Abstract: A magnetic resonance apparatus includes a main magnet (12) which generates a main magnetic field through and surrounding an examination region (14). The main magnetic field contains inhomogeneities which affect image quality. A gradient coil assembly (22) generates gradient magnetic fields across the examination region, which contain higher order harmonics causing inhomogeneities in the gradient magnetic field. A multi-axis shim set (23) is selectively excited in order to cancel both the main magnetic and gradient magnetic field inhomogeneities. More particularly, DC currents are applied by a shim coil power supply (25) to cancel the main magnetic field inhomogeneities. AC current pulses are superimposed on the DC currents by the shim coil power supply (25) in order to correct the gradient magnetic field inhomogeneities.

Abstract: A magnetic field generator for MRI comprises a pair of plate yokes opposed to each other with space in between. A magnet is provided in each of the opposed surfaces of the pair of plate yokes. A column yoke is connected to the pair of plate yokes. An enhancing member made of magnetic material and having a curved slant face is provided in an inner surface of the connection between the column yoke and the plate yokes at a place farthest from the magnet. A first bolt of magnetic field adjusting bolts is screwed into the plate yoke, with a tip of the first bolt contacting the enhancing member. A second bolt of the magnetic field adjusting bolts is screwed into the plate yoke, with a tip of the second bolt contacting an upper end face of the column yoke. An engaging portion for engagement of the enhancing member with the yoke may be provided.

Abstract: A magnet for magnetic resonance imaging has an interior working space within the magnet frame sufficient to accommodate a physician and a patient. Because the physician is positioned inside the magnet frame, the physician has unimpeded access to the patient. Elements of the magnet frame desirably encompass a room, and the magnet frame may be concealed from view of a patient within the room. Preferred embodiments facilitate MRI imaged guided surgery and other procedures performed while the patient is being imaged, and minimize claustrophobia experienced by the patient. Also provided is a magnet having field coils disposed about the of pole portions of the magnet.

Abstract: A pair of pole assemblies (12, 14) are disposed on opposite sides of an examination region (10). A ferrous flux return path includes an overhead ferrous structure (18) above the upper pole assembly and a lower ferrous structure (20) adjacent to the lower pole assembly. The pole assemblies include a Rose ring assembly (34) having axially spaced segments (40, 42) with an axial gap (44) in between. Magnetic attractive forces between an annular magnet (30) and the Rose ring urge the Rose ring and the magnet into an axially centered alignment. The magnet of the upper pole piece assembly (12) is attracted toward the overhead ferrous structure (18) and toward the lower pole piece assembly (14), as well as toward the Rose ring. The magnet is also positioned such that the magnetic attractive forces between the magnet and the Rose ring dominate magnetic attractive forces between the overhead ferrous structure and the magnet.

Abstract: A magnetic field generating apparatus for a magnetic resonance imaging (MRI) system adopting a C-shaped open magnet support structure includes a pair of polygonal permanent magnets disposed parallel to each other in the horizontal plane, with a predetermined imaging area therebetween; a pair of polygonal magnetic pole plates, stacked to face each other on the inner sides of the pair of permanent magnets, respectively; a pair of yokes to which the permanent magnets are fixed; at least one column for connecting the two yokes at one ends thereof with the imaging area being interposed between the yokes, wherein the at least one column together with the pair of yokes forms a closed path of the magnetic field; shims disposed at the vertexes of the polygonal magnetic pole plates, the shims having a polygonal shape, wherein the shims of one of the polygonal magnetic pole plates face those of the other polygonal magnetic pole plate; peripheral permanent magnets disposed along the edges of the polygonal magnetic pole p

Abstract: A short radio frequency coil for magnetic resonance imaging of the head to provide a homogeneous magnetic field including parallel conductors forming a first cylindrical portion with end rings supporting the conductors which then continue at an angle to form the frustum of a cone and further continuing to form a reduced diameter second cylindrical portion. A third conductive end ring of reduced diameter positioned at the open end of the second cylindrical portion supports the end of the conductors and provides a reduced diameter opening. An asymmetrical coil surrounds portions of the radio frequency coil including the first cylindrical and frustum of a cone portions.

