Researchers at the University of Hong Kong showed that biological tissue can be imaged with greater sensitivity through remote sensing of photoacoustic signals than through conventional photoacoustic imaging techniques. The researchers demonstrated a near-infrared (NIR), photoacoustic remote sensing microscopy (PARS) technique for noncontact imaging of lipids. The PARS technique enabled broad detection bandwidth, deep penetration depth, and a high signal-to-noise ratio for the imaging of biological samples, in addition to enabling noncontact implementation. The developed microscopy technique could complement the multispectral techniques of PARS systems and provide useful alternatives for diverse biomedical studies, the researchers said. Conventional photoacoustic microscopy (PAM) techniques use ultrasonic transducers to collect photoacoustic signals. To ensure effective signal detection, water or a gel is often added between the transducer and the tissue sample, and contact with water or gel can damage the sample. Also, acoustic transducers have limited center response frequency and detection bandwidth, which can reduce PAM’s sensitivity to broadband information. Instead of using a transducer to detect acoustic signals, PARS uses a laser. This laser source, which is confocal with the excitation beam, serves as the probe beam. The probe beam detects elasto-optical refractive index modulation caused by photoacoustic pressure. The reflection intensity of the probe beam is monitored by the user, and the corresponding photoacoustic signals are analyzed. The all-optical detection of acoustic signals eliminates direct contact between the PARS system and the sample, helping to protect the integrity of the sample. Further, because the acoustic signals are optically sensed, the detection bandwidth can be transferred from an ultrasonic transducer to a photodiode. This enables broadband detection of photoacoustic signals and can improve detection sensitivity and signal-to-noise ratio. Mechanism of photoacoustic remote sensing. Courtesy of Guyue Hu, PARS Mechanism, 2023. To build the NIR PARS system, the researchers used a 1.7-μm, thulium-doped fiber laser as the pump beam and a 1.5-μm, continuous-wave laser as the detection beam. The detection beam was made confocal with the pump beam in order to detect initial ultrasonic pressure. The researchers imaged two forms of pure lipid samples and analyzed the corresponding power spectrum density of the photoacoustic signals. The pump beam selectively stimulated the C-H bond in the lipids. The pump wavelength was selected to overlap the first vibrational overtone of the C-H bond in response to the intensive absorption of lipid molecules. The strong absorption efficiency of the C-H bond at the first overtone region enabled a high signal-to-noise ratio of about 55 dB under moderate irradiation and detection conditions. The researchers found that optical detection via PARS provided a broader frequency response than that of a conventional transducer. The researchers took advantage of the noncontact nature of PARS and the deep penetration depth at the NIR region to perform tissue-scale lipid imaging on a model organism and mouse brain slices. The resulting images showed good contrast and signal-to-noise ratio, demonstrating that PARS can provide high-performance imaging capability on the tissue scale. “Photoacoustic remote sensing microscopy achieves label-free imaging that can target specific molecular bonds,” professor Kenneth K.Y. Wong said. “Optical detection of ultrasonic signals provides noncontact operation and a broader frequency response. Meanwhile, the photoacoustic remote sensing microscopy shows high performance for lipid distribution mapping on the tissue scale.” The research was published in Advanced Photonics Nexus (www.doi.org/10.1117/1.APN.2.2.026011).