GE Signa HD: cold head down, helium level falling
What happened
The cold head stopped working and the helium level in the magnet dropped to 33.1%, as reported in the case.
What was checked
- Helium level in the magnet, read at 33.1% in this case.
- Helium level tracked as a trend while the magnet sat without a working cold head, since boil-off continues even when the scanner is idle.
Cause
Cold head failure: without it the helium is no longer recondensed and the level falls steadily. What exactly caused the cold head to fail was not established in the case.
The cause is probable and was not fully confirmed.
What was done
Cold head replacement was planned. Until the repair, a helium top-up from a transport dewar was recommended — the engineers put it at about 12% of the dewar volume for an HD. They also noted that, per the manufacturer's documentation, quench risk is raised below roughly 55% helium, and that a quench can happen at any time, including during the fill itself. The plan was judged probable — the repair had not yet been completed.
Safety
Assume the magnet can quench at any moment: keep the MR room oxygen monitor active, keep the emergency vent path clear, and let helium transfers be done only by trained cryogen personnel.
Lesson: A failed cold head starts a countdown on the helium level — trend it daily, arrange the top-up and the cold head replacement early, and plan the fill assuming a quench is possible at any time.
Covered in the course: MRI Service — Level 2: Fault-Finding →
Written from an anonymised case shared by service engineers. It is not a manufacturer instruction and grants no authorisation. Independent note; names are trademarks of their owners.