title: Multi-frequency electrical impedance tomography system with automatic self-calibration for long-term monitoring.
abstract: Electrical Impedance Tomography (EIT) is a safe medical imaging technology, requiring no ionizing or heating radiation, as opposed to most other imaging modalities. This has led to a clinical interest in its use for long-term monitoring, possibly at the bedside, for ventilation monitoring, bleeding detection, gastric emptying and epilepsy foci diagnosis. These long-term applications demand auto-calibration and high stability over long time periods. To address this need we have developed a new multi-frequency EIT system called the KHU Mark2.5 with automatic self-calibration and cooperation with other devices via a timing signal for synchronization with other medical instruments. The impedance measurement module (IMM) for flexible configuration as a key component includes an independent constant current source, an independent differential voltmeter, and a current source calibrator, which allows automatic self-calibration of the current source within each IMM. We installed a resistor phantom inside the KHU Mark2.5 EIT system for intra-channel and inter-channel calibrations of all voltmeters in multiple IMMs. We show the deterioration of performance of an EIT system over time and the improvement due to automatic self-calibration. The system is able to maintain SNR of 80 dB for frequencies up to 250 kHz and below 0.5% reciprocity error over continuous operation for 24 hours. Automatic calibration at least every 3 days is shown to maintain SNR above 75 dB and reciprocity error below 0.7% over 7 days at 1 kHz. A clear degradation in performance results with increasing time between automatic calibrations allowing the tailoring of calibration to suit the performance requirements of each application.
authors.forename: Hun
authors.initials: H
authors.lastname: Wi
authors.forename: Harsh
authors.initials: H
authors.lastname: Sohal
authors.forename: Alistair Lee
authors.initials: AL
authors.lastname: McEwan
authors.forename: Eung Je
authors.initials: EJ
authors.lastname: Woo
authors.forename: Tong In
authors.initials: TI
authors.lastname: Oh
mesh_terms: D002138:Calibration; D017097:Electric Impedance; D055615:Electrical Equipment and Supplies; D004867:Equipment Design; D007091:Image Processing, Computer-Assisted / Q000295:instrumentation / Q000379:methods / Q000592:standards; D019047:Phantoms, Imaging; D059629:Signal-To-Noise Ratio; D014054:Tomography / Q000295:instrumentation / Q000379:methods / Q000592:standards