B_0 Mapping and Image Reconstruction in Low Field Permanent Magnet MRI Systems

摘要

Low field permanent magnet MRI systems have relatively large B_0 inhomogeneities, which lead to image distortions when using fast Fourier transform (FFT)-based reconstructions. It is possible to correct for those distortions in model-based (MB) image reconstruction, but existing techniques rely on having an accurate B_0 map, which is challenging to acquire and can also change if the magnet temperature drifts. Therefore, image-based B_0 mapping would provide an attractive solution. Standard techniques assume that the phase in a pixel of the image is given by the phase of the sampled signal at the echo-top. However, for very strong inhomogeneities this assumption does not hold, and leads to inaccurate B_0 maps. This work presents an iterative B_0 mapping approach in which B_0-induced phase changes during sampling are corrected for by MB image reconstruction.