The feasibility and safety of magnetic resonance imaging (MRI)-controlled transurethral ultrasound therapy were demonstrated recently in a preliminary human study in which a small subvolume of prostate tissue was treated prior to radical prostatectomy. Translation of this technology to full clinical use, however, requires the capability to generate thermal coagulation in a volume up to that of the prostate gland itself. The aim of this study was to investigate the parameters required to treat a full 3D human prostate accurately with a multi-element transurethral applicator and multiplanar MR temperature control.Methods:
The approach was a combination of simulations (to select appropriate parameters) followed by experimental confirmation in tissue-mimicking phantoms. A ten-channel, MRI-compatible transurethral ultrasound therapy system was evaluated using six human prostate models (average volume: 36 cm3) obtained from the preliminary human feasibility study. Real-time multiplanar MR thermometry at 3 T was used to control the spatial heating pattern in up to nine planes simultaneously. Treatment strategies incorporated both single (4.6 or 8.1 MHz) and dual (4.6 and 14.4 MHz) frequencies, as well as maximum acoustic surface powers of 10 or 20 W cm−2.Results:
Treatments at 4.6 MHz were capable of coagulating a volume equivalent to 97% of the prostate. Increasing power from 10 to 20 W cm−2 reduced treatment times by approximately 50% with full treatments taking 26 ± 3 min at a coagulation rate of 1.8 ± 0.4 cm3 min−1. A dual-frequency 4.6/14.4 MHz treatment strategy was shown to be the most effective configuration for achieving full human prostate treatment while maintaining good treatment accuracy for small treatment radii. The dual-frequency approach reduced overtreatment close to the prostate base and apex, confirming the simulations.Conclusions:
This study reinforces the capability of MRI-controlled transurethral ultrasound therapy to treat full prostate volumes in a short treatment time with good spatial targeting accuracy and provides key parameters necessary for the next clinical trial.
The authors thank Lauren Persaud, Xitij Patel, Greg Togtema, and Stephen McCormick for their technical assistance. Financial support was received from the National Institutes of Health (NIH) (1R21CA159550). Dr. Bronskill and Dr. Chopra are shareholders in Profound Medical, Toronto. Dr. Burtnyk is currently employed by Profound Medical. The authors alone are responsible for the content and writing of the paper.
II. MATERIAL AND METHODS
II.A. Patient-specific prostate geometries
II.B. Multiplanar MR thermometry control algorithm for transurethral ultrasound therapy
II.C. MRI-compatible transurethral ultrasound therapy system
II.D. Tissue-mimicking phantom experiments
II.E. Whole-prostate treatments in tissue-mimicking phantoms
II.F. Numerical simulations of whole-gland treatment using MRI-controlled transurethral ultrasound therapy
II.G. Safety considerations in surrounding tissues
II.H. Assessment of results
III.A. Parameters for whole-prostate treatments from simulations
III.B. Whole-gland treatments in tissue-mimicking gel phantoms
III.B.1. Experimental MR thermometry feedback control of the whole prostate gland
III.B.2. Single frequency ultrasound exposures
III.B.3. Effects of dual-frequency exposures on treatment accuracy and safety
III.C. Simulations of rectal wall and pelvic bone heating: protection of surrounding tissue structures during whole-gland treatments
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