Martin Ryschka

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.

A high accuracy broadband measurement system for time resolved complex bioimpedance measurements.

Bioimpedance measurements are useful tools in biomedical engineering and life science. Bioimpedance is the electrical impedance of living tissue and can be used in the analysis of various physiological parameters. Bioimpedance is commonly measured by injecting a small well known alternating current via surface electrodes into an object under test and measuring the resultant surface voltages. It is non-invasive, painless and has no known hazards. This work presents a field programmable gate array based high accuracy broadband bioimpedance measurement system for time resolved bioimpedance measurements. The system is able to measure magnitude and phase of complex impedances under test in a frequency range of about 10-500 kHz with excitation currents from 10 µA to 5 mA. The overall measurement uncertainties stay below 1% for the impedance magnitude and below 0.5° for the phase in most measurement ranges. Furthermore, the described system has a sample rate of up to 3840 impedance spectra per second. The performance of the bioimpedance measurement system is demonstrated with a resistor based system calibration and with measurements on biological samples.

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.

Measurements of Electrode Skin Impedances using Carbon Rubber Electrodes ? First Results

Non-invasive bioimpedance measurement as a tool in biomedical engineering and life sciences allows conclusions about condition and composition of living tissue. For interfacing the electronic conduction of the instrumentation and the ionic conduction of the tissue, electrodes are needed. A crucial point is the uncertainty arising from the unknown, time-varying and current density depend Electrode Skin Impedance (ESI). This work presents ESI measurements using carbon rubber electrodes on different human test subjects. The measurements for this work are carried out by employing a high accuracy Bioimpedance Measurement System (BMS) developed by the authors group, which is based on a Field Programmable Gate Array (FPGA) System on Chip (SoC). The system is able to measure magnitude and phase of complex impedances using a two- or four-electrode setup, with excitation currents from 60 μA to 5 mA in a frequency range from about 10 kHz to 300 kHz. Achieved overall measurement uncertainties are below 1%.

Multi-frequency Electrical Impedance Tomography for Intracranial Applications

Electrical impedance tomography (EIT) is a functional real-time imaging technique based on measurement and reconstruction of electrical impedance distributions.

In-Ear Pulse Wave Measurements: A Pilot Study

The measurement of the Pulse wave Arrival Time (PAT) has proven to be a vital tool in medical diagnosis. Whereby most PAT measurements are carried out at ex- tremities, this work proposes the interior of the ear as a new site. Due to pressure variations inside the auditory canal a pulse wave can be measured by using a pressure sensor or by simple in-ear headphones. To verify the signal origin, a reflectance photoplethysmograph (PPG) measurement inside the ear is carried out. All sensors are integrated for accurate and comfortable fit, in a custom made mould.

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.

A micro Electrical Impedance Tomography System for Vessel Studies

Electrical Impedance Tomography (EIT) is a real-time imaging modality that measures and reconstructs the spatial impedance distributions inside an object under test. Based on the injection of small well know alternating currents (AC) and the measurement of resulting voltages, EIT has no known hazards and is even painless for human beings. Advantages of EIT over other classical imaging modalities are functional real-time imaging and portability, without utilizing ionic radiation. Until now EIT is mainly used as medical imaging technique, as well as for industrial and geophysical applications.