Tong-In Oh

Performance evaluation of KHU Mark2 parallel multi-frequency EIT system

We describe a new parallel multi-frequency EIT system, KHU Mark2. It is based on the impedance measurement module (IMM), which comprises a single-ended constant current source and a voltmeter. Each IMM has an FPGA for its independent operations including current injection at multiple frequencies, voltage amplification, ADC, digital phase-sensitive demodulation and intra-networking with a main controller of the system. The main controller is based on a DSP and an isolated USB for its connection to a PC. There is an FPGA-based intranet controller, which arbitrates data exchanges between the DSP and multiple IMMs. Unlike its precursor, KHU Mark1, it is a true parallel system with no switching for both current injection and voltage sensing. The small size of the IMM results in a much reduced dimension of a multi-channel system. The KHU Mark2 can be assembled in any channels between 1 and 64. Depending on a chosen application, we custom design an analog backplane that interfaces multiple IMMs with electrodes. Special care was given to the system calibration to maximize its performance in frequency-difference EIT imaging as well as time-difference. Flexibility is the key improvement factor compared with the KHU Mark1. The new system can accommodate any current injection and voltage sensing protocol including the optimal injection current pattern. Reduced size and new internal architecture significantly improved mechanical as well as electrical stability of the system.

Electric Field Mapping in ex vivo Anisotropic Muscle Tissue Using DT-MREIT

Accurate coverage of tissue with a sufficiently large electric field is one of the key conditions for successful electroporation. Magnetic resonance electrical impedance tomography (MREIT) provides a means to map the electric filed distribution during electroporation. To estimate the electric field strength, the magnetic flux density data induced by the electroporation pulses are measured from MREIT scans during electroporation. Since biological tissues such as skeletal muscle are anisotropic, we propose a novel MREIT technique to map the electric field in anisotropic as well as isotropic regions. We utilize the anisotropic conductivity estimation method based on the lately developed DT-MREIT technique where diffusion tensor imaging is combined with MREIT. To estimate the current density in an optimal way, we adopted the projected current density estimation algorithm. From ex vivo experiments using bovine muscle tissues, we found that the new method produces electric field maps with a wider coverage of electroporation than the previous method. The results suggest that it is important to properly handle the effects of the tissue anisotropy for more accurate mapping of electric field during electroporation.

Microelectrode array analysis of hippocampal network dynamics following theta-burst stimulation via current source density reconstruction by Gaussian interpolation.

BACKGROUNDnMultielectrode arrays (MEAs) have been used to understand electrophysiological network dynamics by recording real-time activity in groups of cells. The extent to which the collection of such data enables hypothesis testing on the level of circuits and networks depends largely on the sophistication of the analyses applied.nnnNEW METHODnWe studied the systemic temporal variations of endogenous signaling within an organotypic hippocampal network following theta-burst stimulation (TBS) to the Schaffer collateral-commissural pathways. The recovered current source density (CSD) information from the raw grid of extracellular potentials by using a Gaussian interpolation method increases spatial resolution and avoids border artifacts by numerical differentials.nnnRESULTSnWe compared total sink and source currents in DG, CA3, and CA1; calculated accumulated correlation coefficients to compare pre- with post-stimulation CSD dynamics in each region; and reconstructed functional connectivity maps for regional cross-correlations with respect to temporal CSD variations. The functional connectivity maps for potential correlations pre- and post-TBS were compared to investigate the neural network as a whole, revealing differences post-TBS.nnnCOMPARISON WITH EXISTING METHOD(S)nPrevious MEA work on plasticity in hippocampal evoked potentials has focused on synchronicity across the hippocampus within isolated subregions. Such analyses ignore the complex relationships among diverse components of the hippocampal circuitry, thus failing to capture network-level behaviors integral to understanding hippocampal function.nnnCONCLUSIONSnThe proposed method of recovering current source density to examine whole-hippocampal function is sensitive to experimental manipulation and is worth further examination in the context of network-level analyses of neural signaling.

Gated Conductivity Imaging using KHU Mark2 EIT System with Nano-web Fabric Electrode Interface

Abstract: Electrical impedance tomography(EIT) can produce functional images with conductivity distributions asso-ciated with physiological events such as cardiac and respiratory cycles. EIT has been proposed as a clinical imagingtool for the detection of stroke and breast cancer, pulmonary function monitoring, cardiac imaging and other clinicalapplications. However EIT still suffers from technical challenges such as the electrode interface, hardware limita-tions, lack of animal or human trials, and interpretation of conductivity variations in reconstructed images. Weimproved the KHU Mark2 EIT system by introducing an EIT electrode interface consisting of nano-web fabric elec-trodes and by adding a synchronized biosignal measurement system for gated conductivity imaging. ECG and res-piration signals are collected to analyze the relationship between the changes in conductivity images and cardiacactivity or respiration. The biosignal measurement system provides a trigger to the EIT system to commence imag-ing and the EIT system produces an output trigger. This EIT acquisition time trigger signal will also allow us to oper-ate the EIT system synchronously with other clinical devices. This type of biosignal gated conductivity imagingenables capture of fast cardiac events and may also improve images and the signal-to-noise ratio (SNR) by using signalaveraging methods at the same point in cardiac or respiration cycles. As an example we monitored the beat by beatcardiac-related change of conductivity in the EIT images obtained at a common state over multiple respiration cycles.We showed that the gated conductivity imaging method reveals cardiac perfusion changes in the heart region of theEIT images on a canine animal model. These changes appear to have the expected timing relationship to the ECGand ventilator settings that were used to control respiration. As EIT is radiation free and displays high timing res-olution its ability to reveal perfusion changes may be of use in intensive care units for continuous monitoring of car-diopulmonary function.Key words: Electrical Impedance Tomography(EIT), Gated Conductivity Imaging, ECG, Respiration, EIT ElectrodeInterface