Abstract: The present invention provides a magnet assembly (30a, 30b) comprising a pair of drive coils (31a, 31b; 32a, 32b; 33a, 33b) symmetrically disposed with respect to a reference plane. The reference plane is located approximately equidistant between the drive coils. Each of the drive coils has a current flowing through it which is in the opposite sense to the current flowing through the other drive coil, such that a primary magnetic field is generated in a plane which is parallel to and substantially coincident with the reference plane. The assembly may comprise further compensating coils (44a, 45a, 46a) which are configured to generate a magnetic field to improve homogeneity of the primary magnetic field. The drive coils and the compensating coils are generally in the shape of a racetrack and are preferably superconducting coils.

Abstract: The present invention is a saturated ferromagnetic structure for controlling the homogeneity of the primary magnet field of a magnet in a Magnetic Resonance Imaging system. The structure has a plurality of coaxial laminations disposed thereon about a central axis of the structure, such that the structure is magnetically saturated and generates a magnetic field parallel to the central axis. In use the structure functions to improve the homogeneity of the primary magnetic field. The structure further allows for a MRI system with a less confined patient space than is currently available with known MRI systems.

Abstract: The present invention relates to magnets and to magnetic resonance imaging systems. The magnet is open with magnetic coils arranged in quadrant, separated about two perpendicular planes, a midplane and a plane of reflection, and wherein the windings are configured such that, in operation, current flow is symmetrical about the plane of reflection and anti-symmetrical about the midplane, to produce a nett magnetic field at the center in a direction perpendicular to the plane of reflection.

Abstract: The present invention is an improved magnet coil former for use in a magnetic resonance imaging (MRI) system. The improved magnetic coil former reduces the strength of the magnetic fields generated proximate the coils. This enables the coils to be driven at higher currents. The former consists of a substantially magnetically saturated ferromagnetic material and is arranged such that the compensating coil is magnetically attracted towards the former in a direction radially outward from the axis of the coils thereby reducing the collapsing hoop stress on the compensating coil. The former is made of Iron or other suitable ferromagnetic material. The former facilitates construction of a more compact MRI system.

Abstract: A pulse tube refrigerator comprises a fixed pressure casing (3) containing cold parts and a removable pressure casing (2) containing serviceable parts. The fixed pressure casing (3) and the removable pressure casing (2) are coupled together via a joining member (1). During cooling operation the joining member (1) is arranged such that refrigerant fluid flows between the fixed and the removable casings. During servicing the joining member is arranged to cut off flow of refrigerant fluid between the fixed and removable casings, such that refrigerant in the fixed casing is trapped and parts in the removable casing are accessible for servicing. After servicing, substantially pure refrigerant fluid is pumped into the removable casing (2), the fixed and removable casings are re-joined and the joining member (1) is arranged such that refrigerant fluid flows between the casings again.

Abstract: The present invention relates to the mounting of magnet coils as used in Nuclear Magnetic Resonance systems. In particular, the mounting scheme is relevant to gradient magnet assembly of such systems. Using the present invention hard contact between gradient magnet assembly and a cryostat does not occur. This allows a reduction in the transmission of mechanical vibrations and any resultant disturbance. Furthermore vibration of the gradient magnet assembly is reduced enabling improved system performances.

Abstract: Described herein is a temperature control system for cooling magnetic elements (14) in MRI apparatus (10). The control system comprises a wax (16) in contact with the elements (14) which is substantially maintained at its phase transition temperature between a solid state and a liquid state, but in a substantially solid state. A sensor (18) is immersed in the wax (16) and operates to provide a signal on the change of state of the wax (16). The sensor (18) is connected to a controller (20) which controls the operation of a heating element (26) also immersed in the wax (16) to control the temperature thereof. When the MRI apparatus is operational, heat is generated by the magnetic elements (14) is used to change the wax (16) to a liquid, this change being detected by the sensor (18) which sends signals to the controller (20) to turn off the heating element (26).

