Imaging of Gastric Emptying with Electrical Impedance Tomography

Electrical Impedance Tomography uses surface electrical measurements to image changes in the conductivity distribution within a medium. This technique was applied to the measurement of gastric motility by imaging conductivity changes in the abdomen after ingestion of liquid. Preliminary results show good correlation with typical clinical measurements. composition, and caloric content have a large effect on the gastric motility. Experimental Procedure. Our EIT data acquisition system[1] uses 16 electrodes for both current injection and voltage measurement, and is able to read the induced voltage at all electrodes for one injection pattern in 40 ms. In this experiment, a current injection of 1 mA at 13 kHz was used (this is approximately 10% of the threshold for cutaneous perception.) In order to reject any conductivity changes due to cardiac activity, all data acquisition was gated at 100 ms after the QRS peak. Introduction Electrical Impedance Tomography (EIT) is an imaging technique which calculates the electrical conductivity distribution within a medium from electrical measurements made from a series of electrodes on the medium surface. A low amplitude alternating current is successively injected across pairs of electrodes while the voltage differences produced on the remaining electrodes are measured. Using this data, the conductivity distribution is estimated by the resolution of a non-linear system of equations. We are particularly interested in dynamic EIT (i.e. the measurement of changes in the conductivity distribution, rather than the conductivity itself) because it is better able to compensate for measurement errors, and is more sensitive to small conductivity changes. The electrodes were evenly spaced around the abdomen of the subject, at the level of the stomach. The experiment was begun three hours after the subject had finished his meal. A reference set of measurements was acquired, and subsequently a conductivity change was induced by the subject drinking 355 ml of Coca-Cola. Over the next hour, a set of measurements was acquired every five minutes. Using this data the conductivity changes associated with his gastric activity were calculated. Image Reconstruction EIT promises to be of interest in studying physiological processes which modify the electrical conductivity of the body, resulting from the movement of liquids or gasses (e.g. cardiac, respiratory, and gastric activity.) In this paper we present results obtained from initial investigation into the measurement of gastric motility using EIT. Reconstruction of conductivity changes was accomplished using the modified Newton-Raphson algorithm[3]. From our measured data we construct a matrix of voltage ratios f, such that f v 1 i,j 16 ij ij k