Magnetic resonance local coil

The present embodiments relate to a magnetic resonance local coil with a receive antenna for receiving magnetic resonance signals. The magnetic resonance local coil also includes a transmission unit for transmitting magnetic resonance signal data generated on the basis of the magnetic resonance signals via a data transmit antenna of the magnetic resonance local coil to a signal data receiving unit of a magnetic resonance tomography systems. The transmission unit is provided, at least in sections, with screening with a first metal coating and a first dielectric coating.

This application claims the benefit of DE 10 2011 006 493.1, filed on Mar. 31, 2011.

BACKGROUND

The present embodiments relate to a magnetic resonance local coil.

In a magnetic resonance tomography system, conventionally the body to be examined is exposed, with the aid of a basic magnetic field system, to a relatively high basic magnetic field of 3 or 7 tesla, for example. A magnetic field gradient is also applied with the aid of a gradient system. High-frequency excitation signals (HF signals) are then emitted by way of a high-frequency transmission system using suitable antenna devices in order to tip the nuclear spin of atoms that have been excited in a resonant manner by the high frequency field. The nuclear spin of the atoms is tipped by a defined flip angle in relation to the magnetic field lines of the basic magnetic field. This high-frequency excitation or the resulting flip angle distribution is also referred to below as nuclear magnetization or “magnetization.” During relaxation of the nuclear spin, high-frequency signals (e.g., magnetic resonance response signals (“magnetic resonance signals” for short)) are emitted and received by suitable receive antennas and further processed. The raw data acquired in this way may be used to reconstruct the desired image data.

The emission of the high-frequency signals for nuclear spin magnetization may be performed by a “whole-body coil” or “body coil.” A typical design of this coil is a cage-like antenna (e.g., a birdcage antenna) including a plurality of transmit rods arranged running parallel to the longitudinal axis around a patient chamber of the tomography system, in which a patient is present during the examination. At an end face, the antenna rods are respectively connected to each other in a capacitive manner in a ring shape.

Local coils may be used to receive the magnetic resonance response signals from the object under examination. The local coils are receive antenna assemblies including at least one receive antenna element (e.g., in the form of conductor loops). During the examination, the local coils are arranged relatively close to the body surface and if possible, directly on the organ or body part of the patient to be examined. The receive antenna elements may be embodied as a coil. Unlike larger antennas arranged at a greater distance from the patient, local coils have the advantage of being arranged closer to the areas of interest. This reduces the noise component resulting from the electrical losses within the patient's body, which has the result that the signal-noise ratio of the local coil may be better than that of the more remote antenna.

The magnetic resonance signals received by the receive antenna elements may be pre-amplified in the local coil and conducted out of the central region of the magnetic resonance system via cables and sent to a screened receiver in an MRI signal processing device. The received data are digitized and further processed for the imaging.

The cabling of the local coils is, for example, not desired, since the cables cannot be simply run from the patient table to the evaluation device. The cables are perceived as disruptive by the staff, and the patient table with the patient and local coil mat is moved, the cables thus being guided loosely. Therefore, the handling of local coils may be simplified if the data transmission from the local coils to the magnetic resonance tomography system is wireless. It is advantageous, even at the local coil, for the magnetic resonance signals to be provided in analog form and digitized prior to the wireless transmission. Since a circuit of this kind occurs in the “field of view” of the magnetic resonance tomography system (e.g., in the measuring field), the circuit is screened. For example, digital circuits may be susceptible to interfering radiation and may even themselves cause interfering emissions. The function of the circuits may be impaired by the strong field of the high-frequency transmitter for the transmission of the excitation signals. High-frequency emissions emitted by the circuit may be received by the adjacent high-sensitivity receive antenna elements of the local coils and interfere with the reception of the magnetic resonance signals. In addition to improving the electrical properties, the screening may also provide mechanical protection for the circuit.

As standard, digital circuits are screened (e.g., shielded) with an electrically conductive cover connected to a grounding surface. However, in a magnetic resonance device, a particular problem is created by the necessary compatibility between the screening and the alternating fields used. For example, the low-frequency gradient fields that may occur with frequencies of up to 100 kHz may induce unwanted eddy currents in the screening. These eddy currents cause secondary magnetic fields, severe heating due to ohmic losses and vibrations due to Lorentz forces. Shadowing or displacement of the high-frequency fields used during transmission and reception (e.g., the excitation signals and the magnetic resonance signals) may be kept low. The screening may be configured such that these high-frequency fields are not distorted such that the field strength drops in sections.

SUMMARY AND DESCRIPTION

The present embodiments may obviate one or more of the drawbacks or limitations in the related art. For example, a magnetic resonance local coil with improved ease-of-handling that may be used without difficulty in the measuring field of a magnetic resonance tomography system is provided.

