title: High-intensity focused ultrasound for ex vivo kidney tissue ablation: influence of generator power and pulse duration.
abstract: OBJECTIVE
  The therapeutic application of noninvasive tissue ablation by high-intensity focused ultrasound (HIFU) requires precise physical definition of the focal size and determination of control parameters. The objective of this study was to measure the extent of ex-vivo porcine kidney tissue ablation at variable generator parameters and to identify parameters to control lesion size.
  
  
  METHODS
  The ultrasound waves generated by a cylindrical piezoceramic element (1.04 MHz) were focused at a depth of 100 mm using a parabolic reflector (diameter 100 mm). A needle hydrophone was used to measure the field distribution of the sound pressure. The morphology and extent of tissue necrosis were examined at generator powers of up to 400 W (P(el)) and single pulse durations of as long as 8 seconds.
  
  
  RESULTS
  The two-dimensional field distribution resulted in an approximately ellipsoidal focus of 32 x 4 mm (-6 dB). A sharp demarcation between coagulation necrosis and intact tissue was observed. Lesion size was controlled by both the variation of generator power and the pulse duration. At a constant pulse duration of 2 seconds, a generator power of 100 W remained below the threshold doses for inducing a reproducible lesion. An increase in power to as high as 400 W induced lesions with average dimensions of as much as 11.2 x 3 mm. At constant total energy (generator power x pulse duration), lesion size increased at higher generator power.
  
  
  CONCLUSIONS
  This ultrasound generator can induce defined and reproducible necrosis in ex-vivo kidney tissue. Lesion size can be controlled by adjusting the generator power and pulse duration. Generator power, in particular, turned out to be a suitable control parameter for obtaining a lesion of a defined size.
authors.affiliation: Department of Urology, University Hospital Mannheim, Faculty of Clinical Medicine Mannheim, 68135 Mannheim, Germany. axel.haecker@chir.ma.uni-heidelberg.de
authors.forename: Axel
authors.initials: A
authors.lastname: Häcker
authors.forename: Kai Uwe
authors.initials: KU
authors.lastname: Köhrmann
authors.forename: Thomas
authors.initials: T
authors.lastname: Knoll
authors.forename: Sigrun
authors.initials: S
authors.lastname: Langbein
authors.forename: Annette
authors.initials: A
authors.lastname: Steidler
authors.forename: Oliver
authors.initials: O
authors.lastname: Kraut
authors.forename: Ernst
authors.initials: E
authors.lastname: Marlinghaus
authors.forename: Peter
authors.initials: P
authors.lastname: Alken
authors.forename: Maurice Stephan
authors.initials: MS
authors.lastname: Michel
mesh_terms: D000818:Animals; D066298:In Vitro Techniques; D007668:Kidney / Q000473:pathology / Q000601:surgery*; D013552:Swine; D014464:Ultrasonic Therapy / Q000295:instrumentation*; D014465:Ultrasonics