Reconstruction of the opto-mechanical properties of the intervertebral disc with photoacoustic imaging

The intervertebral disc (IVD) is an essential component of the spine, serving as a shock absorber that enables flexibility and movement. Due to aging, the IVD undergoes degeneration, characterised by dehydration, which can lead to conditions like degenerative disc disease (e.g. herniated discs, spinal stenosis), resulting in pain, stiffness, and nerve damage. This degeneration primarily stems from changes in its intrinsic composition, in terms of water and collagen content, the main chromophores. Quantitative photoacoustic imaging is explored as a method for assessing the state of the disc. A numerical study was conducted using a digitised pig’s disc and opto-mechanical properties extracted from the litterature, with measurement configurations compatible with experimental setups. A non-linear model-based reconstruction approach was implemented, utilising the adjoint variables method to express gradients, and Monte Carlo simulations to solve the Radiative Transfer Equation involved in the forward and adjoint problems. Various scenarios were studied and a method that does not rely on the prior knowledge of the Grüneisen coefficient was proposed. The results demonstrate the feasibility of using photoacoustic measurements to assess the IVD’s composition, achieving significantly improved quantification compared to conventional methods.