TwinTree Insert

18-02 Incidental Hazards

ncidental hazards are created by the static magnetic field usually covering an el­lips­oid region around the isocenter of the magnetic resonance machine (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 gardens 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 signs must be posted (Figures 18-05 and 18-06).

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

Figure 18-06:
North American classification symbols for implants and de­vi­ces. Signs and re­gu­la­tions are different in different coun­tries or jurisdictions.

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.

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

spaceholder smallredIt is of special interest for the observer and con­noisseur of bu­reau­cra­tic and ad­mi­ni­stra­ti­ve 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 or car close to the engine the field strength can reach 9000 Gauss (0.9 Tes­la).

spaceholder redConstant education and training 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 accompanied by train­ed per­son­nel.