High-frequency bandpass filter for a coil body with a cavity, MR unit

Description

Background of the invention

[0001] The invention relates to an MR apparatus with a transmitting and / or receiving arrangement with a high-frequency bandpass filter.

[0002] For transmitting and receiving devices of MRI instruments, e.g., NMR spectrometers, bandpass filters with a frequency band in the two- to three-digit MHz range are required. Conventional bandpass filters for this application are typically manufactured using wound cylindrical coils and / or milled helical resonators in cast or milled housings on individual printed circuit boards. [1] discloses such a bandpass filter design. The frequency adjustment necessary due to tolerances is carried out using trimmer capacitors. The active and passive units are typically interconnected using coaxial cables and coaxial connectors. However, such bandpass filters require a high degree of manual effort during filter assembly and subsequent adjustment. In addition, a variety of coils or helical resonators and trimmer capacitors must be kept in stock, which increases manufacturing costs. The entire assembly is also susceptible to mechanical shocks and vibrations and exhibits a certain temperature dependence. [7] discloses a bandpass filter of the dielectric resonator type with a symmetrical damping point characteristic.The bandpass filter is provided with an input means that feeds a microwave input signal into a cavity 41, with dielectric resonators 20, 21 that form bandpass filters for an input signal. [8] discloses a loop-gap resonator. The resonator comprises microwave resonance structures with a field configuration that lies between "lumped" and "distributed" and is intended to be an alternative to dielectric and acoustic surface wave resonators at low microwave frequencies. [9] discloses a split-ring resonator for magnetic resonance from 200-2000 MHz. Object of the invention

[0003] The object of the invention is to propose a compact RF bandpass filter that has a high quality factor, high electrical power handling capability and at the same time a low impedance level, and is easy and inexpensive to manufacture. Description of the invention

[0004] This problem is solved according to the invention by an MR apparatus with a transmitting and / or receiving arrangement with a high-frequency bandpass filter according to claim

[0005] According to the invention, the inductor comprises an electrically conductive coil former with a cavity that is substantially RF-tight. "Substantially RF-tight" means that the magnetic field propagates within the coil former and the stray field outside the coil former is minimal. The RF tightness is achieved through the shape and material of the coil former. Thus, in the resonator according to the invention, the magnetic flux is essentially completely (> 99%, preferably > 99.9%) contained within the coil former, minimizing the stray field outside the coil former.

[0006] Resonators with a cavity inductor are already used in various applications as tunable resonant circuits [2], [3], [4], [5], [6]. RF tightness is not required and therefore not present.

[0007] In the present invention, the resonator is not used as a tunable resonant circuit, but rather as an RF filter, preferably with a fixed resonant frequency. Due to the resonator's cavity-enclosing shape, it exhibits a high Q factor and thus a high electrical resistance at the resonant frequency. The inventive shape and the associated large volume of the coil former result in both low electrical and low thermal resistance (outside the resonant frequency). This achieves high power handling, particularly in combination with the capacitor's high voltage withstand capability.

[0008] The geometry according to the invention ensures a small inductance of the coil body and thus, together with a large capacitance, a low impedance level.

[0009] Part of the bandpass filter according to the invention can be easily manufactured in the form of a machine-assemblable printed circuit board. The printed circuit board can contain additional active electronics for the MR apparatus (e.g., switches, amplifiers, etc.), which simplifies the design and makes it cost-effective to manufacture.

[0010] The resonator's capacitance can comprise one or more capacitors and forms an electrical resonant circuit with the coil former. The coil former is preferably metallic, particularly made of aluminum, and in a preferred embodiment has an electrically conductive outer wall and a central electrically conductive support element, which form, for example, a toroid-shaped cavity. The coil former can be in the form of a milled or cast housing, or a combination thereof.