A magnetic resonance local coil according to the present embodiments may include one or more receive antennas to receive the magnetic resonance signals.

In addition, the magnetic resonance local coil includes a transmission unit in order to transmit magnetic resonance signal data generated on the basis of received magnetic resonance signals via a data transmission channel to a signal data receiving unit of a magnetic resonance tomography system. “Transmission unit” may be a circuit arrangement connected to a receive antenna and a data transmit antenna. The transmission unit may convert the magnetic resonance signals into transmittable magnetic resonance signal data or prepare the data to be transmitted in a suitable way for transmission to the signal data receiving unit. The magnetic resonance signal data are also physical high-frequency signals. The term “magnetic resonance signal data” may nevertheless be used in order to differentiate the signals prepared for transmission from the original magnetic resonance signals. The prepared signals may be digital data.

The transmission unit may, for example, include a digital-analog converter to digitize the magnetic resonance signals. The transmission unit may include a modulator in order to prepare the magnetic resonance signal data such that the transmission frequencies for transmission to the signal data receiving unit of the magnetic resonance tomography system lie outside the frequency ranges used for the raw data acquisition for the imaging or other working frequency ranges such as, for example, the low-frequency gradient fields of the magnetic resonance tomography system. This provides that the magnetic resonance tomography system does not suffer interference from the transmission of magnetic resonance signal data via the data transmission channel.

The transmission of the magnetic resonance signal data from the magnetic resonance local coil to the signal data receiving unit may, as described above, be performed wirelessly. The magnetic resonance local coil includes, as part of the data transmission channel, at least one data transmit antenna, via which the magnetic resonance signal data are emitted. The transmission may also take place via optical waveguides, since, unlike electrical connecting cables, optical waveguides may be designed as relatively thin and flexible and hence also offer an advantage over conventional technology. Optical transmission with significantly higher carrier frequencies is also possible so that the bandwidth is greater, and the number of cables may be lower with the same transmission rate. Therefore, a data transmission channel encompasses a suitable interface for the connection of an optical waveguide. However, unless stated otherwise and without restricting the invention hereto, in the following, it will be assumed that the magnetic resonance signal data are transmitted wirelessly.

Between the receive antenna and the transmission unit or as an input stage of the transmission unit, the local coil may include a preprocessing unit. This preprocessing unit may be used to prepare the often very weak analog magnetic resonance signals received by the receive antenna for further processing and optional digitization. This may be amplification of the measured magnetic resonance signals, filtering or another data processing operation such as mixing with another frequency or modulation.

According to the present embodiments, the transmission unit is provided at least in sections with screening with a first metal coating and a first dielectric coating. The first dielectric coating is advantageously disposed between the first metal coating and the transmission unit or components or assemblies of the transmission unit so that the first dielectric coating effects electric insulation of components of the transmission unit and the electrically conductive metal coating. This at least two-coating design with a dielectric coating in addition to the metal coating makes it possible to embody the metal coating, which exerts the actual electrically screening function, such that the problems with the screening described in the introduction (e.g., the induction of eddy currents by the gradient fields or the shadowing of the high-frequency signals required for the imaging) may be minimized, and the screening functions effectively (e.g., so that the functioning of the transmission unit is not impaired by the fields of the magnetic resonance tomography system, and electromagnetic emissions from the transmission unit do not impair the functioning of the magnetic resonance tomography system). For example, the metal coating may be very thin, which impedes eddy currents. Other advantageous designs are also explained below. In addition, the dielectric coating achieves the desired increased mechanical stability and, at the same time, the components are better protected against environmental influences such as, for example, condensation. Consequently, this increases the protection for the circuit overall and reduces the failure risk.

Since the local coils are disposed in a strong basic magnetic field of the magnetic resonance tomography system (e.g., as homogeneous as possible), the metal coating is advantageously made of a non-magnetic material. This may be a material with a low relative permeability μr(e.g., in the range of 1). Examples of suitable non-magnetic materials are copper, tin, aluminum and silver. Suitable alloys (e.g., be ironless) may also be used.

The present embodiments also include a magnetic resonance tomography system with at least one magnetic resonance local coil.

With a method according to the present embodiments for producing a magnetic resonance local coil of this kind, the magnetic resonance local coil may, for example, be designed in a conventional way in that the magnetic resonance local coil is equipped with one or more receive antennas, a transmission unit and at least one data transmit antenna. According to the present embodiments, the transmission unit is provided with screening by the application, at least in sections, of a dielectric coating and the application thereover, at least in sections, of a metal coating.

The claims in one claim category may be further developed in analogy to the dependent claims in another claim category.

