RU2778634C2 - Passive radio-frequency shimming resonator for homogenization of field of radio-frequency antenna device for transmission mode and reception mode - Google Patents

Abstract

FIELD: radio engineering.

SUBSTANCE: group of inventions relates to the field of magnetic resonance imaging (hereinafter – MRI) systems. A passive radio-frequency (hereinafter – RF) shimming resonator has the first resonance capability and the second resonance capability, wherein the passive RF shimming resonator contains a switching device made with the possibility of switching between the first and the second resonance capability in accordance with a transmission mode (TX mode) and a reception mode (RX mode) of RF field emitted by RF antenna device of MRI system.

EFFECT: provision of a passive RF shimming resonator for homogenization of RF field emitted by RF antenna device of MRI system.

15 cl, 14 dwg

Description

The field of technology to which the invention belongs

The present invention relates to the field of magnetic resonance imaging (MRI) systems, in particular to the field of homogenization of the radio frequency (RF) field emitted by the RF antenna device of the magnetic resonance imaging (MRI) system.

The invention relates in particular to a passive radio frequency (RF) shim resonator for homogenizing an RF field emitted from an RF antenna device in a magnetic resonance imaging (MRI) system.

The invention also relates to a patient bed or patient mattress for use in a magnetic resonance imaging (MRI) system.

The invention also relates to a radio frequency (RF) antenna device for generating and/or receiving a radio frequency field for use in a magnetic resonance imaging (MRI) system, the radio frequency antenna device comprising a coil housing and the aforementioned passive radio frequency (RF) shim resonator for RF homogenization field radiated by a radio frequency antenna.

The invention also relates to a magnetic resonance imaging (MRI) system comprising a main magnet for generating a static magnetic field, a system of magnetic gradient coils for generating gradient magnetic fields superimposed on a static magnetic field, and at least one radio frequency (RF) antenna device for generating an RF field.

The invention particularly also relates to a magnetic resonance imaging (MRI) system comprising a main magnet for generating a static magnetic field, a system of magnetic gradient coils for generating gradient magnetic fields superimposed on a static magnetic field, and at least one radio frequency (RF) ) an antenna device for generating an RF field.

The invention, in particular, also relates to a method for improving the homogenization of the RF field emitted by the RF antenna device of a magnetic resonance imaging (MRI) system.

State of the art

In high-field magnetic resonance imaging (MRI), image quality often suffers from poor B1 field uniformity caused by RF field wave propagation effects. Thus, field non-uniformity continues to be a serious problem in modern high-field magnetic resonance (MR) examination systems, in particular in the latest magnetic resonance examination systems with strong magnetic fields in 3T or 7T. Among the various RF coils designed to address this problem, birdcage coils have been introduced to improve MRI field uniformity, with bars running in directions that are non-linear compared to the longitudinal direction of the RF coils.

To overcome the above problem and improve the uniformity of electromagnetic fields, a very pragmatic, simple and cheap solution to this problem is the use of dielectric spacers, which can have a smoothing effect on the transmitted field. Similarly, local passive resonators can act similarly to dielectric spacers. Therefore, such artificial dielectric and/or narrow band permeable materials can be further used in MRI to improve field uniformity and reduce the Specific Absorption Rate (SAR) for a particular patient.

The disadvantage of these known methods is that these materials have a different optimal shape for maximum transmission and reception efficiency. Accordingly, the effect on the transmit mode (TX mode) and the receive mode (RX mode) may be different, i.e. a favorable effect on the electromagnetic field in TX mode can have a negative effect on RX mode, and vice versa. This applies even more to the design of the coil, in which the antenna rods do not extend in the longitudinal direction of the RF coil. In contrast, independent bar structures optimized for TX mode and RX mode can be provided. In this case, the symmetry of the standard RF coil compared to the standard birdcage coil is broken, as there are usually two orientations for the non-linear passage of the RF coil wires.

US Patent Application US2013/0165768 discloses a chest coil with an RF corrective coil element. During magnetic resonance recording (i.e., in receive mode), the RF correction element acts on the local B1 field to homogenize the B1 field as much as possible.

A single resonator to simultaneously optimize the RF field for efficient transmission and reception is not possible.

Disclosure of the essence of the invention

The object of the invention is to provide a passive radio frequency (RF) shim resonator for homogenizing the RF field emitted by the RF antenna device of a magnetic resonance imaging (MRI) system, patient bed or patient mattress for use in a magnetic resonance imaging (MRI) system, radio frequency (RF) an antenna device for generating and/or receiving a radio frequency field for use in a magnetic resonance imaging (MRI) system, a magnetic resonance imaging (MRI) system, a method for improving the homogenization of a radio frequency field emitted by a radio frequency antenna device of a magnetic resonance imaging (MRI) system, allowing MRI with optimized RF fields for receive mode (RX mode) and transmit mode (TX mode).

