Although there is evidence for a multitude of different effects of static magnetic fields, particularly on macromolecules and flowing charges, studies published to date provide no indication of adverse health effects at whole body exposures of up to 4 T.

However, a substantial potential hazard in clinical routine, which cannot be prevented by complying with this reference value either, are metallic and especially ferromagnetic objects such as

• coins,
• scissors,
• and drip stands.

These objects are strongly accelerated in the stray field of the magnet and thus turn into dangerous projectiles.

This hazard can only be minimised by providing patients and possible escorts with detailed information. Furthermore, the operator of an MRI system is obliged to post warning signs alerting people to the danger.

A fundamental biophysical effect of electromagnetic radiofrequency fields is the heating of tissue. The reason for this are electric currents induced in tissues, leading to power deposition and thus to heating due to electrical resistance of tissues. The energy absorbed by the body per mass and time is referred to as specific absorption rate (SAR, in watts per kilogram).

The relevant parameter to assess the physiological effect of radiofrequency fields is the temperature rise in tissues which does not only depend on local power absorption and exposure period, but also on the thermal conduction and tissue perfusion. The two last-mentioned factors lead to a quick temperature equilibrization in the case of regional heating of the body.

Investigations on humans have not revealed any adverse health effects from radiofrequency exposure of the torso and head yielding a maximum body temperature rise of 1 °C.

The SSK has recommended basic restrictions for temperature and limits for the specific absorption rate derived from these, compliance with which is ensured by complex monitoring systems during patient examinations.

Rapidly alternating magnetic gradient fields may cause stimulation of peripheral muscles and nerves as well as stimulation of the heart muscle with the possible consequences ranging from extrasystoles to ventricular fibrillation. When using ultra-fast MRI techniques under experimental conditions, contractions occurred in chest area, shoulders, hips, buttocks and nose.

As it is not clear whether nerve or muscle cells are stimulated in these cases, such occurrences are generally referred to as magnetostimulation. From theoretical considerations and experimental observations, however, it is known that stimulations are only possible when the stimulus intensity exceeds a certain threshold and that cardiac thresholds are considerably higher (by a factor of about ten) than those required for the stimulation of peripheral nerves and muscles.

In the above-stated recommendation of the SSK, exposure limits are set which have to be complied with during patient examinations in order to avoid the stimulation of peripheral nerve and muscle cells. Compliance with these limits is ensured during patient examination performed at an approved MRI system by a control system.

In addition, considerable noise, which may require the use of hearing protection devices, can be generated by rapidly alternating gradient fields.

Prior to the MRI examination of patients with passive implants such as

• vascular clips or clamps
• intravascular filters or stents,
• artificial heart valves,
• intravenous catheters or ports,
• orthopaedic prostheses,
• plates or screws as well as
• intrauterine coils

it has to be clarified whether these implants are ferromagnetic or whether they contain ferromagnetic components.

Evaluating the situation is more difficult in the case of metallic inclusions such as splinters or bullets, the materials of which are mostly unknown. In these cases, MRI examinations should not be carried out. This also applies to patients in whom critical tissue or organ structures are at risk due to potential dislocations of metallic inclusions or implants.

The risk potential is lower when inclusions or implants have grown in well and are not near critical structures. In such cases, MRI examinations are carried out in many places. However, these examinations should be limited to urgent medical indications.

MR examinations are contraindicated in patients with active implants unless otherwise specified on the patient implant card or by the attending physician.

In addition to the risks described above, the MRI system may modify or impair the operation of these devices and may thereby result in a hazard to the patient.

Important examples to be mentioned in this context are electric and mechanic components of pacemakers and infusion pumps. Furthermore, a range of active implants (such as cochlear implants) use small magnets for activation, programming or controlling functionality which means that an interaction between the implant and the fields used in MRI can cause the device to malfunction.

MRI examinations during pregnancy should be performed only after a critical risk/benefit analysis and at the lowest possible exposure levels and shortest possible exposure time.

Especially during the first three months of pregnancy, particularly strict indication criteria have to be applied.