Gradient coil system for a nuclear resonance tomography apparatus

Claim 1

A gradient coil system for a nuclear magnetic resonance tomography apparatus, said apparatus including means for generating a fundamental magnetic field, in a volume, in the direction of the z-axis of an x-y-z coordinate system, said gradient coil system having a symmetry plane in the x-y plane of said x-y-z coordinate system and comprising: first saddle coil means for generating an x-gradient field in said volume and second saddle coil means for generating a y-gradient field in said volume; each of said first and second saddle coil means containing four main coils which are split arc saddle coils arranged on the same imaginary cylindrical surface having a radius ρ; each of said main coils of said first and second saddle coil means consisting of two elementary saddle coils one inside the other; each elementary saddle coil having a front coil arc substantially parallel to said x-y plane and spaced a distance Z therefrom, a rear coil arc substantially parallel to said x-y plane and spaced a distance H therefrom, and two straight coil sections connecting said front and rear coil arcs, said straight coil sections extending in the z-direction and defining an aperture angle α therebetween in the x-y plane and an angle φ in the x-y plane relative to the x-axis, and each elementary saddle coil having a predetermined number of ampere turns AT; said straight coil sections and said rear arcs of the two elementary coils forming each main coil being disposed substantially against each other; a pair of auxiliary saddle coils allocated to each of the main coils of each of said first and second saddle coil means; each auxiliary saddle coil having a front auxiliary coil arc substantially parallel to said x-y plane and spaced a respective distance Z aux therefrom, a rear auxiliary coil arc substantially parallel to said x-y plane and spaced a respective distance H aux therefrom, and two straight auxiliary coil sections connecting said front and rear auxiliary coil arcs, said straight auxiliary coil sections extending in the z-direction and defining a respective auxiliary coils aperture angle α aux therebetween and a respective angle φ aux in the x-y plane relative to the x-axis, and each auxiliary saddle coil having a respective predetermined number of ampere turns AT aux ; said straight auxiliary coil section and said rear auxiliary arcs of the auxiliary saddle coils forming a pair being disposed substantially against each other; each of said auxiliary coil arcs being disposed on a same further imaginary cylindrical surface having a radius ρ aux , with ρ≠ρ aux ; each of said auxiliary coils being angularly centered relative to the main coil allocated thereto; and the quantities AT, Z, H and φ which define the gradient field for each of said elementary saddle coils forming said main coils and the respective quantities AT aux , Z aux , H aux , and φ aux which define the gradient field for each auxiliary coil being matched to each other so that noise terms C(n,m) of a series development of said gradient fields are substantially eliminated.