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H Scharfetter

Publications and source records attributed to H Scharfetter.

At least 19 recordsLinked to original sources
PubMed2006

A multifrequency magnetic induction tomography system using planar gradiometers: data collection and calibration.

We developed a 14-channel multifrequency magnetic induction tomography system (MF-MIT) for biomedical applications. The excitation field is produced by a single coil and 14 planar gradiometers are used for signal detection. The object under measurement was rotated (16 steps per turn) to obtain a full data set for image reconstruction. We make measurements at frequencies from 50 kHz to 1 MHz using a single frequency excitation signal or a multifrequency signal containing several frequencies in this range. We used two acquisition boards giving a total of eight synchronous channels at a sample rate of 5 MS s(-1) per channel. The real and imaginary parts of DeltaB/B(0) were calculated using coherent demodulation at all injected frequencies. Calibration, averaging and drift cancellation techniques were used before image reconstruction. A plastic tank filled with saline (D = 19 cm) and with conductive and/or paramagnetic perturbations was measured for calibration and test purposes. We used a FEM model and an eddy current solver to evaluate the experimental results and to reconstruct the images. Measured equivalent input noise voltage for each channel was 2 nV Hz(-1/2). Using coherent demodulation, with an integration time of 20 ms, the measured STD for the magnitude was 7 nV(rms) (close to the theoretical value only taking into account the amplifier's thermal noise). For long acquisition times the drift in the signal produced a bigger effect than the input noise (typical STD was 10 nV with a maximum of 35 nV at one channel) but this effect was reduced using a drift cancellation technique based on averaging. We were able to image a 2 S m(-1) agar sphere (D = 4 cm) inside the tank filled with saline of 1 S m(-1).

Calibration↗
PubMed2004

Fast calculation of the sensitivity matrix in magnetic induction tomography by tetrahedral edge finite elements and the reciprocity theorem.

Magnetic induction tomography of biological tissue is used to reconstruct the changes in the complex conductivity distribution by measuring the perturbation of an alternating primary magnetic field. To facilitate the sensitivity analysis and the solution of the inverse problem a fast calculation of the sensitivity matrix, i.e. the Jacobian matrix, which maps the changes of the conductivity distribution onto the changes of the voltage induced in a receiver coil, is needed. The use of finite differences to determine the entries of the sensitivity matrix does not represent a feasible solution because of the high computational costs of the basic eddy current problem. Therefore, the reciprocity theorem was exploited. The basic eddy current problem was simulated by the finite element method using symmetric tetrahedral edge elements of second order. To test the method various simulations were carried out and discussed.

Brain Diseases↗
PubMed2004

Measurement of liver iron overload by magnetic induction using a planar gradiometer: preliminary human results.

The measurement of hepatic iron overload is of particular interest in cases of hereditary hemochromatosis or in patients subject to periodic blood transfusion. The measurement of plasma ferritin provides an indirect estimate but the usefulness of this method is limited by many common clinical conditions (inflammation, infection, etc). Liver biopsy provides the most quantitative direct measurement of iron content in the liver but the risk of the procedure limits its acceptability. This work studies the feasibility of a magnetic induction (MI) low-cost system to measure liver iron overload. The excitation magnetic field (B0, frequency: 28 kHz) was produced by a coil, the perturbation produced by the object (deltaB) was detected using a planar gradiometer. We measured ten patients and seven volunteers in supine and prone positions. Each subject was moved in a plane parallel to the gradiometer several times to estimate measurement repeatability. The real and imaginary parts of deltaB/B0 were measured. Plastic tanks filled with water, saline and ferric solutions were measured for calibration purposes. We used a finite element model to evaluate the experimental results. To estimate the iron content we used the ratio between the maximum values for real and imaginary parts of deltaB/B0 and the area formed by the Nyquist plot divided by the maximum imaginary part. Measurements in humans showed that the contribution of the permittivity is stronger than the contribution of the permeability produced by iron stores in the liver. Defined iron estimators show a limited correlation with expected iron content in patients (R < or = 0.56). A more precise control of geometry and position of the subjects and measurements at multiple frequencies would improve the method.

Computer Simulation↗
PubMed2002

Fluid volume changes and LBNP response after simulated weightlessness with varied oral sodium supply.

