Angle Distribution Measurement of Scattered Light Intensity From Needle-Shaped Crystals in a Magnetic Field for Gout Diagnosis

Monosodium urate (MSU), which causes gout, forms needle-shaped crystals with large diamagnetic susceptibility anisotropy that orient in a magnetic field of up to 500 mT. The light transmittance and reflectance of MSU suspensions change under an applied magnetic field. To use this phenomenon to diagnose gout, it would be necessary to collect light scattering information from all angles, including transmission and reflection. This is because the scattered light intensity from MSU crystals is different in forward, sideward, and backward directions under near-infrared (NIR) light (785 nm), which has high biopermeability and is used in NIR spectroscopy. Therefore, we measured the angular distributions of light scattering by MSU crystals before and after applying a magnetic field and evaluated their characteristics. In the case of MSU crystals of the size found in gout patients, the scattered light intensity decreased at all angles under an applied magnetic field compared with that without a magnetic field. The change ratio of the scattered light intensity was large in the forward and backward directions and relatively small in the sideward directions. This was ascribed to the combined effects of the decrease in the scattering cross section caused by the magnetic orientation and the accompanying change in diffraction behavior. Our findings indicated that for gout diagnosis, the incident light should be horizontal and that if transmission measurement is possible, a detection angle of about 30° is suitable. If transmission measurement is not practical, it is better to avoid a detection angle of around 90°.