Biomedical subjects

L Trahms

Publications and source records attributed to L Trahms.

At least 19 recordsLinked to original sources

PubMed2002

Imaging characteristics of different multichannel magnetocardiographic systems.

In this study a comparison of multichannel magnetocardiographic systems is performed with respect to the "detectable" information content. We investigate the lead-field matrices, the slope of the singular values and the source spaces of three different devices: the VectorView (Neuromag: magnetometer-gradiometer mixed device) of the BioMag Laboratory, Helsinki University Central Hospital (HUCH), the arrangement of electronically coupled magnetometers of the Physikalisch-Technische Bundesanstalt Berlin (PTB) and a virtual sensor geometry which was optimized for an improved slope of the singular values at the Institute of Biomedical Engineering, Karlsruhe.

Adult↗

PubMed2002

Investigation of gastrointestinal transport by magnetic marker localization.

The method of magnetic marker monitoring MMM with a biomagnetic measurement system allows the tracking of the transport of solid drug forms with high spatiotemporal resolution. This technique enables to display the path and the disintegration of the magnetic marked pharmaceutical dosage form via the decrease of its magnetic moment. The method was used to assess exogenous factor on the esophageal transport of orally administered solid drug forms in five healthy volunteers. In a drug delivery study we have applied the MMM to monitor the path of a tablet during the gastrointestinal passage and to visualize the disintegration in the small intestine by the magnetic moment of the tablet. The signal to noise ratio of the measurement data was improved by ICA filtering.

Acetaminophen↗

PubMed2002

Time-delayed decorrelation for the identification of cardiac artifact components in MEG data.

The time-delayed decorrelation (TDD) Independent Component Analysis (ICA) applied to continuous multichannel magnetoencephalographic (MEG) recordings is a straightforward calculation in the time domain: Its main ingredient is a calculation of the correlation between signals having different time shifts with respect to each other. The equivalent mathematical representation in the frequency domain and a simple simulation show that TDD exploits different complex frequency spectra of the sources. Applying ICA to 80-channel whole head MEG data obtained under visual stimulation we can assign five of the 80 source components to the cardiac artifact. Their complex spectra exhibit a randomly distributed phase showing that TDD is capable of isolating quasi-periodic signals even for the case of an unspecified phase.

Algorithms↗

PubMed2002

MEG-analysis using the Hilbert transform.

Event-related fields (ERFs) measured by magnetoencephalography (MEG) upon visual stimulation are analysed by Hilbert transformation. The Hilbert transform of real-valued measured ERF is an analytic complex signal, represented by phase and amplitude. The temporal behaviour of the derivative of the phase, i.e. the instantaneous frequency, allows to distinguish time intervals containing meaningful signal from noise. On the basis of both phase and amplitude, energies and latencies of single event ERFs can be determined.

Evoked Potentials, Visual↗

PubMed2001

Non-invasive single-trial monitoring of human movement-related brain activation based on DC-magnetoencephalography.

Neuroimaging techniques, such as fMRI, PET and near-infrared spectroscopy, monitor task-related neuronal activations in the brain indirectly through the associated neurovascular/metabolic responses. To assess the primary neuronal activations directly, magnetoencephalography was combined here with a mechanical modulation of the head-to-sensor position and signal separation via independent component analysis. In all of five subjects this approach allowed to monitor the time evolution of DC fields (<0.1 Hz) over the left hemisphere related to complex finger movements of the right hand alternating with rest periods (30 s each). Throughout the recording period of 30 min, stable task-related DC fields were recordable in a single-trial mode, i.e. without any averaging. DC-MEG opens up the possibility of analysing non-invasively cortical DC-activity also in stroke, migraine or epilepsy patients.

Adult↗

PubMed2001

Magnetic marker monitoring of disintegrating capsules.

Magnetic marker monitoring was studied for its applicability to investigate the in vivo fate and behavior of disintegrating magnetically marked dosage forms. As a model, hard gelatin capsules were filled with an effervescent mixture of lactose, ascorbic acid and sodium hydrogen carbonate containing 1.3 mg black iron oxide as a magnetic label. The accuracy of the localization procedure whilst calculating all parameters of the dipole in one fitting procedure was checked in phantom experiments where the capsules were moved in well-defined paths with respect to the measurement device. The calculated position coordinates of the capsules deviated between less than 2 mm up to 8 mm from the expected position values depending on the distance between the sensor area and the capsule's path. Further experiments on the in vitro disintegration of the capsules showed that the value of the magnetic moment of the capsules can serve as a measure for their disintegration behavior. In vivo monitoring of the capsules was performed in eight experiments where a healthy volunteer swallowed each time one of the capsules. It was found that the in vivo disintegration behavior of the capsules corresponds well to their disintegration observed in water of about 37 degrees C.

