Analytic calculations of the E-fields induced by time-varying magnetic fields generated by cylindrical gradient coils

Bowtell, Richard; Bowley, R. M.

doi:10.1002/1522-2594(200011)44:5<782::aid-mrm16>3.0.co;2-7

Cited by 46 publications

(57 citation statements)

References 12 publications

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“…It has been shown that for typical gradient coil switching frequencies and tissue conductivity, scalar potentials can contribute significantly to the electric fields produced within biological tissues (12,19). Very recently an exact analytic solution (including scalar potential effects) to the problem of calculating the electric fields induced within an infinite conducting cylinder from an arbitrary cylindrical gradient coil was presented (20). Accurate calculation of induced electric fields within realistic representations of the human body requires finite element electromagnetic models.…”

Section: Figmentioning

confidence: 99%

A comparison between human magnetostimulation thresholds in whole‐body and head/neck gradient coils

Chronik

Rutt

2001

Magnetic Resonance in Med

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Gradient coil magnetostimulation thresholds were measured in a group of 20 volunteers in both a whole-body gradient coil and a head/neck gradient coil. Both coils were operated using both x and y axes simultaneously (xy oblique mode). The waveform applied was a 64-lobe trapezoidal train with 1-ms flat-tops and varying rise times. Thresholds were based on the subjects' perception of stimulation, and painful sensations were not elic- The mean threshold curves were combined with the gradient system performance curves to produce operational limit curves. The operational limit curves for the head/neck coil system were verified to be higher than those of the whole-body coil; however, the head/neck system was also found to be physiologically limited over a greater range of its operation than was the body coil. Subject thresholds between the two coils were not well correlated. Magn Reson Med 46: 386 -394, 2001.

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Section: Figmentioning

confidence: 99%

A comparison between human magnetostimulation thresholds in whole‐body and head/neck gradient coils

Chronik

Rutt

2001

Magnetic Resonance in Med

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“…The largest electric fields experienced by patients occur below the eyes of the coil according to calculations given by Bowtell and Bowley [2]. Here the direction of the vector potential follows the circulation of the current about the eyes, leading to large, circulating electric fields close to the surface of the patient coming from the vector potential.…”

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confidence: 98%

Controlled E-field gradient coils

Mansfield

Bowley

Haywood

2003

Magn Reson Mater Phy

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Peripheral neural stimulation is a major problem in current gradient coil designs. Induced current problems in patients relate directly to gradient strength and modulation frequency. Current designs of gradient coil tend to limit ultra-high-speed imaging methods such as echo-planar imaging through the effect of induced currents which produce tingling sensations and involuntary muscle twitch. Neural stimulation could also trigger epileptic fits and/or cardiac fibrillation. For reduction of induced currents, an important aspect is the coil geometry. It is desirable to design the gradient coil in such a way as to prevent closed loop circulating currents within the body. Preliminary results using a four-sector gradient coil with rectangular geometry, operating in a low mutual coupling mode, indicate significant reduction in the E-field within the subject volume of the coil. Reduction in induced currents in the patient allows safer operation at higher magnetic field strengths together with faster scans currently prohibited through neural stimulation effects in standard coil geometries.

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“…Echo-planar imaging employs gradient coils capable of a maximum amplitude of 20mT/m, a minimum rise time of 0.1ms, a slew rate of 200T/m per second, and a duty cycle of 50%-60% [94]. However, the switching rate is limited for safety consideration because fastswitching gradients induce a time-varying electric field that may cause peripheral nerve stimulation (PNS) at sufficiently high amplitudes [103][104][105]. Parallel imaging methods (pMRI) offer a decrease in acquisition time while maintaining the same resolution and signal to noise ratio (SNR).…”

Section: Image Acquisition and Reconstruction Methodsmentioning

confidence: 99%