Abstract: A patient alignment device is disclosed for use in open magnet MRI having a movable patient table to move a patient within the open magnet MRI in order to position a desired patient scan area accurately within the field-of-view (FOV) of the open magnet MRI. A first alignment light is placed on the outer periphery of the upper magnetic structure of the open MRI to emit a light beam in longitudinal alignment with a center point of the FOV. A second alignment light is positioned to emit a second light beam downwardly from the upper magnetic structure to indicate an interior reference point of the FOV. The second alignment light is then used to reposition a patient within the open magnet MRI without having to withdraw the patient from the open magnet MRI.

Abstract: Image artifacts produced by residual magnetization in elements of the gradient field amplifier system are reduced by driving the residual magnetization to a preselected value at the end of each imaging pulse sequence. Reset gradient pulses are produced by the gradient field amplifier system after each signal is acquired to drive the residual magnetization to a selected value. A number of different reset gradient waveforms may be used, and they may be produced in a number of different ways.

Abstract: A magnet assembly for a magnetic resonance imaging apparatus, has a magnet with a patient receptacle therein, the magnet producing a basic magnetic field having a homogeneity volume within the patient receptacle. Auxiliary components in the form of coils or ferromagnetic elements are disposed relative to the magnet to selectively spatially displace the homogeneity volume, either by selectively activating current flowing through the respective coils, or by selective positioning of the ferromagnetic elements.

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

Abstract: A permanent magnet arrangement has a pair of permanent magnet groupings supported by a C-shaped yoke. Each magnet grouping comprises a series of circular, concentrically-arranged permanent magnets in a support frame. An adjusting mechanism allows each circular magnet to be individually aligned and adjusted with respect to the remaining magnets. The circular magnets can be formed as solid disks, toroidal solids or from individual magnet elements arranged in circular rings. A backing plate supports each magnet grouping and attaches the grouping to the yoke.

Abstract: A single-sided magnet for magnetic resonance imaging has a main field assembly including numerous small outer field coils arrayed around a main axis. The main field assembly may also include a central field coil coaxial with the main axis inside the array of outer field coils, as well as shim coils, also coaxial with the main axis.

Abstract: An object of the present invention is to provide an MRI magnetic field generator with which the magnetic field uniformity within the imaging field of view of the air gap can be adjusted easily and safely, without having to detach the gradient coils, after the MRI magnetic field generator has been installed with gradient coils already mounted on a pair of magnetic pole pieces. After the holder disk 13 has been placed on and fastened to the base disk 11, the magnetic material pellets or permanent magnet pellets for adjusting the magnetic field uniformity are inserted into the required pits disposed in a specific pattern, and if needed, spacers are placed in the gaps between [the pellets] and the cover disk 14 to position everything, after which the cover disk 14 is screwed on to hold everything together. The resulting shim holder 10 is aligned with and fastened to the gradient coil 6 or its support member.

Abstract: Interventional MRI imaging leads to the development of open magnets such as for example C-magnet 1, in which it is correspondingly more difficult to generate a large field in order to obtain a good signal-to-noise ratio. To overcome this, a pre-polarizing unit 7 is provided which utilizes a HTSC magnet, for example of YBCO, which is brought adjacent to the patient to assist in generating the main field, and then rapidly withdrawn to position 8, in order to enable the r.f. signals to be detected in the presence of the field of the C-magnet 1.

Abstract: A laminate tile pole piece for an MRI. Each laminate tile has a trapezoidal or annular sector shape. The trapezoidal shape allows the tiles to be attached side by side to form a multiple concentric annular array pole piece without using oddly shaped edge filler tiles needed to fill a circular pole piece with square tiles. The pole piece is made by placing a plurality of tiles into a mold and filling the mold with an adhesive substance to bind the plurality of tiles into a unitary tile body. The unitary tile body is then removed from the mold and attached to a pole piece base to form the pole piece. The mold cavity surface preferably has a non-uniform contour. The bottom surface of the unitary tile body forms a substantially inverse contour of the mold cavity surface contour.

Abstract: In an MRI apparatus for medical purposes it is desirable to minimize the length of the magnet system for generating the main field, inter alia in order to keep the accessibility of the region to be examined in the imaging volume as high as possible for the attending staff. In order to save costs it is also desirable to make the diameter of the magnet system as small as possible. According to the invention a short, actively shielded magnet system is obtained in that the turns of the magnet system are situated in an imaginary U-shaped space 43 with the opening of the U facing the axis 35. The current component in the bottom 37 of the U has a value which is smaller than that of the current component in the limbs 47a, 47b of the U. The inner space 49 of the U does not contain a current for generating a contribution to the main field in the measuring space 29 of the apparatus.

