Roman Kusche

A FPGA-Based Broadband EIT System for Complex Bioimpedance Measurements—Design and Performance Estimation

Electrical impedance tomography (EIT) is an imaging method that is able to estimate the electrical conductivity distribution of living tissue. This work presents a field programmable gate array (FPGA)-based multi-frequency EIT system for complex, time-resolved bioimpedance measurements. The system has the capability to work with measurement setups with up to 16 current electrodes and 16 voltage electrodes. The excitation current has a range of about 10 µA to 5 mA, whereas the sinusoidal signal used for excitation can have a frequency of up to 500 kHz. Additionally, the usage of a chirp or rectangular signal excitation is possible. Furthermore, the described system has a sample rate of up to 3480 impedance spectra per second (ISPS). The performance of the EIT system is demonstrated with a resistor-based phantom and tank phantoms. Additionally, first measurements taken from the human thorax during a breathing cycle are presented.

An in-ear pulse wave velocity measurement system using heart sounds as time reference

Abstract Pulse wave measurements provide vital information in medical diagnosis. For this reason, a measurement system is developed for determining the transient time of the pulse wave between the heart and the ear. To detect pressure variations in the sealed ear canal, caused by the arriving pulse wave, an in-ear sensor is developed which uses heart sounds as time reference. Furthermore, for extracting the heart sounds from the pressure measurements and calculating the pulse wave transient time, a MATLAB-based algorithm is described. An embedded microcontroller based measurement board is presented, which realizes an interface between the sensor and the computer for signal processing.

Aortic Pulse Wave Velocity Measurement via Heart Sounds and Impedance Plethysmography

The determination of the physical characteristic of the human arterial system, especially the stiffness of the aorta, is of major interest for estimating the risk of cardiovascular diseases. The most common measurement technique to get information about the state of the arterial system is the pulse wave analysis. It includes the measurement of the pulse wave velocity inside the arteries as well as its morphologically changes when propagating through the arteries. Since it is difficult to detect the pulse wave directly at the aorta, most available devices acquire the pulse wave at the extremities instead. Afterwards, complex models and algorithms are often utilized to estimate the original behavior of the pulse wave inside the aorta. This work presents an impedance plethysmography based technique to determine the aortic pulse wave velocity. By measuring the starting time of the pulse wave directly at its origin by the acquisition of heart sounds and the arrival time at the end of the aorta non-invasively via skin electrodes, unreliable complex models or algorithms aren’t necessary anymore to determine the pulse wave velocity. After describing the measurement setup and the problem-specific hardware system, first measurements from a human subject are analyzed and discussed.

A Bioimpedance-Based Cardiovascular Measurement System

Bioimpedance measurement is a biomedical technique to determine the electrical behavior of living tissue. It is well known for estimating the body composition or for the electrical impedance tomography. Additionally to these major research topics, there are applications with completely different system requirements for the signal acquisition. These applications are for example respiration monitoring or heart rate measurements. In these cases, very high resolution bioimpedance measurements with high sample rates are necessary. Additionally, simultaneous multi-channel measurements are desirable. This work is about the hardware and software development of a 4-channel bio-impedance measurement system, whereat all channels are galvanically decoupled from each other. It is capable of measuring 1000 impedance magnitudes per second and per channel. Depending on the chosen measurement configuration, impedance changes down to mΩ ranges are feasible to be detected. To enable the usage in a variety of different research applications, further biosignals like photoplethysmography, electrocardiography or heart sounds can be acquired simultaneously. For electrical safety purposes, an implemented galvanically isolated USB interface transmits the data to a host PC. The impedance measurements can be analyzed in real-time with a graphical user interface. Additionally, the measurement configuration can easily be changed via this GUI. To demonstrate the system’s usability, exemplary measurements from human subjects are presented.

Respiration Monitoring by Combining EMG and Bioimpedance Measurements

A common technique to measure diaphragm electrical activation is the acquisition of the occurring electromyography signals using surface electrodes. A significant problem of this technique is its sensitivity against motion artifacts. Forces or vibrations can influence the electrode skin contacts, which generate changes of the electrodes’ half-cell voltages as well as the electrode skin impedance. Both effects result in noise in the same frequency range as the typical electromyography signal and therefore it’s hard to separate these distortions from the desired signal. Another technique to detect muscle contractions is the electrical impedance myography. By applying a small alternating current to the tissue of interest and measuring the occurring voltage drop, the bioimpedance is determined. It contains the information about muscle contractions and relaxations. The major advantage of this technique is, that the impedance information is coded as the amplitude modulation of that voltage drop. The frequency of the excitation current is typically chosen in the range of tens of kHz and thus can easily be separated from the above mentioned artefacts. This work describes a measurement setup which is capable of acquiring electromyography as well as bioimpedance signals simultaneously, sharing the same electrodes. An analog circuit is presented which combines the information of both measurement techniques, allowing their common analog-to-digital conversion by a single converter. The system is capable to acquire 1000 bioimpedances/electromyography samples per second with a resolution of 24 bits. First measurements show that signal distortions, caused by vibrations at the electrodes, are attenuated significantly in bioimpedance measurements.

Analyzing superparamagnetic iron oxide nanoparticles (spions) using electrical impedance spectroscopy

Conventional methods to evaluate the size of superparamagnetic iron oxide nanoparticles (SPIONs) and their coatings used in magnetic particle imaging (MPI) include photon cross-correlation spectroscopy (PCCS) [1], atomic force microscopy (AFM) [1] and transmission electron microscopy (TEM) [2]. There is however still a potential for improvement as they are expensive and only able to analyze small sample quantities. In this work, a new method using electrical impedance spectroscopy is evaluated. With this method, it is possible to analyze macroscopic samples at low costs.