This problem is solved by a passive radio frequency (RF) shim resonator for homogenizing the RF field emitted by a radio frequency antenna device of a magnetic resonance imaging system, the passive radio frequency shim resonator having a first resonant capability and a second resonant capability, while the passive radio frequency shim resonator comprises a switching device configured with the possibility of switching between the first and second resonant capacity in accordance with the transmission mode (TX mode) and the reception mode (RX mode) of the radio frequency field emitted by the radio frequency antenna device of the magnetic resonance imaging system.

This problem is further solved by a patient bed or patient mattress for use in a magnetic resonance imaging (MRI) system, in which the patient bed or patient mattress contains the aforementioned passive radio frequency (RF) shim resonator for homogenizing the RF field emitted by the RF antenna device of the magnetic resonance imaging system. resonance imaging (MRI).

This object is also solved by a radio frequency (RF) antenna device for generating and/or receiving an RF field for use in a magnetic resonance imaging (MRI) system, in which the RF antenna device comprises a coil housing and the aforementioned passive radio frequency (RF) shim resonator for homogenization An RF field radiated from an RF antenna device, wherein the passive RF shim resonator is located within the coil housing.

This problem is also solved by a magnetic resonance imaging (MRI) system comprising a main magnet for generating a static magnetic field, a system of magnetic gradient coils for generating gradient magnetic fields superimposed on a static magnetic field, and at least one radio frequency (RF) antenna device for generating an RF field, wherein the magnetic resonance imaging (MRI) system also comprises the above-mentioned patient bed or patient mattress.

In addition, this problem is solved by a magnetic resonance imaging (MRI) system containing a main magnet for generating a static magnetic field, a system of magnetic gradient coils for generating gradient magnetic fields superimposed on a static magnetic field, and at least one radio frequency (RF) antenna an RF field generating device, wherein the at least one RF antenna device is the RF antenna device mentioned above.

This problem is also solved by a method for improving the homogenization of an RF field emitted by an RF antenna device of a magnetic resonance imaging (MRI) system, comprising the steps of providing a passive RF shim resonator having a first resonant capability and a second resonant capability according to a transmission mode (TX- mode) and reception mode (RX mode) of the RF antenna device, determine the transmission mode (TX mode) and the reception mode (RX mode) of the RF antenna device of the magnetic resonance imaging system, and switch the passive RF shim resonator between the first and second resonant capability according to the transmission mode (TX mode) and the reception mode (RX mode) of the RF field emitted by the RF antenna device of the magnetic resonance imaging system.

Thus, by switching the passive RF shim resonator between TX mode (or transmit mode) and RX mode (or receive mode), along with the corresponding modes of operation of the magnetic resonance imaging system, an optimal configuration can be achieved for both transmit mode and and for the receive mode. This is possible because in MRI, transmission and reception are performed sequentially. By means of a switching device, a suitable configuration of the passive RF shim resonator can be selected to improve the performance of the MRI system in each case. Various approaches may be taken to provide different resonant capabilities, such as providing the resonant capabilities independently or using at least partially the same structures. Thus, the passive RF shim cavity as a whole can be adapted to various applications by performing a switching operation. In this context, it is preferable that the first and second resonant capabilities are provided in accordance with the transmission mode (TX mode) and the reception mode (RX mode) of the RF field emitted from the RF antenna device of the magnetic resonance imaging system. Thus, the passive RF shim cavity can be switched between a transmit state and a receive state, each optimized for a respective component of the rotating field. A passive RF shim resonator can be incorporated into various components of an MRI system, such as a patient bed, coil housing, or patient mattress. The passive RF shim resonator can be supplied alone or in combination with at least one ceramic dielectric spacer. Accordingly, an optimal configuration for both the transmit mode and the receive mode of the magnetic resonance imaging system can be achieved. For the transmit mode, the uniformity of the B1+ field and the Specific Absorption Rate (SAR) are optimized, while in the receive mode, the uniformity of the B1+ field and the signal-to-noise ratio (SNR, from the English Signal-to-Noise) are optimally increased ratio). Spatial variations of the RF field emitted by the RF antenna, i.e. transmission coil of the MRI system can be reduced.

By switching device is meant a set of individual switching units that are switched to collectively achieve homogenization of the RF field emitted from the RF antenna device of the magnetic resonance imaging system. However, the switching device may comprise only one switching unit.

According to a modified embodiment, the passive RF shim resonator comprises one or more electromagnetically coupled resonant loops, wherein the switching device comprises at least one switching unit located in one or more electromagnetically coupled resonant loops, the switching device being configured to control such so that at least one switching unit switches one or more electromagnetically coupled resonant loops to provide the first and second resonant capability with one or more electromagnetically coupled resonant loops. Thus, depending on the transmit mode (TX mode) or receive mode (RX mode) of the RF coil, at least one switching unit can be used to select and activate the resonant loops in the desired manner. The resonance loops for the transmit mode (TX mode) and the receive mode (RX mode) may share at least a portion of their loops. Alternatively, the resonant loops for the transmit mode (TX mode) and the receive mode (RX mode) may be separate resonant loops.

