Object ID: 00000018WIA30500870GYZ
Topic ID: id_40024964 Version: 1.9
Date: Jul 26, 2022 12:01:24 PM

Magnetic field basics

The main magnet is a stable and very intense magnetic field.

Note: that the MR magnet is always on even when the system is not acquiring scan data. The only exception to this is if service has ramped down the magnet or it has been quenched. The main safety issues regarding the static magnetic field include the potential for biological effects, the potential for attraction of ferromagnetic objects, and the potential for a quench of the cryogens.

Though it is generally accepted that no published evidence exists supporting cumulative or long-term negative effects of EMF exposure, it is advisable for pregnant MR workers to exercise extra precaution in limiting their exposure as much as possible. Health effects increase with increasing magnetic field strength. The existence of local regulations establishing upper limits for MR workers may not apply to pregnant MR workers, although no epidemiological evidence exists supporting negative effects of EMF exposure on the health of a pregnant worker or her fetus. The User is responsible for determining whether local or country legislation may exist establishing occupational limits for exposure to EMF. If such limits exist it is the User’s responsibility to ensure they are being observed.

To ensure safe operation of your system, for both you and your patient, you must understand several components of your system. Your system includes the following magnetic fields:

Static Magnetic Field (the magnet)
Gradient Magnetic Fields (the gradients)
Electromagnetic Fields (the EMF)

The following definitions are from IEC60601-2-33 and are used throughout Magnetic Field Basics section. Not all modes of operation apply to all GEHC MR scanners.

Normal Operating Mode: mode of operation of the MR equipment in which none of the outputs have a value that can cause physiological stress to patients.
First Level Controlled Operating Mode: mode of operation of the MR equipment in which one or more outputs reach a value that can cause physiological stress to patients which needs to be controlled by medical supervision.
Second Level Controlled Operating Mode: mode of operation of the MR equipment in which one or more outputs reach a value that can produce significant risk for patients, for which explicit ethical approval is required (i.e. a human studies protocol approved to local requirements).


Operating mode	Static magnetic field limits
Normal Opoerating Mode	≤3.0T
First Level Controlled Operating Mode	> 3.0T and ≤ 8.0T
Second Level Controlled Operating Mode	> 8.0T

The magnetic field is classified in normal controlled operating mode.

A magnet produces invisible lines of force that extend beyond the magnet that are called the fringe field. The size of the fringe field depends on the strength of the magnet and whether or not it is shielded. Active and inactive shielding are used to reduce or tighten the fringe field.

CAUTION

For some patients or MR workers, rapid movement of the head while in the magnetic field may cause dizziness, vertigo, or a metallic taste in their mouth. None of these motion effects are considered to be hazardous, provided they do not cause the worker to fall. IEC 60601-2-33: 201.7.9.2.101 (k) It is recommended that the patient and the MR worker endeavor to remain still while in the region of high static magnetic field. The MR worker should always vacate the area of the static magnetic field when duties do not require otherwise.

The tesla to gauss conversion is 1 tesla = 10,000 Gauss.

The magnetic field exerts force on susceptible materials and biomedical implants and can create hazards. There are two critical zones: the Security Zone and the Exclusion Zone. Each zone has specific restrictions regarding people and materials.

Warning

IEC 60601-2-33:201.7.9.2.101(e), IEC 60601-2-33: 201.7.9.2.101 It is your responsibility to ensure permanent creation of the Security Zone and the Exclusion Zone and to establish rules for access. Ensure occupational exposure to static magnetic field complies with local requirements.