Abstract: An MRI system includes a magnet which produces the main polarizing magnetic field. Variations in strength of this field are corrected by a temperature compensation system that calculates a compensating flux needed to maintain the field at constant strength. The compensating flux is calculated from changes in sensed magnet temperature and a magnet temperature coefficient. One or more correction coils are wound around the magnet and driven with the current necessary to produce the compensating flux.

Abstract: A magnet, such as an open or closed magnet, has a first assembly with at least one superconductive main coil and with a first vacuum enclosure enclosing the main coil(s). A first cryocooler coldhead has a rigid first housing and is generally vertically aligned. A first flexible bellows is vertically aligned, has a first end attached to the first housing of the first cryocooler coldhead and has a second end attached to the first vacuum enclosure of the first assembly.

Abstract: In order to provide a magnetic field stabilization method, magnetic field generating apparatus and magnetic resonance imaging apparatus that is not affected by the temperature distribution in an installation space, in stabilizing a magnetic field of a magnetic field generating apparatus 2 having a pair of permanent magnets facing each other with a space interposed and yokes constituting a magnetic circuit for the permanent magnets, heat quantities to be applied to a plurality of positions are individually controlled (172, 174) based on the temperatures detected at a plurality of positions 154, 158 on the yoke to stabilize the respective temperatures at the plurality of positions.

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: A coil system for magnetic resonance systems that surrounds a generally cylindrical examination volume, has coil plies formed by X-gradient coils arranged symmetrically relative to the YZ plane and spaced from one another, and Y-coils arranged symmetrically relative to the XZ plane and spaced from one another, and Z-coils arranged symmetrically relative to the XY plane and spaced from another. An indirect cooling with embedded cooling conduits through which a coolant flows is achieved by cooling conduits integrated in the interspaces of the conductor structure of the coil plies for cooling the conductor structures in a coil ply lying therebelow and/or thereabove.

Abstract: The concentration of an analyte in a fluid is measured by a quartz crystal microbalance sensor device in the fluid containing a concentration of the analyte. The sensor device exhibits a resonant frequency representative of the concentration of the analyte in the fluid. A quartz crystal microbalance reference device has a resonant frequency lower, by a predetermined amount, than the initial resonant frequency of the sensor device before the sensor device is exposed to the analyte. A signal is derived based on the difference of the resonant frequencies of the sensor and the reference device, and a first counter counts a predetermined number of cycles of the difference signal frequency to derive a sample time period. A second counter counts cycles of a clock signal frequency during the sample time period to derive a count representative of the difference signal frequency.

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: 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: 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: 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: 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: Biplanar, symmetrical electromagnets for providing a homogeneous magnetic field. The magnets have coils disposed in two parallel planes. The coils in the two planes are identical. The radii and Ampere-turns of the coils are selected so that a magnetic field between the planes is homogeneous. One preferred embodiment has 6 coils, with 3 coils in each plane. Other embodiments have 8, 10, 12, or more coils. The method of making the coils begins with an equation for the spherical harmonic coefficients describing the fields from a coil as a function of radius, position and Ampere-turns. For a magnet with K coils, the first K-1 even spherical harmonic coefficients are set equal to zero (the odd coefficients are zero due to symmetry of the magnet). This produces a set of equations that, when solved, provides the radii, positions, and Ampere-turns of the K coils. The method can be used to design a biplanar, symmetrical electromagnet with any even number of coils.

Abstract: A magnet for magnetic resonance imaging has an interior working space within the magnet frame sufficient to accommodate a physician and a patient. Because the physician is positioned inside the magnet frame, the physician has unimpeded access to the patient. Elements of the magnet frame desirably encompass a room, and the magnet frame may be concealed from view of a patient within the room. Preferred embodiments facilitate MRI imaged guided surgery and other procedures performed while the patient is being imaged, and minimize claustrophobia experienced by the patient.

