Theoretical definition and experimental validation of an correction factor to standardize the absolute magnitude of simulated and clinical spatially-resolved diffuse reflectance spectra
Résumé
In the context of optical biopsy for the diagnosis of skin carcinoma, spatially resolved diffuse reflectance (SR-DR) spectroscopy is widely used to discern healthy from lesional tissues. The estimation of diagnostically relevant optical properties by means of inverse problem solving is one way to exploit the acquired clinical spectra. This method requires the comparison between the latter spectra collected with a medical device (MD), and the ones generated by the photons transport numerical simulations. However, this comparison is typically limited to shape comparison (spectra are normalized before a term-by-term comparison) due to non-standardization of the experimental DR spectra, for which magnitude depends on the multifiber optics probe geometry and on a preliminary calibration measurement performed on a spectralon DR standard illuminated at a given distance. This study proposes to establish a corrective factor to overcome this dependence, and thus obtain clinical spectra whose intensity unit is identical to the simulated ones, i.e., the ratio between photons sent by the emitting fiber and captured by the collecting fibers. The photometric calculations leading to a theoretical value of this factor for various calibration measurement geometries are presented. Experimental validations performed on optical phantoms (with optical properties confirmed from double integrating sphere measurements) using an existing SR-DR MD reveal encouraging fitting between experimental and simulated calculation of such corrective factor. Those results highlight the interest of the method for the standardization of clinically acquired DR spectra i.e. their comparison in terms of absolute magnitudes.