椎葉 倫久¹, 岡田 長也², 内田 武吉³, 黒澤 実⁴, 竹内 真一⁵

Michihisa SHIIBA¹, Nagaya OKADA², Takeyoshi UCHIDA³, Minoru KUROSAWA⁴, Shinichi TAKEUCHI⁵

¹桐蔭横浜大学大学院・日本学術振興会特別研究員DC, ²本多電子株式会社, ³産業技術総合研究所計測標準研究部門, ⁴東京工業大学大学院, ⁵桐蔭横浜大学

¹Toin University of Yokohama·Research Fellow of Japan Society for the Promotion of Science, ²Honda Electronics Company Limited, ³National Institute of Advanced Industrial Science and Technology, ⁴Tokyo Institute of Technology, ⁵Toin University of Yokohama

キーワード : hydrophone, hydrothermally synthesized lead zirconate titanate (PZT) polycrystalline film, Ti front plate, backing material, high-intensity ultrasound

近年，高強度の超音波を用いた様々な治療法や診断法が利用されている．しかし，強力な超音波を生体内に照射するためには，生体への安全性の観点からも，また効率的な利用という観点からも正確な音場，音圧，音響強度の計測が重要である．我々は，チタン製前面板（受音面）の裏面に水熱合成チタン酸ジルコン酸鉛（PZT）多結晶膜を成膜し，音響キャビテーションの発生を伴う強力な超音波の音場で測定をしても壊れないように工夫した耐音響キャビテーションの堅牢型ハイドロホンの開発を行ってきた．しかし，低固有音響インピーダンス背板を用いた従来の堅牢型ハイドロホンでは受波感度が高周波領域ほど低下してしまい，実際の超音波波形を十分な忠実度で表現できないという問題点があった．そのため，Masonの等価回路に基づく数値シミュレーションを実施して周波数特性の改善策を検討した結果，固有音響インピーダンスが約20 MRaylの背板材料を用いることでフラットな受信感度の周波数特性を得られることがわかった．我々は固有音響インピーダンスが20 MRayl付近の背板を有する新しい堅牢型ハイドロホンを開発した．新しい堅牢型ハイドロホンでは受波感度が高周波領域で低下することを抑制できた．このハイドロホンを用いると従来の堅牢型ハイドロホンよりも忠実に超音波波形の非線形歪を表現できることがわかった．強力な超音波により音響キャビテーションが発生している超音波洗浄器の水槽内で，各ハイドロホンの超音波曝露実験を行った結果，新しい堅牢型ハイドロホンは市販のハイドロホンの約10倍の耐久性を持っていると考えられた．

Recently, a number of diagnosis and treatment methods employing high-intensity ultrasound have been developed and used clinically. However, it is important from the standpoint of protecting human tissue and the standpoint of effective utilization of high-intensity ultrasound to accurately measure sound field, sound pressure, and acoustic intensity. Thus, we have been developing an anti-cavitation hydrophone that can withstand measurement in a high-intensity ultrasound field with inertial acoustic cavitation by deposition of a hydrothermally synthesized lead zirconate titanate (PZT) poly-crystalline film on the reverse side of the titanium front plate. However, our conventional anti-cavitation hydrophone with low specific acoustic impedance backing had limitations, i.e., the receiving sensitivity decreased with an increase in frequency, and it could not describe the actual ultrasound waveform with high fidelity. Therefore, we considered improving the frequency characteristics of the receiving sensitivity by performing numerical simulations based on the Mason’s equivalent circuit model and one-dimensional acoustic transmission line model. It was found that flat frequency characteristics of the receiving sensitivity could be obtained by using a backing with about 20 MRayl specific acoustic impedance. We then developed a new anti-cavitation hydrophone using a backing with about 20 MRayl specific acoustic impedance based on the simulated results. The decrease in receiving sensitivity in the high-frequency range could be suppressed in our newly fabricated anti-cavitation hydrophone. Furthermore, durability tests of each hydrophone under ultrasound exposure were performed in a water tank containing ultrasound cleaner with generation of inertial acoustic cavitation. It was found that our new anti-cavitation hydrophone had a 10-fold longer lifetime as compared with a commercial needle-type hydrophone.