DE202007015620U1 - Resonator segments for generating a homogeneous B1 field in ultrahigh-field magnetic resonance tomography - Google Patents

Abstract

Resonator segments for generating a homogeneous B1 field in ultrahigh field magnetic resonance tomography,
Each with a metallically conductive or metal base plate (1),
With two metal-conducting or metal conductor strips (4) and (5) as high-frequency near-field interaction elements,
- With a dielectric, consisting of a first dielectric plate (2) and a second dielectric plate (3) between the conductor strips (4) and (5) and the base plate (1), whereby the conductor strips (4) and (5) and the base plate (1) are galvanically isolated from each other,
Each planar resonator segment (109) is constructed so that it can be individually controlled as a stand-alone antenna module and can be used both individually and as a modular component of a complex, switchable antenna device,
The two conductor strips (4) and (5) together forming a resonance line antenna device in the form of a resonant dipole,
- Wherein the total mechanical length of the resonance line of the dipole thus formed from the sum of the individual lengths of the respective conductor strips ...

Description

The The invention relates to an antenna device for generating a homogeneous high-frequency magnetic field (B1 field) to initiate the spin excitation in the examination volume of an ultrahigh field magnetic resonance tomograph (MRI: Magnetic Resonance Tomograph, or MRB: Magnetic Resonance Imaging and MRI: magnetic resonance imaging). The antenna device is made from numerous (eg eight to sixteen) individual electromagnetically largely decoupled resonator segments in the circumference of the cylindrical examination volume are evenly distributed and can be controlled separately by a control device. The Antenna device serves temporally one after the other on the one hand as a near-field transmitting antenna for initiating the spin excitation of the protons in each case to be examined object and on the other hand as a near-field receiving antenna for Reception of the high-frequency echo signals. The individual resonator segments The antenna device extend in its longitudinal direction parallel to the longitudinal axis of the basic field magnet. With the help of power generators fed Resonator segments is momentarily pulsed for image acquisition, local, if possible homogeneous, high-frequency magnetic field (B1 field) for the deflection of the nuclear spins in the objects to be examined inside the examination volume generated. In the pauses in the pulse, the high-frequency electromagnetic Signals of being returning to the starting position with the same Resonatorsegmenten received and the computer-based evaluation Ultrahigh-field Magnetic Resonance Tomography supplied for image generation.

Next the high-frequency magnetic field (B1 field) for the deflection of being (the actual signal generation) are still more static magnetic fields necessary. To localize the individual signals are magnetic Gradient fields are used and used to align being generated by the main field coils, very strong, homogeneous, static Magnetic field with magnetic flux densities ranging up to several Tesla.

With increasing magnetic flux density, finer structures can be displayed. However, at the same time the technical effort to generate increases of these high magnetic flux densities.

For diagnostic Purposes are used as a compromise between the achievable image resolution and the associated acquisition and operating costs currently typically magnetic flux densities in the range of about 1.5 Tesla used. Since 2006 are new purchases increasingly machines with magnetic flux densities of 3.0 Tesla established. The University Marburg has since 2005 a commercial 7-Tesla device (Siemens) for experimental Head studies. At Forschungszentrum Jülich, a plant is scheduled for completion by 2008 to be built with 9.4 tesla. In physical research become high field devices used with values of magnetic flux densities up to 20 Tesla.

In dependence however, the magnetic flux density of the static main field changes also the frequency of the precession movement the core, the so-called Larmor frequency. This corresponds largely the interaction frequency of the high-frequency magnetic field (B1 field) for deflection the being of the protons involved in the image generation. It is the relationship is linear. At a strength of the magnetic flux density of the main field is 7 Tesla the Larmor frequency of the hydrogen protons is about 300 MHz. This means, that the antenna device for generating a homogeneous high-frequency Magnetic field (B1 field) for initiation of spin excitation in the examination volume a high-field magnetic resonance tomograph to this with increasing magnetic flux density increasing Larmor frequency must be adapted. sufficed so far for Low-field equipment still relatively easy to produce high-frequency coils as appropriate Interaction elements, so must at high field or even ultra-high field devices increasingly low in inductance Near-field antennas are used to produce the required high-frequency Build up magnetic field (B1 field). The special design of the Structure of the antenna device used Resonatorsegmente has So a decisive influence on the quality of the modern high field devices achieved pictures. The resonator segments need not only sufficiently intense high frequency fields (Input power per resonator segment short-term approx. 1 kW) without delay can generate You need to this high impulse output even as evenly as possible in the entire examination volume into a high-frequency magnetic flux density.

For very high values of magnetic flux density and thus very high Larmor frequencies, conventional birdcage coils for the test volume with moderately narrow copper strips as rungs have a relatively high inductance, which requires very low capacitance values to bring the coil assembly into resonance as a whole. This is problematic for several reasons. It creates a high voltage potential across the capacitors, which can lead to a local electrical stray field, which can bring high-frequency energy in the form of heat in the patient to be examined. Thus, this effect contributes to increase the absorbed SAR rate, which eventually leads to an extension of the time required for imaging. In addition, considerable electromagnetic radiation loss to be compensated may occur, which must additionally be applied by the feeding power generators, as a result of which the signal-to-noise ratio (SNR) of the received signals and thus the image quality subsequently deteriorate significantly.

Numerous variants are available for the high field or ultrahigh field antenna devices required for this purpose. A suitable high frequency coil assembly for imaging a subject volume using a high magnetic flux density magnetic resonance tomograph that can be operated at substantially high frequencies is disclosed in U.S. Patent Nos. 4,774,830 DE 10 2004 027 026 A1 (Method and apparatus for very high field strength magnetic resonance imaging systems).

