Biomedical subjects

T Weyh

Publications and source records attributed to T Weyh.

5 recordsLinked to original sources

PubMed2000

Influence of pulse configuration and direction of coil current on excitatory effects of magnetic motor cortex and nerve stimulation.

UNLABELLED: This paper describes the influence of pulse configuration and current direction on the excitation of the hand-associated motor cortex and the median nerve by magnetic stimulation. Monophasic and biphasic current pulses with the same peak rise time of 80 micros and a maximum rate of current change (dI/dt) were discharged through an eight-shaped coil of the stimulator used (Dantec MagPro). Two current directions with opposite orientation in the coil axis were studied. FINDINGS: (1) for both, cortex and nerve stimulation, biphasic stimuli were more effective and elicited compound muscle action potentials (CMAPs) with lower thresholds and larger amplitudes. (2) Using biphasic pulses the direction of the currents in the first phase of the pulse did not influence the CMAP amplitude. (3) Using monophasic pulses induced currents oriented postero-anteriorly in the motor cortex or orthodromically along the nerve axis elicited larger CMAPs than currents in the opposite orientation. (4) Pulse configuration did not influence the CMAP-latencies and by this the stimulation site (cortex, nerve). CONCLUSION: Monophasic stimuli are useful to investigate excitation effects which are dependent on the current direction. The application of biphasic stimuli with their stronger excitation effects might be advantageous when patients with high cortical thresholds or deep lying nerves shall be investigated.

Action Potentials↗

PubMed1995

The boundary effect in magnetic stimulation. Analysis at the peripheral nerve.

The optimal stimulus position for a figure-8-shaped coil for magnetic stimulation of the ulnar nerve at the wrist was not coincident with the optimal electrical stimulus point but was shifted 18.3 mm to the ulnar side (P < 0.01). For the median nerve the optimal stimulus site was 9.6 mm radial to the optimal position for electrical stimulation (P < 0.05). This shift of the stimulus point for magnetic stimulation is significantly smaller after interposition of a homogenous electrically conducting medium between coil and arm but not changed after interposition of distilled water. This so-called boundary effect is therefore due to the different conductivities of the medium interposed between coil and nerve. It may also distort precise localisation of other excitable structures such as cranial nerves, nerve roots and cortical areas by means of magnetic stimuli. The amplitudes of the compound muscle action potentials elicited with identical magnetic stimulus strength were larger after the interposition of isotonic solution between coil and skin but not after interposition of distilled water. Consideration of the boundary effect provided an improved response amplitude to magnetic stimulation, but this could not adequately compensate for its poor localisation compared to electrical stimulation.

Adult↗

PubMed1994

[High-performance biosignal amplifier for electrophysiologic studies for special safety requirements].

For the recording of minute bioelectric signal voltages in the fields of medical research and diagnostic applications, a high-performance amplifier is needed to provide the signal levels necessary for a significant analysis. Irrespective of the concrete application, the major parameters required for a comparative evaluation of such systems are sensitivity, bandwidth and noise. Some applications also make special demands on electrical safety measures. With the aim of using an amplifier with a particularly powerful magnetic stimulator, and in view of the special requirements for brain voltage recording in a fully conscious patient, a universally applicable amplifier system has been developed that incorporates safety measures that ensure virtually risk-free application. The stimulation artefact can be suppressed completely by means of automatic offset regulation of the preamplifier. The circuit elements in electrical contact with the organism are separated from the rest of the measuring system by a high-voltage insulation barrier. The main innovation and central subject of this article is an active protective system with an extremely short response time, which reliably cuts off the relevant amplifier circuits as soon as the ceiling signal level is exceeded. In accordance with relevant regulations, the user is warned of any first failure by a program-controlled self-test run by the protective unit and done automatically when the amplifier is activated. Exhaustive tests with a prototype have confirmed, among other things, that the integration of the new protective device does not reduce the amplifier output to any measurable extent.

Amplifiers, Electronic↗

PubMed1993

Artifact reduction in magnetic stimulation.

A new biopotential amplifier is described in which the stimulus artifact from recordings of electrically or magnetically evoked biopotentials is minimized by a special active filter. The amplifier is tested in different experiments and the recordings are compared to standard EMG recordings.

Action Potentials↗

PubMed1993

[Development, optimization and evaluation of new instruments for magnetomotor stimulation of nerve fibers].

Over the past few years, magnetomotive nerve stimulation--an almost painless procedure--has become increasingly important for both diagnosis and therapy. However, the actual efficiency of this method is highly dependent on the technology available. For this reason, current research programmes are focussed on the development of new devices. Individual components can be optimized only with respect to a particular application. As a means of stimulating deeper-lying nerve fibres, a powerful air coil was developed. A second, optimally focussed, air coil permits the selective stimulation of closely defined areas. For the operation of the two coils, a power source has been specially designed to provide particularly high-energy pulses, as well as an option for repetitive stimulation. This unit was designed to comply with relevant regulations, and incorporates effective safety functions, some employing redundancy backup. Within the performance limits of the device, the configuration of the stimulation pulses is to a large extent freely programmable. In addition to standard applications, this power supply unit opens up new fields of application for magnetomotive stimulation. The first series of tests carried out with this new stimulation system confirmed--among other things--the achievement of the desired depth effect of the air coil. By virtue of its highly focussed effect, the second coil has proved to be particularly suitable for stimulating the motor cortex.

Electromagnetic Fields↗