Toomas Parve

Lock-in measurement of bio-impedance variations

Abstract Electrical impedance measured between suitably placed electrodes can give a reasonable amount of information about the basic physiological parameters of the patient’s body and its organs and vital systems. High noise immunity of the lock-in demodulation technology allows one to obtain, from the bio-impedance variations, the signals corresponding to breathing and heart beating, even using the handles of the veloergometer or other similar equipment as the electrodes. From these signals it is possible to get not only simple physiological parameters like the breathing rate and heart rate, but also to derive more complex parameters like the tidal volume and minute volume of respiration, and stroke volume and cardiac output of the heart. The application specific integrated circuit (ASIC) has been designed for electrical bio-impedance measurements in portable and implantable equipment.

Improvement of Lock-in Electrical Bio-Impedance Analyzer for Implantable Medical Devices

A novel solution for improving lock-in electrical bio-impedance (EBI) analyzers by suppressing errors caused by higher odd harmonics of rectangular wave pulses is proposed. The solution is based on a newly developed method of shortening the rectangular full duty cycle pulses, which are commonly used in correlation-type lock-in EBI measurements. Comparison of the proposed method to a common method of regular rectangular waveforms is given. The results show that the measurement errors of the three-element EBI equivalent become several tens of times smaller when shortening the excitation and reference signals separately by different values, for example, by 30 deg and 18deg. Shortening of only a single signal (either excitation or reference) can also give significant progress. Improved accuracy of the EBI phasor measurement makes the proposed method reliable for most clinical applications. Due to its simplicity, the method is appropriate for on-chip realization in implantable devices

An implantable analyzer of bio-impedance dynamics: mixed signal approach

An on-chip implantable lock-in analyzer of variations of the electrical bio-impedance has been designed and pilot versions of the ASIC based analyzer have been fabricated and tested. A mixed signal approach, as the most suitable way for accomplishing a low voltage and low power ASIC for applications in portable and implantable devices for various biomedical applications, is discussed.

Synchronous Sampling and Demodulation in an Instrument for Multifrequency Bioimpedance Measurement

Direct sampling of known carriers is the preferred digital method for measuring biomodulation of tissue impedance. Due to limited resolution and conversion rate of analog-to-digital converters and limited processing power of available digital processors and/or lack of energy resources, conventional discrete-Fourier-transform-based algorithms are not efficient in small medical devices. Knowing exactly the frequencies of carriers (and excitations), an energy-saving fast signal processing method can be developed and implemented. When sampling synchronously with a carrier, it is possible to minimize the complexity of calculations and to introduce a digital-to-analog feedback for enhancement of resolution by digitizing only the small variations between adjacent samples. The proposed system is qualified on proprietary hardware.

An implantable analyzer of bio-impedance dynamics: mixed signal approach [telemetric monitors]

An on-chip implantable lock-in analyzer of variations of the electrical bio-impedance has been designed, and pilot ASICs have been fabricated and tested. A mixed signal approach as the most suitable way for accomplishing a low-voltage and low-power ASIC for use in implantable devices for various biomedical applications is discussed in this paper.

Using of chirp excitation for bioimpedance estimation: Theoretical aspects and modeling

In this paper, theoretical considerations and simulation results of a method for quick estimation of the bioimpedance (BI)vector over a wide range of excitation frequencies are discussed. This method uses a chirp signal with linearly modulated frequency for excitation. The response signal is processed using cross-correlation techniques together with following fast Fourier transform (FFT). As the result, the complex spectrum of the response signal is attained in the range of frequency, determined by the spread of chirp pulse. This spectrum characterizes the frequency response of the impedance under study.

A sampling multichannel bioimpedance analyzer for tissue monitoring

The paper focuses on principles of designing of a multichannel bioimpedance analyzer based on simultaneous multisine measurement. The measurement task arises due to the need to monitor patients during and after heart surgery operation performing MIMO (multiple-input-multiple-output) bioimpedance measurement. Frequencies of the simultaneously applied sinusoidal excitations must be close but simultaneously varied in a larger range (e.g. from 1 kHz up to 10 MHz). The main idea of the proposed approach is that the use of a rather specific signal system (frequencies of sinusoidal excitations are related as integers and sampling frequencies are properly related/adapted to them) makes it possible to separate responses to different excitations from the measured summary signals by means of a quite simple filter and different (under) sampling rates.

Modification of pulse wave signals in electrical bioimpedance analyzers for implantable medical devices

The problems of application of pulse wave signals in electrical bioimpedance analyzers foreseen for using in implantable medical devices as diagnostical means are discussed in this paper. The main problem arises at measurement of phasor parameters by the aid of rectangular pulse wave signals. The specific measurement errors appear due to presence of higher harmonics in the spectra of pulse waveforms. These errors are discussed in two cases, in the case of full cycle rectangular waveform, and in the case of using the shortened pulses introduced specially for reduction of errors.

Design concepts of instruments for vector parameter identification

The design concepts for phase-sensitive measuring instruments, which are based on two-phase synchronous detection and known as vector analyzers, are proposed. The main idea of the design is to use the advantages of both analog and digital signal processing methods by combining the synchronous detection and averaging procedures with the integrating analog-to-digital and functional digital-to-analog conversion principles. The reference channel of the vector analyzer is built up on the basis of the adaptive third-order phase-locked loop (PLL). >

Broadband spectroscopy of dynamic impedances with short chirp pulses

An impedance spectrum of dynamic systems is time dependent. Fast impedance changes take place, for example, in high throughput microfluidic devices and in operating cardiovascular systems. Measurements must be as short as possible to avoid significant impedance changes during the spectrum analysis, and as long as possible for enlarging the excitation energy and obtaining a better signal-to-noise ratio (SNR). The authors propose to use specific short chirp pulses for excitation. Thanks to the specific properties of the chirp function, it is possible to meet the needs for a spectrum bandwidth, measurement time and SNR so that the most accurate impedance spectrogram can be obtained. The chirp wave excitation can include thousands of cycles when the impedance changes slowly, but in the case of very high speed changes it can be shorter than a single cycle, preserving the same excitation bandwidth. For example, a 100 kHz bandwidth can be covered by the chirp pulse with durations from 10 µs to 1 s; only its excitation energy differs also 10(5) times. After discussing theoretical short chirp properties in detail, the authors show how to generate short chirps in the microsecond range with a bandwidth up to a few MHz by using digital synthesis architectures developed inside a low-cost standard field programmable gate array.