Abstract: A region of interest of a subject (20) on a subject support (18) is positioned above a ferrous pedestal (16, 116) that is supported on a ferrous floor yoke portion (76, 176). A lower imaging coil assembly (50) including a lower gradient coil (52), a radio frequency coil (54), and a lower pole piece (58) are disposed between the pedestal and a region of interest of the subject. An upper imaging coil assembly (40) including an annular gradient coil (42, 142) and an upper annular pole piece (44, 144) is supported from a ceiling ferrous yoke member (74, 174). The upper imaging coil assembly is supported by supports (70, 170) which are moved by drives (72, 78, 172) to raise and lower the upper imaging coil assembly. A laser gauging system (80, 180) gauges the position of the upper imaging coil assembly such that, with a control circuit (82), the upper imaging coil assembly is accurately repositioned at preselected imaging positions.

Abstract: A magnet for magnetic resonance imaging of the type arranged to provide fields suitable for imaging in an imaging region outside of the physical structure of the magnet, such as a magnet including plural concentric superconducting coils, generates a field having near-zero radial curvature at one location. Shim coils are actuated to adjust this location in synchronism with tuning of the RF transceiver for slice selection, so that when each slice is selected, that slice is coincident with the location of near-zero radial curvature.

Abstract: A magnet for magnetic resonance imaging has an interior working space within the magnet frame sufficient to accommodate a physician and a patient. Because the physician is positioned inside the magnet frame, the physician has unimpeded access to the patient. Elements of the magnet frame desirably encompass a room, and the magnet frame may be concealed from view of a patient within the room. Preferred embodiments facilitate MRI imaged guided surgery and other procedures performed while the patient is being imaged, and minimize claustrophobia experienced by the patient.

Abstract: A magnet structure, particularly for Nuclear Magnetic Resonance imaging machines, and of the type having a bearing structure for means for generating magnetic fields inside a volume, a cavity or similar space, defined by the structure, is made to be modular at least for part of the bearing structure, and/or at least for part of the other construction elements.

Abstract: A permanent magnet for nuclear magnetic resonance image detection, includes a magnetic structure having a yoke and magnetic poles, so shaped as to delimit or enclose a cavity, at least a part of a volume of the cavity forms a compartment for receiving at least a part of a body under examination, and at least a part of the volume of the cavity is permeated by a static magnetic field having specific intensity and homogeneity characteristics; the magnetic structure has at least one open side, parallel to the static magnetic field; the magnetic structure has at least two opposite main poles, lying face to face, transverse to the open side, the static magnetic field being generated therebetween; in the vicinity of the open side, the magnet has a device for correcting the static magnetic field generated between the main poles, the correction device including an element for increasing the magnetic potential near the open side, and over a predetermined depth therefrom, transverse to the open side, without reducing th

Abstract: A method for shimming a magnetic resonance imaging magnet requiring higher magnetic field homogneity on a small imaging volume and by magnetic field measurements on a larger volume providing small measurement error and practical field strength measurements and utilizing the spherical harmonic coefficients of the measurements.

Abstract: The instant invention is directed toward a method and an apparatus for mapping the magnetic field strength or flux density of a magnetic source. The invention provides a robotically controlled sensor which is moved in a controlled manner throughout an area of flux density and the variations in the flux density are recorded. The position and recorded magnetic field strength at numerous points throughout the test area are recorded and the data is assembled into a graphical representation the end result of which is a multi-dimensional mapping of the magnetic field strength as a function of distance from the source. The testing method and apparatus provide a convenient methodology for accurately determining dosimetry and tissue penetration of therapeutic magnetic devices.

Abstract: Magnetic apparatus for MRI/MRT probes and methods for construction thereof are disclosed. One embodiment includes a pair of opposed magnet assemblies defining an open region therebetween, a transmitting RF coil having at least a portion thereof disposed within the open region, at least one receiving RF coil disposed within the open region and X,Y and Z gradient coils. At least one of the X,Y and Z gradient coils is disposed outside of the open region. Another embodiment of the apparatus includes a single magnet assembly having a first surface and a second surface opposing the first surface, a transmitting RF coil having at least a portion thereof opposing the first surface, at least one receiving RF coil and X,Y and Z gradient coils. At least one of the X,Y and Z gradient coils opposes the second surface. In another embodiment the magnet assembly generates a permanent z-gradient magnetic field and therefore includes only X and Y gradient coils, at least one of which opposes the second surface.

