Cryogens, Quench and Oxygen Depletion in MRI — What Is Measured, What the Guidance States, Where the Vent Path Has Failed, and Where the Evidence Stops

Cryogens, Quench and Oxygen Depletion in MRI — What Is Measured, What the Guidance States, Where the Vent Path Has Failed, and Where the Evidence Stops

MRIninja Knowledge Base | Child Page — MRI Safety (deep dive S12) Version 1.0 — October 2026

Parent page: MRI Safety — Master Page (hazard table in Section 3). Builds on: Magnetic Field, Hardware, and Homogeneity (why the magnet needs cryogens) · MR Site Zones and Access Control (zones, cryogen venting zone, emergency access) · Projectile and Torque Hazards in the Static Field (projectile incidents that lead to a quench) · Claustrophobia, Sedation, Monitoring, Equipment and Emergencies (resuscitation and removal of the patient)

1. Scope, Status and Boundaries

This page gathers what the sources read state on the cryogens of a superconducting magnet and on a quench: the hazards named by the guidelines, what has been measured during a quench, what has happened when the vent path failed, the effects of oxygen depletion, the question of whether to quench the magnet in an emergency, and what the guidance says to do afterwards. It does not repeat the physics of the magnet (Magnetic Field, Hardware, and Homogeneity), the definition of the zones and of the cryogen venting zone (MR Site Zones and Access Control), the response to a collapse or the equipment for resuscitation (Claustrophobia, Sedation, Monitoring, Equipment and Emergencies), or the projectile incidents themselves (Projectile and Torque Hazards in the Static Field).

Reading status. The page rests on the 2013 ACR guidance document (summary of a PDF copy), a review tool that restates it, Section 2.7.1 of the fifth edition (July 2026) of the UK MHRA guidelines (quotations), one experimental measurement of a planned quench (abstract and main text), one analysis of regulatory adverse-event reports, a manufacturer’s field safety notice, a trade-news report of a regulator’s alert, a gas supplier’s information sheet on oxygen-deficient atmospheres, one local procedure and one blank risk-assessment template. MHRA Chapter 6 and Appendix 1 (the requirements for site management and working practices for cryogens), the ACR Manual on MR Safety chapter on emergencies, ACR Appendix 3, the standards (IEC 60601-2-33), the guidance of any national authority other than the MHRA, and any manufacturer’s installation specification for quench pipes were not read. No systematic review, no incident database with denominators and no model or measurement of the oxygen concentration reached in a scan room after a quench was found.

What this page does not do. It gives no specification for a vent, a monitor or an alarm, and no emergency procedure for any facility. Statements that are the page’s own inference are marked Reasoning; where a source needed to complete a statement was not found they are marked (to be sourced).

2. Sources Read and What They State

Table 1. Documents used, what they were used for and how they were read.

3. Cryogens in the Magnet and the Volume of Gas They Can Produce

The MHRA guidelines state that superconducting magnets routinely use low-temperature liquefied gases such as helium to maintain superconductivity, and name the hazards as asphyxiation in oxygen-deficient atmospheres, cold burns, frostbite and hypothermia from the intense cold, and explosion following over-pressurisation from the large volume expansion of the liquid on evaporation. They state that in many systems the quantity of helium is large enough that the gas produced by normal boil-off cannot be contained and must be vented to the outside, that some systems use so little helium that they can be completely sealed, and that there should be no hazards provided that all potential sources of helium and nitrogen are managed, including through venting directly to the outside of the building [3]. The ACR guidance notes that helium (and, in older magnets, nitrogen) is not flammable and does not pose a fire hazard directly, but that liquid oxygen can result from supercooled air in the vicinity of the released gases [1].

The authors of the quench measurement quote as background that 1 L of liquid helium becomes 757 L of gas and that clinical scanners hold 1,500 to 2,000 L [4]; a conference abstract on magnet design gives 2,000 to 3,000 L for conventional niobium-titanium magnets [11]. Reasoning. With the first figures, release of a full inventory of 1,500 to 2,000 L would correspond to about 1.1 to 1.5 million litres (1,100 to 1,500 m³) of gas at ambient conditions; the sources do not say what fraction of the inventory is present at the time of a quench, how fast it is released, what volume of room it would enter if the vent failed, or what concentration of oxygen would result (to be sourced). The two inventory figures differ, and neither is the label of any particular magnet.

