(P082) Photoacoustic Imaging for Improved Intraoperative Visualization of Prostate Brachytherapy Seeds

Muyinatu Bell, PhD, Emad Boctor, PhD, Nathanael Kuo, PhD, Jin Kang, PhD, Danny Y. Song, MD; Johns Hopkins University

Purpose: Current ultrasound technology offers suboptimal intraoperative visualization of brachytherapy seeds. Photoacoustic imaging has the promise to enhance seed localization compared with standard ultrasound imaging. Photoacoustic imaging utilizes light transmitted into tissue; metallic seeds absorb the transmitted light, undergo thermoelastic expansion, and generate sound waves that are detected with a conventional transrectal ultrasound probe. Excellent seed contrast is expected in photoacoustic images, because the optical absorption of brachytherapy seeds is orders of magnitude larger than that of the surrounding tissue, and this difference is larger than the corresponding acoustic echo difference. We conducted an institutionally approved canine study to investigate the in vivo feasibility of intraoperative seed visualization with photoacoustic imaging.

Materials and Methods: Brachytherapy seeds, coated with black ink, were transperineally inserted into a live canine prostate. A transperineal, interstitial, fiberoptic light delivery method, coupled to a 1064-nm Nd:Yag laser, was used to emit laser into the prostate containing the seeds. The resulting acoustic waves were detected with a conventional transrectal ultrasound probe (Ultrasonix, BPL9-5/55, Richmond, BC, Canada) connected to a clinical ultrasound scanner (Ultrasonix Sonixtouch). Raw prebeam-formed photoacoustic data were collected with a Sonixdaq data acquisition unit that was connected to a probe port on the ultrasound scanner. Ultrasound and postoperative CT images of the implanted seeds were acquired and analyzed to confirm seed locations in the photoacoustic images.

Results: Multiple brachytherapy seeds were visualized within the in vivo prostate within American National Standards Institute (ANSI) laser safety limits for human exposure (100 mJ/cm²). Seeds that were difficult to localize in standard ultrasound images were more visible in coregistered photoacoustic images. the average seed contrast in the photoacoustic images was 20–30 Db for energy densities ranging from 8–84 mJ/cm² when the short-lag spatial coherence beamformer was applied to the raw data. There was excellent agreement between photoacoustic, ultrasound, and CT image.

Conclusion: Photoacoustic imaging is a promising complement to ultrasound imaging for intraoperative brachytherapy seed visualization. Work is ongoing for eventual translation into human clinical trials.