Early detection of neoplastic changes before metastasis occurs remains a critical objective in clinical cancer diagnosis and treatment. Excisional biopsy and histopathology is currently the gold standard for cancer diagnosis. However, it can suffer from false negative rates due to sampling errors. Optical imaging technologies can provide real-time imaging of human tissues in vivo with resolutions approaching that of histopathology and are able to reveal the biochemical and/or molecular information; therefore they could significantly improve the identification of malignancies at curable stages. The ability to assess tissue architectural morphology (such as the alterations in glandular or stromal morphology) and molecular information (such as up-regulation of receptors and over-expression of enzymes), in vivo and in real time, without the need for tissue excision, would be a major advance in cancer diagnostics and therapy.
Optical coherence tomography (OCT) is an emerging medical diagnostic imaging technology that enables micron-scale, cross-sectional imaging of microstructure in biological tissues in situ
and in real time [1
]. OCT imaging is analogous to B-mode ultrasound, except that OCT is performed by measuring the echo time delay and intensity of reflected or backscattered light rather than sound. An optical beam is scanned across the tissue and the backscattered light is measured as a function of axial depth and transverse position. In this way, OCT can generate cross-sectional, tomographic images of subsurface tissue microstructure. Three-dimensional tissue morphology can be formed by stacking a series of two-dimensional OCT tomograms. OCT can perform imaging with resolutions approaching that of conventional histopathology, but without the need of tissue removal. Standard-resolution OCT technology has image resolutions of 10–15 μm, and ultrahigh resolutions of 1–2 μm has been achieved using state-of-the-art laser technologies [3
]. OCT imaging can be performed using noninvasive or minimally invasive optical delivery systems such as microscopes, handheld probes, endoscopes, catheters, laparoscopes, and needles that enable noninvasive or minimally invasive internal body imaging [4
]. OCT imaging can be performed in real time, thus allowing the guidance of conventional excisional biopsy or real-time assessment of tissue pathologies.
In contrast to the anatomical information provided by OCT, fluorescence imaging provides the biochemical and metabolism information [7
]. Therefore there are great interests to combine these two modalities to provide both the structural and functional information to assess the biological tissue more comprehensively and enhance the disease detection capability. Kuranov et al.
combined OCT and laser induced fluorescence (LIF) using aminolevulinic acid (ALA) to improve the identification of tumor boundaries in the cervix [10
]. Pan et al.
reported that ALA fluorescence-guided endoscopic OCT could enhance the efficiency and sensitivity of early bladder cancer diagnosis [12
]. In an animal model studies, they demonstrated that the specificity of fluorescence detection of transitional cell carcinoma was significantly enhanced by fluorescence-guided OCT (53% vs. 93%), and the sensitivity of fluorescence detection also improved by combination with OCT (79% vs. 100%) [13
]. Tumlinson et al.
have developed a combined OCT and LIF spectroscopy imaging catheter for in vivo
mouse colon imaging [14
]. This miniaturized 2 mm diameter catheter has been used to monitor the disease progression in mouse colon longitudinally, and is able to identify colorectal adenomas in murine models [15
]. Podoleanu et al.
have reported the combined OCT and confocal laser scanning ophthalmoscopy (SLO) with integrated simultaneous fluorescence detection using Indocyanine green (ICG), and demonstrated the synergy between the functional and anatomic information will provide a more complete view of the pathologic conditions of a variety of macular diseases [18
Using the fluorescence contrast agents which target to specific molecular processes, fluorescence molecular imaging (FMI) could reveal molecular information associated with specific disease development. Previous research has shown that cancerous tumors can be identified with fluorescent markers [21
]. In a study done in mice, it was shown that adenomatous polyps in the colon express 36% more of the proteolytic enzyme cathepsin B than normal tissue [23
]. The adenomas in the animals injected with 2 nmole of cathepsin B probe showed a remarkably higher target to background contrast. On average, contrast was about 2 fold higher in large adenomas. Only after administration of the imaging probe, these lesions were highly conspicuous and even small adenomas, about 50 μm in diameter, could be readily identified [23
In this research, we combined the OCT with FMI to investigate the correlation between OCT structural features and FMI molecular information. The system demonstrated co-registered en face OCT and FMI imaging with ~ 10 μm resolution. Relationships of FMI intensity and dye concentration as well as FMI intensity and target fluorescence tube depth are studied. The capability of imaging biological tissue was demonstrated by imaging the mouse colon cancer model ex vivo using molecular contrast agent targeting glycoprotein over-expression.