Initial investigations of the application of linear signal transfer theory to tissue optics

Rheumatoid arthritis affecting the small joints-in particular the fingers-has advantageous geometry for the transmission of near-infrared (NIR) light. Examination of the optical properties of tissues has revealed that as a result of changes to the capsule and synovial fluid there is a considerable increase in photon scattering already in the early stages of the disease-in particular around 685 nm. This suggests the appropriateness of analysing the photon density profile resulting from punctiform irradiation of the joint, In a first approximation, the point spread function of transmitted photon density is confirmed to be proportional to a Gauss distribution, as suggested by Arridge. In accordance with the linear signal transfer theory, therefore, it is possible to establish a virtual transfer system described by a first-order differential equation. (The tissue optical conditions mu (a) << mu'(s) and mu (a) approximate to constant (mu (a)= absorption coefficient) were assumed). The parameter mu'(s) (= reduced scattering coefficient) was determined by linear approximation of the Gauss distribution to the calculated or measured point spread function. For selected patient data, the mu'(s) was determined in healthy and diseased finger joints (e.g. 10.1 cm(-1) and 26.8 cm(-1), respectively), and the results were in good agreement with those obtained experimentally.