[0011] Preferably, the cavity of the coil former is rotationally symmetrical with a toroidal surface, i.e., a surface of revolution with a hole in its center (the axis of rotation passes through the hole and does not intersect the surface of revolution). The cross-section of the body of revolution parallel to the axis of rotation can be arbitrarily shaped and is preferably rectangular with rounded edges on at least one side. This results in a partially hollow cylindrical cavity. The axis of the hollow cylinder forms the resonator axis. "Partially hollow cylindrical" therefore also includes, among other things, a cylinder with at least partially rounded edges. In this case, the cavity cross-section tapers parallel to the resonator axis, or the cavity cross-sections perpendicular to the resonator axis are of different sizes for different positions along the resonator axis.

[0012] According to the invention, the resonator has a cover element which, together with the coil former, defines the cavity. The cover element is a substantially RF-tight, optionally planar element which is optionally placed on the open area of ​​the coil former. The cover element can be attached to the coil former, for example, by screws, presses, clamps, or soldering.

[0013] Preferably, the cover element is designed as a printed circuit board. Alternatively, another metal part (or metallized part) can be used as the cover element.

[0014] The cover element is preferably electrically insulating and is largely provided, in particular coated, with an electrically conductive material on the side facing the cavity.

[0015] Preferably, the side facing the cavity (underside) is completely coated with an electrically conductive material, except for areas where capacitors may be provided. If the capacitors are arranged on the cover element, the cover element cannot be completely coated, as this would short-circuit the capacitors.

[0016] A continuously coated underside of the lid element can be provided, for example, if the capacity is located in the support element (su).

[0017] In a particularly preferred embodiment, the cover element has at least one through-contact that connects one of the contact surfaces of the capacitor and the coil body to the signal input or signal output.

[0018] The through-contact connects to the support element either directly or via a conductive coating on the side of the cover element facing the cavity. If the capacitor assembly comprises multiple capacitors, the through-contact connects one contact surface of each capacitor to the coil element.

[0019] In a particularly preferred embodiment, the cover element is a printed circuit board.

[0020] Preferably, an electrical input line is provided for coupling the signal into the signal input and an electrical output line is provided for coupling the signal out of the signal output, which are preferably arranged on the outside of the cover element.

[0021] The signal input and output can be located at a single contact point. The signal is then coupled out at the same port (contact point of the resonator to the input or output line) as it is coupled in (1-port). If the resonator is contacted as a 1-port, it can be used as a series or parallel resonant circuit.

[0022] Alternatively, the signal input and output can be located at different contact points. The signal is then coupled out at a different port than the one it is coupled in at. If the resonator is contacted as a two-port, it is better suited as a bandpass filter.

[0023] According to the invention, the capacitor is arranged within the cavity. This reduces electromagnetic losses.

[0024] In this embodiment, the side of the cover element facing the cavity is preferably coated with an electrically conductive material such that the coating comprises two galvanically isolated partial coatings, one partial coating being galvanically connected to a first contact surface of the capacitor and the other partial coating being galvanically connected to a second contact surface of the capacitor.

[0025] Additionally, the capacitor can also be located outside the cavity. This offers advantages for tuning the resonator in its installed state. The cover element is then preferably coated with an electrically conductive material on both the side facing the cavity (underside) and the side facing away from the cavity (topside). The degree of coating depends on the quantity and arrangement of the capacitors outside the cavity.

[0026] According to the invention, an electrically conductive intermediate layer is arranged between the side of the cover element facing away from the cavity and the side facing the cavity. This intermediate layer galvanically connects the radially outer contact surfaces of various capacitors in the arrangement with a low impedance. The intermediate layer serves to electromagnetically seal the cavity, particularly when the capacitors are located within the cavity, since in this case the side facing the cavity cannot be completely coated with an electrically conductive material. This minimizes the stray field outside the cavity and thus optimizes the RF shielding.

[0027] The galvanic connection is preferably made via blind-hole contacts to the side of the cover element facing the cavity and via the through-contacts of the cover element. Through-contacts can also be used instead of blind-hole contacts.