According to a modified embodiment, the passive RF shim resonator comprises a plurality of electromagnetically coupled resonant loops, the resonant loops being arranged in a two-dimensional (2D) or three-dimensional (3D) grid structure, forming an artificial dielectric, the switching device comprising a plurality of switching blocks arranged in several electromagnetically conjugated resonant loops, wherein the switching units are configured to switch one or more electromagnetically conjugated resonant loops to provide the first and second resonant capability using one or more electromagnetically conjugated resonant loops. Based on the grid structure of the resonant loops, a homogeneous passive RF shimming resonator can be provided. This facilitates the fabrication of a passive RF shim resonator. In addition, the grid structure makes it easier to optimize the RF field of an MRI system because the RF field can be easily controlled along the entire coordinate axis. Switching blocks can be provided in the branches of the grid structure, depending on the desired shape of the resonant loops.

In accordance with a modified embodiment, the resonant loops are located in the branches connecting the connecting points of the grid structure. Accordingly, resonant loops can be formed and activated by connecting branches of the grid structure. Thus, depending on the transmit mode (TX mode) or receive mode (RX mode), different sets of branches can be connected to form resonant loops. The resonance loops for the transmit mode (TX mode) and the receive mode (RX mode) may be independent of each other. However, it is also possible that the transmit mode (TX mode) and receive mode (RX mode) resonant loops at least partially share branches of the grid structure. It is also possible that some of the branches are not used at all. The branches may have detuning components or outlines so that corresponding branches with appropriate detuning abilities can be activated in the desired manner.

In accordance with a modified embodiment, the passive RF shim resonator comprises one or more electromagnetically coupled transmission lines, wherein the switching device comprises at least one switching unit located in one or more electromagnetically coupled transmission lines and is configured to be controlled so that at least one switching unit switched one or more electromagnetically coupled transmission lines to provide the first and second resonant ability with one or more electromagnetically coupled transmission lines. Transmission lines may contain various kinds of switching units made of electronic components such as pin diodes, Schottky diodes or transistors. For example, pin diodes can switch a transmission line at a significant induced voltage. The switching operation may be performed to adapt the behavior of the transmission line to the transmit phase and to the receive phase. The two switching units can be placed in an anti-parallel configuration to provide full control in both directions. In addition, switching units may be provided within a branch in a sub-branch parallel to another sub-branch. Accordingly, a branch may be disabled, for example, during the transmit phase and may be active during the receive phase.

According to a modified embodiment, the passive RF shim resonator comprises one or more coupled dielectric resonators, wherein the switching device comprises at least one switching unit located in one or more coupled dielectric resonators, the switching device being operable to be controlled such that at least one switching unit switched one or more conjugated dielectric resonators to provide the first and second resonant power with one or more conjugated dielectric resonators. Thus, in order to provide the first and second resonant capabilities in accordance with the transmission mode (TX mode) and the reception mode (RX mode) of the RF field emitted by the RF antenna device of the magnetic resonance imaging system, one dielectric resonator or a plurality of conjugated dielectric resonators.

According to a modified embodiment, the switching device comprises at least one switchable resonant circuit. The resonant circuit can be used to adapt the resonant frequency depending on the transmit mode (TX mode) or the receive mode (RX mode). The resonant circuit may include any switching unit suitable for switching the resonant circuit between different resonant frequencies, thereby homogenizing and/or optimizing the RF field. Due to the presence of resonant abilities, i.e. resonant frequency of the resonant circuit, the behavior of the passive RF shim resonator can be changed by switching between the first and second resonant capability according to the transmit mode (TX mode) and receive mode (RX mode) of the RF field emitted by the RF antenna device of the magnetic resonance imaging system . Switching of the passive RF shim resonator can be accomplished with switching units including electrical components such as pin diodes, Schottky diodes, or transistors.

According to a modified embodiment, the switching device is an actively controlled switching device, wherein the passive RF shim resonator comprises a control unit, the control unit being configured to actively switch the switching device between a first and a second resonant capability in accordance with a transmission mode (TX mode) and receiving mode (RX mode) of the RF field emitted by the radio frequency antenna device of the magnetic resonance imaging system. Accordingly, the switching device can be controlled to switch the passive RF shim cavity in accordance with a transmit mode (TX mode) and a receive mode (RX mode) of the RF field. The active switching of the switching device provides a high degree of freedom in the control of the switching units. It is desirable that the control unit be synchronized with the control system of the transmit mode (TX mode) and the receive mode (RX mode) of the magnetic resonance imaging system. The switching units can be, for example, pin diodes, Schottky diodes or transistors actively controlled by the control unit. A transistor may be actively controlled via its control terminal, while diodes may be actively controlled, for example, by applying a switching voltage between their terminals.

