Characterization of tissue-simulating polymers for photoacoustic vascular imaging

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Photoacoustic tomography (PAT) is a maturing imaging technique which combines optical excitation and acoustic detection to enable deep tissue sensing for biomedical applications. Optical absorption provides biochemical specificity and high optical contrast while ultrasonic detection provides high spatial resolution and penetration depth. These characteristics make PAT highly suitable as an approach for vascular imaging. However, standard testing methods are needed in order to characterize and compare the performance of these systems. Tissue-mimicking phantoms are commonly used as standard test samples for imaging system development and evaluation due to their repeatable fabrication and tunable properties. The multi-domain mechanism behind PAT necessitates development of phantoms that accurately mimic both acoustic and optical properties of tissues. While a wide variety of materials have been used in the literature, from gelatin and agar hydrogels to silicone, published data indicates that poly(vinyl chloride) plastisol (PVCP) is a promising candidate material for simulating tissue optical and acoustic properties while also providing superior longevity and stability. Critical acoustic properties of PVCP phantoms, including sound velocity and attenuation, were measured using acoustic transmission measurements at multiple frequencies relevant to typical PAT systems. Optical absorption and scattering coefficients of PVCP gels with and without biologically relevant absorbers and scatterers were measured over wavelengths from 500 to 1100 nm. A custom PAT system was developed to assess image contrast in PVCP phantoms containing fluid channels filled with absorbing dye. PVCP demonstrates strong potential as the basis of high-fidelity polymer phantoms for developing and evaluating PAT systems for vascular imaging applications.