The fiber-optic probe is an essential component of many quantitative fluorescence spectroscopy systems, enabling delivery of excitation light and collection of remitted fluorescence in a wide variety of clinical and laboratory situations. However, there is little information available on the role of illumination--collection geometry to guide the design of these components. Therefore we used a Monte Carlo model to investigate the effect of multifiber probe design parameters--numerical aperture, fiber diameter, source--collection fiber separation distance, and fiber-tissue spacer thickness--on light propagation and the origin of detected fluorescence. An excitation wavelength of 400 nm and an emission wavelength of 630 nm were simulated. Noteworthy effects included an increase in axial selectivity with decreasing fiber size and a transition with increasing fiber-tissue spacer size from a subsurface peak in fluorophore sensitivity to a nearly monotonic decrease typical of single-fiber probes. We provide theoretical evidence that probe design strongly affects tissue interrogation. Therefore application-specific customization of probe design may lead to improvements in the efficacy of fluorescence-based diagnostic devices.
[Show abstract][Hide abstract] ABSTRACT: We theoretically demonstrated a new optical imaging technique based on reverse-time migration (RTM) for reconstructing optical structures in homogeneous media for the first time. RTM is a powerful wave-equation-based method to reconstruct the image of the structure by modeling the wave propagation inside the media with both forward modeling and reverse-time extrapolation. While RTM is commonly used with acoustic seismic waves, this paper represents the first effort to develop optical RTM imaging method for biomedical research. To refine the image quality, we further developed new methods to suppress the low-wavenumber artifact (LWA). When compared with the conventional means for LWA suppression such as Laplacian filtering, illumination normalization, and the ratio method, our new derivative-based and power-image methods are able to significantly reduce LWA, resulting in high-quality reconstructed images with sufficient contrasts and spatial resolutions for structure identification. The optical RTM imaging technique may provide a new platform for non-invasive optical imaging of structures in deep layers of tissues for biomedical applications.
"Here we use the Monte Carlo (MC) method to calculate the spatial distribution of Raman sensitivity depending on the measurement geometry and the optical properties of the sample. Similar calculations have been performed previously for fluorescence measurements [21,22]. MC models for Raman scattering have been previously developed by Enejder et al.  for predicting the effect of measurement geometry on the detected Raman signal of blood. "
[Show abstract][Hide abstract] ABSTRACT: We present a Monte Carlo model, which we use to calculate the depth dependent sensitivity or sampling volume of different single fiber and multi-fiber Raman probes. A two-layer skin model is employed to investigate the dependency of the sampling volume on the absorption and reduced scattering coefficients in the near infrared wavelength range (NIR). The shape of the sampling volume is mainly determined by the scattering coefficient and the wavelength dependency of absorption and scattering has only a small effect on the sampling volume of a typical fingerprint spectrum. An increase in the sampling depth in nonmelanoma skin cancer, compared to normal skin, is obtained.
"As the separation between the excitation and emission fibers increases by a distance d, the penetration depth increases by a distance approximately d/2 . As the separation increases however, the collection efficiency also decreases exponentially . Therefore, the single channel design provided the least depth of penetration while also the greatest collection efficiency. "
[Show abstract][Hide abstract] ABSTRACT: Combined optical coherence tomography (OCT) and laser-induced fluorescence (LIF) endoscopy has shown higher sensitivity and specificity for distinguishing normal tissue from adenoma when compared to either modality alone. Endoscope optical design is complicated by the large wavelength difference between the two systems. A new high-resolution endoscope 2 mm in diameter is presented that can create focused beams from the ultraviolet to near-infrared. A reflective design ball lens operates achromatically over a large wavelength range, and employs TIR at two faces and reflection at a third internal mirrored face. The 1:1 imaging system obtains theoretically diffraction-limited spots for both the OCT (1300 nm) and LIF (325 nm) channels.
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