Recent Advances in Photoacoustic Tomography

Abstract

Photoacoustic tomography (PAT) that integrates the molecular contrast of optical imaging with the high spatial resolution of ultrasound imaging in deep tissue has widespread applications in basic biological science, preclinical research, and clinical trials. Recently, tremendous progress has been made in PAT regarding technical innovations, preclinical applications, and clinical translations. Here, we selectively review the recent progresses and advances in PAT, including the development of advanced PAT systems for small-animal and human imaging, newly engineered optical probes for molecular imaging, broad-spectrum PAT for label-free imaging of biological tissues, high-throughput snapshot photoacoustic topography, and integration of machine learning for image reconstruction and processing. We envision that PAT will have further technical developments and more impactful applications in biomedicine.

Imaging approaches for monitoring three‐dimensional cell and tissue culture systems

“…Excitation wavelengths in photoacoustic imaging range from UV to mid-IR, with photoacoustic infrared (PAIR) being a popular choice due to the absorption contrast achieved in the IR region, and the reduced scattering of IR light for deeper tissue penetration [154,161]. However, the water absorption signal in the IR region often drowns out other endogenous signals.…”

“…However, the water absorption signal in the IR region often drowns out other endogenous signals. As a result, PAM has expanded into wavelengths including mid-infrared [174] (unfortunately resulting in reduced lateral resolution), other detection schemes utilising interrogation beams from other wavelengths including UV ( [154,156,175]), while PACT has wavelengths in the X-ray [176][177][178] and radiowaves/microwaves (thermoacoustic tomography [TAT]) [179][180][181]. The most promising method, mid-infrared PAM (ULM-PAM), targeting the mid-infrared vibrational modes, while achieves high lateral resolution, determined by the wavelength of UV laser rather than the mid-infrared, and reduces IR-water signals.…”

“…Both tools employ (a) a (usually pulsed) photon source with illumination optics, (b) an ultrasound detectors/transducers and (c) a signal processing and reconstruction device [165]. In PAM, volumetric images are formed by raster scanning light and sound across a sample whereas in PACK/PAT images are formed by inverse reconstructions of the detected signal induced by wide-field illumination [154].…”

“…The working principle of photoacoustic (optoacoustic) imaging (PAM) is the photoacoustic effect, where ultrasonic waves (sound waves) are generated (propagating in the light‐absorbing region at speeds of approximately 1500 m/s [153, 154] via non‐radiative transient thermoelastic expansion, due to the absorption of temporally varying electromagnetic radiation. This photoacoustic effect can be used as a vibrational imaging tool to detect ultrasonic waves, detected as an electrical signal using ultrasonic transducers [153], as a result of spectroscopic absorption, achieving multi‐contrast imaging of molecules, including exogenous labels and endogenous such as lipids, proteins and nucleic acids.…”

“…The amplitude of photoacoustic signal is proportional to the concentration, the optical absorption coefficient and the thermal coefficient of the photoabsorber molecules [153, 155]. The scattering of acoustic waves is an order of magnitude smaller in tissues than light/photons [154] achieving rich contrast and high spatial resolution, with deeper penetration than electron or optical imaging, proving photoacoustic detection advantageous as a tool to visualise the structural, functional and molecular information of biological samples and tissues [153, 154]. PAM is suitable for highly absorbing and scattering tissues, and has a greater tissue probing depth than FTIR spectroscopy, and the ultrasound detection has improved signal‐to‐noise ratio than FTIR as ultrasound is attenuated less than light [156].…”