Fig. 3

EIT measurement principle and global impedance method. a The 16-electrode ring arrangement is shown. Current (i) is injected between electrodes e1−e2 and the voltage measured on the remaining pair of electrodes (a, left). The injecting pair of electrodes is successively changed until all 16 pairs are covered (a, right). b The reference homogeneous measurement (vh: empty bladder), is subtracted from the bladder inhomogeneous measurement data (vi: BCmax), and mapped to impedance change (ΔZ) with a reconstruction matrix (R). A global impedance (GI) value is calculated for each EIT frame by summing every pixel value (ΔZ_i,j). To apply the global impedance method to the estimation of BCmax and RU, at least two measurement cycles are needed. A calibration curve is generated from the first measurement cycle assuming a linear regression. The volume is estimated using the data from the second measurement cycle