There is evidence on body fluid volume effects of head-down tilt bed rest and altered oral sodium supply, but the combined impact of both has not been investigated in detail. We therefore studied circulatory adaptation to 8 days -6 degrees head down bed rest (HDBR) with different levels (-140 to -430 mM/d) of oral sodium load (SL). We expected decreased extracellular volume and increased aldosterone and PRA levels with low sodium load, and hypothesized that these effects get exaggerated with additional HDBR, also influencing lower body suction (LBNP) responses. Variations in sodium status seem to influence plasma but not interstitial volume, confirming recent results of another group who used different experimental conditions.

Journal Article↗
PubMed2001

Assessing abdominal fatness with local bioimpedance analysis: basics and experimental findings.

OBJECTIVE: Abdominal fat is of major importance in terms of body fat distribution but is poorly reflected in conventional body impedance measurements. We developed a new technique for assessing the abdominal subcutaneous fat layer thickness (SFL) with single-frequency determination of the electrical impedance across the waist (SAI). SUBJECTS AND MEASUREMENTS: The method uses a tetrapolar arrangement of surface electrodes which are placed symmetrically to the umbilicus in a plane perpendicular to the body axis. Twenty-four test subjects (12 male, 12 female) underwent SAI and abdominal magnetic resonance imaging (MRI). The SFL below the sensing electrodes was determined from MRI and correlated with the SAI data at four different frequencies (5, 20, 50 and 204 kHz). RESULTS: A highly significant linear correlation (r2=0.99) between SFL and SAI over a wide range of the abdominal SFL was found. Separate regression models for female and male subjects did not differ significantly, except at 50 kHz. CONCLUSION: SAI represents a good predictor of the SFL and provides an excellent tool for the assessment of central obesity.

Abdomen↗
PubMed2001

Sensitivity maps and system requirements for magnetic induction tomography using a planar gradiometer.

We evaluated analytically and experimentally the performance of a planar gradiometer as a sensing element in a system for magnetic induction tomography. A system using an excitation coil and a planar gradiometer was compared against a system with two coils. We constructed one excitation coil, two different sensing elements and a high-resolution phase detector. The first sensor was a PCB square spiral coil with seven turns. The second sensor was a PCB planar gradiometer with two opposite square spirals of seven turns, with a distance between centres of 8 cm. Theoretical sensitivity maps were derived from basic equations and compared with experimental data obtained at 150 kHz. The experimental sensitivity maps were obtained measuring the perturbation produced by a brass sphere of 12 mm in empty space. The advantage of using a gradiometer is that it can be adjusted to give a minimum signal for homogeneous objects, while increasing the sensitivity to local perturbations of the conductivity. Results show that a system using a planar gradiometer as detector has less demanding requirements for the electronic system than a system using simple coils.

Electric Impedance↗
PubMed2001

Magnetic induction tomography: hardware for multi-frequency measurements in biological tissues.

Magnetic induction tomography (MIT) is a contactless method for mapping the electrical conductivity of tissue. MIT is based on the perturbation of an alternating magnetic field by a conducting object. The perturbation is detected by a voltage change in a receivercoil. At physiologically interesting frequencies (10 kHz-10 MHz) and conductivities (< 2 S m(-1)) the lower limit for the relative voltage change (signal/carrier ratio = SCR) to be resolved is 10(-7)-10(-10). A new MIT hardware has been developed consisting of a coil system with planar gradiometers and a high-resolution phase detector (PD). The gradiometer together with the PD resolves an SCR of 2.5 x 10(-5) (SNR = 20 dB at 150 kHz, acquisition speed: 100 ms). The system operates between 20 and 370 kHz with the possibility of extending the range up to 1 MHz. The feasibility of measuring conductivity spectra in the beta-dispersion range of biological tissues is experimentally demonstrated. An improvement of the resolution towards SCR = 10(-7) with an SNR of > or = 20 dB at frequencies > 100 kHz is possible. On-line spectroscopy of tissue conductivity with low spatial resolution appears feasible, thus enabling applications such as non-invasive monitoring of brain oedema.

Brain Edema↗
PubMed2000

Kinetics of the metal cations magnesium, calcium, copper, zinc, strontium, barium, and lead in chronic hemodialysis patients.