According to a modified embodiment, the switching device is a passive switching device, wherein the switching device is configured to passively switch between the first and second resonant capability in accordance with a transmit mode (TX mode) and a receive mode (RX mode) of the RF field emitted by the RF antenna device of the magnetic resonance imaging system. Thus, the transmission field B1 can be used to switch the switching units. This can be done in a similar manner as is known in the art, for example to switch local pick-up coils. Thus, the transmission field B1 can be used, for example, to switch part of the resonator. Switching of the passive RF shim cavity can be done with electrical components such as pin diodes, Schottky diodes, or transistors.

According to a modified embodiment, the first resonant capability is to achieve RF coil transmit field optimization and the second resonant capability is to achieve RF coil receive field homogenization, or vice versa.

Brief description of the drawings

These and other aspects of the invention will become apparent and will be explained with reference to the embodiments described below. However, such embodiments do not necessarily represent the full scope of the invention, and therefore reference is made here to the claims to interpret the scope of the invention.

In FIG. 1 is a schematic illustration of a portion of a first, preferred embodiment of a magnetic resonance imaging (MRI) system.

In FIG. 2 is a schematic side view of a patient bed with a passive RF shim resonator and a test object supported thereon, according to the second embodiment.

In FIG. 3 is a schematic plan view of the patient bed shown in FIG. 2 with its passive RF shim resonator.

In FIG. 4 is a schematic plan view of a passive dipole antenna for use in a passive RF shim resonator according to a third embodiment.

In FIG. 5 is a schematic plan view of a combination of a passive dipole antenna with a resonant loop for use in a passive RF shim resonator according to the fourth embodiment.

In FIG. 6 is a schematic top view of an arrangement of several resonant loops forming a passive RF shim resonator in a two-dimensional structure, according to a fifth embodiment.

In FIG. 7 is a detailed plan view of one resonant loop with control terminals, according to the sixth embodiment.

In FIG. 8 is a schematic plan view of a passive RF shim resonator in a 2D grid structure, according to the seventh embodiment.

In FIG. 9 is a schematic perspective view of a passive RF shim resonator in a 3D grid structure according to an eighth embodiment.

In FIG. 10 is a schematic side view of a passive RF shim resonator with three resonant loops stacked one above the other in a ninth embodiment.

In FIG. 11a is a schematic side view of a passive RF shim resonator with a first internal structure according to the tenth embodiment.

In FIG. 11b is a schematic side view of a passive RF shim resonator with a second internal structure, according to an eleventh embodiment.

In FIG. 12 is a schematic view of an object of interest placed on a patient bed with a passive RF shim resonator, in accordance with any of the previous embodiments, with dark spots resulting from the generation of the MR image.

In FIG. 13 is a schematic view according to FIG. 12 indicating the formation of various dark spots formed when an MR image is generated for the transmit mode (TX mode) and for the receive mode (RX mode) of the magnetic resonance imaging system.

Implementation of the invention

In FIG. 1 shows a schematic illustration of a portion of a magnetic resonance imaging (MRI) system 110 comprising an MR scanner 112 in accordance with the first, preferred embodiment. The magnetic resonance imaging system 110 of the first embodiment is described herein in general terms as the basis for all discussed embodiments.

The magnetic resonance imaging system 110 includes a main magnet 114 for generating a static magnetic field. The main magnet 114 has a central opening which provides an examination space 116 around a central axis 118 for an object 120 to be examined, typically a patient, to be placed inside. The investigated object 120 usually lies on the bed 142 of the patient. Accordingly, in this embodiment, the central hole and hence the static magnetic field of the main magnet 114 are horizontally oriented in accordance with the central axis 118 and the patient bed 142. In an alternative embodiment, the orientation of the main magnet 114 may be different, for example, to provide a static magnetic field with a vertical orientation. In addition, the magnetic resonance imaging system 110 includes a magnetic gradient coil system 122 for generating gradient magnetic fields superimposed on a static magnetic field. The system 122 of magnetic gradient coils is located concentrically within the opening of the main magnet 114, as is known in the prior art.

In addition, the magnetic resonance imaging system 110 includes a radio frequency (RF) antenna device 140 configured as a whole body coil with a tubular body. The RF antenna device 140 is also sometimes referred to as the RF coil device. In an alternative embodiment, RF antenna device 140 is in the form of a head coil or any other suitable type of coil for use in magnetic resonance imaging systems 110. The RF antenna device 140 is designed to apply an RF magnetic field to the area of interest 116 during the phases of the RF transmission, i. in the TX mode of the magnetic resonance imaging system 110, to excite the nuclei of the test object 120 to be covered by the MR images. The RF antenna device 140 is also designed to receive magnetic resonance signals from excited nuclei during the RF reception phases, i. in the RX mode of the magnetic resonance imaging system 110 . In the operational state of the MRI system 110, the RF transmission phases and the RF reception phases alternate. RF antenna device 140 is positioned concentrically within the opening of main magnet 114. Cylindrical RF shield device 124 is positioned concentrically between magnetic gradient coil array 122 and RF antenna device 140. Thus, RF shield device 124 coaxially surrounds RF antenna device 140. In some RF embodiments, the antenna device 140 and the RF shielding device 124 are integrally formed.

