CN210864013U

CN210864013U - Magnetic resonance antenna

Info

Publication number: CN210864013U
Application number: CN201920592556.2U
Authority: CN
Inventors: J.尼斯特勒; R.拉扎尔
Original assignee: Siemens Healthcare GmbH
Current assignee: Siemens Healthcare GmbH
Priority date: 2018-04-27
Filing date: 2019-04-26
Publication date: 2020-06-26
Anticipated expiration: 2029-04-26
Also published as: DE202018002146U1

Abstract

The utility model relates to a magnetic resonance antenna, it has capacitive and inductive antenna tank circuit component and at least one high frequency switch element, utilizes high frequency switch element, confirms at least one in the antenna tank circuit component of the self-resonance frequency of antenna can switch between accessible state and the unable state to the high frequency to change the self-resonance frequency of antenna. The at least one high-frequency switching element is here at least one field-effect transistor (FET) and/or at least one microelectromechanical system (MEMS).

Description

Magnetic resonance antenna

Technical Field

The utility model relates to a magnetic resonance antenna.

Background

Modern magnetic resonance systems usually operate with a plurality of different antennas (also referred to below as coils) for transmitting high-frequency pulses for exciting nuclear magnetic resonances and/or for receiving induced magnetic resonance signals. Usually, magnetic resonance systems have a large, usually stationary, system-mounted so-called whole-body coil (also called body coil or BC) and a plurality of small local coils (also called surface coils or LC). Unlike whole-body coils, local coils are commonly used to record detailed images of a body part or organ of a patient that is relatively close to the body surface. For this purpose, local coils are applied directly at the body part in which the region to be examined is located. When such a local coil is used, in many cases, transmission is performed using a whole-body coil (as a transmission coil) fixedly installed in the magnetic resonance apparatus, and an induced magnetic resonance signal is received using the local coil (as a reception coil). In order to keep the coils from interacting, the receiving coil can be detuned in the transmitting phase and the transmitting coil can be detuned in the receiving phase. When detuned, the self-resonant frequency of the respective antenna is adjusted.

Magnetic resonance antennas with a so-called birdcage structure are often used as whole-body coils. Such antennas usually have a plurality of antenna longitudinal rods arranged on a cylindrical surface and extending in parallel, which are each directed at high frequencies by means of an antenna end ring on the end side

Are connected with each other. The antenna longitudinal rod and the antenna end ring can in principle be constructed in any desired shape. In many cases, conductor tracks are applied to a flexible conductor track film, which conductor tracks are wound cylindrically around a measurement volume in which an examination object is located during an examination. For a whole-body coil, the birdcage structure typically extends around a patient accommodating space in which the patient is positioned during measurements. For local coils in the form of a birdcage construction, a measurement volume is often used to accommodate the head or other limb of the patient in order to examine just that region.

In order to detune such a magnetic resonance antenna with a birdcage structure, it is proposed in US 8237442B2 to detune by means of diodes.

SUMMERY OF THE UTILITY MODEL

The to-be-solved technical problem of the present invention is to provide a magnetic resonance antenna with a birdcage structure, which can be as simple as possible, low cost and effectively detune its self-resonant frequency. The detuning is in particular carried out with as little electrical power as possible. Furthermore, a corresponding method for detuning the self-resonant frequency of such an antenna is specified.

The above technical problem is solved accordingly by the subject matter of the present invention.

To this end, according to the present invention, a magnetic resonance antenna is provided, which has capacitive and inductive antenna tank elements and at least one high-frequency switching element, with which at least one antenna tank element, which determines the self-resonance frequency of the antenna, can be switched for high frequencies between a passable state and a non-passable state in order to change the self-resonance frequency of the antenna. The at least one high-frequency switching element is here at least one field-effect transistor (FET) and/or at least one microelectromechanical system (MEMS).

According to the invention, detuning can be provided effectively in the body coil for persons to be examined of different body weights or for exciting different nuclei. Detuning requires only minimal electrical power. Field effect transistors and MEMS components are typically controlled by voltage, whereas diodes require current control. In the case of voltage control, the switching element remains cooled, so that less control power is required.

