Actively shielded, cylindrical gradient coil system with passive RF shielding for NMR devices

Abstract

A gradient coil system has a cylindrical section in a central region, which contains no conductor elements and has a maximum outer radius that is larger than a minimum inner radius of conductor elements of a main gradient coil. An outer radius of this cylindrical section is only insubstantially smaller or equal in size to a minimum inner radius of a shielding coil in this axial range. The free space in the center of the gradient coil system is used to insert a passive RF shield, whose radius in a central region becomes larger over a certain length than its radius in outer regions. The RF shield is constructed from at least three partial sections, which are electrically interconnected. The actively shielded gradient coil system maximizes the volume of the RF region without loss of gradient coil system performance.

Claims

1. An actively shielded, cylindrical gradient coil system for use in an MR (=magnetic resonance) spectrometer, the spectrometer having a main field magnet, which generates a main magnet field aligned in a direction of a z-axis, wherein, when current flows, the gradient coil system generates a Z-gradient field in a measurement volume through which the z-axis passes whose zero crossing is located at a center of the measurement volume, the gradient system comprising: at least one main gradient coil, said main gradient coil being constructed from at least two cylindrical partial coil systems, axially spaced from one another in the z-direction by a length L1 and symmetrically with respect to the center of the measurement volume, axes of said cylindrical partial coil systems being collinear with the z-axis, wherein said cylindrical partial coil systems are at least partially constructed from electrical conductor sections wound with a maximum outer radius R1gradient.sub.out.sup.max around the z-axis; at least one active shielding coil, at least one of said at least one active shielding coil being constructed from electrical conductors on a minimum inner radius R1shield.sub.in.sup.min around the z-axis, with R1shield.sub.in.sup.min>R1gradient.sub.out.sup.max, wherein the gradient coil system has no electrical conductor elements disposed in a hollow cylindrical section along said axial length L1, symmetrically with respect to the center of the measurement volume in a radius range between a minimum inner radius R1gradient.sub.in.sup.min of the main gradient coil and R1shield.sub.in.sup.min; and a passive RF shield, said passive RF shield being constructed from at least three electrically interconnected partial sections, of which two partial sections are disposed around the z-axis with a maximum outer radius R1hf.sub.out.sup.max, wherein a third partial section with an axial length L2, a minimum inner radius R2hf.sub.in.sup.min and a maximum outer radius R2hf.sub.out.sup.max is disposed around the z-axis between said two partial sections, wherein R1hf.sub.out.sup.max<R1gradient.sub.in.sup.min, R1gradient.sub.out.sup.max<R2hf.sub.in.sup.min<R2hf.sub.out.sup.max and L2<L1.

2. The gradient coil system of claim 1, wherein R2hf.sub.out.sup.max<R1shield.sub.in.sup.min.

3. The gradient coil system of claim 1, wherein R2hf.sub.in.sup.min≥1.1.Math.R1gradient.sub.out.sup.max and R2hf.sub.out.sup.max≥0.8.Math.R1shield.sub.in.sup.min.

4. The gradient coil system of claim 1, wherein R2hf.sub.in.sup.min≥R1gradient.sub.out.sup.max+3 mm and R2hf.sub.out.sup.max≥R1shield.sub.in.sup.min−3 mm.

5. The gradient coil system of claim 1, wherein electrical conductor sections wound around the z-axis are constructed from wire with a round cross section.

6. The gradient coil system of claim 1, wherein electrical conductor sections wound around the z-axis are constructed from strip conductors.

7. The gradient coil system of claim 1, wherein electrical conductor sections wound around the z-axis are constructed from electrically conductive layers coated on dielectric formers.

8. The gradient coil system of claim 1, wherein at least two pairs of said axially spaced cylindrical partial coil systems of said main gradient coil have differing minimum inner radii.

9. The gradient coil system of claim 1, wherein at least two partial sections of said passive RF shield have differing minimum inner radii.

