Arrangement to control antenna elements

1. Field of the Invention

The present invention concerns an arrangement to control an antenna arrangement as well as a device for signal splitting and a transmission method, for a magnetic resonance examination to be implemented.

2. Description of the Prior Art

Presently techniques known as “transmit-sense” techniques are under discussion for use in magnetic resonance examinations of patients in order to excite nuclear spins in a narrowly limited range in a targeted manner.

This technique enables the effective integral of the radio-frequency magnetic field for examination to be directed only in a narrowly limited region of interest for the excitation of the nuclear spins, while also enabling the integral of the radio-frequency magnetic field (which integral is effective for the excitation of the nuclear spins) to be homogeneously adjusted in the examination region.

Multiple transmitters are used in this technique, each of which is connected with an antenna element of a transmission antenna arrangement. For example, the transmission antenna arrangement is fashioned in the form of a birdcage antenna. A radio-frequency signal is emitted via each transmission element in a transmission time period. In their time curve, the radio-frequency signals differ entirely from one another in terms of their voltages, frequencies and phases.

Today circularly-polarized transmission signals are preferably used for examination.

FIG. 3 shows an exemplary arrangement of a circularly-polarized transmission signal according to the prior art.

A radio-frequency transmission signal SS arrives at a radio-frequency splitter SPLIT as an amplified transmission signal VSS via a transmission power amplifier PA. The radio-frequency splitter SPLIT has two inputs In 1 , IN 2 as well as two outputs Out 1 , Out 2 , wherein the amplified transmission signal VSS is connected to a first input In 1 .

The radio-frequency splitter SPLIT divides the transmission signal VSS in two, into essentially equal transmission signals SS 1 and SS 2 , with the two transmission signals SS 1 and SS 2 being phase-shifted relative to one another by 90°.

A first transmission signal SS 1 arrives at a first connection A 1 of an antenna arrangement ANT while a second transmission signal SS 2 arrives at a second connection A 1 of the antenna arrangement ANT. The two connections A 1 , A 2 form respective feed points of the antenna arrangement ANT.

The antenna arrangement ANT is fashioned to radiate a circularly-polarized transmission signal when two transmission signals (ideally phase-shifted by 90°) are fed to it via the feed points.

In this example the antenna arrangement ANT is fashioned as what is known as a birdcage resonator. The two connections A 1 and A 2 are arranged offset on a ferrule of the antenna ANT such that a circularly-polarized transmission signal is radiated by the transmission signals phase-shifted by 90°.

FIG. 4 shows an exemplary embodiment according to the prior art, based on FIG. 3 of the radio-frequency splitter SPLIT.

The radio-frequency splitter SPLIT has a first input In 1 , a second input In 2 , a first output Out 1 and a second output Out 2 . The amplified transmission signal VSS is connected at the first input In 1 .

The first input In 1 is connected via a first inductor L 1 with the first output Out 1 while the second input In 2 is connected via a second inductor L 2 with the second output Out 2 . It is also possible to magnetically couple the inductors L 1 , L 2 .

The first input In 1 is connected via a first capacitor C 1 with the second input In 2 while the first output Out 1 is connected via a second capacitor C 2 with the second output Out 2 .

The second input In 2 is connected via a resistor Z with a reference potential (here the ground) such that constant power components reflected in the outputs Out 1 , Out 2 are compensated in the resistor Z.

The first output Out 1 is connected with the first connection A 1 of the antenna arrangement ANT while the second output Out 2 is connected with a second connection A 2 of the antenna arrangement ANT.

The values of the inductors L 1 , L 2 , the coil coupling and the values of the capacitors C 1 , C 2 are selected such that the desired phase difference of 90° between the two transmission signals SS 1 and SS 2 results at the outputs Out 1 , Out 2 of the frequency splitter SPLIT.

Here the first transmission signal SS 1 that is fed via the first output Out 1 to the first connection A 1 as a first feed point at the ferrule of the antenna ANT has a phase position of 90°.

The circularly-polarized transmission signal is formed by the phase-shifted control at the aforementioned connections A 1 , A 2 .

In magnetic resonance apparatuses that are operated with magnetic field strengths of greater than or equal to three Tesla and that use circularly-polarized transmission signals, currents are induced in the tissue of the subject due to different relative permittivities (dielectric constants) and conductivities of the human tissue.

Depending on the type of the tissue and after distribution of the currents, these currents cause an amplification or reduction of the exciting radio-frequency magnetic field. These effects disadvantageously affect the examination.

An object of the present invention to provide an improvement in examination results with low effort in magnetic resonance apparatuses of the aforementioned type.

