Journal of Medical Ultrasonics (2020) 47:3–11
https://doi.org/10.1007/s10396-019-00964-0

ORIGINAL ARTICLE–PHYSICS & ENGINEERING

Discrimination of thoracic spine from muscle based on their difference
in ultrasound reflection and scattering characteristics
Tomohiro Yokoyama1 · Shohei Mori2
Hiroshi Kanai2,1· Mototaka Arakawa1,2· Eiko Onishi3· Masanori Yamauchi3 ·Received: 10 April 2019 / Accepted: 28 June 2019 / Published online: 21 August 2019
© The Japan Society of Ultrasonics in Medicine 2019

Abstract
Purpose Medical ultrasound is often used to specify the puncture position during epidural anesthesia. However, visualizationof the thoracic spine is difficult because of the complex structure, i.e., it is difficult to determine whether the thoracic spineor muscle is depicted. Therefore, this study aims to distinguish bone from muscle tissue using the differences in reflection
and scattering characteristics of ultrasound.
Methods We experimentally investigated the difference in signals received from bone and muscle. We proposed a newparameter utilizing the ratio of the amplitude of the received signals averaged in a wide range around the ideal delay line
and that only along the ideal delay line, to emphasize the bone.
Results First, we confirmed the difference in signals received from bone and muscle tissue by basic experiments. We also investigated the difference by in vitro experiments using chicken thigh and in vivo experiments in humans. In both experiments, the proposed method succeeded to clearly depict bone, suppressing the depiction of muscle, compared with conventional B-mode imaging.
Conclusion Using the difference in the characteristics of reflection from bone and scattering from muscle tissue, we could
distinguish bone from muscle tissue with the proposed method.
Keywords Ultrasound imaging · Epidural anesthesia · Bone · Muscle · Scattering · Reflection

Introduction
Epidural anesthesia is a local anesthesia. A needle is punctured from the spinal gap into the epidural space, and thenan anesthetic is injected during surgery. It has the following
three advantages compared to general anesthesia. First, thestress response is suppressed, and immunity is maintained
during surgery for the lower limbs and lower abdomen. Second, pain after surgery can be drastically reduced by inserting a catheter that administers medication continuously.
Third, the incidence of postoperative cardiovascular and* Mototaka Arakawa
arakawa@ecei.tohoku.ac.jp
1

Graduate School of Biomedical Engineering, Tohoku
University, Sendai 980‑8579, Japan

2

Graduate School of Engineering, Tohoku University,
Sendai 980‑8579, Japan3

Department of Anesthesiology and Perioperative Medicine,
Tohoku University School of Medicine, Sendai 980‑8575,
Japan

respiratory complications decreases [1]. Therefore, epidural
anesthesia greatly contributes to analgesia during surgery
and early recovery after surgery.
Gap identification is important for epidural anesthesia;
however, it relies on the doctor’s experience and palpation. The failure rate of epidural anesthesia was reported as
6–25% [2–4], and the proportion of patients who complainedof back pain and mental distress was reported as 22% and14%, respectively [5].
Before performing epidural anesthesia in clinical practice,practitioners often observe ultrasound images of the spine[6] to confirm the puncture position, but a clear image cannot be obtained [7]. Although various methods have beendeveloped for using medical ultrasound in epidural anesthesia [8, 9], puncture of the needle under real-time ultrasonic guidance is still difficult due to the low capability ofultrasound imaging in bone. Automatic identification of the
epidural space and extraction of vertebrae on epidural anesthesia using ultrasound have been studied [10–12], but thosestudies involved the lumbar spine, which has a large gap andsimple structure. In addition, ultrasound imaging related to13Vol.:(0123456789)

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Journal of Medical Ultrasonics (2020) 47:3–11

bone has been increasingly used in the field of orthopedic
surgery [13, 14], and methods using the distortion generated
by the application of pressure [15, 16] or the phase symmetry [17] have been proposed. However, the spine, which has
to be measured during the epidural anesthesia, is in a deeper
position. In addition, the thoracic spine has a complicatedstructure and its visualization is difficult.