10. The gradient coil system of claim 1, wherein at least one of said cylindrical partial coil systems is constructed from a plurality of electrical conductor sections stacked in a radial direction.

11. The gradient coil system of claim 1, wherein said cylindrical partial coil systems of said main gradient coil and at least one active shielding coil are completely enclosed by said passive RF shield with an exception of an incoming supply cable opening.

12. The gradient coil of claim 11, wherein radial inner surfaces and the axial end faces of said cylindrical partial coil systems of said main gradient coil as well as radial outer surfaces and axial end faces of said at least one active shielding coil are enclosed by said passive RF shield.

13. The gradient coil system of claim 1, wherein said passive RF shield is shaped to enclose a region of space that is impenetrable to RF radiation and from which RF radiation cannot escape.

14. The gradient coil system of claim 11, wherein an RF impenetrability of said passive RF shield is achieved by capacitive overlapping elements of said passive RF shield, by soldering, by compression and/or by gluing with electrically conductive adhesive.

15. The gradient coil systems of claim 1, wherein said passive RF shield is mounted on a substrate or is mounted on a substrate by vacuum deposition, sputtering, CVD, galvanic coating, printing, painting and/or gluing.

16. The gradient coil system of claim 1, wherein at least two of said electrically interconnected partial sections of said passive RF shield are disposed cylindrically symmetrically around the z-axis with a maximum outer radius R1hf.sub.out.sup.max, wherein a third partial section with an axial length L2 and a minimum inner radius R2hf.sub.in.sup.min as well as a maximum outer radius R2hf.sub.out.sup.max is disposed around the z-axis between those two partial sections and that, between said third partial section and each of those two other partial sections, a transitional section is disposed, which interconnects partial sections disposed on different radii along an axial length L8.

17. The gradient coil system of claim 16, wherein said transitional section comprises a conical element.

18. An MR spectrometer having the gradient coil system of claim 1, further comprising an RF transmit and/or receive coil system which is disposed inside said radius R2hf.sub.in.sup.min along an axial length L3<L2 and symmetrically with respect to the center of the measurement volume.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The figures show:

(2) FIG. 1 a schematic cross section through a first, particularly simply constructed embodiment of the inventive actively shielded cylindrical gradient coil system;

(3) FIG. 2 an embodiment of the inventive gradient coil system containing a transmit and/or receive coil system;

(4) FIG. 3 an embodiment with a main gradient coil comprising three pairs of partial coil systems on different radii and a passive RF shield with three partial sections on different radii;

(5) FIG. 4 a further inventive gradient coil system, in which the partial coil systems of the main gradient coil are each manufactured from multiple layers of conductors and the RF shield defines recesses in the region of the conductors;

(6) FIG. 5 an embodiment of the inventive gradient coil system, which is completely surrounded by the passive RF shielding;

(7) FIG. 6 an inventive gradient coil system, which is entirely surrounded by the passive RF shielding, wherein the gradient shielding coil consists of two spaced partial coil systems in a central region;

(8) FIG. 7 an embodiment with an RF-impenetrably sealed RF region;

(9) FIG. 8a RF shield for an inventive gradient coil system, which is mounted on the outer side of the substrate; and

(10) FIG. 8b RF shield, which is mounted on the outer side of a substrate, wherein the transition between the two radii of the RF screening is constituted as a cone.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(11) The following detailed description presents examples that disclose specific details and are intended to explain, not to restrict, in order to provide a more in-depth understanding of the present teachings. For a person skilled in the art who has read this disclosure, it is, however, obvious that other examples according to the present teachings, which deviate from the specific details disclosed in this document, are protected by the attached claims. Moreover, descriptions of devices and methods known from the prior art may have been omitted for reasons of clarity. Such methods and equipment obviously lie within the scope of protection of the present teachings.

(12) The terminology used herein exclusively serves the purpose of describing certain examples and is not intended to restrict. The defined expressions are additional to the technical and scientific meanings of the defined expressions as they are usually understood and accepted in the technical field of the present teachings.

