There are multiple national and international bodies that regulate MRI exposure doses for patients, volunteers and staff.
- Medicines and Healthcare Products Regulatory Agency (MHRA) for the use of MRI in patients and the exposure of staff.
- International Electrotechnical Commission (IEC) (2010): provides standards for manufacturers of medical MRI equipment to follow
- International Commission on Non-Ionizing Radiation Protection (ICNIRP): guidance for occupational and general public exposure.
The limits are specific to different modes of scanning. These are:
- Routine / normal operating mode: normal patient studies
- Specific / controlled operating mode: specific studies outside of the normal operating mode. Patient must have panic button with free verbal contact and constant visual contact
- Research / experimental operating mode: examinations conducted at levels above controlled operating mode. These require approval by an ethic committee.
MR safety marking*
| MR Safe | “an item that poses no known hazards resulting from exposures to any MR environment. MR Safe items are composed of materials that are electrically nonconductive, nonmetallic, and nonmagnetic” |
| MR Conditional | “an item with demonstrated safety in the MR environment within defined conditions.” |
| MR Unsafe | “an item that poses unacceptable risks to the patient, medical staff or other persons within the MR environment.” |
* Definitions from the ASTM international standard F2503-13
Static magnetic field
MHRA guideline for whole body exposure of patients
| Mode | Tesla |
| Normal | 4.0 |
| Controlled | 8.0 |
| Research | No limit |
| Pregnant | < 2.5 |
Staff – MHRA guideline for exposure of staff
< 2T for whole body
< 5T for limbs
Over 24h average exposure should not exceed 0.2T
Controlled area
5 Gauss line (0.5 mT) is drawn around the room in which the static magnetic power is greater than or equal to 5 Gauss. Patients / staff with contraindications (pacemakers etc.) to MRI should not enter this area.
Radiofrequency (RF) fields
RF fields cause microwave heating. This is due to the oscillating electromagnetic fields creating electrical currents in patient tissues that then produces heat.
Specific absorption ratio (SAR) = RF energy deposited per mass of tissue (Watts / kg).
1 SAR = 1 W/kg = whole body temperature rise of 0.5 degrees
Legislation
Current recommendations are set by the International Committee on Non-ionising Radiation Protection (ICNIRP) (2014).
(scroll sideways to view whole table if required)
| Localised temperature limits (°c) | |||||
| Operating mode | Rise of body core temperature (°c) | Head | Trunk | Extremities | SAR (W/kg) |
| Normal | 0.5 | 38 | 39 | 40 | 2 |
| Controlled | 1 | 38 | 39 | 40 | 4 |
| Restricted | 2 | 39 | 40 | 41 | > 4 |
Factors that affect SAR
The risk of a rise in body temperature is increased in certain patients:
- Patients who are unable to thermoregulate adequately (e.g. heart failure, fever, pregnancy (risk of fetal heating), patients taking medication that affects thermoregulation (vasodilators, tranquilisers and diuretics)
- Patients in a cast
- Patients who are unable to communicate any heat sensations
| Higher SAR | Lower SAR |
| Large body parts | Small body parts |
| High static fields (higher Tesla. Doubling the magnetic field from 1.5 T to 3 T leads to a 4x increase in SAR) | Low static fields |
| 180° RF pulse | 90° RF pulse |
| Spin echo: large RF pulses applied very rapidly means higher deposition of energy | Gradient echo: smaller RF pulses. Although these are also applied rapidly the net deposition of energy is lower than in spin echo |
| Time-of-flight MRA: although this is a gradient echo the RF pulses are applied at a very high speed | |
| High conductivity tissues (blood, brain, liver, CSF) | Low conductivity tissue (bone marrow, fat) |
Techniques to reduce SAR
- Increase the TR (apply the RF pulses less rapidly)
- Reduce the flip angle
- Reduce the number of slices in each acquisition
- Reduce the number of echoes in multiecho sequences
- Reduce room temperature and dress patients in light clothing
- Alternate high and low SAR sequences
Gradient fields
ICNIRP and MHRA Limits for exposure to patients
- Normal operating mode: 80% of median perception threshold
- Controlled operating mode: 100% of median operating
- Research operating mode: none available but suggested limit of 120% of median perception threshold
1. Induced currents and voltages
These time-varying gradient fields cause eddying currents in conductive tissues and cause stimulation e.g.:
- Peripheral nerve stimulation
- Involuntary muscular contraction
- Breathing difficulties
- Ventricular fibrillation
For static fields greater than 3T:
- Flashes of light on the retina
- Vertigo
- Nausea
- Sensation of metallic taste
The voltages induced may also affect devices:
- Cardiac pacemakers
- Cochlear implants
- ECG monitors
2. Acoustic Noise
- The fast-switching magnetic fields in gradient echo sequences create loud noises.
