MRI Quenching and Venting
MRI Quenching Vents
Let’s start with the basics about a quench system venting. MRI systems use cryogens (usually helium) to cool the magnet in the MR scanner. This cooling can drastically reduce the amount of power that’s required to control the MR scanner. With temperatures hundreds of degrees below freezing, cryogens are cold enough to freeze human tissue within seconds, and pose serious cold burn and frostbite hazards. As cryogens are released to keep the magnet cool, they evaporate into odorless, colorless and tasteless gases. Most of the gas is recaptured, but some escapes during the process. These gases are still extremely cold and are normally vented safely out of the building.
An MRI quench pipe, like US Tubing clamp or flange connect systems, is used to safely expel helium out of the building quickly. If the magnet overheats, due to an accident, a scheduled ramp down, a broken magnet, or a piece of metal getting to the magnet, you will need to shut it off right away. As the magnet loses its superconducting capacity, the cryogens immediately begin to boil and expand inside the cryostat. The expanding cryogens increase the pressure and temperature until, in a matter of seconds, the pressure-relief valve gives way. The expanding cryogens erupt from the cryostat into the quench pipe, continuing their dramatic expansion as they barrel through the pipe to the discharge point - usually a specifically designed vent hood (see below).
Helium Quench Pipe Systems
An MRI quench pipe is vital part of the overall system. During a quench, a large volume of super cold helium gas needs to be safely vented outside. Failure of the cryogenic vent can result in this cold gas going into the magnet room or another portion of the building. MRI quench pipes must hold up to the drastic shift in temps from ambient to that of liquid helium within seconds. Failure of proper helium MRI quench pipes could result in helium leaking into the room and jeopardizing patient safety.
It’s also important to check out the discharge point. All roof-discharging quench pipes should stand up off of the roof surface in order to keep debris from being knocked into them. Usually a 36-inch minimum is required. You should also make sure it’s high enough so drifting snow can’t block the discharge.
Although specifications vary for MRI Quench pipe for venting, typically the pipe is constructed of heavier gauge stainless-steel and fully welded at all seams. Depending on the requirements, a secure, airtight clamping system can be effective.
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Some further specifications include:
Cryogen (Quench) Vent Pipe
Provide a dedicated cryogen vent pipe (quench pipe) run as directly as possible to the outdoors.
The vent pipe must meet the pressure and diameter requirements of the MRI system manufacturer and is to be fully insulated to the point of discharge. At the discharge, the vent pipe must provide a weather-head to prevent the introduction of horizontally driven precipitation. Discharge direction should be downward. Cryogenic gas vent discharge should be located 25 feet from any air intake or operable window. The 25 foot radius exclusion zone should be clearly marked and staff and contractor personnel should be restricted from working in the area until they have been educated to the risks of cryogenic gasses escaping the vent pipe.
Emergency Exhaust System
Provide a dedicated emergency exhaust system to exhaust the MRI Scanning Room in the event of a cryogen discharge into the room. The system shall be activated either automatically by the MRI alarm panel (or oxygen sensor for MRI systems without a quench alarm relay) or a manual wall switch, one located inside the MRI Scanning Room and another located at the operator’s console in the MRI Control Room. Locate the grille for the exhaust system in the ceiling to the rear of the MRI Scanner Room, opposite the location of the MRI Scanner Room door.