Visualization target in cross section by ultrasound.

target surface which is tangential to the ellipse. Moreover,they depict plural ellipses which consist of a combination ofmany points of transmit and receive positions. These methods
are characterized by high performance in rendering the target
surface when a single target exists in the region of interest(ROI); however, when plural targets exist in the ROI, theestimated target surface is shallower than the actual surface,
severely deteriorating the performance of these methods.
A new imaging method is herein proposed by which thevertebral surface in the thoracic area is depicted using
received radio frequency (RF) data at each element of an
ultrasonic probe. In the proposed method, the signal to noise

Experimental methods

Figure 1(a) shows the process of epidural anesthesia in which
the anesthetic needle is tapped. Since spinous processes
extend caudally, an examiner inserts the anesthetic needle
into the epidural space in a lateral direction to avoid theseprocesses. Figure 1(b) depicts a paramedian section ofvertebrae in the thoracic area. The aim of the present study
was to depict the acoustic window between vertebral arches
in the thoracic area by using ultrasound, as shown in
Fig. 1(c). In an in vivo experiment, the ultrasound image ofthe thoracic vertebrae depicted by the proposed method wascompared with a conventional B-mode image, showing thesuperiority of the proposed method.
2.1 Conventional B-mode imaging
Figure 2 shows a schema of conventional B-mode imaging
method using a linear probe. The output signal Si,r(t) of theprobe for observation point Ri,r located at a distance of r
along the i-th scan beam is given by the delayed sum of thereceived RF signals {si, j (t)} of each element in the ultrasoundprobe, that is,
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
23r þ r2 þ ðxi % xj Þ2 2r
X% 5; ð1ÞSi;r ðtÞ ¼si; j 4t þccjwhere xi and xj are, respectively, the lateral positions of thei-th transmit beam and the j-th received element, si, j (t) is
the signal received by the j-th element in response to theultrasound transmission along the i-th scan line, and c is theacoustic velocity.
However, since conventional B-mode imaging assumes
that the reﬂected direction from the object coincides with theincident direction on the object and since the delay values in

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K. Takahashi et al.

Eq. (1) are predesigned based on this assumption, it isdiﬃcult to obtain a high intensity when the echo comes froman inclined specular reﬂector, as shown in Fig. 2, resulting in
low brightness of a inclined specular target surface, such as avertebral surface.
2.2 Conventional envelope method
In the present study, the proposed method employs bi-static
data acquisition as follows: In the conventional envelope
method,19) an ultrasound pulse is transmitted spherically froman element, and echoes from targets are acquired separately ateach element of a probe, as shown in Fig. 3(a). The length ofthe path from the transmit-position Xi to the receive-position
Xj is determined by summation of the lengths Xi Pi and Pi Xj .
Therefore, the reﬂection point on a target surface should belocated on an ellipse, the foci of which are at the positions ofXi and Xj and the long axis of which has the same length as
the path length Xi Pi þ Pi Xj .
By scanning the transmitted position Xi along the probe,
the conventional envelope method depicts plural ellipseswhich consist of many points of transmit position Xi andreceive position Xj, and ﬁnally estimates the target surface as
the outer envelope of the ellipses. However, when there areplural targets in the ROI, the target surface estimated by theconventional envelope method is restricted to a shallow
depth, as shown in Fig. 3(a). Since the method selects one
receive-position Xj from all positions {Xj}, the path length
Xi Pi þ Pi Xj has a minimum. Thus, position XjA is selected but
Xj and XjAA are not, as shown in Fig. 3(b).
2.3 Proposed method with limited width and directionof transmit beam
A modiﬁed envelope method by limiting the width anddirection of the transmit beam, as shown in Fig. 3(c), isherein proposed. In the proposed method, a narrow transmit
beam rather than a spherical beam is employed. Alternatively, the transmitted beam is set to be inclined by θ. For
the resultant plural ellipses which consist of many points oftransmit position Xi and receive position Xj, the arc of eachellipse is employed only within the width of the transmit
beam. The grid size is 56 and 50 mm in the lateral and depth
directions, respectively. The compound image is constructedby the summation of several images depicted by the proposed
method using various incident angles {θ} of the transmit
beam. Thus, the proposed method restricts the number ofassumed targets that reﬂect ultrasound and can depict ellipses
in the area of a gap. Moreover, in the proposed method, thereceived signal intensity acquired by the narrow transmit
beam is greater than that acquired by the spherical beam.
Therefore, the proposed method can obtain signals on deeper
target surfaces.
2.4 In vivo experiment of thoracic vertebrae
RF signals of each element in the ultrasonic probe wereacquired for the thoracic vertebrae of a healthy 23-year-old
male using an ultrasound diagnostic device (Prosound alpha10, Hitachi-Aloka). The resultant image obtained by the
proposed method was compared with a conventional B-mode
image. The employed linear array probe consisted of 192
elements with an element pitch of 0.2 mm. The transmit
frequency, sampling frequency, and focal distance were
7.5 MHz, 40 MHz, and 30 mm, respectively. The inclined
angle of transmit beam θ ranged from −20 to 20°, the angle
interval being 5°.

