A new concept of concomitant field free, low-field SQUID MRI.

We report a new MRI concept using a magnetic field configuration that eliminates the concomitant field effect, especially for low-field MRI. In low field MRI, the image acquisition time and/or the image resolution are constrained by small image encoding magnetic field gradients. Image encoding gradients always entail the undesirable concomitant fields according to the Maxwell equations. These fields are necessarily proportional to the encoding gradients, and result in various image artifacts. To reduce or correct the artifacts, the gradients should be small so that the magnetic field variation in field of view is a small fraction of the static homogeneous field. We obtained a field configuration such that the magnitude of the field changes monotonically along one direction by removing the constraint of uniaxial spin precession and linearity of field gradients. This field configuration is used to encode projections and the image is obtained from back projection reconstruction. A current configuration was obtained to produce this field configuration. An implementation design is proposed based on expanding the field and current configuration in terms of harmonics.