Abstract: A method and apparatus for determining a characteristic of an earth formation surrounding a borehole in which a pulsed nuclear magnetic resonance (NMR) tool is received. A static magnetic field is produced in the borehole using at least two spaced-apart magnets in the NMR tool. The static magnetic field has a first region of substantially uniform magnetic intensity at a first location in the borehole, the first location in the borehole having a first temperature. The NMR tool is moved to a second location in the borehole having a second temperature, and a static magnetic field having a second region of substantially uniform magnetic is produced. The at least two spaced-apart magnets are controllably moved relative to each other wherein the first region of substantially uniform magnetic intensity and the second region of substantially uniform magnetic intensity are substantially equal in size and distance from the NMR tool.

Abstract: Magnetic resonance apparatus includes a magnet system for generating a steady magnetic field in a measuring space, which steady magnetic field is oriented mainly parallel to one of the axes of an orthogonal coordinate system, a gradient system with gradient coil systems and with a controlled source for supplying the gradient coil systems with excitation currents with a predetermined variation in time. The gradient coil systems include a number of linear gradient coil systems, each of which is arranged to generate a main gradient field which is dependent on the location of the field in the measuring space in such a manner that a magnetic field formed by superposition of one of the main gradient fields on the steady magnetic field can be described, as a function of the coordinates of the coordinate system, as a series containing first-order terms and higher-order terms, the first-order terms having predetermined coefficients which are equal to zero for two of the coordinates.

Abstract: A magnetic resonance imaging suite is sheathed with plates (32, 34, 36) of iron or other ferrous material. The plates define projections (42, 44, 54, 54', 68) in alignment with each other on opposite ceiling and floor or wall surfaces. A pair of magnetic pole pieces (10, 10'; 50, 50'; 60, 60') are surrounded by superconducting electromagnetic coils (12, 12'; 52, 52'; 62, 62'). The pole pieces are positioned between the ferrous plates in axial alignment. When current flows through the electromagnetic coils, magnetic flux flows between the pole pieces. The ferrous wall sheathing or other ferrous constructions define a flux return path. The pole pieces are magnetically attracted toward each other and are each magnetically mirrored in and attracted toward the adjacent ferrous flux return path.

Abstract: In order to provide a phased-array coil having a highly homogeneous sensitivity region, an opposing coil set 10 composed of a pair of gutter-shaped coils 10a and 10b whose concave surfaces face each other, an opposing coil set 20 composed of a pair of gutter-shaped coils 20a and 20b whose concave surfaces face each other, an opposing coil set 30 composed of a pair of gutter-shaped coils 30a and 30b whose concave surfaces face each other, and an opposing coil set 40 composed of a pair of gutter-shaped coils 40a and 40b whose concave surfaces face each other are disposed around the center axis J of an imaginary cylinder at an angular spacing of 45.degree., with each coil overlapping the adjacent ones by about 10% of the coil surface area, to be assembled into a cylindrical shape as a whole.

Abstract: Primary superconducting coils (50) generate a magnetic field through an examination region (10). Stabilizing coils (70) are magnetically coupled with the magnetic field generated by the primary coils. A primary persistence switch (60) and a stabilizing coils persistence switch (72) are opened when the primary coils are connected to a current source (62) to ramp-up the magnetic field. The persistence switches are closed, disconnecting the primary coils from the current source and connecting the primary coils and the stabilizing coils into closed loops. As the magnetic flux generated by the primary coils fluctuates as the primary coils stabilize, the changing flux induces currents in the stabilizing coils. The currents induced in the stabilizing coils generate an offsetting magnetic flux such that the net magnetic flux generated by the primary and stabilizing coils is held constant.

Abstract: A device for examining by nuclear magnetic resonance a volume having small depth, including means for creating a static homogeneous magnetic field B.sub.0 in which a volume to be examine is positioned, a system for creating magnetic field gradients in three directions of space, and a radio frequency transmission/reception system. The means for creating the static magnetic field B.sub.0, the field gradient system, and the radio frequency transmission/reception system are situated on a same side of an open surface, while the volume to be examined is situated on the other side of the surface. The device can be applied to magnetic resonance imaging.