The coil assembly used therein has a number of conductors, arranged cylindrically and around a patient tunnel of the MRB system are provided over a number of capacitive elements located between the respective ends the conductor (and these interconnecting) are arranged, wherein the multiple conductors and the multiple capacitive elements provide a high-pass birdcage configuration form and over a number of dynamic disconnect switches, each dynamic disconnect switch forming a parallel resonant circuit parallel to a associated capacitive element lying electrically coupled is.

The described device is a significant improvement in comparison to the conventional ones Systems and should excite at Larmor frequencies up to 300 MHz enable.

A disadvantage of this invention, however, is the need for circuit breakers for each resonator element, capacitively coupled to each other at the ends resonators, which are difficult to be excited independently and thus the use of resonant TEM lines in the form of floating conductor strips without metal ground plane ( only one virtual mass line is used). The use of semiconductor devices (PIN diodes) to build the necessary here numerous circuit breakers (for each resonant line) can limit the reliability sustainable, since typically with high frequency pulses very high pulse power (in kW range) is used, which is for semiconductor devices in the Over time becomes increasingly harmful (degradation by local overloads in the crystal lattice). Overall, the realization according to the DE 10 2004 027 026 A1 too complicated for the desired purpose, thus too expensive and prone to failure.

By the resonance - related uneven current distribution over the transversal course of each operated in quarter wavelengths Resonant lines is the high frequency resulting from this device Magnetic field also relatively inhomogeneous, causing unwanted artifacts in imaging can cause.

One about that Examination volume homogeneously distributed B1 field, however, would undisturbed pictures without complicated correction calculations.

A method for homogenizing a B1 field and magnetic resonance system is described in US Pat DE 10 2004 013 422 A1 described. For this purpose, the measurement is carried out in different iteration steps.

In a first iteration step, measurement data are initially recorded which represent a B1 field distribution in at least part of the examination volume of an MRB system. Then, with the help of a computer, an automatic B1 homogeneity analysis based on the acquired measurement data is performed. Then, from a number of possible homogenization actions, a specific homogenization action is automatically performed on the basis of the B1 homogenization analysis. The iteration is terminated when the diagnosed homogeneity is sufficient for an intended magnetic resonance measurement. In particular, the field distortion caused by the object to be examined should also be compensated with this method. For this purpose it is according to the DE 10 2004 013 422 A1 requires the MRB system to have a suitable memory device with numerous homogenization actions associated with particular homogenization deviation classes. To control the homogenization action, the help of a homogenization controller is needed.

The homogenization action is to be carried out by a suitable adjustment or adjustment of the parameters of the various components of the MRB system. Thus, specifically resonating antenna structures can be switched in order to achieve a specific B1 field predistortion. Similarly, changes in the B1 field should be achieved by adjusting the measurement sequence parameters. In addition, a targeted change in the dielectric environment of the body by means of water pillows or other dielectric elements by a dielectric environment modifying intervention of the operator to take place. Thus, in order to perform the homogenization action by means of a special homogenization action control device and an output device (for example a prompt generation unit), information about cushion size or about certain types of dielectric pads and the exact positioning data thereof is to be forwarded to the personnel.

The Although the process is interesting, it is too labor intensive and needed for the respective iteration steps in total too much time. In addition, the Effort to stockpile the customizable dielectric Pillow practice, in particular with regard to the achievable Advantages of piecewise improved homogeneity, which is very local in nature and can be achieved in other ways leaves. For the Physical lab may bring benefits to this procedure for the clinical However, practice is all harmful, what the for needed the investigation Time also extended. The key to rapidly achieving crisp, sharp images using high-field or ultrahigh field magnetic resonance tomographs lies in the provision of suitable antennas for the examination volume. This can be avoided in advance many problems that otherwise would have to be cleaned up again with tricky procedures.

An already quite useful arrangement for generating high-frequency B1 fields in the MRB with surface current antennas is in DE 103 34 170 B3 given. This invention relates to an arrangement for generating and / or detecting radiofrequency B1 fields in an examination volume of a magnetic resonance tomograph, the arrangement comprising a surface coil tubularly constructed from a conductive band capacitively closed at at least two adjacent ends wherein the ratio of length to width of the band is less than or equal to 30 and wherein the examination volume outside the tape-enclosed space area is subsequently arranged on an outer surface of the surface coil, so that the B1 field lines generated by the surface coil are substantially parallel in the examination volume run to this outer surface. As a result, an MRB transmission and reception arrangement with improved properties is created, in particular in which the distribution of the B1 field can be influenced.

The in the DE 103 34 170 B3 presented Flächenstromantenne comes in the physical mode of action to the Resonatorsegmenten presented below quite close. However, there are some drawbacks recognizable, whereby the Flächenstromantennene in the DE 103 34 170 B3 does not seem to be better. The Flächenstromantennene uses no planar, but a rather voluminous design. Top and bottom are located at a relatively large distance from each other. The band is once completely guided around, capacitively closed at two adjacent ends and is operated in resonance.

By this special shape is the one for the examination volume for generating the high-frequency magnetic field relevant part of the surface current extraordinarily short. The total length the volume is significantly shorter as a half wavelength, if you go to the rough estimate Looking at the unfolded structure, the one to the End capacitively loaded dipole corresponds. After folding will be this length not just halved, as the top and bottom are about the same length are, but the relatively large Distance between top and bottom leads to a further shortening of the for field generation in the study volume effective length. Though can also use this surface current certainly be used in principle in high-field magnetic resonance tomographs, the examination volume acquired by this antenna is due to However, given by the design length extension still higher Larmor frequencies too small. Therefore, this design for ultrahigh field Magnetic resonance tomographs certainly not suitable.

