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M Burghoff

Publications and source records attributed to M Burghoff.

At least 19 recordsLinked to original sources
PubMed2005

A template-free approach for determining the latency of single events of auditory evoked M100.

The phase of the complex output of a narrow band Gaussian filter is taken to define the latency of the auditory evoked response M100 recorded by magnetoencephalography. It is demonstrated that this definition is consistent with the conventional peak latency. Moreover, it provides a tool for reducing the number of averages needed for a reliable estimation of the latency. Single-event latencies obtained by this procedure can be used to improve the signal quality of the conventional average by latency adjusted averaging.

Evoked Potentials, Auditory↗
PubMed2004

Technique for the direct measurement of DC-like magnetic biosignals demonstrated by the cold reflex of the abdomen.

Very low frequency dc-like signals, such as the cold reflex, could only be measured up to now by moving the subject repeatedly, up to the magnetic detector. PTB's novel magnetically shielded room BMSR 2, together with a low noise 16 channel SQUID magnetometer, allow the recording of dc-like signals without moving the subject; these are direct measurements. The total observed magnetic drifts are limited by 1/f-noise and external disturbances to a value below 6 pT/h. The measurement is continuous in time, therefore provides frequency resolution from dc to several kHz. This allows us to also observe the changing pattern between two different static magnetic states. As an example, the measurement of the cold reflex of the abdomen is shown and discussed. Not only the expected cold reflex, but other periodic and spontaneous signals from the human body can be seen with this method.

Abdomen↗
PubMed2004

Spatial distribution of cardiac magnetic vector fields acquired from 3120 SQUID positions.

An extended measurement of the magnetic vector field of the human heart is presented. It is acquired by sequential recordings, shifting a 16 SQUID vector magnetometer across 195 positions over a healthy subject's thorax. The magnetocardiographic (MCG) signals were synchronized using a simultaneously measured ECG channel. The registration of the field extends over a volume of 1000 mm x 600 mm x 420 mm sampled at 3120 SQUID positions. We present diagrams of the vector amplitude of selected points in 6 planes at increasing distances from the frontal thorax. Each plane contains 76 vector points. Additionally, we measured the vector field at 126 points lateral to the chest. At the edge points of the measurement volume, the absolute value of the magnetic vector signal amplitude exceeds 0.3 pT in all measurement points. The dataset provides an excellent base to study dedicated MCG detection or rejection methods. Examples where rejection of the heart signal is necessary are magnetoencephalography, magnetoneurography and fetal MCG. The knowledge of the spatio-temporal distribution of the magnetic vector field of the heart supports the development and comparison of multi-SQUID systems and will be used to create new MCG interpretation and representation algorithms.

Body Surface Potential Mapping↗
PubMed2004

Separation of fetal and maternal magnetocardiographic signals in twin pregnancy using independent component analysis (ICA).

The identification of fetal and maternal signals in magnetocardiograms (MCG) is central to data preprocessing and a prerequisite for data analysis and assessment. This is usually done by creating a template of the signal to be identified and marking data segments correlating to this template before averaging. This procedure is not only cumbersome, but may also lead to problems when there are several overlapping signals of interest such as in MCG recording in single or, more so, in twin pregnancy. Independent component analysis (ICA), which uses higher order statistics to decompose the signal into statistical independent components, has already been used in single pregnancies to distinguish between maternal and fetal signals. We applied the ICA algorithm TDSEP to 9 data sets of twin pregnancies acquired between the 28th and 38th week of pregnancy. Resulting ICA components can be used for further data analysis, e.g., for finding robust triggers or estimating the heart rate and its variability of the twins. The results showed that the maternal and fetal components can be separated from each other as well as from other sources of noise and artifacts. Differences between averaged ICA time curves and averaged raw data are not significant. Limitations include a concurrence of heart rates and changes in signal morphology due to gross movement. Nonetheless, ICA offers a fast and efficient approach for the preprocessing of MCGs with multiple signals of interest.

Electrocardiography↗
PubMed2004

Discrimination of multiple sources using a SQUID vector magnetometer.

