Doc menu

5. Radio-Frequency Radiation (B1)

5.1  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. 7

ICNIRP7 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.

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

All patients should be weighed prior to scanning in accordance with manufacturers guidelines.  Short bore magnets will also require the patient’s height to be recorded.

5.1.1   SAR Limits

SAR limits have been defined by the International Electrotechnical Commission (IEC)9 and ICNIRP5. The MHRA2 recommends that departments make themselves familiar with the SAR limits used by their system from both the IEC standard and the manufacturer’s user 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 the SAR is not reduced in adverse conditions  i.e. in high ambient temperatures and high relative humidity.  The HPA4 recommends 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.

13695342-exclamation-point.jpg MR operators should ensure that a good airflow is passing through the MR scanner while patients are in situ.

13695342-exclamation-point.jpg MR Operators should be aware of the acceptable limits of humidity and ambient temperature for each scanner.

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

13695342-exclamation-point.jpg 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.

5.2  Induced current burns
 

5.2.1  

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.

5.2.2  

Care should be taken to ensure that cables should be correctly positioned and to 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.

5.2.3  

Patient’s skin should be insulated from the bore of the magnet and staff should ensure that there is no skin to skin contact.

5.2.4  

Staff should visually inspect patients after imaging to look for any areas of skin redness that may develop into a burn.

5.2.5  

All incidents of induced current burns should be reported as outlined in Section 17.3 of this document.  It is good practice to provide all patients with an after-care leaflet as it is often the case that a burn will develop after the patient has left the department.

13695342-exclamation-point.jpg Careful positioning technique is essential in order to avoid any skin to skin contact.

13695342-exclamation-point.jpg Burns from poor patient and cable positioning are entirely avoidable with good MR practice.

Content tools

Accessibility controls

Text size

AA A

Colour