Patent ID: 7123014

Claim:
Magnetic gradient field projection apparatus for generating a magnetic field along a projection axis, in a defined sample volume offset from and outside said apparatus, said projection apparatus comprising: gradient field generating means arranged to generate a substantially linear axial gradient component in said sample volume, said gradient field generating means comprising a substantially circular first coil positioned so that the center of the first coil is displaced along the axis of the first coil away from the centre of the sample volume by a distance approximately equal to half of the radius r 1 of the first coil, such that the second derivative of the axial gradient component is substantially equal to zero at the center of the sample volume, a further coil, the axis of the first coil being substantially coincident with the axis of said first coil, and distance of said further coil from the center of the sample volume, and the radius of the further coil, being selected such the angle subtended at the center of the sample volume by each of said first and further coils is substantially the same; the distance of said further coil from the center of the sample volume being substantially equal to half of the radius r 2 of the further coil, the first and further coils being connected so that, in use, the relation ( r 2 r 1 ) 4 = - I 2 ⁢ N 2 I 1 ⁢ N 1 is substantially satisfied where I 1 and I 2 are the currents flowing through said first coil and said further coil respectively, and N 1 and N 2 are the numbers of turns of said first coil and said further coil respectively, the radii of the first and further coils being mutually different, the currents in the first and further coils flowing in opposite directions.