(13) The expressions “one, a and the” cover both singular and plural forms unless the context unambiguously states the contrary. Thus, for example “a device” covers one device and/or a plurality of devices.

(14) The expressions “substantial” or “substantially” used in the description and in the attached claims signify “within acceptable limits and degrees.”

(15) The expressions “approximate” or “approximately” mean “within acceptable limits or an amount as understood by an average person skilled in the art.” For example, “approximately equal” means that an average person skilled in the art considers the elements that are being compared to be identical.

(16) The expression “in particular” merely emphasizes a subset of a set without explicitly restricting the total population of the set. For example, the set “cylinders, in particular circular cylinders,” comprises the set of all cylinders of any cross-sectional shape and merely emphasizes those with a circular cross-sectional shape as being especially suitable.

(17) An inventive actively shielded gradient coil system for use in an MR spectrometer with one main field magnet, which generates a main magnet field aligned in the direction of a z-axis, is disposed cylindrically around the z-axis and comprises one main gradient coil, which is constructed from at least two cylindrical partial coil systems, at least one cylindrical shielding coil, and at least one passive RF shield, wherein the at least two partial coil systems of the main gradient coil are constructed from electrical conductors on a radius R1gradient, and are spaced in the z-direction along an axial length L1, and the at least one shielding coil is constructed from electrical conductors on a radius R1shield.

(18) In many embodiments, in particular those with electrical conductor elements made of wire, the radial extent of the main gradient coil and the shielding coil is so large that the difference between the minimum inner diameter and the maximum outer diameter can no longer be considered approximately equal. For that reason, in particular, for the embodiments with non-negligible radial dimensions, a minimum inner radius R1gradient.sub.in.sup.min and R1shield.sub.in.sup.min as well as a maximum outer radius R1gradient.sub.out.sup.max and R1shield.sub.out.sup.max are assigned respectively in each case. In the case of circularly cylindrical gradient coil systems, in particular, the minimum inner radius is equal to the inner radius and the maximum outer radius is equal to the outer radius. For embodiments, in particular circularly cylindrical ones, with approximately the same inner and outer radii of the partial coils, R1gradient.sub.in.sup.min=R1gradient.sub.out.sup.max=R1gradient.

(19) The following conditions apply here:

(20) R1gradient.sub.out.sup.max<R1shield.sub.in.sup.min, i.e. the main gradient coil can be constituted inside the shielding coil. This facilitates both simple installation during manufacturing as well manufacturing on a single former for the shielding coil and the gradient coil in which the conductor elements are mounted on the inner side and the outer side. Technically, this condition is necessary for separation of the functionality into field generation in an inside space, in particular, in the measuring volume, and active shielding of the gradient field in an outside space, in particular, outside the NMR probe.

(21) Moreover, R1shield.sub.out.sup.max is determined by the maximum possible dimensions of the gradient coil system that can still be installed in the NMR probe or in the shim system of the MR spectrometer. R1gradient.sub.in.sup.min is determined by the requirements for performance of the gradient coil system and the dimensions and performance of the transmit and/or receive coil system.

(22) The gradient coil system furthermore consists of at least one passive RF shielding system, which is cylindrical in sections, its electrically conductive elements being located within at least two minimum inner radii Rihf.sub.in.sup.min and two maximum outer radii Rihf.sub.out.sup.max, wherein i is a natural number greater than or equal to two. Analogously with the radii of the gradient coils, if the inner and outer radii are approximately equal, the latter will be considered Rihf.sub.in.sup.min=Rihf.sub.out.sup.max=Rihf, which is, in particular achieved by thin, circularly cylindrical RF shields.

(23) The RF shield is composed of at least three partial sections, wherein two of these partial sections have the radius R1hf and, between these two, a third partial section with an axial length of L2 is inserted symmetrically with respect to the center of the measuring volume along radius R2hf.

