Bargeman Fonseca, Martina Paula; (2017) Quantitative photoacoustic tomography: experimental phantom studies. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Photoacoustic tomography (PAT) is a promising non-invasive imaging modality exhibiting high resolution, good contrast and specificity to light-absorbing molecules (chromophores). One of the outstanding challenges the technique faces is that PAT images, though dependent on optical absorption, are not its direct representation because they are coloured by the unknown light fluence. Theoretical studies have succeeded in quantifying optical absorption and chromophore concentration by employing model-based inversions (MBI) that can deal with the non-linearity of the problem and the fluence-related distortion. However, experimental translation has been scarce. The aim was to perform quantitative PAT (qPAT) in a rigorous experimental phantom study to show that highly-resolved 3D estimation of chromophore distributions can be achieved. The first consideration was finding a tissue-relevant and stable matrix material and chromophores. Thermoplastic PVCP was fully assessed. Its stability, intrinsic optical properties, thermoelastic efficiency and low-frequency acoustic properties were suitable. The limitation was the lack of photostability of embedded pigments. Separately, we fully characterised aqueous solutions of sulphate salts and found them to be suitable chromophores for qPAT and potential surrogates for oxy- and deoxyhemoglobin. For a phantom made of sub-mm tubes filled with sulphate solutions in an intralipid-rich background, 3D high resolution estimates of chromophore concentrations were obtained through an efficient diffusion-approximation MBI. Uncertainties in optical inputs of the MBI were tackled by assessing in silico their effect on quantification accuracy and then mitigated in the designed experiment through careful measurements. A faithful representation of the multiwavelength photoacoustic tomography images was sought by employing broadband, near-omnidirectional and high-sensitivity sensors and a detection configuration and reconstruction that overcame the limited-view problem. Estimation of the chromophore ratio, analogous to the much sought-after blood oxygenation, gave a mean absolute error of 3.4 p.p., whilst normalised estimates of the two main chromophore distributions gave errors of 13.2% and 17.2%.

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