The above object is achieved in accordance with the present invention by an arrangement to control an antenna arrangement in a magnetic resonance examination that has a device for signal splitting, which has an input and first and second outputs. A radio-frequency transmission channel is supplied at the input of the device for signal splitting, which contains phase-shaping components with which the transmission signal supplied at the input is divided into a first transmission signal at the first output and a second transmission signal at the second output. The first and second transmission signals have a phase difference therebetween that is adjusted by the phase-shaping components, by virtue of these components being switchable so as to be respectively activated or deactivated in order to modify the phase difference. The antenna arrangement has first and second connections for the first and second transmission signals, respectively, and is configured to radiate a circularly-polarized field as soon as the two transmission signals at the respective connections exhibit a phase difference of 90°.

Not only is a circularly-polarized transmission signal used for examination in the present invention; rather, examination results are acquired via time-shifted usage of further linearly-polarized or anti-circularly-polarized transmission signals.

An improvement in the examination results is achieved via superimposition of the individual examination results or via the superimposition of the partial images acquired in the respective examination.

A device for signal splitting is used in the inventive arrangement to control antenna elements of a transmission antenna arrangement in a magnetic resonance apparatus. This device for signal splitting has an input and at least one first and one second output, wherein a radio-frequency transmission signal is connected at the input.

The first output of the device for signal splitting is connected with a first connection of an antenna arrangement and the second output of the device for signal splitting is connected with a second output of the antenna arrangement. The antenna arrangement is fashioned for radiation of a circularly-polarized transmission signal as soon as transmission signals that possess a phase difference of 90° relative to one another are present at the two connections of the antenna arrangement.

The device for signal splitting has phase-shaping components with which the transmission signal supplied via the input is divided into a first transmission signal and a second transmission signal. A first transmission signal formed in this manner arrives at the first connection of the antenna arrangement via the first output of the device for signal splitting while a second transmission signal so formed arrives at the second connection of the antenna arrangement via the second output of the device for signal splitting.

A difference between the phase position of the first transmission signal and that of the second transmission signal can be achieved via the phase-shaping components of the device for signal splitting, such that a circularly-polarized sum emission field is formed by radiation from the antenna arrangement.

In addition to the phase-shaping components, the device for signal splitting has additional switchable components. These are arranged such that the phase-shaping components can be activated and deactivated. The phase position of the two signals that are present at the outputs of the device for signal splitting is altered by the activation or deactivation of the phase-shaping components, such that a linearly-polarized transmission signal or an anti-circularly-polarized transmission signal is formed by radiating via the antenna arrangement.

FIG. 1 illustrates an embodiment of an arrangement according to the invention to form a circularly-polarized transmission signal.

FIG. 2 illustrates an embodiment of the inventive device for signal splitting, based on FIG. 1 .

FIG. 3 shows the exemplary arrangement described above to form a circularly-polarized transmission signal according to the prior art.

FIG. 4 shows the exemplary embodiment of the radio-frequency splitter SPLIT according to the prior art that is described above and based on FIG. 3 .

FIG. 1 shows an embodiment of an inventive arrangement to form a circularly-polarized transmission signal.

A radio-frequency transmission signal SS arrives as an amplified transmission signal at a radio-frequency splitter SPLIT 1 via a transmission power amplifier PA.

The radio-frequency splitter SPLIT 1 has two inputs In 11 , IN 21 as well as two outputs Out 11 , Out 21 , wherein the amplified transmission signal VSS is connected at a first input In 11 .

The radio-frequency splitter SPLIT 11 divides the transmission signal VSS into two essentially identical transmission signals SS 11 and SS 21 , wherein the two transmission signals SS 11 and SS 21 are preferably phase-shifted by 90° relative to one another.

As an alternative to this, the radio-frequency splitter SPLIT 1 alternatively divides the transmission signal VSS into two essentially identical transmission signals SS 11 and SS 21 , with the two transmission signals SS 11 and SS 21 exhibiting further phase differences or no phase difference relative to one another.

A first transmission signal SS 11 arrives at a first connection A 1 of an antenna arrangement ANT while a second transmission signal SS 21 arrives at a second connection A 1 of the antenna arrangement ANT. The two connections A 1 , A 2 form respective feed points of the antenna arrangement ANT.

The antenna arrangement ANT is fashioned to radiate a circularly-polarized field when two transmission signals (ideally phase-shifted by 90°) are fed to it via the feed points.

In this example the antenna arrangement ANT is thus fashioned as a birdcage resonator. The two connections A 1 and A 2 are arranged on a ferrule of the antenna ANT such that a circularly-polarized transmission signal is formed upon irradiation via the transmission signals SS 11 and SS 21 phase-shifted by 90°.

