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A magnetic resonance imaging magnet includes a ferromagnetic frame. A pair of generally toroidal superconducting coil units overlie interfaces of side walls incorporated in the frame. Each coil unit may include a vessel having hollow support extensions extending into recesses in the side walls. The

A magnetic resonance imaging magnet includes a ferromagnetic frame. A pair of generally toroidal superconducting coil units overlie interfaces of side walls incorporated in the frame. Each coil unit may include a vessel having hollow support extensions extending into recesses in the side walls. The coil units may further include elongated, low-thermal conductance supports disposed within the support extensions. The frame may include pole stems projecting inwardly from the side walls, and the coils may be disposed in close proximity to the pole stems. Cryocoolers may be mounted to the frame so that the cryocoolers are substantially mechanically isolated from the coils of the coil units, but are in thermal communication therewith. The cryocooler mountings may be arranged for convenient servicing and installation of the cryocoolers.

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The invention claimed is: 1. A magnetic resonance imaging magnet comprising: (a) a ferromagnetic frame including a pair of generally vertical side walls spaced apart from one another, and flux return members extending between said walls, said frame having a generally horizontal axis extending betwe

The invention claimed is: 1. A magnetic resonance imaging magnet comprising: (a) a ferromagnetic frame including a pair of generally vertical side walls spaced apart from one another, and flux return members extending between said walls, said frame having a generally horizontal axis extending between said walls and having a medial plane transverse to said axis; and (b) a pair of superconducting coil units disposed between said side walls on opposite sides of said medial plane, each said coil unit including a coil of a superconducting material substantially coaxial with said axis and a vacuum vessel surrounding the coil, and (c) a pair of ferromagnetic pole stems projecting inwardly from said side walls toward said medial plane and extending at least partially through said coil units, said magnet having a patient-receiving space encompassing said medial plane, said coils and said frame cooperatively providing a magnetic field of at least about 0.5 T in an imaging volume within said patient-receiving space, said coils being spaced from said inner faces of said side walls by about 6 inches or less, said coils being spaced radially from said pole stems by about 5 inches or less. 2. A magnet as claimed in claim 1 wherein said patient-receiving space has a width in the direction of said axis of at least about 15 inches. 3. A magnet as claimed in claim 1 wherein said side walls are spaced apart from one another by about 65 inches or less. 4. A magnet as claimed in claim 1 wherein said magnetic field is uniform to 1 part in 107 or better throughout a region of said imaging volume at least 25 cm in diameter. 5. A magnetic resonance imaging magnet comprising a ferromagnetic frame including a first wall and a first pole stem projecting from said wall, the magnet further comprising a first coil unit including a primary field coil of superconducting material encircling the pole stem, a vacuum vessel surrounding the primary field coil, and a thermal shield within said vacuum vessel, said primary field coil being spaced radially from said pole stem by about 5 inches or less and spaced from said wall by about 6 inches or less. 6. A magnet as claimed in claim 5 wherein said frame includes a second wall and a second pole stem projecting from said second wall toward said first pole stem. 7. A magnet as claimed in claim 6 further comprising a second coil unit including a second primary field coil of superconducting material encircling the second pole stem, a second vacuum vessel surrounding the second primary field coil, and a second thermal shield within the second vacuum vessel. 8. A magnetic resonance imaging magnet comprising: (a) a ferromagnetic frame including a pair of walls spaced apart from one another, each said side wall having an inner face facing in an inward direction toward the other wall, and flux return members extending between said walls, said frame having an axis extending between said walls, each said wall having an array of recesses surrounding the axis, each said recess extending into the wall from the inner face thereof; and (b) a pair of superconducting coil units overlying the inner faces of the walls, each said coil unit including a coil of a superconducting material substantially coaxial with said axis and a vacuum vessel surrounding the coil, said vessels having hollow support extensions extending into said recesses, each said coil unit further including elongated coil supports disposed within said support extensions. 9. A magnet as claimed in claim 8 further comprising ferromagnetic pole stems projecting inwardly from said walls along said axis so that said coil units surround said pole stems. 10. A magnet as claimed in claim 9 wherein said coils are spaced from said pole stems by about 5 inches or less. 11. A magnet as claimed in claim 9 wherein said pole stems have ends remote from said walls, the magnet further comprising ferromagnetic pole caps separate from said pole stems overlying said ends of said pole stems. 12. A magnet as claimed in claim 8 wherein said coils are spaced from said inner faces of said side walls by about 6 inches or less. 13. A magnet as claimed in claim 8 wherein said recesses have inwardly-facing floor surfaces remote from the inner faces of said walls, and wherein said support extensions have end walls, said end walls of said support extensions bearing on said floor surfaces of said recesses, said elongated coil supports transmitting axially-directed magnetic forces applied to said coils through said end walls and floor surfaces to side walls. 14. A magnet as claimed in claim 13 wherein each said wall includes a main member having bores therein and fillers disposed in said bores so that each said bore forms one of said recesses and the filler disposed in each said bore defines the floor of such recess. 15. A magnet as claimed in claim 14 wherein said main members define outer faces of said walls and each said bore extends from the inner face of the wall to the outer face of such wall, and wherein each said filler extends into the main member from the outer face of the wall. 16. A magnet as claimed in claim 14 wherein said fillers include abutment portions defining the floors of the recess and wherein said abutment portions are pivotably connected to said main members. 