[0028] The capacitor can comprise several capacitors connected in parallel. The capacitors are preferably arranged symmetrically, and in particular rotationally symmetrically, around the resonator axis. The arrangement of the capacitors is preferably chosen such that they cover the smallest possible area. This effectively suppresses other (undesirable) resonances. A circular ring structure in which the capacitors are arranged as close together as possible is particularly preferred. This reduces the capacitance between the supporting element and any electrically conductive intermediate layer that may be present.

[0029] In a particular embodiment, the capacitor is formed by an insulator disk coated with electrically conductive material, in particular a ceramic disk. The coating forms the contact surfaces of the capacitor. Preferably, in this embodiment, the insulator disk is arranged between the support element and the cover element. The insulator disk is then coated with an electrically conductive material on its two opposite sides and mechanically connects the cover element to the coil former. This results in a particularly compact capacitor design. The first contact surface is preferably electrically connected to the underside of the cover element.

[0030] The insulator disk is preferably designed as a ring with a central via.

[0031] To improve the filtering effect, a particularly preferred embodiment of the high-frequency bandpass filter according to the invention provides that the resonator arrangement comprises at least two resonators that are coupled to each other. In coupled resonators, the input and output lines are preferably connected to ports of different resonators.

[0032] Preferably the resonators are coupled via their magnetic fields, wherein the cavities of each of the at least two resonators are connected to each other via openings in the coil bodies.

[0033] Alternatively, coupling via passive elements (coils, capacitors, lines) is also possible.

[0034] The center frequency of the RF bandpass filter according to the invention is preferably in the two- to three-digit MHz range. This can be achieved, for example, by the following characteristics of the resonator: diameter of the cavity: 32 mm - 90 mm, height of the cavity: 7 mm - 22 mm, resulting inductance of the coil former: 1.5 nH - 7.0 nH.

[0035] The power handling capability of the resonators is limited by the power handling capability of the capacitors and the insulation of the cover element. The RF bandpass filter according to the invention is suitable for power levels greater than 500 W and can therefore be advantageously used for HR (High Resolution) and MAS (Magic Angle Spinning) MR applications.

[0036] Further advantages of the invention will become apparent from the description and the drawing. Likewise, the aforementioned and further elaborated features can each be used individually or in any combination according to the invention, provided that these combinations remain within the scope of protection of the appended claims. The embodiments shown and described are not to be understood as an exhaustive list, but rather serve as examples for the description of the invention. Detailed description of the invention and drawing

[0037] Fig. 1 shows a cross-section of a resonator of an RF bandpass filter according to the invention with capacitors arranged inside the cavity. Fig. 2 shows the projection of the components of the resonator. Fig. 1 Fig. 3 shows a cross-section of a resonator arrangement with the resonator made of Fig. 1and a 1-port connector and a schematically represented signal flow. Fig. 4 shows the projection of the components of the resonator arrangement from Fig. 3 Fig. 5 shows an equivalent circuit diagram of the resonator arrangement. Fig. 3 Fig. 6 shows a cross-section of a resonator arrangement with the resonator made of Fig. 1 and a 2-port connector and a schematically represented signal flow. Fig. 7 shows the projection of the components of the resonator arrangement from Fig. 6 Fig. 8 shows an equivalent circuit diagram of the resonator arrangement. Fig. 6 Fig. 9 shows a cross-section of a resonator of an RF bandpass filter according to the invention with capacitors arranged inside and outside the cavity. Fig. 10 shows a cross-section of a resonator of an RF bandpass filter according to the invention with a capacitor designed in the form of a central insulator disk. Fig. 11 shows a schematic representation of the signal flow in the resonator. Fig. 10Fig. 12 shows a perspective sectional view of a coil element of a particularly preferred embodiment of the RF bandpass filter with coupled resonators according to the invention. Fig. 13 shows the projection of the components of the resonator arrangement with coupled resonators. Fig. 14 shows an equivalent circuit diagram of the resonator arrangement with coupled resonators.

