梅村 晋一郎

Shin-ichiro UMEMURA

東北大学医工学研究科

Graduate School of Biomedical Engineering, Tohoku University

キーワード : high-intensity focused ultrasound, absorption, ultrasonic frequency, spatial selectivity, heat conduction, blood perfusion

超音波のもつエネルギーは分子レベルでは希薄であるが，それを熱として蓄積することにより，生体組織に不可逆変化をもたらすことができる．強力集束超音波（high-intensity focused ultrasound: HIFU）による治療はその応用に相当する．生体はそのずり粘性により超音波エネルギーを熱に変換するが，その変換効率すなわち吸収係数は超音波周波数におよそ比例するので，超音波が治療目的組織まで生体中を伝播する距離に応じて，周波数を選択する必要がある．超音波照射時間を，熱伝導による拡散が目立たない時間内，例えば，数MHzの超音波をFナンバー1程度に集束する場合，数sに設定すれば，集束超音波の空間的選択性を生かすことができる．一方，治療目的外組織の過熱を防ぐには，一般に，1～数分の血液の灌流による熱的時定数の程度の冷却時間を超音波照射間におく必要があり，これがHIFU治療のスループットを低めている．本稿では，これらを原理から説明するとともに，HIFUを照準し治療を監視するイメージング手段や，治療スループットを向上させる研究についても言及する．数式は，あくまで説明の補助手段とし，それによらずとも本質を捉えられるよう書いた．

Although ultrasonic energy is relatively thin at the molecular level, it can cause irreversible change to biological tissue after it is accumulated as heat. Thermal treatment using high-intensity focused ultrasound (HIFU) is such an application. Biological tissue converts ultrasonic energy to heat due to its shear viscosity. Because its conversion ratio, i.e., absorption, is approximately proportional to the ultrasonic frequency, the frequency has to be chosen according to the propagation distance in tissue to the target tissue to be treated. By choosing a HIFU exposure duration within the time in which the heat conduction has not taken place significantly, e.g., within a few seconds for focused ultrasound at a few MHz with an f-number of 1, the spatial selectivity of focused ultrasound can be utilized for the treatment. To suppress overheating outside the target tissue, a long cooling time between each consecutive HIFU exposure, in the order of the time constant of blood perfusion ranging from 1 to several minutes, is necessary. This significantly lowers the throughput of HIFU treatment. These are explained in this paper based on the principles of ultrasound and heat transfer. The imaging methods for aiming HIFU and monitoring the treatment as well as research to enhance the treatment throughput are also explained. They are explained without relying too much on equations, which are only used as a supplement.