Method and device for suppressing interference emissions in magnetic resonance systems

Abstract

Magnetic resonance imaging (MRI) scanner, wherein the MRI scanner (1) comprises a magnet unit (10), a transmitter and a transmitting antenna for exciting nuclear spins, wherein the MRI scanner (1) further comprises a transmission interference suppression device (70) with a transmission interference suppression control unit (72), a sensor (71) and a transmission interference suppression antenna (60), wherein the transmission interference suppression device (70) is designed to receive an excitation signal from the transmitter for exciting nuclear spins with the sensor (70), to determine a transmission interference suppression signal as a function of the received excitation signal from the transmitter with the transmission interference suppression control unit (72) and to transmit it via the transmission interference suppression antenna (60), such that the excitation signal transmitted by the transmitter via the transmitting antenna is attenuated at a predetermined location outside the MRI scanner (1), characterized in thatthat the sensor (71) is located in a far field of the transmitting antenna and the predetermined location of attenuation is the location of the sensor (71).

Description

[0001] The invention relates to a magnetic resonance imaging (MRI) scanner, comprising a transmission interference suppression device with a transmission interference suppression controller, a sensor, and a transmission interference suppression antenna. The transmission interference suppression device is designed to receive an excitation signal from the transmitter to excite nuclear spins using the sensor, to determine a transmission interference suppression signal as a function of the received excitation signal from the transmitter using the transmission interference suppression controller, and to transmit this signal via the transmission interference suppression antenna.

[0002] Magnetic resonance imaging (MRI) scanners are imaging devices that use a strong external magnetic field to align the nuclear spins of a sample and then stimulate them to precess around this alignment using an alternating magnetic field. The precession, or return of the spins from this excited state to a lower-energy state, in turn generates an alternating magnetic field that is received by antennas.

[0003] Magnetic gradient fields are used to imprint a spatial coding on the signals, which subsequently allows the received signal to be assigned to a volume element. The received signal is then evaluated, and a three-dimensional imaging representation of the object under investigation is generated. Local receiving antennas, so-called local coils, are preferably used to receive the signal. These are positioned directly next to the object under investigation to achieve a better signal-to-noise ratio. The receiving antennas can also be integrated into a patient's examination table.

[0004] Magnetic resonance imaging (MRI) scanners require high-frequency shielding for two reasons. Firstly, high-frequency pulses with kilowatt power are generated to excite the nuclear spins; these are only partially absorbed by the patient. Secondly, radio waves exiting the patient passageway are emitted into the surrounding area and must therefore be shielded to comply with emission limits.

[0005] Conversely, the magnetic resonance signals received for imaging are extremely weak. To achieve a sufficient signal-to-noise ratio (SNR), shielding against external interference is necessary.

[0006] Therefore, in state-of-the-art technology, elaborate shielded cabins are installed around magnetic resonance imaging scanners to reduce both emissions and immissions.

[0007] A magnetic resonance imaging scanner with active noise suppression and a method for its operation are known from publication WO 2019 / 06867 A2.

[0008] With such devices, it is problematic to achieve stable interference suppression under different interference influences.

[0009] It could therefore be a task of the invention to improve interference suppression.

[0010] The problem is solved by a magnetic resonance tomograph according to claim 1 and by a method according to claim 7 for operating the magnetic resonance tomograph.

[0011] The magnetic resonance imaging scanner according to the invention comprises a magnet unit, a transmitter, and a transmitting antenna for exciting nuclear spins. Typically, the field magnet of the magnet unit is designed as a superconducting magnet because of the achievable field strengths of the static magnetic field B0, which determines the resonance frequency or Larmor frequency of the nuclear spins.

[0012] The magnetic resonance imaging scanner according to the invention further comprises a transmission interference suppression device with a transmission interference suppression controller, a sensor, and a transmission interference suppression antenna. The transmission interference suppression device is designed to use the sensor to detect an excitation signal from the transmitter to excite nuclear spins in a far field of the transmitting antenna. The far field is defined as an alternating electromagnetic field generated by the transmitting antenna in which the H and E fields are in phase and which typically predominates at a distance greater than one, two, or multiple times the wavelength.