Suitable field effect transistors are, for example, Junction Field Effect Transistors (JFETs) or insulating layer field effect transistors (IGFETs, MISFETs), in particular Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). Which advantageously switches largely without power or losses.

Another possible advantage of the invention is that the function of the resonator is optimally matched to varying loads. This applies to both whole body resonators and local coils.

The following description contains particularly advantageous embodiments and further embodiments of the invention.

According to one embodiment of the invention, the at least one high-frequency switching element is arranged in parallel with one of the inductance or capacitance, which makes it possible to bridge it effectively and in a targeted manner.

According to a further embodiment of the invention, at least one high-frequency switching element is arranged in series with one of the inductance or capacitance, which makes it possible to switch it on or off in a targeted manner. The inductance or capacitance may also be only a part of the cross section of the antenna end ring or the antenna longitudinal rod; the inductance or capacitance may be arranged, for example, in a recess of the conductive surface.

A part of the high-frequency switching element may also be a diode, which may be switched simply by means of a direct current.

The magnetic resonance antenna according to the invention can be used in particular for detuning the self-resonance frequency of the magnetic resonance antenna, wherein at least one of the antenna resonant circuit elements is switched for high frequencies by means of at least one high-frequency switching element in order to change the self-resonance frequency of the magnetic resonance antenna from a first operating magnetic resonance frequency to a second operating magnetic resonance frequency.

For example, the first operating magnetic resonance frequency is a frequency for exciting hydrogen nuclei, and the second operating magnetic resonance frequency is a frequency for exciting nuclei other than hydrogen nuclei.

In particular, a person with a first operating magnetic resonance frequency and a different body weight than the second operating magnetic resonance frequency can be tuned.

According to a further embodiment of the invention, at least one of the antenna resonant circuit elements can be switched between a passable state and a non-passable state for high frequencies by means of a high-frequency switching element alone.

Drawings

Further possible features and advantages of the invention result from the following description of embodiments in accordance with the accompanying drawings. Wherein the content of the first and second substances,

figure 1 shows a perspective schematic view of a three-dimensional wire model of a known antenna with a birdcage structure having eight longitudinal rods,

figure 2 shows an example of a circuit according to the invention as a schematic circuit diagram,

figure 3 shows as a schematic circuit diagram an example of a switching element according to the invention in an open switching state,

fig. 4 shows an example of a switching element according to the invention in the closed switching state as a schematic circuit diagram.

Detailed Description

Fig. 1 shows the general structure of a birdcage structure in the form of a simple three-dimensional line model. The birdcage structure is formed here by a number of equidistant, parallel running antenna longitudinal rods arranged on a cylindrical surface. These longitudinal rods are connected to one another at the end sides for high frequencies by means of antenna end rings 3, 4, respectively. "for high-frequency connections" means in this context that there is no mandatory galvanic (galvanosche) connection, but only a connection through which high-frequency currents can pass. As is shown in fig. 1, and as is often the case, for example, in magnetic resonance antennas according to a birdcage structure, in the antenna end loops there are in each case resonance capacitors 5 between two connection points of adjacent antenna longitudinal struts 2. For greater clarity, only the entire structure of the outer side of the virtual cylinder around which the line model extends is shown here.

In the known example shown in fig. 1, the end rings 3, 4 are each circular. Alternatively, however, the end rings 3, 4 can also be formed by straight sections which each extend between two antenna longitudinal rods 2. Then, for example, in an embodiment with eight longitudinal rods, the antenna will have an octagonal cross-section.

Such a birdcage structure can in principle have any number of longitudinal rods 2. Thus, a smaller antenna may have, for example, only 6 longitudinal rods. In the presently developed embodiment, which is not shown in the figures for greater clarity, the birdcage structure has 16 longitudinal rods.

The magnetic resonance antenna 1 is connected to a high-frequency pulse generator 7 via a supply line 6. The supply lines 6 are connected to one of the end rings 4 on the left and right of the resonant capacitor 5, respectively. Via this supply line 6, not only are high-frequency pulses fed in during the transmit mode, but also the received magnetic resonance signals are acquired during the receive mode.