Abstract: In an MR system, a mounting assembly is provided at each end of the bore of the main magnet for supporting a cylindrical gradient coil assembly within the bore. Each mounting assembly comprises first and second brackets, the first bracket being provided with a concave bearing surface having a specified curvature. The first bracket is fixably joined to the main magnet proximate to the lower side of the bore, and the second bracket is provided with a convex bearing surface which is selectively similar to the curvature of the concave bearing surface of the first bracket. The second bracket is attached to a lower portion of the gradient coil, so that the convex bearing surface extends longitudinally outward from the gradient coil assembly. A vibration attenuator is positioned between the convex and concave bearing surfaces of each mounting assembly, the vibration attenuator acting to transmit a static force between the two bearing surfaces to support the gradient coil assembly within the bore.

Abstract: Permanent magnet assemblies are disclosed for use in medical applications, particularly permanent magnet assemblies for use in Magnetic Resonance Imaging (MRI) and/or Magnetic Resonance Therapy (MRT) to produce a volume of substantially uniform magnetic field in a restricted part of the patient's body in a region either adjacent to the surface of one permanent magnet assembly or between a set of a first and second permanent magnet assemblies, leaving open access to the patient's body. The assemblies consist of a plurality of annular concentric magnets spaced-apart along their axis of symmetry. A method for constructing such annular permanent magnetic assemblies is disclosed, using equi-angular segments permanently magnetized.

Abstract: Permanent magnet assemblies and hybrid magnetic apparatus are disclosed for use in medical applications, particularly permanent magnet assemblies for use in Magnetic Resonance Imaging (MRI) and/or Magnetic Resonance Therapy (MRT) to produce a volume of substantially uniform magnetic field in a restricted part of the patient's body in a region either adjacent to the surface of one permanent magnet assembly or between a set of a first and second permanent magnet assemblies, leaving open access to the patient's body. The assemblies consist of a plurality of annular concentric magnets spaced-apart along their axis of symmetry. A method for constructing such annular permanent magnetic assemblies is disclosed, using equi-angular segments permanently magnetized.

Abstract: Pole faces for an open magnetic resonance (MR) imaging system are constructed from a number of blocks having laminate sheets. The laminate sheets of the blocks are aligned, such that they are parallel to the flux of an applied time-changing magnetic field. Each laminate sheet is constructed of an amorphous material such that there is no predominant magnetic field alignment. This construction reduces eddy currents induced in the pole faces, and reduces residual magnetization, thereby reducing artifacts in an acquired image.

Abstract: Yoked permanent magnets are disclosed for use in medical applications, particularly permanent magnets for use in magnetic resonance imaging (MRI), magnetic resonance therapy (MRT), and interventional magnetic resonance imaging (IMRI) to produce a volume of substantially uniform magnetic field in a part or organ of a patient's body disposed in an open region between a set of a first and second opposing permanent magnet assemblies, leaving open access to a part or organ of a patient's body. Each of the assemblies consists of a plurality of coaxial concentric permanent magnets. The magnets within each of the assemblies may be spaced-apart along their common axis of symmetry and may be tilted with respect to the common axis of symmetry. The magnets include an open yoke made from a ferromagnetic material for increasing the magnetic field strength within the imaging volume by providing a path for closing magnetic flux lines of the opposing permanent magnet assemblies.

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

Abstract: A laminate tile pole piece for an MRI, a method and an apparatus for manufacturing laminate tile metal pole pieces for an MRI. Each laminate tile has a trapezoidal or annular sector shape. The trapezoidal shape allows the tiles to be attached side by side to form a multiple concentric ring pole piece without using oddly shaped edge filler tiles needed to fill a circular pole piece with square tiles. The laminate tiles are formed by unwinding a metal ribbon, guiding the ribbon through an adhesive bath, winding the ribbon on a polygonal bobbin, such as a rectangular bobbin, to form a coil with at least one flat side, removing the coil from the bobbin, cutting the coil into laminate bars and shaping the laminate bars into trapezoidal or annular sector shaped laminate tiles. The apparatus contains an adhesive bath, a polygonal shaped bobbin, bobbin side plates for guiding the ribbon onto the bobbin and pressure plates for controlling the thickness of the coil.