17. A magnet as claimed in claim 8 wherein each said elongated support of each said coil unit includes an elongated first member having a coil end attached to the coil and extending generally axially outwardly from said coil end to an intermediate support end, an elongated second member having an outer end connected to the intermediate support end of the first member, the second member extending generally axially inwardly from its outer end to an inner end, and an elongated third member having inner end connected to the inner end of the second member, the third member extending generally axially outwardly from the inner end of the third member to an outer end, the outer end being mechanically connected to the vessel. 18. A magnet as claimed in claim 17 wherein, within each said elongated support, at least one of said members is pivotally attached to another one of said members and the coil end is pivotally attached to the coil so that the coil end of the first member is free to move relative to the outer end of the third member in a radial direction transverse to the axial direction. 19. A magnet as claimed in claim 17 wherein the second member of each said elongated support is in the form of a tubular shell and the first member is disposed within such shell. 20. A magnet as claimed in claim 17 wherein each said coil unit further includes a generally toroidal heat shield disposed within the vessel of such unit and surrounding the coil of such unit, and support-to-shield thermal conductors connected between said shield and to a location on each said elongated support adjacent the inner ends of the second and third members. 21. A magnetic resonance imaging magnet comprising: (a) a pair of superconducting coil units aligned with a generally horizontal axis, each said coil unit including a coil of a superconducting material substantially coaxial with said axis, a vacuum vessel surrounding the coil, and a thermal shield within the vacuum vessel surrounding the coil, said coil units being spaced apart from one another along said axis so that the coil units lie on opposite sides of a patient-receiving space; and (b) at least one cryocooler mounted in thermal communication with the coil units but substantially mechanically isolated from the coils of the coil units. 22. A magnet as claimed in claim 21 wherein the at least one cryocooler includes a pair of cryocoolers, each said cryocooler being associated with one of said coil units. 23. A magnet as claimed in claim 22 further comprising a ferromagnetic frame including a pair of generally vertical side walls spaced apart from one another and extending transverse to said axis, each said side wall having an inner face facing in an inward direction toward the other side wall, and flux return members extending between said sidewalls, each said coil unit overlying the inner face of one of said side walls. 24. A magnet as claimed in claim 23 wherein said frame further includes a pair of ferromagnetic poles projecting from said inner faces of said side walls along said axis, said poles extending through said coil units. 25. A magnet as claimed in claim 23 wherein each said coil unit is mounted to said frame. 26. A magnet as claimed in claim 22 wherein each said cryocooler includes a high-temperature thermal extraction element, each said unit further comprising a flexible heat conductor connected between the high-temperature thermal extraction element and the thermal shield. 27. A magnet as claimed in claim 22 wherein each said coil unit includes a cryogen reservoir in thermal communication with the coil of such unit, and wherein the cryocooler associated with each said coil unit includes a low-temperature thermal extraction element disposed in proximity to the cryogen reservoir for contact with cryogen vapor from the cryogen reservoir. 28. A magnet as claimed in claim 22 wherein the cryocooler associated with each coil unit is mounted above the coil unit. 29. A magnet as claimed in claim 28 wherein said cryocoolers are mounted to the side walls of said frame. 30. A magnetic resonance imaging magnet comprising: (a) a ferromagnetic frame including a pair of generally vertical side walls spaced apart from one another, each said side wall having an inner face facing in an inward direction toward the other side wall, a pair of poles projecting inwardly from said side walls along an axis so that said poles define a patient-receiving space therebetween, and flux return members extending between said sidewalls; (b) a pair of superconducting coil units, each said coil unit including a coil of a superconducting material substantially coaxial with said axis, a generally toroidal vacuum vessel coaxial with the axis surrounding the coil and extending around one of said poles, and a thermal shield within the vacuum vessel, each said coil unit further including a service port chamber disposed above one of said poles; and (c) a pair of cryocoolers mounted to the frame in thermal communication with the coil units through said service port chambers. 31. A magnetic resonance imaging magnet as claimed in claim 30 wherein said frame includes a pair of upper flux return members extending between said side walls, said upper flux return members being spaced apart from one another in a direction transverse to said axis so that said upper flux return members define an upper space above said axis, said service ports being disposed in end regions of said upper space above said poles, a central region of said upper space above said patient-receiving space being clear of said service ports. 32. A magnetic resonance imaging magnet comprising a pair of superconducting coil units spaced apart from one another along a substantially horizontal axis, each said coil unit including a coil of a superconducting material extending generally in a vertical plane coaxial with the axis and a vessel surrounding the coil, said vessel having a wall, each said coil unit further including a plurality of elongated straps extending between the wall of the vessel and the coil, said straps extending generally in tangential directions in a plane perpendicular to said axis, said straps being disposed around the circumference of the coil, said straps holding the coil substantially coaxial with said axis. 33. A magnet as claimed in claim 32 wherein said vessel includes a plurality of elongated projections, each said projection extending generally in a tangential direction transverse to the axis, each said strap extending within one of said projections. 34. A magnet as claimed in claim 32 wherein said straps are arranged in pairs, the straps of each said pair extending in opposite, generally tangential directions from the coil to the vessel. 35. A magnet as claimed in claim 32 further comprising a ferromagnetic frame including a pair of generally vertical side walls spaced apart from one another along said axis, each said side wall having an inner face facing in an inward direction toward the other side wall, said coil units overlying said inner faces of said side walls, said frame further including flux return members extending between said sidewalls and ferromagnetic poles projecting inwardly from said inner faces of said side walls along said axis.