Abstract
Optical coherence tomography (OCT) is an imaging modality based on the inherent backscattering of light within different tissue types. OCT was introduced in the early 1990s and has since become a standard diagnostic tool in ophthalmology. Images can be acquired non-destructively, in real time and three dimensions at micrometer resolution. Only recently, OCT has been increasingly recognized in other fields such as neuroimaging. Here we present a multimodal imaging approach using a custom-built visible light optical coherence microscope (OCM) combined with a fluorescence imaging mode for the evaluation of different tumor compartments in glioblastoma (GB) samples retrieved during 5-aminolevulinic acid (5-ALA) fluorescence-guided surgery. 18 biopsies of ten GB patients were imaged using the visible light OCM system, providing the three-dimensional morphologic structure of the tissue on a cellular level (axial resolution in brain tissue 0.88 µm, penetration depth 100 µm). Attenuation coefficients, i.e., indicators for light penetration and scattering, were calculated for each sample. Tumor-specific contrast enhanced by 5-ALA was evaluated in co-registered images from the fluorescence channel. Samples were ultimately processed for histopathologic work-up and compared to OCM findings. Three different groups of biopsies could be defined based on quantitative 5-ALA fluorescence [normalized between 0 and 1], attenuation [mm-1] and histological H&E stainings: tumor core (n=8; fluorescence [median ± standard deviation]=0.72±0.13, attenuation [median ± standard deviation]=3.9±0.66), infiltration zone (n=6; fluorescence=0.5±0.19, attenuation=4.4±0.5), and adjacent brain parenchyma (n=4; fluorescence=0.3±0.12, attenuation=5.0±0.79). Concurrent increase in fluorescence intensity and cell density was significantly associated with tissue malignancy (tumor core: 3849±1028 nuclei/mm²; brain parenchyma: 1364±236 nuclei/mm². p=0.024). Furthermore, a negative correlation between fluorescence and attenuation coefficient was detected (r=0.51, p=0.032). Aforementioned results suggest that this multimodal imaging setup is a promising approach for non-destructively investigating the three-dimensional morphologic structure at microscopic resolution whilst at the same time leveraging tumor-specific contrast through 5-ALA fluorescence.
Revealing brain pathologies with multimodal visible light optical coherence microscopy and fluorescence imaging