Air-cooling of direct-coupled ultrasound applicators for interstitial hyperthermia and thermal coagulation

The feasibility of using air-cooling to improve the thermal penetration of direct-coupled interstitial ultrasound (US) applicators was investigated us ing biothermal simulations, bench experiments, phantom testing, and in vivo thermal dosimetry. Two applicator configurations using tubular US transduc ers were constructed and tested. The first design, intended for simultaneou s thermobrachy-therapy, utilizes a 2.5 mm OD transducer with a central lume n to accommodate a radiation source from remote afterloaders. The second ap plicator consists of a 2.2 mm OD transducer designed for coagulative therma l therapy. Both designs provide cooling of the inner transducer surface by the counterflow of chilled air or CO2 gas through the annulus of the enclos ed applicator. The average convective heat transfer (h(a)) 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 (h(a) > 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 wi th air-cooling (4.8 L min(-1), airflow 10 degrees C), effectively doubling and tripling the thermal penetration in vivo. Similar improvements were dem onstrated at higher temperatures with the thermal coagulation applicator. B iothermal simulations, which modeled the physical, thermal, and acoustic pa rameters of the air-cooled applicator and surrounding tissue, were also use d 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 tran sfer, and also agreed with in vivo results. These theoretical and experimen tal results clearly show air-cooling controls the transducer surface temper ature, significantly increases thermal penetration, and produces a greater treatment volume for direct-coupled US applicators in hyperthermia and ther mal coagulation. (C) 1998 American Association of Physicists in Medicine. [ S0094-2405(98)02112-9].