Siemens Essenza: resonance frequency drift after shim tray work
What happened
After work on the shim tray, the magnet's resonance frequency drifted and the system would not shim back to specification.
What was checked
- Field measurement (flat-shot) confirmed the frequency had shifted.
- Inspection of the shim tray showed how the plates had actually been stacked.
Cause
The engineers found that the shim plates had been reinstalled incorrectly: without their plastic spacers, stacked directly on top of one another. The changed geometry distorted the field and pulled the resonance frequency off.
What was done
The plates were re-laid in the correct order with the plastic spacers in place; then the field measurement and shim iteration were carried out per a procedure from the manufacturer's service documentation, done by authorised personnel. The frequency returned to nominal — as reported in the case.
Safety
The magnet's static field is present at all times, even with the system powered down: work at the bore only with non-ferromagnetic tools and under strict MR access control.
Lesson: Reassembly details such as spacer placement are part of the calibration — rebuild shim hardware exactly as documented before you start measuring anything.
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.