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7. Radio-Frequency radiation (B1)

7.1 Biological effects

Exposure to radio-frequency (RF) radiation results in increased oscillation of molecules and generation of heat.  Dissipation of this heat occurs through the dilatation of blood vessels and increased blood flow. Avascular structures are therefore less efficient in removing this heat. RF exposure of patients is usually characterized by means of the ‘specific energy absorption rate’ (SAR), which is defined as the average energy dissipated in the body per unit of mass and time.17

The International Commission on Non-Ionizing Radiation Protection  (ICNIRP) summarises: For whole-body exposures, no adverse health effects are expected if the increase in body core temperature does not exceed 1°C.  In the case of infants and persons with cardiocirculatory impairment, the temperature increase should not exceed 0.5°C. With regard to localised heating, it seems reasonable to assume that adverse effects will be avoided with a reasonable certainty if temperatures in localised regions of the head are less than 38°C, of the trunk less than 39°C, and in the limbs less than 40°C.17

However, good practice should mean that RF deposition should be minimised in all patients. An accurate patient weight and height (if required) should be input into the system and manufacturer software will alert scanner operators to high SAR sequences.

  • All patients should be weighed prior to scanning in accordance with manufacturers guidelines.
  • The patient’s height may also need to be recorded depending on the manufacturer’s guidelines.

7.1.1 SAR Limits

SAR limits have been defined by the International Electrotechnical Commission (IEC)20 and ICNIRP16.  MHRA2 recommends that departments make themselves familiar with the SAR limits used by their system from both the IEC standard and the manufacturer’s users manual. The use of different operating modes with regard to the varying SAR levels should be recorded within the local rules.  Departments should also be aware that the IEC SAR limits are set assuming moderate environmental conditions of relative humidity and ambient temperature. There is a risk of overheating the patient if SAR is not reduced in adverse conditions   ie in high ambient temperatures and high relative humidity. PHE15 recommend that departments follow the ICNIRP guidelines for RF fields for each operating mode and additionally that an upper temperature limit be specified for the experimental operating mode.

  • MR operators should ensure that a good airflow is passing through the MR scanner while patients are in situ. 
  • MR operators should be aware of the acceptable limits of humidity and ambient temperature for each scanner.

Note: this information should be provided within the manufacturer’s literature.

  • MR operators should be aware of the different operating modes available on systems, and their importance in ensuring that SAR levels remain as low as reasonably possible.  Local rules should provide clear guidance on the use of such operating modes.

7.2 Induced current burns

7.2.1 Burns will occur when patients are positioned in such a way to create a conductive loop pathway, for example where thighs meet or when hands are clasped. Poor positioning of the patient and associated leads and sensors are the cause of many burns.

7.2.2 Care should be taken to ensure that cables should be correctly positioned and avoid them touching patients. The cables should not be crossed, looped or allowed to lie diagonally across patients. Ideally cables should lie parallel and as close to the centre of the bore as possible, and should not touch the bore of the magnet at any point.

7.2.3 Patient skin should be insulated from the bore of the magnet and staff should ensure that there is no skin to skin contact. Foam pads 1-2 cm thick should be used to insulate patients from cables, the bore and between limbs.2

7.3  Contact burns

Contact burns may occur in patients where there is contact with metallic objects that act as conductors such as coils, cables, monitoring equipment, transdermal patches. Contact burns have been reported due to metallic fibres in clothing.21 There is a reported case of a patient receiving a serious burn in MRI due to the paramagnetic ink in a wristband. 22

  • Careful positioning technique is essential in order to avoid any skin to skin contact.
  • Clothing should be checked to ensure it is safe and ideally patients changed into suitable hospital provided clothing.
  • Burns from poor patient and cable positioning are entirely avoidable with good MR practice.
  • Staff should visually inspect patients after imaging to look for any areas of skin redness that may develop into a burn.
  • All incidents of burns should be reported as outlined in Section 3.5 of this document.
  • It is recommended that patients are provided with an after care leaflet as it is possible that a burn will develop after the patient has left the department.

7.3.1 Transdermal patches

Some transdermal patches contain metal within the backing which could potentially become conductive, leading to skin burns. Heating may also pose a problem for some medicinal patches leading to an overdose due to more of the medicine being released into the skin

  • Transdermal patches should be removed prior to the patient entering the scan room if they contain, or may possibly contain metal within the backing or maybe affected by heat.
  • Patients should be advised to bring a spare patch. We recommend that this  information is included within the patient information literature

7.3.2 Make up, piercings, tattoos

Non-medical objects such as piercings and make-up which have high iron oxide content may cause burning and, wherever possible, should be removed both for patient safety and diagnostic image quality. Some tattoos also have a high ferrous content and patients should be counselled regarding the possibility of local burns and asked to report any discomfort immediately and scanning stopped. Consideration should also be given to patients who have hair extensions as there are certain types that are bonded or tied to the hair using metal components.

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