4. What Has Been Measured During a Quench

The only measurement read is of a planned quench of a 4-T research scanner at decommissioning. The maximum rate of change of the magnetic field was about 0.36 T/s at the magnet centre, about 18 s after the quench started, and about 0.02 T/s at the bore entrance, about 12 s after it; the field fell to zero in about 40 s; the authors state that these values are about 60 times (centre) and about 1,000 times (entrance) below the IEC cardiac-stimulation threshold of 20 T/s and judge cardiac stimulation unlikely to be a concern for this system [4]. Helium gas leaving the quench pipe was about 35 K at 0.10 m from the outlet and below 50 K for almost 2 minutes, about 150 K at 1.10 m, and ambient temperature was recovered about 3 minutes after the quench; the authors note that PTFE gaskets common in flanged pipes lose flexibility below 194 K and many PTFE products are not certified below 33.2 K, and they recommend restricting access around the pipe terminal and certifying pipe materials for low temperature [4]. Figure 1 shows the durations measured.

Figure 1 — Durations measured during a planned quench of one 4-T research system (not generalisable)

The ACR guidance states that quenching the magnet and having the field dissipate could easily take more than a minute [1]. Reasoning. The measured decay in one system (about 40 s) and the ACR statement (more than a minute) are different quantities from different sources and do not contradict each other; neither gives a decay time for a given magnet. The measurement is of one 4-T system, one pipe geometry and one helium volume; the thermal camera saturated at its lower limit; no pipe failure and no oxygen concentration were measured. It supports the statement that the field does not vanish at once and that the gas outside the outlet is very cold, not any statement about the room.

5. When the Vent Path Fails

A manufacturer’s field safety notice of April 2018 states that during a quench, if the helium vent path is impeded, helium can enter the examination room and displace oxygen, with a potential for death or serious injury (suffocation) if people are not evacuated in time; it concerned systems with a 3-inch metal burst disk, states that the customer cannot tell whether a system has the affected disk, and describes inspection and replacement by the manufacturer [6]. A trade-news report of an alert by the UK regulator describes two events at Scottish hospitals: in one, rainwater blown into the vent pipe pooled in a bend and froze, and when the magnet quenched the trapped helium spilled into the examination room with no staff present and no injury; in the other, the vent pipe was narrower than the vendor specified and the regulator’s analysis found that a quench could have raised internal pressure to 250 per cent of design; no injuries were reported, and the alert advised checking that external quench pipes meet vendor specifications [7]. The ACR guidance states that cryogenic gases may vent partially or completely into the scan room and that visible white clouds or fog near the magnet can signal this [1].

A 10-year analysis of voluntary adverse-event reports contains a report of a third spontaneous quench in three years (cause and consequence not given), a patient and gurney pulled into the magnet where the technician quenched the magnet to release them (fractures of the foot, ankle and leg), a service engineer with serious cryogenic burns of the hand, and a field service engineer who went into cardiac arrest under anaesthesia for treatment of a cryogen burn, classified by the authors as a death attributed to the MR system; cryogen burns were grouped with fires among non-radiofrequency thermal events, 57 of 906 thermal events (6 per cent) [5].

Reasoning. The sources show that a vent path can fail by an installation defect, a maintenance defect (freezing of water) or a component, and that the manufacturer and the regulator considered death possible; the news report and the notice describe no injury caused by gas in a room. The sources read give no rate of quench, of vent failure or of oxygen depletion; the reports of the adverse-event analysis are voluntary, without a denominator, and the injuries described occurred during service work and an intentional quench, not from a vent failure. The absence of reported injury is not an estimate of risk, and the single fatal case concerned burns in a service engineer.