Farther must be considered that in the examination volume normally already very cramped space conditions prevail anyway. The surface current antenna must be with their back inside of the examination volume far enough from the metal of the magnetic resonance tomographs be removed because otherwise the stray field of capacitive coupled Ends over the metal is heavily attenuated or the resonance frequency is too much detuned. It comes to a distance the top and the bottom so now the distance from the metal Add the study volume.

A (relatively thin) insulating layer must still be applied to the examination object, since the patient's skin must not have direct contact with the metal of the surface antenna. However, the observed spatial distances in a cylindrical examination volume become effective both above and below as well as right and left, ie the examination volume available for the object to be examined is further restricted. The in the DE 103 34 170 B3 described Flächenstromantenne is despite useful high-frequency magnetic properties due to their voluminous design altogether too short and too thick to name two significant disadvantages.

The specified in claim 1 invention is based on the problem improved Near field antennas for operating frequencies in the decimeter range (0.3 GHz to 3 GHz), designed as in the array or individually to be operated from each other electromagnetically largely decoupled resonator segments with the lowest possible height for an antenna device to produce a homogeneous as possible, ie despite existing, the Magnetic field influencing objects, over almost the entire examination volume in its intensity as evenly distributed, high-frequency magnetic field of a high field or ultra-high-field magnetic resonance tomography with magnetic flux densities of 7 Tesla and more to provide.

This Problem is solved by the features listed in the protection claim 1 solved.

The particular advantages of the invention are that the resonator segments over almost the entire, for the interaction with the high-frequency magnetic field effective course have a magnetic field generating current density with almost equal intensity. Thereby will be without further technical measures in the examination volume largely homogeneous high-frequency magnetic field generated. This can simultaneously improve both the quality of the pictures improved as well as for the time required to produce these images is shortened. Another advantage is that the resonator segments have a low height exhibit. As a result, the existing examination volume generously designed be without the technical effort to generate the static Basic field increase.

One Another advantage is that the resonator segments compared to Free space wave impedance are low impedance mismatched and therefore, much like in magnetic loop antennas, a of the neighboring objects have little influenceable Strahlnahnahfeld own. Significantly fewer field distortions occur due to the patients or inhomogeneous objects of investigation are caused. Thereby can additional time-consuming and labor-intensive homogenization measures omitted.

One Another advantage is that the resonator segments by the low-impedance Mismatch with each other only very low (typically less as - 15 dB) are coupled. Another advantage is that the resonator segments with respect to the far field to low impedance and thus mismatched are. This means that those of the resonator segments in operation outgoing electromagnetic radiation decays very quickly with the distance.

One Another advantage is that the resonator segments by their embodiment as a modified single-ended strip lines (microstrip line) already have a relatively large mass metallization by design, which emanates from the resonator segments in operation electromagnetic Noise, similar to a Farraday cage, outward largely shielded.

One Another advantage is that the resonator segments separate, independent units are, making the entire antenna device modular can be and the individual Resonatorsegmente if necessary easily can be exchanged. So can with a differently selected Value of the basic magnetization z. B. the same basic construction the antenna device are maintained and it will then turn on the changed one Larmor frequency matched resonator segments used.

to explanation The invention are given drawings, which are described in more detail below become. It shows

1 schematically the typical structure of a magnetic resonance tomograph,

2 schematically and by way of example the typical arrangement of the resonator segments in the examination volume,

3 schematically the cross section of a resonant line in unbalanced stripline technique in the longitudinal direction with a length of the resonator, which corresponds to half a wavelength,

4 schematically the cross section of a resonant line in unbalanced stripline technique in the longitudinal direction with a length of the resonator, which corresponds to about three quarters of the wavelength,

5 FIG. 2 schematically shows the cross section of a distributed resonator segment according to the invention in a modified asymmetrical stripline technique in the longitudinal direction with a length of the resonator which here corresponds to approximately three quarters of the wavelength, FIG.

6 3 shows a schematic representation of the spatial representation of a distributed resonator segment in a modified asymmetrical stripline technique in the longitudinal direction with a length of the resonator, which here corresponds to approximately three quarters of the wavelength,

7 schematically the cross section of a distributed resonator segment according to the invention in a modified asymmetrical stripline technique in the longitudinal direction with a length of the resonator, which corresponds to about three quarters of the wavelength, with used for fine tuning capacitors between conductor strip and ground plane,

8th FIG. 2 schematically shows the spatial representation of a distributed resonator segment in modified asymmetrical stripline technique in the longitudinal direction with a length of the resonator which corresponds to approximately three quarters of the wavelength, with capacitors between conductor strip and ground plane used for the fine-tuning, FIG.

9 1 schematically shows the cross section of a distributed resonator segment according to the invention in modified asymmetrical stripline technique in the longitudinal direction with a length of the resonator which corresponds to approximately three quarters of the wavelength, with capacitors used for fine tuning between respectively the end and the underside of the conductor strip;

10 FIG. 2 schematically shows the spatial representation of a distributed resonator segment in modified asymmetrical stripline technique in the longitudinal direction with a length of the resonator which corresponds to approximately three quarters of the wavelength, with capacitors used for fine-tuning between respectively the end and the underside of the conductor strip, FIG.

