th  - 1988 - 51st ENC Conference

[WED 11-10 I SELF SHIELDED GRADIENT COILS AND THEIR APPLICATIONS
TO IMAGING: P. B. Roemer *1, W. A. Edelstein 1, G. H. Glover 2. (1) GE Corporate Research and
Development Center, Schenectady, NY 12345. (2) GE Medical Systems, Milwaukee, WI 53188.
Gradient induced eddy currents adversely affect many imaging and volume spectroscopy techniques. In this
talk we cover three aspects of gradient coil design and their applications: i) the nature of gradient induced
eddy currents and their correctable and uncorretable components; ii) the design of self-shielded gradient coils
and limits to which they can reduce eddy currents; iii) the impact of eddy currents on imaging and selective
volume spectroscopy experiments.
We show that unshielded gradient coils of conventional design produce eddy currents on the order of 20%
of the gradient field. Shaping of the gradient waveform is typically used to compensate for eddy currents and
this technique in general can only be used to correct a single point in space. Other points in the imaging
volume will have time dependent field errors on the order of a few tenths of a percent. We show that this
level of eddy currents can adversely affect many imaging experiments such as multi-echo multi-slice, cardiac
cine, phase contrast angiogrophy and volume selective spectroscopy. By using self-shielded gradient coils it
is readily possible to reduce eddy currents by at least another factor of 30, making them almost undetectable.
Results are presented for coils we have designed and operated that range from small fast gradients (20 G/cm
100 usec risetime, 15 cm bore) operated in small bore imaging systems to whole body imaging gradients
(1G/cm, 500 usec, 66 cm bore).
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