The screening is designed such that the screening screens the transmission unit as a whole or at least parts of the transmission unit (e.g., at least those parts of the transmission unit, in which digitization of the signals and further digital processing takes place).

The screening may be embodied as an enclosure correspondingly enclosing the transmission unit as a whole or at least parts of the transmission unit. The enclosure may consist wholly or partially of the first metal coating with the associated dielectric coating. For example, the components to be screened may be surrounded substantially completely by the dielectric coating and a metal coating disposed there over.

In one embodiment, the screening includes or forms an elongated screening enclosure (e.g., with a length-width ratio of greater than or equal to 5). In one embodiment, the screening enclosure includes an enclosed first end face and a second end face lying opposite the first end face including at least one opening for connection to the receive antenna. A further opening on the second end face may be used for connection with the data transmit antenna. With this embodiment, therefore, all the signals are introduced and removed at an open end face, and the most interference-intensive circuitry parts such as, for example, an analog/digital converter may be arranged close to the closed end face in the screening enclosure. The dimensions of the screening enclosure are selected so that the diameter of the end faces is smaller than the length of the screening enclosure. With respect to an outside dimensions, the screening enclosure may be embodied with a rectangular cross section extending transverse to the longitudinal axis, a round cross section or an elliptical cross section. In the case of a round cross section, the screening enclosure has a cylindrical shape and hence has the external shape of a “tablet tube.” The radius may be substantially smaller than the longitudinal extension of the enclosure.

An elongated slim shape of the screening enclosure of this kind minimizes the repercussion on the magnetic fields of the magnetic resonance tomography system, since the eddy current density induced in a surface substantially scales with the length of the shorter axis of a surface disposed in the magnetic field. A narrow rod such as, for example, a cylindrical tablet tube is heated up less by the penetrating magnetic fields and circumcirculated more gently by high-frequency fields than a wide, cuboidal screening enclosure.

The screening may be embodied such that components (e.g., optionally, also whole assemblies or functional groups) of the transmission unit are screened from each other by the screening. This is possible if, for example, the metal coating of the screening is in contact with a ground of the circuit at selected points.

In one embodiment, the screening enclosure is embodied such that an interior compartment of the screening enclosure surrounded by screening forms an attenuating waveguide. The attenuating waveguide has a cut-off frequency lying in a prespecified frequency spacing under a transmit frequency for transmitting the magnetic resonance signal data. The transmit frequencies may be in a frequency range above 1 GHz. Depending upon the enclosure design, the attenuating waveguide may, for example, be embodied as a rectangular waveguide, round waveguide or as a waveguide with an elliptical cross section. The embodiment as an attenuating waveguide enables intermediate walls or screenings between different circuitry parts that would otherwise be omitted or embodied more simply.

In one embodiment, the cut-off frequency is selected such that the value of the cut-off frequency is 10% below the value of the transmit frequency. If, for example, the value of the transmit frequency is 20 GHz, a cut-off frequency of 18 GHz is selected. The result is that electromagnetic waves with a frequency below 18 GHz in the interior of the screening enclosure may propagate poorly and are greatly attenuated.

In the interior of the screening enclosure, connecting lines connect the individual components of the transmission unit in an electrically conductive manner. Connecting lines of this kind may function as signal bridges and hence reduce the attenuating effect of the attenuating waveguides. Connecting lines of this kind are connected in the interior of the screening enclosure to suitable low-pass filters, and/or band-stop filters and/or high-pass filters. The insertion of filters of this kind or the blocking measures may, for example, suppress the propagation of interference on the magnetic resonance receive frequency (e.g., of about 64 MHz with a 1.5 T magnetic resonance device or about 123 MHz with a 3 T magnetic resonance device). Correspondingly, the filters may be embodied to precisely block these frequencies.

In one embodiment, the magnetic resonance signal received by the receive antenna is initially converted outside the screened area from the magnetic resonance frequency to an intermediate frequency. The signal input line may also be provided with blocking filters for the magnetic resonance frequency on entry into the screened region. If, in addition, the digital output signals are modulated to a very remote frequency (e.g., to a transmit frequency above 1 GHz) or exported via optical waveguides, all the incoming and outgoing lines may be blocked for the magnetic resonance frequency.

In one embodiment, the screening is embodied as multilayer screening (e.g., the screening includes at least one further metal coating and at least one further dielectric coating). In each case, a further dielectric coating is disposed between two metal coatings to insulate the two metal coatings from each other. A more conductive screening of this kind results in particularly low interference emissions. In addition, the design with a plurality of different layers achieves even further increased mechanical stability so that the local coil withstands shock tests well. Coating packets, each including one dielectric coating and one metal coating, may additionally be added in order to further improve the screening.