Abstract: A compact magnet is disclosed for high field, superconducting magnets for use in magnetic resonance spectroscopy. The magnet has at least a first and a second group of coils with the or each coil in the first group having wound to provide a negative current density with respect to the other coils. The first group acts to enable the overall length of the magnet to be reduced while maintaining a homogenous field over a central region of the magnet.

Abstract: A superconducting magnet utilizing a pair of donut-shaped annular magnet assemblies including in each a cryogen pressure vessel with a non-magnetic cylindrical divider forming pairs of concentric chambers. Pairs of axial non-magnetic strut members spaced about the interior of the pressure vessel extend at an angle within the concentric chambers between the end flanges of each assembly to resist forces generated within and between the pair of magnet assemblies.

Abstract: NMR apparatus for achieving construction of improved patient access. Facilities and methods of mobile and fixed site scanning.

Abstract: A very flexible method for designing electromagnets which produce an arbitrary magnetic field. The conductors of the magnet can be constrained to an arbitrary surface or volume. The method provides the lowest power (or shortest wire-length) design given constraints on the desired magnetic field and constraints on where the coils can be located with respect to the desired magnetic field. The method begins with establishing a mesh of nodes and current elements connecting the nodes. The mesh can have any 2 or 3 dimensional shape (e.g. a rectangular grid on a cylinder). The magnet conductors can only be located where current elements are defined. A number of target points are established and a desired magnetic field is defined for each target point. Next, a matrix of coefficients is defined which relates the current in each current element with the magnetic field at each target point. Also, a total power expression is defined for the power consumed in the current elements.

Abstract: A method for shimming main magnetic field in a magnetic resonance imaging apparatus is provided. The method includes generating a radio frequency pulse sequence (200) while a subject is in an examination region (14) of the magnetic resonance imaging apparatus. A reference signal (EC1) which is immune to shim errors is then acquired. Thereafter, a field echo (EC3a) signal is acquired which is sensitive to shim errors. The field echo (EC3a) signal is acquired at a timed interval (T) equal to a multiple of an amount of time it takes for fat and water signals to become in phase. The temporal position of the maximum of the field echo signal is compared to its predicted temporal position (EC3) relative to the reference signal (EC1). The shim term is calculated based on the preceding comparison and an electrical current is applied to one of a gradient offset and a shim coil such that the main magnetic field is adjusted according to the shim term.

Abstract: A homogeneous field electromagnet having at least two coils in each of two regions. One region has a smaller inner radius than the other region so that the magnet is asymmetrical. The magnet has coils with a smaller radius on one side compared to the other side. This provides the benefit of allowing the magnet to be shorter for a given field of view size (compared to a uniform-radius cylindrical magnet). In a preferred embodiment the magnet is short enough so that a patient's heart can be located within the field of view (FOV) while visual access is provided to the patient's face. Specific dimensions are given for the two regions, including lengths, inner radii, and outer radii. The magnet can also include gradient coils and RF electronics for magnetic resonance imaging.

Abstract: The present invention provides a superconducting magnet device for a crystal pulling device comprising a pair of ring-like superconducting coils facing with each other, with the crystal pulling device disposed therebetween, a radiation shield surrounding the superconducting coils, and a vessel surrounding the radiation shield, wherein the vessel on the side facing to the crystal pulling device is made from a nonmagnetic substance, and the vessel on the other side is made from a magnetic substance.

Abstract: A family of NMR coils based on Litz foil conductor groups is disclosed. The simplest embodiment is a two-element Litz foil coil. The foils are joined at node (1) and node (2) and are electrically insulated at crossover (3). When the coil is positioned in a plane perpendicular to a uniform magnetic field, the areas (A) defining two flux sub-windows must be equal.

