shim
Process of making the MRI magnetic field homogeneous across the scan volume. Two layers:
- Passive shim — iron shim plates installed during commissioning to compensate for static field inhomogeneity unique to the magnet and its room. Set once and (mostly) left alone.
- Active shim — electrically driven shim coils that fine-tune the field per scan, including dynamic shim before each volume on advanced sequences.
Why it matters to buyers: Poor shim = artifact and failed advanced sequences (DWI, MR spectroscopy, EPI / fMRI, prostate spectroscopy, breast diffusion). Shim is set at install and verified during PMs. A magnet with shim issues affects every clinical session.
Why it matters to engineers: Shim file is magnet-specific — back it up before any service event that could erase it (cold-head swap, board replacement, software re-image). A lost shim file forces a multi-day re-shim event with the manufacturer field engineer. "Shim coil fault" in service logs is often the shim power supply, not the coil itself — cheap fix first.