Changes in the structure of the nasal mucosa can be a morphological biomarker and therefore helpful for diagnosis and follow-up of various pulmonary diseases such as asthma, cystic fibrosis and primary ciliary dyskinesia. In order to verify that microscopic optical coherence tomography (mOCT) is a valuable instrument for the investigation of those changes, an endoscopic OCT system with microscopic resolution (emOCT) was developed and built for clinical testing. The endoscope is based on a graded-index (GRIN) lens optic and provides a calculated lateral resolution of 0.7 μm and an axial resolution of 1.25 μm. The imaging depth was up to 500 μm in tissue; axially, a lateral range of approximate 250 μm could be covered. B-scans were acquired at 80 Hz with 512 pixels in lateral and 1024 pixels in depth-direction. The diameter of the endoscope decreases over a length of 8 cm from 8 mm at the beginning to 1.4 mm at the end and is small enough to observe the mucous membrane in the human nasal concha media and inferior down to the nasopharynx. The emOCT workstation was designed to meet German electrical, optical and biological safety standards. The applicability of the endoscope could be demonstrated in vivo. Mucus transport, glands, blood and lymphatic vessels could be visualized.
Raman microscopy has the potential of creating a molecular contrast in unstained tissue with high specificity at sub-cellular resolution. To overcome the low signal level in raster-scanned microscopy, commonly non-linear Raman techniques are employed. Imaging is usually performed in the CH-stretch region around 3000cm-1 with Raman contrast by spectral unmixing of the broadband lipid and protein Raman modes. Imaging in the fingerprint region around 1500cm-1 can yield higher information density due to the more specific molecular signatures. However, the problem is that in the fingerprint region the Raman signals are generally lower.
We present Raman imaging in the fingerprint region of unstained tumorous tissue samples from human pharynx biopsies employing the time-encoded (TICO) technique. We chose pharynx tissue because this sample location can be accessed by future TICO-Raman endoscopes. In our fiber-based TICO-Raman system the stimulated Raman gain (SRG) signal is encoded in time by a Raman pump laser (1065nm, 600ps pulse length) synchronized to a wavelength-swept Fourier-Domain Mode-Locked (FDML) fiber laser. We sample the fingerprint region between 1300cm-1 – 1900 cm-1 with shot-noise limited sensitivity by employing a dual-balanced, digital and analogue balanced detector. The Stokes laser is a newly developed dispersion compensated FDML laser providing ultra-low noise and long coherence length at 400kHz sweep frequency and 100nm span around 1300nm. We investigate the possibility of applying higher instantaneous powers to increase the signal-to-noise ratio for imaging by lowering the relative shot-noise level suitable for detecting weak, narrowband Raman transitions in the fingerprint region.
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