| 摘要 |
A magnetic resonance imaging system having an exterior cryogenic enclosure containing a device for creating an intense main magnetic field in a usable interior tunnel space an RF exciter, a set of solenoid gradient windings in a cylindrical space around the interior tunnel space and electronic control circuits. The cryogenic enclosure includes an interior cylindrical space vacuum at room temperature having the set of windings therein, at least one thermal screen in a temperature range of 20° K to 80° K, a cold box below 5° K, and a former supporting the exciter for creating an intense main magnetic field. To reduce acoustic noise and cryogenic losses, an additional envelope is between the set of windings and the vacuum enclosure, the additional envelope being of a conductive material having electrical resistivity at least 7×10−8 &OHgr;·m and having a characteristic frequency no more than the characteristic frequencies of each of the other components of the exterior cryogenic enclosure. |