In addition, the magnetic resonance imaging system 110 includes a block 130 for reconstructing MR images from received MR signals, and a block 126 for controlling the magnetic resonance imaging system with a monitor block 128 designed to control the functions of the MR scanner 112, as is known in the art. Control lines 138 are installed between the magnetic resonance imaging system control unit 126 and the RF transmitter unit 134, which provides RF energy with magnetic resonance radio frequency to the RF antenna device 140 via the RF switching unit 136 during RF transmission phases. In turn, the RF switching unit 136 is also controlled by the MRI control unit 126, and for this purpose, another control line 132 is installed between the MRI control unit 126 and the RF switching unit 136. During the RF reception phase, the RF the switching unit 136 directs the MR signals from the RF antenna device 140 to the MR image reconstruction unit 130 after pre-amplification.

In FIG. 2 and 3 show a patient bed 142 according to the second embodiment. The patient bed 142 may be used in conjunction with the magnetic resonance imaging system 110 of the first embodiment.

The patient bed 142 of the second embodiment includes a passive radio frequency (RF) shim resonator 144 for homogenizing the RF field radiated by the RF antenna device 140. The test object 120 may be placed on the patient bed 142 as described above.

Passive RF shim resonator 144 contains several electromagnetically coupled resonant loops 146 and a control unit 148. The resonant loops 146 are provided in a linear arrangement with three groups 150 of resonant loops 146, with each group 150 containing two resonant loops 146 that are arranged concentrically.

Each resonant loop 146 contains several coupling capacitors 152 and a pin diode 154 as a switching unit. Each pin diode 154 is located between the control ports 156. The control ports 156 are connected to a control unit 148 which actively controls the voltage on the pin diodes 154 through the control ports 156 to make the pin diodes 154 conduct or not. Thus, the control unit 148 controls all of the pin diodes 154, with the pin diodes 154 generally considered to be switching devices. The connection of control ports 156 to control unit 148 is not shown in FIG. 2 and 3. Thus, with the pin diode 154 conducting, the corresponding resonant loop 146 becomes conductive, i. a circulating current can flow through the corresponding resonant loop 146. If the pin diode 154 is not conducting, then the corresponding resonant loop 146 is interrupted, thereby interrupting the flow of circulating current through the corresponding resonant loop 146.

The control unit 148 controls the pin diodes 154 to switch the electromagnetically coupled resonant loops 146 to provide the first and second resonant capability in accordance with the transmit mode (TX mode) and the receive mode (RX mode) of the RF field emitted by the RF antenna device 140 of the system 110 magnetic resonance imaging.

The first and second resonant capabilities are provided in accordance with the transmission mode (TX mode) and the reception mode (RX mode) of the RF field emitted by the RF antenna device 140 of the magnetic resonance imaging system 110. Therefore, the passive RF shim cavity 144 can be switched between transmit mode and receive mode, with the first and second resonant capabilities optimized for the corresponding rotating RF field. For the transmission mode, the B1+ field homogeneity and the Specific Absorption Rate (SAR) are optimized to reduce the spatial variation of the RF field emitted by the RF antenna device 140, i. the transmitting coil of the 110 MPT system. In addition, in the receive mode, you can increase the uniformity of the B1- field and the signal-to-noise ratio (SNR, from the English Signal-to-Noise Ratio).

In FIG. 4 shows a passive dipole antenna 158 for use in a passive RF shim resonator 144 according to a third embodiment. The passive dipole antenna 158 and the passive RF shim cavity 144 according to the third embodiment can be used together with the MRI system 110 according to the first embodiment. The general principles of the passive RF shim resonator 144 of the second embodiment, as discussed above, apply to both the passive dipole antenna 158 and the passive RF shim resonator 144 of the third embodiment.

As can be seen from FIG. 4, the passive dipole antenna 158 includes two separate transmission lines 164 that are electromagnetically coupled. Each of the transmission lines 164 is provided with a pin diode 154. In accordance with the second embodiment, each pin diode 154 is located between the control ports 156, which are connected to the control unit 148, which actively controls the voltage on the pin diodes 154 through the control ports 156, to make pin diodes 154 conductive or not. The control unit 148 is not shown in FIG. 4. Thus, with a conductive pin diode 154, the corresponding transmission line 164 is put into an active state, i.e. the dipole antenna 158 is activated. If the pin diode 154 is not conducting, then the corresponding transmission line 164 is interrupted.

The passive RF shim resonator 144 according to the third embodiment includes a plurality of passive dipole antennas 158, which are typically located in the plane of the patient's bed 142.

In FIG. 5 shows an antenna structure 160 for use in a passive RF shim resonator 144 according to the fourth embodiment. The antenna structure 160 for use in the passive RF shim resonator 144 according to the fourth embodiment can be used in conjunction with the magnetic resonance imaging system 110 of the first embodiment. The general principles of the passive RF shim cavity 144 of the second and/or third embodiment, as discussed above, also apply to the antenna structure 160 and to the passive RF shim cavity 144 of the fourth embodiment.

