Yes, they’re rare, but they’re also guaranteed to happen at least once (during final decommissioning of each superconducting MRI). I’m talking about a quench... the event during which the liquefied cryogen within a superconducting MRI boils rapidly, bursting forth from the scanner vessel. The MRI scanner system is provided with a number of safety features to mitigate the risks of a quench, but these equipment features - without appropriate site design and installation - won’t provide appropriate protection for your building and its occupants.
MRI Safety Week was just a couple weeks ago, and we responded to a number of questions about what designers, engineers, or facility managers should be doing for facility safety of MRI installations. One nearly constant response from us was to inspect and evaluate cryogen safety issues related to MRI.
An MRI quench which put an imaging center at risk.
Most facilities are unaware of two vital facts about the quench pipe for their MRI. 1) In virtually all cases the MRI vendor is only responsible for the maintenance of that part of the quench pipe within the RF enclosure (and then only for the duration of the warranty or service contract period). 2) The MRI vendor makes it the owner’s duty to annually inspect the quench pipe for condition.
Should the cryogen venting system fail, there are tremendous risks to patients and personnel, the MRI scanner itself, and the building in which the magnet is housed. The annual quench pipe inspection can help mitigate these risks. In order to help guide a thorough inspection process, RAD-Planning has a set of inspection criteria which we use when asked to provide quench pipe inspections.
The first step is to obtain the MRI manufacturer's most contemporary quench pipe standards for construction, pressure drop, insulation, and discharge design. Design criteria have changed over the years, so it is recommended to have the most contemporary standards for each MRI system. Have these available for comparison during your physical inspection.
Review your quench pipe designs, intended and – if you have them – ‘as builts’:
Identify specified materials, insulation, vapor barrier, and connections, in the original design documents or shop drawings.
Review design calculations for pressure drop and minimum pipe sizes based on length of run, direction, and bends. The further the quench pipe travels, and the more bends in the route between the magnet and the discharge point, the larger the pipe will need to get to meet the vendor’s pressure drop specifications.
Check original designs against current design criteria from MRI manufacturer and identify any discrepancies.
Inside the MRI scanner room (magnet room):
Pop ceiling tiles as needed to expose the quench pipe to where it penetrates (exits) the RF shield.
Survey the length of the exposed quench pipe for materials and construction (note any differences from designs). If as-built condition is different, verify pressure drop calculations based on as-built condition.
Check mechanical connections of the pipe, itself.
Are bolts tight?
Are band-clamps tight?
Are hangers secure?
(N.B. When checking sturdiness of the connections, do not move the pipe too much, as it could potentially damage the RF shield at the point where it penetrates)
Check for signs of condensation / water damage. These could be condensation trails on the quench pipe, itself, or water marks on the tops of removed ceiling tiles.
Check quality / quantity of pipe insulation and vapor barrier. The requirement for insulation from the MRI scanner to the discharge point is one of the areas where there has been significant change in vendor design criteria over the years.
Check for drip-tray, if specified.
Interior to the building, but beyond the MRI scanner room:
Check any readily expose-able length of interior quench pipe, particularly horizontal (above ceiling) runs, with same criteria as within the MRI scanner room.
Exterior to building, at the discharge point:
Check discharge pipe diameter against design & pressure-drop calculations.
Check the screen at the discharge end of your quench pipe for condition and hole size.
Check for warning signage at discharge point (standards usually call for hazard signage at the discharge point).
Check for marked identified exclusion radius from discharge point. The exclusion distance from the discharge point will vary depending upon your MRI’s specifications and your discharge condition. A distance of 25 feet (8 meters) radial distance between the discharge point and occupy-able areas will conform with most magnet’s requirements.
Check flashing / seal integrity at roof / wall penetration. Evidence of water in the magnet room may be the result of improper sealing where the pipe exits the building.
If vertical, 'through roof' discharge, make sure that discharge has at least 135-degree turn-down at end, 180-degree is preferred. 90-degree horizontal discharges are not sufficiently weather protected from wind-driven rain.*
If horizontal, 'through wall' discharge, make sure that horizontal length of discharge pipe has positive drainage slope to discharge point.
Whenever possible, have your quench pipe inspection coordinated with vendor PM of their portion of the system. Have them open the quench pipe at the MRI scanner to look for the presence of water, or evidence of past accumulation of water (mineral scaling or discoloration), at the 'elbow' transition piece. Look for signs of insects or animals. Look for debris / litter.
If any of the above inspection criteria fail, it is advisable to get a fiberoptic scope of the interior of the quench pipe immediately prior to making any corrections. Since much of the quench pipe is typically inaccessible, evidence of problems at one end may be symptoms of a problem that occurs in the middle.
The above inspection applies just to the quench pipe portion of the cryogen protections that ought to be in place. In addition to the quench pipe, MRI facilities should also check their oxygen detector, overpressure relief, and exhaust fan.
With the appropriate documentation for your MRI’s quench pipe, the above basic quench pipe inspection can often be completed in an hour. With the other cryogen protection components, perhaps a half-day.
If there have been changes from your MRI manufacturer's quench pipe design criteria between the original suite build-out and today (or if you've switched magnets and the quench pipe design criteria for the new magnet weren't reviewed for the switch), I recommend that you contact the manufacturer of your current magnet and ask them for guidance. If they can't, or won't, advise you, it may be wise to get an MRI suite design expert to help you identify the best approach for correction and/or remediation.
* One major vendor of superconducting MRI systems has, as their standard 'through roof' design, a 90-degree discharge with a 45-degree end chamfer to the pipe. It only takes a wind speed of between 20 and 25 mph to drive rain at more than a 45-degree angle, defeating this chamfered 'protection'. Despite the fact that this is the quench pipe discharge design provided by one MRI vendor (all of the others have moved away from this design), it is my opinion that this is insufficient discharge end protection and I recommend to my clients that this detail get changed.