(24) The following conditions apply for radii of the passive RF shield: R1hf.sub.out.sup.max<R2hf.sub.in.sup.min, R1hf.sub.out.sup.max<R1gradient.sub.in.sup.min and R1gradient.sub.out.sup.max<R2hf.sub.in.sup.min. Furthermore, the axial dimensions are L2<L1. This results in an actively shielded gradient coil system with a passive RF shielding, which in a central region has an outward extending recess. Because at least one transmit and/or receive coil system is located in this central region of the NMR probe, the performance of the latter is improved by the increased size of the volume available as compared with the NMR probes of the prior art with cylindrical RF shield on the radius Rlhf.

(25) An embodiment is especially preferred that consists of one main gradient coil comprising exactly two partial coil systems and one single shielding coil, wherein the passive RF shield has as small a radial distance as possible to the shielding coil in the central area and as small radial distance as possible to the gradient coils in the edge areas.

(26) For this embodiment, the condition R2hf.sub.out.sup.max<R1shield.sub.in.sup.min applies in addition to the conditions stated above. This embodiment can be implemented technically extremely easily and makes it possible to provide a large volume for the RF coil system without restricting positioning of the conductor element of the shielding coil. In this way, efficient shielding of the gradient fields toward the outside can be ensured. If the shielding coil is mounted on a former and the latter has a small wall thickness, there is only a slight loss of performance of the transmit and/or receive coil system in this embodiment. It is shown schematically as a cross section in FIG. 1.

(27) In particular, such a gradient coil system can preferably be manufactured from the following components: 1. From four components comprising an RF shielding on a substrate, two partial coil systems of the main gradient coil, each on a tube-shaped substrate, and one shielding coil on another tube-shaped substrate. This manufacturing method can be used for all common methods of manufacturing gradient systems. In particular, it is suitable for gradients wound from wire, but also for electrically conductively coated formers. If the gradient coils are manufactured from cut metal tubes, the formers can be omitted at least for part of the elements. 2. From three components each comprising one or more partial coil systems of the main gradient coil on the inner side and half of the shielding coil (cut through the xy-plane) on the outer side, as well as RF shield on a substrate. Here, it should be noted that manufacturing a shielding coil from two halves can be seen as substantially identical to manufacturing on a single cylindrical substrate, if the two halves touch to a substantial degree in the central section. This manufacturing method reduces the degrees of freedom of positioning of the components and can thus minimize waste if the manufacturing technique can ensure correct positioning of the conductor element on both substrates. 3. The gradient coil system can be manufactured on the inner and outer sides of a single substrate into which the RF shield is inserted. In this case, the RF shield is usually assembled from individual parts or mounted on an insulation layer inside the gradient coil. This manufacturing method is, in particular, suitable for electrically conductively coated formers and, with manufacturing by machine, results in a high yield of gradients without complex positioning of the main gradient coil with respect to the shielding coil. 4. Another possibility is to manufacture the gradient coil system on the inner and outer side of a “half-shell shaped” substrate cut along the longitudinal axis. Here, the design of the gradient system can, in particular, be designed in such a way that no or only very few electrically conductive connections between the “half shells” are required. This can be achieved, for example, by having the current exchanged multiple times between the main gradient coil and the shielding coil across the cut edge. The RF shielding can be manufactured on a third substrate, which is inserted in both half shells or which is mounted on an insulation layer inside the half shells.

(28) Other manufacturing methods consisting of more elements, in particular, for the RF shield, can also have advantages under certain circumstances, in particular, if the RF shield is not manufactured from a continuous, thin, electrically conductive layer, but from sections with capacitive coupling between adjacent elements.

