Active coil to shift a volume of uniform magnetic field

1. A method for operating a magnetic resonance imaging (MRI) system, the method comprising:

operating gradient coils to acquire magnetic resonance (MR) signals encoding a first MRI image over a first region inside a main magnet of the MRI system in which at least a portion of a subject is placed, the first region being located within a volume of magnetic field where a field inhomogeneity is below a defined threshold;

energizing an active coil to shift the volume of magnetic field such that a second region inside the main magnet of the MRI system is located within the shifted volume of magnetic field, at least a portion of the second region being located outside of the volume of magnetic field before the volume of magnetic field has been shifted and at least a portion of the first region being located outside of the shifted volume of magnetic field after the volume of magnetic field has been shifted; and

operating the gradient coils to acquire MR signals encoding a second MRI image over the second region.

2. The method of claim 1 , wherein energizing the active coil comprises:

energizing the active coil while operating the gradient coils to acquire the second MRI image.

3. The method of claim 1 , wherein energizing the active coil comprises:

energizing the active coil prior to operating the gradient coils to acquire the second MRI image.

4. The method of claim 1 , wherein energizing the active coil comprises:

driving the active coil with at least 2 amperes of electrical current.

5. The method of claim 1 , wherein energizing the active coil comprises:

cooling the active coil by running liquid coolant to the active coil.

6. The method of claim 1 , further comprising:

operating a shim coil to compensate susceptibility differences inside the portion of the subject located within the shifted volume of magnetic field with inhomogeneity below the defined threshold.

7. The method of claim 6 , wherein operating the shim coil further comprises:

operating the shim coil while operating the gradient coils to acquire MR signals encoding the second MRI image.

8. A magnetic resonance imaging (MRI) system, comprising:

a housing having a bore in which at least a portion of a subject to be imaged is placed;

a main magnet accommodated by the housing and configured to generate a volume of magnetic field with inhomogeneity below a defined threshold to form an MRI image over a region located within the volume of magnetic field;

an active coil that when energized causes the volume of magnetic field to shift in location and transform in shape;

pulse generating coils to generate and apply radio frequency (RF) pulses in sequence to scan the portion of the subject;

gradient coils to provide perturbations to the volume of magnetic field such that MRI signals encoding an MRI image are acquired in response to the applied RF pulses; and

a control unit coupled to the MRI system and configured to:

operate the gradient coils to acquire MR signals encoding a first MRI image over a first region inside the main magnet in which the portion of the subject is placed, the first region being located within the volume of magnetic field in which a field inhomogeneity is below the defined threshold to form the first MRI image;

energize the active coil to shift the volume of magnetic field with inhomogeneity below the defined threshold such that a second region inside the main magnet of the MRI system is located within the shifted volume of magnetic field in which the field inhomogeneity is below the defined threshold, at least a portion of the second region being located outside of the volume of magnetic field before the volume of magnetic field has been shifted and at least a portion of the first region being located outside of the shifted volume of magnetic field after the volume of magnetic field has been shifted; and

operate the gradient coils to acquire MR signals encoding a second MRI image over the second region being located within the shifted volume of magnetic field in which the field inhomogeneity is below the defined threshold to form the second MRI image.

9. The MRI system of claim 1 , wherein the active coil and the gradient coils are integrated into one mechanical assembly.

10. The MRI system of claim 9 , wherein the integrated coil assembly includes one liquid cooling system for both the active coil and the gradient coils.

11. The MRI system of claim 8 , wherein the active coil is constructed as a removable module configured to be mounted along with a gradient assembly that houses the gradient coils.

12. The MRI system of claim 11 , wherein the active coil and the gradient coil are separately cooled by respective liquid cooling systems.

13. The MRI system of claim 12 , wherein the cooling system incorporates a reservoir holding liquid helium or liquid nitrogen.

14. The MRI system of claim 8 , wherein the active coil is constructed on an RF coil assembly configured to be mounted inside the gradient coils.

15. The MRI system of claim 14 , wherein the active coil and the gradient coils are separately cooled by respective liquid cooling systems.

16. The MRI system of claim 15 , wherein the cooling system incorporates a reservoir holding liquid helium or liquid nitrogen.

17. The MRI system of claim 8 , further comprising:

a group of shimming coils placed around the housing and configured to compensate susceptibility differences inside the portion of the subject located within the volume of magnetic field.

18. The MRI system of claim 8 , wherein the control unit is further configured to energize the active coil to shift the volume of magnetic field wherein variations in magnetic field strength within the volume of magnetic field are below the defined threshold.

19. The MRI system of claim 8 , wherein the control unit is further configured to energize the active coil to shift the volume of magnetic field wherein a free induction decay (FID) signal from the volume of magnetic field has a spectral width that is below the defined threshold.