Abstract: Although magnet coil windings are distributed discontinuously, the distribution of magnetic fields generated actually approaches a desired continuous distribution. The desired continuous distribution is defined analytically, as closely as possible. A system for acquiring magnetic resonance data has a coil segment formed by winding a conductor on a bobbin representing an axial direction. A magnetic field is generated by supplying current into the conductor. Winding positions at which the conductor is wound turn-by-turn on the bobbin are determined in agreement with a specified current step value and sequentially from a winding positioned at an outermost end of the coil segment in the axial direction. Additionally, a shunt element for shunting the current carried turn-by-turn is arranged in the coil segment in relation to a pattern of turns of the coil segment by winding the conductor into a plurality of current flows through a plurality of shunt paths.

Abstract: A magnetic resonance imaging system includes a lower body which has a flat upper surface, and contains a first magnetic pole, an upper body which has a lower surface disposed in opposed relation to the upper surface of the lower body, and contains a second magnetic pole, a bed disposed adjacent to the lower body, and a pair of support posts which are provided respectively on right and left sides of the lower body, and interconnect the lower body and the upper body. A space, formed between the lower surface of the upper body and the upper surface of the lower body, is open at least toward the bed between the pair of right and left support posts.

Abstract: NMR apparatus for achieving construction of improved patient access. Faciliites and methods for mobile and fixed site NMR scanning.

Abstract: A magnet for use in medical magnetic resonance studies and having a pair of ferromagnetic pole pieces supported by a ferromagnetic yoke which establishes a flux return path. columns of the yoke are configured to maintain the magnetic field within the columns approximately constant in every cross section of the columns and to maximize access to the magnet gap.

Abstract: An open architecture magnetic resonance imaging permanent magnet assembly with moveable magnets and adjustable field inclination with automatic magnetic field homogeneity shimming to enhance imaging and positioning of the magnetic field about the portion of the patient to be imaged while enabling interventional physician access.

Abstract: A magnet resonance imaging system is described having an open magnet structure that provides a remote region of background field homogeneity for imaging that is open and accessible. The system includes spatial encoding gradient coils and a rf coil. The open magnet structure has an planar active shimming coil array that provides a remote region of enhanced field homogeneity with open geometry for easy accessibility to a patient. Preferably, the active planar shimming coil array has one or more pair of current loops arranged and constructed in a plane separating the open magnet structure from the remote region, a first loop having current in a first direction to provide a magnetic flux (i.e., magnetization vector) away from the plane and a second loop having current in the opposite direction to provide a magnetic flux (i.e., magnetization vector) toward the plane.

Abstract: A gradient coil (1) to generate switched magnetic field gradients in an nuclear spin resonance (NMR) device consisting of one or several current paths (a, b) following windings and being arranged on the surface of a geometric body is characterized in that at least two electrical current paths (a, b) are provided for, running geometrically essentially parallel to each other and being electrically connected in parallel, which current paths cross n times per winding, wherein n is an integer with n.ltoreq.8, preferably n=1 or n=2. In this way, eddy current generation by current re-distributions on the gradient coil conductor strips and thereby a time-dependent distortion of the generated magnetic field gradient inside the volume of interest of the NMR apparatus is considerably reduced.

Abstract: A mobile nuclear magnetic resonance apparatus has a shielding arrangement having at least one primary shielding coil and at least one auxiliary shielding coil that is activated at least during transport of the nuclear magnetic resonance apparatus. This allows shielding of the stray field during transport in a simple way while still allowing qualitatively good exposures to be produced.

Abstract: A snap-in, whole-body radio frequency coil (38) for increasing the diameter of a bore (12) of a toroidal magnetic resonance imaging apparatus includes a plurality of longitudinally extending coil elements (42) having element ends (44). The longitudinally extending coil elements are disposed on a flat, flexible, non-conductive plastic sheet (40) which is capable of being rolled and disposed concentricly in an interior diameter of the bore to form a thin-walled, bird-cage type radio frequency coil. Adjacent element ends are connected with a first capacitance C.sub.0 to resonate the coil at a certain frequency. To permit mechanically switching the resonant frequency of the coil, protruding metal contacts (60) are electrically connected to the element ends. An adjustment end ring includes separated metal pads (66) disposed on a non-conducting ring substrate (64). Adjacent metal pads are connected with second capacitances C.sub.1.

