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6. Timevarying (gradient) magnetic fields (dB/dt)

6.1 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 gradients’ fields change rate. The main concerns associated with time-varying magnetic fields are biological effects and acoustic noise.

6.2 Biological effects

6.2.1 Subjecting the human body to time-varying electromagnetic fields leads to induced electric fields and circulating currents in connective tissues. 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. 

6.2.2 Peripheral nerve and muscle stimulation

At low frequencies, induced currents can produce stimulation of nerve and muscle cells.19  The body is most sensitive at up to about 5 KHz. Extreme cases can result in limb movement or ventricular fibrillation.
 
6.2.3  Implant interaction

Time-varying magnetic field gradients can interact with implants. This may result in device heating and vibration.2

  • Reference to exposure limits and relevant standards should be made and can be found in the MHRA guidelines.2

6.3  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.  PHE15 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.

  • It is recommended that departments provide adequate hearing protection to ALL patients and others remaining in the scan room eg carers, anaesthetic staff, etc.
  • Radiography staff should be trained in the selection and fitting of hearing protection.
  • MR operators should be aware of noise reducing protocols and trained in their utilisation particularly for those patient groups who are sensitive.

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