BACKGROUND: Dialysis patients are at risk of developing trace element imbalances. To further elucidate the origin of these potential trace element imbalances, plasma and dialysis fluids concentrations of the elements barium (Ba), calcium (Ca), copper (Cu), lead (Pb), magnesium (Mg), strontium (Sr) and zinc (Zn) of seven maintenance dialysis patients were investigated. PATIENTS AND METHODS: In each hemodialysis session 10 to 15 samples of each, whole blood and dialysis liquid before and after passing the artificial kidney were collected. Concentrations of elements were determined by inductively coupled plasma mass spectrometry following strict quality control schemes to guarantee the accuracy and precision of the results. RESULTS: Plasma concentrations of Cu and Zn continuously increased during hemodialysis. Plasma Cu remained within the reference range for healthy adults, whereas plasma Zn was always at or below the reference range in our patients. The behavior of Ca and Sr exhibited extraordinarily strong similarities both in plasma and dialysis liquids, although concentrations of Sr are approximately 2000 times lower. Plasma Ca and Sr were at or above the upper level of the reference range. Plasma Mg concentrations decreased during clinical treatment, but were at the end of dialysis still more than 50% higher than the high end of the reference range. Although concentrations of Ba in dialysis fluids were approximately 10 times lower than in plasma, plasma Ba concentrations (approximately 23 microg/l) were significantly elevated compared to plasma Ba of healthy adults. Initial concentrations of Pb in plasma (0.74 microg/l) were increased by approximately 15% during the clinical treatment and were always higher than the high limit of the reference range. Dialysis liquids had approximately the same Pb concentrations (0.5 to 1.3 microg/l) as found in the plasma of our patients but with higher concentrations at the inlet of the dialyzer. CONCLUSION: This study could give an insight into the kinetics of trace element concentrations during dialysis, the clinical relevance of which needs to be further elucidated.

Adult↗
PubMed1999

Inductively coupled wideband transceiver for bioimpedance spectroscopy (IBIS).

Most measurement devices for bioimpedance spectroscopy are coupled to the measured object (tissue) via electrodes. At frequencies > 500 kHz, they suffer from artifacts due to stray capacitances between electrode leads as well as between the ground and object. The noninvasive measurement of the brain conductivity is hardly possible with surface electrodes. These disadvantages can be obviated by inductive coupling. The aim of this work was the development of a wideband transceiver for inductive impedance spectroscopy. In order to define its specifications, a feasibility study has been carried out with a simulation model for three different coil systems above a homogeneous conducting plate. According to simulation results, all systems render it possible to resolve conductivity changes down to 10(-3) (omega m)-1 at frequencies > 50 kHz. The transceiver electronics must then provide a resolution of > or = 1 microV and an excitation current of up to 1 A. The realized receiver matches these specifications with an S/N ratio of 22 dB at 1 microV in the frequency range of 50 kHz to 5 MHz.

Brain Chemistry↗
PubMed1999

Exchange of alkali trace elements in hemodialysis Patients: a comparison with Na(+) and K(+).

BACKGROUND: In the past, nephrologists have been troubled by electrolyte disturbances and consequently focused their attention on the importance of maintaining the concentrations of electrolytes within the normal range. However, information about the potential role of trace elements in chronic renal failure is scarce. METHODS: During hemodialysis sessions, the concentrations of the five alkali metal cations lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs) have been determined in plasma and dialysis fluids of chronic hemodialysis patients by inductively coupled plasma mass spectrometry (Li, Rb, Cs) and by ion-sensitive electrodes (Na, K). Strict quality control schemes were applied to all analytical procedures to ensure accuracy and precision of the results. RESULTS: The plasma concentrations of the elements Li, Cs, Rb, and K distinctly decreased to 29, 50, 69, and 71%, respectively, of their initial values during hemodialysis. Simultaneously, the concentrations of these elements in dialysis fluids at the outlet of the dialyzer increased approximately 13-fold for Rb, 11-fold for Li, 3-fold for Cs, and 2-fold for K as compared with the inlet values. The concentrations of Na in plasma and dialysis fluids were almost identical and did not change during hemodialysis. CONCLUSIONS: Li, Rb, and Cs were depleted in hemodialysis patients, although the plasma concentrations of these trace elements still remained within the reference ranges for healthy adults. Consequently, further studies are needed to elucidate the clinical importance and long-term effects of these trace element imbalances - for example, CNS disturbances associated with diminished concentrations of Rb - in hemodialysis patients.

