title: Multifrequency ultrasound transducers for conformal interstitial thermal therapy.
abstract: Control over the pattern of thermal damage generated by interstitial ultrasound heating applicators can be enhanced by changing the ultrasound frequency during heating. The ability to change transmission frequency from a single transducer through the use of high impedance front layers was investigated in this study. The transmission spectrum of multifrequency transducers was calculated using the KLM equivalent circuit model and verified with experimental measurements on prototype transducers. The addition of a quarter-wavelength thick PZT (unpoled) front layer enabled the transmission of ultrasound at two discrete frequencies, 4.7 and 9.7 MHz, from a transducer with an original resonant frequency of 8.4 MHz. Three frequency transmission at 3.3, 8.4, and 10.8 MHz was possible for a transducer with a half-wavelength thick front layer. Calculations of the predicted thermal lesion size at each transmission frequency indicated that the depth of thermal lesion could be varied by a factor of 1.6 for the quarter-wavelength front layer. Heating experiments performed in excised liver tissue with a dual-frequency applicator confirmed this ability to control the shape of thermal lesions during heating to generate a desired geometry. Practical interstitial designs that enable the generation of shaped thermal lesions are feasible.
authors.affiliation: Sunnybrook and Women's College Health Sciences Centre, Department of Medical Biophysics, University of Toronto, Toronto, ON M4N 3M5, Canada. rajiv.chopra@utoronto.ca
authors.forename: Rajiv
authors.initials: R
authors.lastname: Chopra
authors.forename: Chris
authors.initials: C
authors.lastname: Luginbuhl
authors.forename: F Stuart
authors.initials: FS
authors.lastname: Foster
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Bronskill
mesh_terms: D000818:Animals; D002056:Burns / Q000209:etiology / Q000517:prevention & control; D002417:Cattle; D003198:Computer Simulation; D017076:Computer-Aided Design; D004867:Equipment Design / Q000379:methods; D019544:Equipment Failure Analysis; D006801:Humans; D066298:In Vitro Techniques; D008099:Liver / Q000528:radiation effects; D008954:Models, Biological; D009369:Neoplasms / Q000628:therapy; D013814:Thermal Conductivity; D014159:Transducers; D014464:Ultrasonic Therapy / Q000009:adverse effects / Q000295:instrumentation* / Q000379:methods*; D014465:Ultrasonics