## 3 Potential of Various Probe Designs for Epithelial Precancer Diagnosis
### Effect of Orientation of Fibers  
In this section, we study the effect of the orientation of optical fibers on the epithelium sensitivity. Using the Monte Carlo simulations it has been established [12] that change in the bevel angle of the source as well as collection fibers not only affects the amount of the collected photons but also governs the depth selection. To verify the effect of the bevel angle of the collection fiber on the depth sensitivity, MC simulations have been performed for a two layered phantom irradiated by a fiber oriented normal to the phantom. The spatially resolved epithelial sensitivity for multiple bevel angles \((10,30,50,70^{\circ})\) of the collection fibers (Figure 1a) were obtained from the simulations and demonstrated in Figure 1b. As expected, increased bevel angle of the collection fibers was observed to yield better epithelial sensitivity. We further evaluated three probe geometries, commonly found in the literature [11; 15]: Probe geometry-I: unbeveled source and detector fibers placed normal to the phantom surface (Figure 2a). Probe geometry-II: beveled source and detector fibers (bevel angle \(\beta=45^{\circ}\)) on the phantom surface (Figure 2b). Probe geometry-III: beveled source and detector fibers (bevel angle \(\beta=45^{\circ}\)) with the source and detector fibers oriented in opposite directions as shown in Figure 2c. While the probe geometry-I has already been thoroughly studied in [4; 6; 8; 9; 16], we have included it in our analysis for completeness.  
Figure 2d-f depicts the 2-d spatial distribution of reflectance obtained from the MC simulations for the three probe geometries. The asymmetry in the collected SRR as observed in Figure 2e,f is due to asymmetry in the source-detector configurations in probe geometry-II and III respectively. The plot of \(\mathcal{E}\) along the x- and y- axes are depicted in Figure 2g-i.  
The most commonly used probe geometry (I) has a sharp drop in \(\mathcal{E}\) along both axes (Figure 2g). The probe geometry-II, with the beveled source and detection fibers, shows substantially improved \(\mathcal{E}\) along x-axis (Figure 2h). The best epithelial the sensitivity is obtained with probe geometry-III (Figure 2i), where significant values of \(\mathcal{E}\) are observed upto 1 mm. The numerical analysis shows that the probe-geometry III is the most suitable for epithelial pre-cancer detection. While for internal organs it is not feasible to employ such a geometry, it can be used for early detection of cancer in the external organs such as skin and oral cavity. Hence, in our study we have utilized probe geometry-II.  
Figure 1: (**a**) Source-detector geometry in which detector fibers are at \(10^{\circ},30^{\circ},50^{\circ},70^{\circ}\) bevel angles, (**b**) variation of epithelium sensitivity with bevel angles and source-detector distances, plot with red curve shows spatially resolved epithelium sensitivity at a \(10^{\circ}\) bevel angle and similarly, blue, black and magenta curves represent epithelium sensitivity at \(30^{\circ},50^{\circ}\), and \(70^{\circ}\) bevel angles respectively (color online only).