Adult↗

PubMed2001

Magnetoneurography of evoked compound action currents in human cervical nerve roots.

OBJECTIVE: A measurement protocol for magnetoneurography (MNG) is established which allows the non-invasive localization and tracing of evoked compound action currents propagating along cervical nerve roots in man. METHODS: Inside a magnetically shielded room either both median or both ulnar nerves of healthy subjects were conventionally electrostimulated in alternation. Evoked magnetic responses were recorded using a multichannel SQUID-detector with a planar measuring area centered over the neck. Simultaneously, electric surface potentials were recorded using cervical bipolar electrode montages. RESULTS: Upon median (ulnar) nerve stimulation somatosensory evoked magnetic fields up to 20 fT (10 fT) amplitude were detected propagating over the cervical transforaminal root entry zone, with corresponding electrical surface potentials of 1.5 microV (0.5 microV). Furthermore, the signal-to-noise ratio of the spatiotemporal magnetic field mappings in median nerve stimulation experiments allowed dipolar source reconstructions and tracing of the propagation of the compound action currents along nerve root fibers. CONCLUSION: Magnetoneurography allows tracing of the propagation of evoked compound action currents along cervical roots in healthy subjects with millisecond temporal and high spatial resolution. Thus, MNG offers a sensitivity appropriate to serve as a clinical diagnostic tool for localizing focal neuropathies of cervical nerve roots.

Electric Conductivity↗

PubMed2001

Tracing of proximal lumbosacral nerve conduction--a comparison of simultaneous magneto- and electroneurography.

OBJECTIVE: The reconstruction of nerve impulse conduction along proximal lumbosacral plexus and nerve roots is compared using simultaneous magneto- and electroneurography. METHODS: In 3 healthy subjects the left tibial nerve was electrostimulated at the ankle. Evoked magnetic fields and electric surface potentials were measured simultaneously over the lumbosacral spine using a multichannel SQUID-detector with a planar measuring area and 25 surface electrodes covering a comparable area centered around L4. Based on either magnetic field or electric potential maps the depolarization front of the evoked compound action currents (CAC) was spatio-temporally reconstructed using a simple equivalent current dipole model in a half-space volume conductor. RESULTS: The mean signal-to-noise ratio in the magnetic (electric) recordings was around 4 (8). Yet, the localization quality for the propagating CAC was lower for electric than magnetic recordings. The local nerve conduction velocity was around 47 m/s (calculated from magnetic data), but fluctuated unphysiologically for electric data. CONCLUSION: In comparison to electroneurography, an anatomically reasonable localization of evoked compound action currents propagating in lumbosacral roots can be obtained by magnetoneurography.

Adult↗

PubMed2001

Magnetocardiographic intra-QRS fragmentation analysis in the identification of patients with sustained ventricular tachycardia after myocardial infarction.

The aim of this study was to investigate if magnetocardiographic (MCG) analysis of cardiac micropotentials within the QRS complex can identity patients prone to ventricular arrhythmias, and to compare it to MCG time-domain, late-field analysis. The study population consisted of 136 patients with remote MI, 53 with and 83 without a history of VT. After averaging and high pass filtering of multichannel MCG signals, time-domain parameters describing the end-QRS activity and fragmentation index M and score S describing the whole QRS complex were computed. Fragmentation and time-domain parameters differed between the VT and control groups: fragmentation index M was 12 +/- 3 versus 9 +/- 2 (P <0.001), fragmentation score S was 83 +/- 42 versus 56 +/- 21 (P < 0.001), and filtered QRS duration was 144 +/- 32 versus 114 +/- 19 ms (P < 0.001) in VT and control groups, respectively. A combination of fragmentation parameters yielded 87% sensitivity and 61% specificity in VT identification. Corresponding figures for a time-domain parameter combination were 81% and 72%. Sensitivity of time-domain analysis was 88% and specificity was 75% in a subgroup with anterior MI. In multivariate analysis, fragmentation and time-domain analyses discriminated VT patients from controls independently of the extent of coronary artery disease or left ventricular dysfunction. MCG in postinfarction patients reveals pathology associated with propensity to ventricular arrhythmias inside and not only at the end of the QRS complex. MCG seems most accurate in the anterior infarct location.

Aged↗

PubMed2001

Variability of the QRS signal in high-resolution electrocardiograms and magnetocardiograms.