(a)(b)(c)Fig. 3. (Color online) (a, b) Schemas of the conventional envelope
method. In the conventional envelope method, the path length X! P!;|0 þ
P!;|0 X|0 which is the shortest in (b) is selected. (c) Schema of the proposed
method with limited width and direction of the transmit beam.

3.

Results

Figure 4(a) illustrates the vertebral surface and muscle tissue
on a conventional B-mode image. Figure 4(b) shows the Bmode image of the thoracic vertebral surface. Figure 4(c)shows the signal power of the echoes from the vertebral
surface and muscle tissue with respect to the various transmit
beam directions {θ}, where ten measurement positions
were assigned, as shown in Fig. 4(b). The echoes from thevertebral surface were higher in intensity compared withthose from the muscle tissue.
Figures 5(a)–5(i) show the images obtained by theproposed method. Especially when transmit beam direction
θ was 15° in Fig. 5(h) or 20° in Fig. 5(i), the proposed
method clearly depicted the vertebral surface without aneﬀect of the muscle tissue as compared with the conventional
B-mode image in Fig. 4(b).

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K. Takahashi et al.

Fig. 4. (Color online) (a) Vertebral surface and muscle tissue assumed on
B-mode image. (b) Conventional B-mode image of the surface of thoracic
vertebrae. Crosses show the assigned points on the vertebral surface (red)and muscle tissue (blue). (c) The received power depends on the inclined
angle θ of the transmitted beam. (The reﬂected angle coincides with theincident angle.)

Figure 6(a) shows a conventional B-mode image of thethoracic vertebral surface. In the B-mode image, the vertebral
surface at depth of about 26 mm was depicted, but theinﬂuence due to the muscle tissue at depths from 10 to 23 mm
was not negligible.
Figure 6(b) shows the images obtained by the conventional envelope method. In this method, a narrow transmit
beam was employed. The red line shows the outer envelope,which is tangential to the depicted plural ellipses. Thismethod estimates the vertebral surface at a shallow depth
because it selects the shortest path length as the long axisof an ellipse.
Figure 6(c) shows a compounded image of Figs. 5(a)–5(i),the images obtained by the proposed method. In theFig. 5. Surface of the thoracic vertebrae estimated by the proposed
method using width and direction of the transmit beam. The results when thetransmit beam angle is −20, −15, −10, 0, 5, 10, 15, 20° from (a) to (i),respectively.

compounded result, the vertebral surface was depicted as
being smoother than that of each angle result of Fig. 5
acquired by the proposed method. The white boxes inFigs. 6(a) and 6(b) show the assigned area of the vertebral
surface and muscle tissue. Inside each box, the average power
was calculated and the contrast ratio deﬁned by the ratio of
the power in the vertebral surface area to the power in themuscle tissue area was calculated. The contrast ratio was10.2 dB in Fig. 6(a) and 31.1 dB in Fig. 6(b). Thus, theproposed method was able to more clearly depict thevertebral surface than possible with conventional B-mode
imaging.
4.

Discussion

A clearer image was obtained by the proposed method thanby the B-mode imaging method. This result shows that themeasurement plane was not perpendicular to the vertebral

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K. Takahashi et al.

This study intended to localize the gap between vertebrae by
the clear depiction of the thoracic vertebral surface. Therefore, the suppression of the contribution of soft tissue will
facilitate administration of thoracic anesthesia by the accurate
depiction of the vertebral surface.
The frame rate of the proposed method is about 1=9 of theconventional B-mode imaging method because nine images
are used to acquire a compound image. Since the movement
due to the heartbeat is imperceptible in the practice ofthoracic anesthesia, the proposed method is useful for theobservation and localization of the vertebral surface.
5.

Conclusions

Herein, we proposed an envelope method with limited widthand direction of the transmit beam, depicted the surface of thethoracic vertebrae in vivo, evaluated the ability of depiction
and compared the results of this method with B-modeimaging. From the results of this study, it was shown thatthe proposed method can clearly and extensively depict thevertebral surface. As explained above, the possibility wassuggested that the proposed method is suitable as an auxiliary
technique for visualization of the vertebral surface to safely
achieve epidural anesthesia.

Fig. 6. (Color online) (a) Conventional B-mode image for the thoracic
vertebral surface. (b) Surface of the thoracic vertebrae estimated by theconventional envelope method. The red line shows the outer envelope as theestimated vertebral surface. (c) Surface of the thoracic vertebrae estimated by
the proposed method. The results of all angles are compounded. The whiteboxes show the assigned area of the vertebral surface and that of the muscle
tissue.

surface, as shown Fig. 4(c), which may also occur in clinicalpractice because of the diﬃculty of vertical access to thevertebral surface. Even though this condition exists in the
proposed method, this method was successful in depicting thevertebral surface. This shows the robustness of the visualization of the vertebral surface by the proposed method.
The proposed method selectively depicts the vertebral
surface with the suppression of the contribution of soft tissue.

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