[0038] Fig. 1 and Fig. 2 show a cross-section or a projection of a first embodiment of a resonator 10 of an RF bandpass filter according to the invention 12. The resonator 10 includes an inductor in the form of a coil body. 11 with a cavity 9 is formed and forms a resonant circuit with the capacitors 1. The resonator 10 has a capacitance provided by the capacitors arranged inside the cavity 9. 1 is formed.

[0039] The coil former 11 of the illustrated embodiments is rotationally symmetrical, wherein the cavity 9 of the coil former 11 is essentially a hollow cylinder with partially rounded edges in the lower region of the coil former 11, i.e., on the side of the coil former 11 opposite the cover element 7. The coil former 11 has a large electrically conductive surface area and thus a low electrical resistance.

[0040] The coil body 11 comprises a central electrically conductive support element. 2 (radially inner, i.e., near-axis, part of the in Fig. 1 (in cross-section U-shaped wall), an outer wall 13 (lower and radially outer, i.e., farther from the axis, part of the in Fig. 1 (shown in cross-section as a U-shaped wall). The resonator 12 includes a cover element. 7, which closes the coil body 11 to form a housing.

[0041] The support element 2 and the outer wall 13 are made of an electrically conductive material. The support element 2 is preferably hollow and has a central through-opening. 8 The cover element 7 is preferably a conductively coated insulator, in particular a printed circuit board, the underside of which (the side facing the cavity 9) is coated with an electrically conductive layer. 5 (horizontal dashes in Fig. 2 ) is provided, in particular coated. Between the two contact surfaces 1a, 1b of the capacitor 1, the underside of the cover element 7 has no electrically conductive coating. The electrically conductive layer 5 comprises, in the case of the Fig. 1 and Fig. 2 Therefore, partial coatings in the illustrated embodiment 5a, 5b, which are galvanically isolated from each other, wherein the first partial coating 5a is galvanically connected to a first contact surface 1aThe capacitors 1 are connected and electrically at a reference potential (e.g., ground potential). The second partial coating 5b is galvanically connected to a second contact surface. 1b The capacitors 1 are connected. In general, the arrangement of the capacitors 1 is chosen so that they cover the smallest possible area in order to effectively suppress interfering resonances.

[0042] In order to optimally seal the cavity 9 electromagnetically despite the uncoated surface 15, an electrically conductive intermediate layer can be placed between the top and bottom of the cover element 7 (i.e., inside the cover element 7). 6 (vertical dashes in Fig. 2 ) are provided which galvanically connect the radially outer contact surfaces 1a of the capacitors 1 with each other at a low impedance. In the case of the Fig. 1 and Fig. 2In the illustrated embodiment, this galvanic connection is made via the first partial coating 5a and blind hole contacts open towards the cavity 9. 3. Except for the openings in the through contact 4, the cavity 9 is electromagnetically sealed.

[0043] In the present embodiment, the capacitors 1 are arranged in a circular ring structure (i.e., at the same distance from a resonator axis). 14) evenly distributed around the resonator axis 14, as shown Fig. 2 shown. This allows the capacitors 1 to be arranged as close together as possible, thereby reducing the capacitance between the support element 2 and the electrically conductive intermediate layer 6. The partial coatings 5a, 5b of the electrically conductive layer are accordingly ring-shaped and concentric and are covered by a layered ring-shaped surface. 15separately. Alternatively, a single ring capacitor (with 2 ring-shaped contact surfaces) could be used.

