There are a lot of NMR imaging techniques, and I'm not an expert, but my understanding (based on discussion with experts) is that the main magnetic field is sta

There are a lot of NMR imaging techniques, and I'm not an expert, but my understanding (based on discussion with experts) is that the main magnetic field is static during all MRI scans. The gradient fields from electromagnets change, though, to alter the Larmor frequency across the thing being imaged.