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: Method and apparatus for eliminating mutual inductance effects in resonant coil assemblies in which a plurality of coils are situated in sufficiently close proximity to create small mutual inductances between the coils. The method comprises evaluating the mutual inductances using a T, star or other transformation of the relevant parts of the circuit, thereby isolating the mutual inductances in such a way that series capacitances may be introduced to tune out the mutual inductances at a common frequency, reducing the coil array to a synchronously tuned circuit.

Abstract: Proposed is an improvement in an opposed-magnet magnetic circuit assembly with permanent magnets suitable for use, for example, in an MRI instrument comprising a pair of upper and lower permanent magnets to form a magnetic-field gap space therebetween, a pair of magnetic-field adjustment plates each on the surface of the permanent magnet to face the gap space and a pair of back yokes each on the back surface of the permanent magnet. According to the first aspect of the invention, a combination of a gradient coil and a shimming plate is mounted on the magnetic-field adjustment plate with intervention of several pieces of shim members and, further, another set of second shim members are bonded to the surface of the shimming plate facing the gap space with an object to improve the uniformity of the magnetic field in the gap space.

Abstract: Magnet arrangement for a one-sided NMR tomography system having a permanent magnetic ring (4) of an outer radius R.sub.a and an inner radius R.sub.i and being magnetized axially in the direction of the z-axis and extending in the axial direction up to a plane E (z=z.sub.E) for generating a homogeneous magnetic field B in a measuring volume (2) is characterized in that, in a radial region R<R.sub.i, at least one further permanent magnetic field-generating element, which is rotationally symmetric, is arranged with respect to the plane E on the same side as the permanent magnetic ring (4) at an axial distance from plane E, whereby a depression V is formed on the surface of the magnet arrangement (1) facing plane E and that the permanent magnetic field-generating elements generate at least 90%, preferably 99% of the homogeneous magnetic field B in the measuring volume (2).

Abstract: The invention relates to a process for magnetic resonance examinations, in particular nuclear spin tomography, whereby a static magnetic field is produced in an examination zone and at least one gradient magnetic field is superposed, whereby unwanted alternating electromagnetic forces are produced, whereby by means of magnetostriction, forces opposing the electromagnetic forces are produced by the same gradient magnetic field and these forces act longitudinally and/or radially and/or around the circumference of the examination zone. Furthermore, the invention relates to a device for magnetic resonance examinations, in particular nuclear spin tomography, with a first generating unit for a static magnetic field, a second generating unit (20) for at least one gradient magnetic field and a support unit (10) for the second generating unit (20), whereby a magnetostrictive material system (11, 12) is provided on the support unit (10).

Abstract: Annular portions are arranged on mutually confronting surfaces at circumferential edge portions of low-temperature containers, the annular portions forming annular containing portions whose cross section is rectangular. Not only a side coil among a group of coils forming a collective coil body is disposed inside the containing portion, but also a gradient magnetic field unit, a radio-frequency transmission coil, and a reception coil are disposed inside a recessed portion so as to be integrated with one another into a single body, the recessed portion being formed so as to be surrounded by the corresponding annular portion. The side coil supplies more than half a designed magnetomotive force. Since a distance J of the mutually confronting low-temperature containers, i.e.

Abstract: A magnetic resonance imaging method comprises exciting spins in an investigation volume via a radio frequency pulse and the signal produced thereby is subsequently read-out after appropriate spatial encoding following a short echo time te. Repetition of the sequence required for spatial encoding occurs in a time interval tr under appropriate variation of the spatially encoding gradient. The radio frequency pulse utilized for excitation has a short time duration and therefore a large excitation band width so that a projection image through a thick projection slice is produced under application of no slice selection gradient or with only a weak slice selection gradient. The time interval tr is sufficiently minimized that the image recording time required for recording of an image lies in the range of one EKG cycle or below. This process of recording a projection image is continuously repeated and a contrast medium bolus administered during the course of this sequential recording.