The design of the multilayer (including at least one dielectric coating and one metal coating) screening may take different forms. Part of the screening may also be formed by a printed circuit board with a dielectric substrate that bears the conductor structure of the circuit arrangement and the components on one side and which is coated on the other side with a metal coating functioning as screening.

In order to achieve effective screening, the metal coating(s) of the screening may be electrically connected to a specific screening potential (e.g., the zero potential such as the ground of the actual circuit). If the circuit arrangement of the transmission unit is, for example, arranged on a printed circuit board, contact between the metal coating(s) and a ground surface of the printed circuit board may, for example, be established.

The application of the dielectric coating onto the transmission unit or the components thereof may, for example, take place by potting with a suitable dielectric potting compound (e.g., an epoxy-containing potting compound). An injection-molding process, for example, may be used. Alternatively, a dielectric coating may be applied by laminating-on an insulating film. In one embodiment, a polyimide film may be used for this. The lamination may, for example, be performed by film deep-drawing. In both cases, the dielectric coating may have an insulation thickness of 50 to 500 μm.

Both potting and film lamination permit a very flat design that facilitates a high integration density. Both variants also facilitate the direct reproduction of the superficial topography that may, for example, be determined by the components on a printed circuit board. In the case of potting, for example, a planar surface that may be further built upon may be provided.

For example, with potting any shape of screening enclosure desired may be achieved (e.g., the above-described oblong enclosure shape such as an elongated cylindrical shape). For example, a printed circuit board bearing the components of the transmission unit (e.g., already with an elongated slender shape) may be cast all round in the desired shape.

In another act, the metal coating is applied. The application of the metal coating may take place by various suitable technologies.

For example, a metal coating may be applied in that a metal-clad film is laminated-on. For example, a metal coating with a thickness of 100 nm to 5 μm may be created. In an alternative process, the application of a thin metal coating of this type is performed, for example, by vapor-deposition.

The metal coating may be reinforced in a second act, for example, by galvanic deposition. This enables the metal coating to be brought to a coating thickness of 5 to 50 μm. Even if the metal coating includes a plurality of thin layers, a reinforced metal coating of this kind is considered to be a single metal coating since the metal layers are not separated as individual coatings by a dielectric.

In one embodiment, the thickness of the metal coatings is selected such that the thickness at least corresponds to the skin depth of the electromagnetic waves to be screened. The skin depth δ is obtained according to the following

In equation (1), f is the frequency of the electromagnetic waves to be screened, μois the permeability constant of the vacuum, μrfor the relative permeability of the material of the metal coating, and σ is the electric conductivity of the metal of the metal coating. With copper as a material, for a frequency of 1 MHz, a skin depth of 66 μm is obtained, and for a frequency of 100 GHZ, 200 nm is obtained.

When the screening is configured with two or more metal coatings for the screening of electromagnetic waves of different frequencies, the two or more metal coatings may have different thicknesses; the thickness of the individual metal coatings may vary according to the frequency band to be screened.

A dielectric coating may be embodied free of structures or, as mentioned above, emulate the topography formed by the components of the transmission unit. In one embodiment, the dielectric coating is provided with structures (e.g., textures). The structures may be μ-structures (e.g., structures in the μm range). A structure or texture of this kind improves the scatter of electromagnetic waves through the dielectric coating in question and hence the screening effect. In one embodiment, the structures have a structure size of from 1 to 500 μm (e.g., up to 200 μm).

The structures may be embodied without any preferred direction and, in this case, effect a scatter of electromagnetic waves with no preferred direction. The structures may have periodic structures with respect to an elongation direction and hence improve the scatter in a selected preferred direction. The structure may, for example, have a saw-tooth shape in the elongation direction. In the case of the screening embodied as an elongated screening enclosure, this elongation direction may be the direction of the longitudinal extension of the screening enclosure.

In the case of a design with a plurality of dielectric coatings, this may also be differently structured or non-structured.

A metal coating may be created over a large area without any structuring. In one embodiment, at least one metal coating is provided with structures. Structuring a metal coating is also able to achieve the suppression of eddy currents in the metal coating. The structures of the metal coatings may be μ-structures, where the structures may have a structure size of from 1 μm to 5000 p.m.

The structures of the metal coatings may also have any shape (e.g., be embodied as regular or irregular). The structures may form periodic grid structures in an elongation direction. The eddy-current-damping effect of the structures may be particularly optimized in a preferred direction.

In the case of a configuration with a plurality of metal coatings, the plurality of metal coatings may also be structured differently or non-structured.