(29) In another preferred embodiment, the outer radius of the RF shield can also be larger than the inner radius of the active shielding coil in a central section of the length L2, that is, R2hf.sub.out.sup.max≥R1shield.sub.in.sup.min. This is possible if the design of the gradient does not include a central region of the length L6, in which the main gradient coil nor the shielding coils have conductor elements. In this case, the volume for the transmit and/or receive coil systems is maximized. However, usually, the efficiency of the active gradient shielding is slightly reduced, so that when the gradient field is switched quickly, more eddy currents are induced in the electrically conductive structures having radii greater than R1shield.sub.out.sup.max. This must be counteracted by using suitable materials with a high electrical resistance and/or non-conductive materials near the gradient coil system or by adapting the design of the gradient, which generates eddy currents that affect the measuring volume less. As a rule, the following still applies: R2hf.sub.out.sup.max=R1shield.sub.out.sup.max.

(30) Furthermore, an embodiment is especially preferred in which the length L2 is greater than the length L3 of at least one transmit and/or receive coil system. This permits the lowest possible performance losses by RF shielding while simultaneously achieving good efficiency of the gradient coil system. FIG. 2 schematically shows such a gradient coil system in its simplest form. An NMR probe usually comprises more than one transmit and/or receive coil system. In this case, the length L3 refers to the magnetic length of one of these coil systems. There are various definitions for the magnetic length of transmit and/or receive coil systems, which, however, within the scope of this invention can be interpreted as approximately the same values. One of these definitions for magnetic length is the half-value length of the RF magnetic field on the z-axis.

(31) In another embodiment, the partial coil systems of the main gradient coil are manufactured on more than one radius and the RF shield is manufactured on more than two radii. This has the advantage of providing more flexibility for the design of the gradient, improving the linearity and shielding of the gradient and at the same time further increasing the volume available for the RF region for the given specifications of the gradient coil system.

(32) FIG. 3 schematically shows a gradient coil system in which the RF shielding has an inner radius R2hf.sub.in.sup.min over the length L2. Along two sections of L2/2≤|z|≤L4/2 that are symmetrical with the center of the measuring volume, it has a radius R3hf, where R3hf.sub.out.sup.max<R2hf.sub.in.sup.min. Along the remaining length, the RF shielding has a radius R1hf, wherein in this example R1hf.sub.out.sup.max<R3hf.sub.in.sup.min applies. In principle, further graduations of the RF shielding are possible. These do not need to be disposed symmetrically with respect to the magnetic center of the probe either.

(33) In a further preferred embodiment, the partial coil systems of the main gradient coil occupy a smaller area. This further maximizes the volume for the RF region. This reduced occupation of the area can, in particular, be achieved if multiple layers of conductors are radially stacked one on top of the other. This is, in particular, advantageous for gradients wound from wire because it is technically simple to solve by manufacturing grooves in a substrate for accommodating gradient wires and the gradient is wound densely packed in these grooves. Alternatively, this can be achieved using multilayer PCBs or by nesting multiple layers of tubes one inside the other. In this case, each radius occupied by a conductor must be viewed as an independent gradient coil on radius Rigradient, wherein i is a natural number (positive integer). The z-positions of different partial coil systems can therefore also overlap.

(34) In FIG. 4, such a gradient coil system is represented schematically in cross section, wherein in this specific embodiment the RF shielding has radius R2hf in an interval −L2/2≤z≤L2/2, and radius R3hf in two further intervals L2/2<|z|<L4/2, radius R4hf in intervals L4/2≤|z|≤L5/2, and radius R1hf over the remaining length. In FIG. 4, R2hf=R4hf. This does not necessarily have to apply and is merely used to illustrate an especially preferred variant. Generally, all radii can be different. Moreover, in FIG. 4, the axial extent of the various gradient coils at approximately the same z-position is shown to be of equal size. This does not have to be ensured generally, but is especially easy to implement, for example, for wire gradients when wires are wound in grooves. Another simple way of implementing a wire gradient is to wind the wires into a “sphere packing” so that each layer contains one wire less than the previous one and is offset by half of the wire diameter in z-direction.

