High-Homogeneity NMR and MRI Magnets With Minimized Fringe Field

This paper demonstrates the capabilities of a computer program to design magnets with exquisite field homogeneity and minimal fringe field. The program adjusts magnet parameters to reduce (typically to zero) selected coefficients cn in the power series expansion B
_z
(z) = Σc
_n
z
ⁿ
, with n ranging from -21 to +16 ( n ≠ - 2 or -1) and c
_n
evaluated by multiple differentiations of the on-axis field of a solenoid. Presented are illustrative MRI magnets of 1-m I.D. with field homogeneity of 12th order and fringe field annulled through order as high as 21st. ΔB/B(0) is 0.1-0.2 ppm within a 30-cm sphere. The volume of the OSHA “hazard zone” (|B| > 5 is as little as 0.5% that of an uncompensated magnet.