In one embodiment, at least one metal coating is embodied as the data transmit antenna. This results in a particularly simple design. The magnetic resonance local coil may, for example, include two metal coatings, of which one metal coating is connected for grounding to the ground of the printed circuit board forming the transmission unit. The second metal coating is at least temporarily embodied as separable from the ground layer and so, may be used as a data transmit antenna during operation.

In one embodiment, passivation is applied to a metal coating. The passivation provides corrosion protection for the screening. The passivation may, for example, take place by the galvanic deposition of tin. The passivation may, for example, have a layer thickness of from 1 to 5 μm.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1is a schematic diagram of a magnetic resonance tomography system1. The magnetic resonance tomography system1includes a magnetic resonance tomography scanner2with an examination chamber8or patient tunnel disposed therein. A patient bed7may be introduced into this patient tunnel8so that, during an examination, a patient O or test subject lying on the patient bed7may be positioned at a specific position inside the magnetic resonance tomography scanners2relative to the magnetic system and high-frequency system arranged therein. Alternately, the patient bed7may also be moved between different positions during a measurement.

Components of the magnetic resonance tomography scanners2are a basic field magnet3, a gradient system4with magnetic field gradient coils in order to apply optional magnetic field gradients in the x-, y- and z-directions, and a whole-body high-frequency coil5(or body coil).

The magnetic resonance tomography system1also includes a control device10and a terminal20. The control device10is connected via a terminal interface17to the terminal20so that an operator may control the entire magnetic resonance tomography system1via the terminal20. In one embodiment, the terminal20is a computer with a keyboard, one or more screens and further input devices such as, for example, a mouse or similar so that a graphical user interface is available to the operator.

The control device10includes, for example, a gradient control unit11that may include a plurality of subcomponents. This gradient control unit11connects the individual gradient coils with gradient control signals SGx, SGy, SGz. The gradient control signals SGx, SGy, SGzare gradient pulses that may be set during a measurement to precisely stipulated temporal positions and with a precisely specified temporal course. The control device10also includes a high-frequency transmit/receive unit12in order to feed high-frequency pulses HF into the body coil as MR excitation signals. The reception of magnetic resonance signals induced in the patient O may, for example, also take place via the whole-body coil-high-frequency coil5and the high-frequency transmit/receive unit12.

The magnetic resonance signals are received by at least one local coil6disposed close to the patient O. The local coil6is configured to convert the received magnetic resonance signals (the actual raw data) into magnetic resonance signal data RD (e.g., in raw data in digitized form prepared for wireless transmission) and to transmit the magnetic resonance signal data RD wirelessly to further components of the magnetic resonance tomography system1via a data transmit antenna (not shown inFIG. 1). One embodiment of a local coil of this kind is explained below with reference toFIG. 2.

The control device10includes a signal data receiving unit13with a signal data antenna18. The signal data receiving unit13uses the signal data antenna18to receive the magnetic resonance signal data RD sent by the data transmit antenna of the local coil6. The received magnetic resonance signal data RD, which are optionally appropriately processed in the signal data receiving unit13(e.g., demodulated and/or decoded), are sent to a reconstruction unit14that constructs image data BD therefrom in the usual way. The reconstruction unit14stores the image data BD at a memory (not shown) and/or sends the image data BD via the interface17to the terminal20so that the operator may see the image data BD. The image data BD may also be stored and/or displayed and evaluated at other places via a network NW.

The gradient control unit11, the HF transmit/receive unit12and the receive unit13for the local coils6are each controlled in coordination by a measuring control unit15. This uses suitable commands to provide that a desired gradient pulse is transmitted by suitable gradient control signals SGx, SGy, SGzand in parallel, controls the HF transmit/receive unit12, such that an HF pulse or a whole HF pulse train is transmitted. The magnetic resonance signals at the local coils6may be read and further processed by the HF receive unit13, and/or any signals at the whole-body coil5may be read and further processed by the HF transmit/receive unit12at the correct time. The measuring control unit15may specify the corresponding signals for the other components of the control device10according to a prespecified control protocol P. The control protocol P stores the control data that is to be set during a measurement.

In one embodiment, a plurality of control protocols P for different measurements is stored in a memory (not shown). This may be selected by the operator via the terminal20and optionally varied in order to have a suitable control protocol P available for the currently desired measurement, with which the measuring control unit15may work. Alternatively, the operator may also call up control protocols P via a network NW (e.g., from a manufacturer of the magnetic resonance tomography system1), then optionally modify and use the control protocols P.

The basic course of a magnetic resonance tomography measurement of this kind and the named components of a magnetic resonance tomography systems are known to the person skilled in the art and so will be not be discussed in any further detail here. Otherwise, a magnetic resonance tomography scanner2of this kind and the associated control device10may also include a plurality of further components that also will not be explained in detail here. The magnetic resonance tomography scanner2may also have a different structure (e.g., with a patient chamber open at the side).

