Multi-channel bioimpedance spectroscopy based on orthogonal baseband shifting

Abstract

Objective. Multi-channel bioimpedance spectroscopy (BIS) systems typically sample each channel’s impedance sequentially using multiplexers and a single analog-to-digital converter. These systems may lose their real-time capability with an increasing number of channels, especially for low excitation frequencies. We propose a new method, called orthogonal baseband shifting (OBS), for high-speed parallel BIS data acquisition at multiple excitation frequencies with low hardware and computational effort. Approach. Similar to orthogonal frequency-division multiplexing, used for digital data transmission, OBS systems use channel-specific orthogonal carrier frequencies to modulate the voltage response of the tissue. Given a suitable choice of carrier frequencies, the modulated signals of all channels sum up without loss of information and cross-talk. The fast Fourier transform (FFT) of the summed signal reveals a spectrum of non-overlapping, interleaved BIS data from which the corresponding BIS data of each channel can be calculated. Main results. In simulations, the system design requires a minimum signal-to-noise ratio of 30 dB to achieve amplitude errors below 1% and phase errors below 0.8°. The hardware realization, called ‘AixBIS’, has been evaluated for impedance measurements between 0.1 and 10 Ω with multi-frequency excitations between 45 and 180 kHz. The impedance values acquired had an averaged precision of 3.67 mΩ, which is only 0.65‰ in relation to the measured impedance. The phase had a mean precision of 0.46°. Moreover, in vitro measurements achieved 140 full spectrum acquisitions per second. The impedance change measured in a silicone heart phantom showed a high correlation of 0.83 with the ventricles volume change (flow). Significance. The proposed method enables very fast impedance acquisition of all channels. A complete measurement is performed in the time of a single FFT acquisition, which is equal to the resolution bandwidth of the FFT. In addition, portable and low-power multi-channel BIS devices profit from highly reduced hardware effort. The outstanding performance of OBS measurements with the AixBIS system have the potential for in vivo BIS measurements in real-time.

Advances of deep learning in electrical impedance tomography image reconstruction

“…Electrical impedance tomography (EIT) is a non-invasive imaging method for reconstructing the distribution or changes in electrical properties by applying a safe alternating current excitation, measuring the surface voltage signal, and using a reconstruction algorithm. With the advantages of being radiation-free and inexpensive, and allowing real-time imaging, it has been extensively utilized in geophysical imaging, multiphase flow monitoring, and biomedical imaging (Menden et al, 2021a;Menden et al, 2021b;Hsu et al, 2021;Jiang et al, 2021). In terms of medical OPEN ACCESS EDITED BY applications, EIT is regarded as a functional imaging method compared with traditional computed tomography (CT) and ultrasound, which reflect the pathophysiological information of the human body through impedance changes.…”

Advances in electrical impedance tomography and bioimpedance including applications in COVID-19 diagnosis and treatment

“…Relatively fewer advances have been made in performing true multi-frequency studies, principally because of difficulties and uncertainties inherent in system calibration using multiple frequencies and electrodes. The work presented by Menden et al (Menden et al, 2021b) presents a method for performing multiple frequency measurements using orthogonal baseband shifting (OBS). OBS may prove a viable path to obtaining spectroscopic EIT information using an efficient hardware configuration.…”