Effects Of Coil Position Research Articles

Effect of Coil Position on Magnetization Dynamics of Multilayered Hard Disk Writer Yokes

Three partially built hard disk writer structures with active coil windings located at different positions relative to the yoke were studied by time resolved scanning Kerr microscopy. All three Cartesian components of the magnetization were recorded simultaneously. Time resolved signals recorded near the pole tip showed that the component of magnetization parallel to the driving field had the same rise time irrespective of the location of the coil winding, but largest amplitude when the active coil was located near the middle of the yoke. Dynamic images obtained from three devices showed “flux beaming” in a channel parallel to the driving field. The magnetic contrast was strongest when the active coil was located near the centre of the yoke, while relaxation after removal of the excitation was most complete when the active coil was located near the confluence region. These results confirm the need for a multiturn coil to ensure effective flux propagation along the entire length of the yoke.

Cortical Transcranial Magnetic Stimulation of the Frontalis Muscle: Characteristic Features and Details on Motor-Evoked Potentials of the Forehead

The effect of coil position on the head surface along the interaural line on motor-evoked potentials (MEPs) of the frontalis muscle due to cortical transcranial magnetic stimulation was investigated in 16 healthy subjects. Bilateral reproducible responses could be observed in all subjects investigated at coil positions varying from 2 to 12 cm lateral to the vertex. MEP amplitudes of the frontalis muscle offered no significant side differences neither in amplitude nor in onset latency. Despite a considerable overlap, a statistically significant separation (p < 0.0001) of the two areas from which reproducible MEPs of upper and lower mimetic muscles could be elicited was evident for the calculated mediolateral center with the frontalis muscle area being placed more medially. Our findings are in accordance with anatomical studies indicating a predominance of corticonuclear descending fibers to lower but not to upper facial motoneurons.

Effect of coil position and stimulus intensity in transcranial magnetic stimulation on human brain

Evoked spinal cord potentials (ESCPs) from the cervical and high thoracic epidural space following transcranial magnetic stimulation were recorded from eight subjects in awake and anesthetized condition. Motor evoked potentials (MEPs) from the right abductor digiti minimi (ADM) and rectus femoris (RF) muscles were simultaneously recorded during voluntary contraction. The stimulus intensity was at 30% above the MEPs threshold of the ADM when the coil center was fixed on 10–20 international Cz position. In awake condition, multiple ESCP components (greater than 3) were recorded from the cervical epidural space but no or minimal components were recorded from the upper thoracic epidural space. When the coil was moved anteriorly so that the posterior edge of the coil was positioned on Cz, the amplitude of the first ESCP component was significantly increased ( P < 0.02) and shortened (not significant) at cervical levels. In addition, several ESCP components were more evident at high thoracic levels. Although the amplitude of the ADM was not enhanced, that of the RF was enhanced. During general anesthesia with volatile anesthetics (sevoflurane), only the first component of the ESCPs (D-wave) was elicited. Its amplitude was enhanced ( P < 0.02) when the coil edge was fixed on Cz, similar to results in awake condition. This enhancement of the first ESCP component was accompanied by enhancement of those recorded from the high thoracic epidural space. However the amplitude of D-wave was the same in the two different coil positions when the stimulus intensity was set at 100% of the output. These results suggest that at low stimulus intensity, positioning the coil edge on Cz is optimal in inducing D-wave effectively but at high stimulus intensity, D-wave generation can be achieved in either if the two different coil position.

Mapping location of excitation during magnetic stimulation: effects of coil position.

We determined the location of excitation for different positions of a round and butterfly coil during in vitro magnetic stimulation of cut peripheral nerves. We analyzed the conditions under which excitation occurs, either at the termination or at the peak of the field gradients (first spatial derivative of the electric field). These results were then compared to predictions about the location of excitation sites from a theoretical model of magnetic stimulation of finite neuronal structures. Excitation along a straight nerve occurred at terminations when 1) a coil was positioned close to the end of a nerve (at least one diameter length from the end), 2) a nerve ended in a finite terminating impedance much greater than the axial resistance of the nerve, 3) the induced electric field was of sufficient magnitude, pointing in a direction away from the axis of a nerve. Excitation occurred at the negative peak of the field gradients along a nerve when 1) a coil was positioned far away from the ends of a nerve, 2) there were no geometric or volume conductor inhomogeneities around a nerve, and 3) it was of sufficient magnitude. Threshold strengths for excitation at terminations were significantly lower than that for field gradient excitation and comparable to that due to geometric and volume conductor inhomogeneities.