Motion-based dynamic light delivery to minimize laser-related thermal damage while preserving photoacoustic image quality.

Abstract

Significance: Photoacoustic imaging has the potential to be integrated into surgical guidance systems. However, biosafety from prolonged laser exposure can limit the maximization of signal-to-noise ratios. Although cooling strategies can potentially mitigate thermal impact, the associated adverse effects necessitate an alternative strategy.

Aim: We introduce a dynamic light delivery strategy that displaces a light source in a controlled manner during photoacoustic imaging, which is expected to both minimize laser-related thermal damage and maintain the image quality achievable with stationary light delivery.

Approach: Monte Carlo simulations were performed to determine the impact of light source displacement on local energy density. A dynamic light delivery device was designed, prototyped, and evaluated with an experimental phantom to determine image quality. To assess potential laser-related thermal damage, in vivo swine liver was exposed to laser light delivered with 750-nm wavelength, nanosecond pulses, and 32.4 mJ median pulse-to-pulse energy for 20-min total duration, under both stationary and dynamic light delivery. The exposed liver samples were excised, followed by categorical grading and quantitative depth measurements of resulting hemorrhage observed in H&E liver sections.

Results: Energy densities at the simulated tissue surface were 1.85 lower with dynamic rather than stationary light delivery. As target depth was varied from 14 to 53 mm, the median signal-to-noise and generalized contrast-to-noise ratios ranged 24.60 to 38.76 and 0.96 to 1.00, respectively, with stationary light delivery and 23.06 to 37.47 and 0.96 to 1.00, respectively, with dynamic light delivery, with no statistically significant differences between light delivery approaches ( $p>0.05$ ). Histopathology of excised liver samples revealed mild hemorrhage with stationary light delivery that was reduced to minimal hemorrhage with dynamic light delivery, quantified as median hemorrhage depths reduced from 0.79 to 0.16 mm (i.e., 80% hemorrhage depth reduction).

Conclusions: Dynamic light delivery is a promising approach to mitigate potential laser-related damage.