# Mueller Matrix-Based Approach for the Ex Vivo Detection of Riboflavin-Treated Transparent Biotissue
## 3 Results
### Mueller Matrix Measurements Ex Vivo  
Porcine samples prepared as described in Section 3.1 were used for the study presented in this work. A distinction was made between the four measurement conditions as specified in Table 2. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Measurement Condition** & **Riboflavin** & **UV Irradiation** \\ \hline I & \(\times\) & \(\times\) \\ II & \(\times\) & \(\checkmark\) \\ III & \(\checkmark\) & \(\times\) \\ IV & \(\checkmark\) & \(\checkmark\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The different measurement conditions specify the different treatments of the porcine cornea samples with UV irradiation and riboflavin. Noticeable corneal collagen cross-linking (CXL) should only take place in the last group of samples (IV), i.e., the established procedure for photo-induced cross-linking. The remaining conditions are intended for reference and cross-checking. A total of nine cornea samples were available in the first series of measurements. Through the multiple  
Figure 4: Raw image taken with the polarization state HH for PSG (first letter) and PSA (second letter) as an example. The cornea is mounted on a specific polymer chamber to prevent it drying out which is achieved by the refilling of Dextran 500 solution. The back of the holder can also be seen as the cornea is transparent. In the central part, it becomes obvious that a region of the image is overexposed which needs to be excluded from later analysis. The black, nearly circular areas in the image show the liquid access to the chamber. The two boxes indicate from which areas of the images the used measurement data are. The blue area consists of the cornea without the reflection while the red area also contains the reflection spot. Figure 3: Photograph of the setup for MM measurements of porcine corneas in reflection mode. Since the cornea has a curved surface, the sample cannot be aligned in parallel to the illumination beam. Reproduced from [38]. sample use, a sample size of n = 4 can be achieved in all measurement conditions. Five cornea samples were available for the group without riboflavin exposure but with UV radiation. This is because one sample was not used several times to rule out the influence of multiple use on the measurement. The other cornea samples were measured first without UV irradiation and then with UV irradiation. Care was taken to keep the timing for the preparation of the measurements constant. This was important to minimize the influence of drying out of the cornea samples during the measurement. The results of the measurements are displayed in Figure 5. There, the mean values obtained for each MM entry are plotted with the standard deviation. For the calculation, we used the whole images as the input. The series of measurements were grouped for samples that were not prepared with riboflavin (conditions I and II) and those that were prepared with riboflavin (conditions III and IV). It is obvious that even for the small number of samples investigated, the MM entries for the elements \(M(2,1)\), \(M(2,2)\), \(M(2,3)\), \(M(3,2)\), \(M(3,3)\), and \(M(4,4)\) differed between the two groups as a consequence of CXL due to the application of riboflavin and UV irradiation, whereas they are quite similar for all other MM entries. Matching to the results of the measurement in transmission mode, these elements also show the largest mean values, with \(M(2,2)\), \(M(2,3)\), \(M(3,2)\), \(M(3,3)\) representing the entries constructed from linear polarized light states. In contrast, \(M(4,4)\) was obtained from circular polarized light states. Especially in \(M(2,2)\) and \(M(3,3)\), the results between the measurements with and without riboflavin differed even for the small number samples. However, the measurement data for conditions III and IV should differ from each other as well, as in the former case, UV irradiation was not applied and the effects of the cross-linking should be smaller. As the area of the holder does not contain information about the CXL process, it can be omitted.