Abstract: The present invention relates to the mounting of magnet coils as used in Nuclear Magnetic Resonance systems. In particular, the mounting scheme is relevant to gradient magnet assembly of such systems. Using the present invention hard contact between gradient magnet assembly and a cryostat does not occur. This allows a reduction in the transmission of mechanical vibrations and any resultant disturbance. Furthermore vibration of the gradient magnet assembly is reduced enabling improved system performances.

Abstract: Apparatus is provided for generating an accurate magnetic field in a magnetic resonance imaging device. The device comprises a number of permanent magnet assemblies (1) each of which is in thermal contact with a plate (5) having good thermal conductivity, the plate (5) is positioned between said permanent magnetic assemblies (1) and gradient coils (3). The plate (5) has connected thereto one or more temperature sensors (7) which provide a signal to control circuit (8) which is arranged to control the operation to a plurality of thermoelectric heat pumping devices (9) which are connected to a plate (5) so that the overall heat generated by the heat pumping devices (9) and the gradient coils (3) is kept constant and is dissipated by a yoke (10) of the magnetic resonance imaging apparatus.

Abstract: The present invention relates to a cooling system for a cryostat incorporating a pulse tube refrigerator 8. The cryostat usually consists of several interior cylindrical or oval-shaped vessels for storing liquids. The cryostat for MRI/NMR applications and related fields usually comprises an outer vacuum case, a nitrogen can or a radiation shield instead, and a further radiation shield and the helium vessel housing 4 the superconducting magnet 2. The magnet usually is made of NbTi, Nb3Sn, or some HTC conductor or a combination of both. The present invention uses a pulse tube refrigerator 8, the so-called pulse tube cooler, which essentially comprises an empty tube and a further tube housing the regenerator material for the cooler, which is inserted into any opening of a cryostat. The opening is defined as e.g. the neck-tube 6 or any other location required for accessing the cryostat, e.g.

Abstract: A temperature control system for a permanent magnetic assembly in a permanent magnetic system comprises thermoelectric heat pumping devices which are in thermal contact with a yoke of the permanent magnet system. The heat pumping devices are controlled by an electronic circuit which controls the current that flows through them, so that the desired temperature of the permanent magnetic assemblies is achieved. Each thermoelectric heat pumping device includes a spring-loaded plunger which is mounted in a hole in the yoke. Heat is exchanged from or to the yoke via ambient air by natural convection or by forced air. Alternatively heat may be exchanged from the yoke via a cooling water circuit.

Abstract: An apparatus for and method of control of a switch (102) in an electrical power converter (100) using pulse width modulation to regulate output voltage (112) and current The method allows precise output voltage regulation to be achieved whilst accurately controlling the proportion of load-current supplied by multiple modules connected in parallel. A ramped waveform (408), consisting of a component representing the instantaneous current and a component representing the input voltage applied since the switch (102) switched on in the current PWM cycle, is compared with an error signal (406) to determine the width of each PWM pulse.

Abstract: There is disclosed a parallel loaded series resonant converter 300 suitable for supplying DC to a three axis gradient amplifier in an MRI system. In conventional resonant circuits, output voltage is regulated by controlling the switching frequency of an H-bridge or half bridge switching arrangement and the resonant circuit (334, 335) is under more stress at no load than at full load. The voltage across the resonant capacitor (334) is a function of the switching frequency, the load and the voltage applied across the resonant circuit (VA-VB). In the present invention, the peak current at no load is reduced to lower than the peak current at full load by controlling the voltage applied to the resonant circuit (VA-VB) rather than controlling the switching frequency.

