Erste untersuchungen zur anwendung der linearen signalübertragungstheorie in der auswertung diaphanoskopischer untersuchungen am beispiel der rheumadiagnostik

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 μ_a ≪ μ′_s and μ_a ≈ constant (μ_a = absorption coefficient) were assumed). The parameter μ′_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 μ′_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.