Adult↗
PubMed1998

A model of artefacts produced by stray capacitance during whole body or segmental bioimpedance spectroscopy.

We have developed a novel model for the simulation of artefacts which are produced by stray capacitance during bioimpedance spectroscopy. We focused on whole body and segmental measurements in the frequency range 5-1000 kHz. The current source was assumed to by asymmetric with respect to ground as is the case for many commercial devices. We considered the following stray pathways: 1, cable capacitance; 2, capacitance between neighbouring electrode leads; 3. capacitance between different body segments and earth; 4, capacitance between signal ground of the device and earth. According to our results the pathways 3 and 4 cause a significant spurious dispersion in the measured impedance spectra at frequencies > 500 kHz. During segmental measurements the spectra have been found to be sensitive to an interchange of the electrode cable pairs. The sensitivity was also observed in vivo and is due to asymmetry of the potential distribution along the segment with respect to earth. In contrast to previously published approaches, our model renders possible the simulation of this effect. However, it is unable to fully explain the deviations of in vivo measured impedance spectra from a single Cole circle. We postulate that the remaining deviations are due to a physiologically caused superposition of two dispersions from two different tissues.

Artifacts↗
PubMed1997

Influence of ionic shifts during dialysis on volume estimations with multifrequency impedance analysis.

During dialysis the ion concentrations in many body fluids change significantly. The influence of these changes on the accuracy of volume measurements with bioimpedance spectroscopy is investigated by the following procedure: Plasma ion concentrations and impedance spectra (5-500 kHz) are measured during six standard haemodialyses. Intracellular ion concentrations are estimated using a multi-compartment model. Intra- (ICV) and extracellular (ECV) volumes are calculated using a fluid distribution model (FDM) based on Hanai's mixture theory. The input variables of the FDM are intra- and extracellular resistance data that have been fitted from impedance spectra with a Cole-Cole model. Resistivity changes (RCs) due to concentration changes of Na+, K+, Cl-, HCO3- and unspecified intracellular ions are estimated. The FDM is corrected for the RCs. Corrected ICVs and ECVs are calculated and compared with uncorrected values. The range of relative RCs between the start and end of the dialyses is -3.2% to 1.4% in the ECV and -3.7% to 1.7% in the ICV. From the RCs, volume estimation errors of -1.0% to 1.9% (ECV) and -1.2% to 2.1% (ICV) relative to the initial values have been calculated. At the end of dialysis, the percentage of the error with respect to the volume change is < 15% for the ECV but > 20% for the ICV. Consequently, a correction of the FDM for RCs is necessary to obtain more reliable ICV data.

Body Fluids↗
PubMed1997

Effect of postural changes on the reliability of volume estimations from bioimpedance spectroscopy data.

Bioimpedance spectroscopy (BIS) has been suggested for the assessment of fluid shifts between intracellular (ICV) and extracellular volume (ECV) during dialysis. The electrical tissue parameters are estimated by fitting a Cole-Cole model to the impedance data. Those parameters are used for the calculation of ICV and ECV with a fluid distribution model (FDM). We investigated whether postural changes cause artifacts in the volume data measured with a commercial BIS system. This is of importance at the beginning of dialysis, when the patient lies down for treatment. Volume estimations were performed during tilt table experiments with 11 healthy volunteers. Impedance spectra (5 to 500 kHz) were recorded for the total body as well as for body segments (leg and arm) during three phases: (1) 30 minutes resting in a supine position after standing; (2) 30 minutes 70 degrees head up tilt; and (3) a 30-minute resting period in a supine position. ECV and ICV were estimated with a commercially utilized FDM which is based on Hanai's mixture theory. A monoexponential function was fitted to the data for extracting the time constants and the extrapolated steady state values of the volume changes. The ECV and ICV data changed significantly during all three periods, that is, a steady state could not be reached within 30 minutes. During phase 1 the ECV decreased by 1.8 +/- 0.7%, in the tilt phase it increased by 3.8 +/- 1.1%, and in phase 3 it decreased again by 2.9 +/- 1%. The ICV increased by 3.6 +/- 2.4% during phase 1 and decreased by 6.8 +/- 5.1% during tilting; in phase 3 it increased by 4.6 +/- 1.7%. The time constants were 36.4 +/- 12.7 minutes (ECV) and 10.8 +/- 5.4 minutes (ICV) during phase 3. Segmental measurements revealed that the legs contribute significantly to the measured volume changes. The absolute volume changes in ICV and ECV differed significantly in all phases, and the same was found for the time constants during phases 1 and 3. From this discrepancy it is concluded that the measured volume changes are artifacts that are caused by extracellular fluid redistribution. Furthermore, it appears unlikely that the measured fluid shifts actually occur between ECV and ICV in the absence of osmotic changes in the body fluids. The validity of the method for a reliable assessment of volume changes during dialysis appears questionable, as dialysis-induced volume changes lie in the same range as the orthostatically-induced spurious volume changes.