Abstract: A magnetic resonance imaging suite is sheathed with plates (32, 34, 36) of iron or other ferrous material. The plates define projections (42, 44, 54, 54', 68) in alignment with each other on opposite ceiling and floor or wall surfaces. A pair of magnetic pole pieces (10, 10'; 50, 50'; 60, 60') are surrounded by superconducting electromagnetic coils (12, 12'; 52, 52'; 62, 62'). The pole pieces are positioned between the ferrous plates in axial alignment. When current flows through the electromagnetic coils, magnetic flux flows between the pole pieces. The ferrous wall sheathing or other ferrous constructions define a flux return path. The pole pieces are magnetically attracted toward each other and are each magnetically mirrored in and attracted toward the adjacent ferrous flux return path.

Abstract: A magnetic resonance device has a main field magnet system which generates a steady magnetic field in an examination zone. The main field magnet system includes a yoke structure which consists of two yoke plates and a yoke wall interconnecting the two yoke plates, a pole block which is arranged inside the yoke device, and below an examination zone, and a coil which is arranged within the yoke system and above the examination zone. In order to ensure that a patient present in the examination zone can also undergo further treatments and is freely accessible during an MR examination, the inner space of the coil as well as the space between the coil and the examination zone remains free from components of the magnet system, the coil being constructed so as to be ring-shaped and encloses the examination zone essentially in such a manner that the patient is freely accessible from above and from the sides.

Abstract: A magnet resonance imaging system is described having an open solenoidal magnet structure that provides a remote region of background field homogeneity for imaging that is open and accessible. The system includes spatial encoding gradient coils and a rf coil. The open magnet structure has a pair of primary solenoidal coils and a bias coil system located within the primary coil geometry. The primary and bias coils emit a total additive flux in an imaging region to generate a resulting background magnetic field profile that provides a remote region of substantial field homogeneity with open geometry for easy accessibility to a patient.

Abstract: This invention relates to a technique for compensating for the inhomogeneity of the field generated by the RF coil (B1) in a nuclear magnetic resonance experiment. Current techniques for achieving accurate flip angles with non-uniform B1 transmit fields, are based upon modulation of the RF waveform. Inherent disadvantages of any RF-based compensation is an increased pulse length and/or increased RF power. Moreover, for some important applications, e.g. multi-slice excitation, no suitable pulses are known. We present an alternative strategy involving a Bz field whose spatial variation is correlated with that of the B1 field. This spatial correlation between the fields allows Bz-based compensation for the effects of B1 inhomogeneity. Successful operation over a wide bandwidth and range of B1 intensities may be achieved without any modification of the RF pulses. An alternative approach for compensating for B1 inhomogeneity is to apply a rapid oscillatory phase-modulation to an existing RF pulse waveform.

Abstract: The invention concerns a magnet system (1) for charging of a superconducting magnet coil (3). The magnet system (1) is characterized by control means (4) for reading-in and controlling which read-in data of a storage medium (7) associated with the superconducting magnet (3) into a power supply (2) and which control the charging of the superconducting magnet coil (3) by the power supply (2) using the read-in data. The magnet system (1) facilitates a fully automatic, safe and rapid charging of a superconducting magnet coil (3) and guarantees better reproduceability and an optimal charging process.

Abstract: Magnetic field homogeneity correction system including coils for a superconducting magnet assembly are wound on the same coil forms as the axially spaced main magnet coils such that they are superimposed radially to simplify the system including design requirements for associated passive shimming and reduce transverse magnetic forces therebetween, and improve magnetic field homogeneity.

