Patent ID: 7786730

Claim:
A nuclear magnetic resonance machine comprising means for creating an intense main magnetic field B 0 in a usable interior space in the form of a tunnel with axis Z, means for radio-frequency excitation and for processing radio-frequency signals emitted in response by a body or object placed in said usable interior space, and a set of solenoidal gradient coils for superimposing on the intense magnetic field B o components of a additional magnetic field, said gradient coils being incorporated into tubes disposed in an annular cylindrical space situated between an exterior cryogenic enclosure containing said means for creating an intense magnetic field B 0 and said usable interior space, the diameter of the gradient coils being inscribed in a thickness of said annular cylindrical space, the solenoidal gradient coils being adapted to produce a first field gradient X in a first radial direction (x) of the machine perpendicular to the axis Z and a second field gradient Y in a second radial direction (y) of the machine also perpendicular to the axis Z, the first direction (x) being perpendicular to the second direction (y), and the machine including amplifiers for simultaneously energizing said solenoidal coils with algebraic sums of currents corresponding to the two gradients, characterized in that a solenoidal coil whose diameter is inscribed in the cylindrical annular space and that produces a field gradient Z in a direction (z) parallel to the main field B 0 is coaxial with another solenoidal coil inscribed in the same tube of this annular cylindrical space and producing a field gradient X in a direction (x) perpendicular to the direction (z), in that it further comprises between said exterior cryogenic enclosure and said annular cylindrical space a cylindrical screen along the axis Z comprising a conductive material cooled to a temperature less than or equal to 77 K and chosen to have resistivity less than or equal to 10 -8 Ω·m such that negligible induced currents are generated in the exterior cryogenic enclosure while the eddy currents induced in the cylindrical screen contribute to reinforcing the gradients produced by said solenoidal gradient coils.