Emergency Magnet Rampdown

warning personnel Injury or equipment damage The Emergency magnet rampdown procedures in this section may create dangerous situations or damage equipment Perform Emergency magnet rampdown only in emergency situations (such as when a person is trapped between a magnetic object and the magnet). In situations where there is no immediate threat, refer to the appropriate Magnet and Cryogen manual for your system and follow the non-emergency rampdown procedure.

warning Asphyxiation Hazard! Emergency magnet rampdown will cause the magnet to quench. If venting system fails the magnet room may fill with cryogen gas. Before performing emergency magnet rampdown, ensure all room ventilation systems are on and functioning. ensure the magnet room door is propped open (or the hatch in a mobile van). After initiating emergency magnet rampdown, exit the room immediately and Remain near the floor, where oxygen is present. Stay outside of the magnet room until the quench is over and safe room oxygen levels are present.

Perform the following procedures in the order listed until emergency rampdown is achieved.

notice If you are unable to ramp down the magnet after following all the procedures in this section, contact your MR regional support engineer for assistance.

1. Magnet Rundown Unit (MRU): Press and release the red Rundown button on either the primary or remote MRU (the Rundown button will remain depressed for 30 seconds). The magnet will quench and the magnetic field will begin to decay. Field decay times may vary by magnet type—refer to the Magnet and Cryogen manual for the exact times for your system.

   Figure 1. MRU

   

   note:

   The illustration above shows an MRU example. Other models may be used on some systems. Refer to the Magnet and Cryogen manual for your system.

2. Engage the shim lead.

   note:

   This method will only work for magnets with shim leads (actively shimmed magnets). If your magnet does not have shim leads, go to vacuum break method, below. Shim lead configuration may vary by magnet type. Refer to the Magnet and Cryogen manual for your system.

   1. Unlock the Shim Lead Assembly from its disengaged position.

      notice As the magnet pressure rises in the next step, do not open V2 valve to release pressure. The increasing pressure is required to rupture the burst disk. This will cause the magnet to quench.

   2. Rapidly lower and engage the Shim Lead Assembly in the Sav-Con Connector.

3. Break the magnet vacuum to ramp the magnet down.

   notice Performing the vacuum break procedure will take the magnet out of service for at least one month and could permanently damage the magnet. This procedure should only be performed when all other rampdown attempts have failed.

   note:

   Vacuum port location and cover configuration vary by magnet type. Refer to the Magnet and Cryogen manual and service documentation for your system.

   1. Locate the Emergency Evacuation Release Tool (46-260852G3) for all magnet types (except S3 with Magnashield which uses 46-281847G1).

   2. Ensure all room ventilation systems are on and the magnet room door is propped open.

   3. Remove enclosure cover from the magnet cover exposing the vacuum port.

      Figure 2. Vacuum Port Enclosure Cover Removal

      

      note:

      For MR systems with a Salt block magnet cover configuration (shown below) or any other system (e.g., Vectra, Contour, Openspeed, etc.) for which the vacuum port is not easily accessible, call your GE Healthcare Service Representative for enclosure removal.

      Figure 3. Salt Block Magnet Cover Example

      

   4. Remove the protective dust cover from the vacuum port.

      Figure 4. Vacuum Port Dust Cover Removed

      

   5. Screw the handle of the Emergency Evacuation Release Tool into the plug of the vacuum port.

      Figure 5. Vacuum Release Tool Attachment

      

      note:

      If the magnet enclosure prevents the vacuum break tool from screwing into the vacuum port, use a non-magnetic 5 mm Allen Wrench to remove the two M6 screws that secure the magnet cover and then remove the bridge cover (the table may need to be moved for bridge removal).

      Figure 6. Magnet Cover Screws

      

   6. Lift the Emergency Evacuation Release Tool handle until it is parallel to the floor. This will engage the tool and break the vacuum seal. Leave the handle in this position. The vacuum break tool must stay attached to the vacuum port until pressure is equalized inside and outside of the magnet vessel.

      Figure 7. Vacuum Release Tool Engaged

      

      The inrush of air into the magnet vacuum may be loud. The magnet will quench in about 30 seconds.