FPGA-Based Voltage and Current Dual Drive System for High Frame Rate Electrical Impedance Tomography

Abstract

Electrical impedance tomography (EIT) is used to image the electrical property distribution of a tissue under test. An EIT system comprises complex hardware and software modules, which are typically designed for a specific application. Upgrading these modules is a time-consuming process, and requires rigorous testing to ensure proper functioning of new modules with the existing ones. To this end, we developed a modular and reconfigurable data acquisition (DAQ) system using National Instruments' (NI) hardware and software modules, which offer inherent compatibility over generations of hardware and software revisions. The system can be configured to use up to 32-channels. This EIT system can be used to interchangeably apply current or voltage signal, and measure the tissue response in a semi-parallel fashion. A novel signal averaging algorithm, and 512-point fast Fourier transform (FFT) computation block was implemented on the FPGA. FFT output bins were classified as signal or noise. Signal bins constitute a tissue's response to a pure or mixed tone signal. Signal bins' data can be used for traditional applications, as well as synchronous frequency-difference imaging. Noise bins were used to compute noise power on the FPGA. Noise power represents a metric of signal quality, and can be used to ensure proper tissue-electrode contact. Allocation of these computationally expensive tasks to the FPGA reduced the required bandwidth between PC, and the FPGA for high frame rate EIT. In 16-channel configuration, with a signal-averaging factor of 8, the DAQ frame rate at 100 kHz exceeded 110 frames s (-1), and signal-to-noise ratio exceeded 90 dB across the spectrum. Reciprocity error was found to be for frequencies up to 1 MHz. Static imaging experiments were performed on a high-conductivity inclusion placed in a saline filled tank; the inclusion was clearly localized in the reconstructions obtained for both absolute current and voltage mode data.

A Versatile and Reproducible Multi-Frequency Electrical Impedance Tomography System

“…As has been observed in existing EIT systems [36], this apparent decrease is as a result of low frequency drifts, likely from temperature changes in the current source, rather than degradation of the performance of the system, Figure 4. Overall, the SNR was lower than figures reported for other systems, which are capable of measuring with SNR above 90 dB [12,14]. The amplitude of the voltages recorded was 2.66 mV, orders of magnitude less than the input range of the EEG amplifier used (≈250 mV), and did not take full advantage of the high dynamic range offered by these amplifiers.…”

“…The versatility of the system as a whole is largely as a result of this wide operating range of the Keithley 6221. This range covers the majority of EIT applications, with the exception of those operating at high frequencies >100 kHz, such as breast imaging [14]. …”

“…Existing EIT systems include the KHU [12], fEITER [13], Dartmouth EIT System [14], Xian EIT system [15], Swisstom Pioneer Set (Swisstom AG, Switzerland) and systems previously developed in our laboratory at UCL [16,17]. Typically these systems have at least 32 electrodes with voltages recorded in parallel to increase frame rate, Table 1.…”

Prostate Cancer Detection Using Composite Impedance Metric

“…The electrical impedance acquisition system [22] used in this study is capable of applying current or voltage signals at frequencies ranging from 100 Hz to 10 MHz; however, the parasitic impedances associated with the multiplexer and the passive probe cable (~0.5m long), were the limiting factors in selecting the frequency range for this study (1 kHz – 1 MHz). Measured signals can also be averaged at an averaging factor of 2 to 16384.…”

“…To achieve the ultimate goal of developing an intraoperative surgical margin assessment system for use during RALP, this study reports the results from an ex vivo human prostate study performed using an electrical impedance acquisition system [22], with a microendoscopic impedance probe [23]. The microendoscopic probe measures 11 mm in diameter making it suitable for introduction into the surgical cavity through one of the abdominal ports used by the robot to introduce surgical instruments into the operating site.…”