Shimming in MRI can be performed by all the following EXCEPT:

Shimming in MRI is a crucial process designed to optimize the homogeneity of the magnetic field within the imaging volume, which enhances the quality of the images obtained. Each of the methods listed is associated with how shimming can be achieved, except for the one that suggests turning the shim coil off and on rapidly.

Turning the shim coil off and on rapidly does not contribute to improving the uniformity of the magnetic field. In fact, for effective shimming, the shim coils must be continuously operational and adjusted to provide a stable, consistent magnetic field. The shim coils work by redistributing magnetic field strength across various locations in the imaging area, and intermittent operation would introduce instability rather than improve field homogeneity.

In contrast, adding current to the gradient coils contributes to the optimization of field uniformity by adjusting the spatial magnetic field gradients, which can aid in achieving better shimming. Similarly, adding metal elements to different coils within the shim coil can also help in adjusting the magnetic field characteristics. Changing the current in the shim coil directly impacts the magnetic field produced by those coils, which is a fundamental part of the shimming process. These methods are all intended to fine-tune and improve the field homogeneity necessary for high-quality MRI imaging.