cold head
Two-stage cryocooler that maintains the MRI magnet's cold shields at ~40 K, limiting helium boil-off from the inner cryogen reservoir at ~4 K. Runs continuously, paired with a remote helium compressor that delivers high-pressure helium to the cold-head heat exchangers. Sealed-bore / zero-boil-off magnet designs use the same physics with reduced cryogen-replacement burden.
Why it matters to buyers: Cold-head neglect is the #1 cause of expensive MRI ownership cost on conventional helium-fill magnets. Service interval ~2–3 years (closer to 2 in warm rooms or on 24/7 scanning duty). Skipped PMs → rising helium boil-off → cryogen-refill cost rises sharply → eventually a quench. A healthy 1.5T Symphony-class boil-off is < 0.03 L/hr; > 0.05 L/hr means the cold head is tired.
Why it matters to engineers: Audible compression cycle + temperature monitoring + boil-off-rate trend are the end-of-life signals. Before any cold-head swap: warm up the compressor, have the helium-recharge logistics on-site, and confirm the shim file is backed up.