Therefore, to depict the surface of the thoracic spine, we
proposed a method for improving the mismatch of the timedelay addition caused by the reflection of ultrasound at thesurface of the thoracic spine [18] by applying the envelope
method [19], in which the transmitted and received positions of ultrasound signals are not regarded as the same,and the range point migration (RPM) method [20, 21]. As

Fig. 1  Schematic diagram of a the transmitted wave from each piezoelectric element of the ultrasound probe to form the focus beam andb–f reflected waves and received signals from various targets. The
upper and lower figures in b–f show contour plots of envelope amplitudes of received signals at received elements and reflected waves

from the targets, respectively. Depth is calculated from the delay
times of the received signals and the assumed sound velocity. Lower
parts in the figures correspond to the deeper parts. b Point scatterer. c
Muscle parallel to the probe. d Bone parallel to the probe. e Muscle
tilted to the probe. f Bone tilted to the probe

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Journal of Medical Ultrasonics (2020) 47:3–11

a result, it was possible to depict the surface of the thoracic
spine, which was unclear in B-mode images. However, there
still remains the problem of muscle present in a shallower
position than the thoracic spine also being depicted. In thepresent study, we investigate the difference in the reflection
and scattering characteristics of ultrasound from the thoracic
spine and muscle to depict the thoracic spine with the muscle suppressed. Then, we propose a method to identify the
position of the thoracic spine utilizing the difference.

Principle
Difference in scattering and reflection
between bone and muscle
Piezoelectric elements of an ultrasonic probe are usuallyused for the transmission and reception of ultrasound.
Focused waves formed by a linear array probe are used toobserve the thoracic spine. We examined differences inultrasonic scattering and reflection characteristics at muscle
and bone using the received signal at each receive element
before delay addition for one transmission beam. To form
the focused wave, ultrasound waves are transmitted fromouter elements to the center element in order, as shown inthe schematic diagram of Fig. 1a. Here, the arc drawn withthe black line represents the wavefront of the transmitted
wave from each element, and the yellow part represents thehigh-amplitude part caused by focusing, which is the main
lobe. Usually, sidelobes are also formed around the main
lobe, although they are not drawn in Fig. 1a.
Figures 1b–f are schematic diagrams of scattered orreflected waves from various targets (lower figures) andenvelope amplitudes of the received waveforms at received
elements by irradiating the usual focus wave in the ultrasound diagnosis apparatus (upper figures). The delay timeat each element for the transmission of the focused wave
was corrected as the timing of the transmission from thecenter element became 0. The transmission delay time ateach element and the distance from the target to the receiving element were taken into account when drawing theseschematic diagrams. The received signal at each element
becomes a convolution between the transmitted wave andall scattered or reflected signals. Here, it was assumed thatmuscle tissue and bone were much larger than the wavelength of the ultrasound.
When the target is a point scatterer, a scattering wave
is emitted from the scatterer and received at received elements, as illustrated in Fig. 1b. Figure 1c illustrates thereceived waves from the muscle parallel to the probe. Figure 1d illustrates the received waves from a bone parallel to the probe. Considering the scattering and reflection
properties in the muscle tissues and bone surface, it is5thought that scatterers sparsely exist in muscle tissues andscatterers densely exist at the surface of bone. In the caseof muscle tissue, each received wave is the same as thatfrom the point scatterer, although the amplitude of thereflected waves except for the focal point is small owing
to the low reflectance. On the other hand, in the case ofbone, since the bone has a high reflectance, not only themain lobe of incident waves to the bone but also sidelobesof those are strongly reflected at the surface. As a result, a
lot of reflected waves having high intensity reflected fromdifferent positions on the surface of the object are received
at different times. Therefore, compared to the received signals from the point scatterer and muscle, those from bone
spread in the shallower region at the outer received elements caused by the effect of the sidelobes.
Next, we consider cases where the targets are tilted tothe probe. Figure 1e illustrates the received waves frommuscle tilted to the probe. The received wave from the
focal point is the same as that from the muscle parallel tothe probe in Fig. 1c. The times that scattered waves arereceived from positions other than the focal position are
different from those in the case of muscle tissue parallelto the probe, but the intensity of these scattered waves ismuch lower than that of the scattered waves from the focal
point. Therefore, scattered waves from muscle are almost
independent of the inclination of the target.
Figure 1f illustrates the received waves from a bonetilted to the probe. Ultrasound reflects specularly at thesurface of the bone. Received waves with high amplitude
are observed at the lateral position depending on the inclination of the bone and those widely spread in the depth
direction at the opposite received elements.
In muscle tissue, received signals with high amplitude
are always obtained around the center receiving element
regardless of the tilt of the object. With bone, due to specular reflection, received signals with high amplitude arereceived by the receiving element in the direction corresponding to the inclination of the object. These characteristics are useful to discriminate bone and muscle using thereceived waveforms at the received elements.

