# Validation of an RF Image System for Real-Time Tracking Neurosurgical Tools
## 3 Experimental Validation and Results  
### Experimental Setup  
Once the system was implemented and calibrated, its performance as a cranial surgery navigation tool was assessed. The targeted application consists of detecting the position of the surgical tool within the cranial area, so that the surgical team is provided with proper guidance and assistance. In order to emulate such a scenario, a setup imitating an operating room for cranial surgical interventions with robotic tools was prepared, as shown in Figure 8. As it can be seen, a structure was designed to hold the antenna system, allowing to house the 3-D-printed cranium in the exact center. Ad hoc connection of the antennas to the VNA and the computer was installed, as explained in the prior sections, to allow the proper running of the microwave image system. Finally, a UR5 robotic arm was used to emulate the surgeon's moves. The robotic arm was configured to hold the clinical tool, which is intended to navigate towards the critical area within the cranium.  
For the experimental validation, an accurate and detailed 3-D-printed model for the cranium was used (horizontal section dimensions 128 \(\times\) 170 mm\({}^{2}\)), which had a hole on the left side of the forehead to enable the entrance of the surgical tool, thereby allowing us to simulate intracranial surgery. A piece of plastic filled with water was used to imitate a tumor, which was placed inside the cranium, in the inner center forehead area (Figure 9). The cranium was placed in the center of the antenna system, so that the cranium's center coincided with the center of the antenna coordinate system, as can be seen in the picture. With this configuration, the tumor was slightly displaced from the center of the coordinate system, with the tumor's center position at roughly (0, 26) mm coordinates considering the framework in Figure 9. The tumor had 25 \(\times\) 25 \(\times\) 25 mm\({}^{3}\) dimensions.  
Figure 6: Calibration measurement with a metallic cylinder.  
Figure 7: Obtained images for the measurement with the metallic cylinder: (**a**) DAS; (**b**) DMAS.  
The navigation sequence for the experimental validation consisted of the surgical tool being approached to the tumor, throughout the cranium's hole, following a spatially diagonal straight line (also including the approach in the \(z\) coordinate, which is not considered in the 2-D images provided by the microwave image system) at the same time that the microwave image system was tracking the tool position. The tool path started at a position sufficiently far from the tumor (more than 150 mm away), and it was planned to finish as close as possible to the tumor's center; thus resembling the trajectory followed during an actual intervention. Although the tool may be handled by the surgeon in real operations, the robotic arm was used to hold it during this experiment so that the trajectory and navigation sequence could be accurately controlled, and reference coordinates throughout the trajectory could be obtained in a reliable manner. A picture of the tool entering the cranium and the antenna system can be seen in Figure 10.  
Figure 8: Experimental setup.  
Figure 9: Schematic representation of the cranium (yellow), the hole (black), the tumor (pink), the antennas (green), and the coordinate system.