Abstract
Ultrasound-modulated optical tomography is an emerging biomedical imaging modality which uses the spatially localised acoustically-driven modulation of coherent light as a probe of the structure and optical properties of biological tissues. In this work we begin by providing an overview of forward modelling methods, before deriving a linearised diffusion-style model which calculates the first-harmonic modulated flux measured on the boundary of a given domain. We derive and examine the correlation measurement density functions of the model which describe the sensitivity of the modality to perturbations in the optical parameters of interest. Finally, we employ said functions in the development of an adjoint-assisted gradient based image reconstruction method, which ameliorates the computational burden and memory requirements of a traditional Newton-based optimisation approach. We validate our work by performing reconstructions of optical absorption and scattering in two- and three-dimensions using simulated measurements with 1% proportional Gaussian noise, and demonstrate the successful recovery of the parameters to within ±5% of their true values when the resolution of the ultrasound raster probing the domain is sufficient to delineate perturbing inclusions.
Highlights
T HE wavelength-dependent optical absorption and scattering coefficients of biological tissues provide clinically valuable information regarding tissue function and composition
In photo-acoustic tomography (PAT) a pulsed laser illuminates the tissue, and regions of optical absorption undergo thermo-elastic expansion, generating an ultrasound wave which is detected on the surface of the medium [22]
PAT can achieve higher transverse spatial resolution than Ultrasound-modulated optical tomography (UOT), as it is not dependent on the ability to focus an acoustic field to a particular point in the medium, though the achievable resolution is typically depth dependent as high frequency components in the measured acoustic field are attenuated by tissue [24]
Summary
T HE wavelength-dependent optical absorption and scattering coefficients of biological tissues provide clinically valuable information regarding tissue function and composition. In PAT a pulsed laser illuminates the tissue, and regions of optical absorption undergo thermo-elastic expansion, generating an ultrasound wave which is detected on the surface of the medium [22] Techniques such as acoustic time-reversal are used to reconstruct the original absorbed energy distribution. PAT can achieve higher transverse spatial resolution than UOT, as it is not dependent on the ability to focus an acoustic field to a particular point in the medium, though the achievable resolution is typically depth dependent as high frequency components in the measured acoustic field are attenuated by tissue [24] Both techniques require reconstruction methods to quantitatively map the optical properties of tissues [25]–[28]. The practical capabilities of PAT and UOT can be seen as largely complimentary: UOT offers the potential to achieve the recovery of absorption and scattering with millimetric resolution, at significant depth, and PAT more readily achieves sub-millimetre resolution in the absorption coefficient at shallower depths
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