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Effects of gradient anisotropy in MRI

✍ Scribed by Dr. Bernd Aldefeld; Peter Börnert

Publisher
John Wiley and Sons
Year
1998
Tongue
English
Weight
1007 KB
Volume
39
Category
Article
ISSN
0740-3194

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✦ Synopsis

A gradient system is anisotropic if the impulse responses of at least two of the gradient channels, x, y, or z, differ from each other. Such an undesired condition may arise, for example, from differences between the gradient channels with respect to eddy currents or from unbalanced time delays in the electronic components. Depending on the degree of anisotropy, the actual gradient then deviates from the nominal, desired gradient under certain oblique orientations during the transient periods of gradient switching. The adverse consequence is degradation of image quality, such as distortion, ghosting, and blurring. In this paper, a theoretical analysis is given of the basic effects. Furthermore, the implications for the MRI process and possible correction methods are described. The effects of anisotropy are shown experimentally for echo-planar imaging and two-dimensional selective RF excitation with spiral gradient pulses.

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