Intra-operative Fluorescence Imaging of ALA-induced PpIX using the Double Ratio Imaging Technique in Malignant Brain Tumours

Abstract

Contrast in fluorescence signals measured in vivo from tumour-selective photosensitizers, is blurred by spatial variations in optical properties of the tissue, its intrinsic fluorescence and geometrical parameters of the fluorescence measurement setup. A method using two wavelength excitation and two wavelength regions detection, the Double Ratio technique, corrects the measured signals for the influence of variations in absorption, scattering and setup geometry, leaving a result depending only on the photosensitizer concentration. A fluorescence imaging system based on this technique, was used intra-operatively during removal of malignant brain tumours from patients that received ALA pre-operatively. Comparison of preliminary imaging results with fibre spectroscopic measurements and histological results, show that the Double Ratio fluorescence imaging system can reveal differences in ALA-induced Protoporphyrin IX concentrations, where normal fluorescence imaging fails.