Emerging concepts in functional and molecular photoacoustic imaging.

Providing the specific imaging contrast of optical absorption and excellent spatial scalability across the optical and ultrasonic dimensions, photoacoustic imaging has been rapidly emerging and expanding in the past two decades. In this review, I focus on a few latest advances in this enabling technology that hold the potential to transform in vivo functional and molecular imaging at multiple length scales. Specifically, multi-parametric photoacoustic microscopy enables simultaneous high-resolution mapping of hemoglobin concentration, oxygen saturation and blood flow-opening up the possibility of quantifying the metabolic rate of oxygen at the microscopic level. The pump-probe approach harnesses a variety of photoinduced transient optical absorption as novel contrast mechanisms for high-specificity molecular imaging at depth and as nonlinear excitation strategies for high-resolution volumetric microscopy beyond the conventional limit. Novel magneto-optical and photochromic probes lead to contrast-enhanced molecular photoacoustic imaging through differential detection.