The variability of electric and magnetic signals from the heart during the depolarization phase is investigated. A signal processing method is developed, which provides estimates for the beat-to-beat variability of the QRS-complex. The method is based on the decomposition of the depolarization signal into bandpass signals by means of the Morlet wavelet transform. The beat variability of the depolarization signal is estimated by normalized variances of the envelope and instantaneous frequency of bandpass signals. Time intervals of the bandpass filtered depolarization signals having a high signal-to-noise ratio are selected applying an analysis based on phase statistics. The method was tested by computer simulation and experimental data taken from electrocardiographic and magnetocardiographic measurements of healthy persons and patients prone to malignant ventricular tachycardia (VT) or ventricular fibrillation (VF). Results suggest that the calculated variance parameters permit the characterization of beat variable depolarization signals and distinguish VT/VF patients from healthy persons.

Algorithms↗

PubMed2000

Non-invasive magnetoneurography for 3D-monitoring of human compound action current propagation in deep brachial plexus.

Compound action current (CAC) propagation along nerve fibers running deep in the human brachial plexus was 3D-visualized based on non-invasive 49-channel superconducting quantum interference device (SQUID) magnetoneurography. Spatio-temporal mappings over the upper thoracal quadrant of magnetic fields (<100 fT) evoked upon alternating median and ulnar nerve stimulation in seven healthy volunteers showed consistently smoothly propagating dipolar patterns for both the CAC depolarization and repolarization phases. Multipolar current source reconstructions (i) distinguished spatially CAC propagation pathways along either median or ulnar plexus fibers, allowed (ii) to calculate local conduction velocities ( approximately 56 m/s) and (iii) even to estimate the CAC extension along the nerve fibers (depolarization phase: approximately 11 cm). Thus, for deep proximal nerve segments magnetoneurography can provide a detailed tracing of neural activity which is a prerequisite to localize non-invasively focal nerve malfunctions.

Action Potentials↗

PubMed2000

Hyperventilation-induced human cerebral magnetic fields non-invasively monitored by multichannel 'direct current' magnetoencephalography.

Self-paced hyperventilation (HV) induces slow cerebral magnetic field changes which were monitored and mapped continuously over 15 min using 49-channel DC-coupled ('direct current') magnetoencephalography (DC-MEG) based on a modulation technique. In nine/nine healthy subjects HV caused an increase (range: 1.1-6.2 pT) of the mean global DC-MEG field strength which slowly decayed after HV termination (mean time constant: 2 min). The complex HV-related field patterns were distinctly different from mainly dipolar somatosensory evoked field maps (N20m) in four/four subjects. Thus, current sources in the primary somatosensory cortex need not regularly dominate DC-field changes as had been previously considered. Rather, DC-MEG enabled the monitoring of a widely distributed HV-induced enhanced cortical excitability which may serve as model to study epileptic or post-anoxic cerebral hyperexcitability.

Adult↗

PubMed2000

Identification of post-myocardial infarction patients with ventricular tachycardia by time-domain intra-QRS analysis of signal-averaged electrocardiogram and magnetocardiogram.

A new time-domain analysis method, which quantifies ECG/MCG intra-QRS fragmentation, is applied to parts of the QRS complex to identify post-myocardial infarction patients with ventricular tachycardia. Three leads of signal-averaged electrocardiograms and nine leads of magnetocardiograms were band-pass filtered (74 Hz to 180 Hz). The filtered signals showed fragmentation in the QRS region, which was quantified by the number of peaks M and a score S, that is the product of M and the sum of the peak amplitudes. Both parameters were determined for the first 80 ms of the QRS complex and the total QRS complex in each channel. For classification, the mean-values of the parameters M and S of the three electrical leads and the nine magnetic leads were calculated. Late potential and late field analyses were performed for the same signals. 31 myocardial infarction patients were included, 20 of them with a history of documented ventricular tachycardia (VT). Identification of VT patients using the SAECG led to better results (sensitivity 95%, specificity 91%) considering the entire QRS complex than with the standard late potential analysis suggested by Simson (sensitivity 90%, specificity 73%). For the SAMCG and the entire QRS complex results using the parameters S and M are also better (sensitivity 95%, specificity 100%) than for the late field analysis (sensitivity 90% and specificity 100%). For the first 80 ms, the performance of the parameters M and S is only slightly decreased.

Adult↗

PubMed2000

Independent component analysis of noninvasively recorded cortical magnetic DC-fields in humans.