6. Oxygen Depletion and Oxygen Monitoring

A gas supplier’s information sheet, not written for MRI, gives the effects of reduced oxygen in persons at rest: at 19 per cent some adverse effects that may not be noticeable; at 15 to 19 per cent impaired thinking and attention, faster pulse and breathing and reduced coordination, often without the person noticing; at 12 to 15 per cent poor judgment, faulty coordination and abnormal fatigue; at 10 to 12 per cent very poor judgment, breathing impairment and possible fainting within minutes without warning; and below 10 per cent inability to move, near-immediate fainting, unconsciousness, convulsions and death. It states that workplace regulations commonly regulate work in atmospheres below 19 to 19.5 per cent oxygen and that attempts to rescue someone in an oxygen-deficient atmosphere without supplied air most often result in additional victims [8].

The sources read differ on oxygen monitoring in the scan room. The ACR guidance states that room oxygen monitoring was discussed by its panel and rejected at that time because industry experts considered the technology not sufficiently reliable, in particular during power outages [1]; the review tool restates that oxygen sensors and alarms may not accurately report hypoxic conditions in certain failure modes [2]. A United Kingdom risk-assessment template lists, as arrangements a unit might record, an oxygen monitor in each magnet room with the alarm panel in the control room and an emergency extract fan designed to activate on oxygen depletion [10], and the MHRA text read lists quench pipes and oxygen and humidity monitors among the items a safety audit may check [3].

Table 2. Oxygen monitoring: statements by source.

Reasoning. The documents are not in direct contradiction (one is a 2013 panel’s judgement on the reliability of the technology, the other a template describing a United Kingdom arrangement) but they cannot both be applied as written. None of the sources read gives the oxygen concentration reached after a quench with a failed vent, the time to reach it, the reliability of any monitor, or evidence that a monitor or an extract fan has prevented harm (to be sourced). The effects in the first paragraph are those of an industry information sheet for persons at rest and are not a tolerance limit for patients, children or sedated persons.

7. Whether to Quench the Magnet in an Emergency

The ACR guidance states that quenching the magnet (superconducting systems only) is not routinely advised for cardiac arrest or other medical emergencies, because the field may take a long time to dissipate and a quench can theoretically be hazardous, so that ideally one should evacuate the magnet room; the staff are advised to use the time to start life support and move the patient to a location where the field is too weak to be a medical concern [1]. For fires, it states that quenching the magnet during a response to an emergency or fire should not be a requirement if trained MR personnel can keep responders out of Zones III and IV and the 5-gauss line, that if responders must enter, a decision to quench should be very seriously considered, that the field may be present and fully operational during a fire, and that after a quench the restrictions stay until designated MR personnel have verified that the static field is no longer detectable or no longer hazardous [1]. The review tool restates these points as: do not quench for emergencies where people can be evacuated expediently; a quench may not immediately remove the field; trained MR personnel must verify field reduction before admitting unscreened responders [2].

A local procedure states that the emergency quench button is for a fire the staff cannot contain and for a person pinned or trapped by a non-removable ferromagnetic object, that once initiated a quench cannot be stopped and may cause permanent damage to the magnet, and that the operator should evacuate the room if possible and close its door [9]. The adverse-event analysis contains a report in which a technician quenched the magnet to release a patient and gurney pulled into it [5]. The statement of a professional association that the magnetic field cannot be turned off during a cardiac arrest is recorded on Claustrophobia, Sedation, Monitoring, Equipment and Emergencies. Reasoning. The sources agree that quench is a last resort whose benefit (collapse of the field) is delayed and whose cost includes gas release and loss of the magnet; they identify as indications only a pinned or trapped person and a fire in which responders must enter. They give no measured time from the quench button to a field low enough to free a person, no comparison of outcomes with and without quench, and no data on injuries caused by an intentional quench (to be sourced). The question of closing the door of the room after evacuation appears in the local procedure and is not addressed in the other texts read, so it cannot be treated as a recommendation.

