## 3 Results
Due to the 180\({}^{\circ}\) back-scattering arrangement, the incident light transmits the sample twice. Forward-scattered light is reflected back by the coated surface. Gold absorbs in the blue spectral range. This is indicated by the drop around 450 nm in each reflectance spectrum of Figure 4a. Reflectivity increases between 500 nm and 700 nm and remains constant from 700 nm to 950 nm. The highest reflectivity of the shown spectra is detected for OCT. GM has the lowest reflectivity in the VIS spectral region. This is in accordance with the color-coding in Figure 3(2a,4a). Pixels in the WM area appear reddish and GM is rather bluish. Artefacts, such as the three air bubbles and the tissue fold, that appear darker in the brightfield images have lower reflectivity values as well. However, the created images (Figure 3(2a,4a)) are based on reflectivity at 750 nm. The chosen wavelength lies in the optical window of tissue that ranges from 600 nm to 940 nm [69]. This spectral region is independent of the main absorber in blood-perfused tissue, hemoglobin [70]. Therefore, the created images do not contain information regarding the chemical composition of the tissue section but represent the reflectivity of the sample at 750 nm. Similar results were obtained by images based on the reflectivity in the NIR spectral region at 1150 nm (R\({}_{1150}\)) (Figure 3(2b,4b)). This wavelength lies also in an optical window around 1100 nm to 1350 nm [71]. Compared to the VIS spectra, NIR spectra fluctuate across the entire wavelength region (Figure 4b). This could be a result of the NIR spectrometer's lower sensor sensibility (extended wavelength InGaAs detector) compared to the VIS spectrometer (Si photodiodes). The signal-to-noise ratio becomes poorer with increasing wavelengths. Anyhow, these fluctuations could be the reason for poorer contrast and lateral resolution of the NIR images compared to the VIS images. Images of the tissue section that are based on chemical composition were obtained by the evaluation of the Raman spectra. As already mentioned in the introduction, intensity peak ratios as well as the sum of two intensity peak ratios of lipid and protein peaks have been identified to distinguish GM and WM. Representative Raman spectra of WM, GM, and OCT are shown in Figure 5. Peaks used to create Raman images are highlighted within the figure. The corresponding assignments of intensity peaks to lipid and protein are indicated in Table 3.  
Figure 5: Preprocessed Raman spectra (averaging over 9 spectra) of white matter (WM), grey matter (GM), and optimal cutting temperature compound (OCT), offset +150 counts (GM), +300 counts (WM). The majority of the peaks are located in the fingerprint region between 700 cm\({}^{-1}\) and 1800 cm\({}^{-1}\)[75]. The high-wavenumber region (2800-3050 cm\({}^{-1}\)) contains CH-stretching vibrations [76]. The evaluated peak ratios of Raman intensities to distinguish GM and WM were adapted from literature and include the following: I\({}_{1299}\)/I\({}_{1439}\)[55], I\({}_{1330}\)/I\({}_{1344}\)[35], I\({}_{1441}\)/I\({}_{1659}\)[34], I\({}_{1442}\)/I\({}_{1662}\)[33], I\({}_{1443}\)/I\({}_{1008}\)[46], I\({}_{2845}\)/I\({}_{2930}\)[56], I\({}_{2885}\)/I\({}_{2888}\))/I\({}_{2930}\)[46]. The best results to distinguish GM, WM, and OCT within the global scan were obtained with an intensity peak ratio of I\({}_{1444}\)/I\({}_{1660}\) (Figure 3(c)), where 1444 cm\({}^{-1}\) is primarily assigned to lipids (CH\({}_{2}\), CH\({}_{3}\) vibration modes) and 1660 cm\({}^{-1}\) corresponds to proteins (Amide I band). The highest peak ratios were achieved for OCT (yellow pixels). This can be explained by the fact that the Raman spectrum of OCT does not show a peak around 1660 cm\({}^{-1}\). The peak ratio is higher in WM than in GM. This is confirmed by the peak characteristics within the shown Raman spectra. The ventricles do not show color contrast. The reason, therefore, is the broad fluorescence background of the glass signal at approximately 1400 cm\({}^{-1}\) when using 785-nm laser excitation [27].