[0013] The sensor can be, for example, an antenna, such as a conductor loop, into which a voltage is induced by a magnetic component of an excitation signal. The excitation signal is defined as any high-frequency alternating magnetic field emitted by the magnetic resonance imaging (MRI) scanner to influence the alignment of nuclear spins in the patient within the magnetic field B0. Typically, the excitation signal is emitted in the form of time-limited, usually short excitation pulses. The sensor transmits a signal from the excitation signal to the transmission interference suppression control. This signal can be, for example, a voltage or a current proportional to the induced current, or, in the case of an electric antenna, to the voltage generated by the electric field. Alternatively, the signal could already be digitized by an analog-to-digital converter.A sensor can also be designed to detect a polarization of the field, for example for the H-field by two orthogonal antenna loops or for the E-field by two dipoles aligned perpendicular to each other.

[0014] The transmission interference suppression control is designed to determine a suppression signal based on the received excitation signal from the transmitter and to transmit this signal via the suppression antenna. This attenuates the excitation signal transmitted by the transmitter via the transmitting antenna at a predetermined location outside the magnetic resonance imaging (MRI) scanner. This can be achieved, for example, by using the suppression antennas to generate a signal through phase shifting and amplification / attenuation. This signal weakens or cancels out the excitation signal at the sensor location through destructive interference. For instance, an optimization process could be used to vary the phase shift and attenuation parameters until the field strength at the sensor is minimized.

[0015] Advantageously, the sensor arranged in the far field according to the invention leads to the interference field not only being reduced or minimized at the location of the sensor itself, but also for a wave propagating from the source of interference beyond this location into the surroundings.

[0016] The method according to the invention comprises the step of receiving information about the excitation signal. Preferably, the information relates to the amplitude or phase of the excitation signal. The information can, for example, be the signal from an antenna loop acting as a sensor.

[0017] In a further step of the inventive method, the transmission interference suppression control determines a transmission interference suppression signal depending on the information such that, when the transmission interference suppression signal is transmitted via the transmission interference suppression antenna, the field strength of the excitation signal is reduced at a predetermined location. An exemplary method has already been described with regard to the device claim. In a further step of the process, the transmission interference suppression device sends the transmission interference suppression signal via the transmission interference suppression antenna.

[0018] The method according to the invention shares the advantages of the magnetic resonance tomograph according to the invention.

[0019] Further advantageous embodiments are specified in the dependent claims.

[0020] In one conceivable embodiment of the magnetic resonance imaging (MRI) scanner according to the invention, the predetermined location is defined by a regulatory-predetermined distance for an EMC limit value. For this location, a limit value for the electric and / or magnetic field intensity that must not be exceeded is predetermined by regulatory requirements. This could, for example, be a predetermined distance of more than 5 m or more than 10 m from the transmitting antenna. Alternatively, the limit could be a barrier restricting access to the MRI scanner, such as a partition, wall, door, or similar structure.

[0021] The sensor advantageously ensures that the limit values ​​are reliably adhered to outside the boundary by means of the restriction.

[0022] In one possible embodiment of the magnetic resonance graph according to the invention, the transmission interference suppression control has a signal connection to the transmitter. This signal connection is designed to receive information about the excitation signal. This can be, for example, a high-frequency transmission line that carries a signal of the excitation signal attenuated by a predetermined factor, for example, by a factor of 40 dB, 60 dB, or more, from the transmitter's final stage to the transmission interference suppression control. It is also conceivable to transmit the unamplified signal from the final stage to the transmission interference suppression control via the high-frequency transmission line. Digital or wireless transmission is also possible. It is essential that the information transmitted to the transmission interference suppression control is not altered by external factors, such as the patient, during transmission. The transmission interference suppression control is designed to determine the transmission interference suppression signal based on the received information.For example, the transmission interference suppression signal can be obtained by attenuation / amplification and / or phase shifting.