A new method to identify the position of bone
To identify the position of the thoracic spine, we utilize thecharacteristics that reflected waves from bone spread in thedepth direction at the outer received elements and scatteredwaves from muscle follow the ideal delay-time distributionof the point scatterer. We use the received waveform obtainedfor each transmission beam i of the linear electronic operation method [22], in which the multiple ultrasonic beams aretransmitted while shifting the transmission positions. The
B-mode image is obtained by summing the received waveforms with the addition of a delay corresponding to the depth

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Journal of Medical Ultrasonics (2020) 47:3–11

Fig. 2  Schematic diagram of theconcept of the procedure in the
proposed method. a The green
line is the ideal delay-time
distribution from the point scatterer and Pline (d, i) is the sumof the amplitude on the line. b
The zone within the blue frame
is the region for calculation of
Pzone (d, i). w(j) is a windowfunction. d means depth calculated from the delay time ofreceived signals

at each sampling point for each transmission beam i. This
depicts all boundaries between media with different acoustic impedances. Muscle tissue and bone, in particular, aredepicted with high intensity. Here, we propose a new method
to emphasize only the depiction of bone.
Figure 2 schematically shows the concept of the proposed
method. Here, we explain using the schematic diagram of
the received waveform obtained from bone. Figure 2a
sketches contour plots of envelope amplitudes of received
waves from bone and an ideal delay time from the point

scatterer to calculate Pline (d, i). Pline (d, i) is the parameter
indicating the sum of envelope signals Ai (t, j) along the ideal
delay time 𝜏(d, j) from the point scatterer at the depth d forthe transmitted beam number i at the n received elements.

Pline (d, i) =n−1∑

Ai {c𝜏(d, j), j},

(1)

j=0

where c is the sound speed. Ai (t, j) is obtained by the envelope detection of the rf signals.

Fig. 3  Experimental results using wire and acrylic plate as targets. A B-mode image. B Received signals for the beam at the lateral position of
18 mm. a Wire. b Acrylic plate parallel to the probe. c Acrylic plate tilted to the probe

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Fig. 4  Experimental results for chicken thigh with bone. A B-mode image. B Received signals for the beam at the lateral position of 18 mm. a
Muscle parallel to the probe. b Bone parallel to the probe. c Muscle tilted 18° to the probe. d Bone tilted 7° to the probe

Figure 2b sketches contour plots of envelope amplitudes of received waves from bone and a region to calculate
Pzone (d, i) as follows. Pzone (d, i) is the parameter indicating
the average value of the sum of the envelope amplitude
Ai (t, j) within the width WΔd at the center of the depth dfor the transmitted beam number i. Here, W is the numberof samplings and Δd is the sampling interval. A windowfunction w(j), as shown in Fig. 2, is multiplied to reduce theamplitudes obtained around the center receiving elements
since the scattering waves from muscle tissues have highamplitude around center received elements, as shown in
Fig. 1c and e.

�∑n−1 �
1 ∑ (W−1)∕2
w(j)
A
j)
+
kΔd,
j}
{c𝜏(d,
j=0
k=−(W−1)∕2 i
W
Pzone (d, i) =
.
∑n−1
w(j)
j=0

(2)
Scattered waves from muscle follow the ideal delay-time
distribution for scattered waves from the point scatterer. On
the other hand, reflected waves from bone also contain theother components. Thus, Pline of muscle tissue and bone arealmost the same, and Pzone of bone becomes larger than thatof muscle tissue.