The following explains the design of one embodiment of the magnetic resonance local coil6with reference toFIG. 2.

The magnetic resonance local coil6includes a receive antenna22for receiving magnetic resonance signals, a preprocessing unit26for preprocessing the magnetic resonance signals, a transmission unit24for converting the magnetic resonance signals into magnetic resonance signal data RD, and an energy source50for supplying the transmission unit24and the further components of the local coil6with energy. The magnetic resonance local coil6also includes a data transmit antenna28for wireless transmission of the magnetic resonance signal data RD to the signal data antenna18of the control device10(seeFIG. 1).

The preprocessing unit26includes a preamplifier52that initially amplifies the magnetic (e.g., very weak) resonance signals received by the receive antenna22. The magnetic resonance signals amplified by the preamplifier52are fed to a mixer54that effects a frequency conversion of the magnetic resonance signals so that the signals leaving the mixer54lie within a frequency range, in which the magnetic resonance signals do not interfere with the magnetic resonance tomography measurement and other components of the magnetic resonance tomography system1.

The transmission unit24is accommodated in a screening enclosure42structured according to the present embodiments. The screening enclosure42has a substantially cylindrical shape with a first end face46and a second end face44. The cylindrical screening enclosure has, for example, a length of 10 cm and a diameter of 5 cm. The first end face46of the cylindrical screening enclosure42is enclosed, while the second end face44of the cylindrical screening enclosure42has openings, through which connecting leads92ato92dextend from the transmission unit24to the preprocessing unit26, the energy source50and the data transmit antenna28.

On an inlet side, the transmission unit24includes a filter unit56, with which interference signals on the connecting leads92ato92dare blocked so that the interference signals cannot reach the other components of the transmission unit24and interfere with the function of the other components or, vice versa. No interfering signals may escape from the transmission unit24to the outside.

This filter unit56includes a first low-pass filter58athat provides that no high-frequency interference signals from the energy source50or via the connecting lead92amay penetrate the transmission unit24, or no interference signals may leak out. The filter unit56includes two band-stop filters60a,60bfor the connecting leads92b,92cto the preprocessing unit26. A high-pass filter62aprovides that only the desired magnetic resonance signal data modulated to the transmit frequency may reach the data transmit antenna28from the transmission unit24.

In addition to the filter unit56, the transmission unit24includes a power supply64, a clock generator66, an analog/digital converter68and a modulator70.

The power supply64is connected to the energy source50and supplies the clock generator66, the analog/digital converter68and the modulator70with electrical energy via a connecting line94a.

Two low-pass filters58b,58care looped into the connecting line94ain order to provide that no high-frequency signals, which may become coupled in the connecting lines94a, exert a negative influence on the function of the clock generator66, the analog/digital converter68or the modulator70.

An output of the clock generator66is connected by a connecting line94bvia a band-stop filter60cto the band-stop filter60aof the filter unit56. From there, the output of the clock generated66is connected via the connecting lead92bto the mixer54. Thus, a clock pulse for converting the frequency of the magnetic resonance signals amplified by the preamplifier52is provided to the desired mixed frequency. The band-stop filter60cblocks unwanted frequencies that may become coupled into the connecting line94b. An output of the clock generator66is connected via a connecting line94cto the analog/digital converter68and the modulator70in order to provide this with the appropriate clock pulse as well.

On the input side, the analog/digital converter68is connected by the connecting line94dvia a band-stop filter60d, the band-stop filter60bof the filter unit56, and the connecting lead92cto the output from the mixer54. The band-stop filter60dprovides that unwanted frequencies cannot become coupled into the connecting line94dbetween the analog/digital converter68and the filter unit56.

The magnetic resonance signal data digitized by the analog/digital converter68are fed to the modulator70. The modulator70uses the clock signal of the clock generator66to generate digitized magnetic resonance signal data that are fed via the connecting line94eand via the filter unit56and further via the connecting lead92dinto the data transmit antenna28. In order to be able to exclude any unwanted frequencies, two high-pass filter62b,62care looped into the connecting line94e.

The digitized magnetic resonance signal data transmitted by the data transmit antenna28are, as described above, received and further processed by the antenna18of the control device10(seeFIG. 1).

An explanation of an exemplary design of a screening30of the transmission unit24according to the present embodiments and an embodiment of a method for production of the screening are described with reference toFIGS. 3 to 8.

