# Validation of an RF Image System for Real-Time Tracking Neurosurgical Tools
## 2 Materials and Methods
### System Hardware  
Figure 1 (left) shows the microwave imaging system. Briefly, it is made up of 16 wide-band twin antennas located at equally-spaced points throughout a circumference with a diameter 34 cm--i.e., one antenna every 22.5\({}^{\circ}\)--around the area where the object under study is located, in this case a cranium 3-D model. This configuration leaves a space of 20 cm to place the model in the center of the circumference. Each antenna is held by a 3-D-printed holding piece, and the 16 antenna-holder pairs are screwed onto the top face of a wood board. All the antennas are connected by means of a high-frequency switching network to Port 1 (the port under use) of a Vector Network Analyzer (VNA) to perform all measurements (Rhode and Schwarz ZNLE6). An Arduino Due microcontroller is used to control the electronic switches so that only one antenna is active at the same time. Both the switches and the control system are located in the bottom face of the wood board, at a sufficiently far position under the antennas in order to avoid interferences in the measurement process. The whole system is controlled with a computer which executes Python script that automates the entire process. This script is in charge of the communication with both the VNA and the microcontroller. It therefore controls which antenna is connected to the network analyzer at each time, as well as the measurement files transfer between the VNA and the computer. It is also in charge of loading the measurement data into a MATLAB script that performs the signal processing and generates the images for the surgical navigation. The different parts of the system will be further described in the next paragraphs. The wideband antennas constitute one of the key parts of the measurement system. The antennas are the front part of the microwave imaging system since they are responsible for signal transmission and reception, i.e., for data acquisition. In this case, they must have wideband behavior to be able to transmit the frequency information of the narrow time-domain pulses (which are similar to radar pulses), which are the basis of the imaging system. In this sense, the so-called Vivaldi antennas are one of the most widely used options in microwave imaging systems, for they offer a very large bandwidth and are slightly directive, concentrating the radiation in their aperture pattern (Kal  
all consecutively, one after another. The designed microwave imaging system uses a total of 16 antennas, which means that five switches are needed for a proper connection of the antennas to the VNA. In the selected configuration, one of the switches acts as the central one, which selects one of the remaining four switches, being each one of them connected to four antennas. In particular, we used the ZSWAA-63 DR+ switch from Mini-Circuits. It is a high-speed, low-losses switch, based on CMOS technology, which is internally adapted to 50 \(\Omega\) in the 1 MHz to 6 GHz frequency range, making it perfectly suitable for the frequency and switching speed requirements of our imaging system. The selection of the active output is made with three control voltages, which must follow the truth table provided by the manufacturer in the switch's datasheet, always avoiding not supported combinations. The control and DC power supply subsystem manages the power supply and control voltages for the switches. All the system is powered with the voltage supplied by the computer's USB port. The control signals for the switches are provided by an AT91 SAM3X8E microprocessor, embedded in the Arduino Due platform. DB9 connectors were used to connect the supply voltage and the control voltages of the switches with a four-core twisted shielded cable to reduce noise and interference. Control and power supply circuits were designed in order to make possible the connection between the microcontroller and the switch connectors, as well as a series of LED diodes were included to indicate the selected antenna at each moment. With this setup, the emitted power by the antennas is less than 1 mW.