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In addition, we introduce the parameter Spower (d, i). Thisparameter is Bpower (d, i), which is the power of the B-mode
image, multiplied by the ratio of Pzone (d, i) and Pline (d, i).
}{Pzone (d, i)
Spower (d, i) = Bpower (d, i) ⋅,(3)
Pline (d, i)

Spower (d, i) is large for bone and low for muscle tissue. Therefore, the bone can be emphasized and the muscle can besuppressed in the image composed by Spower (d, i) comparedwith the B-mode image.

Experimental conditions
We observed the received signals in fundamental experiments using a molybdenum wire and an acrylic plate, inin vitro experiments using chicken’s thigh, and in in vivo
experiments with the human thoracic spine. Then, weapplied the proposed method to in vitro experiments using
chicken’s thigh and to in vivo experiments with the humanthoracic spine.
First, we verified the difference in signals received fromthe wire and the acrylic plate as the point scatterer and thespecularly reflecting object, respectively. The diameter of thewire was 0.2 mm, and the thickness of the plate was 40 mm.
Next, we examined the difference in the scattering andreflection characteristics of ultrasound from muscle andbone using a chicken thigh with bone. This chicken thighwith bone was wide compared with the probe, whose width
was 36 mm. The chicken thigh was set at the depth of 30 mm
in a water tank filled with physiological saline solution.
Finally, to compare the proposed method with the conventional B-mode image, we measured a paramedian image ofthe human thoracic spine centered on the thoracic gap, whichshould be depicted during the actual epidural anesthesia.
We used an ultrasound diagnosis apparatus (ProSound
α-10, Aloka) and a linear array probe with 181 elementswith a pitch of 0.2 mm. We used the focus beams by setting
the number of transmit beams, the transmit frequency, thesampling frequency, and the focal distance as 181, 10 MHz,
40 MHz, and 30 mm, respectively, in the ultrasound diagnosis apparatus. The temperature of the water was 23° C during
the water tank experiment. Subjects A and B were healthy
23-year-old males, and subject C was a healthy 21-year-old
male.

Journal of Medical Ultrasonics (2020) 47:3–11(A) the B-mode images and (B) the received signals for thetransmitted beam at the lateral position of 18 mm. Figure 3ashows the result for the wire. Only the waves along the ideal
delay time from the point scatterer were acquired. Figure 3b
shows the results for the acrylic plate parallel to the probe.
Waves deviating from the ideal delay time from the point
scatterer were also observed in the shallower region on theouter elements. Figure 3c shows the results for the acrylic
plate tilted to the probe. Spread of the received waves to theshallower region was observed at the received elements on
the right side. Waves with high amplitude were observed at
the received elements on the left side. These were caused by
the specular reflection at the surface of the acrylic plate. In
the right side in Fig. 3B–c, three waves were observed in thedepth direction. In principle, it must be continuous because
the scattering points were densely distributed at the surface
of bone. This might have been caused by the apodization of
the transmitted wave in the ultrasound diagnosis apparatus.

In vitro experiment
The results for the chicken thigh with bone are shown in
Fig. 4. Figure 4 shows the results of (A) the B-mode imagesand (B) the received signals for the transmitted beam at thelateral position of 18 mm. Figure 4a, b, c, and d show theresults for muscle parallel to the probe, bone parallel to theprobe, muscle tilted 18° to the probe, and bone tilted 7° tothe probe, respectively.

Results and discussion
Fundamental experiments
We examined the difference in the signals received fromthe wire and the acrylic plate. Figure 3 shows the results of

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Fig. 5  Depiction of chicken thigh with bone for the region including
both muscle and bone. a B-mode image. b Image yielded by the proposed method