A sectional view of a transmission unit24is shown in schematic form inFIG. 3. The transmission unit24includes a printed circuit board72, on which components74are arranged. The components, when interconnected, take over, for example, the functions of the power supply64, the clock generator66, the analog/digital converter68, the modulator70or other components or functional groups of the transmission unit24. The components74are arranged on an upper side of the printed circuit board72. A rear side of the printed circuit board72is provided with a continuous coating made of an electrically conductive material forming the ground76(e.g., the ground potential of the circuit). The printed circuit board72includes two contact openings80that are filled with electrically conductive material and hence permit contact with the ground76from the front side of the printed circuit board72.

In a first act according toFIG. 3, the printed circuit board72provided with the components74is coated with a first dielectric coating34. The dielectric coating34may, for example, have a thickness of 50 to 500 micrometers. The coating may, for example, be performed by injection molding, where, for example, an epoxy resin is used. Alternatively, an insulation film (e.g., a polyimide film) may be laminated-on. As a result, the printed circuit board72with the components74is provided with a first dielectric coating34with a substantially constant thickness so that the surface topography that is established by the components74is reproduced on the top side.

In a second act (seeFIG. 4), two passage openings78are introduced into the dielectric coating34in a laser beam ablation procedure in order to expose the contact openings80in the printed circuit board72. Alternatively, the passage openings78may also be formed in that, prior to the application of the dielectric coating34, in the first act, a sacrificial coating is applied in the region of the contact openings80on the upper side of the printed circuit board72and removed after the application and curing of the dielectric coating34in order to expose the contact openings80.

In a third act (seeFIG. 5), to form a first metal coating32, a first very thin metal layer82is applied. This first metal layer82is, for example, made of copper and is applied by vapor phase deposition. The first metal coating32has a thickness of up to 500 nm, for example. The first metal layer82has an electric connection with the ground76through the contact openings78and the passage openings80.

In a fourth act (seeFIG. 6), a second metal layer84is applied to the first metal layer82by galvanic deposition to complete the first metal coating32. The second metal layer84is, for example, also made of copper so that the two metal coatings82,84form a film32. For the galvanic deposition of the second metal layer84, the ground76on the rear side of the printed circuit board72may be used as an electrical connection. The second metal layer84reinforces the thickness of the metal coating32to up to 50 μm.

Alternatively, the first metal coating or the first layer of the first metal coating may be applied by laminating-on a metal-clad film. The metal-clad film may be coated with copper.

In a fifth act (FIG. 7), a second dielectric coating34′ is applied to the first metal coating32. The second dielectric coating34′ may be made of the same material as the first dielectric coating34and applied in the same way.

Prior to the application of a second metal coating32′, the second dielectric coating34′ is again provided with contact openings or through-openings100extending as far as the first metal coating32. The same technologies as those in act2(seeFIG. 4) may be used to introduce the contact openings78.

The second metal coating32′, which is made up of a first metal layer86and a second metal layer88, is applied to the second dielectric coating34′. The first metal layer86is again applied to the second dielectric coating34′ by vapor phase deposition. The first metal layer of the second metal coating32′ is connected to the first metal coating32in an electrically conductive way the through-openings100and thus also to the ground76on the rear of the printed circuit board72. To complete the second metal coating32′, a second metal layer88is applied to the first metal layer86again by galvanic deposition until the second metal coating32′ has the desired thickness of between, for example, 5 and 50 μm. The second metal coating32′ (e.g., the metal coatings86,88thereof) may also be made of copper.

In a sixth act (FIG. 8), passivation40is applied to the second metal layer88of the second metal coating32′. The passivation40is made of tin, which is deposited galvanically on the second metal layer88of the second metal coating32′ until a layer thickness of from 1 to 5 μm is achieved. Alternatively, a non-metallic passivation that is applied, for example, by spraying or immersion may be selected. The passivation40provides protection from moisture and mechanical damage.

FIG. 9shows a schematic section through a further exemplary embodiment of a transmission unit24, the components74of which are potted in a potting compound to form the dielectric coating34. The potting creates a planar surface, on which, as described above, a metal coating32that is connected in an electrically conductive manner through contact openings80to an outside ground surface76of a printed circuit board72, on which the components74are arranged, is applied.

Potting with potting compound enables any shape of the screening enclosure42to be created. In one embodiment, a substantially cuboidal screening enclosure42with a length of, for example, 10 cm, a width of 5 cm and a height of 5 mm may be created.

Alternatively, the potting compound for forming the dielectric coating34may be applied such that the potting compound forms a substantially cylindrical screening enclosure42(as explained in connection withFIG. 2), in which the printed circuit board72with the components74is accommodated.

An elongated shape of this kind (e.g., cuboidal or cylindrical) minimizes the repercussion on the magnetic fields oriented in all the spatial directions, since the induced current density in a surface positioned in the magnetic fields is substantially determined by the length of the shortest axis of the shape. A body embodied as a narrow rod is heated to a less degree than a cuboid with a more quadratic cross section.