This configuration involves linearly polarized HF supply, i.e. a high-frequency field (also referred to as B) generated by high-frequency pulses₁The field) is linearly polarized inside the magnetic resonance antenna 1. However, this arrangement is not feed dependent and may be at B₁Any polarization of the field. Thus, for example, a supply in which one supply line is connected to the resonance capacitor and the other supply line is connected to a ground shield (Masseschild) can also be realized.

The body coil in a birdcage embodiment is formed, for example, from a number of flat, running conductor tracks through which an HF current flows, which together with capacitors having different capacitances form an electrical network. The resulting network usually has one or more resonance frequencies at which the applied high frequencies can be converted with maximum efficiency into an excitation that is effective for Magnetic Resonance (MR). The system ideally behaves as a purely ohmic load in the resonance state, as the supply point of the guided high frequencies of the magnetic resonance antenna 1 is viewed to the outside.

In the resonant state, the losses in the HF energy transmission due to the supply cable are advantageously minimal due to the absence of reflections, the efficiency in converting the HF power into MR effective work (Arbeit), in particular for spin excitation, being the highest, and furthermore the homogeneity of the HF power distribution in the resonator is the best.

In practice, however, the resonance frequency is not only related to the intrinsic properties of the resonator, but also to the loading of the resonator. The load of the (whole) body resonator is usually constituted by the human body to be examined. Ideally, the resonator (in particular the magnetic resonance antenna 1 and the patient therein) is arranged in advance such that it achieves its natural resonance at maximum load and works optimally in this state, which, according to the system and definition, corresponds for example to a patient in a prone position with a weight of 80 or 120kg, the head of which is in front of the magnetic resonance antenna 1 (voraus). For all other load cases, the resonators are more or less mismatched.

In principle, the mismatch caused by the load can be compensated reactively (reaktiv) at the lowest point of the antenna and transformed to 500hm according to the previously common external "Body-Tune-Box" design. Although this will alleviate the problem of power generation and efficiency of power transfer along the supply line, the uniformity of the power distribution inside the resonator 1 is always disturbed by the load.

With the magnetic resonance antenna described herein, the resonator can be adjusted in a remotely controlled manner for patients of different weights and/or different nuclei. By means of the adjustment, on the one hand a good impedance matching of the supply point of the resonator can be achieved, and on the other hand the current distribution along the electrical structure of the resonator is to be adjusted within certain limits for the respective measurement situation.

Tuning of the resonator according to the invention, which is adjustable from the outside for the electrical path, can be used for the purpose of optimizing the homogeneity, or possibly also for adjusting the antenna profile specifically for a specific task.

For this purpose, it is proposed that the geometry and structure of the resonator be dynamically adjusted by means of connected semiconductor elements and that the resonant frequency or current distribution thereof be influenced in this way.

In particular, the following variants can be implemented, which can be seen in fig. 2:

"connection capacitance":

for example, (at least) one capacitor C may be used_ERA and a switching element T_ER ¹A are connected in series.

At the switching element T_ER ¹The open, i.e. non-conducting, switch position of a, the influence of the connected capacitor on the resonance frequency and the local current distribution is minimal. The resonance frequency of the (overall) system is shifted upwards towards higher frequencies. The current flow at that location is concentrated and the phase of the current is shifted.

At the switching element T_ER ¹Closed, i.e. conducting, switch position of a, connected capacitor C_ERA has the greatest effect on the resonant frequency and local current distribution. And connected capacitor C_ERThe now fully active capacitance of a corresponds to the resonance frequency of the (overall) system 1 moving downwards towards lower frequencies. The current track is wider at this location. Smaller than now in playThe reactance of (3) corresponds to a small change in the phase jump of the current.