As can be seen from FIG. 5, the antenna structure 160 of the fourth embodiment includes a passive dipole antenna 158 according to the third embodiment and a resonant loop 146 according to the second embodiment. The resonant loop 146 is surrounded by transmission lines 164 of the passive dipole antenna 158. The details of the passive dipole antenna 158 of the third embodiment and the resonant loop 146 of the second embodiment apply as discussed above.

The control ports 156 of the passive dipole antenna 158 and resonant loop 146 are connected to a control unit 148 which actively controls the voltage across all pin diodes 154 through the control ports 156 to make each of the pin diodes 154 conduct or not. Control unit 148 is not shown in FIG. 5. The respective resonant loop 146 and the passive dipole antenna 158 are controlled to provide the first and second resonant capabilities in accordance with the transmit mode (TX mode) and the receive mode (RX mode) of the RF field radiated by the RF antenna device 140 of the magnetic resonance system 110 tomography.

In FIG. 6 shows a passive RF shim cavity 144 according to the fifth embodiment. The passive RF shim cavity 144 according to the fifth embodiment can be used together with the magnetic resonance imaging system 110 of the first embodiment. The general principles of the passive RF shim resonator 144 of the second to fourth embodiments, as already discussed above, also apply to the antenna structure 160 and to the passive RF shim resonator 144 of the fifth embodiment.

As can be seen from FIG. 6, the passive RF shim resonator 144 of the fifth embodiment comprises a plurality of resonant loops 146 arranged in a plane with two resonant loops 146 placed side by side in the longitudinal direction and three resonant loops 146 arranged one above the other in the vertical direction. This arrangement of resonant loops 146 is shown by way of example only.

In an alternative embodiment, at least some of the resonant loops 146 are replaced with antenna structures 160 of the fourth embodiment and/or passive dipole antennas 158 of the third embodiment.

In FIG. 7 shows a resonant loop 146 and a passive RF shim resonator 144 according to the sixth embodiment. The passive RF shim resonator 144 according to the sixth embodiment can be used together with the magnetic resonance imaging system 110 of the first embodiment. The general principles of the passive RF shim cavity 144 of the previous embodiments, as already discussed above, also apply to the passive RF shim cavity 144 of the sixth embodiment.

The passive RF shim cavity 144 of the sixth embodiment is generally configured in accordance with the passive RF shim cavity 144 of the second embodiment. Passive RF shim resonator 144 may include multiple electromagnetically coupled resonant loops 146, as described below.

The resonant loop 146 contains two coupling capacitors 152. A tuning capacitor 162 is connected in parallel with each coupling capacitor 152. As a switching unit, a pin diode 154 is provided connected in series with each tuning capacitor 162. Each pin diode 154 is located between the control ports 156. The control ports 156 are connected to a control unit 148 which actively controls the voltage on the pin diodes 154 through the control ports 156 to make the pin diodes 154 conduct or not. The control unit 148 is not shown in FIG. 7.

Thus, with pin diode 154 conducting, the corresponding resonant loop 146 is configured differently, since the coupling capacitance is the sum of the capacitance of the corresponding coupling capacitor 152 and the capacitance of the corresponding tuning capacitor 162. For pin diodes 154 in the non-conducting state, the coupling capacitance includes only the capacitance capacitor 152 connection.

The control unit 148 controls the pin diodes 154 to switch the resonant loop 146 to provide the first and second resonant capability in accordance with the transmit mode (TX mode) and the receive mode (RX mode) of the radio frequency field radiated by the RF antenna device 140 of the system 110 magnetically resonant tomography.

In FIG. 8 shows a passive RF shim resonator 144 in a 2D grid structure according to the seventh embodiment, forming an artificial dielectric 166. The passive RF shim resonator 144 according to the seventh embodiment can be used together with the magnetic resonance imaging system 110 of the first embodiment. The general principles of the passive RF shim cavity 144 of the previous embodiments, as discussed above, also apply to the antenna structure 160 and to the passive RF shim cavity 144 of the seventh embodiment.

Passive RF shim resonator 144 of the seventh embodiment is a two-dimensional grid structure with multiple branches 170 that are connected at connection points 172 at each intersection of branches 170. Each branch 170 may comprise a resonant circuit 174. Accordingly, each branch 170 may be loaded with branch capacitors 176 or branch inductors 178 to carry out tuning to perform independent homogenization of the B+ field and the B- field in accordance with the transmit mode (TX mode) and receive mode (RX mode) of the RF field radiated by the RF antenna device 140 of the magnetic resonance system 110 tomography.

