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 B0 field and without self-shielding can be quite susceptible to perturbations from external magnetic sources. B0 variation in such a system located close to a subway station was measured to be greater than 0.7 μT by both MRI and a fluxgate magnetometer. This B0 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 B0 variations were effectively removed using the navigator method.