Optimal oblique light illumination for photoacoustic microscopy beyond the diffusion limit

Abstract

To image beyond the quasi-ballistic photon regime, photoacoustic tomography systems must rely on diffuse photons; however, there still exists an optimal illumination pattern that results in the largest number of photons reaching a target at a given depth. Many photoacoustic imaging systems incorporate weak optical focusing through oblique or dark-field illumination, but these systems are not often optimized for deep light delivery. Multiple parameters and constraints, particularly for in vivo imaging, need to be considered to determine the optimal illumination scheme for a given system: beam diameter, incident angle, pulse repetition rate, laser fluence, and target depth. Monte Carlo simulations of varied beam geometries and incident angles show the best optical illumination schemes for different imaging depths. Further an analytic model based on the diffusion theory provides a rapid method of determining the optimal beam size and incident angle for a given target depth and agrees well with the simulations. The results reveal the most efficient optical focal position to maximize the number of photons delivered to a target depth, therein maximizing the PA signal. The principles and results discussed here are not limited to the system investigated, but can be applied to other system configurations to improve the photoacoustic signal strength.

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