A disturbance of the
field homogeneity, because of magnetic material (inside or outside the patient), technical problems or scanning at the edge of the field.
When images were obtained in a progression from the center to the edge of the
coil, the
homogeneity of the field observed by the imaged volume, changes when the distance from the center of the volume increase.
The same problem appears by scanning at a distance from the
isocenter in left-right direction or too large
field of view.
There are different types of bad image quality, the images are noisy, distorted or the
fat suppression doesn't work because of badly set shim currents.
E.g. by using an IR sequence, changes in the T1 recovery rates of the tissues are involved. The
inversion time at the center of the imaged volume is appropriate to suppress fat, but at the edge of the
coil the same
inversion time is sufficient to suppress water. Since the
inversion time is not changed, the T1 recovery rates will increase.
Take a smaller imaging volume (and for
fat suppression a volume
shimming), take care that the imaged region is at the center of the
coil and that no magnetic material is inside the imaging volume.
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Further Reading:
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