(35) For certain applications of NMR, not only the reduction of coupling between the transmit and/or receive coil system and the gradient coil system is relevant but also suppression of the NMR background signal, which is due to wire or conductor insulations, adhesives, or substrate material in the gradient. This background signal is generated by excitation and reception of NMR signals. In the best case, it results in a reproducibly altered baseline of the NMR spectra, which can be corrected numerically. This is the case, in particular, when the background signal is weak and the corresponding NMR lines are very broad. In the worst case, however, the background signal contains relatively strong narrow NMR lines that cannot be corrected. To avoid this background signal as completely as possible, it is advisable to disconnect the gradient coil system completely from the RF region. To achieve that, it may be necessary to enclose the gradient on the inner side, outer side, and at the end face using RF shielding. RF shielding can also be implemented in the region of the gradient supply cable in order to prevent coupling or reception of the background signal in this region. In this context, it must be noted that usually only components with an almost identical static magnetic field can contribute to the NMR background signals as otherwise the Larmor frequency of the nuclei of the background is shifted with respect to the spectrum to be measured to such an extent that it ends up outside the measurement frequencies because the static field of the main magnets exhibits a plateau which is substantially symmetrical with respect to the measuring range and having an amplitude dropping steeply outside that range.

(36) FIG. 5 schematically illustrates a variant of this embodiment in which the gradient coil system is fully enclosed by an RF shield. Here, the RF shield consists of elements 3a on the inner side of the gradient coil system, elements 3b on the end faces, and elements 3c on the outer side of the gradient coil system, as well as an enclosure 6 of the gradient supply cables. The RF shield forms a sealed gradient region 7 into which essentially no RF radiation can penetrate as long as the RF shield achieves a sufficient level of attenuation of the RF radiation. This can be achieved particularly well by electrically closed RF shielding if the conductor thickness is larger than several skin depths at the relevant frequencies and temperatures. Alternatively, a structured RF shield can also be designed such that it has sufficient RF impenetrability against the relevant frequencies. However, structured shielding is usually not sufficiently impenetrable to RF that it fully prevents artifacts produced by the background signal.

(37) End face elements and elements shaped to the lateral cylinder surface can, for example, be connected by capacitive overlapping, soldering, compression, gluing with conductive adhesive, etc. to keep RF impenetrability of the RF shielding to the required level.

(38) FIG. 6 shows an embodiment in which the RF shielding is implemented in such a way that for the central region of the RF shielding (3d) R2hf.sub.out.sup.max R1shield.sub.out.sup.max applies. As a result, the volume of the RF region relevant to the performance of the transmit and/or receive coil systems can be increased even further. To achieve this, the shielding coil must have a symmetrical region at its center of length L6, which contains no conductors. In this case, the radius R2hf.sub.out.sup.max is identical to the radius of the RF shield of a NMR probe without an integrated gradient coil system or with a gradient coil system without RF shielding.

(39) In the above mentioned variants of RF shields, the material that generates the background signal is “packed in” by the RF shielding. As an alternative thereto, an RF-impenetrable RF-shield can be implemented inside the gradient system by sealing the RF-region towards the outside. This prevents, for example, NMR background signals resulting from materials outside the actual RF region from being received. I.e., the RF region is encapsulated in such a way that no signal can be received outside this region. For this reason, the end faces of the RF region must be as impenetrable as possible to RF. However, because a measuring sample usually has to be inserted into the probe, this can only be completely achieved on one side. The insertion opening can be closed, for example, using a waveguide, which is operated below its cutoff frequency and thus exhibits exponential attenuation for the RF waves. This is illustrated schematically in FIG. 7. Moreover, the RF supply cables must be inserted through the RF shielding in such a way that they cannot emit any radiation in the outside space.

(40) In an especially preferred embodiment, the RF shield is mounted on a former (for example by vacuum deposition, sputtering, CVD, galvanic coating, gluing, clamping, printing, or painting). This has the advantage that no internal coating has to be carried out and the RF shield is merely mounted on the outer sides. This is technically considerably easier to implement. In addition, structuring of the RF shield on an outer lateral cylinder surface is also technically simpler. Structuring can improve the “recovery behavior” of the gradient coil system because the eddy currents induced in the RF shield are reduced due to electrical interruptions in the RF shield. Many different variants of patterned RF shields that can be adapted to the inventive geometry are mentioned in the literature. The inventive execution of the RF shield explicitly covers all concepts known from the prior art for implementing passive RF shielding.