If the two transmission signals SS 11 and SS 21 now exhibit a phase shift deviating by 90° from one another, a linearly-polarized or an anti-circularly-polarized field is formed upon the irradiation.

FIG. 2 shows an embodiment of the inventive device for signal splitting SPLIT 1 according to the invention, based on FIG. 1 .

The radio-frequency splitter SPLIT 1 has a first input In 11 , a second input In 21 , a first output Out 11 and a second output Out 21 . The amplified transmission signal VSS 1 is connected at the first input In 11 .

The first input In 11 is connected via a first inductor L 11 with the first output Out 11 while the second input In 21 is connected via a second inductor L 21 with the second output Out 21 . It is also possible to magnetically couple the inductors L 11 , L 21 .

The first input In 11 is connected via a first capacitor C 12 with the second input In 21 while the first output Out 11 is connected via a second capacitor C 21 with the second output Out 21 .

The second input In 21 is connected via a switch SW 5 and via a first resistor Z with a reference potential (here ground), such that constant power components reflected at the outputs Out 11 , Out 21 are compensated in the resistor Z.

The first output Out 11 is connected with the first connection A 1 (not shown here) of the antenna arrangement ANT while the second output Out 2 l is connected with the second connection A 2 (not shown here) of the antenna arrangement ANT.

The inductors L 11 and L 21 as well as the capacitors C 12 and C 21 serve to divide the supplied transmission signal VSS that is present at the input In 11 of the device for signal splitting SPLIT 1 into a first transmission signal SS 11 and a second transmission signal SS 21 .

The first transmission signal SS 11 arrives via the first output Out 11 of the device for signal splitting SPLIT 1 at the first connection A 1 (not shown here) of the antenna arrangement ANT. The second transmission signal SS 21 arrives via the second output Out 21 of the device for signal splitting SPLIT 1 at the second connection A 2 (not shown here) of the antenna arrangement ANT.

The two transmission signals SS 11 and SS 22 are essentially identical, but exhibit a phase difference relative to one another. The phase difference is set by the inductors L 11 and L 21 and the capacitors C 12 and C 21 as phase-shaping components.

According to the invention, additional switchable components SW 1 through SW 5 are provided in the device for signal splitting SPLIT 1 with which the phase-shaping components L 11 , L 21 , C 12 and C 21 can be activated or deactivated.

The phase difference between the two transmission signals SS 11 and SS 21 provided for the antenna arrangement ANT is altered by the activated or deactivated phase-shaping components L 11 , L 21 , C 12 and C 21 . The goal is to vary the phase difference of the two transmission signals SS 11 and SS 21 such that a circularly-polarized field or an anti-circularly-polarized field or a linearly-polarized field is alternately formed by radiation from the antenna arrangement ANT.

For this purpose a first switch SW 1 is connected to the first inductor L 11 and a second switch SW 2 is connected in parallel to the second inductor L 21 . A third switch SW 3 is likewise connected to the first capacitor C 12 and a fourth switch SW 4 is connected in parallel to the second capacitor C 21 .

If the switchable components SW 1 through SW 4 are open, the values of the inductors L 11 and L 21 , the coil coupling and the values of the capacitors C 12 and C 21 are selected such that a desired phase difference of 90° results between the two transmission signals SS 11 and SS 21 at the two outputs Out 11 and Out 21 of the frequency splitter SPLIT 1 . The circularly-polarized transmission signal is formed upon irradiation via the activation of the antenna arrangement ANT with a 90° phase shift of the two transmission signals SS 11 and SS 21 .

The following polarizations of a transmission signal radiated via the antenna arrangement are achieved by the selectively occurring opening or closing of the switches SW 1 through SW 5 :

A switching from a circularly-polarized mode to a linearly-polarized mode is shown in this exemplary embodiment.

With correspondingly designed phase-shaping and switchable components as well as their circuiting it is also possible to form an anti-circular mode and, in addition to the linear mode shown here, a linear mode that is perpendicular to the shown linear mode.

The switches SW 1 through SW 5 are preferably fashioned as PIN diodes.

In summary, different polarized fields are used for magnetic resonance examinations. For example, a linearly-polarized field is used in a first time segment, a circularly-polarized field is used in a following second time segment and a further linearly-polarized field is used in a further following third time segment.

Each of these fields excites other currents in the body, such that a respective different examination result is acquired in each time segment.

These evaluation results are subjected to an additional evaluation individually and/or in an overlapping manner. For example, cancellations of the radio-frequency magnetic field in respective partial images may be present, but at respectively different geometric points. These cancellations can be reduced by the superimposition of the partial images. An improved sum image is formed by the superimposition of the individual partial images.

Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.