Stimulation threshold comparison of time-varying magnetic pulses with different waveforms.

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.

Increased time rate of change of gradient fields: effect on peripheral nerve stimulation at clinical MR imaging.

PURPOSE: To increase gradient stimulation from 100% to a fixed 120% level and to assess patient acceptance of the degree of peripheral nerve stimulation (PNS) at magnetic resonance (MR) imaging. MATERIALS AND METHODS: Two hundred ten patients underwent MR imaging of various body regions according to clinical indications. An additional three-dimensional fast low-angle shot sequence with the 120% stimulation level was performed. A patient questionnaire was distributed after MR imaging to document the presence, degree, and location of PNS. Degree was measured with an 11-point scale (score range, 0-10). Age was analyzed between the sexes for significant statistical differences. Furthermore, correlation between location of examination and location and degree of stimulation was performed. To determine stimulation discomfort relative to other factors typically present at MR imaging, the degree of discomfort due to room temperature, size of magnet bore, acoustic noise, examination time, and heating sensation was determined for comparison, as well. RESULTS: Thirty-five (16.7%) patients reported a stimulation sensation during imaging in one or more locations, while six (2.9%) felt very uncomfortable local stimulation (score of 8-10). No significant difference between male and female patients regarding age, sex, and appearance or degree of stimulation sensation could be detected. No significant correlation between location of examination and location and degree of stimulation was recorded. Compared with other side effects, PNS was considered relatively unimportant. CONCLUSION: The 120% gradient stimulation level seems acceptable for routine clinical imaging with this gradient system, since only 2.9% of patients experienced very uncomfortable local stimulation.

Comparison of the threshold for peripheral nerve stimulation during gradient switching in whole body MR systems.

PURPOSE: To compare thresholds for peripheral nerve stimulation from gradient switching in whole body magnetic resonance (MR) equipment of different design. MATERIALS AND METHODS: Threshold data obtained in three experiments were reformatted into a single joint data set describing thresholds for anterio-posterior (AP) gradient orientation and Echo Planar Imaging (EPI) waveforms with bipolar ramp times between 0.07 and 1.2 ms. Reformatting included the use of: a) the rate of change of the maximum field in the patient space as a measure of gradient output, b) lowest observable thresholds, c) lognormal distribution of thresholds, and d) equal standard deviation (SD) of all samples. RESULTS: The joint data fit a hyperbolic threshold function. The residues were not significantly different between experiments. CONCLUSION: Then expressed in appropriate format, the thresholds for peripheral nerve stimulation in volunteers for whole body MR equipment can be described with a hyperbolic threshold curve with rheobase 18.8 +/- 0.6 Tesla/second and chronaxie 0.36 +/- 0.02 milliseconds.