Simulated assessment of light transport through ischaemic skin flaps

Currently, free flaps and pedicled flaps are assessed for reperfusion in post-operative care using colour, capillary refill, temperature, texture and Doppler signal (if available). While these techniques are effective, they are prone to error due to their qualitative nature. In this research, we explore using different wavelengths of light to quantify the response of ischaemic tissue. The assessment provides us with indicators that are key to our goal of developing a point-of-care diagnostics device, capable of observing reduced perfusion quantitatively. We set up a detailed optical model of the layers of the skin. The layers of the model are given appropriate optical properties of the tissue, with due consideration of melanin and haemoglobin concentrations. We simulate 24 models of healthy, perfused tissue and perfusion-deprived tissue to assess the responses when illuminated with visible and near-infrared wavelengths of light. In addition to detailed fluence maps of photon propagation, we propose a simple mathematical model to assess the differential propagation of photons in tissue; the optical reperfusion factor (ORF). Our results show clear advantages of using light at longer wavelengths (red, near-infrared) and the inferences drawn from the simulations hold significant clinical relevance. The simulated scenarios and results consolidate the belief of a multi-wavelength, point-of-care diagnostics device and inform its design for quantifying blood flow in transplanted tissue. The modelling approach is applicable beyond the current research, wherein other medical conditions that can be mathematically represented in the skin can be investigated. Through these, additional inferences and approaches to other point-of-care devices can be realised.