High-speed wide-field photoacoustic microscopy with enhanced sensitivity

Photoacoustic microscopy (PAM), an emerging biomedical imaging technology, has provided the label-free imaging of biomolecules with the aid of superior optical contrast in them. A pulsed laser with wavelength of 532 nm is widely used to visualize the microvascular structures of small animal or human in vivo. Employing a customized 1-axis high-speed scanning device and ring-shaped ultrasound transducer (RUT), we developed a high-speed PAM system which enables wide field-of-view (FOV) imaging with high sensitivity. The optical and ultrasonic beam can be automatically aligned without opto-ultrasound beam combiner (OUC) by transmitting the laser through the hole in the transducer. Especially the transducer used in our system was designed to have a long focal length and a large sensing area to achieve wide-field imaging with high sensitivity. To verify the high sensitivity of the developed system, we performed ultrasonic pulse-echo experiments on quartz and PA in vitro imaging on leaf skeletons. Compared to the conventional beam alignment method which adopts OUC, the enhancement of signal-to-noise ratio (SNR) was 15 dB and 12 dB in those experiments, respectively. In addition, the high-speed scanner was designed to scan up to 12 mm long at resonant frequency of 80 Hz and to cover the wide sensing area of the transducer. By utilizing these specifications, the microvascular structures in a whole mouse ear could be imaged within 30 seconds without any additional image registration. The developed system could be a practical imaging platform for future studies in microvasculatures such as hemodynamic changes, oxygen saturation, and metabolism.