Evaluation of Mouse Models of Colorectal Cancer Using Optical Coherence Tomography and Laser Induced Fluorescence Spectroscopy

Abstract

Colorectal cancer (CRC) is the third leading cause of cancer related deaths. Rodent models of CRC are useful for evaluating diagnostic tools, therapeutics, and disease progression; however, an appropriate imaging tool is needed. Optical coherence tomography (OCT) is a non-destructive imaging modality readily packaged into small diameter endoscopes. Using a near- infrared light source, structural images are generated from index of refraction mismatches with resolutions of 2-15 mm at imaging depths of up to 1.3 mm. In contrast, laser-induced fluorescence (LIF) spectroscopy provides information about biochemical composition, exciting tissues with ultraviolet to green wavelengths of light to measure fluorescence emission from endogenous fluorophores such as NADH, collagen, and porphyrin.We apply OCT and LIF to mouse models of CRC, beginning with a comprehensive ex-vivo evaluation of normal mouse gastrointestinal (GI) tract in various strains and ages and secondarily sampled colorectal neoplasia and inflammatory bowel disease (IBD) using a combined in-air OCT/LIF system. A set of characteristic features of OCT images were developed for normal esophagus, small intestine, and colon; preliminary image feature criteria were also developed for colorectal neoplasia and IBD. LIF characterized the endogenous fluorescence of mouse GI tract, with spectral features corresponding to collagen, NADH, and hemoglobin. In the IBD sample, LIF emission displayed potentially diagnostic peaks at 635 and 670 nm, consistent with increased porphyrin production by bacteria associated with IBD.Next, endoscopic OCT/LIF was evaluated in an in-vivo serial study using a prototype 2 mm diameter endoscope to image the lower colon of ApcMin and control mice. Adenoma development over OCT imaging timepoints was characterized as a progressive mucosal thickening to frank mass formation. LIF spectral comparisons revealed decreased 405 nm intensity and the presence of a peak at 680 nm over adenoma.In a final study, ultrahigh resolution OCT (UHR OCT) was used to serially image the lower colon of azoxymethane treated A/J mice to monitor CRC progression and determine OCT's capability of identifying early disease. A panel of blinded mouse colon pathology experts assigned a diagnosis based on the OCT images, which was then compared to a histological diagnosis assigned by a blinded pathologist. At the final imaging timepoint, 95% of adenomas and 23% of gastrointestinal intraepithelial neoplasia (GIN, 38% protruding GIN and 9% non-protruding GIN) were correctly diagnosed. The panel identified 68% of disease foci (95% adenoma, 76% protruding GIN, and 13% non-protruding GIN). Over the OCT imaging timepoints, disease progression followed a typical succession, with normal or GIN preceding adenoma. Endoscopic UHR OCT enabled accurate diagnosis of adenomas, identification of protruding GIN, and non-destructive visualization of CRC progression, providing a tool for cancer research in animal models.