Image Reconstruction: Off-Resonance Effects and Correction

1. INTRODUCTION When put into the magnetic field, a tiny but detectable fraction of the protons present in water in the body will align with this strong, steady magnetic field. During imaging, the tissue of interest is occasionally subjected to a brief electromagnetic radio-frequency (RF) pulse, essentially tilting the aligned protons off the axis of the main magnetic field. The protons then precess around the main field axis at a known frequency proportional to the main magnetic field. That field can be manipulated to identify the location of certain types of activity. The rate of precession can be used, for example, to encode the spatial position of the protons for imaging, by creating a gradient in the magnetic field along one direction [1]. The protons then precess at different frequencies depending on their location, as the magnetic field that they experience is due to both the main magnetic field and the applied imaging gradient. Signal intensity and spatial localization in MRI rely on a uniform magnetic field existing throughout the sample of interest. Much effort is devoted to properly designing and shimming the main magnetic field to achieve uniform intensity.