"connection inductance":

one or more capacitors C as connections_ERAlternatively or additionally, at least a part of the current-guiding structure of the resonator may be slotted in strips parallel to the main flow direction of the HF current in one, but preferably in a plurality of positions, of the resonator. The strip thus formed can be interrupted at a suitable location transversely to the direction of current flow. May include T_ER ²-d、T_ER ²-e、T_ER ²-f switching element T_ER ²Located to the interruption, these switching elements can be electrically switched on and off individually from the outside.

At the switching element T_ER ²-d、T_ER ²-e、T_ER ²The open, i.e. non-conducting, switching position of at least one of the switches f, in which the surface available for the HF current is reduced, due to the strip of the current path being interrupted by the open switching element. This corresponds to a local increase in inductance. Thus, the amplitude and phase of the HF current change locally, and the resonance frequency of the resonator changes globally, moving towards smaller values.

At the switching element T_ER ²(including T)_ER ²-d、T_ER ²-e、T_ER ²-f) closed, i.e. conducting, switching position of at least one of the switching elements, the switching element is closed and the HF current can thus pass through the switching element smoothly. Thereby, the total inductance of the strip is reduced, the current intensity is maximized, and the resonance frequency of the (overall) system is increased.

It is also conceivable that the switching elements are not connected in series with the respective reactance of the resonator, the discrete capacitance or a part of the inductive conductor track, but in parallel. The mirror-inverted situation is thus correspondingly obtained compared to the situation already depicted.

Figure 2 schematically illustrates the application of a birdcage resonator in a bandpass configuration. The figures show only a part of a birdcage resonator, namely the central one (of the rods usually present) 2 and the segments of the end ring structures 3 and 4 extending vertically to the left and right.

The basic idea of an electrically variable geometry can be used on any HF structure, for example in an Aldeman-Grant resonator, a TEM resonator or a simple HF loop, independently of whether it is a whole-body resonator or a local resonator.

In fig. 2, the surface having the capacitor (F1, F2, F3, etc.) represents the basic structure of the birdcage resonator.

According to fig. 3 and 4, a field effect transistor T_D ^K、T_ST ^K、T_ER ^KAnd a switch group T_ER ¹、T_ER ²、T_ST ¹、 T_ST ²、T_ST ³Of a field effect transistor T_ER ¹-a/T_ER ¹-b/T_ER ¹-c、T_ER ²-d/T_ER ²-e/T_ER ²-f、T_ST ³-g/ T_ST ³-h/T_ST ³I can be switched on and/or off separately from the outside. Instead of field effect transistors, it is also possible to use a micro-electromechanical system which is not shown here, i.e. the field effect transistors can be replaced completely or partially by micro-electromechanical systems. Any other switching element, such as a diode, may also be used, but may not contribute to all the advantages according to the invention.

Fig. 3 shows the voltage U applied to the switching element 8_oWhich turns the switch off, i.e. the switching element 8 is impermeable for HF currents (i.e. impermeable for high frequencies).

Fig. 4 shows the voltage U applied to the switching element 8_gWhich closes the switch, i.e. the switching element 8 is passable for HF currents (i.e. passable for high frequencies).

Voltage U_oAnd U_gIn particular, an input voltage (gate-source voltage) between the gate and the source of a field effect transistor is to be understood. In general, the field can be determined according toThe current-voltage characteristic curve associated with the effect transistor is used for deriving the voltage U suitable for switching_oAnd U_gIn particular its direction (sign) and intensity (magnitude). These values can be derived in particular from the threshold voltage (english threshold voltage) of the field effect transistor. The threshold voltage is in this case generally the gate-source voltage of the field effect transistor, at which a significant current flows relative to the maximum collector or drain current.

In contrast to diodes, no current flows here when the switch is switched on and off (except for a possible negligible current for recharging the electrodes of the field effect transistor (umauldung)), but only via the voltage U_oOr U_gAnd (5) controlling.

By means of, for example, switching field effect transistors T_D ^KAnd a switch group T_ER ¹Of a field effect transistor T_ER ¹A to T_ER ¹-c, the capacitors in one or both end rings 3, 4 can be switched on and off.

