## 4 Results and Discussion
### Technical Evaluation
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Quality Index**} & \multirow{2}{*}{**Detector**} & \multicolumn{2}{c}{**880 nm**} & \multicolumn{2}{c}{**810 nm**} & \multicolumn{2}{c}{**770 nm**} & \multicolumn{2}{c}{**660 nm**} & \multicolumn{2}{c}{**Wavelength**} & \multicolumn{2}{c}{**Detector**} \\ \cline{3-13}  & & **Me** & **RI** & **Me** & **RI** & **Me** & **RI** & **Me** & **RI** & **_p_-Value** & _p_-Value** \\ \hline \multirow{3}{*}{\begin{tabular}{c} Perfusion \\ index \\ \end{tabular} } & Proximal & 0.31 & 0.03 & 0.29 & 0.03 & 0.28 & 0.02 & 0.31 & 0.02 & 0.00 & * & \\  & Distal & 1.41 & 0.08 & 1.40 & 0.05 & 1.45 & 0.04 & 1.41 & 0.08 & 0.01 & * & 0.00 & * \\ \hline \multirow{3}{*}{Skewness} & Proximal & 0.84 & 0.04 & 0.85 & 0.05 & 0.84 & 0.06 & 0.83 & 0.05 & 0.51 & & \\  & Distal & 0.23 & 0.15 & 0.23 & 0.13 & 0.22 & 0.14 & 0.22 & 0.13 & 0.95 & 0.00 & * \\ \hline \multirow{3}{*}{Kurtosis} & Proximal & 2.33 & 0.19 & 2.36 & 0.22 & 2.39 & 0.26 & 2.38 & 0.16 & 0.69 & & \\  & Distal & 1.89 & 0.06 & 1.90 & 0.09 & 1.90 & 0.11 & 1.90 & 0.13 & 0.79 & 0.00 & * \\ \hline \multirow{3}{*}{\begin{tabular}{c} Skewness freq. \\ domain \\ \end{tabular} } & Proximal & 2.47 & 0.00 & 2.47 & 0.00 & 2.47 & 0.00 & 2.47 & 0.00 & 0.92 & & \\  & Distal & 2.47 & 0.00 & 2.47 & 0.00 & 2.47 & 0.00 & 2.47 & 0.00 & 0.11 & & \\ \hline \multirow{3}{*}{\begin{tabular}{c} Kurtosis freq. \\ domain \\ \end{tabular} } & Proximal & 7.12 & 0.00 & 7.12 & 0.00 & 7.12 & 0.00 & 7.12 & 0.00 & 0.92 & & \\  & Distal & 7.12 & 0.00 & 7.12 & 0.00 & 7.12 & 0.00 & 0.11 & & 0.51 & \\ \hline \multirow{3}{*}{Entropy} & Proximal & 2.09 & 0.09 & 2.08 & 0.07 & 2.09 & 0.08 & 2.11 & 0.11 & 0.98 & & \\  & Distal & 2.24 & 0.03 & 2.24 & 0.03 & 2.24 & 0.03 & 2.24 & 0.04 & 0.96 & & 0.00 & * \\ \hline \multirow{3}{*}{\
\begin{tabular}{c} Zero crossing rate \\ \end{tabular} } & Proximal & 0.0010 & 0.0000 & 0.0010 & 0.0000 & 0.0010 & 0.0002 & 0.0010 & 0.0000 & 0.00 & * & \\  & Distal & 0.0010 & 0.0002 & 0.0010 & 0.0002 & 0.0010 & 0.0002 & 0.0010 & 0.0000 & 0.08 & 0.00 & * \\ \hline \multirow{3}{*}{
\begin{tabular}{c} Signal-to-noise ratio \\ \end{tabular} } & Proximal & 23.82 & 2.71 & 22.76 & 9.23 & 22.33 & 7.79 & 22.05 & 1.99 & 0.61 & & \\  & Distal & 24.22 & 4.37 & 24.86 & 6.92 & 27.56 & 7.59 & 28.12 & 8.76 & 0.12 & & 0.00 & * \\ \hline \multirow{3}{*}{Relative power} & Proximal & 0.76 & 0.02 & 0.76 & 0.02 & 0.76 & 0.02 & 0.75 & 0.02 & 0.97 & & 0.01 & * \\  & Distal & 0.74 & 0.02 & 0.74 & 0.03 & 0.75 & 0.03 & 0.74 & 0.03 & 0.97 & & \\ \hline \hline \end{tabular}
* * Significant difference defined by a _p_-value < 0.05.  
\end{table} 
Table 3: Signal quality indices comparison of the detectors and wavelengths. (PSD) in this band compared to the PSD in the overall signal 0-8 Hz, provides a measure of the signal quality (Expected value > 0.85) [28]. This index was around 0.75 for all the signals included in the analysis, which is good enough, though it is not sufficient to do signal comparison. In addition to the analysis of the pulsatile signals from the proximal and distal detectors, the acquisition of the DC components from all three photodiodes is needed for the sensor's applications. For instance, Figure 9 shows the DC level recorded from all photodetectors at 810 nm. Further evaluation, changing the phantom's fluid oxygen level, will assess the response of the DC signals for the subsequent calculation of oxygenation parameters. The ZenTBI system fulfilled the design requirements for acquiring PPG and NIRS signals, which were methodically evaluated in an in vitro setup. This study showed the morphology of the pulsations and calculated some quality indices to evaluate the signals. Even though the signals' quality depends on the subject (in vitro/in vivo), the PPG signals acquired from the phantom using artificial blood successfully led to the extraction of quality measurements. Still, there is always room for improvement in the acquisition system design.