Object ID: 00000018WIA30800870GYZ
Topic ID: id_40022397 Version: 1.8
Date: Jul 26, 2022 1:17:10 PM

Cryogen and quench concerns

With superconductive MR systems, another concern related to the static magnetic field is a quench of the cryogens. A superconductive magnet uses cryogens to super-cool the electrical conductor that creates the magnetic field. Temperatures as low as -269°C (-452°F) are achieved to create the proper environment within the magnet. A quench, which is a sudden boil-off of the entire volume of cryogenic liquid, causes a rapid loss of the static magnetic field.

Liquid cryogen hazards

Cryogens come in large vacuum containers called dewars. Liquid helium is generally used for cooling purposes, although some service procedures also require liquid nitrogen. Nitrogen dewars weigh from 400 to 500 pounds when full. Helium dewars weigh from 700 to 800 pounds. In addition to large dewars, there may be smaller helium gas cylinders present. This helium gas is used to fill the magnet to proper cryogen levels. Special considerations should be observed when dealing with cryogens.

CAUTION

Leaking helium or nitrogen gas will displace oxygen. The ambient air oxygen concentration may then be insufficient for human respiration. The limit of the air oxygen concentration should meet national laws or regulations.

CAUTION

The following information defines the proper handling of cryogens. IEC 60601-2-33: 201.7.9.2.101 (f)

Dewars and cylinders should not be tipped or heated, nor should the valves be tampered with.
The cryogens boil off as they cool the magnet wires and must be replenished periodically by qualified personnel. The rate of boil-off should be monitored by checking the cryogen level meter found on the system cabinet.
Contact with the cryogenic liquids or gas could result in severe frostbite; care should be taken when in proximity to these substances. The wearing of protective clothing is essential during all work in conjunction with liquefied cryogens. Such clothing consists of
Safety gloves
Work gloves
Face shield
Laboratory coat or overalls (cotton or linen)
Non-magnetic safety shoes
Dewars should be stored in a well-ventilated area. Cryogens could be accidentally released in gaseous form, resulting in an asphyxiation hazard.
All dewars and gas cylinders must be non-magnetic.
Gas cylinders should be stored upright and secured to the wall with a chain with the metal protective top in place. (If a cylinder falls over or the valve is knocked off, the container may act like a rocket; a full cylinder has enough power to penetrate walls.)
Because the cylinder’s metal cap may be magnetic, the cap should always be removed before bringing the cylinder into the magnet room. IEC 60601-2-33: 201.7.9.2.101 ff
If possible, all personnel should stay out of the magnet room when a qualified service engineer is filling cryogens in the magnet. If personnel must be present, they must wear proper gloves, a face shield, and ear protectors.
A qualified service engineer should be present any time cryogens are transported within the hospital or added to the magnet.
It is crucial that ventilation and cryogenic systems be kept in good repair and checked regularly to ensure proper functionality.
Flammable materials must not be brought near the cryogen containers.
You are responsible for establishing and following a procedure, in accordance with your local and federal requirements (in the US: OSHA 29 CFR 1910.36), that includes possible evacuation of the MRI area, if flammable materials are identified near cryogenic gases. If grease, oil, or other combustible material is present in the vicinity of the containers, the escape of cryogenic gases can lead to the formation of a potentially combustible liquid due to liquefaction of air and concentration of oxygen.

Quench vent failure hazards

ID # US256628

Warning

IEC 60601-2-33: 201.7.9.3.101 dd In the unlikely event of a quench and vent failure, a procedure needs to be in place to evacuate the patient and all personnel from the magnet room. Failure to follow these precautions can result in serious injury (e.g., asphyxiation, frostbite, or injuries due to panic).

LHe will transform into a gaseous phase if a vacuum breach occurs. When this happens, the normal room temperature in the vacuum space will cause the liquid helium to quickly boil away and must be vented out of the building. Never install a magnet to an uncompleted vent and if a vent line has a failure, do not perform any service operations on the magnet and system.

LHe will also transform into a gaseous phase when the electrical energy stored in the magnet cryostat is released. A rapid release of wire current is defined as a magnet quench. A quench can occur when part of the superconducting coil enters the normal (resistive) state. When this happens, the electric current dissipates as heat and the liquid Helium quickly boils away and must be vented out of the building.

The very cold temperatures, dynamic forces, and internal pressures produced by this expanding gas must be accommodated by the customer supplied cryogen vent system.

Room safety is ensured when a quality and compliant cryogen vent system is connected to the GE magnet to the customer supplied cryogen vent. The cryogen vent will allow the Helium gas to flow out of the MR scan room to unoccupied spaces.

Cryogen management is a safety issue. All GE and Medical Device suppliers must follow IEC regulations.

Quenches are indicated by a loud noise, warning message, or the tilting of an image on the display screen. A quench is a hazard only if the vent fails. This would present a potential asphyxiation hazard to both the patient and personnel.

It is critical to have a well-planned method to quickly evacuate the patient and personnel from the magnet room should a quench occur.

If a quench occurs and the vent does not fail:
- The cryogen vent rapidly emits a helium exhaust plume.
- The level of helium displayed on the meter falls.
- Prescan fails.
- The system emits no gradient sound.
If a quench occurs and the vent fails:
- The system releases white clouds of cryogen vapor into the magnet room.
- Frost forms above the magnet.
- The level of helium displayed on the meter falls.
- The oxygen monitor emits an alarm.