3D conformal MRI-guided transurethral ultrasound therapy: results of gel phantom experiments

Abstract

MRI‐guided transurethral ultrasound therapy shows promise for minimally invasive treatment of localized prostate cancer. Previous in‐vivo studies demonstrated the feasibility of performing conservative treatments using real‐time temperature feedback to control accurately the establishment of coagulative lesions within circumscribed prostate regions. This in‐vitro study tested device configuration and control options for achieving full prostate treatments. A multi‐channel MRI compatible ultrasound therapy system was evaluated in gel phantoms using 3 canine prostate models. Prostate profiles were 5 mm‐step‐segmented from T2‐weighted MR images performed during previous in‐vivo experiments. During ultrasound exposures, each ultrasound element was controlled independently by the 3D controller. Decisions on acoustic power, frequency, and device rotation rate were made in real time based on MR thermometry feedback and prostate radii. Low and high power treatment approaches using maximum acoustic powers of 10 or 20 W.cm−2 were tested as well as single and dual‐frequency strategies (4.05/13.10 MHz). The dual‐frequency strategy used either the fundamental frequency or the 3rd harmonic component, depending on the prostate radius. The 20 W.cm−2 dual frequency approach was the most efficient configuration in achieving full prostate treatments. Treatment times were about half the duration of those performed with 10 W.cm−2 configurations. Full prostate coagulations were performed in 16.3±6.1 min at a rate of 1.8±0.2 cm3.min−1, and resulted in very little undertreated tissue (<3%). Surrounding organs positioned beyond a safety distance of 1.4±1.0 mm from prostate boundaries were not damaged, particularly rectal wall tissues. In this study, a 3D, MR‐thermometry‐guided transurethral ultrasound therapy was validated in vitro in a tissue‐mimicking phantom for performing full prostate treatment. A dual‐frequency configuration with 20 W.cm−2 ultrasound intensity exposure showed good results with direct application to full human prostate treatments.