Abstract: A magnetic resonance imaging scanner includes a pair of opposing pole pieces (20, 20') disposed symmetrically about an imaging volume (24) facing one another. The pair of opposing pole pieces (20, 20') includes a first ferrous pole piece (20) which has a front side (22) facing the imaging volume (24) and a back side (28). Also included is a second ferrous pole piece (20') which also has a front side (22) facing the imaging volume (24) and a back side (28). A magnetic flux return path (30) extends, remotely from the imaging volume (24), between a point adjacent the back side 28 of the first pole piece (20) and a point adjacent the back side 28' of the second pole piece (20)'. A source of magnetic flux generates a magnetic flux through the imaging volume (24), the pair of opposing pole pieces (20, 20'), and the magnetic flux return path (30). An array of annular hoops (40) and (40') are integrated with the pair of opposing pole pieces (20, 20') for homogenizing the magnetic flux through the imaging volume (24).

Abstract: An open yoked magnetic apparatus for producing a substantially homogenous magnetic field. The apparatus includes an open ferromagnetic yoke, and two complementary permanent magnetic assemblies attached to opposing sides of the yoke. Each magnet assembly includes a ferromagnetic pole-piece and a first magnetic structure disposed between the pole-piece and the yoke, at least one annular ferromagnetic collimator concentric with said pole-piece and radially separated therefrom by a gap, and at least a second magnetic structure radially separated from the first magnetic structure by a gap and disposed between the at least one collimator and the yoke. The magnetic structures may be solid structures made from magnetic material or may be segmented or may include a plurality of magnetic blocks. A method for designing such an open yoked magnetic apparatus with improved properties is also disclosed.

Abstract: The gradient coil system in a conventional MRI apparatus is optimized in respect of the shielding effect by the shielding coil of the system. Consequently, the system generally is not optimized in respect of the Lorentz forces occurring in the system, resulting in noise of a level such that it is annoying to the users. In order to avoid such noise, the system can be force-optimized. Because the shielding effect is partly lost in that case, however, eddy currents and hence disturbing noise would occur again. However, if the eddy currents are made to occur in acoustically insulated eddy current conductors which also have a large time constant for the decay of the eddy currents, the adverse effects of these eddy currents are adequately counteracted and the disturbing noise is reduced to an adequate extent. Particularly, the (cold) radiation shields of a cryogenic magnet system, arranged in vacuum, can be used as eddy current conductors. The vacuum then constitutes the acoustic insulation.

Abstract: Asymmetric, compact superconducting magnets for magnetic resonance imaging are provided. The magnets have a homogeneous region (the "dsv") which can be located close to one end of the magnet so as to reduce the sensation of claustrophobia experienced by patients undergoing MRI procedures. The magnets can be designed using a hybrid process in which current density analysis is performed to obtain an initial coil configuration which is then refined using non-linear optimization techniques to obtain a final coil configuration. The hybrid method can incorporate various constraints, including, the location and size of the dsv, the uniformity and strength of the B.sub.0 field, stray field strengths outside of the superconducting magnet, and field strengths within the magnet's coils. The hybrid technique can also be used to design compact symmetric superconducting magnets.

Abstract: A magnetic structure for generating magnetic fields to be used in nuclear magnetic resonance image detection, having means (1, 101, 201, 301, 101', 201', 101", 201", 3, 3') for generating a magnetic field with the characteristics required to obtain valid images inside a predetermined tridimensional area (102), being at least a part of a cavity (2), which is at least partially contained in the magnetic structure. According to the invention, the means for generating the magnetic field consist of at least one coil (3, 3'), of the resistive or superconductive type.

Abstract: A gradient system for a nuclear spin resonance (NMR) tomograph with a main magnet system producing a homogeneous magnetic field directed along a z-axis inside a measuring volume, wherein the main magnet system provides access to the measuring volume along the z-axis and transversely to the z-axis, and wherein the gradient system comprises two first partial gradient systems which are located at opposite sides of a central plane perpendicular to the z-axis, across the measuring volume. At least one of the two partial gradient systems is movable between at least two positions along the z-axis. In this way, with a small constructional size and with a simple manufacturing procedure, a gradient system is obtained with high performance capability with respect to generated gradient strength for given ampere turns, high linearity of the generated gradient fields, minimum inductance and minimum electrical resistance of the conductor segments.