[0044] The RF bandpass filter 12 according to the invention can be connected via ports P, P1, P2 to be connected to a component of an MR apparatus. For this purpose, the ports include a signal input for coupling in a signal to be filtered and / or a signal output for coupling out a filtered signal. Resonator 1 can be configured as a 1-port (signal input and signal output implemented in a single port P), as shown in Fig. 3 and Fig. 4 shown. A corresponding equivalent circuit diagram is in Fig. 5 depicted. Fig. 6 and Fig. 7 An alternative circuit using two separate ports P1 and P2 (2-port) is shown. A corresponding equivalent circuit diagram is shown in Fig. 8Both connection options (1-port and 2-port) are feasible with all embodiments 12, 12', 12" described here and are shown using the first embodiment 12 as an example. Figs. 3 to 7 shown. In the first embodiment of the RF bandpass filter 12, the ports P, P1, P2 are connected via through contacts. 4 with the second partial coating 5b galvanically connected. The through contacts 4 are thus in galvanic contact with the top and bottom of the cover element 7, but are not galvanically connected with the intermediate layer 6. An input line 17 and an output line 18 Lead from port P or ports P1, P2 of the RF bandpass filter 12 to the component to be connected (not shown). The input line 17 and the output line 18 are preferably designed as conductor tracks on the side of the cover element 7 facing away from the cavity.

[0045] In Fig. 3 and Fig. 6The signal flow for the 1-port and 2-port variants of the RF bandpass filter 12 is shown. The RF signal to be filtered is coupled via the conductive parts of the surface of the cover element (input line 17) and passes through the through-contact 4 in the cover element. At the lower end of the through-contact 4, the RF signal splits. High-frequency components of an RF signal to be filtered, coupled in via the input line 17, are routed from the signal input port P or P1 via the second partial coating 5b of the cover element 7, the capacitors 1, and the first partial coating 5a of the cover element 7 to a reference potential (e.g., ground potential). Low-frequency components of the RF signal to be filtered are routed from the signal input port P or P1 via the second partial coating 5b of the cover element 7, the capacitors 1, and the first partial coating 5a of the cover element 7 to a reference potential (e.g., ground potential).P1 is guided via the second partial coating 5b of the cover element 7, the support element 2 along the inner wall of the cavity 9, the outer wall 13 of the coil former 11 to the underside of the cover element 7 and a reference potential (e.g., ground potential). For frequencies near the resonator's resonant frequency, the resonator exhibits high impedance. The corresponding components of the RF signal to be filtered oscillate within the resonator and can be coupled out of the RF bandpass filter 12 via the signal output port P or P2. Since the frequency of the RF signals to be filtered is in the MHz range, signal transmission occurs only via the electrically conductive surfaces of the resonator 2.

[0046] Fig. 9 shows a cross-section of a second embodiment of a resonator 10' of an RF bandpass filter according to the invention 12' with a coil former 11'a capacitance provided by capacitors 1 arranged inside the cavity 9 and by capacitors arranged outside the cavity 9 1' is formed.

[0047] The first contact surfaces are 1a, 1a' the capacitors 1, 1' arranged inside and outside the cavity 9 via further through contacts 3' galvanically connected to each other. Likewise, the second contact surfaces 1b, 1b' of the capacitors 1, 1' arranged inside and outside the cavity 9 are galvanically connected to each other. In contrast to the through contacts 4, the additional through contacts 3' are not only galvanically connected to the top and bottom of the cover element 7, but also to the intermediate layer 6. The additional through contacts 3' therefore also fulfill the function of the blind hole contacts 3. Fig. 1The input line 17 is galvanically connected to the second contact surfaces 1b of the capacitors 1, 1' and may need to be routed around the first contact surfaces 1b' of the capacitors 1' located outside the cavity 9. Alternatively, the input line 17 can also be routed to the contact surface 1b' via a further intermediate layer (not shown).

