Using Walsh functions in electrical impedance tomography

The authors propose a measurement method using Walsh functions in 32-electrode electrical impedance tomography. By injecting currents in spatial Walsh function patterns, one can exploit the simplicity of pulse type injection and obtain the optimal signal-to-noise ratio or distinguishability of sinusoidal injection. Once boundary voltage data for the injection of all 31 patterns of Walsh functions are measured, all 31 optimal current patterns can be computed. One can also synthesize the voltage data due to the injection of the optimal current patterns without actually injecting the optimal current patterns. It is pointed out that the use of Walsh functions simplifies the design of current sources since only two levels of current (+1 and -1) are required whereas sinusoidal injection requires a DAC to produce many different values of currents. Compared to diagonal or neighboring types of pulses as injection current patterns, Walsh function injection provides much better information about the interior of the subject since Walsh functions simulate low as well as high spatial frequency patterns. Walsh functions also have the advantage of computational simplicity since they take the value of only +1, -1, or zero.