  Recently we demonstrated an integrated photoacoustic (PA) and ultrasound
(PAUS) system using a kHz-rate wavelength-tunable laser and a swept-beam
delivery approach. It irradiates a medium using a narrow laser beam sweeping at
high repetition rate over the desired imaging area, in contrast to the
conventional PA approach using broad beam illumination at a low repetition. One
significant advantage of this approach is that the fundamental problem of
decoupling local light absorption at a point from optical fluence at the same
point can be solved. Here, we present a fluence compensation method and
demonstrate its performance in phantom studies. We adopted analytic fluence
models, extending diffusion theory for the case of a pencil beam obliquely
incident on a medium, and developed robust methods to estimate medium optical
parameters using PA measurements acquired from multiple irradiation positions.
We conducted comprehensive simulation tests and phantom studies using
well-known contrast-agents to validate the reliability of the fluence models
and spectral corrections.
