Grueneisen Relaxation Photoacoustic Microscopy

The temperature-dependent property of the Grueneisen parameter has been employed in photoacoustic imaging mainly to measure tissue temperature. Here we explore this property using a different approach and develop Grueneisen relaxation photoacoustic microscopy (GR-PAM), a technique that images nonradiative absorption with confocal optical resolution. GR-PAM sequentially delivers two identical laser pulses with a microsecond-scale time delay. The first laser pulse generates a photoacoustic signal and thermally tags the in-focus absorbers. When the second laser pulse excites the tagged absorbers within the thermal relaxation time, a photoacoustic signal stronger than the first one is produced, owing to the temperature dependence of the Grueneisen parameter. GR-PAM detects the amplitude difference between the two colocated photoacoustic signals, confocally imaging the nonradiative absorption. We greatly improved axial resolution from $45\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ to $2.3\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ and, at the same time, slightly improved lateral resolution from $0.63\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ to $0.41\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. In addition, the optical sectioning capability facilitates the measurement of the absolute absorption coefficient without fluence calibration.