INSTITUTE OF PHYSICS PUBLISHING PHYSICS IN MEDICINE AND BIOLOGY Phys. Med. Biol. 46 (2001) 1785–1798 www.iop.org/Journals/pb PII: S0031-9155(01)21837-5 Comparison of modelled and observed in vivo temperature elevations induced by focused ultrasound: implications for treatment planning Kathleen Mahoney, Todd Fjield 1 , Nathan McDannold, Greg Clement and Kullervo Hynynen Department of Radiology, Brigham and Women’s Hospital and Harvard Medical School, 221 Longwood Avenue, Room 013, Boston, MA 02115, USA Received 9 February 2001, in final form 11 April 2001 Abstract Two numerical models for predicting the temperature elevations resulting from focused ultrasound heating of muscle tissue were tested against experimental data. Both models use the Rayleigh–Sommerfeld integral to calculate the pressure field from a source distribution. The first method assumes a source distribution derived from a uniformly radiating transducer whereas the second uses a source distribution obtained by numerically projecting pressure field measurements from an area near the focus backward toward the transducer surface. Both of these calculated ultrasound fields were used as heat sources in the bioheat equation to calculate the temperature elevation in vivo. Experimental results were obtained from in vivo rabbit experiments using eight-element sector-vortex transducers at 1.61 and 1.7 MHz and noninvasive temperature mapping with MRI. Results showed that the uniformly radiating transducer model over-predicted the peak temperature by a factor ranging from 1.4 to 2.8, depending on the operating mode. Simulations run using the back- projected sources were much closer to experimental values, ranging from 1.0 to 1.7 times the experimental results, again varying with mode. Thus, a significant improvement in the treatment planning can be obtained by using actual measured ultrasound field distributions in combination with backward projection. (Some figures in this article are in colour only in the electronic version; see www.iop.org) 1. Introduction Modelling of the thermal fields produced by ultrasound in tissue is important both in the area of diagnostic ultrasound, where it can aid in establishing safety levels, and in the rapidly growing field of focused ultrasound (FUS) surgery, where it is used for treatment planning and establishing safety thresholds. In FUS surgery, the goal is to heat a small tissue volume above 1 Currently at Transurgical, Inc., 220 Belle Meade Road, Setauket, NY 11733, USA. 0031-9155/01/071785+14$30.00 © 2001 IOP Publishing Ltd Printed in the UK 1785