Access Restriction
Magnetic field distribution (fringe field) The stray magnetic field outside the bore of the magnet is known as the fringe field and this is a 3 dimensional field measured in Gauss. MRI systems are shielded to confine the fringe field within the scan room. Magnetic fields less than 5 Gauss are inconsequential to MRI safety. In most systems the 5 Gauss field is confined within the scan room, so the fringe field does not affect any area external to the magnet room.
The 30 Gauss field demarcates the point where projectile hazards become significant and only MRI compatible equipment can safely enter this region. Each MRI system has its own unique fringe field due to varying magnetic design, shielding characteristics, and field inhomogeneity. Each site must be supplied with a schematic that clearly defines the fringe field of the magnet. The schematic must demarcate the 30 Gauss and 5 Gauss lines.
This section summarizes the different zones of a UCSF MR suite and points out specific safety issues of greatest concern. At UCSF, each MRI site is divided into 4 safety zones based on the American College of Radiology guidelines:
- Zone 1: General public area outside the MR environment. This area is the reception and waiting areas.
- Zone 2: Area between Zone 1 (Public Access) and the strictly controlled Zone 2 (Control Room) and Zone 3 (Magnet). This is the area just outside of the restricted area Zone 3. This is the area of travel that patients are brought into their procedure.
- Zone 3: Control Room. All access to Zone 3 is to be strictly restricted with access to regions within it controlled by and entirely under the supervision of MR personnel. This zone is restricted from general public access by a reliable restricting method that can differentiate between MR and non-MR personnel.
- Zone 4: Magnet Room. No individual is allowed in the scan room without being supervised by trained MRI personnel. The scan room door is always locked when unattended. When the scan room door is opened, the MRI Safety barrier must be implemented at all times by MRI personnel (MRI Safety Barrier docx). Only MR compatible equipment approved by the MR Safety Committee may be brought into Zone 4. The MR technologists must be able to directly observe and control via line of sight the entrances or access to Zone 4 from their normal positions when stationed at their desks in the scan control Room.
All patient transfer will occur in the designated Zone 2 holding or induction area if available. For sites without a designated Zone 2 holding or induction area, patient transfer may occur in Zone 3. Before a patient is transported into the scan room (Zone 4), a verbal timeout process including all staff members assigned to the patient must occur:
- Identifying correct patient
- Review of the patient MRI screening form
- Screening and review of MRI Safety information with ancillary staff. Check for ferrous objects
- Screen and wand patient for loose ferrous objects or contraindicated devices such as drug patches, non-conditional ECG, EEG leads, Foley catheters with temperature probes
For MRI exams requiring anesthesia, a verbal timeout with the participation of all staff members assigned to the patient must occur to include the completion of the MRI Safety: Pre Anesthesia Timeout Form (docx).