By means of a pair of switch sets T_ST ¹Can switch on and off one or more capacitors C in the rod 2_ST。

Here, in the example shown, the entire surface available for HF current also changes when the capacitor is switched. In another possible embodiment, not shown in the drawings, a plurality of parallel-connected capacitors and their associated switching elements can be arranged discretely next to one another at one capacitor location. In circuits of such combinations of multiple capacitors closely together, only the total capacitance of such an arrangement is explicitly changed, without implying a change of the capacitor C when switching takes place_STThe current surface beside (F2 or F5).

In contrast, switch group T_ER ²Of a field effect transistor T_ER ²-d and T_ER ²-e and T_ER ²F surface F affecting only the HF current trajectory₁. This means an increase in the inductance of the resonator. Thereby, on the one hand, the resonance frequency is globally changedThe rate, on the other hand, locally influences the amplitude (magnitude) and phase of the HF current.

Field effect transistor T_ST ²And T_ST ³Locally varying the width of the surface of the bar 2 (for T)_ST ²Is F₂And F₃Width in between) and as a result of this, the inductance of the rod can be controlled in this way. This means that, on the one hand, the resonance frequency of the resonator is changed and, as a result, the local current intensity and phase are changed, which in turn has an effect on the imaging properties of the resonator.

It should be repeated for the sake of completeness that the switching element may be connected not only in series with the reactance of the resonator, but also in parallel with the reactance of the resonator.

In the right-hand part of the diagram, a field effect transistor T is drawn by way of example_ST ^K、T_D ^KAnd T_ER ^K. Field effect transistor T_D ^KIn the closed state, the capacitor C is replaced by its (small) inductance_ERThe inductance is obtained by a loop as a switching element and its input line itself.

Field effect transistor T_ST ^KAnd T_ER ^KThe parallel inductance of the conductor track inductance is switched in and the total reactance of the line is reduced accordingly. The HF current flowing through thereby experiences an expansion of its path, thereby influencing its amplitude and phase.

As much flexibility as possible in the electrical adjustability of the resonator is possible by any combination of the series and parallel connection possibilities of the switching elements and the reactances, but the complexity of the structure may also increase as a result.

Furthermore, it should be noted that the possibilities proposed here for remote control of the internal parameters of the MR antenna are also applicable to very effectively detuning the resonator, which is important for the interaction and interaction of the individual elements in the MR antenna composite structure with one another. In particular for spectral applications and at high and particularly high field strengths, it is generally advantageous to effectively and specifically detune the resonant structure of the respective antenna. For this purpose, the detuning described here by controlling the "electrical geometry", i.e. the entire arrangement 1, by switching its capacitance and/or inductance, is very suitable.

Claims

1. A magnetic resonance antenna, characterized in that the magnetic resonance antenna has capacitive and inductive antenna tank elements and at least one high-frequency switching element, with which at least one of the antenna tank elements determining the self-resonance frequency of the magnetic resonance antenna can be switched for high frequencies between a passable state and a non-passable state in order to change the self-resonance frequency of the magnetic resonance antenna, wherein the at least one high-frequency switching element is at least one field effect transistor and/or at least one microelectromechanical system.

2. A magnetic resonance antenna according to claim 1, characterized in that the antenna tank element is an inductance.

3. A magnetic resonance antenna according to claim 1, characterized in that the antenna tank element is a capacitor.

4. A magnetic resonance antenna according to claim 2, characterized in that at least one high frequency switching element is arranged in parallel with one of the inductances.

5. A magnetic resonance antenna according to claim 3, characterized in that at least one high frequency switching element is arranged in parallel with one of the capacitances.

6. A magnetic resonance antenna according to claim 2, characterized in that at least one high frequency switching element is arranged in series with one of the inductances.

7. A magnetic resonance antenna according to claim 3, characterized in that at least one high frequency switching element is arranged in series with one of the capacitances.

8. A magnetic resonance antenna as claimed in claim 1, characterized in that the high-frequency switching elements are switched in both antenna end loops of the magnetic resonance antenna such that the same capacitance is present in both antenna end loops.

