MRI Safety and precautions
Metal and magnetic field
Due to the presence of a strong magnetic field, certain materials may present a functional or even a vital risk:
- Projectile effect (attraction by a static magnetic field and acceleration, with speeds of up to several meters per second): ferromagnetic material (if in doubt about the ferromagnetic nature of a metal object, a test can be carried out using a small magnet)
- Displacement of intra-corporeal metallic foreign objects: Intraocular metallic foreign body (metal worker, history of ballistic orbit trauma, old intra-cranial aneurysm clips)
- Perturbed functioning of certain devices: cardiac pacemaker, neurostimulators, cochlear implant, derivation valves.
In regard to prostheses, non ferromagnetic materials with no electrical activity (titanium and its alloys, nitinol, tantalum, etc.) carry no particular risks in relation to magnetic field. For low magnetic prostheses (orthopedic material), a delay of 6 to 8 weeks after implantation is advised to avoid displacing the material.
Heart valves are generally MR compatible.
In all cases, it is advisable to check the MR compatibility of the material (see http://www.mrisafety.com/), particularly when operating in high fields: some devices carry no risks at 1.5 T but can be dangerous at a higher field.
Gradient strength and switching
Rapid switching of the magnetic field gradients can trigger peripheral nerve and muscular stimulation. Stimulation of the heart, which can be dangerous, occurs at a higher level than for the peripheral nerves.
Echo-planar sequences are those most likely to cause this type of adverse effect, as they put the greatest strain on the gradients, with ascents and descents at high frequencies and strengths.
RF and SAR
SAR corresponds to the amount of radiofrequency energy deposited in the patient, which may result in heating. It is measured in W/kg (which explains the need to specify the patient’s weight before the exam).
SAR is proportionate to the square of the strength of the static magnetic field and the square of the flip angle. It can be reduced:
- by using quadrature coils with lower transmission volumes
- by optimizing the sequence parameters (increasing TR, reducing the number of slices, flip angle, echo train length).
SAR standards exist to limit the maximum acceptable dose for patients under MR scanning (IEC 60601-2-33 standard). The safety standards are designed to ensure that no tissue is subjected to a temperature increase of over 1°C.
The other risk from RF exposure is that of skin burns provoked by the induced current in a conducting loop. These burns may occur in contact with electric leads forming a loop (ECG monitoring in particular), metal devices (skin patches, body piercing, dental appliances) or when there is skin contact (hands on the stomach, calves touching).
SAR (Specific Absorption Rate)
SAR value in W/kg is of the type:
- B0 = static magnetic field amplitude
- B1 = RF pulse amplitude
- α = flip angle
- D = cyclic ratio (fraction of the duration of the sequence during which the RF waves are transmitted)
- ρ = density
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