Abstract: An open superconductive magnet useful in magnetic resonance imaging (MRI) applications. The magnet has two spaced apart assemblies, wherein each assembly has a superconductive shielding coil spaced longitudinally outward and apart from a superconductive main coil. A magnetizable pole piece is spaced apart from the coils, has a radially-outer portion at least partially radially overlapping the main coil, and has a longitudinally-inner portion which projects longitudinally inward past the longitudinally outer end of the main coil.

Abstract: An improved MRI magnet structure is created by utilizing a lower pole mounting support with one or more vertically extending yokes. The one or more vertically extending yokes mount an upper pole mounting structure including an upper pole mounting ring and at least one spoke connecting the ring to at least one of the vertically extending structural supports. The average effective spacing of the upper pole mounting structure is greater than the air gap between the upper and lower pole thereby resulting in improved uniformity of the B.sub.o field at the patient imaging volume. In a preferred embodiment, a pie-shaped cryostat encompasses a plurality of radially distributed superconducting electromagnets for generating a more uniform field in the patient imaging volume. The patient imaging field uniformity in some embodiments can be improved by the use of shim coils and/or shim rings associated with either the upper or lower poles to further modify the field within the patient imaging volume.

Abstract: Proposed is an improvement in an opposed-magnet magnetic circuit assembly with permanent magnets suitable for use, for example, in an MRI instrument comprising a pair of upper and lower permanent magnets to form a magnetic-field gap space therebetween, a pair of magnetic-field adjustment plates each on the surface of the permanent magnet to face the gap space and a pair of back yokes each on the back surface of the permanent magnet. According to the first aspect of the invention, a combination of a gradient coil and a shimming plate is mounted on the magnetic-field adjustment plate with intervention of several pieces of shim members and, further, another set of second shim members are bonded to the surface of the shimming plate facing the gap space with an object to improve the uniformity of the magnetic field in the gap space.

Abstract: A device for the safe discharge of a superconducting magnet coil (4) which is located inside a cryostat (2) and superconductively short-circuited by a superconducting switch (7) providing at or in the cryostat (2) a transmitter (10) of electromagnetic energy being located at a temperature level considerably above the cryogenic temperature of the superconducting magnet coil (4), which transmitter can be switched on quickly from outside, and by arranging in the region of the superconducting switch (7) at the cryogenic temperature level of the superconducting magnet coil (4) a receiver (20) for the emitted electromagnetic energy, which transmits the received energy directly or indirectly to a heating device (15) of the superconducting switch (7) or which activates an auxiliary energy source (41) in the region of the superconducting switch (7) , which on its turn effects a heating of the superconducting switch (7), so that it becomes normal-conducting and causes a discharge process of the superconducting magnet c

Abstract: The present invention provides is an improvement in a magnetic circuit system with a pair of opposite permanent magnets to construct an MRI instrument comprising a pair of upper and lower permanent magnets connected with yokes to form an air gap space therebetween, in which the patient under MRI inspection is kept lying, a pair of shimmed pole pieces each mounted on the permanent magnet to face the air gap and a pair of gradient coils each mounted on the shimmed pole piece to face the air gap. The improvement comprises mounting a cancellation magnetic plate, which is a thin plate of a magnetic material having a coercive force of 0.1 to 500 Oe, on the gradient coil to face the air gap with an object to cancel the residual magnetization of the shimmed pole piece by the gradient magnetic field. The cancellation magnetic plate has a thickness not exceeding 0.5 mm and has a base area which is 5 to 75% relative to the base area of the gradient coil.

Abstract: Procedures for designing magnets, including superconducting magnets, shim magnets, and gradient magnets for magnetic resonance systems, are provided. The procedures involve the use of a simulated annealing procedure in which weighted spherical harmonics are included in the procedure's error function. The procedure has resulted in the development of previously unknown magnet designs. In particular, superconducting magnets have been designed which include at least one coil in which the current flow is opposite to that in adjoining coils. Such reversed flow in combination with a relatively large number of coils, e.g., more than 6 coils, have enabled the development of short, yet homogeneous, whole body magnets for use in magnetic resonance imaging (MRI).