Accordingly, as seen from FIG. 8, each of the branches 170 is provided with a pair of switching units 154 in an anti-parallel configuration. As shown in FIG. 8, branches 170 with various configurations can be used to provide the first and second resonant capabilities in accordance with the transmission mode (TX mode) and the reception mode (RX mode) of the RF field emitted by the RF antenna device 140 of the magnetic resonance imaging system 110. Branches 170 may be connected to form, for example, resonant loops 146 in passive RF shim resonator 144.

In FIG. 9 shows a passive RF shim resonator 144 in a three-dimensional grid pattern providing an artificial dielectric 166 according to an eighth embodiment. Moreover, the principles of the two-dimensional passive RF shim resonator 144 of the seventh embodiment are applicable.

In FIG. 10 shows a passive RF shim resonator 144 with three resonant loops 146 stacked one above the other in a ninth embodiment. The three resonant loops 146 may be identical to any of the resonant loops of the previous embodiments. Alternatively, the three resonant loops 146 may be formed by appropriately switching the passive RF shim cavity 144 with a three-dimensional grid structure according to the eighth embodiment.

In FIG. 11a and 11b show passive RF shim resonators 144 with first and second internal structures according to the tenth and eleventh embodiments. As shown in FIG. 11a and 11b, passive RF shim resonators 144 of the tenth and eleventh embodiments are made with various different patterns of 2D or 3D grid structures adapted in wavelength to MRI frequency and body size. In addition, the integration of branch inductors 176 or branch capacitors 174 in combination with local E-shields or ceramic dielectric solid or liquid structures can be envisaged.

In FIG. 12 and 13 show a passive RF shim resonator 144 with a patient bed 142 and a passive RF shim resonator 144, object of interest 120, i. the person in this example is positioned on patient bed 142 for examination. As shown in FIG. 12 and 13, B-fields in different directions of rotation can be applied, resulting in black spots 180. These black spots 180 are located differently for the transmit mode (TX mode) and the receive mode (RX mode) of the RF antenna device 140 .

While the invention has been illustrated and described in detail in the drawings and in the foregoing description, such illustration and description is to be considered by way of example and not as limiting; the invention is not limited to the disclosed embodiments. Those skilled in the art may recognize and practice other variations of the disclosed embodiments of the invention based on examination of the drawings, description, and appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the use of the singular does not exclude plurality. The fact that certain measures are listed in mutually distinct dependent claims does not mean that a combination of these measures cannot be used to advantage. Any reference designations in the claims should not be construed as limiting the scope of the invention.

List of reference designations

110 - magnetic resonance imaging system (MRI system)

112 - magnetic resonance (MR) scanner

114 - main magnet

116 - RF study space

118 - central axis

120 - investigated object

122 - magnetic gradient coil system

124 - RF shielding device

126 - MRI system control unit

128 - monitor unit

130 - MR image reconstruction unit

132 - control line

134 - RF transmitter unit

136 - RF switching unit

138 - control line

140 - radio frequency (RF) antenna device

142 - patient bed

144 - passive RF shim resonator

146 - resonant loop

148 - control unit

150 - group of resonant loops

152 - coupling capacitor

154 - pin diode, switching block

156 - control port

158 - dipole antenna

160 - antenna structure

162 - tuning capacitor

164 - transmission line

166 - artificial dielectric

170 - branch

172 - connection point

174 - resonant circuit

176 - branch capacitor

178 - branch inductor

180 - black spot

Claims (33)

