Magnetic field homogeneity

One of the key concepts in assessing the quality of a magnet is that of magnetic field homogeneity, as it will relate to image quality and the presence of artifacts.Field homogeneity refers to the uniformity of the main magnetic field when no patient is present, measured in parts per million (ppm).  Magnetic field homogeneity needs to be optimized to reduce spatial distortion, blurring, shading, intensity loss, curved slice profile and banding artifacts. These may become more noticeable and important in large field-of-view studies, during gradient echo or fast/echo-planar sequences and images using spectral fat suppression techniques

Homogeneity needs to be optimized on an individual / patient-by-patient basis by “shimming” the magnetic field. Shimming involves the removal of magnetic field inhomogeneities and can be achieved by the use of shim coils, which superimpose small corrective field differences on the main field. Higher levels of homogeneity are required in MR spectroscopy as the proton resonances are often separated by less than 1ppm, and both static and temporal stability of the field need to be considered. Temporal stability refers to the consistency of the field strength over time, and effects image resolution.

Normal magnets contain various magnetic field inhomogeneities, which is one of the sources effecting T2*. This results in protons precessing at different Larmor frequencies and subsequently dephasing. Dephasing from the inhomogeneity contributions to T2* can be overcome by generating spin echoes, whereas signal loss owing to true T2 relaxation is irreversible.