Investigation of power and frequency for 3D conformal MRI-controlled transurethral ultrasound therapy with a dual frequency multi-element transducer

Abstract

Transurethral ultrasound therapy uses real-time magnetic resonance (MR) temperature feedback to enable the 3D control of thermal therapy accurately in a region within the prostate. Previous canine studies showed the feasibility of this method in vivo. The aim of this study was to reduce the procedure time, while maintaining targeting accuracy, by investigating new combinations of treatment parameters. Simulations and validation experiments in gel phantoms were used, with a collection of nine 3D realistic target prostate boundaries obtained from previous preclinical studies, where multi-slice MR images were acquired with the transurethral device in place. Acoustic power and rotation rate were varied based on temperature feedback at the prostate boundary. Maximum acoustic power and rotation rate were optimised interdependently, as a function of prostate radius and transducer operating frequency. The concept of dual frequency transducers was studied, using the fundamental frequency or the third harmonic component depending on the prostate radius. Numerical modelling enabled assessment of the effects of several acoustic parameters on treatment outcomes. The range of treatable prostate radii extended with increasing power, and tended to narrow with decreasing frequency. Reducing the frequency from 8 MHz to 4 MHz or increasing the surface acoustic power from 10 to 20 W/cm² led to treatment times shorter by up to 50% under appropriate conditions. A dual frequency configuration of 4/12 MHz with 20 W/cm² ultrasound intensity exposure can treat entire prostates up to 40 cm³ in volume within 30 min. The interdependence between power and frequency may, however, require integrating multi-parametric functions in the controller for future optimisations.

• canine prostate
• 3D temperature feedback controller
• transurethral
• ultrasound thermal therapy

Acknowledgements

The authors thank Stephen McCormick, Ilya Kobelevskiy and James Kendall for their technical assistance, and the reviewers for suggesting investigation of temperature-dependent tissue attenuation.

Declaration of interest: Financial support was received from the Terry Fox Foundation, the Ontario Institute of Cancer Research, the Ontario Research Foundation and the Health Technology Exchange. Drs Bronskill, Chopra and Burtnyk are shareholders in Profound Medical, Toronto. The authors alone are responsible for the content and writing of the paper.