[0023] In one conceivable embodiment of the magnetic resonance imaging scanner according to the invention, the information about the excitation signal specifies its amplitude and / or phase. For example, the signal can be proportional to the excitation signal, or a digitally sampled signal with a sampling frequency above the minimum sampling rate defined by Nyquist. However, it is also conceivable to specify the maximum amplitude and frequency as well as a phase in the case of a time-constant signal. Hybrid forms are also possible, such as the amplitude, frequency, and phase of a carrier wave, as well as a baseband signal to be modulated.

[0024] Advantageously, by transmitting the information to the transmission interference suppression control, the latter can adapt the interference suppression signal to the current excitation signal.

[0025] In one possible embodiment of the magnetic resonance imaging scanner according to the invention, the information about the excitation signal is proportional to a magnetic field generated by the transmitting antenna. In other words, the transmission interference suppression device receives information about the excitation signal that indicates, or is proportional to, the magnetic field strength of the excitation signal emitted by the transmitting antenna.

[0026] Between an output signal generated by the transmitter and the magnetic field strength produced by the transmitting antenna, significant differences in magnetic field strength can occur, for example, at different frequencies or with different loads from a patient. These differences directly influence the field strength of the wave propagating into the room. Active interference suppression can therefore be better adjusted with more precise knowledge of the magnetic fields.

[0027] In one conceivable embodiment of the magnetic resonance imaging scanner according to the invention, the transmission interference suppression device comprises a directional coupler. The directional coupler is arranged on a signal link between the transmitter and the transmitting antenna, such that the current intensity of a current flowing towards the transmitting antenna is detected by the directional coupler as a signal. Preferably, the directional coupler is arranged close to, i.e., less than 10% of a wavelength of the center frequency of the excitation signal on the signal link, at a feed point or feed point of the transmitting antenna. It is also conceivable that two directional couplers are arranged on the signal link, each detecting the current flow of the excitation signal traveling towards the antenna and the reflected excitation signal, so that a difference between them can be calculated.

[0028] The directional coupler allows for the advantageous detection of the current flow that directly influences the fields. Positioning it near the feed point minimizes phase shifts between current and voltage, while two opposing directional couplers enable the direct detection and consideration of the reflected current.

[0029] In one possible embodiment of the magnetic resonance imaging scanner according to the invention, the transmission interference suppression device comprises a high-frequency amplifier arranged in close proximity to the transmission interference suppression antenna. It is considered to be in close proximity if the distance from the feed point of the transmission interference suppression antenna is less than one-tenth of the wavelength of the center frequency of the excitation signal on the signal line. Preferably, the high-frequency amplifier is a current source amplifier.

[0030] The close proximity, especially in the case of a power source amplifier, ensures that the transmitted interference suppression signal essentially corresponds to the signal fed to the high-frequency amplifier by the interference suppression control. This particularly minimizes frequency dependencies.

[0031] In one possible embodiment of the magnetic resonance imaging (MRI) scanner, the interference suppression device comprises multiple sensors and / or interference suppression antennas at various locations. For example, the sensors and / or interference suppression antennas can surround the transmitting antenna or the MRI scanner in a plane, preferably horizontal, in which people or other potentially susceptible devices are located. However, it is also conceivable that the arrangement only partially surrounds the transmitting antenna to prevent interference in a specific direction requiring protection, or completely encloses it in all directions to prevent any interference whatsoever.The transmission interference suppression system is designed to determine a transmission interference suppression signal based on the excitation signal from the transmitter received by the majority of sensors and to transmit this signal via the transmission interference suppression antennas, thus attenuating the excitation signal transmitted by the transmitter via the transmitting antenna at the sensor locations. For example, it is conceivable that the transmission interference suppression control uses an optimization process to determine the phase shifts and attenuation of the individual transmission interference suppression signals in such a way that the total energy at the sensors is minimized through interference.

