微小脳波測定に関する基礎的検討

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Electrocerebral inactivity in the diagnosis of brain death is usually determined by finding no electroencephalogram (EEG) with an amplitude greater than noise level inherent in an amplifier when it is recorded at a higher gain. A surface biopotential electrode contains two interfaces composed of skin/electrolyte (jelly) and electrolyte/metal (electrode), each forming a noise source. In electroencephalography we should consider these noises generated in the electrodes in addition to noises generated in the amplifier itself. In this paper, the effects of this electrode noise on low-amplitude EEG measurement were examined analytically. After A/D conversion (12 bits, 2 kHz) of noise signals from an extremely low noise amplifier, its power spectral density S(f) was obtained by means of FFT and auto-correlation. All noise signal measurement was done in the electromagnetic shielding chamber in order to minimize external interference, Inter-electrode resistance R(f) was measured with a synchronous rectification circuit. The formula of the equivalent noise resistance Rn=S(f)/4kT (where, k : Boltzmann constant, T : room temperature in Kelvin), gives us the amount of ohms of resistance to generate the thermal noise corresponding to the measured S(f). Rn/R is a parameter derived from normalization by R. When Rn/R=1, measured noise signal contains the thermal noise only. And Rn/R >1 indicates existence of excess noise in addition to the thermal noise. S(f) measured with a pair of electrodes placed face to face via jelly was almost equal to that of the amplifier noise. Mean Rn/R of the skin (scalp)/metal interface noise was 104at 10Hz, showing existence of such excess noises as 1/f or tissue noises. The results suggest that it is necessary to take these excess noises into consideration in the measurement of low-amplitude EEG.
東海大学の論文
2002-03-30

微小脳波測定に関する基礎的検討

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