In
homogeneity is the degree of lack of
homogeneity, for example the fractional deviation of the local
magnetic field from the average value of the field. Inhomogeneities of the static
magnetic field, produced by the scanner as well as by object
susceptibility, is unavoidable in
MRI. The large value of gyromagnetic coefficient causes a significant
frequency shift even for few parts per million field in
homogeneity, which in turn causes distortions in both geometry and intensity of the MR images.
Manufacturers try to make the
magnetic field as homogeneous as possible, especially at the core of the scanner. Even with an ideal
magnet, a little in
homogeneity is always left and is caused in addition by the
susceptibility of the imaging object.
The geometrical
distortion (displacement of the
pixel locations) are important e.g., for some cases as stereotactic surgery. Displacements up to 3 to 5 mm have been reported. The
second problem is the undesired changes in the intensity or brightness of pixels, which may cause problems in determining different tissues and reduce the maximum achievable
image resolution.