Abstract: A magnet arrangement for a diagnostic nuclear magnetic resonance apparatus has a first system for generating a uniform magnetic field in a first imaging volume, the first imaging volume being arranged inside the first system, and a second system for generating a uniform magnetic field in a second imaging volume is in interactive connection with the first system. The second imaging volume is arranged between the first system and the second system.

Abstract: A superconducting annular electro-magnet for MRI purposes includes an annular winding within which a generally cylindrical space is provided. The winding adjacent each end of the space is formed with a chamfer to provide flaring at each end of the space.

Abstract: A magnetic resonance imaging (MRI) magnet includes different-diameter, longitudinally-proximate, first and second coil form segments. First and second superconductive coils are wound, respectively, around the first and second coil form segments. A first spacer ring is attached to the first coil form segment and longitudinally abuts a lateral side of the first superconductive coil, and a second spacer ring circumferentially surrounds and is attached to the second coil form segment and longitudinally abuts a lateral side of the second superconductive coil. Two or more circumferentially-spaced-apart plate members are attached to, and extend radially outward from, the first and second spacer rings.

Abstract: The magnetic field assembly of a magnetic resonance imaging device includes an annular superconducting magnet (10) which is mounted within a toroidal vacuum vessel (24). Two cylindrical members (26, 46) define an annular gap (58). A shim set (60) for shimming the uniformity of the magnetic field is mounted in the gap (58). The shim set includes a plurality of shim trays (62), each of which defines a plurality of shim receiving pockets (64). A bottom wall (81) of each pocket is spaced from the cylindrical member (26) by side walls (76) and feet (78) to minimized heat flow. Shim tray covers (68) have flanges (90) and narrow contact surfaces (94) which are cammed against surfaces (61) of spacers (56) to press the cover on the tray and the tray feet against the cylinder (26) while minimizing heat flow through the spacers to the shims.

Abstract: The invention describes apparatus for, and a method of, active acoustic screening and combined active acoustic and magnetic screening, of magnetic field coils, which pass a time varying current and which reside in relatively high static magnetic fields, typically greater than about 0.1 T. The invention is particularly well suited for acoustic screening of magnetic field gradient coils used in Magnetic Resonance Imaging (MRI). In a preferred embodiment a closed loop carrying current is arranged such that two different parts of the loop are mechanically coupled, dimensioned and arranged with respect to one another such that Lorentz forces experienced by the magnetic equipment are substantially reduced and preferably cancelled.In a different embodiment the invention is modified so as to permit simultaneous magnetic and acoustic screening.

Abstract: A magnet for use in medical magnetic resonance studies and having a pair of ferromagnetic pole pieces supported by a ferromagnetic yoke which establishes a flux return path. Columns of the yoke are configured to maintain the magnetic field within the columns approximately constant in every cross section of the columns and to maximize access to the magnet gap.

Abstract: An apparatus and method for generating homogenous electromagnetic fields within a volume. The homogeneity provided may be for magnetic and/or electric fields, and for field magnitude, radial gradient, or higher order radial derivative. The invention comprises conductive pathways oriented about a desired region of homogeneity. A corresponding apparatus and method is provided for substantially canceling the electromagnetic field outside of the apparatus, comprising a second set of conductive pathways placed outside the first set.

Abstract: A magnet resonance imaging system is described having an open magnet structure that provides a remote region of background field homogeneity for imaging. The system includes spatial encoding gradient coils and an rf coil. The open magnet structure has a primary magnet with two primary pole pieces connected by a primary ferromagnetic core for internal flux. Each primary pole piece has a surface facing away from the primary magnet. Located between the two primary pole pieces is a bias magnet having two bias pole pieces connected by a bias ferromagnetic flux pathway for internal flux. The bias magnet has polarity opposite to the primary magnet. Each bias pole piece has a surface facing away from the bias magnet and in a direction the same as the surfaces of the primary pole pieces. Superposition of the magnetic fields of the primary magnet and bias magnet provide a region of substantial background field homogeneity remote from the magnets in said direction.