We apply a recently developed multivariate statistical data analysis technique--so called blind source separation (BSS) by independent component analysis--to process magnetoencephalogram recordings of near-dc fields. The extraction of near-dc fields from MEG recordings has great relevance for medical applications since slowly varying dc-phenomena have been found, e.g., in cerebral anoxia and spreading depression in animals. Comparing several BSS approaches, it turns out that an algorithm based on temporal decorrelation successfully extracted a dc-component which was induced in the auditory cortex by presentation of music. The task is challenging because of the limited amount of available data and the corruption by outliers, which makes it an interesting real-world testbed for studying the robustness of ICA methods.

Acoustic Stimulation↗

PubMed2000

Conversion of magnetocardiographic recordings between two different multichannel SQUID devices.

Comparison of biomagnetic measurements performed with different multichannel magnetometers is difficult, because differing sensor types and locations do not allow measurements from the same locations in respect to the body. In this study, two transformation procedures were utilized to compare magnetocardiograms (MCG) recorded with two different multisensor systems. Signals from one sensor array were used to compute parameters of a multipole expansion or minimum-norm estimates at 1-ms steps over the cardiac cycle. The signals of the second sensor array were then simulated from the computed estimates and compared against measured data. Both the multipole- and the minimum-norm-based transformation method yielded good results; the average correlation between simulated and measured signals was 93%. Thus, the methods are useful to compare MCG recordings performed using differing sensor configurations, e.g., for multicenter patient studies. This study provides the first empirical basis for assessing the transformation of MCG data of differing devices by general model-based field reconstructions.

Biomedical Engineering↗

PubMed1999

Non-invasive long-term recordings of cortical 'direct current' (DC-) activity in humans using magnetoencephalography.

Recently, biomagnetic fields below 0.1 Hz arising from nerve or muscle injury currents have been measured non-invasively using superconducting quantum interference devices (SQUIDs). Here we report first long-term recordings of cortical direct current (DC) fields in humans based on a horizontal modulation (0.4 Hz) of the body and, respectively, head position beneath the sensor array: near-DC fields with amplitudes between 90 and 540 fT were detected in 5/5 subjects over the auditory cortex throughout prolonged stimulation periods (here: 30 s) during which subjects were listening to concert music. These results prove the feasibility to record non-invasively low amplitude near-DC magnetic fields of the human brain and open the perspective for studies on DC-phenomena in stroke, such as anoxic depolarization or periinfarct depolarization, and in migraine patients.

Acoustic Stimulation↗

PubMed1999

Magnetometry of injury currents from human nerve and muscle specimens using superconducting quantum interferences devices.

Acute lesions of polarized membranes lead to slowly decaying ('near-DC') injury currents driven by the transmembrane resting potential gradient. Here we report the first recordings of injury-related near-DC magnetic fields from human nerve and muscle specimens in vitro using Superconducting Quantum Interference Devices (SQUIDs) operated in a conventional magnetically shielded room in a clinical environment. The specimen position was modulated sinusoidally beneath the sensor array by a non-magnetically fabricated scissors lift to improve the signal-to-noise ratio for near-DC fields. Depending on the specimen geometry the field patterns showed dipolar or quadrupolar aspects. The slow decay of human nerve and muscle injury currents was monitored for several hours from a distance of a few centimeters. Thus DC-magnetometry provides a sensitivity which might allow the remote detection of injury currents also in vivo.

Animals↗

PubMed1999

Magnetocardiographic analysis of the two-dimensional distribution of intra-QRS fractionated activation.

The spatial distribution of high-frequency components in magnetic signals during the QRS complex of the human heartbeat was investigated. Cardiomagnetic signals were recorded simultaneously using 49 first-order magnetogradiometer channels of a multi-SQUID system with a low noise power density. The QRS fragmentation score S, as a measure of the fragmentation of the bandpass-filtered QRS complex, was examined for its sensitivity and specificity to discriminate 34 healthy volunteers, 42 post-myocardial infarction patients and 43 patients with coronary heart disease and with a history of malignant sustained ventricular tachycardia or ventricular fibrillation. The multichannel information was visualized by two-dimensional mapping of the score values of the single channels. By averaging the score values for the seven central channels, S7, the score values of all 49 channels, S49, and calculating the standard deviation for all 49 channels, D49, a higher sensitivity and specificity for detecting patients with ventricular tachycardia (VT) or ventricular fibrillation (VF) was reached than by analysis of a single channel. Combination of these parameters furnishes a sensitivity of 90% and a specificity of 70% for identifying patients prone to VT/VF. The results were compared with diagnostic information obtained from the QRS duration of the signal as well as with results obtained by modified QRS integral mapping.

Adult↗