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Benjamín Sánchez Terrones

Benjamín Sánchez Terrones

Ph.D. Thesis title:
BROADBAND ELECTRICAL IMPEDANCE SPECTROSCOPY FOR DYNAMIC ELECTRICAL BIO-IMPEDANCE CHARACTERIZATION

Author:
Benjamín Sánchez Terrones

Director:
Ramon Bragós / Gerd Vanders

Reading date:
13/01/2012

Abstract:
The electrical impedance of biological samples is known in the literature as Electrical Bioimpedance (EBI). The Electrical Bioimpedance enables to characterize physiological conditions and events that are interesting for physiological research and medical diagnosis. Although the Electrical Bioimpedance weakness is that it depends on many physiological parameters, on the other hand, it is suitable for many medical applications where minimally invasive and real-time measurements with simple and practical implementations are needed.

 

The Electrical Impedance Spectroscopy (EIS) techniques based on broadband excitations are expected to help to understand various unsolved problems in biomedical applications. Broadband EIS opens up the possibility to reduce drastically the measuring time for acquiring EBI time-variations but, at the same time, measuring in a short time compromises the EBI accuracy. The way to overcome this intrinsic loss of accuracy relies on the design of the appropriate time/frequency input excitation properties and the use of the suitable spectral analysis processing techniques.

 

The presented thesis covers the topics related to study of broadband excitations for Impedance Spectroscopy in biomedical applications and, more specific, the influence of the multisine excitation time/frequency properties on the impedance spectrum accuracy and its optimization. Furthermore, an advanced fast signal processing method has been implemented to process in real-time EBI data corrupted by transients, a common situation when measuring in a short measuring time. Despite being the goal to apply all this knowledge for myocardial tissue regeneration monitoring, at the moment of drafting the thesis, any of the research projects that have supported this thesis have issued functional beating tissue. For that reason, the theory presented has been validated by a set of experimental measurements over animals and patients where the impedance spectrum time-varying properties were pretended to be characterized.

 

The thesis presents novel findings of relevance of a successful application of broadband EIS in two different measurement campaigns where it has been put in practice: (1) within the collaboration of the pneumology and cardiology service from Hospital Santa Creu i Sant Pau for in-vivo human lung tissue characterization, and (2), within the measurement of animal healthy myocardium tissue electrical impedance including its dynamic behavior during the cardiac cycle.

 

As a result the thesis research, two manuscripts have been already accepted for publication and one has been submitted:

 

B. Sanchez, G. Vandersteen, R. Bragos and J. Schoukens, Optimal Multisine Excitation Design for Broadband Electrical Impedance Spectroscopy, Meas. Sci. Technol. 22 115601, 2011,  doi:10.1088/0957-0233/22/11/115601.

 

The aim of this work is to solve the problem of the optimal multisine excitation design for accurate Electrical Bioimpedance estimation. The a priori knowledge of the impedance spectrum enables to optimize the excitation in the frequency domain. As a result, the impedance spectrum accuracy is maximized for a discrete number of excited frequencies under energy and measuring time constraints. The main finding of the work reported in this paper is the study of the contribution of the multisine amplitudes and frequencies distributions to the Electrical Bio-Impedance accuracy for characterizing electrical impedance relaxations.

 

B. Sanchez, J. Schoukens, R. Bragos and G. Vandersteen, Novel Estimation of the Electrical Bioimpedance using the Local Polynomial Method. Application to in-vivo real-time Myocardium Tissue Impedance Characterization during the Cardiac Cycle, Accepted for publication IEEE Trans. Biomed. Eng., July 2011.

 

This paper presents a novel approach for impedance spectrum estimation, based on the Local Polynomial Method (LPM) for real-time time-varying Electrical Bio-Impedance (EBI) characterization. The LPM efficiently rejects the leakage error’s influence on the impedance frequency response estimation when the EBI is under the effect of transients. The theory and instrumentation is supported by a set of validation measurements compared to a commercial impedance analyzer. Experimental results from in-vivo myocardium tissue electrical impedance measurements show that the estimation of the impedance spectrum can be improved with respect to the classical spectral analysis methods based on windows, where the leakage is reduced at low frequencies without increasing the calculation time. The results obtained present novel findings of relevance of a successful application of the fast LPM for real-time myocardium tissue electrical impedance characterization. Further research should enable to detect how the systolic and diastolic function as well as cardyomiocyte contractile activity change with time during an ischemia process.

 

B. Sanchez, G. Vandersteen, R. Bragos and J. Schoukens, Survey of the Strengths and Weaknesses of Periodic Broadband Signals for Electrical Impedance Spectroscopy: A Comparative Study, Journal of Electroanalytical Chemistry, Submitted September 2011.

 

In contrast to the Electrical Impedance Spectroscopy (EIS) technique based on frequency sweep approach, broadband EIS offers the advantage of simultaneous frequency impedance response data collection. This is due to the fact that broadband excitation spreads the energy at several frequencies at the same time, which is reflected into a short measuring time because only a single time record is required to measure the frequencies of interest. However, the weakness of such kind of fast EIS measuring techniques is its intrinsic loss of accuracy. At the end, not only the total measuring time will be determined by the final accuracy desired, but to the broadband signal applied and its time/frequency features as well as the system time properties and the signal processing techniques used. This manuscript presents a comprehensive review of the major time/frequency features of four different periodic broadband excitations suitable for Impedance Spectroscopy applications. Theory of the comparison based on the Impedance Spectrum Signal-to-Noise Ratio (SNR) has been supported by a set of simulations and experimental measurements obtained when measuring with a custom impedance analyzer are provided.