  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.
