The University of Hong Kong used a 1.5μm thulium-doped fiber laser in confocal with a pump beam to detect the initial ultrasonic pressure

Photoacoustic imaging is a cutting-edge technology that uses light and sound to create images of the inside of the body. When pulsed laser illuminates the surface of biological tissue, part of the photon energy is absorbed by the tissue to produce heat. This increase in heat causes the thermoelastic expansion of the tissue, releasing energy in the form of ultrasonic waves. By scanning the samples and collecting the corresponding photoacoustic signals, the researchers were able to reconstruct 2D or 3D images of the biological tissue.

Usually, ultrasonic transducers collect photoacoustic signals. But because sound waves are attenuated in air, water or ultrasonic gel is usually added between the tissue sample and the transducer to ensure the sensitivity of the signal detection. Such physical contact or immersion can have significant effects on biological samples, greatly limiting the applicability of traditional photoacoustic imaging in many practical scenarios. On the other hand, due to the inherent structure and material characteristics of ultrasonic transducers, their central response frequency and detection bandwidth are limited, which may reduce the sensitivity of the system in broadband signal detection. Given these limitations, traditional photoacoustic imaging needs to be updated to perform high-quality photoacoustic studies.

Photoacoustic remote sensing imaging is a new photoacoustic imaging mode. Unlike conventional acoustic detection, which uses ultrasonic transducers, photoacoustic remote sensing uses another laser beam to detect acoustic signals. Specifically, another laser source is used as a probe beam in confocal with the excitation beam. When the sample absorbs energy to produce initial pressure, the refractive index of the sample surface changes instantaneously due to the elastic optical refractive index modulation. By monitoring the reflection intensity of the probe beam, the corresponding photoacoustic signal can be analyzed. All-optical detection of the acoustic signal eliminates direct contact with the sample. At the same time, thanks to optical sensing.

Based on these insights, a team of researchers from the University of Hong Kong recently reported near infrared photoacoustic remote sensing microscopy for non-contact imaging of lipids. According to Advanced Photonics Nexus, the team's photoacoustic remote sensing microscope uses a 1.7μm thulium-doped fiber laser as a pumping beam to selectively excite C-H bonds in lipids. At the same time, another 1.5μm continuous wave (CW) laser is used as a detection beam in confocal with the pump beam to detect the initial ultrasonic pressure. The optical detection of ultrasonic signal eliminates the need of ultrasonic transducer and realizes remote sensing of photoacoustic signal. At the same time, this method provides a wider detection bandwidth and improves the detection sensitivity and signal-to-noise ratio of the system.

In their experiment, the team first presented imaging results of two forms of pure lipid samples via photoacoustic remote sensing and analyzed the corresponding power spectral densities of the signals. They found that the optical detection method can provide a wider frequency response than conventional transducers. They also performed photoacoustic remote sensing imaging of biological samples, including nematodes and brain slices, showing good contrast and signal-to-noise ratio, demonstrating high performance imaging capabilities at the tissue scale.

"Photoacoustic remote sensing microscopy enables tag-free imaging that can target specific molecular bonds," commented corresponding author Kenneth KY Wong, professor of engineering at the University of Hong Kong. He added: "Optical detection of ultrasonic signals provides contactless operation and a wider frequency response. At the same time, photoacoustic remote sensing microscopy shows high performance in lipid distribution mapping at the tissue scale." The newly developed technology has great application potential in all kinds of biomedical research.

Source: Laser Net