Measurement of high frequency electricaltransfer impedances from biological tissues

Abstract

Two novel methods are compared for achieving the isolation required to perform high frequency transfer impedance measurements on biological tissue. The first uses sinusoids and the second pulsatile current injection. Sinusoidal current injection offers the higher accuracy but both give similar performance when used to model tissue in terms of a Cole-Cole equation.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.

Aspects of Using Chirp Excitation for Estimation of Bioimpedance Spectrum

“…The respective Bode diagram has two real poles f p1 , f p2 , and two zeros f z1 , f z2 (Fig. 2b), spread typically over the frequency range from some kHz up to several MHz (Nebuya et al 1999;Pliquett et al, 2000;Grimnes & Martinsen, 2008).…”

Noise Cancellation Using Adaptive Filter for Bioimpedance Signal

“…To obtain complete information, measurement of the whole frequency response of the complex impedance[ Fig. 1(c)] is necessary [2]. Fig.…”

Estimation of the size of air emboli detectable by electrical impedance measurement

“…The resistance between the electrode and the skin is 134fL and that of the tissue existing between the electrodes is taken to be 25f~ (NEBUYA et al, 1999). Then we obtain R=25+(134x2)=293fL if we measure the impedance at a rate of 1 ms to detect bubbles moving fast, the bandwidth Author's biography SATORU NEBUYA was born in Tokyo in 1964.…”

“…However, the change in impedance caused by a bubble is expected to be very small and of short duration, so that in viva detection will be difficult. Accordingly, an impedance measurement system with high accuracy and speed has been developed (NEBUYA et al, 1999) and tested in vitro. The current paper shows that the equipment developed can detect bubbles in a saline tank, and the smallest detectable size of an air embolus is estimated.…”