Fluorescence confocal endomicroscopy in biological imaging

Abstract

In vivo fluorescence microscopic imaging of biological systems in human disease states and animal models is possible with high optical resolution and mega pixel point-scanning performance using optimised off-the-shelf turn-key devices. There are however various trade-offs between tissue access and instrument performance when miniaturising in vivo microscopy systems. A miniature confocal scanning technology that was developed for clinical human endoscopy has been configured into a portable device for direct hand-held interrogation of living tissue in whole animal models (Optiscan FIVE-1 system). Scanning probes of 6.3mm diameter with a distal tip diameter of 5.0mm were constructed either in a 150mm length for accessible tissue, or a 300mm probe for laparoscopic interrogation of internal tissues in larger animal models. Both devices collect fluorescence confocal images (excitation 488 nm; emission >505 or >550 nm) comprised of 1024 x 1204 sampling points/image frame, with lateral resolution 0.7um; axial resolution 7um; FOV 475 x 475um. The operator can dynamically control imaging depth from the tissue surface to approx 250um in 4um steps via an internally integrated zaxis actuator. Further miniaturisation is achieved using an imaging contact probe based on scanning the proximal end of a high-density optical fibre bundle (~30,000 fibres) of <1mm diameter to transfer the confocal imaging plane to tissue in intact small animal organs, albeit at lower resolution (30,000 sampling points/image). In rodent models, imaging was performed using various fluorescent staining protocols including fluorescently labelled receptor ligands, labelled antibodies, FITC-dextrans, vital dyes and labelled cells administered topically or intravenously. Abdominal organs of large animals were accessed laparoscopically and contrasted using i.v. fluorescein-sodium. Articular cartilage of sheep and pigs was fluorescently stained with calcein-AM or fluorescein. Surface and sub-surface cellular and sub-cellular details could be readily visualised in vivo at high resolution. In rodent disease models, in vivo endomicroscopy with appropriate fluorescent agents allowed examination of thrombosis formation, tumour microvasculature and liver metastases, diagnosis and staging of ulcerative colitis, liver necrosis and glomerulonephritis. Miniaturised confocal endomicroscopy allows rapid in vivo molecular and subsurface microscopy of normal and pathologic tissue at high resolution in small and large whole animal models. Fluorescein endomicroscopy has recently been introduced into the medical device market as a clinical imaging tool in GI endoscopy and is undergoing clinical evaluation in laparoscopic surgery. This medical usage is encouraging in-situ endomicroscopy as an important pre-clinical research tool to observe microscopic and molecular system biologic events in vivo in animal models for various human diseases.