3Pb5-10
Effect of difference in shear modulus of phantom ondisplacement distribution induced by acoustic radiation
force of focused ultrasound
集束超音波の音響放射圧によって生じたファントムの変位分布に対する剛性率の影響
Erika Numahata1†, Shin-ichiro Umemura1, and Shin Yoshizawa2,3(1Grad. School of Biomed. Eng., Tohoku Univ.; 2Grad. School of Eng., Tohoku Univ.;
3SONIRE Therapeutics)
沼畑瑛里香 1†, 梅村晋一郎 1, 吉澤晋 2,3(1 東北大院 医工, 2 東北大院 工, 3 ソニア・セラピューティクス)
1. Introduction
High-intensity focused ultrasound (HIFU)
treatment is one of the noninvasive treatments forcancer. In this treatment, the tumor is thermallycoagulated by focusing ultrasound generated outside
the body onto the target. With this method, thetreatment region cannot be directly observed by
optical method. Therefore, the prediction of theHIFU treatment region in advance by ultrasoundimaging is important for the efficacy and safety ofthe treatment.
The previous study1), showed that acoustic
radiation force (ARF) imaging is a useful indicatorof heat generated in the HIFU focal region. Thisestimation method is based on the approximationthat the distribution of HIFU heat source isproportional to that of the ARF induced by a HIFU
short burst. Since ARF induces a tissue displacement,
the displacement distribution is proportional to theARF distribution, when neglecting the effect of shear
wave propagation. Therefore, the distribution ofHIFU heat source was estimated from the ultrasonicmeasurement of the tissue displacement distribution.
However, this method still has some issues inquantitativity. In this study, tissue mimicking
phantoms with different shear moduli were used toevaluate the effect of shear modulus on theestimation of the radiation force distribution.

proportional to the acoustic intensity. Assuming thatthe acoustic attenuation is proportional to theabsorption, the displacement induced by F can be
regarded as proportional to Q.
The shear wave propagation velocity cs whena shear wave propagates is expressed as,(1)𝑐𝑠 = √𝐺 ⁄𝜌 ,where G is the shear modulus of elasticity (stiffnessmodulus) and ρ is the density. Therefore, the stiffness
modulus is proportional to the square of the shear
wave propagation velocity.
In summary, assuming that the displacement
induced by ARF is caused only by shear deformation,
the displacement is inversely proportional to thestiffness modulus and square of the shear wave
propagation velocity.
2.2. Experimental setup and Sequence
The experimental setup is shown in Fig. 1. A
push beam for ARF was generated by a 128-channel
linear probe (Hitachi Aloka Medical) connected to an
ultrasound imaging system (Verasonics, Vantage256), driving the central 20 elements and setting thefocus at a depth of 16 mm from the surface of thephantoms (OST ELPT-018). The phantoms haveshear wave velocities of 1.15, 1.6, and 3.2 m/s. By
using the same linear probe and imaging system, RFdata is acquired.

2. Materials and Methods
2.1. HIFU heat source estimation
When a tissue is exposed to ultrasonic waves,
ARF is generated. ARF F and the acoustic intensity
I are proportional.
On the other hand, the heat Q generated byHIFU exposure can be approximated to beerika.numahata.p3@dc.tohoku.ac.jp

Fig. 1

Schematic of experimental setup.

Fig. 2 shows the ultrasound exposure
sequence for push beam transmission and RF dataacquisition. A push beam at a voltage of 50 V and aduration of 90 µs at a frequency of 6 MHz wasfocused to the phantom, and a single plane wave at
7.5 MHz was transmitted from the imaging probe
before and after the push beam exposure. The axial
displacement is calculated by applying the 2D
combined autocorrelation method2) between theframes before and after the push beam exposure. It
was measured that it took 416-418 µs to transmit an
imaging pulse after the end of the push beam
exposure in this experiment.

Fig. 2 Ultrasound exposure sequence.
3. Results and Discussion
Fig. 3 shows the displacement distributions in
the three phantoms with different shear wave
propagation velocities, about 400 µs after the end of
the push beam. In all phantoms, the displacement in
the shallower area than the focal point at a depth of
16 mm took the maximum value. Since the locationof the maximum displacement shifts toward theshallower region as the shear wave propagation
proceeds3), so the larger the shear modulus is, theshallower the location of the maximum displacement
should be. In Fig. 3, the phantom with a shear wavepropagation velocity of 3.2 m/s, which has thehighest shear modulus among the three, shows themaximum displacement about 10 mm in front of thefocal point.

Fig. 3 Displacement distributions in the threephantoms with shear wave velocities of (a) 1.15m/s, (b) 1.6 m/s, and (c) 3.2 m/s.

Fig. 4 shows the averaged values of themeasured displacements over the ROI (from 9.0 to
11.0 mm and -1.0 to 1.0 mm in axial and lateral
directions, respectively) in each phantom. The
average displacements in the phantoms with shear
wave propagation speeds of 1.15, 1.6, and 3.2 m/s
were 13.1 ± 0.8, 10.6 ± 0.2, and 5.4 ± 0.4 µm,respectively. Comparing these values, it is seen thatthe ratio of the average displacement was close to theinverse ratio of the shear wave propagation velocity,rather than that of the shear modulus. The halfwidths of the displacement distribution in thephantoms with shear wave propagation speeds of
1.15, 1.6, and 3.2 m/s were 1.4, 1.4, and 2.9 mm,respectively.
The effect of the shear wave propagation wasclearly shown in the result of the half-width in the
3.2 m/s phantom. The shear wave propagation may
have caused the larger average displacement in the
3.2 m/s phantom than that in the 1.6 m/s phantom
divided by the ratio of shear modulus. The
measurement of the acoustic absorption coefficients
will be also needed to compare the displacements indetail.

Fig. 4 Average displacements in ROI in the threephantoms with different shear modulus.
4. Conclusion
In this study, the quantitative performance ofARF imaging was evaluated by measuring thedisplacement in phantoms with different shear
modulus for the quantitative estimation of HIFU heatsource distribution. While a negative correlation
between the displacement and shear modulus wasobserved, further study is needed for the detailanalysis to consider the effect of shear wave
propagation.
References
1. H.Yabata, et al.: Jpn. J. Appl. Phys. 60, (2021)
SDDE23
2. T. Shiina, et al.: J. Med. Ultrason. 29, (2002)
119.
3. M. L. Palmeri et al.: IEEE Trans. Ultrason.
Ferroelect. Freq. Contr. 52, (2005) 1699.