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18-02 Incidental Hazards

Incidental hazards are created by the static magnetic field usually covering an el­lips­oid region around the isocenter of the magnetic resonance imager (Figure 18-03 and Figure 18-04). The range of this fringe or stray field depends on the field strength of the system, the type of magnet, and the kind of shielding used.

Figure 18-03: A magnetic resonance imaging site. A Safety zone 1: reception area; B Safety zone 2: patient changing and resting area; C Safety zone 3: control rooms; D Safety zone 4: magnet room. Zones C and D are strictly off-limits to non-authorized personnel.

Figure 18-04: The fringe field around the magnetic re­so­nan­ce system may stretch into ad­ja­cent rooms and floors. Never forget that the mag­ne­tic memory of credit and si­mi­lar cards, as well as magnetic de­vi­ces such as tapes, will be erased by MR magnets. Leave home without them or leave them outside the magnet room. (500 Gauss = 0.05 Tesla).

The fringe field around the magnetic re­so­nan­ce system may stretch into ad­ja­cent rooms, floors, even garden and parking places outside the building. It both influences electronic equipment and can be a possible hazard to persons passing by. The official 5-Gauss safety line encircles the area that pacemaker carriers should not enter. This region can stretch beyond the magnet room itself. The en­tran­ce to the magnet room should be visible to the systems operator. Ap­pro­pri­ate warning sign must be posted (Figures 18-05a and 18-05b).

In this case, warning signs or si­mi­lar notices should be displayed out­side the magnet room, in neigh­bor­ing rooms on the same floor, and on floors above and below. This danger has been reduced by shielded mag­nets.

It is of special interest for the observer and connoisseur of bureaucratic pro­ce­du­res that the 5-Gauss limit is ten times higher than the average earth magnetic field, but lower than the magnetic field in electric trains such as subways (up to 7 Gauss). The fields measured on the sur­face of the receiver of a telephone are 35 Gauss and of an audio headset 100 Gauss – and that in an electric train close to the engine the field strength can reach 9000 Gauss (0.9 Tes­la).

Figure 18-05a: Danger and prohibition symbols and signs used worldwide at MR installations.

Figure 18-05b: North American classification symbols for implants and de­vi­ces. Signs and re­gu­la­tions are different in different coun­tries or jurisdictions. Note that for clinical MR examinations "MR conditional" devices must be con­si­der­ed unsafe if the contrary has not been proven. Devices declared “MR Safe” for 1.5 Tesla are not necessarily safe at higher fields.

Ultralow- and low-field magnets possess a limited stray field of sometimes less than one meter radius from the isocenter. The stray field of large bore, high field systems may cover a radius of 15 or 20 meters, unless the magnet is heavily shielded (cf. Table 03-03).

Constant education of everybody involved in MR imaging is vital. Every per­son working or entering the magnet room or adjacent rooms with a magnetic field has to be instructed about the dangers. This should include the in­ten­si­ve care staff, and maintenance, service and cleaning personnel, as well as the crew at the local fire station. The best protection against this danger is not to allow per­son­nel other than those directly involved in patient examinations, i.e., the operator and the radiologist, into the magnet room by building the room with a closed and controlled access.

As a general rule, access to the magnet room should be limited to trained per­son­nel or to thoroughly screened patients and visitors who are acommpanied by trained personnel.