Image correction during large and rapid B0 variations in an open MRI system with permanent magnets using navigator echoes and phase compensation

An open permanent magnet system with vertical B-0 field and without self-shielding can be quite susceptible to perturbations from external magnetic sources. B-0 variation in such a system located close to a subway station was measured to be greater than 0.7 mu T by both MRI and a fluxgate magnetometer. This B-0 variation caused image artifacts. A navigator echo approach that monitored and compensated the view-to-view variation in magnetic resonance signal phase was developed to correct for image artifacts. Human brain imaging experiments using a multislice gradient-echo sequence demonstrated that the ghosting and blurring artifacts associated with B-0 variations were effectively removed using the navigator method. (C) 2009 Elsevier Inc. All rights reserved.