Magnetostimulation in MRI

Irnich, Werner; Schmitt, Franz

doi:10.1002/mrm.1910330506

Cited by 90 publications

(113 citation statements)

References 8 publications

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“…The true amplitude is given by the integral of Eq. [11]. Dividing it by the same integral but now over Ϯ T/2 yields a correction factor F cor according to Eq.…”

Section: Inserting the Normalization Parameters Into Eq [14] Yieldsmentioning

confidence: 99%

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Stimulation threshold comparison of time‐varying magnetic pulses with different waveforms

Irnich¹,

Hebrank²

2008

Magnetic Resonance Imaging

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Purpose: To clarify whether sinusoidal pulses possess lower thresholds than rectangular ones at perception threshold, a statement often made that contradicts the theory of stimulation. Materials and Methods:The results of a nerve stimulation study with 65 volunteers and with trapezoidal and sinusoidal gradient pulses were used to apply the combination of the electric field, induced in the tissue of the human body, with the "Fundamental Law of Electrostimulation." This law claims that the waveshape of a pulse is not essential as long as the amplitude of the pulse does not decrease below rheobase (rheobase condition). Results:If the rheobase condition is applied to sinusoidal waveforms and the pulse duration and amplitude is corrected accordingly, both trapezoidal and sinusoidal gradient pulses have identical threshold amplitudes as a function of pulse duration. Conclusion:The "Fundamental Law of Electrostimulation," including the "rheobase condition," proved to be a good basis for describing magnetic field stimulation (magnetostimulation) and that application of it to magnetostimulation is suitable as the basis for describing magnetic field stimulation with various waveforms. For nonrectangular pulses, pulse durations and pulse amplitudes must be corrected according to the "rheobase condition." The exponential Blair Equation is less suited to be applied in magnetostimulation. NATIONAL AND INTERNATIONAL BODIES have discussed possible hazards due to gradient switching in magnetic resonance imaging (MRI) systems and have formulated limiting exposure levels of magnetic fields (1). KeyHowever, there is still interest in understanding the process of stimulation in human subjects exposed to timevarying magnetic fields as they are used in MRI. The need to avoid painful peripheral nerve stimulation (PNS) or, even worse, cardiac stimulation, due to rapidly switched magnetic fields sets an upper limit on the magnetic field gradient strengths that can be employed in fast MRI. It is generally recognized that in whole-body imaging the ygradient coil causes PNS at the lowest rates of gradient change (2,3). A search of the literature of which electric fields can stimulate the heart revealed that the rheobase field strength of the heart for single monophasic stimuli is Ϸ60 V/m (4), and thus, 10 times higher than the lowest assumed threshold for 20 m diameter nerves with 6.2 V/m (5). Due to the fact that cardiac magnetostimulation has not yet been reported in humans and is not expected with the hardware currently available, the following considerations are focused on PNS, the first sensations that appear in whole-body MRI.Since publication of our first article on electrostimulation by time-varying magnetic fields (4), it is mostly accepted in the MRI literature that it is the electric field and not the current density that is responsible for magnetostimulation and that there is a hyperbolic relationship between pulse duration and the electric field characterized by the terms rheobase and chronaxie as they were introduced by Lapicq...

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“…The true amplitude is given by the integral of Eq. [11]. Dividing it by the same integral but now over Ϯ T/2 yields a correction factor F cor according to Eq.…”

Section: Inserting the Normalization Parameters Into Eq [14] Yieldsmentioning

confidence: 99%

“…This view is not self-evident, as one of our articles on this topic was initially rejected because of the use of "old-fashioned" terms and laws (11). Instead, a capacitor-based model was recommended that is still in use, and which is known as the "Blair model" (12).…”

mentioning

confidence: 99%

Stimulation threshold comparison of time‐varying magnetic pulses with different waveforms

Irnich¹,

Hebrank²

2008

Magnetic Resonance Imaging

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“…The expressions and variables described above are new, and it is more common in the literature to express magnetostimulation thresholds in terms of the change in magnetic flux density, ⌬B, as a function of (4,5). Because gradient strength and magnetic flux density are always related by a constant factor (6), Eq.…”

Section: Theorymentioning

confidence: 99%

“…[2]. ⌬B min was previously introduced in this context by Irnich et al (4). The definition of dB/dt min is implicit in the forms introduced by Bourland et al (5), and using their notation, dB/ dt min would be equal to (b/c), where b is ⌬B min and c is the chronaxie time ( c ).…”

Section: Theorymentioning

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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“…Regulatory standards (6,30 -32) presently adjust for stimulation threshold differences between sinusoidal and trapezoidal pulse trains (16,18,43,46,48,49) by defining d (see Eq. [3]) as the half-period of a sinusoid or the ramp time (from minimum to maximum gradient amplitude) of a trapezoid.…”

Section: Theoretical Stimulation Modelsmentioning

confidence: 99%

Review of Patient Safety in Time-Varying Gradient Fields

Schaefer¹,

Bourland²,

Nyenhuis³

2000

J. Magn. Reson. Imaging

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In magnetic resonance, time-varying gradient magnetic fields (dB/dt) may stimulate nerves or muscles by inducing electric fields in patients. Models predicted mean peripheral nerve and cardiac stimulation thresholds. For gradient ramp durations of less than a few milliseconds, mean peripheral nerve stimulation is a safe indicator of high dB/dt. At sufficient amplitudes, peripheral nerve stimulation is perceptible (ie, tingling or tapping sensations). Magnetic fields from simultaneous gradient axes combine almost as a vector sum to produce stimulation. Patients may become uncomfortable at amplitudes 50%-100% above perception thresholds. In dogs, respiratory stimulation has been induced at about 300% of mean peripheral nerve thresholds. Cardiac stimulation has been induced in dogs by small gradient coils at thresholds near Reilly's predictions. Cardiac stimulation required nearly 80 times the energy needed to produce nerve stimulation in dogs. Nerve and cardiac stimulation thresholds for dogs were unaffected by 1.5-T magnetic fields.

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Magnetostimulation in MRI

Cited by 90 publications

References 8 publications

Stimulation threshold comparison of time‐varying magnetic pulses with different waveforms

Stimulation threshold comparison of time‐varying magnetic pulses with different waveforms

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

Review of Patient Safety in Time-Varying Gradient Fields

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