[0048] Fig. 10 shows a cross-section of a third embodiment of a resonator 10" of an RF bandpass filter according to the invention 12" with a capacitor 1", which is designed in the form of a central insulator disk (e.g., an insulating film or a ceramic disk) that is coated on both sides with an electrically conductive material (capacitor 1"). The coatings of the insulator disk form contact surfaces 1a", 1b" of the capacitor 1". In this embodiment, the underside 5 of the cover element 7 can be completely coated. The through contact 4"In this embodiment, the through-contact 4 is centrally located and contacts one of the two contact surfaces 1a", 1b" of the capacitor 1" (here, by way of example, the lower contact surface 1b"). In contrast to the embodiments described above, the through-contact 4" here contacts the support element 2 directly and not via a partial coating of the underside of the cover element 7. In this embodiment, the support element 2 is preferably solid.

[0049] Fig. 11The corresponding signal flow is shown as an example for a 1-port variant. High-frequency components of an RF signal to be filtered, coupled in via input line 17, are routed from port P via the through-contact 4", the capacitor 1", the support element 2 along the inner wall of the cavity 9 of the outer wall 13 of the coil former 11" to the underside of the cover element 7 to a reference potential (e.g., ground potential). Low-frequency components of an RF signal to be filtered, coupled in via input line 17, are routed from port P via the through-contact 4" and the coating 5 on the underside of the cover element 7 to a reference potential (e.g., ground potential). The portion of the RF signal to be filtered that is not filtered out in this way is coupled out of the RF bandpass filter 12" via port P.

[0050] The resonators 10, 10', 10" can each function individually as an RF bandpass filter 12, 12', 12" according to the invention (simple resonator arrangement) or be coupled with other resonators 10, 10', 10" to form a more complex resonator arrangement 20 in order to improve the filter effect.

[0051] Fig. 12 Figure 1 shows a perspective sectional view of a coil former 11'' of a particularly preferred embodiment of the RF bandpass filter 12'" according to the invention with such a resonator arrangement 20 with coupled resonators 10. The coil former 11'" comprises several sub-coil formers 11a, 11b, 11c which have openings 21are electrically and mechanically interconnected so that the magnetic fields can couple together, resulting in particularly good performance. The coil former 11'"', comprising the three sub-coil formers 11a, 11b, 11c, is preferably manufactured from a single piece (e.g., as a single milled part) and is preferably closed with a single cover element (not shown) comprising three sub-cover elements that have the components of the cover elements of individual resonators, as described above.

[0052] Fig. 13 Figure 1 shows the projection of the components of the resonator arrangement 20 with coupled resonators 10. The two outer resonators are each connected as a 1-port. The entire resonator arrangement 20 is thus again wired as a 2-port. An equivalent circuit diagram of the resonator arrangement 20 is shown in Fig. 14 depicted. Reference symbol list

[0053] 1 Capacitor inside cavity 1' Capacitor outside cavity 1" Capacitor in support element 1a, 1a', 1a" First contact surface of the capacitor 1b, 1b', 1b" Second contact surface of the capacitor 2 Electrically conductive support element 3 Blind hole contact 4 Through contact 4" Central through contact 5 Electrically conductive layer (coating) 5a First partial coating 5b Second partial coating 6 Electrically conductive intermediate layer 7 Cover element (PCB) 8 Through opening 9 Cavity 10 Resonator with capacitance in the cavity (simple resonator arrangement) 10' Resonator with capacitance inside and outside the cavity (simple resonator arrangement) 10" Resonator with capacitance in the support element (simple resonator arrangement) 11, 11' 11", 11‴ Coil former 11a, 11b, 11c partial coil body 12, 12', 12",12'" HF bandpass filter 13 Outer wall 14 Resonator axis 15 Uncoated area 17 Electrical input line 18 Electrical output line 20 Resonator arrangement with multiple coupled resonators 21 Opening in coil body PPort with signal input and signal output P1Port with signal input P2Port with signal output , Reference list