[0032] In one conceivable embodiment of the magnetic resonance imaging scanner according to the invention, the transmission interference suppression signal comprises a plurality of components. For example, these components can be signals that differ in phase and amplitude. The transmission interference suppression device is designed to transmit each component via one of the plurality of transmission interference suppression antennas. For example, the transmission interference suppression device can comprise a plurality of high-frequency amplifiers that amplify the components and transmit them via signal lines to a plurality of transmitting antennas, through which the transmission interference suppression signal or its components are radiated.

[0033] In this advantageous way, the transmission jamming device can achieve a reduction in radiation in many predetermined directions.

[0034] In one possible embodiment of the method according to the invention, the method further includes the step of transmitting a test pulse with the transmitter via the transmitting antenna.

[0035] The test pulse is a time-limited, predetermined high-frequency wave train with known amplitude, frequency, and phase. Preferably, the amplitude is significantly smaller than that of an excitation signal, for example by more than 20 dB, 40 dB, or 60 dB. In a further step, the sensor detects the field strength of the high-frequency electromagnetic field caused by the test pulse at the location of the sensor, for example an E-field or H-field depending on the sensor.

[0036] In a further step, the transmit interference control preferably determines a transfer function between the transmitting antenna and the sensor. This transfer function is defined as at least an attenuation of the amplitude or a proportionality factor between the amplitude of the test pulse and the preceding field strength detected by the sensor, as well as a delay or phase shift of the detected signal relative to the test pulse. The step of determining the transmit interference signal is then performed depending on the transfer function. For example, the transmit interference signal can be determined by applying the inverse of the defined transfer function and the inverse of a transfer function between the sensor and the transmit interference antenna to an excitation signal.

[0037] In one conceivable embodiment of the method according to the invention, the transmission interference suppression device comprises a plurality of sensors and transmission interference suppression antennas. The sensors and transmission interference suppression antennas surround the transmitting antenna in several directions, or preferably completely, at least in one plane or in every spatial direction, in order to reduce the propagation of interference in multiple directions. In the detection step, the plurality of sensors detect the field strength of the test pulse, and in the determination step, the transmission interference suppression control determines a plurality of transfer functions for the plurality of sensors. As already described for a single sensor, the subsequent step involves determining the transmission interference suppression signal as a function of the plurality of transfer functions.

[0038] The determined transfer function advantageously simplifies and speeds up the determination of the transmission interference suppression signals.

[0039] In one possible embodiment of the method according to the invention, the method includes the step of detecting the field strength of the excitation signal with the sensor. In the step of determining the transmission interference suppression signal, an optimization algorithm minimizes the energy of the excitation signal received by the sensor as a function of the detected field strength.

[0040] An optimization process can advantageously reduce the total interference radiation from the excitation signal.

[0041] The properties, features and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more easily understood in connection with the following description of the exemplary embodiments, which are explained in more detail in conjunction with the drawings.

[0042] They show: Fig. 1 a schematic representation of a magnetic resonance imaging scanner with a transmission interference suppression device according to the invention; Fig. 2 schematically a possible embodiment of the transmission interference suppression device in detail; Fig. 3 schematically shows another exemplary embodiment of the transmission interference suppression device in detail; Fig. 4 a schematic flowchart for an exemplary embodiment of the method according to the invention.

[0043] Fig. Figure 1 shows a schematic representation of an embodiment of a magnetic resonance tomograph 1 with a transmission interference suppression device 70 according to the invention.

[0044] The magnet unit 10 includes a field magnet 11 that generates a static magnetic field B0 for aligning the nuclear spins of samples or the patient 100 within a recording area. The recording area is characterized by an extremely homogeneous static magnetic field B0, with the homogeneity referring in particular to the magnetic field strength or magnitude. The recording area is nearly spherical and arranged within a patient tunnel 16 that extends longitudinally 2 through the magnet unit 10. A patient bed 30 can be moved within the patient tunnel 16 by the traversing unit 36. The field magnet 11 is typically a superconducting magnet capable of generating magnetic fields with a magnetic flux density of up to 3 T, and even higher in the latest devices. However, for lower field strengths, permanent magnets or electromagnets with normal-conducting coils can also be used.

