荒井 修¹, 東 隆^{2, 3}, 川畑 健一², 村垣 善浩⁴, 伊関 洋⁴

Osamu ARAI¹, Takashi AZUMA^{2, 3}, Ken-ichi KAWABATA², Yoshihiro MURAGAKI⁴, Hiroshi ISEKI⁴

¹東京女子医科大学大学院先端生命医科学系専攻, ²株式会社日立製作所中央研究所, ³東京大学大学院工学系研究科バイオエンジニアリング専攻, ⁴東京女子医科大学先端生命医科学研究所

¹Department of Advanced Biomedical Engineering and Science, Graduate School of Medicine, Tokyo Women's Medical University, ²Central Research Laboratory, Hitachi, Ltd., ³Department of Bioengineering, School of Engineering, The University of Tokyo, ⁴Institute of Advanced Biomedical Engineering and Science, Tokyo Women's Medical University

キーワード : focused ultrasound surgery, targeting, predict focal spot, acoustic radiation force, displacement mapping

目的：安全かつ確実な強力集束超音波（high-intensity focused ultrasound: HIFU）治療のために，事前に焦点位置を確認できる技術の開発が望まれている．本研究では，熱変性を引き起こさない低いエネルギーのHIFUパルスを照射し，音響放射力によって生じた変位を超音波で撮像して焦点を可視化する．対象と方法：照射終了から変位観察を行うまでの時間に関して，感度と正確度それぞれに最適な条件が異なる可能性がある．そこで，パルス照射終了から焦点撮像までの時間間隔を0.3 ms，0.8 ms，1.3 msとして変位画像を撮像し，描出された焦点領域を実際のHIFU照射による熱凝固領域と比較した．実験試料はブタ肝臓とした．結果と考察：どの条件でも十分な感度が得られることを確認した．一方，正確度については，0.3 msの撮像であれば，サイズ1.1 mm×3.6 mmのHIFU熱凝固領域を0.8 mmの正確度で予測できたが，撮影時刻が遅くなると，変位分布の変化により正確度と精度が低下することを確認した．この結果はせん断波の伝搬によって説明できた．焦点を正確に描出するためには，せん断波速度を考慮した条件設定で変位を撮像する必要があることが明らかになった．結論：焦点イメージングによって，術者は，熱凝固が起こる領域を事前に予測できるようになる．この技術は，HIFU照射のターゲティングの課題を克服するものであり，さらに超音波装置を使ったコンパクトなシステムなので，HIFU治療をより多くの医療機関，疾患へ普及させられる可能性を持っている．

Purpose: Development of a technique to confirm the focus position in advance is desired for safe and reliable treatment with high-intensity focused ultrasound (HIFU). The purpose of this study is to induce displacement by means of acoustic radiation force generated by a low-energy HIFU pulse to make displacement images using ultrasound, and visualize the focal spot. Subjects and Methods: The optimal interval between the end of exposure and observation of displacement may differ in the case of sensitivity and accuracy. Hence, we set the time interval between the end of exposure and imaging of the focal spot to 0.3 ms, 0.8 ms, and 1.3 ms to compare the predicted focal spot with the actual thermocoagulation spot by HIFU. The experiment sample was porcine liver. Results and Discussion: The sensitivity was sufficient at every time interval. On the other hand, accuracy decreased as the time interval increased. A thermocoagulation spot with a size of 1.1 mm×3.6 mm was predicted with accuracy of 0.8 mm at 0.3 ms, but due to a change in displacement distribution, the accuracy and precision decreased at 0.8 ms and 1.3 ms. This result was explained with shear wave propagation. It became clear that it was necessary to consider the speed of shear waves in order to visualize the focal spot correctly when configuring the conditions for making displacement images. Conclusion: Using focal spot imaging, the operator can predict the thermocoagulation spot in advance. This technique overcomes the issue of targeting the focal spot during HIFU exposure, and since it is a compact system using an ultrasonic device, use of HIFU treatment may become more widespread at medical institutions for a wider range of diseases.