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4. Time-varying (gradient) magnetic fields (dB/dt)


Time-varying magnetic field gradients in MR systems provide position-dependent variation in magnetic field strength.  The gradients are pulsed and the faster the sequence of imaging, the greater the gradient fields change rate.  The main concerns associated with time-varying magnetic fields are biological effects and acoustic noise.

4.2  Biological effects


Subjecting the human body to time-varying electromagnetic fields leads to induced electric fields and circulating currents in connective tissues.  At frequencies above 1MHz, a reactive element begins to be significant and at frequencies above 30 MHz, the wavelength begins to influence the electric field and current distribution.7 Induced electric currents can be sufficiently large to interfere with normal function of nerve cells and muscle fibres.  An example of this is the sensation of flashes of light caused by induced currents stimulating the retina.  A more serious response is that of ventricular fibrillation.

4.2.2 Peripheral nerve and muscle stimulation

At low frequencies, induced currents can produce stimulation of nerve and muscle cells.8 The body is most sensitive at up to about 5 kHz.  Extreme cases can result in limb movement or ventricular fibrillation.  Reference to exposure limits should be made and can be found in the MHRA guidelines.2

4.3 Acoustic noise

Acoustic noise caused by the gradient coils switching on and off during the scan can reach unacceptable levels. In general, the higher the field strength, the higher the acoustic noise level, but this effect is not exponential and is also dependent on pulse sequence.  The HPA4 reports that the threshold of instantaneous and permanent acoustic trauma normally associated with exposure to impulsive noise is 140dB in adults – children may have a lower threshold and maximum peak levels of 120dB are advised.

13695342-exclamation-point.jpg Departments should provide adequate hearing protection to ALL patients.  The MHRA2 also advises that ear protection should be provided for others in the scan room if the levels reach 80dBA.

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