[0045] Furthermore, the magnetic unit 10 includes gradient coils 12, which are designed to superimpose variable magnetic fields in three spatial directions onto the magnetic field B0 in order to spatially differentiate the captured imaging areas in the investigation volume. The gradient coils 12 are typically coils made of normally conducting wires that can generate mutually orthogonal fields in the investigation volume.

[0046] The magnet unit 10 also includes a body coil 14, which is designed to radiate a high-frequency signal supplied via a signal line into the examination volume and to receive resonance signals emitted by the patient 100 and transmit them via a signal line. In the following, the term transmitting antenna refers to an antenna through which the high-frequency signal is emitted to excite the nuclear spins. This can be the body coil 14, but also a local coil 50 with a transmitting function.

[0047] A control unit 20 supplies the magnet unit 10 with the various signals for the gradient coils 12 and the body coil 14 and evaluates the received signals.

[0048] The control unit 20 thus has a gradient control 21 which is designed to supply the gradient coils 12 with variable currents via supply lines, which provide the desired gradient fields in the investigation volume in a time-coordinated manner.

[0049] Furthermore, the control unit 20 includes a high-frequency unit 22 designed to generate a high-frequency pulse with a predefined temporal profile, amplitude, and spectral power distribution to excite magnetic resonance of the nuclear spins in the patient 100. Pulse powers in the kilowatt range can be achieved. The excitation signals can be radiated into the patient 100 via the body coil 14 or via a local transmitting antenna.

[0050] A controller 23 communicates via a signal bus 25 with the gradient controller 21 and the high-frequency unit 22.

[0051] A local coil 50 is arranged on the patient 100 as a first receiving coil, which is connected to the high-frequency unit 22 and its receiver via a connecting line 33. However, it is also conceivable that the body coil 14 is a first receiving antenna within the meaning of the invention.

[0052] The magnetic resonance imaging (MRI) scanner 1 has a transmission interference suppression device 70 according to the invention. This device has a sensor 71 or preferably a plurality of sensors 71 designed to detect a high-frequency signal with the Larmor frequency of the MRI scanner, in particular scattered radiation from an excitation signal of the MRI scanner, and to transmit it as a signal to the transmission interference suppression control unit 72. These sensors can be, for example, magnetic or electrical antennas or other detectors for high-frequency alternating electric and / or magnetic fields. The sensor(s) 71 are arranged at least in one spatial direction relative to the transmitting antenna in which a reduction of the interference radiation from excitation signals is to be achieved.Preferably, the sensors 71 enclose the magnetic resonance imaging (MRI) scanner 1 at least in one plane, for example the horizontal plane, or completely in all spatial directions, in order to reduce the propagation of interference radiation from the MRI scanner 1 into the surroundings. The sensor(s) 71 are arranged in a far field of the transmitting antenna in which the electric and magnetic fields of the high-frequency alternating electromagnetic field are in phase and emitted electromagnetic waves propagate in space. Since the sensor(s) 71 are located in the far field, the measured value of the sensor 71 also allows for a good determination of the field strength behind the sensor as seen from the transmitting antenna.

[0053] By arranging the sensor at a distance corresponding to a predetermined distance for a limit value, compliance with this limit value can be ensured with the transmission interference suppression device 70 according to the invention.

[0054] The interference suppression antenna 60 is preferably located near the transmitting antenna in the patient tunnel 16, for example, at or around the opening in the case of several interference suppression antennas. The interference suppression antenna is thus positioned on the propagation path of the electromagnetic wave between the transmitting antenna and the sensor 71. The same applies to multiple transmitters.

[0055] Fig. Figure 2 schematically shows a possible embodiment of the transmission interference suppression device 70 in detail.

[0056] In Fig. Figure 2 shows only one sensor 60; however, the transmission interference suppression device 70 preferably has a plurality of sensors 60, as shown below. Fig. 3 indicated.

