A cutting-edge technique called photoacoustic imaging mixes sound and light to produce images of the inside of the body. When biological tissue is illuminated by a pulsed laser, a portion of the photon energy is absorbed by the tissue and converted to heat. The tissue expands thermoelastically as the temperature rises, releasing energy in the form of ultrasonic vibrations. In order to recreate 2D or 3D images of the biological tissue, researchers scan the sample and gather the relevant photoacoustic signals.
The photoacoustic signals are typically collected via ultrasonic transducers. To ensure signal detection sensitivity, water or ultrasonic gel must typically be added between the tissue sample and the transducer due to the attenuation of sound waves in air. Such direct physical touch or immersion may have profound impacts on biological samples, severely restricting the practical applications of traditional photoacoustic imaging.
The core response frequency and detection bandwidth of ultrasonic transducers, on the other hand, are constrained due to their inherent structural and material qualities, which may lower the system’s sensitivity in terms of detecting broadband signals. Due to these drawbacks, conventional photoacoustic imaging needs to be updated in order to support high-quality photoacoustic research.
A brand-new photoacoustic imaging technique is photoacoustic remote sensing imaging. Photoacoustic remote sensing uses a different laser beam to detect acoustic signals as opposed to traditional acoustic detection, which uses ultrasonic transducers. In particular, a different laser source is used to focus the probe beam so that it is concentric with the excitation beam. The elasto-optical refractive index modulation causes the sample surface’s refractive index to change instantly when the sample absorbs energy to create initial pressure.
The associated photoacoustic signals can be examined by keeping an eye on the probe beam’s reflection intensity. Direct contact with the sample is avoided when acoustic impulses are detected entirely optically. The detection bandwidth may be readily moved from the small ultrasonic transducer to a larger photodiode thanks to optical sensing, which offers the option to further enhance the detection sensitivity and signal-to-noise ratio of the system.
A research team from the University of Hong Kong recently revealed near-infrared photoacoustic remote sensing microscopy for non-contact imaging of lipids based on the aforementioned observations. The team’s photoacoustic remote sensing microscope, which is detailed in Advanced Photonics Nexus, uses a 1.7-m thulium-doped fibre laser as the pump beam to specifically excite the C-H bond in lipids. In order to detect the initial ultrasonic pressure, a second 1.5-m continuous-wave (CW) laser is used as the detection beam that is confocal with the pump beam. Remote sensing of photoacoustic signals is made possible by the optical detection of ultrasonic waves, which does away with the need for ultrasonic transducers. While doing so, the system’s detection sensitivity and signal-to-noise ratio are enhanced by the method’s wider detection bandwidth.
The team’s investigation began with a photoacoustic remote sensing imaging demonstration of two types of pure lipid samples, followed by an analysis of the matching power spectrum density of the signals. They discovered that, in comparison to a typical transducer, the optical detecting approach might offer a wider frequency response. Additionally, they used photoacoustic remote sensing imaging to examine biological samples such as nematodes and brain slices. The results demonstrated good contrast and signal-to-noise ratios, proving the ability to carry out high-performance imaging at the tissue scale.
“Photoacoustic remote sensing microscopy achieves label-free imaging that can target specific molecular bonds,” remarks corresponding author Kenneth K. Y. Wong, Professor of Engineering at the University of Hong Kong. He adds, “Optical detection of ultrasonic signals provides non-contact operation and a broader frequency response. Meanwhile, the photoacoustic remote sensing microscopy shows high performance for lipid distribution mapping on the tissue scale.”
The newly developed technique suggests great application potential in a variety of biomedical research.
