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Developments in QPAT promise clearer views of tissues

16 Jan 2024

SPIE review of quantitative photoacoustic tomography highlights current challenges being tackled.

Quantitative photoacoustic tomography (QPAT) creates 3D images of tissues and blood vessels, by combining laser-induced photoacoustic signals and ultrasound detection.

Clinical translation of QPAT will be a further step in the adoption of photoacoustic (PA) methods, techniques which have for that last decade been recognised as attractive optics-based alternatives to MRI and CT scans.

Research into medical applications of PA as a family of imaging techniques continues to deliver breakthroughs, such as the use of miniaturized PA modules able to operate within endoscopes that exploit advances in both optical hardware and multimode fibers.

QPAT brings its own particular challenges, however. Unlike direct photoacoustic microscopy, a tomographic method involves unfocused light illuminating a whole region of interest, with an array of unfocused detectors used to record the resulting acoustic time series.

A review article in SPIE Journal of Biomedical Optics has now surveyed the optical and image generation aspects of the QPAT technique, to identify some of the current strengths and weaknesses of the method.

"It surveys the current thinking regarding two related problems," commented co-author Tanja Tarvainen from the University of Eastern Finland. "What is the best way to describe light propagation and its interaction with biological tissue mathematically? Given photoacoustic measurements, what can we learn about the optical properties of tissue, or indeed the related and more clinically relevant properties such as blood oxygenation?"

The review, authored by Tarvainen and Ben Cox from University College London, discusses the factors needed to model the propagation of light and sound in tissues, including the radiative transfer equation (RTE) describing how light interacts with tissue; and the Grüneisen parameter, linking the optical energy absorbed by tissues to the acoustic pressure distribution.

Honoring Lihong Wang, biomedical optics pioneer

Since photoacoustics involves estimating the concentrations of light-absorbing molecules in biological tissues, it requires what the authors term an "acoustic inverse problem" of determining the pressure distribution from a photoacoustic time series; and an "optical inverse problem" where the distributions of optical parameters are estimated from the absorbed optical energy density.

The JBO review focuses on the second of these, reviewing the current best solutions: either directly estimating chromophore concentrations from absorbed optical energy density, or calculating the concentrations from the spectroscopic data via the absorption coefficients.

Practical implementation of QPAT is another of the article's topics, discussing complications such as the effect of optical scattering, variations in optical energy absorption by the tissues, the intensive computational methods needed, and uncertainties in the parameters used as inputs to the models.

"Although QPAT is a promising methodology for providing high-resolution 3D images of physiologically relevant parameters, there are many computational modeling-based challenges that need to be tackled before the technique can be developed as a standard clinical or preclinical tool," said Tanja Tarvainen.

 The review appears in a special issue of JBO honoring Lihong V. Wang as a pioneer in biomedical optics.

Now at Caltech, Lihong Wang's work on photoacoustic methods has included steps towards 3D functional imaging of the human brain, alongside applications of the technique to breast cancer examination, small animal studies, and ways to maximise the useful information extracted from the inherently weak photoacoustic signals.

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