9. A magnetic resonance antenna according to claim 7, characterized in that a plurality of mutually parallel capacitances in an antenna end loop or in an antenna longitudinal rod, respectively, can be switched individually and independently of each other by means of a high-frequency switching element respectively connected in series with each of the plurality of mutually parallel capacitances.

10. A magnetic resonance antenna according to claim 1, characterized in that in the electrically conductive surface in the antenna end ring, along the longitudinal direction of the antenna end ring, a plurality of high-frequency switching elements are arranged in recesses of the electrically conductive surface.

11. A magnetic resonance antenna as claimed in claim 1, characterized in that a plurality of high-frequency switching elements are arranged side by side in connection with at least one electrically conductive surface in the longitudinal rod of the antenna, viewed in the longitudinal direction of the longitudinal rod of the antenna, in series with the electrically conductive surface.

12. A magnetic resonance antenna as claimed in claim 1, characterized in that in the line direction, in parallel with the electrically conductive surface in the longitudinal rod of the antenna, there are arranged high-frequency switching elements in the longitudinal direction of the longitudinal rod of the antenna for bridging the electrically conductive surface.

13. A magnetic resonance antenna according to any one of claims 1 to 12, characterized in that a part of the high frequency switching element is a diode.

14. A magnetic resonance antenna according to any one of claims 1 to 12, wherein the high frequency switching element is opened or closed with respect to a high frequency current flowing through the high frequency switching element by the intensity and/or direction of a voltage applied to the high frequency switching element.

15. A magnetic resonance antenna according to any one of claims 1 to 12, wherein the first operating magnetic resonance frequency of the magnetic resonance antenna is a frequency for exciting hydrogen nuclei and the second operating magnetic resonance frequency of the magnetic resonance antenna is a frequency for exciting nuclei other than hydrogen nuclei.

16. A magnetic resonance antenna according to any one of claims 1 to 12, wherein the first operating magnetic resonance frequency of the magnetic resonance antenna is tuned to a person having a different weight than the second operating magnetic resonance frequency of the magnetic resonance antenna.

17. A magnetic resonance antenna according to any one of claims 1 to 12, wherein only the first and second operating magnetic resonance frequencies are adjustable.

18. A magnetic resonance antenna according to any one of claims 1 to 12, wherein in addition to the first and second operating magnetic resonance frequencies, at least one further operating magnetic resonance frequency is adjustable.

19. A magnetic resonance antenna according to any one of claims 1 to 12, characterized in that a plurality of high frequency switching elements are provided to determine at least one of the antenna tank elements of the self-resonance frequency of the magnetic resonance antenna for high frequency interruption to change the self-resonance frequency of the antenna.

20. A magnetic resonance antenna according to any one of claims 1 to 12, characterized in that the antenna longitudinal rods and the antenna end rings connecting the antenna longitudinal rods at the end sides for high frequency are arranged in a birdcage structure.

21. A magnetic resonance antenna according to any one of claims 1 to 12, characterized in that the antenna longitudinal rods extend parallel to each other.

22. A magnetic resonance antenna according to any one of claims 1 to 12, characterized in that the magnetic resonance antenna has an antenna longitudinal rod and an antenna end ring connecting the antenna longitudinal rod at the end side for high frequencies.

23. A magnetic resonance antenna according to any one of claims 1 to 12, characterized in that at least one of the antenna tank elements is switchable between a passable state and a non-passable state for high frequencies by means of high-frequency switching elements, respectively, alone.

24. A magnetic resonance antenna according to any one of claims 1 to 12, characterized in that the antenna tank elements, which are part of the longitudinal rod of the antenna or part of the end rings of the antenna, are individually switchable by means of high-frequency switching elements, respectively.

25. A magnetic resonance antenna according to any one of claims 1 to 12, characterized in that antenna resonant circuit elements which are each switchable by a high-frequency switching element are arranged in the antenna longitudinal rod.

26. A magnetic resonance antenna according to any one of claims 1 to 12, characterized in that antenna tank elements, each switchable by a high-frequency switching element, are arranged in an antenna end ring.