Erste Untersuchungen zur Anwendung der linearen Signalübertragungstheorie in der Auswertung diaphanoskopischer Untersuchungen am Beispiel der Rheumadiagnostik - 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 μ a « μ' s and μ a = constant (μ a = absorption coefficient) were assumed). The parameter its' (= 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.