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| Paper IPM / Cognitive Sciences / 14225 |
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Introduction: In order to achieve higher image quality and speed, it is preferred to use high-strength, high-slew-rate gradient coils in imaging
applications1. The peripheral nerve stimulation (PNS), however, limits this fast gradient switching where it may result in a mild to intolerable
discomfort for the patient. IEC standards put limits on the gradient output to operate in the normal safe mode. For biological tissues, the IEC standard
for safe gradient switching is given by the following formula as the upper limit: ôô¤âôô = .8 à 20(ô¶âô)(1 + 360(ô¤ô)âô (ô¤ô)). Here ôô¤âôô, which
is a hyperbolic function of the duration of stimuli(t), is the allowed intensity of the gradient pulse for the normal safe mode2. According to3, the
required intensity for reaching a discomfortable state is 50much as the mild threshold. PNS may happen at lower gradients because of eddy currents in the vicinity of metallic foreign objects, adjacent to
sensitive locations. Such objects can intensify and concentrate the induced current by the gradient switching. In this study we evaluate the effect of
eddy current on defining the safe mode to prevent the PNS.
Theory and Methods: Eddy currents, although oppose the increase of the magnetic flux along the metallic object, in coplanar points outside of the
object further intensifies the magnetic field. The effect of eddy current is highest, when a conductor with a coin-geometry is placed perpendicular to
the time-varying magnetic field. It will result in the maximum induced current in the conductor and the maximum resultant dipole moment.
Considering a coin with a radius âaâ and thickness âdâ, placed in the transverse plane during the rise time of a gradient pulse, the induced current
density can be calculated, by combining Faradayâs and Ohmâs laws as in (Eq.1). Applying the dipole-approximation (Eq.2) (Eq.3), the overall
magnetic field is approximated for coplanar points (Eq.4) and its time variations expressed as Eq.5.
Result: To consider the maximum possible adverse effect of eddy current, we assumed a large diameter of about 30 mm with a thickness of d=1.5
mm for a metallic foreign object made from a highly conductive material with ôª = 10ô¬¼ôµ. The maximum value for the ôô¤ô¯§âôô was then estimated
through Eq.5 for common rise-times. The variation of normalized ôô¤ô¯§âôô in tô° = 200μs versus the radial distance from the coin origin (in and out of
the coin plane) is shown in Fig.1. The new safe mode values for applicable ôô¤ô¬´âôô, are calculated and shown in figure.2 under the aforementioned
circumstances.
Discussion and conclusion: According to Eq.5 the applied gradient intensity reaches its maximum value in the coin plane where = ô½ . According to
Fig.1 which denotes the ((ôô¤ô¯§ ôô â ) (ôô¤ ô¬´
â âôô)) (t = 200μs) versus ô, this extra gradient drops fast with a 1âôô¬· factor such that at radial distance
ô = 2ô½, the external metallic object intensifies the applied ôô¤ô¬´âôô only by a factor of 1.10. So if the external objects located far enough from the
sensible location; there is no major threat and discomfort for the patient. Fig.2 shows that for ôô = 100ô¤ô¯¦, 200ô¤ô¯¦, 300ô¤ô¯¦, the ôô¤ô¯§âôô (ô¢ô¢ô¢) can
respectively reach 2.76,1.88,1.58 times greater values and therefore may change the comfort zone with painful or even intolerable situation for the
patients. The interesting outcome of the study is that the new limit for ôô¤ô¬´âôô (considering the metal induced eddy current) is much less sensitive to
gradient rise time compared to original limit set by the IEC standard (Fig.2). The current analysis is valid not only for non-magnetic metals but also
for most ferromagnetic objects as they are saturated in the static magnetic field.
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