11 schematically the cross section of a concentrated resonator segment according to the invention in modified asymmetrical stripline technique in the longitudinal direction with a length of the resonator, which corresponds to about three quarters of the wavelength, wherein for the electrical extension of the resonant line additional coils are connected between the end of the respective resonant line and the ground plane,

12 schematically the spatial representation of a concentrated Resonatorsegmentes in modified asymmetrical stripline technique in the longitudinal direction with a length of the resonator, which corresponds to about three quarters of the wavelength, wherein for electrically extending the resonant line additional coils are connected between the end of the respective resonant line and the ground plane,

13 schematically the cross section of a concentrated resonator segment according to the invention in modified single-ended stripline technique in the longitudinal direction with a length of the resonator, which corresponds to about three quarters of the wavelength, wherein for the electrical extension of the resonant line additional coils between the end of the respective resonant line and to the fine-tuned capacitors between coils and Ground surface are connected,

14 schematically the spatial representation of a concentrated Resonatorsegmentes in modified asymmetrical stripline technique in the longitudinal direction with a length of the resonator, which corresponds to about three quarters of the wavelength, wherein for electrically extending the resonant line additional coils between the end of the respective resonant line and to the fine-tuned capacitors between coils and Ground surface are connected.

To explain the invention, the most important components of a magnetic resonance tomograph must first be roughly localized. The typical structure of a magnetic resonance tomograph is in 1 shown. The housing ( 101 ) of the magnetic resonance tomograph substantially encloses the cylindrical, (with great technical effort cooled (superconducting) main field coils ( 102 ), which are constructed to produce a homogeneous static magnetic field based on massive Helmholtz coil arrangements and extended by smaller auxiliary coils to compensate for field distortions. Furthermore, coil arrangements for generating magnetic gradient fields ( 103 ), which can generate magnetic fields with a linearly increasing intensity profile in the three spatial coordinate directions in order to obtain a local assignment of the respective spatial coordinates of the individual spin-pulse signals or echo signals. The patient ( 106 ) or the object to be examined are located on a sliding carriage ( 107 ) and are for examining imaging through a mostly approximately cylindrical opening ( 105 ) are pushed so deeply into the magnetic resonance tomograph that the body site to be examined or the object site to be examined is preferably located in the middle of the actual examination volume (FIG. 108 ), which only a small part of the total volume of the housing ( 101 ) of the magnetic resonance tomograph.

Within the examination volume is in the immediate vicinity of the patient ( 106 ) the antenna device with the individual resonator segments ( 109 ) for generating the high-frequency B1 field and for receiving the echo signals radiated with approximately the Larmor frequency of the proton spins which precirculate the starting position.

In 2 is a schematic and exemplary of the typical cylindrical arrangement of the individually controllable modular resonator segments ( 109 ) in the examination volume ( 108 ) in the cross section of the cylinder axis to recognize. In this illustration, it is also clear that the resonator segments used here according to the invention ( 109 ) by their structure in modified unbalanced strip line are relatively flat. If the Resonatorsegmente significantly thicker, as z. For example, in the case of the above-mentioned surface current antenna constructions, the portion of the examination volume available to the patient is greatly reduced from all sides at the same time, which should significantly promote the already existing claustrophobia fears.

To explain the particular mode of action of the new resonator segments presented here, a classical line dipole with a total length of half a wavelength is initially considered. The 3 schematically shows the cross section of such a resonant line in asymmetric stripline technique in the longitudinal direction with a length of the resonator, which corresponds to half a wavelength.

Microstrip technology or microstrip line technology or unbalanced stripline technology are some of the numerous different names for one and same line technology with planar structure and means that itself a conductor strip with a certain width in a given Distance over an extensive base metallization is located as a ground plane. The ratio determines the width of the conductor strip to the distance from the ground plane the Characteristic impedance of this line. The wider the conductor strip and each less the distance of this conductor strip from the ground plane, the more lower is the characteristic impedance of the line.

Between the conductor strip and the ground surface there is usually a substrate material or dielectric with a certain permittivity number which carries both metallizations. This permittivity number also has an influence on the characteristic impedance. With increasing permittivity, the value of the characteristic impedance of the unbalanced stripline decreases. In the limiting case, this dielectric may also predominantly be air, but then the arrangement must use some additional dielectric auxiliary supports due to the omission of the supporting function of the substrate material. The 2 schematically shows the cross section through such resonator segments ( 109 ) in the described unbalanced stripline technique, which are constructed self-supporting on a substrate material with predetermined overall width as independent modular units. The respective metallizations for the conductor strips (only the main resonant lines are outlined) and the ground plane are shown as thick black strips in cross-section.

In the 3 The illustration shown shows a resonator segment in asymmetrical stripline technology in the cross section of the longitudinal axis, ie in comparison to the representation in FIG 2 turned 90 degrees. The respective conductor strips ( 14 ) and ( 15 ) are on top of a substrate ( 12 ) with height (H3). The metallization of the ground plane ( 11 ) has therefore of the actual conductor strips ( 14 ) and ( 15 ) the distance of this height (H3). The conductor strips ( 14 ) and ( 15 ) are at the feed point ( 16 ) adjacent but not directly connected. They are each equipped with a coordinated feed line consisting of two conductors ( 17 ), which are formed for example by the inner lines of a twinaxial line, which the ground surface below the feed point ( 16 ) and their shielding outer jacket ( 18 ) at the point of breakdown with the ground plane ( 11 ) of the unbalanced stripline is electrically connected. Recktanz elements, which, as the expert knows, necessarily have to be used for impedance matching, are not shown here in the schematic pictures for the sake of clarity!

The two conductor strips ( 14 ) and ( 15 ) form the actual resonance lines. They are the same length and run from the feeding point ( 16 ) until the abrupt end of the substrate. Since at the ends of a metallic conductor strip is no longer, because the conductor strips ( 14 ) and ( 15 ) end there, there can be no high-frequency current more there. The respective abrupt mechanical end of the conductor strips ( 14 ) and ( 15 ) thus determines the electrical boundary conditions for the resonant structure. Is in the feed point ( 16 ) is now supplied with a high-frequency alternating voltage whose frequency is exactly half the wavelength (or a multiple thereof) of the total length of the two conductor strips ( 14 ) and ( 15 ) formed resonant line, so forms the in 3 shown arrangement the well-known classic half-wave dipole in unmodified unbalanced stripline technology.

