An adaptive current tomography using voltage sources

Structured Design Methodology to Achieve a High SNR Electrical Impedance Tomography

In this paper, we present a methodology for designing the main circuit building blocks of an electrical impedance tomography (EIT) system. In particular, we derive equations that map system-level EIT specifications to the performance requirements of each circuit block. We also review the circuit architectures that are best suited for meeting a given set of performance requirements. Our proposed design methodology is focused on maximizing the EIT system's signal-to-noise ratio while minimizing total power consumption.

Video rate electrical impedance tomography of vascular changes: preclinical development

Peripheral vasculature disease is strongly correlated with cardiovascular-associated mortality. Monitoring circulation health, especially in the peripheral limbs, is vital to detecting clinically significant disease at a stage when it can still be addressed through medical intervention. Electrical impedance tomography (EIT) maps the electrical properties of tissues within the body and has been used to image dynamically varying physiology, including blood flow. Here, we suggest that peripheral vasculature health can be monitored with EIT by imaging the hemodynamics of peripheral vessels and the surrounding tissues during reactive hyperemia testing. An analysis based on distinguishability theory is presented that indicates that an EIT system capable of making measurements with a precision of 50 microV may be able to detect small changes in vessel size associated with variations in blood flow. An EIT system with these precision capabilities is presented that is able to collect data at frame rates exceeding 30 fps over a broad frequency range up to 10 MHz. The system's high speed imaging performance is verified through high contrast phantom experiments and through physiological imaging of induced ischemia with a human forearm. Region of interest analysis of the induced ischemia images shows a marked decrease in conductivity over time, changing at a rate of approximately -3 x 10(-7) S m(-1) s(-1), which is the same order of magnitude as reported in the literature. The distinguishability analysis suggests that a system such as the one developed here may provide a means to characterize the hemodynamics associated with blood flow through the peripheral vasculature.

A high-precision voltage source for EIT

Electrical impedance tomography (EIT) utilizes electrodes placed on the surface of a body to determine the complex conductivity distribution within the body. EIT can be performed by applying currents through the electrodes and measuring the electrode voltages or by applying electrode voltages and measuring the currents. Techniques have also been developed for applying the desired currents using voltage sources. This paper describes a voltage source for use in applied-voltage EIT that includes the capability of measuring both the applied voltage and applied current. A calibration circuit and calibration algorithm are described which enables all voltage sources in an EIT system to be calibrated to a common standard. The calibration minimizes the impact of stray shunt impedance, passive component variability and active component non-ideality. Simulation data obtained using PSpice are used to demonstrate the effectiveness of the circuits and calibration algorithm.

Propagation of measurement noise through backprojection reconstruction in electrical impedance tomography

A framework to analyze the propagation of measurement noise through backprojection reconstruction algorithms in electrical impedance tomography (EIT) is presented. Two measurement noise sources were considered: noise in the current drivers and in the voltage detectors. The influence of the acquisition system architecture (serial/semi-parallel) is also discussed. Three variants of backprojection reconstruction are studied: basic (unweighted), weighted and exponential backprojection. The results of error propagation theory have been compared with those obtained from simulated and experimental data. This comparison shows that the approach provides a good estimate of the reconstruction error variance. It is argued that the reconstruction error in EIT images obtained via backprojection can be approximately modeled as a spatially nonstationary Gaussian distribution. This methodology allows us to develop a spatial characterization of the reconstruction error in EIT images.

Comparison of the electrical and dielectric behavior of wet human cortical and cancellous bone tissue from the distal tibia

We compared the electrical and dielectric properties of wet human cortical and cancellous bone tissue from the distal tibia. The relationships between the electrical and dielectric behavior of cortical and cancellous bone and variables of measurement such as frequency and direction (orientation) were evaluated. In general, the resistivity of human cortical bone was found to be approximately 3.1 times more than that of cancellous bone in the longitudinal direction and least 25 times more in the transverse directions. The dielectric permittivity of human cortical bone was approximately 3.9 times that of cancellous bone in the longitudinal direction, whereas in the transverse directions it was about 0.65 times greater. Measurement variables such as frequency and orientation appeared to affect cancellous and cortical bone differently. Strong correlations were found between the electrical properties in one direction and the electrical properties in the other two directions; however, these relationships differed for the samples of cortical and cancellous bone.

Development of a real-time adaptive current tomograph

Following the successful development of a multiple-drive electrical impedance tomography system OXPACT-II featuring a voltage-driven current method for in vitro studies, research work currently being undertaken at the EIT research group in Oxford is aimed at developing a real-time multiple-drive adaptive system, called the Oxford Brookes Adaptive Current Tomograph Mark-III (OXBACT-III) which will operate at several frequencies in between 10-160 kHz. The objective of this system development is to enable EIT clinical studies to be undertaken based on the adaptive current method. One of the most important issues addressed in the new system design is to achieve high data acquisition speed while maintaining sufficient system accuracy. This paper will describe the overall data acquisition system structure and relevant system performance specifications.