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3. Static magnetic field (B0)

3.1  Definition

Static magnetic field (B0) is dependent on the field strength of the magnet.

The SI unit used for measuring magnetic field strength is the tesla; its symbol is T.

A magnetic field of one tesla (1T) is approximately 30,000 times as powerful as the Earth's magnetic field.

3.2  Controlled Areas

The static magnetic field and the shielding define the controlled areas.  Typically, MR departments will define two controlled areas the controlled area and the inner controlled area.

3.2.1   Controlled Area

The controlled area is where the static magnetic field, B0, may exceed 0.5 mT2 (often referred to as the 5 Gauss line: 5 Gauss = 0.5mT).  The controlled area will incorporate the inner controlled area, the control room and typically, preparation room, patient changing rooms and reporting rooms.  Ideally the limit of the 0.5mT line should be marked out on the floor as a safety guide for staff.

13695342-exclamation-point.jpgAccess should be restricted to the controlled area by a permanently locked door, ideally with a keypad entry.

3.2.2   Inner Controlled Area

The inner controlled area is where the static magnetic field may exceed 3mT.  This is typically the scan room itself and this area should be clearly marked and access controlled by means of a lockable door.

13695342-exclamation-point.jpg Due to the hazards of the static magnetic field described in this section, resuscitation of patients should take place outside the inner controlled area.  Local rules should outline specific procedures to reflect this.

3.3  Bio-effects

The interaction of the static magnetic field (B0) with the body and its functions may result in the creation of electrical potentials, currents generated by body movements and the possible displacement of naturally generated currents within the body by B0.  Electrical potentials and related effects during physical movements within static magnetic field gradients may induce sensations of vertigo, nausea, phosphenes and a metallic taste in the mouth.3  The Health Protection Agency HPA  offers the following advice regarding the movement of Patients and Volunteers in the static field:

13695342-exclamation-point.jpg “The Biological Effects most likely to occur are the production of vertigo-like sensations and these acute effects are associated with movement in the static field.  The sensitivity to these effects varies considerably between individuals.  Patients and volunteers should be moved slowly into the scanner, to avoid the possibility of vertigo and nausea.”4

Further recommendations, guidance and exposure limits relating to Bio- effects of B0 can be viewed in the following publications:

Health Protection Agency HPA Protection of Patients and Volunteers Undergoing MRI Procedures, 2008.4

International Committee on Non-Ionising Radiation (ICNIRP) Guidelines on Limits of Exposure to Static Magnetic Fields 2009. 5

International Committee on Non-Ionising Radiation (ICNIRP) Amendment to the ICNIRP Statement on Medical Magnetic Resonance (MR) Procedures: Protection of Patients 2009. 6

All the above are accessible on the SCoR website MRI Practice Page: https://www.sor.org//practice/cross-sectional-imaging/mri  (accessed 27th February 2013)

3.4  Projectile and attractive forces

The potential hazard of the projectile effect of ferro-magnetic material in a strong magnetic field must be taken very seriously.  The HPA reports that serious incidents have occurred including a patient fatality when items such as ferro-magnetic oxygen cylinders have inadvertently been brought into the scan room. 4

13695342-exclamation-point.jpgExtreme caution must be employed and strict screening protocols in place and adhered to, to ensure ferro-magnetic items do not enter the controlled area.  Loose metallic objects can reach considerable velocities.

3.4.1   Ancillary Equipment

Caution should be exercised when assessing, purchasing and authorising equipment for patient support in the MR environment.  Equipment must be assessed for MR compatibility and clearly labelled as indicated in section 3.5.  Departments should ensure that an up to date inventory is kept of all equipment for use in the MR environment.

3.5  Labelling of Equipment

All equipment used or stored within the MR environment should be clearly labelled as one of the following:

MR safe

MR conditional

MR unsafe

All staff should be conversant with the labelling system and understand the conditions for use. Further information on labelling of equipment can be viewed in the MHRA guidelines.2 Departments should take into account the different field strengths when labelling equipment.  For example, if a department is equipped with two MR units of 1.5T and 3T only equipment which is safe in both units should be labelled as ‘MR Safe’.  Clear guidance for use should be marked on equipment that is MR conditional.

3.6  Procurement

The MR Responsible Person and/or the MR Advisor must be involved in all procurement decisions relating to items for use in the controlled area.

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