We lead in this arrangement now a suitable high-frequency magnetic field probe (S) evenly spaced (D) parallel to the substrate surface in the x-direction and we detect the location-dependent value of the absolute value the total generated by the resonance line magnetic flux density (A) at this distance, the well-known location-dependent course of the results Absolute amount of the magnetic flux density of a half-wave dipole (A1), which at the same time also the distributed course of the conductor strip flowing high frequency amperage reflects because the current generated by the current-carrying conductor strip magnetic Flux density is directly proportional to the amperage in the conductor strip.

If one were to use such a resonator segment directly and unmodified for imaging in MRT, then the problems would be prepro programmed. Only in the middle area of the longitudinal axis (in the x-direction), ie near the entry point ( 16 ), the value of the absolute value of the magnetic flux density (A1) is reasonably equal. Then, the value of the absolute value of the magnetic flux density (A1) falls according to the course of a cosine function corresponding to the open ends of the conductive lines (FIG. 14 ) and ( 15 ) down quickly. The useful range in the examination volume is severely limited. Nevertheless, in many cases so far the procedure is the same, because one does not know any better.

The essence of the invention is now to first significantly lengthen the resonant lines in a first step and to operate a resonant line at a length that is more than half the wavelength and less than a whole wavelength and then in a second step that part of the resonant line which causes the cosinusoidally decreasing intensity values of the magnetic flux density to be mechanically or electrically separated from the B1 field to be generated such that only the regions of the strip lines are used as resonant lines with the examination volume ( 108 ), Which produce approximately equal values of the absolute value of the high-frequency magnetic flux density (B1 field) in the field space.

4 Displays a resonant line as explained 3 , but now according to the invention with significantly longer serving as a resonant line conductor strip ( 24 ) and ( 25 ), wherein the total length of both lines is more than half the wavelength and less than a whole wavelength. These are each equipped with a coordinated feed line consisting of two conductors ( 27 ), which are formed for example by the inner lines of a twinaxial line, which the ground surface below the feed point ( 26 ) and their shielding outer jacket ( 28 ) at the point of breakdown with the ground plane ( 21 ) of the unbalanced stripline is electrically connected. With the help of appropriately adapted supply lines ( 27 ), which serve as resonance lines operated conductor strips ( 24 ) and ( 25 ) in the feed point ( 26 ) supply electromagnetic energy, results in the location-dependent course of the absolute value of the magnetic flux density of a more than half-wave and less than full-wave dipole (A2), which also reflects the distributed course of the high-frequency current flowing in these conductor strips because the magnetic flux density generated by the respective current-carrying conductor strip is directly proportional to the current intensity in the conductor strip. The absolute value of the high-frequency magnetic flux density now has a sufficiently high value over a wide range and is, except for a small indentation of the intensity in the region of the feed point (FIG. 26 ) In terms of amount largely the same size. The imaging area is significantly increased. However, also falls in the 4 In the arrangement shown, the value of the magnetic flux density according to the course of a cosine function corresponding to the open ends of the conductor tracks operated as resonance lines (US Pat. 24 ) and ( 25 ) down quickly. These areas can only be used to a very limited extent in the examination volume.

According to the invention, the part of the resonance line which causes the cosinusoidally decreasing intensity values of the absolute value of the magnetic flux density is first mechanically separated from the B1 field to be generated. As a result, only the areas of the strip lines can be used as resonant lines with the examination volume ( 108 ) Interacting, which produce approximately the same values of the absolute value of the high-frequency magnetic flux density in the considered field space. The 5 shows the cross section of such a modified arrangement of the resonator segment according to the invention in the same time also modified asymmetric stripline technique in the longitudinal direction with a mechanical length of the resonance line, which is approximately the total length of in 4 shown resonant line corresponds. As can be clearly seen here, according to the invention the part of the resonant line which causes the cosinusoidally decreasing intensity values of the magnetic flux density is located below the main resonant lines (FIG. 4 ) and ( 5 ) arranged. The location-dependent profile of the absolute value of the high-frequency magnetic flux density of the dipole (A3) according to the invention is obtained along the resonator segment (in the x-direction). The absolute value of the high-frequency magnetic flux density now has a sufficiently high value over a wide range and is, except for a small indentation of the intensity in the region of the feed point (FIG. 6 ) and directly at the respective ends in the middle largely the same. The area of the resonator segment which can be used for imaging is significantly enlarged by this measure. At the ends of the resonator segment, the absolute value of the high-frequency magnetic flux density initially remains at a high level and then drops relatively abruptly.

The course of the absolute value of the high-frequency magnetic flux density of the dipole (A3) according to the invention can be explained very clearly in such a way that the respective values of the amounts of the magnetic flux density decreasing cosinusively towards the ends are superimposed as if they were each from the length (L1) of the main resonance lines ( 4 ) respectively. ( 5 ) over the course of the length (L2) of the idle conductor track ends mirrored. This means to the relatively high value of the amount of magnetic flux density at about the middle of the length (L1) of the main resonance lines ( 4 ) respectively. ( 5 ), the vanishingly small value of the magnitude of the magnetic flux density at the end of the idling conductor track is superimposed and does not change the total value there. The situation is different at the point where the main resonance lines ( 4 ) respectively. ( 5 ) are bent downwards. Since the corresponding fields overlap with significant values and homogenize thereby additionally the overall course of the high-frequency magnetic flux density over the length of the entire resonator segment.

