Coherence-gated photoacoustic remote sensing microscopy.

Photoacoustic remote sensing microscopy (PARS) represents a new paradigm within the optical imaging community by providing high sensitivity (>50 dB in vivo) non-contact optical absorption contrast in scattering media with a reflection-mode configuration. Unlike contact-based photoacoustic modalities which can acquire complete A-scans with a single excitation pulse due to slow acoustic propagation facilitating the use of time-gated collection of returning acoustic signals, PARS provides depth resolution only through optical sectioning. Here we introduce a new approach for providing coherence-gated depth-resolved PARS imaging using a difference between pulsed-interrogation optical coherence tomography scan-lines with and without excitation pulses. Proposed methods are validated using simulations which account for pulsed-laser induced initial-pressures and accompanying refractive index changes. The changes in refractive index are shown to be proportional to optical absorption. It is demonstrated that to achieve optimal image quality, several key parameters must be selected including interrogation pulse duration and delay. The proposed approach offers the promise of non-contact depth-resolved optical absorption contrast at optical-resolution scales and may complement the scattering contrast offered by optical coherence tomography.