Characterisation of a PVCP based tissue-mimicking phantom for Quantitative Photoacoustic Imaging

Photoacoustic imaging can provide high resolution images of tissue structure, pathology and function. As these images can be obtained at multiple wavelengths, quantitatively accurate, spatially resolved, estimates for chromophore concentration, for example, may be obtainable. Such a capability would find a wide range of clinical and pre-clinical applications. However, despite a growing body of theoretical papers on how this might be achieved, there is a noticeable lack of studies providing validated evidence that it can be achieved experimentally, either in vitro or in vivo. Well-defined, versatile and stable phantom materials are essential to assess the accuracy, robustness and applicability of multispectral Quantitative Photoacoustic Imaging (qPAI) algorithms in experimental scenarios. This study assesses the potential of polyvinyl chloride plastisol (PVCP) as a phantom material for qPAI, building on previous work that focussed on using PVCP for quality control. Parameters that might be controlled or tuned to assess the performance of qPAI algorithms were studied: broadband acoustic properties, multiwavelength optical properties with added absorbers and scatterers, and photoacoustic efficiency. The optical and acoustic properties of PVCP can be tuned to be broadly representative of soft tissue. The Grüneisen parameter is larger than expected in tissue, which is an advantage as it increases the signal-to-noise ratio of the photoacoustic measurements. Interestingly, when the absorption was altered by adding absorbers, the absorption spectra measured using high peak power nanosecond-pulsed sources (typical in photoacoustics) were repeatably different from the ones measured using the low power source in the spectrophotometer, indicative of photochemical reactions taking place.