(41) Due to the typical dimensions in NMR applications, electrical connections between the individual sections of the RF shielding are considerably easier to implement on the outer side than on the inner side of the substrate. This is particularly the case for their electrically conductive connection, for example, by soldering when mounting on the outer side of the substrate. Likewise, RF shielding structured on a flexible printed circuit board (PCB) can be simply mounted from the outside onto a former, e.g. by gluing or clamping.

(42) Moreover, the former can be manufactured from material with high thermal conductivity (aluminum oxide, aluminum nitride, silicon nitride, or silicon carbide in ceramic, polycrystalline form, or as a single crystal, e.g. sapphire) making it possible to cool the RF shield efficiently. This is necessary, in particular, for probes with cryogenically cooled RF coil systems, in order to keep the noise contribution of the RF shield as small as possible. Furthermore, this is also an advantage for gradients that have to operate with high currents because cooling can be performed during operation, e.g. by the coolant tubes embedded in the former or mounted on it. This way, the duty cycle and permissible maximum current of an inventive gradient system can be increased.

(43) It is particularly easy to manufacture RF shields on a substrate if the transitions between the various radii of the RF shield are beveled. It is technically considerably easier to apply electrically conductive coatings on this type of former. The beveling can be conical; however, it can also contain geometries that are more complicated.

(44) The design of the passive RF shield presented within the scope of this invention can be combined with the various implementation options for reducing eddy currents on RF shields according to the prior art. However, thin metallic layers that have few or no slots are especially preferred. “Thin” means a metallic layer where the thickness of the layer d is of the same magnitude as the electrical penetration depth (skin depth) δ in the relevant frequency range, that is, 0<d<10δ, but in particular 0<d≤3δ.

(45) For slotted shields, capacitive connections between the individual conductive elements are preferably implemented as capacitive overlaps across the slots. This minimizes the radial dimensions. Moreover, capacitive overlaps with small dielectric layer thickness are preferred because the remaining magnetic flux through the remaining gap is smaller than for capacitive connections with the same capacitance values but a larger distance between the conductive elements. In the case of larger RF shields, the capacitive connections can also be carried out using discrete capacitors, which permits greater flexibility in the selection of elements.

LIST OF REFERENCE SYMBOLS

(46) 1a-1f Electrical conductor sections of the partial coil systems of a main gradient coil 2; 2a-2c Active shielding coils 3 Passive RF shield 3a-3e Partial sections of the passive RF shield 4 RF transmit and/or receive coil system z z-axis
Variables List L1 Length of the axial spacing between the partial coil systems of the main gradient coil in which there are no conductor elements between R1gradient.sub.in.sup.min and R1shield.sub.in.sup.min L2 Axial length of the third partial section of the RF shield L3 Axial length of the RF transmit and/or receive coil system L4,5 Axial length of various regions of the RF shield L6 Axial spacing between the two partial coil systems of the shielding coil R1gradient.sub.in.sup.min Minimum inner radius of the main gradient coil R1gradient.sub.out.sup.max Maximum outer radius of the main gradient coil R1shield.sub.in.sup.min Minimum inner radius of the shielding coil R1hf.sub.out.sup.max Maximum outer radius of the at least two partial sections of the RF shield R2hf.sub.in.sup.min Minimum inner radius of the third (central) partial section of the RF shield R2hf.sub.out.sup.max Maximum outer radius of the third (central) partial section of the RF shield R2gradient.sub.in.sup.min Minimum inner radius of a second partial coil system of the main gradient coil R3gradient.sub.in.sup.min Minimum inner radius of a third partial coil system of the main gradient coil R1hf.sub.in.sup.min (i∈) Minimum inner radii of the various partial sections of the RF shielding

LIST OF REFERENCES

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