- Louder noise caused by:
- Higher field strength
- Higher gradient amplitudes
- Machine limit is 140 dB (most don’t exceed 120 dB)
- Hearing protection needed to prevent irreversible damage at 90 dB
Metal Related Hazards
1. Ferromagnetic missile effect
- Caused by static field
- At 1.5T, objects can reach an acceleration 10x that of gravity and can reach speeds of >80 k/h once they reach the centre of the bore, usually where the patient’s head is positioned
2. Migration / rotation of metal in body
- Caused by static field
- May cause ferromagnetic metal containing objects to migrate within the body or rotate to align with the field like the needle of a compass.
- Of particular concern near the eye or ear e.g. shrapnel in the eye causing sub-retinal haemorrhage and blindness
3. Heating of metal objects
- Caused by RF wave
- Thought to be due to resonant antenna effect. RF pulses set up a standing voltage wave in the metal causing tips of wires to undergo rapid heating and burn the patient
- Pacemaker forms conductive circuit
- RF pulse may induce pulses that cause heart to contract
- Resonant antenna effect may cause heating up of wires
- Solutions:
- MR compatible leads
- Place ECG electrodes as close together as possible to minimise area of conductive loop formed
- Braid cables
- Keep cables close to the centre (area of lowest fields)
Contraindications
1. Absolute
- Pacemaker / defibrillator that is non-MRI compatible
- Metallic foreign body in the eye
- Deep brain stimulator
- Bullets or gunshot pellets
- Cerebral aneurysm clips that are non-MRI compatible
- Cochlear implant
- Drug infusion device – might malfunction
2. Relative
- Surgical clips, wire sutures etc
- Joint replacement or prosthesis
- Large patients might not fit into scanner
- Claustrophobic patient
- Significant pain or other conditions that might limit patient’s ability to sit still
- Surgery in previous 6 weeks
- Pregnancy – usually not performed in first trimester
Written by radiologists, for radiologists with plenty of easy-to-follow diagrams to explain complicated concepts. An excellent resource for radiology physics revision.
Emergencies
1. Cardiac Arrest
- Patient is removed from the magnet on to an MR-compatible trolley and taken outside the controlled area
- Here, resuscitation can commence
- Appliances such as oxygen cylinders must not be brought to the patient in the scanner due to the ferromagnetic missile effect
2. Fire
- Non-ferrous carbon dioxide extinguishers should be used
- Fire-fighting equipment should be used only at a distance of 1m or more from the bore
- If fire-fighters definitely need access to the room the magnet must be quenched to switch it off
3. Quench
- If the magnet has to be switched off e.g.:
- Person caught between a metal object and the machine
- Fire
- Quenching the machine involves converting 1000+ litres of liquid helium (which is necessary to cool the magnet) into gaseous helium
- This is a very quick event and the gas needs to be vented out into the atmosphere as quickly as possible
- A quench can cost £10,000 worth of lost helium
- Helium displaces oxygen and so oxygen monitors are used and staff should be evacuated from the whole MR suite as asphyxiation can occur
Σ Summary
- MHRA guideline for patient whole body exposure
- Normal < 4T
- Controlled < 8T
- Research > no limit (needs approval by Ethics committee)
- Pregnant < 2T (usually avoided in first trimester)
- Guideline for staff exposure
- < 2T for whole body
- < 5T for limbs
- Should not exceed 0.2T over 24h
- Controlled area
- Where stray field is greater than or equal to 5 Gauss (0.5 mT)
- Patients / staff with contraindications to MR are excluded from this area
- Radiofrequency Fields
- Cause microwave heating
- Measured by Specific Absorption Ratio (SAR) = Watts / kg
- 1 W/kg = temperature rise of 0.5 °c
- Gradient Fields
- Induced currents and voltages – cause stimulation of peripheral nerves, muscles and possible ventricular fibrillation
- Acoustic noise – maximum machine allowance is 140 dB
- Metal Related Hazards
- Ferromagnetic missile effect – caused by static magnetic field
- Migration / rotation of metal in body – caused by static magnetic field
- Heating of metal objects – caused by RF wave
- Contraindications
- Absolute
- Non-MR compatible pacemaker / cochlear implant
- Metallic foreign body in eye
- Bullets
- Non-MR compatible cerebral aneurysm clips
- Relative
- Surgical clips
- Surgery in previous 6 weeks
- Joint replacement / prosthesis
- Claustrophobic
- Large patient
- Inability to lie still
- Pregnancy – not scanned in first trimester usually
- Absolute
