Deardorff D L, Diederich C J, Nau W H
Department of Radiation Oncology, University of California-San Francisco 94143, USA.
Med Phys. 1998 Dec;25(12):2400-9. doi: 10.1118/1.598451.
The feasibility of using air-cooling to improve the thermal penetration of direct-coupled interstitial ultrasound (US) applicators was investigated using biothermal simulations, bench experiments, phantom testing, and in vivo thermal dosimetry. Two applicator configurations using tubular US transducers were constructed and tested. The first design, intended for simultaneous thermobrachy-therapy, utilizes a 2.5 mm OD transducer with a central lumen to accommodate a radiation source from remote afterloaders. The second applicator consists of a 2.2 mm OD transducer designed for coagulative thermal therapy. Both designs provide cooling of the inner transducer surface by the counterflow of chilled air or CO2 gas through the annulus of the enclosed applicator. The average convective heat transfer (ha) associated with each applicator was determined empirically from curve-fits of radial steady-state temperatures measured in a tissue-mimicking phantom. High levels of convective heat transfer (ha > 500 W m-2 degrees C-1) were demonstrated in both designs at relatively low flow rates (< 5 L min-1). Transient and steady-state radial heating profiles were also measured in vivo (pig thigh muscle) with and without cooling. The therapeutic radius for hyperthermia (41-45 degrees C) was extended from 5-6 mm (without cooling) to 11-19 mm with air-cooling (4.8 L min-1, airflow 10 degrees C), effectively doubling and tripling the thermal penetration in vivo. Similar improvements were demonstrated at higher temperatures with the thermal coagulation applicator. Biothermal simulations, which modeled the physical, thermal, and acoustic parameters of the air-cooled applicator and surrounding tissue, were also used to investigate potential improvements in heating patterns. The simulated radial heating profiles with transducer cooling demonstrated significantly enhanced thermal penetration over the experimental range of convective transfer, and also agreed with in vivo results. These theoretical and experimental results clearly show air-cooling controls the transducer surface temperature, significantly increases thermal penetration, and produces a greater treatment volume for direct-coupled US applicators in hyperthermia and thermal coagulation.
利用生物热模拟、实验台实验、体模测试和体内热剂量测定，研究了采用空气冷却来改善直接耦合间质超声（US） applicator热穿透性的可行性。构建并测试了两种使用管状超声换能器的applicator配置。第一种设计用于同步热近距离治疗，采用外径2.5 mm的换能器，其中心腔可容纳来自远程后装治疗机的放射源。第二种applicator由外径2.2 mm的换能器组成，设计用于凝固性热疗。两种设计均通过冷却空气或二氧化碳气体在封闭applicator的环形空间内的逆流来冷却换能器内表面。通过对在组织模拟体模中测量的径向稳态温度进行曲线拟合，凭经验确定了与每个applicator相关的平均对流换热系数（ha）。在相对较低的流速（<5 L min-1）下，两种设计均显示出高水平的对流换热系数（ha>500 W m-2℃-1）。还在有和没有冷却的情况下在体内（猪大腿肌肉）测量了瞬态和稳态径向加热曲线。热疗（41-45℃）的治疗半径从5-6 mm（无冷却）扩展到空气冷却（4.8 L min-1，气流10℃）时的11-19 mm，有效地使体内热穿透增加了一倍和两倍。热凝固applicator在更高温度下也显示出类似的改善。生物热模拟对空气冷却applicator和周围组织的物理、热和声学参数进行了建模，还用于研究加热模式的潜在改善。带有换能器冷却的模拟径向加热曲线在对流传递的实验范围内显示出热穿透显著增强，并且也与体内结果一致。这些理论和实验结果清楚地表明，空气冷却可控制换能器表面温度，显著增加热穿透，并为直接耦合US applicator在热疗和热凝固中产生更大的治疗体积。