The in the 5 The illustration shown is schematic and not to scale. The picture serves for explanation and the description of the characteristic quantities. In practice, this resonator segment is by no means so clunky, but a slender, robust and elegant structure.

There are used to construct this resonator segment with the present invention distributed dipole (ie, the dipole consists primarily completely - distributed over the space - conductor strips, additional capacitors are only used for fine tuning of the resonant frequency - in contrast to the described below concentrated dipole, in the parts the resonant line is replaced by concentrated components such as coils) first a metal (eg silver plated or gold plated copper) base plate ( 1 ) is used to realize the required ground plane to which a first dielectric plate ( 2 ) with a certain permittivity number and plate height (H1). Above this is a second dielectric plate ( 3 ) with a certain permittivity number and a suitable plate height (H2), which by no means has to be equal to the plate height (H1), but can be independently selected appropriately (eg only half as high). On top, on the sidewalls and on the bottom of this second dielectric plate, the resonant conductor patterns are mounted. This can be z. Example, be a silver-plated copper foil, which is embedded in milled depressions of the second dielectric plate and thus the main resonance lines ( 4 ) respectively. ( 5 ) each having the length (L1), the lateral conductor tracks each having the same length as the height (H2) and the running at the bottom of the second dielectric plate idler conductor ends each having the length (L2) form. The outer jacket ( 8th ) of the twinaxial feed line ( 7 ) is with the bottom plate ( 1 ) soldered. By a through bottom plate and substrates through hole (not shown in the drawing) get the adapted (adjustment elements not shown in the drawing) feeders ( 7 ) to the feeding point ( 6 ) of the two main lines ( 4 ) and ( 5 ). As can easily be seen, the resonant lines are unbalanced strip lines only in the range from the feeding point to the respective distances (L1-L2) from the end of the substrate plates, as known from the literature. For the range beyond that, it is necessary to speak of a modified asymmetrical double-stripe line, since such a strip line with two conductor strips in two dielectric planes is a special case (required here).

An illustrative, schematic spatial representation of the resonator segment according to the invention is shown in FIG 6 given. This illustration shows the unmodified basic element according to the invention of the distributed resonator segment.

Practical experiments have shown, however, that it is desirable to electrically change the rigid, mechanically predetermined length of the resonant lines of the distributed resonator segment, in order to allow the best possible matching of the resonant frequency of the resonator segment with the respective Larmorfrequenz of the MRT. For this purpose, additional voltage-resistant capacitors (CS1) are used, which are connected to each other with the free-running ends of the respective resonant lines, as well as with the bottom plate electrically conductive. As a result, the resonant frequency of the resonator segment can be reduced as a function of the capacitance value of the voltage-resistant capacitors (CS1). The 7 schematically shows the cross section of a distributed resonator segment according to the invention in partially modified and partially unmodified unbalanced stripline technique in the longitudinal direction with voltage-tight capacitors (CS1) used for fine tuning between conductor strip and ground plane.

In the 8th is shown schematically the associated spatial representation of the arrangement. This spatial representation also the problem of increased manufacturing complexity in this approach is clear. For the placement of the voltage-resistant capacitors (CS1), however, two are through the first substrate plate ( 2 ) continuous cutouts or punched to accommodate the voltage-resistant capacitors (CS1) necessary, which requires additional technical effort.

Alternatively, the procedure according to 9 to get voted. There are for fine tuning two capacitors (CS2) between the open ends of the resonant lines and the bottom of the main resonance lines ( 4 ) and ( 5 ) appropriate. This at first glance unusual approach actually works. Between the respective idling line ends and the respective underside of the main residual nance lines ( 4 ) and ( 5 ) there is a significant high-frequency potential difference in the resonance case, which causes a current flow through the capacitors (CS2) and thus becomes effective with respect to the variation of the resonant frequency of the resonator segment. The dielectric strength of the capacitors can be chosen to be lower and the possibility of tuning is more accurate at discrete predetermined capacitance values because the influence of the capacitors on the resonant frequency of the resonator segments is lower in this variant (because of the smaller differential voltage).

In the 10 is shown schematically the associated spatial representation of this arrangement. This spatial representation of a further advantage of this construction is clear, which allows a further variant of a construction of a cheap but highly suitable resonator segment.

For the construction of this new variant of the resonator segment with the dipole distributed according to the invention, a metal base plate ( 1 ) used to realize the required ground plane. On this floor plate ( 1 ), as already described, a first dielectric plate ( 2 ) with a certain permittivity number and plate height (H1). Above this is - and this is new - no second dielectric plate ( 3 ) having a certain permittivity number and a suitable plate height (H2), but this second dielectric plate ( 3 ) is by air as a dielectric with the permittivity number 1 replaced. The resonant conductor structures are made of bent or angled silver-plated or gold-plated copper plates. The used for fine tuning, soldered capacitors (CS2) also serve as a mechanical support and stabilize the entire construction even more. By choosing self-supporting silver-plated or gold-plated copper plates for the resonance lines and the choice of air as a dielectric, the quality of the resonance lines of the resonator segments increases greatly, which both reduces the generator power required for imaging, and positively influences the signal-to-noise ratio and thus improves the picture quality or shortens the total time required for the image acquisition. Since the permittivity number of air has the lowest possible value of 1, the length of the resonator segments optimally effective for the resonant frequency in question is increased to the maximum possible value. This means an even greater possible increase in the examination volume ( 108 ). Manufacturing technology also eliminates the need for recording the capacitors (CS2) in the substrate plate milling or punching, since the second substrate plate is eliminated without replacement, which in turn saves material costs. One according to 10 resonator segment constructed with air as a dielectric ( 109 ) is certainly the optimum in terms of characteristic electrical properties.

