Measurement of bone mineral density via light scattering

In this study we have investigated the potential of optical techniques to monitor changes in bone mineral density (BMD) via changes in scattering coefficient. For each of five bone samples, diffuse reflection and transmission coefficients were measured over the wavelength range 520-960 nm using an integrating sphere and CCD spectrometer. These were converted into optical absorption and scattering coefficients using a Monte Carlo inversion procedure. Measurements were made on samples immersed in formic acid solution for different lengths of time in order to investigate the effect of reduction in BMD on the optical properties. After full demineralization, the optical scattering coefficient fell by a factor 4. From the observed degree of fluctuation of the measurements, we estimate that BMD could be measured with an accuracy of 7% if optical scattering can be measured with an accuracy of 10%. We also report preliminary measurements of bone scattering using optical coherence tomography (OCT). An inter-side variability of 3% is obtained on dry samples with and without overlying periosteum. These results suggest that minimally invasive techniques for measuring optical scattering, such as OCT, may have a role in monitoring regional changes in BMD. This could be an important advance in our understanding of bone remodelling and its relationship to osteoarthritis. Both the integrating sphere and OCT measurements also suggest that light transport in bone is spatially anisotropic. OCT was used to assess probability of obtaining results in vivo.