1. Passive radio frequency (RF) shim resonator (144) for homogenizing the radio frequency field emitted by the radio frequency antenna device (140) of the magnetic resonance imaging (MRI) system (110), and the passive RF shim resonator (144) has a first resonant capability and a second resonant capability, wherein the passive RF shim resonator (144) comprises a switching device configured to switch between the first and second resonant capability, characterized in that said switching is provided in accordance with the transmit mode (TX mode) and the receive mode (RX mode) to adjust the passive RF shim resonator to independently homogenize the components (B1±) of the circularly polarized magnetic field rotating clockwise and counterclockwise, respectively, of the RF of the field emitted by the radio frequency antenna device (140) of the magnetic resonance imaging (MRI) system (110), moreover, the passive radio frequency (RF) shim resonator (144) contains several electromagnetically coupled resonant loops (146), and the resonant loops (146) are located in two-dimensional (2D) or three-dimensional (3D) grid structure to form an artificial dielectric (166), moreover, the switching device is a passive switching device, and the switching device is configured to passively switch between the first and second resonant capacity in accordance with the transmission mode (TX mode) and the reception mode (RX mode) of the radio frequency field emitted by the radio frequency antenna device (140) systems (110) of magnetic resonance imaging. 2. Passive radio frequency (RF) shim resonator (144) according to claim 1, which contains one or more electromagnetically coupled resonant loops (146), and the switching device comprises at least one switching unit (154) located in one or more electromagnetically coupled resonant loops (146), and the switching device is configured to be controlled so that at least one switching unit (154) switches one or more electromagnetically coupled resonant loops (146) to provide the first and second resonant capability with one or more electromagnetically coupled resonant loops (146). 3. Passive radio frequency (RF) shim resonator (144) according to claim 2, in which the switching device contains several switching blocks (154) located in several electromagnetically coupled resonant loops (146), while the switching blocks (154) are configured to switching one or more electromagnetically coupled resonant loops (146) to provide the first and second resonant capacity with one or more electromagnetically coupled resonant loops (146). 4. Passive radio frequency (RF) shim resonator (144) according to claim 3, in which the resonant loops (146) are located in the branches (170) connecting the connection points (172) of the grid structure. 5. Passive radio frequency (RF) shim resonator (144) according to any one of paragraphs. 1-4, which contains one or more electromagnetically coupled transmission lines (164), and the switching device comprises at least one switching unit (154) located in one or more electromagnetically coupled transmission lines (164), wherein the switching device is configured to be controlled so that at least one switching unit (154) switches one or a plurality of electromagnetically coupled transmission lines (164) to provide the first and second resonant capability by means of one or more electromagnetically coupled transmission lines (164). 6. Passive radio frequency (RF) shim resonator (144) according to any one of paragraphs. 1-5, which contains one or more conjugated dielectric resonators, and the switching device comprises at least one switching unit (154) located in one or more coupled dielectric resonators, while the switching device is configured to be controlled so that at least one switching unit (154) switches one or more coupled dielectric resonators to provide the first and second resonant capacity with one or more coupled dielectric resonators. 7. Passive radio frequency (RF) shim resonator (144) according to any one of paragraphs. 2-6, in which the switching device comprises at least one switchable resonant circuit (174). 8. Passive radio frequency (RF) shim resonator (144) according to any preceding claim, wherein the switching device is an actively controlled switching device, wherein passive RF shim resonator (144) contains a control unit (148), while the control unit (148) is configured to actively switch the switching device between the first and second resonant capacity in accordance with the transmission mode (TX mode) and the reception mode (RX mode) of the radio frequency field emitted by the radio frequency antenna device (140) of the system (110) magnetic resonance imaging. 9. Passive radio frequency (RF) shim resonator (144) according to any preceding claim, wherein the first resonant capability is to achieve optimization of the transmitting field of the RF antenna device (140), and the second resonant capability is to achieve optimization of the receiving field of the RF antenna device (140), or vice versa. 10. Bed (142) of the patient for use in the system (110) magnetic resonance imaging, and the patient bed (142) or patient mattress comprises a passive radio frequency (RF) shim resonator (144) for homogenizing the radio frequency field emitted by the radio frequency antenna device (140) of the magnetic resonance imaging system (110), according to any one of paragraphs. 1-9. 11. Mattress of the patient for use in the system (110) magnetic resonance imaging, and the patient bed (142) or patient mattress comprises a passive radio frequency (RF) shim resonator (144) for homogenizing the radio frequency field emitted by the radio frequency antenna device (140) of the magnetic resonance imaging system (110), according to any one of paragraphs. 1-9. 12. A radio frequency (RF) antenna device (140) for generating and/or receiving a radio frequency field for use in a magnetic resonance imaging (MRI) system (110), comprising a coil housing and a passive RF shim resonator (144) for homogenizing the RF field radiated by the RF antenna device (140), according to any one of paragraphs. 1-9, with a passive RF shim resonator (144) located inside the coil housing. 13. A magnetic resonance imaging (MRI) system (110) comprising a main magnet (114) for generating a static magnetic field, a magnetic gradient coil system (122) for generating gradient magnetic fields superimposed on the static magnetic field, and at least one radio frequency (RF) antenna device (140) for generating a radio frequency field, and the magnetic resonance imaging system (110) further comprises a patient bed (142) or a patient mattress according to claim 10 or 11. 14. A magnetic resonance imaging (MRI) system (110) comprising a main magnet (114) for generating a static magnetic field, a magnetic gradient coil system (122) for generating gradient magnetic fields superimposed on the static magnetic field, and at least one radio frequency (RF) antenna device (140) for generating a radio frequency field, and at least one radio frequency antenna device (140) is a radio frequency antenna device (140) according to claim 12. 15. A method for homogenizing the radio frequency field emitted by the radio frequency antenna device (140) according to claim 12 of the magnetic resonance imaging (MRI) system (110), comprising the following steps: providing a passive radio frequency (RF) shim resonator (144) having a first resonant ability and the second resonant capacity in accordance with the transmission mode (TX mode) and the reception mode (RX mode) of the radio frequency antenna device (140), determine the transmission mode (TX mode) and the reception mode (RX mode) of the radio frequency antenna device (140 ) system (110) magnetic resonance imaging, and switching the passive RF shim resonator (144) between the first and second resonant capacity in accordance with the transmit mode (TX mode) and the receive mode (RX mode) to adjust the passive RF shim resonator (144) to independently homogenize the components (B1±) circularly a polarized magnetic field rotating clockwise and counterclockwise, respectively, a radio frequency field emitted by the radio frequency antenna device (140) of the magnetic resonance imaging (MRI) system (110).

Images