Multifrequency time difference EIT imaging of cardiac activities

It has been proven that EIT can distinguish respiration and cardiac activities. However, the quality of EIT images of cardiac activities is poor because the induced/sensed impedance signal from the heart is much weaker than the ventilation signal. For EIT imaging of cardiac activities, induced voltages should differ at different excitation frequencies, owing to the cardiac capacitive reactance. At the same time, there are only few existing EIT systems designed for cardiac imaging. Based on these reasons, we present herein a preliminary study on the frequency response of cardiac activities. Three aspects have been improved in the designed system: i) The designed dual-band belt is shown to maintain good body contact for more than four hours of testing, ii) the recovery time of channel switching was quantified, and the signal recovery time is estimated to be approximately 23 us in the final system, and iii) the electrode belt and the load value were taken into consideration for system correction. The corrected system has an SNR greater than 80 dB for excitation frequencies up to 1 MHz. Five healthy male volunteers were tested in the siting position, and the results showed that the frequency response amplitudes reach maximum values when the excitation frequency is 50 kHz. Therefore, it is possible to conduct a preliminary qualitative analysis of cardiac function using impedance spectroscopy. At the same time, additional details on the impedance of cardiac activities can be obtained using a lower excitation frequency.