High acoustic numerical aperture photoacoustic microscopy with improved sensitivity

Limited by the numerical aperture of ultrasonic detection, optical resolution photoacoustic microscopy (OR-PAM) has not achieved optimal sensitivity. To address this problem, we have developed a high acoustic numerical aperture (∼0.74) OR-PAM (HNA-OR-PAM). Via engineering the acoustic lens, we implement the highest acoustic numerical aperture that a spherical concave lens can achieve. The sensitivity of HNA-OR-PAM is improved to around 160%—the state-of-the-art OR-PAM. Without averaging, the new system can image oxygen saturation in vivo with only 10-nJ pulse energy. The improved sensitivity allows us to image weaker absorbers, penetrate deeper, and reduce nonlinear effects induced by high pulse energy. Moreover, the photoacoustic view angle is augmented to 51.8 deg and makes tilted features more visible. We validate the improved view angle in both a phantom study and brain imaging.