th  - 1987 - 51st ENC Conference

MF71
A DESIGN OF HOMOGENEOUS SELF-SHIELDING GRADIENT COILS
I. J. Lowe
Physics Department, University of Pittsburgh, Pittsburgh, PA
15260 and Department of Biological Sciences, Carnegie Mellon
University, Pittsburgh, PA 15213
Magnetic field gradients are of fundamental importance in NMR
imaging and volume localization techniques. For apparatuses
with cylindrical symmetry, these gradients are usually
generated by discrete coils wound on cylindrical formers (of
radius R). These coils are usually designed using spherical
harmonic or Taylor expansions of magnetic fields due to
current distributions (I). The gradients are usually
satisfactorily linear over a spherical volume of less than
0.5 R. Further, these gradients are normally time dependent
and the fringing magnetic fields generate eddy currents in
the surrounding metal surfaces that can produce undesirable
magnetic field inhomogeneities and time dependences.
We have developed a calculational technique based upon
Maxwell's Equations along with conditions of rotational and
translational symmetries to find the behavior of the desired
magnetic field over all of space. From these fields, the
necessary current density distributions on the surface of a
cylinder of radius R I are derived for x, y, and z gradients.
These gradients are perfectly homogeneous inside the cylinder
for the described current distribution and a long enough
cylinder.
It has been suggested (2-5) that a dynamically driven shield
can be designed to cancel the fringing field outside of a
second cylinder of radius R 2 (R 2 > RI). We further describe
the current distribution on the surface of this second
cylinder that perfectly shields the fringing field of the
first cylinder.
I. F. Romeo and D. I. Hoult, Hag. Res. in Med. ~, &4
(1985).
2. p. Mansfield and B. Chapman, J. Phys. E. Sci. Instr.
I__99, 540 (1986).
3. B. Chapman and P. Mansfield, J. Phys. D. Appl. Phys. I__99,
L129, (1986).
4. P. Mansfield and B. Chapman, J. Magn. Reson. 66, 573
(1986).
5. R. Turner and R. M. Bowley, J. Phys. E. Sci. Instr. I__99,
876 (1986).