Non-Ionising Radiation: MRI Safety

Question 1

Describe how the static field of an MRI scanner is produced

Answer 1

The static field comes from a closed bore superconducting magnet with large electric currents (500-1000A) continuously flowing within it. The superconductivity is maintained by keeping the magnet at 4K.

Question 2

What is the MRI fringe field?

Answer 2

The stray magnetic field that extends beyond the main magnet’s bore.

Question 3

State the hazards associated with the MRI static field

Answer 3

• Projectiles: magnetic force pulls ferromagnetic objects towards the scanner at high speed
• Torque: ferromagnetic objects will align with the magnetic field
• Lenz’s law: conductive metals will experience a force that opposes motion

Question 4

Define translational force

Answer 4

The magnetic attraction experienced by ferromagnetic objects due to the spatial gradient of the static magnetic field.

Question 5

Give the equation for translational force

Answer 5

F = translational force
V = volume
χ = magnetic susceptibility
B = static magnetic field
r = position

Question 6

Where is the magnetic force on ferromagnetic objects strongest in around an MRI scanner?

Answer 6

Near the edge of the bore

Question 7

Define torque

Answer 7

The force on a ferromagnetic object that encourages it to rotate in an attempt to align with a magnetic field.

Question 8

Give the equation for torque

Answer 8

T = torque
V = volume
χ = magnetic susceptibility
B = static magnetic field

Question 9

Where is the torque on ferromagnetic objects strongest around an MRI scanner?

Answer 9

Near the isocentre

Question 10

What are the biological effects of the static magnetic field of an MRI scanner?

Answer 10

Movement in fringe fields can excite certain body tissues, potentially causing:
- Vertigo/dizziness
- ‘Metallic taste’ in mouth
- Magnetophosphenes (induced electric currents stimulating the retina causing the sensation of flashing lights)

Question 11

Describe how the gradient coil in an MRI scanner works

Answer 11

Gradient coils apply spatially varying additions to the static field in the x, y, and z directions.

Question 12

What is the maximum gradient strength of an MRI gradient coil?

Answer 12

The maximum amplitude of the gradient field

Question 13

What is the maximum slew rate of an MRI gradient coil?

Answer 13

The maximum rate of change of gradients

Question 14

State the hazards associated with the MRI gradient field

Answer 14

• Magnetophosphenes (induced electric currents stimulating the retina causing the sensation of flashing lights)
• Peripheral nerve & muscle stimulation
• Discomfort
• Limb movement
• Sensitivity (fibrillation and PNS)

Question 15

Describe how the RF coil in an MRI scanner works

Answer 15

It is a magnetic field that varies very rapidly, transmitting and receiving radiofrequency (RF) signals.

Question 16

What is the main hazard associated with RF fields?

Answer 16

Tissue heating and RF burns

Question 17

How is RF exposure measured?

Answer 17

Using the Specific Absorption Rate (SAR). This is measured in W/kg.

Question 18

What does SAR depend on?

Answer 18

• Sequence type
• Number of slices/echoes
• Repetition time (TR)
• Flip angle
• Patient size
• Conductivity
• RF amplitude

Question 19

Give the equation for SAR

Answer 19

Question 20

What can cause RF contact burns?

Answer 20

• Patient monitoring equipment (ECG pads, pulse oximeters)
• Damaged MRI coils
• Transdermal patches (nicotine, pain relief)
• Clothes (conductive/metallic fibres)

Question 21

Why do patients need to wear ear protection when having an MRI scan?

Answer 21

The gradient coils are switched on/off very rapidly, making the magnet room very noisy.

Question 22

What is the MR controlled access area?

Answer 22

The space around an MRI scanner that has strictly controlled access. Everyone must be screened before entering this area.

Question 23

What is the MR environment?

Answer 23

The three-dimensional volume of space surrounding the MR magnet that contains both the Faraday shielded volume and the 0.5mT field contour (5 Gauss line). This area poses projectile hazards, risk of equipment malfunctioning, and risk to persons with implants. Patients, visitors, and untrained staff should always be supervised in this area.

Question 24

What are the 3 types of MR authorised person?

Answer 24

• Authorised person (non-MR environment)
• Authorised person (MR environment)
• Authorised person (supervisor)

Question 25

Describe the access requirements of each type of authorised person and patients to the MR environment and MR controlled access area

Answer 25

Question 26

Before entering the MR environment, it is vital that all ______, __________, and ________ items are removed.

Answer 26

Metal Electronic Magnetic

Question 27

What should be included in MR local rules?