Journal of Medical Ultrasonics (2020) 47:3–11

9Fig. 6  Results of in vivo experiment in subject A. a B-mode
image. b Received signal frommuscle. c Received signal frombone. d Pzone. e Pzone ∕Pline. f
Spower . g Bpower along yellow
lines in (a) and (f). h Spower
along yellow lines in (a) and (f)Similar received waves were observed for Fig. 4a andc because the transmitted waves were scattered at the surface of the muscle. In Fig. 4b and d, high-amplitude waves
that deviated from the ideal delay-time distribution for the
point scatterer were observed at the outer region of thereceived elements. The inclination of the wavefront wasobserved in Fig. 4d. The waves that scattered and reflectedfrom muscle and bone were similar to those from the wire
and the acrylic plate in Fig. 3, respectively. From theseresults, the scattering and reflection characteristics of muscle and bone were confirmed, respectively.
Figure 5 shows the measurement result for the region
including both muscle and bone. Figure 5a shows the
B-mode image and Fig. 5b shows the result for the proposed parameter Spower (d, i). W was set at 50 in the present
study based on the results in the fundamental experiment,
as shown in Fig. 3. WΔd was 0.96 mm. Although WΔd waslarger than the pulse width, the spatial resolution yieldedby the proposed method was not inferior to that of theconventional B-mode image. The spatial resolution of bonein the B-mode image was lowered by the widening of thereflection signals, as shown in Figs. 3B–b and B–c. Fromthese results, it was demonstrated that bone was emphasized
and muscle tissue was suppressed, although the surface ofmuscle was depicted, which is not a problem because thedepth of the surface of tissues is 0 mm in vivo.

In vivo experiment
Figure 6 shows the results for the human thoracic spine in
subject A. Figure 6a, b, and c show the B-mode image, thereceived waves from the muscle, and the received waves
from the bone, respectively. Figure 6b and c were received
beams for the transmitted beam at the lateral position of
9 mm. From Fig. 6b, the received waves from the muscle
followed the ideal delay-time distribution from the point
scatterer, and they had high amplitude from the 40th to 50th
received elements at the depth of 17 mm. On the other hand,

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Journal of Medical Ultrasonics (2020) 47:3–11

Fig. 7  Results of in vivo experiment for (a)–(d) subject B and(e)–(h) subject C. a B-mode
image. b Image yielded by the
proposed method. c Bpower along
yellow lines in (a) and (b). d
Spower along yellow lines in (a)and (b). e B-mode image. f
Image yielded by the proposed
method. g Bpower along yellow
lines in (e) and (f). h Spower
along yellow lines in (e) and (f)investigating the received waves from the bone in Fig. 6c,waves with high amplitude were observed from the 40th to
96th received elements at the depth of 27 mm, and waves
that deviated from the ideal delay-time distribution of thepoint scatterer were also observed, especially around the20th to 30th received elements. Therefore, the reflection
and scattering properties of bone and muscle were also confirmed, respectively, in the in vivo experiment.
We applied the proposed method to the measured data
for subject A, as shown in Fig. 6. Figure 6a, d, e, and fshow the results of Bpower , Pzone , Pzone ∕Pline , and the proposed parameter Spower ( Pzone ∕Pline × Bpower ), respectively.

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The visualization of the thoracic spine with Pzone was better than that with Bpower ; however, the effect of muscle wasstill large. Using Pzone ∕Pline , the thoracic spine was betteremphasized than using Pzone . Figure 6g and h exhibit Bpowerand Spower along the yellow lines, as shown in Fig. 6a and f.
The proposed method suppressed the depiction of muscle
and clearly depicted the shape of the thoracic spine.
Figure 7 shows the results for subjects B and C. In subject
B, both the muscle tissue and the thoracic spine showed highintensity. In subject C, the thoracic spine was clearly visualized with the proposed method. For all subjects, Spower could
obviously emphasize bone compared with Bpower.

Journal of Medical Ultrasonics (2020) 47:3–11

As for the reason why the proposed method could not
distinguish between muscle tissue and bone in subject B, thefascia in subject B was denser than that in the other subjects,
as seen in the B-mode image in Fig. 7a. Spower increased
because reflection waves from other muscle might have been
included in the calculation of Pzone.

Conclusions
In this paper, we developed a method of depicting the thoracic spine utilizing the difference in reflection and scattering characteristics of ultrasound from bone and muscle. The
reflection and scattering characteristics were confirmed fromthe basic experiments and in vitro experiment. The proposed
method was applied to the human thoracic spine, and bone
was clearly depicted with suppression of the muscle present in the shallower region in three subjects. The proposed
method should be useful for real-time monitoring during
epidural anesthesia.

Compliance with ethical standards
Conflict of interest The authors have no conflicts of interest with regard to the presented research.
Informed consent Informed consent to the protocol was obtained fromcontrol subjects. The study was approved by our institutional ethics
committee for human research.

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