shimming
Process of optimizing magnetic field homogeneity in MRI scanners to improve image quality and reduce artifacts.
Full Definition
Shimming is a critical calibration procedure performed before MRI scanning to achieve uniform magnetic field strength across the imaging volume. The process involves adjusting specialized shim coils or inserting passive shim materials to compensate for magnetic field inhomogeneities caused by the scanner environment, patient anatomy, or metallic objects. Poor shimming results in signal dropout, geometric distortion, and reduced image quality, particularly in areas near air-tissue interfaces such as the orbitofrontal cortex and temporal poles. Both automated and manual shimming techniques are employed depending on the specific imaging requirements.
Usage
Usage note: Distinct from 'tuning' which refers to radiofrequency calibration.
In Context
- "Second-order shimming was applied to reduce susceptibility artifacts in the orbitofrontal region." — MRI protocol documentation
- "The technologist performed manual shimming after automated shimming failed to achieve adequate field homogeneity." — Quality assurance report