[0057] The sensor 60 initially comprises an antenna that converts the high-frequency alternating electric and / or magnetic field of the pulse emitted by the transmitting antenna into a current and / or voltage in a conductor. For example, the antenna can be an induction loop. The electrical signal thus generated is typically amplified within the sensor by a low-noise preamplifier (LNA) before being transmitted via a signal link for further processing in the transmission interference suppression unit 70.

[0058] In the Fig. Figure 2 shows an analog signal processing as an exemplary embodiment.

[0059] The basic concept is that an excitation signal propagating into the environment as an electromagnetic wave is reduced by destructive interference, thus keeping the magnetic resonance imaging (MRI) scanner's emissions into the environment below a regulatory limit. Several possibilities exist for this. Firstly, the sensor 71 can simply serve as a measuring device for the strength of the propagating electromagnetic wave. The transmission interference suppression device must then receive information about the excitation signal via another route, e.g., via a signal line from the high-frequency unit 22 or the control unit 23. This information must be suitable for generating a signal for destructive interference. This could be, for example, the signal fed to a power amplifier in the high-frequency unit 22 to generate the excitation signal, or a damped output signal from the power amplifier.However, it can also be a digitized form of the excitation signal or parameters or signals from which the excitation signal is generated and which define it sufficiently for the generation of a differential signal.

[0060] A scaled excitation signal is then, for example, phase-shifted by the phase converter 73 and subsequently amplified by the high-frequency amplifier 74 before being radiated via the transmit interference suppression antenna 60. The transmit interference suppression controller 72 adjusts the parameters, in this case phase shift and gain, depending on the signal from the sensor 71. It is conceivable that the transmit interference suppression controller 72 minimizes the energy of the sensor signal by varying the phase and gain in an optimization process.

[0061] It would also be conceivable that the sensor signal itself provides the information about the excitation signal, and that the transmission interference suppression signal is generated from this through amplification and phase shifting. Here, too, an optimization method to reduce the energy of the sensor signal is conceivable, but complete cancellation is hardly feasible, since the amplification of the sensor signal would then have to approach infinity, and the loop would become unstable.

[0062] The transmission interference suppression antenna 60 is in turn arranged as close as possible to the transmitting antenna in the patient tunnel 16, for example at, or in the case of several transmission interference suppression antennas 60, around the opening.

[0063] Another conceivable embodiment of the transmission interference suppression device according to the invention is in Fig. 3 shown.

[0064] The embodiment of the Fig. The third version differs in that it incorporates multiple sensors 71 and multiple interference suppression antennas 60. These are distributed as far as possible in different directions relative to the transmitting antenna, in this case the body coil 14, so that emissions in various directions can be optimally suppressed. To supply these multiple interference suppression antennas 60 with different signals, as required for adaptive suppression of interference emissions in different directions, the controllable high-frequency amplifier 74 has multiple independent amplifier channels for amplifying the individual signals.

[0065] In the embodiment in Fig. 3 The transmission interference suppression controller 72 has a signal processing resource, for example a digital signal processor (DSP) or an FPGA. In this exemplary embodiment, the sensors 71 already digitize the signals and forward them to the transmission interference suppression controller 72. As already described in section 3, the sensors 71 digitize the signals and forward them to the transmission interference suppression controller 72. Fig. As described in section 2, the phase shifts and attenuation / gain factors can be determined by an optimization procedure to minimize the energy of the excitation signal received by the sensors 71. The phase shift and gain / attenuation can then be calculated using appropriate digital operations.

[0066] However, it is also conceivable that these steps take place in analog signal processing, with the mixing being carried out, for example, by a crossbar matrix with adjustable couplings and phase shift at intersection points.

[0067] Fig.Figure 4 shows an exemplary flow chart of the inventive method for operating the inventive transmission interference suppression device 70 in an inventive magnetic resonance tomograph 1.

