To improve the spatial resolution of ECT sensors in a concentric annulus measurement area, different number of combined electrodes ECT sensors are designed. For four types of concentric annulus area with inner and outer diameter ratios of 0. 2, 0. 3, 0. 4 and 0. 5, traditional 12 external electrodes (EE) and 12-4, 12-6, 12-8 internal-external electrodes (IEE) are considered. The IEE sensors are implemented with external-opposite-internal ( EOI) and external-opposite ( EO) measuring strategies. The capacitance between different electrode pairs is calculated for typical permittivity distributions by numerical simulation approach. The obtained capacitances are used to reconstruct images using LBP, Tikhonov, and Landweber algorithms, respectively. Based on numerical simulation and experimental investigation, the main results are given as follows. As the ratio of the inner and outer diameters of the concentric annulus area increases, there is an optimal number of inner electrodes which the quality of reconstructed images with the 12-4 electrodes structure are higher in the measurement. When LBP algorithm is used, the EOI excitation strategy has a slight advantage over the EO excitation strategy in imaging accuracy. When the Tikhonov algorithm and Landweber iterative algorithm are used, the EO excitation strategy can effectively reduce the influence of abnormal capacitance, and effectively improve the quality of reconstructed images when combined with IEE sensors in the measurement of concentric annulus areas with different inner and outer diameter ratios.