Systems and methods for shim current calculation

A magnetic resonance (MR) imaging system, comprising:

• an imager comprising an MR magnet and being configured to acquire a forward spatial encoding polarity MR coil image and a reverse spatial encoding polarity MR coil image, the reverse spatial encoding polarity MR coil image being generated from data obtained with a magnetic field gradient that is reversed with respect to the magnetic field gradient with which the forward spatial encoding polarity MR coil image is acquired;
• and
• control circuitry configured to receive the forward spatial encoding polarity MR image and the reverse spatial encoding polarity MR image from the imager and to:
• perform an iterative shift map calculation algorithm in order to determine a pixel shift map based in part on the forward spatial encoding polarity MR coil image, the reverse spatial encoding polarity MR coil image, and a cost function quantifying pixel displacements between the forward and the reverse spatial encoding polarity MR coil images in order to stabilize at least linear first and second order spatial features of the pixel shift map;
• convert the pixel shift map into a magnetic field shift map by determining a magnetic field value corresponding to each pixel in the pixel shift map;
• and
• perform a shim calculation process utilizing the magnetic field shift map as an input in order to determine a level of a shim current passed through a shim coil of the imager in order to shim the MR magnet.

One or more tangible, non-transitory machine-readable media comprising instructions that when executed by a processor, cause the processor to:

• perform an iterative shift map calculation algorithm in order to determine a pixel shift map based in part on a forward spatial encoding polarity magnetic resonance (MR) coil image, a reverse spatial encoding polarity MR coil image, and a cost function quantifying pixel displacements between the forward and reverse spatial encoding polarity MR coil images in order to stabilize at least linear first and second order spatial features of the pixel shift map, wherein the reverse spatial encoding polarity MR coil image is generated from data obtained with a magnetic field gradient that is reversed with respect to the magnetic field gradient with which the forward spatial encoding polarity MR coil image is acquired;
• convert the pixel shift map into a magnetic field shift map by determining a magnetic field value corresponding to each pixel in the pixel shift map;
• and
• perform a shim calculation process utilizing the magnetic field shift map as an input in order to determine a level of a shim current passed through a shim coil of an MR imager in order to shim an MR magnet of the MR imager.