Abstract: Described herein is an improved suspension system for cryostat vessels forming a part of magnetic resonance imaging (MRI) apparatus. The MRI apparatus comprises an outer cylindrical element, an inner cylindrical element mounted within the outer element, and a suspension system for accurately mounting the inner element with respect to the outer element. The suspension system comprises a plate (40) pivotally mounted on the inner cylindrical element (10) for rotation relative thereto and a pair of continuous bands (66, 68) connecting the plate (40) to the outer element. Adjusters are positioned offset from the axis of rotation of the plate for tensioning the bands. Locking means are also provided to retain the plate in a given position relative to the outer element.

Abstract: A magnetic resonance imaging system comprises a magnet (30) for generating a main magnetic field, a gradient coil (34) to superimpose a linearly varying magnetic field over the main magnetic field, and an RF coil (34) forming part of a transmit/receive system for signals which are used to construct an image. A shimset (32) is positioned between the magnet (30) and the gradient coil (34) to provide magnetic field correction and is constructed so as not to have axial and azimuthal symmetry. This is achieved by providing cutout portions (38) in the shimset (32) which are diametrically opposite each other, similar cutouts (40) being provided in the gradient coil (34) which are aligned with the cutout portions (38). The cutout portions (38, 40) accommodate the shoulders of larger patients, therefore reducing the need to extend the bore size of the magnetic resonance imaging system.

Abstract: An open electromagnet including a pair of field coils which respectively comprise juxtaposed poles of the electromagnet between which an imaging volume of substantially homogeneous magnetic field is defined, toroidal shielding coil means arranged on a locus which forms a loop, so that when energized, the said shielding coil means produces a controlling magnetic field which configures a magnetic flux return path for the field coils so that the magnetic flux return path describes the said loop, and a support structure which serves rigidly to support the shielding coil means and the field coils.

Abstract: Cryostat systems comprise first and second liquid interconnected helium baths, and a magnet structure which is immersed in one of the baths. The liquid helium is cooled by means of a pumped refrigeration means. The pumped refrigeration means is replaced with a pulse tube refrigerator which can be located directly within the two baths. The pulse tube refrigerator has a “cold finger” having cold and warm ends. The cold end extracts into the bath housing the coil structure and has heat exchange connected thereto. The pulse tube refrigerator can also serve to cool the radiation shields.

Abstract: The present invention relates to a cooling system for cryostats or transport containers for liquids incorporating a PTR cooler. The cryostat usually consists of several interior cylindrical or otherwise shaped vessels for storing liquids. In particular, the cryostat for NMR applications and related fields usually comprises an outer vacuum case, sometimes also a nitrogen can, as well as one or several radiation shields and the helium vessel housing the superconducting magnet whereas the latter usually is made of NbTi, Nb3Sn, and/or some HTC conductor. It is the aim of the present invention to use a pulse tube cooler by incorporation into an NMR cryostat via an opening, an opening refers to the neck tube itself or to any other means for accessing the cryostat for filling or for energising the magnet or acting conjointly as a pressure relief tube or/and as a drainage outlet e.g.

Abstract: An open electromagnet for use in MRI apparatus includes a pair of field coils which respectively comprise juxtaposed poles between which an imaging volume of substantially homogeneous magnetic field is defined. A support structure, which serves rigidly to support the field coils in spaced apart relationship, includes two steel plates between which the field coils are positioned. The relative position of the field coils and the plates is selected such that forces of magnetic attraction between the field coils are at least partly counter-balanced by forces of attraction between the plates and the field coils. In this manner, the force on the field coils and their associated support structure due to the forces of attraction between the poles is significantly reduced or nullified.

Abstract: A regulated resonant converter comprises a series resonant circuit through which current is switched alternately in opposite directions by a pair of switching transistors, a rectifier via which the switching transistors are fed with d.c. current from an a.c. mains supply input, operation of the switching transistors being controlled by signals generated in a control circuit, in dependence upon a fed-back sample of a convertor output developed in the resonant circuit in accordance with the amplitude of which regulation is effected, and a cross-over voltage associated with current reversal in an inductor which forms a part of the resonant circuit, which cross-over voltage is fed to the control circuit via a differentiator whereby switch operation is effectively enhanced.