## 3 Result and Discussion
### Impedance Monitoring of the Needle Positioning  
The extrapolation of tissue parameters from the fitting analysis with measured impedance spectra, for example, R\({}_{\text{ex}}\), R\({}_{\text{in}}\), and C\({}_{\text{m}}\) may provide better discrimination for tissue types. This fitting analysis needs to measure the impedance at least three different frequencies and to implement the algorithm for extrapolating the circuit parameters. One limitation for this is that the needle should not be moved until the fitting analysis is completely finished. However, we focused on the real-time monitoring of the needle positioning, which requires fast signal analysis and processing, and monitored the impedance at a fixed frequency at a sampling rate of 19 S/s. With this high speed of impedance measurement, the dynamic needle position in different tissues layer could be detected in real-time. The target of the intra-articular injection needle is synovial fluid which has a similar conductivity to the saline (0.9% NaCl) solution [23]. Further, saline is commonly used as a joint irrigation solution. Therefore, we investigated the impedance value of tissue layers relative to the saline. The impedance spectra at three different positions in the specific tissue layer were measured and normalized with respect to the values of the saline (0.9% NaCl) solution in the frequency range of 200 Hz to 50 kHz by using Equation (2). \[Z_{\text{norm}}=\frac{Z_{\text{tissue}}}{Z_{\text{NaCl}}}. \tag{2}\] \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Tissue Type** & **R\({}_{\text{in}}\)(T)** & **R\({}_{\text{in}}\)(T)** & **C\({}_{\text{m}}\)(F)** & \multicolumn{3}{c}{**CPE\({}_{\text{el}}\)**} & \(\chi^{2}\)** \\ \cline{2-7}  & & & & **P** & **T(Ohm)\({}^{-1}\)s\({}^{\text{P}}\)** & \\ \hline Dermis & 7426 \(\pm\) 223.2 & 3608.3 \(\pm\) 766.3 & 4.81 \(\times\) 10\({}^{-10}\)\(\pm\) 1.1 \(\times\) 10\({}^{-10}\) & 0.49 \(\pm\) 0.09 & 3.02 \(\times\) 10\({}^{-06}\)\(\pm\) 1.3 \(\times\) 10\({}^{-06}\) & 0.001 \\ Hypodermis & 6786.6 \(\pm\) 706.5 & 2622.6 \(\pm\) 155.5 & 7.25 \(\times\) 10\({}^{-10}\)\(\pm\) 6.69 \(\times\) 10\({}^{-11}\) & 0.5 \(\pm\) 0.05 & 2.86 \(\times\) 10\({}^{-06}\)\(\pm\) 7.61 \(\times\) 10\({}^{-07}\) & 0.001 \\ Muscle & 2437.6 \(\pm\) 140.4 & 6750.6 \(\pm\) 837.5 & 4.96 \(\times\) 10\({}^{-10}\)\(\pm\) 5.14 \(\times\) 10\({}^{-11}\) & 0.76 \(\pm\) 0.011 & 7.7 \(\times\) 10\({}^{-06}\)\(\pm\) 3.86 \(\times\) 10\({}^{-08}\) & 0.0003 \\ \hline \hline \end{tabular} Each datum point represents the average of the analysis of triplicate values with standard deviation (n = 3).  
\end{table} 
Table 1: Extrapolated value of the circuit parameters from the fitting analysis against the experimentally measured impedance spectra shown in Figure 4.  
From the normalized data of Figure 5, it is observed that the real parts of the impedance of tissues were significantly discriminated from each other at the frequencies higher than 10 kHz. Considering the variation of the real parts of the impedance of the tissue layer decreased with the increase in frequency, we have selected a frequency of 50 kHz for the real-time monitoring of the needle position in the tissue. Figure 6a presents the Student's _t_-test (one-way ANOVA) to evaluate the difference between the real part of the impedance value between different tissue types (n = 3) at 50 kHz. The results show that the type of tissue compartment in the pork meat could be significantly discriminated from each other (p \(<\) 0.001) using the monopolar injection needle and DFT-based impedance measurement system. The deviation between impedance measurements in each layer of different pork tissues at 50 kHz is under 3.7%. While in Figure 6b, the monopolar injection needle was progressed into different tissue layers of the pork, and the real part of impedance at 50 kHz was recorded.