Negligible slightly worse electrical properties, the construction of the Resonatorsegmentes according to 11 on. This is the concentrated dipole already mentioned above, in which a part of the resonance line is replaced by concentrated components (coils). Instead of the bent, at the end idle lines now two coils (SP) on the one hand at the same point of the main resonance lines ( 4 ) and ( 5 ), where in the constructions previously the conductor has been bent at right angles and on the other hand with the ground potential, ie the bottom plate ( 1 ) connected. In this case, the inductance values of the coils (SP) are dimensioned such that exactly the same impedance ratios as before arrive at this connection point in the case of the empty-ended resonant lines. There are two things to consider: firstly, the inductance value of the coils (SP) must compensate for the electrical length of the resonant lines previously extended by the lateral length (H2) and the length (L2) because they no longer exist and, on the other hand the boundary conditions have changed, ie instead of a line running empty at the end, there is now a short-circuited line. This means that in addition to the lengths (H2) and (L2), each of the coils (SP) must in each case compensate for the length of a resonant line which corresponds to a quarter wavelength (quarter-wavelength resonance transformation: the short circuit is converted into an open circuit) , Since in this case no stray fields which can be influenced from the outside exist at the end of an open-circuit line, the high-frequency electrical behavior of such a construction is extremely stable. In the 12 is shown schematically the associated spatial representation of this arrangement. The inductance is schematically outlined here as a planar meander-shaped structure, which is also able to process high pulse current strengths. While the conductor strips used for the resonant lines ( 4 ) and ( 5 ) are manufactured in known asymmetrical stripline technology, the attachment of the meandering coils ( 9 ) not the known standard technique for the production of unbalanced strip conductors, since they are applied laterally on the front edge of the substrates. Thus, again, this is a modified single-ended stripline technique.

Disadvantageous in the 11 respectively. 12 Constructions is that the relatively high inductance value in planar technique can be produced only relatively expensive and possible manufacturing tolerances after the resonant frequencies of the resonator after could influence in part. A possibility for subsequent fine tuning of the resonator segments by means of suitable capacitors appears desirable.

The realization of such a possibility is in 13 shown. This figure shows schematically the cross-section of a longitudinal section of a concentrated resonator segment according to the invention in modified asymmetrical stripline technique with a length of the resonator which corresponds approximately to three quarters of the wavelength. In this case, for the electrical extension of the resonant lines additional coils (SP) in each case between the end of the respective conductor strips ( 4 ) and ( 5 ) are connected as resonant lines in single-ended stripline technique and capacitors (C) used for fine tuning between the respective coil end and the ground plane.

In the 14 is shown schematically the associated spatial representation of this arrangement. The inductance values of the meandering coils attached to the front sides in modified stripline technology ( 9 ), can be selected significantly lower again for this construction, because it is again in electrical terms to a capacitive loaded open end of a resonant line, ie the originally additionally compensated resonant line length of a quarter wavelength can now be omitted. As a result, meandering coils ( 9 ) can be produced very easily in robust, impulse-resistant, planar form. The capacitors necessary for fine tuning can be used, for example, as pulse-solid so-called MIM capacitors ( 10 ) (MIM: metal-insulator-metal) can be produced in a layered structure, which can be easily tuned by local breaking away of the upper metallization (but only in one direction towards lower capacitance values). As a result, with the aid of a standing wave measuring device, it is also easy to carry out a fine adjustment after installation of the resonator segment directly at the place of use.

Claims (10)

