Visualizing Laser Ultrasound Wave Propagation Using a Wollaston-Based Stroboscopic Schlieren Interferometer

Laser ultrasound (LUS)-the generation and detection of ultrasound using photoacoustic generation and optical detection was recently demonstrated to be feasible for noncontact ultrasound imaging on human subjects. However, technical challenges in optical and acoustic system designs remain before a clinical LUS imaging system can be realized. Understanding and experimental validation of the photoacoustically generated wavefields from various LUS sources is crucial in designing future LUS systems. Here, we present wavefield mapping results of LUS excitation at multiple optical configurations using a Wollaston-based stroboscopic Schlieren interferometry system. The system visualizes index of refraction variations within the test media induced by each photoacoustic source, which enables rapid wavefield mapping at multiple time instances to visualize the full wave propagation. The design and calibration of the Schlieren system are presented, followed by visualization of LUS excitation that generated photoacoustic point-sources, plane-waves, and focused waves. Image results from this work will enhance understanding and control of LUS excitation to generate specific wave transmissions for in vivo imaging, which enables broader LUS imaging capabilities and beamforming for clinical translation. Comparable to the development of medical ultrasound, the insights and results presented here will support future LUS system designs to reach sufficient performance for clinical imaging.