Impulsive Noise Removal in Multi-frequency Phase Sensitive Demodulation for Electrical Impedance Tomography

Multi-frequency phase sensitive demodulation plays an important role in electrical impedance tomography (EIT). The phase sensitive detector is an effective tool for discriminating the amplitude and phase of each frequency component signal. It can be considered as the optimal linear filter in case of Gaussian-type noise. In real applications, however, the noise has also rather distinctive impulsive characteristics. It is a challenging task to remove the impulsive noise effectively in multi-frequency phase sensitive demodulation. In this paper, an approach for impulsive noise removal is presented in multi-frequency phase sensitive demodulation, which takes advantage of the periodicity of a multi-frequency signal and the significant properties of the median filter. The current work focuses on the use of two-dimensional (2D) median filter to handle the impulsive noise that existed in the measured signal, i.e. a 2D median filter is employed as the pre-stage signal conditioning module prior to the phase sensitive detector. The main advantages of the proposed method are that: 1) increasing the number of samples within the median filter window, which benefits the consecutive impulsive noise removal, and 2) no additional storage and computational resources are required for hardware system. Two classical methods are selected as comparisons in experiment. Numerical simulations and experimental results demonstrate that the proposed method can effectively eliminate the influence of impulsive noise on multi-frequency phase sensitive demodulation, and is able to achieve outstanding signal-to-noise ratio (SNR) performance.