An elongated shape of this kind enables the screening enclosure42to be embodied as an attenuating waveguide with a cut-off frequency of, for example, 10% below the values of the frequency to be screened. In one embodiment, the value of the frequency to be screened is, for example, 20 GHz (e.g., a value of 18 GHz is selected as the cut-off frequency).

In the exemplary embodiment according toFIG. 9, the screening may be made up several coatings. For example, at least one further dielectric coating is applied to the metal coating, and another second metal coating is applied thereover.

FIG. 10shows a schematic section view of another embodiment of a transmission unit24with a very simple screening. The components74are coated with a potting compound to form the dielectric coating34, such that the topography formed by the upper side of the printed circuit board72is retained. Correspondingly, the metal coating32also has the same topography. A screening30with a structure36is available. This structure36suppresses unwanted eddy currents in the metal coating32. This design substantially corresponds to the design according toFIG. 8after the third act according toFIG. 5.

FIG. 11shows a schematic section view of a further embodiment of a transmission unit24that has been provided with two metal coatings32,32′ and two dielectric coatings34,34′. Both metal coatings32,32′ are connected through the passage openings80to the ground76in an electrically conductive manner on the rear of the printed circuit board72. As in the exemplary embodiment shown inFIG. 10, the topography of the components74on the printed circuit board72is reproduced in the second metal coating32′ and hence forms a structure36that suppresses unwanted eddy currents.

FIG. 12shows a further exemplary embodiment of a transmission unit24in a schematic sectional view. The dielectric coating34is provided with a μ-structure38(e.g., a texture) that is periodic. The periodicity extends along an elongation direction I. The structure38has a saw-tooth shape in the elongation direction I (e.g., the elongation direction does not reproduce the topographies caused by the components74on the printed circuit board72).

A metal coating32is applied to the structured dielectric coating34, in which coating a μ-structure is correspondingly reproduced. The metal coating32may, as described above, include a first metal layer and a second metal layer. The first metal layer is applied by vapor phase deposition, and the second metal layer is galvanically deposited. Alternatively, the metallization may also take the form of the application of a film clad with copper.

The structure38of the dielectric coating34has the effect of improving the scatter of electromagnetic waves. The structural dimensions may be effective if their dimensions are λ/2 to λ/4 of the wavelength of the electromagnetic waves to be screened. The structure36of the metal coating32suppresses unwanted eddy currents. The structures38of the dielectric coating34have structure sizes of, for example, 1 to 500 μm (e.g., to 200 μm).

To provide that, for the protection of the μ-structure, the screening is given a smooth surface, the metal coating32is covered by a second dielectric coating34′ with a smooth outer surface.

FIG. 13shows a schematic top view of a metal coating32that has been provided with a μ-structure37with a periodicity also extending in the elongation direction I. To form the structure37, slot-shaped openings are introduced into the metal layer32so that the dielectric coating34(in an enlarged view) is visible, and the structures37are embodied as periodic grid structures37in the elongation direction I. The structures37may have structure sizes lying between 1 μm and 5000 μm. Structuring of this kind may effectively prevent or at least damp the propagation of eddy currents.

FIG. 14shows a further schematic section through an exemplary embodiment of a transmission unit24, in which the individual components74are screened from each other. Hence, there may be no mutual interference. A dielectric coating34is applied to the upper side of the printed circuit board72, covering all the components of the transmission unit24. A metal coating32is applied to the dielectric coating34only in dedicated areas over certain components of the transmission unit24. Initially, for example, with a laser ablation method or another method (e.g., as was explained in connection withFIG. 4), slot-type contact openings, for example, are introduced at the boundaries of the certain dedicated areas and connected in turn via contact openings to a ground surface76. This provides that different components74or assemblies of the transmission unit24such as, for example, the power supply64, the clock generator66, the analog/digital converter68or the modulator70(seeFIG. 2) are screened from each other and during operation, do not cause any mutual interference from unwanted emissions of electromagnetic radiation.

The designs and methods described above are exemplary embodiments, and the basic principle may also be varied within a wide range by the person skilled in the art without leaving the scope of the invention as defined in the claims. For example, when using a printed circuit board as the basis of the circuit, the rear of the complete printed circuit board or the metallic coating thereof may be provided with at least one further dielectric coating and at least one further metal coating in order provide a multi-coat screening on the rear. The use of the indefinite article “a” or “an” does not preclude the possibility that the features in question may also be present on a multiple basis. Similarly, the term “unit” does not preclude the possibility that the unit includes a plurality of components that may also be spatially distributed.