Design and characterisation of intervertebral disc mimicking phantoms for photoacoustic imaging

Photoacoustics has gained momentum as a new medical imaging technique owing to its ability to benefit from good optical contrast and acoustic resolution. To ease transfer into clinical settings and validate the algorithms, calibrated tissue-mimicking materials (TMM) are required. This paper describes a complete photoacoustic characterisation of a line of titanium dioxide (TiO₂) doped agarose hydrogels whose optical absorption (490-835 nm), reduced scattering (590-815 nm), isobaric heat capacity, mass density, speed of sound and acoustic attenuation were quantified for an agarose concentration up to 4% w/w and a TiO₂ concentration ranging from 0.25 to 1 mg/mL. Empirical constitutive laws as a function of the concentrations of the components were derived, enabling the creation of TMM with tailored properties. Results showed that these phantoms are suitable candidates to mimic the photoacoustic properties of various soft tissues including intervertebral discs (IVD). Photoacoustic probings performed on an IVD-mimicking phantom and six healthy porcine discs demonstrated the ability of these TMM to accurately replicate healthy IVD properties; this could serve as a first step towards an application of photoacoustic imaging to quantifying disc degeneration.