For many biomagnetic applications the discrimination between simultaneously active sources is required. To evaluate the performance of a given SQUID system in this respect, the angle between the signal vectors of different sources is used. If the angle reaches large values, discrimination between the multiple sources is possible. We tested this approach with the first module of a new vector magnetometer system consisting of 19 identical modules. Two examples of measurements illustrate the differentiation of multiple sources, i.e. the fetal and the mother's heart signal, and alpha rhythm and heart signal in MEG recordings. This first module of a vector magnetometer system containing 16 SQUIDs is operated at PTB in the new Berlin Magnetically Shielded Room (BMSR 2) The spatial configuration of the 16 integrated SQUID magnetometers of the module is such that all three vector components of the magnetic field can be calculated in three measurement planes at 1.5 cm, 5 cm, and 10.5 cm above the Dewar bottom, respectively. The SQUID magnetometer channels have a typical white noise level of less than 2.3 fT/square root of Hz1/2 at 1 kHz.

Electromagnetic Fields↗
PubMed2004

A sensor configuration for a 304 SQUID vector magnetometer.

A novel SQUID vector magnetometer system is introduced which has been specially designed for the use inside the strongly magnetically shielded room BMSR-2 of PTB. The system is housed in a dewar with a flat bottom and an inner diameter of Ø 250 mm. The SQUIDs are arranged so that in addition to the usually measured Z-component of the field the horizontal magnetic fields are measured too. A total of 304 DC-SQUID magnetometers are divided up into 19 identical modules. The 16 low-Tc SQUIDs of each module are located in such a way that an estimation of the magnetic field in all three dimensions is possible at three points inside the module. The 57 SQUIDs of the lowest Z plane of all modules form a hexagonal grid with a base length of 29 mm. The design criteria and the physical principle behind the complex SQUID arrangement are explained.

Electromagnetic Fields↗
PubMed2004

Systematic latency variation of the auditory evoked M100: from average to single-trial data.

Standard analyses of neurophysiologically evoked response data rely on signal averaging across many epochs associated with specific events. The amplitudes and latencies of these averaged events are subsequently interpreted in the context of the given perceptual, motor, or cognitive tasks. Can such critical timing properties of event-related responses be recovered from single-trial data? Here, we make use of the M100 latency paradigm used in previous magnetoencephalography (MEG) research to evaluate a novel single-trial analysis approach. Specifically, the latency of the auditory evoked M100 varies systematically with stimulus frequency over a well-defined time range (lower frequencies, e.g., 125 Hz, yield up to 25 ms longer latencies than higher frequencies, e.g., 1000 Hz). Here, we show that the complex filtering approach to single-trial analysis recovers this key characteristic of the M100 response, as well as some other important response properties relating to lateralization. The results illustrate (i) the utility of the complex filtering method and (ii) the potential of the M100 latency to be used for stimulus encoding, since the relevant variation can be observed in single trials.

Acoustic Stimulation↗
PubMed2002

[High resolution EEG and whole head MEG].

A low-noise multichannel EEG-Amplifier-System has been designed and analyzed, which is electromagnetically compatible with SQUIDs. The EEG- together with the MEG-System is operated inside the quiet environment of a magnetically shielded room, hence no electrical 50 Hz-Artifact is detectable. Measurements pointed out that the influence of amplifier- and electrochemical-noise of electrodes to averaged EEG-spectral densities is less than 1% within the frequency range 0.5 to 70 Hz. Thermal noise of the skin-electrode-interface, equivalent to a resistor of 30 k omega, only begins to take effect above 20 Hz, suggesting that there is no need of skin abrasion.

Alpha Rhythm↗
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↗
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

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↗
PubMed1998

Magnetoneurographic 3D localization of conduction blocks in patients with unilateral S1 root compression.

OBJECTIVES: Tibial nerve somatosensory evoked magnetic fields (tSEFs) over the lower back reflect the propagation of compound action currents along fibers of plexus, nerve roots and cauda equina. One clinical perspective for this 'magnetoneurography' is the non-invasive 3D localization of focal slowing or blocks of conduction. Here, first tSEF mappings in 3 consecutive patients with acute unilateral S1 nerve root compression are reported. METHODS: Right and left tibial nerves were electrostimulated in alternation; tSEF responses were recorded using a multichannel SQUID-detector; additionally, spinal and cortical SEP, F-wave and H-reflex studies were performed. RESULTS: In all patients an intraindividual side-to-side comparison of spinal tSEF mappings was obtained: using a dipolar source model compound action currents could be visualized propagating along plexus, nerve roots and cauda equina on the non-affected side whereas on the affected side normally-propagating dipolar field patterns could be recorded only distal to the spinal transforaminal root entrance; this reflects focal slowing or block of conduction in nerve root fibers as indicated by the SEP, F-wave and H-reflex study results. CONCLUSIONS: With a registration time of 15 min a 3D localization of proximal slowing or block of conduction was successfully performed in patients suffering from acute nerve root lesions.

Cauda Equina↗