8. After a Quench or a Release of Gas into the Room

The ACR guidance states that after a quench all personnel and patients should be evacuated from the scan room as quickly as is safely feasible and access restricted until MR service personnel arrive, that the static field may persist despite a quench or partial quench so that responders must not enter without checks, and that signage should inform the public that the magnetic field is active even when power is off [1]. The manufacturer’s notice states that if helium enters the room, all persons should be removed immediately, air circulation and ventilation must not be switched off, and nobody should re-enter until it is confirmed that the oxygen content is at a safe level [6]. The quench measurement recommends restricting access to the area around the pipe terminal, as very low temperatures were recorded at 1.10 m [4]. The review tool restates that access should be restricted if gas clouds or fog are seen [2].

Reasoning. The statements converge on evacuation first, then restriction of access, then verification of the field and of the atmosphere by competent persons before anyone re-enters. The sources do not state how oxygen content is to be confirmed (with a portable monitor, by the installed alarm, by waiting), who is competent to do it, or how long the room should remain closed; they also do not state what to do with a person who has been in a low-oxygen atmosphere beyond the statement that rescue without supplied air often adds victims [8] (to be sourced). The documentation and reporting of the event belong to Incident Learning, Governance, Quality and Jurisdictions in MR Safety.

9. Where the Documents Differ and Open Questions

Table 3. Differences and unresolved questions.

10. Practice: the Arrangements the Sources Support

Reasoning. The following arrangements are the page’s synthesis of statements in the sources and are not themselves a recommendation of any one source. (1) Treat the vent path as a safety-critical item: that it ends outside the building, that it is built to the vendor’s specification, and that it is inspected and kept free of water and obstruction [3][7][4]. (2) Restrict access to the area around the outlet, which is very cold at 1.10 m [4]. (3) Do not quench for a medical emergency if the room can be evacuated; start life support and move the patient [1]. (4) Reserve quench for a person pinned or trapped and for a fire in which responders must enter, and know before the event who decides [1][9]. (5) After a quench or any visible gas, evacuate, restrict access, keep the ventilation running and allow re-entry only after the field and the oxygen content have been confirmed by competent persons [1][6]. (6) Decide locally, with the manufacturer, what oxygen monitoring is used and what it can and cannot report, since the sources differ [1][2][10]. (7) Rehearse the sequence with the persons who would respond (see Claustrophobia, Sedation, Monitoring, Equipment and Emergencies).

11. Limits of the Evidence and What Was Not Read

The guidance is a panel’s statement of 2013 and an extract of a regulator’s guideline; the experimental evidence is one measurement of one planned quench; the evidence of harm is a manufacturer’s notice, a news report of a regulator’s alert and a set of voluntary reports without denominators. No source read gives a rate of quench or of vent failure, a measured or modelled oxygen concentration in a scan room, an evaluation of an oxygen monitor, an extract fan or a quench pipe design, or outcomes of intentional quenches. MHRA Chapter 6 and Appendix 1, the chapter of the ACR Manual on emergencies and ACR Appendix 3, the standards, the guidance of other national authorities and manufacturers’ specifications were not read, so the page cannot say what current requirements are for vents, monitors or extract systems. All statements are those of the documents as returned by summarising tools, and were not checked page by page.

12. Topics Not Covered

Technical specifications of quench pipes, burst discs and run-down units; the refilling and handling of cryogens by service staff; magnet ramping; helium supply and cost; helium-free and low-helium magnets beyond the statement that some systems can be sealed; the toxicology of helium and of nitrogen; the treatment of a person with cold injury or hypoxia; the management of a fire in the MR environment; incident reporting, learning and the rules of individual jurisdictions (Incident Learning, Governance, Quality and Jurisdictions in MR Safety); and the cryogen venting zone in the zone model (MR Site Zones and Access Control).

13. Evidence-Based References

No systematic review and no incident database with denominators was read; the sections below follow the hierarchy of the sources actually read, and the guidance documents rest on consensus without stated evidence grading.

A. Guidelines, Consensus, Standards and Regulation

B. Measurement and Analysis of Reports

C. Manufacturer Notice and News Report of a Regulator’s Alert

D. Industry Information, Local Procedure, Template and Abstract

End of document — Cryogens, Quench and Oxygen Depletion (S12) — MRIninja v1.0 — October 2026. Parent: MRI Safety master.