3Pa5-5
A Basic Study on Effect of Shielding Objects on Focused
Ultrasound Treatment by Acoustic Radiation Force
ImagingErika Numahata1‡, Shin Yoshizawa1,2,3, Shin-ichiro Umemura1,3, Takuro Ishii4,Yoshifumi Saijo1 (1Grad. School of Biomed. Eng., Tohoku Univ.; 2Grad. School ofEng., Tohoku Univ.; 3SONIRE Therapeutics; 4Frontier Research Institute forInterdisciplinary Sciences)
1. Introduction
HIFU is a minimally invasive treatment inwhich ultrasound waves are irradiated from outside
the body and focused inside the body to selectively
heat and coagulate tissue. The problem with thistreatment is that the treated region cannot be seen by
the naked eye. Therefore, it is necessary to estimate
the treatment region in advance using ultrasoundimages to improve the safety and efficacy of thetreatment.
The previous study1) showed that acoustic
radiation force (ARF) imaging is effective as apredictor for the heat which should be generated inthe HIFU focal region. This estimation method isbased on the approximation that the distribution ofHIFU heat source is proportional to the ARF inducedby a very short HIFU exposure.
In actual clinical practice, when a liver cancer
is targeted, the HIFU transducer can be partially
blocked by the ribs, which may result in inadequate
treatment to the targeted region. Therefore, in thisstudy, we estimated the heating distribution in thepresence of shields such as ribs, and investigated acorrection method for such cases.
2. Materials and Methods
2.1. HIFU heat source estimation by ARF
When tissue is exposed to ultrasound waves,
ARF is generated. ARF F is expressed as,
2𝛼𝐼
(1)
𝑐
where α is the acoustic attenuation coefficient, I is
the acoustic intensity, and c is the longitudinal soundvelocity in biological tissue.
The heat quantity Q generated by HIFU
exposure is expressed as,|𝐹| =𝑄 = 2𝛼𝑎 𝐼(2)where 𝛼𝑎 is the acoustic absorption coefficient.
Assuming that most of the attenuation is due toabsorption, the right-hand sides of equations (1) and(2) are proportional. In addition, F can beerika.numahata.p3@dc.tohoku.ac.jp

approximated to be proportional to the tissue
displacement u. Therefore, the heat generation anddisplacement are also approximately proportional,
and the heat generation distribution can be estimatedfrom the displacement distribution.
In the estimation method used in this study,
HIFU is first applied for a short period of time, andthe displacement caused by the ARF is detected byultrasonic echoes. The displacement distribution
obtained from the ultrasound echoes is used as theestimated coagulation area. Next, HIFU at the same
intensity is continuously irradiated for tens ofseconds to generate thermal coagulation. The
process of displacement detection is called theestimation mode, and the process of generating
coagulation and actually performing treatment iscalled the coagulation mode in this study.
2.2. Experimental setup and Sequence
The experimental setup is shown in Fig. 1. A
chicken breast tissue was vacuum compressed in aplastic bag and placed in a water tank filled withdegassed and deionized water at room temperature.
The tissue was placed in room-temperature saline
solution and degassed for 3 hours before enclosed in
the bag. A 256-channel HIFU transducer was placedin the tank and driven at 1 MHz by a driving system(Microsonic). A sector probe was placed in thecentral hole of the HIFU transducer and connected toan ultrasound imaging system (Verasonics) toacquire RF data. Two 12 mm diameter rods wereplaced on one side of the HIFU transducer tosimulate ribs. Before the HIFU exposure experiment,
the acoustic pressure at the focal point was measured
in two patterns, one without the rods and the otherwith the rods. The pressure with the rods was about
10% lower than that without the rods.
The ultrasound exposure sequence is shown inFig. 2. In the estimation mode, RF data wereacquired by transmitting a single plane wave before
and after HIFU exposure, with an interval of 0.8 ms
after HIFU exposure to reduce interference of HIFU
with the ultrasound imaging. A two-dimensional
combined autocorrelation method2) was applied
between frames before and after the HIFU push-beam exposure to calculate axial displacement. In
the coagulation mode, coagulation was generated
with HIFU with a duration of 20 s, whose total
acoustic power (TAP) of 40 W was the same as thepush-beam.

Fig. 1was observed at a depth of 60 to 72 mm. The shift in
the displacement peak of the estimated mode may be
attributed to the overlap of the HIFU beams on theaxial axis as the shear wave propagates3) and also tothe approximation assuming that the displacement is
proportional to the ARF.

Schematic of experimental setup.
Fig. 3 Displacement distributions in chicken breast
induced by HIFU at a TAP of 40 W with a durationof 0.5 ms (a) without rods and (b) with rods.

Fig. 2Ultrasound exposure sequence.

3. Results and Discussion
Fig. 3 shows the results of the estimation mode
without and with rods. Fig. 3(a) shows the result of
the estimation mode without rods, and Fig. 3(b)shows the result of the estimation mode with rods
inserted and the amplifier voltage adjusted so that theintensity at the focal point is the same value as
without rods. Comparing (a) and (b), there was no
significant difference in the magnitude ofdisplacement and the area of displacement despite
the different amplifier voltage values.
Fig. 4 shows the set of results for theestimation and coagulation modes without and withrods. Figs. 4(a) and 4(b) show the results of bothmodes without rods, while Figs. 4(c) and 4(d) show
the results of both modes when rods were inserted
and at different locations than in Figs. 4(a) and 4(b),using the voltage values used in Fig. 3(b).
Comparing the results of the estimation mode, there
was no significant difference in the area ofdisplacement and the magnitude of displacement.
Comparing the results of the coagulation mode, thecoagulated area was almost the same. In both cases,
coagulation was generated at a depth of 65 to 75 mm,while in the estimated mode, a displacement area

Fig. 4Comparison between (a) displacement
distributions without rods, (b) coagulated tissue
without rods, (c) displacement distributions withrods, (b) coagulated tissue with rods.
4. Conclusion
The results suggest that even in the presenceof partial acoustic shielding, it may be possible togenerate coagulation almost the same as that without
the shielding if the HIFU output intensity is adjusted
so that the estimation mode displacements should beequal.
References
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