Multi-channel bioimpedance spectroscopy based on orthogonal baseband shifting

Area of Science:

• Electrical Engineering
• Biomedical Instrumentation
• Signal Processing

Background:

• Sequential sampling in multi-channel bioimpedance spectroscopy (BIS) limits real-time capabilities, especially at low frequencies.
• Existing BIS systems often rely on multiplexers and single analog-to-digital converters, creating bottlenecks for high-speed data acquisition.

Purpose of the Study:

• To introduce a new method, orthogonal baseband shifting (OBS), for high-speed, parallel BIS data acquisition.
• To enable multi-frequency BIS measurements with reduced hardware and computational requirements.

Main Methods:

• Orthogonal baseband shifting (OBS) utilizes channel-specific orthogonal carrier frequencies to modulate tissue voltage responses.
• Modulated signals from all channels are summed, and a fast Fourier transform (FFT) separates the interleaved BIS data.
• The 'AixBIS' hardware realization was developed and evaluated for impedance measurements.

Main Results:

• Simulations indicate a minimum 30 dB signal-to-noise ratio is required for <1% amplitude and <0.8° phase errors.
• The AixBIS system achieved averaged impedance precision of 3.67 mΩ (0.65‰) and phase precision of 0.46° for 0.1-10 Ω measurements.
• In vitro measurements demonstrated 140 full spectrum acquisitions per second, with high correlation (0.83) between phantom impedance changes and volume flow.

Conclusions:

• The OBS method allows for extremely fast impedance acquisition, completing measurements within a single FFT acquisition time.
• Reduced hardware complexity makes OBS suitable for portable, low-power multi-channel BIS devices.
• The AixBIS system's performance suggests significant potential for real-time in vivo BIS measurements.