Answer 27

- Equipment descriptions - Risks of MRI - Controlled areas - Patient management - Equipment management - Incidents and emergencies - Cleaning and security - Research studies - Appendices

Question 28

What are the 5 key roles in the MHRA MRI safety guidelines?

Answer 28

- MR medical director (MRMD) - MR safety officer (MRSO) - MR safety expert (MRSE) - MR operators - MR authorised persons

Question 29

What are the 3 safety labels for MRI?

Answer 29

- MR safe - MR conditional - MR unsafe

Question 30

Define MR safe

Answer 30

Devices/equipment that pose no risk to the patient in the MR environment.

Question 31

Define MR conditional

Answer 31

Devices/equipment that pose no additional risk to the patient when introduced to the MR environment under specified conditions.

Question 32

Define MR unsafe

Answer 32

Devices/equipment that may not be introduced into the MR environment as they pose significant risk to the patient and/or staff.

Question 33

How can MR safe impact scans?

Answer 33

They can significantly impact/degrade image quality.

Question 34

How does the static field impact implants like aneurysm clips and shrapnel?

Answer 34

It can exert a substantial force on them

Question 35

How do static, gradient, and RF fields impact active devices (e.g. cardiac pacemakers and neurostimulators)?

Answer 35

They can cause them to malfunction

Question 36

How do RF fields impact metal implants like hip implants, pacemaker leads, and deep brain stimulators?

Answer 36

It causes them to heat up

Question 37

Give an example of MR safe implants

Answer 37

- Dental fillings - Cataract operations

Question 38

Give an example of MR conditional implants

Answer 38

- Stents - Loop recorders - Cochlear implants - Shunts

Question 39

Give an example of MR unsafe implants

Answer 39

- Some cardiac pacemakers - Some internal cardiac defibrillators - Intra-ocular forein bodies (IOFBs) - Some aneurysm clips

Question 40

Who determines the conditions for an MR conditional implant?

Answer 40

The manufacturer

Question 41

Give 3 examples of conditions for an MR conditional implant

Answer 41

1) Static field strength limits 2) SAR level limits 3) B1+RMS (RF root mean square) limits 4) Spatial gradient limits 5) Slew rate limits

Question 42

What are MR screening forms?

Answer 42

Forms that everyone must fill out before entering the MR environment. Any 'no' answers should be followed up to ensure the person can enter the MR environment.

Question 43

Can pregnant patients get an MRI scan?

Answer 43

There are no known risks from MRI to a foetus or mother, meaning that they can be scanned at any time. However, the MHRA recommends that the examination should be postponed until after birth if possible (particularly if contrast is required).

Question 44

What should be done if there is a fire within an MR controlled area?

Answer 44

- Turn off the electrical power - Remove patients from the MR environment - Raise the fire alarm - If it is safe to do so, attack the fire with a non-ferrous extinguisher - Lock the magnet room door and evacuate the MRI unit

Question 45

What should be done if there is a cardiac arrest within the MR environment?

Answer 45

- Call the resucitation team immediately - Remove the person from the magnet room - Lock the scanner door - The crash trolley and associated equipment is NOT MR safe so should NOT be taken into the magnet room.

Question 46

What is a quench?

Answer 46

The release of cryogens from the magnet using a quench button (found in the control room and magnet room). This destroys the magnetic field. It normally takes ~30 seconds for the magnetic field to be removed following a quench but can take up to 10 minutes.

Question 47

Why should a quench only be done in emergencies?

Answer 47

It is very costly due to the release of cryogens and may damage the scanner (which is also costly).

Question 48

In which situations would a quench be appropriate?

Answer 48

- If a person is being crushed, causing a life or limb emergency - If there is a fire or other unexpected occurrence that requires action and entry to the MR environment by emergency personnel

Question 49

What hazards are associated with cryogens?

Answer 49

- Cold burns - Frostbite - Asphyxiation

Question 50

What can cause cryogen hazards?

Answer 50

Cryogen leakage when the room is unoccupied or during a quench

Question 51

What should be done in the case of a spontaneous quench?

Answer 51

- Evacuate the room immediately - Switch on extractor fans (oxygen alarms will sound if this fails)

Question 52

What are gadolinium contrast agents?

Answer 52

A strongly paramagnetic liquid that facilitates the T1 relaxation of nearby water molecules.

Question 53

Why can gadolinium only be given to patients with proper renal function?

Answer 53

As it is nephrotoxic

Question 54

What hazards are associated with gadolinium contrast agents?

Answer 54

- Nephrogenic Systemic Fibrosis (NSF) - Nausea - Vomiting - Headache - Allergic reactions - Neonatal death (pregnancy-specific)