Resonator segments for generating a homogeneous B1 field in ultrahigh-field magnetic resonance tomography, - each with a metallically conductive or metal base plate ( 1 ), - with two metal-conducting or metal conductor strips ( 4 ) and ( 5 ) as high-frequency near-field interaction elements, - with a dielectric, consisting of a first dielectric plate ( 2 ) and a second dielectric plate ( 3 ) between the conductor strips ( 4 ) and ( 5 ) and the base plate ( 1 ), whereby the conductor strips ( 4 ) and ( 5 ) and the base plate ( 1 ) are galvanically isolated from each other, - wherein each planar resonator segment ( 109 ) is designed so that it can be controlled individually as a separate antenna module and can be used both individually and as a modular component of a complex, switchable antenna device, - wherein the two conductor strips ( 4 ) and ( 5 ) together form a resonance line antenna device in the form of a resonant dipole, - wherein the total mechanical length of the resonance line of the dipole thus formed from the sum of the individual lengths of the respective conductor strips ( 4 ) and ( 5 ), the dipole formed in this way being its feed point ( 6 ) has approximately the middle of the Resonatorsegmentes and with a matched or adapted two-wire feed line ( 7 ), characterized in that - the electrically effective total length of the resonant line of the conductor strip ( 4 ) and ( 5 ) formed dipole is more than half a wavelength and less than a whole wavelength of the respective desired interaction frequency (Larmorfrequenz), when the ends of the dipole form an electrical or idle capacitively charged - and the electrically effective total length of the resonance line of the conductor strip ( 4 ) and ( 5 ) formed dipole more than a whole wavelength and less than one and a half times the wavelength of the respective desired interaction frequency (Larmor frequency), when the ends of the dipole in electrical respect to the bottom plate ( 1 ) are short-circuited - and wherein the resonator segment is designed so that only the high with high frequency current largely uniformly flowed through part of the resonant line, so the part with the respective part length (L1) of the conductor strip ( 4 ) or with the respective partial length (L1) of the conductor strip ( 5 ), in which case this part of the resonance line has a total length ( 2 · L1), which results from the sum (L1 + L1) of the respective partial lengths (L1) and which altogether always measures less than half a wavelength of the respective desired interaction frequency (Larmor frequency), to build up a high-frequency field of the magnetic flux density in the examination volume ( 108 ), ie in the immediately adjacent spatial region, ie at distances of less wavelengths and below, essentially contributes, and - those parts with the respective partial length (L2) of the conductor strips acting as resonance line ( 4 ) and ( 5 ), which cause the cosinusoidally decreasing or fluctuating intensity values of the magnetic flux density, to be separated mechanically or electrically from the B1 field to be generated in such a way that only those with a high level are separated high-frequency current largely uniformly flowed through partial lengths (L1) of the conductor strips ( 4 ) and ( 5 ) as resonance lines with the examination volume ( 108 ) And can then generate approximately equal values of the absolute value of the high-frequency magnetic flux density (B1 field) in the field space. Resonator segments for generating a homogeneous B1 field in the ultrahigh field magnetic resonance tomograph according to claim 1, characterized in that the two from the feed point ( 6 ) most distant, end-to-end ends of the two conductor strips ( 4 ) and ( 5 ) mechanically bow-shaped or U-shaped below the one on the second dielectric plate (FIG. 3 ) remaining high-current areas of these two conductor strips ( 4 ) and ( 5 ), that - the two conductor strips ( 4 ) and ( 5 ) each with a certain partial length (L1) on the examination volume ( 108 ) facing side above the second dielectric plate ( 3 ), then laterally along the height (H2) of this second dielectric plate (FIG. 3 ) are continued to the bottom plate, - finally on the underside of the second dielectric plate ( 3 ) just below and parallel to the remaining above with high high frequency current largely uniformly strongly flowed through partial lengths (L1) of the two conductor strips ( 4 ) and ( 5 ) with a certain length (L2), which is significantly shorter than a quarter wavelength, are continued - and finally end there, - whereby the fields of the sections through which the respective different local values of the current flow with the respective lengths (L1) , (H2) and (L2) of the conductor strips ( 4 ) and ( 5 ) caused magnetic flux density in the direction of the examination volume ( 108 superimposed in such a way that a largely homogeneous high-frequency magnetic field in almost the entire study volume ( 108 ). Resonator segments for generating a homogeneous B1 field in the ultra-high-field magnetic resonance tomograph according to claim 2, characterized in that for fine tuning of the resonant frequency of acting as a resonant line conductor strips ( 4 ) and ( 5 ) additional capacitors (CS1) between the respective ends of the conductor strips ( 4 ) and ( 5 ) and the base plate ( 1 ) are mounted electrically conductive. Resonator segments for generating a homogeneous B1 field in the ultra-high-field magnetic resonance tomograph according to claim 2, characterized in that for fine tuning of the resonant frequency of acting as a resonant line conductor strips ( 4 ) and ( 5 ) additional capacitors (CS2) between the respective ends of the conductor strips ( 4 ) and ( 5 ) and about the central bottom of these conductor strips ( 4 ) and ( 5 ) are mounted electrically conductive. Resonator segments for generating a homogeneous B1 field in the ultrahigh field magnetic resonance tomograph according to claim 1, characterized in that the two from the feed point ( 6 ) most distanced line sections of each at the end of each idle or to the bottom plate ( 1 ) shorted ends of the two conductor strips ( 4 ) and ( 5 ) are electrically replaced by reactance elements such as coils (Sp) or capacitors (C), that - the two conductor strips ( 4 ) and ( 5 ) in each case with the high-frequency current largely uniformly flowed through partial lengths (L1) on the study volume ( 108 ) facing side above the second dielectric plate ( 3 ) and - wherein the remaining lengths of the two conductor strips ( 4 ) and ( 5 ) are respectively equivalently replaced or simulated by appropriate reactance elements in electrical terms. Resonator segments for generating a homogeneous B1 field in the ultra-high-field magnetic resonance tomograph according to claim 5, characterized in that as reactance elements for the two to be substituted in electrical terms from the feed point ( 6 ) most distanced, at the end in each case shorted to the bottom plate two conductor strips ( 4 ) and ( 5 ) only coils (Sp) are used, which directly with the base plate ( 1 ) are electrically connected. Resonator segments for generating a homogeneous B1 field in the ultra-high-field magnetic resonance tomograph according to claim 5, characterized in that as reactance elements for the two to be substituted in electrical terms from the feed point ( 6 ) most distanced, at the end each idle two conductor strips ( 4 ) and ( 5 ) a series circuit of a respective coil (Sp) with a capacitor (C) are used, which directly with the base plate ( 1 ) are electrically connected. Resonator segments for generating a homogeneous B1 field in the ultra-high field magnetic resonance tomograph according to one of the preceding claims, characterized in that instead of the second dielectric plate ( 3 ) only air is used as a dielectric and for achieving mechanical stability as a substitute for this second dielectric plate ( 3 ) dielectric supports with negligible influence on the characteris electrical properties are used. Resonator segments for generating a homogeneous B1 field in the ultra-high field magnetic resonance tomograph according to one of the preceding claims, characterized in that instead of the first dielectric plate ( 2 ) Air is used as a dielectric and for achieving mechanical stability as a substitute for this first dielectric plate ( 2 ) dielectric supports with negligible influence on the characteristic electrical properties. Resonator segments for generating a homogeneous B1 field in the ultrahigh field magnetic resonance tomograph according to one of the preceding claims, characterized in that both instead of the second dielectric plate ( 3 ) as well as the first dielectric plate ( 2 ) air is used as a dielectric and to achieve mechanical stability as a substitute for this second dielectric plate ( 3 ) and for this first dielectric plate ( 2 ) dielectric supports with negligible influence on the characteristic electrical properties.