[0054] [1] "Helical resonator bandpass filter" https: / / coil32.net / design / helix-resonator.html [2] Garbacz et al. "A loop-gap resonator for chirality-sensitive nuclear magneto-electric resonance (NMER)" The Journal of Chemical Physics 145, 104201 (2016) DOI: 10.1063 / 1.4962285 https: / / aip.scitation.org / doi / abs / 10.1063 / 1.496 2285 [3] Bobowski et al. "Permittivity and Conductivity Measured using a Novel Toroidal Split-Ring Resonator" https: / / arxiv.org / pdf / 1901.00994.pdf [4] Crypto Museum "Pulsed Cavity Resonant cavity microphone" https: / / www.cryptomuseum.com / covert / bugs / ec / cavity / index.htm [5] Su et al "Slot Antenna Integrated Re-Entrant Resonator Based Wireless Pressure Sensor for High-Temperature Applications" Sensors 2017, 17, 1963; DOI: 10.3390 / s17091963 https: / / www.mdpi.com / 1424-8220 / 17 / 9 / 1963 / htm [6] Anand et al. "Air Cavities Integrated with Surface Mount Tuning Components for Tunable Evanescent-Mode Resonators" https: / / www.researchgate.net / publication / 306117073 DOI: 10.1109 / MWSYM.2016.7539960 [7] JP2001127502A [8] MEHDIZADEH M ET AL: "Loop-Gap Resonator: A Lumped Mode Microwave Resonant Structure", IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, IEEE, USA Bd. 30, Nr. 12, 1. Dezember 1983 (1983-12-01), Seiten 1059-1064, [9] HARDY W ET AL: "Split-ring resonator for use in magnetic resonance from 200-2000 MHz", REVIEW OF SCIENTIFIC INSTRUMENTS, Bd. 52, Nr. 2, 1. Februar 1981 (1981-02-01), Seiten 213-216,.

Claims

1. MR apparatus comprising a transmission and / or receiving arrangement with a radio-frequency bandpass filter (12, 12', 12", 12'") comprising a resonator arrangement (10, 10', 10", 20) with a signal input, a signal output, and at least one resonator (10, 10', 10"), wherein each resonator (10, 10', 10") has a capacitor which is connected in parallel to an inductor, characterized in that the inductor comprises an electrically conductive coil body (11, 11', 11", 11"") having a cavity (9) which is closed in a substantially RF-tight manner, wherein the resonator (10, 10', 10") has a cover element (7) which, together with the coil body (11, 11', 11", 11""), delimits the cavity (9), wherein the capacitor is arranged within the cavity (9), wherein an electrically conducting intermediate layer (6) is arranged between the side facing away from the cavity (9), and the side of the cover element (7) facing the cavity (9).

2. MR apparatus according to claim 1, characterized in that the cavity (9) of the coil body (11, 11"") is rotationally symmetrical, in particular partially cylindrical.

3. MR apparatus according to any of the preceding claims, characterized in that the cover element (7) is electrically insulating and is at least partially provided, in particular coated, with an electrically conductive material on the side facing the cavity (9).

4. MR apparatus according to any of the preceding claims, characterized in that the cover element (7) has at least one via (4) which connects one of the contact surfaces (1a, 1b) of the capacitor and the coil body (11, 11', 11", 11"") to the signal input or signal output.

5. MR apparatus according to any of the preceding claims, characterized in that the cover element (7) is a printed circuit board.

6. MR apparatus according to any of the preceding claims, characterized in that the capacitor comprises a plurality of parallel-connected capacitors.

7. MR apparatus according to any of claims 1 to 5, characterized in that the capacitor is formed by an insulating disc (1") coated with electrically conductive material, in particular a ceramic disc.

8. MR apparatus according to any of the preceding claims, characterized in that the resonator arrangement comprises at least two resonators (10) which are coupled to one another.

9. MR apparatus according to claim 8, characterized in that the resonators (10) are coupled via their magnetic fields, wherein the cavities (9) of two of the at least two resonators (10) are connected to one another via openings (21) in the coil bodies (2).

10. MR apparatus according to any of the preceding claims, characterized in that the center frequency of the RF bandpass filter is in the two- to three-digit MHz range.