MRI Safety Flashcards
Define the ‘MR Environment’
the three dimensional volume of space surrounding the MR magnet that contains both the Faraday shielded volume and the 0.50 mT field contour (5 gauss (G) line). This volume is the region in which an item might pose a hazard from exposure to the electromagnetic fields produced by the MR equipment and accessories.’
What does MR Safe mean?
‘an item that poses no known hazards resulting from exposure to any MR environment. MR Safe items are composed of materials that are electrically nonconductive, nonmetallic, and nonmagnetic’ *
What does MR Conditional mean?
‘an item with demonstrated safety in the MR environment within defined conditions. At a minimum, address the conditions of the static magnetic field, the switched gradient magnetic field and the radiofrequency fields. Additional conditions, including specific configurations of the item, may be required
What does MR Unsafe Mean?
‘an item which poses unacceptable risks to the patient, medical staff or other persons within the MR environment.’
Who is an ‘MR Authorised Person’?
A suitably trained member of staff authorised to have access to the MR CONTROLLED ACCESS AREA.
Define the MR Controlled Access Area.
A locally defined area of such a size to contain the MR ENVIRONMENT. Access shall be restricted and suitable warning signs should be displayed at all entrances.
Define the MR Responsible Person
Person to whom day to day management of MR Safety is deligated.
This has to be a MR Responsible Person.
The MR RESPONSIBLE PERSON should not take on the role of MR SAFETY EXPERT. Each MR RESPONSIBLE PERSON should retain close contact with other relevant groups or committees responsible for safety and welfare of personnel on site, such as a research ethics committee, local safety committee and local radiation safety committee
Define MR Safety Expert.
The expert should be a designated professional with adequate training, knowledge and experience of MRI equipment, its uses and associated requirements.
The MR Safety Expert will have an advanced knowledge of MRI techniques and an appropriate understanding of the clinical applications of MRI. Ideally they will be a physicist with expertise in MRI. Clinical units should appoint an MR SAFETY EXPERT who acts according to recognised standards ie they should normally have Health and Care Professional Council (HCPC) registration or General Medical Council (GMC) specialist registration.
Who is responsible for ensuring adequate Local Rules are in place?
MR Responsible Person - must consult with the MR Expert and representatives of all MR Authorised Personnel.
What are the three levels of responsibilty/organisation for MR Controlled Access Areas?
1) MR Authorised Person (Supervisor) - ): responsible for supervision of all persons within the MR Environment.
2) MR Authorised Person (Magnet Room/MR Environment): has access to MR Environment but does not supervise others.
3) MR Authorised Person (Non-Magnet room/Non-MR Environment): has access to MR Controlled Access Areas but cannot enter the MR Environment without being under supervision.
Define the MR Safety Supervisor.
Ensures local rules are followed; this is not a requirement in the guidance, it has been put in place locally to mirror the role of an Radiation Protection Supervisor required by IRR17 to provide consistency between departments
Define the MR Clinical Advisor.
Not required in the guidance(1) but included locally to provide advice on the safe and appropriate use of MRI.
Define the MR Operator.
Responsible for the day to day operation of the MRI equipment (with appropriate training, skills and knowledge).
What are the 3 main sources of hazards in the MR Environment?
1) Static Magnetic Field
2) Gradient (time dependant field)
3) RF magnetic field/signal
What are the safety issues associated with the strong, static magnetic field?
1) Extent and gradients of the fringe field - staff should have good understanding of these at their site.
2) Biological Effects:
mainly creating electrical potentials, therefore, currents from body movements and displacement of normal body currents
e.g. vertigo,
Possible hand-eye coordination effects
3) Projectile risk due to high field strenght and high gradients of the fringe field (ferromagnetic materials)
4) torque risk - shape dependant, proportional to the field strength and angle the object is from alignment with the field
5) Lenz effect - induction of current as object moved through field (not large up to 1.5T but can be significant >3T) - can be a concern for mitral and aortic valve replacements
6) Interaction with implantable devices - can affect function as well as movement within the field (if containing ferromagnetic materials) - note pacemakers may experience torque sufficient to cause movement within the chest
7) Interaction with other (peripheral) equipment - can effect function due to interaction with ferromagnetic componants or inducing currents in metal parts (e.g. drug infusion pump)
What are the safety issues associated with time varying field gradients?
1) Periferal Nerve Stimulation (PNS) - induced electric fields and currents in conductive tissues (low gradient varying frequencies) - can cause discomfort or pain; most sensitive at up to 5kHz
2) Muscle stimulation - induced electric fields and currents in conductive tissues - can cause movement of limbs in extreem situations
3) High acoustic noise - due to movement of gradient coils in the scanner
4) Ventricular fibrilaton (AF) - body most sensitive at frequencies of 10-100Hz
5) Implant Interaction - heating and vibration - from induced currents or magnetic interaction with ferromagnetic componants
What are the safety issues associated with Radiofrequencey (RF) fields?
1) mainly thermal heating - burns and contact burns; oscillation of molecules and generation of heat - higher in tissue than in blood, blood can act as a coolant.
Heating effects vary througout the body due to tissue type (e.g. eye has little blood flow, lens has none, more time for heat to disapate; testes are separated from body and considered heat sensitive).
A 1 deg C temp rise is acceptable for normal healthy adult.
Clothing, ambiant temperature and humidity effect thermal dissipation; lower ambient temp and humidity, the faster thermal cooling
2) Heat stress - some patients (e.g. hypertension, pregnant women) are at risk with heating so rate of cooling needs to be considered
3) Burns - contact burns from conductors in contact with patient (e.g. coils, coil leads, ECG connectors, O2 monitoring probes)
What are the most reported MRI adverse incidents in England?
Burns due to RF induced heating.
Name the modes of operation of an MRI scanner and define their maximum whole body average SAR values for RF field exposure (IEC 2010 definitions).
1) Normal Mode - 2W/Kg
2) Controlled Mode - 4W/Kg
3) Research/Experimental Mode - >4W/Kg
What is the partial body - Head - maximum SAR value for Normal and Controlled modes? (IEC 2010 definitions)
3.2W/Kg
What is the partial body - Head - maximum SAR value for Research/Experimental mode? (IEC 2010 definitions)
> 3.2WKg
Who are the two organisations that provide SAR limits?
1) International Electrotechnical Commission (IEC)
2) International Commission on Non-Ionizing Radiation Protection (ICNIRP)
What are the whole body temperature rise restrictions stated in MHRA guidance?
Normal mode = 0.5 degree C
Controlled mode = 1.0 degree C
Experimental mode = >1.0 (<2) degrees C
What are the maximum body temperature restrictions stated in MHRA guidance for the head?
38 degree C for Normal and Controlled mode; >38 degrees C for Experimental mode.
HRA upper limit is 39 degrees C
