Correcting for main magnetic field drift in MRI scanners

摘要

Two 3D interleaved navigator pulse sequences (echo planar pulse sequences) with low flip angle and different echo times are used between imaging of consecutive voxels. A magnetic field map is obtained by complex division of the resultant pair of navigator images. This is used to obtain parameters to adjust the MRI frequency to compensate for B0 drift and shim to compensate for B0 inhomogeneity. Comparison with reference navigator images can also correct for subject motion. The system central frequency is adjusted by adjusting a phase and frequency for a Numerically Controlled Oscillator (NCO) for all radiofrequency (RF) excitation pulses, and for Analogue-to-Digital converter (ADC) pulses for both the parent scanning sequence and the two navigator pulse sequences.