MRI safety in detail

Question 1

What are the main regulatory bodies for MR safety?

Answer 1

• Health Protection Agency (HPA).
• Medicines and Healthcare Products Regulatory Agency (MHRA).
• International Electro technical Commission (IEC).
• International Commission on Non-Ionising Radiation Protection (ICNIRP).

Question 2

What are the 3 operating modes for MRI define by regulatory bodies? What do these correspond to?

Answer 2

• Normal mode: Risk of ill effect to the patient is minimised.
• Controlled mode: Exposure higher than normal mode and some people may experience some effects at this level. However, the risks are minimised and the patient will benefit from the enhanced imaging performance. Scanning required patient monitoring.
• Research mode: Exposure is only restricted to prevent harmful effects. Requires ethics approval and patient monitoring.

Question 3

What are the safety implications on the B_0 field?

Answer 3

• B_0 field constantly on.
• Attraction of ferromagnetic materials resulting in projectiles.
• Eddy current induced forces in non-ferromagnetic materials.
• Altered function of implanted electronics.

Question 4

How are the effects of the B_0 field minimised outside of the scan room?

Answer 4

MRI scanner is housed within a Faraday cage around the perimeter of the room which limits the magnetic field strength outside of the room.

Question 5

What are the MRI safety designations?

Answer 5

• MR safe: An item which poses no hazards in an MR environment.
• MR conditional: An item which has been demonstrated to pose no known hazards in a specified MR environment with specified conditions of use (this can be field dependent).
• MR unsafe: An item which is known to pose hazards in all MR environments.

Question 6

Are implants contraindicative for MRI?

Answer 6

Some implants are while some are not. Various sources of safety information are available to determine this.

Question 7

What is the B_1 field? What are the safety implications of the B_1 field?

Answer 7

• An oscillating RF magnetic field.
• Energy from field can be deposited as heat in the patient resulting in burns.

Question 8

How is RF exposure measured? What does it depend on? What do regulations provide in relation to this?

Answer 8

• Specific absorption rate (SAR). This is the total power absorbed per unit mass of tissue.
• RF power, pulse sequence, field strength, tissue density and tissue conductivity.
• Regulatory SAR limits are given (same from HPA, IEC and ICNIRP).

Question 9

What are the safety implications of the magnetic field gradients in MRI? How can these effects be minimised?

Answer 9

• Noise. Can be minimised with ear defenders and designing pulse sequences which minimise noise.
• Induced currents can stimulate peripheral nerves (tingling through to cardiac stimulation depending on dB/dt).

Question 10

How does the B_0 field vary with mode of operation? How does the SAR limit vary with mode of operation? How does dB/dt vary with mode of operation?

Answer 10

All increase with mode of operation as increasing each increases the risk of a hazard.

Question 11

What are some safety implications of MRI contrast agents? How is safety ensured?

Answer 11

• Potential for reaction from mild through to anaphylactoid.
• Can cause nephrogenic systemic fibrosis (high risk for patients with low kidney function).
• Medical history checked, medical assistance available in the case of allergic reaction and observations of patient after contrast administration.

Question 12

What are the safety implications magnet quenches? How are risks mitigated?

Answer 12

• Large volume of helium gas which can cause an asphyxiant atmosphere.
• Magnet room fitted with quench pipe to vent helium.