Adult↗
PubMed1996

Dynamical control of the dialysis process. Part II: An improved algorithm for the solution of a tracking problem.

An efficient algorithm for the optimization of process parameters during dialysis has been developed. By solving a tracking-problem for prescribed time courses of distinguished variables, it is possible to compute optimal concentrations of electrolytes in dialysate as well as an optimal rate of ultrafiltration. These variables are indirectly influencing the status of the patient and can be directly modelled. They are describing the important exchange processes between blood and dialysate as well as between the different distribution spaces within the patient during dialysis. Their time courses are determined by an individually identifiable patient model. The tracking problem was treated as a dynamic optimization problem, and a continuous descent procedure which is usually employed for solving unconstrained static optimization problems has been adapted in such a manner that it is applicable for the solution of this problem. The used method is characterized by its simple mode of application, short solution time and moderate storage need. Especially in cases of contradictional requirements for desired time courses of model outputs the used optimization method performs well.

Algorithms↗
PubMed1995

Diagnosis of congenital pericardial defects, including a pathognomic sign for dangerous apical ventricular herniation, on magnetic resonance imaging.

OBJECTIVE: To establish criteria for the accurate diagnosis of different forms of left sided pericardial defects on magnetic resonance imaging. Early detection of a partial apical defect is essential as it is potentially fatal. DESIGN: Examination of four children with congenital pericardial defects by magnetic resonance imaging, the results being compared with the features on conventional chest radiography and echocardiography and with published data. RESULTS: Magnetic resonance imaging improved the ability to diagnose and distinguish between complete and partial left sided pericardial defects. A deep myocardial crease was visualised in a patient with apical pericardial defect, indicating the risk of a life threatening ventricular strangulation. A prominent left atrial appendage was, in contrast to many reports, not a reliable sign for partial left sided pericardial defect. CONCLUSIONS: The various forms of congenital left sided pericardial defects cannot reliably be diagnosed in plain chest radiographs or on echocardiography. Their diagnosis and the distinction between partial and complete defects, however, is of clinical importance and can be accomplished more confidently by magnetic resonance imaging.

Adolescent↗
PubMed1995

Development and critical evaluation of an improved comprehensive multicompartment model for the exchange processes during hemodialysis.

An improved comprehensive multicompartment model for the simulation of the most important metabolic state variables in the patient during dialysis is presented. With this approach time courses of urea, creatinine, K+, Na+, Cl-, HCO3-, H+ and CO2 can be predicted. Additionally, osmotic water shifts as well as resting membrane potentials are calculated. The model contains the following extensions compared to classical approaches: For the calculation of osmotic water shifts, not only sodium, but also urea, potassium, chloride and unspecified indiffusable ions are taken into account. Furthermore, hemodynamic aspects are considered by assuming two tissue groups with different perfusion. Thus it is possible to estimate the influence of hemodynamic parameters (e.g. cardiac output or blood flow distribution) on the exchange processes. The model can be adjusted individually by several system parameters. This adjustment is performed by minimizing the sum of the quadratic differences between simulated and measured plasma concentrations of the considered substances. A first validation has been performed successfully with measured data from 18 dialysis patients. After the effective whole-body exchange area of the resting cell membranes for potassium, sodium and chloride had been estimated, rebound effects for those electrolytes could be simulated successfully.

Acid-Base Equilibrium↗