Measurement of high frequency electrical transfer impedances from biological tissues

Introduction: Electrical impedance spectroscopy (EIS) is a non-invasive method for characterising human tissue. Because currents will pass either around or through cells depending on the frequency, EIS can be used to observe the structure and arrangement of cells. EIS has been used to identify precancerous changes in the oesophagus and cervix [1]. Many of the precancerous changes in tissue concern the cell nucleus where frequencies of > 1MHz are needed to gain information. However, methods for measuring impedance in which wired connections to electrodes on the tissue surface are used do not perform well at high frequencies. Parasitic capacitances between wires and between current injection (drive) and voltage measurement (receive) circuitry can produce large errors [2]. Common-mode currents between drive and receive circuits also affect accuracy. The optical isolation of both circuits reduces such problems but requires synchronisation for complex transfer impedance measurement. To achieve synchronisation we suggest two novel methods. The first uses optically isolated sinusoidal excitation using a phase locked loop (PLL) for synchronisation, direct-digital synthesis (DDS) for sinewave generation and a digital signal processor (DSP) for analysis. The second uses pulsatile current injection (impulse system) and obtains spectral information using a Fourier transform.