[0068] In step S50, the transmit jamming device 70 receives information about the excitation signal. In the simplest case, this can be the excitation signal itself or a proportional signal, for example, attenuated by a factor of 20 dB, 40 dB, 60 dB, or more. However, for predetermined excitation signals for known sequences, such as a sinc pulse, it may also be sufficient if the scaling factor, center frequency, phase relationship, and / or duration are provided as information. The baseband signal of the excitation signal and the mixing frequency are also conceivable.

[0069] In step S60, the transmission interference control determines a transmission interference signal depending on the information such that when the transmission interference signal is transmitted via the transmission interference antenna, the field strength of the excitation signal is reduced at a predetermined location.

[0070] One possibility is to calculate the signal using Maxwell's field equation and a known geometry, where the attenuation and phase shift of the excitation signal at the sensor are determined from the known excitation signal. Using this information about the excitation signal, a corresponding transmit interference suppression signal with the inverse phase shift and appropriate gain can then be determined, resulting in negative interference with an attenuation greater than 6 dB, 12 dB, or more. Subsequently, in step S70, the transmit interference suppression signal is transmitted via the transmit interference suppression antenna 60.

[0071] However, it is also conceivable that instead of calculating in step S10 (transmitting), a test pulse is transmitted via the transmitter antenna, and then in step S20, the sensor detects the field strength caused by the test pulse. The sensor can, for example, detect the electrical or magnetic component.

[0072] Finally, in step S30, a transfer function between the transmitting antenna and sensor 71 is determined by the transmission interference suppression device 70 using the known properties of the test pulse and the properties detected by sensor 71. An autocorrelation algorithm is conceivable, for example. Preferably, at least the delay, i.e., the phase shift, and the attenuation are determined.

[0073] Finally, in step S60, the transmission interference suppression signal is determined as a function of the transfer function. As already explained in the calculation, this can be achieved with the transfer function determined by the test pulse by a corresponding inverse phase shift and gain, or more generally by the inverse transfer function.

[0074] Determining the transfer function(s) using a test pulse allows us to capture conditions that are not accessible to calculation, for example, because the patient's characteristics are only partially known.

[0075] It is also conceivable that a transfer function between one or more transmission interference suppression antennas 60 and one or more sensors 71 is determined in the same way. Various variations of the method are then possible. For example, the transmission interference suppression signal can be determined directly from the transfer functions and information about the excitation signal. It is possible that the transfer function(s) are determined once during the installation of the magnetic resonance system 1. Preferably, however, the determination is carried out at least before each image acquisition in order to account for changes caused by the patient.

[0076] It is also conceivable that the transfer functions or their parameters, such as attenuation and phase shift, are continuously adjusted by an optimization process, in which, for example, the energy of the signal detected by the sensors 71, resulting from the excitation signal and the transmission interference signal, is minimized. This simultaneously minimizes the emission of the excitation signal in the vicinity of the magnetic resonance imaging scanner, since the sensors are advantageously already arranged in the far field and thus provide a measure of the fields at a large distance.

[0077] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited by the disclosed examples and other variations can be derived by the person skilled in the art without leaving the scope of protection of the invention. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] WO 2019 / 06867 A2

[0007]

Claims

[1] Magnetic resonance imaging (MRI) scanner, wherein the MRI scanner (1) comprises a magnet unit (10), a transmitter and a transmitting antenna for exciting nuclear spins, wherein the MRI scanner (1) further comprises a transmission interference suppression device (70) with a transmission interference suppression control unit (72), a sensor (71) and a transmission interference suppression antenna (60), wherein the transmission interference suppression device (70) is designed to receive an excitation signal from the transmitter for exciting nuclear spins with the sensor (70), to determine a transmission interference suppression signal as a function of the received excitation signal from the transmitter with the transmission interference suppression control unit (72) and to transmit it via the transmission interference suppression antenna (60) so that the excitation signal transmitted by the transmitter via the transmitting antenna is attenuated at a predetermined location outside the MRI scanner (1), characterized by, that the sensor (71) is located in a far field of the transmitting antenna and the predetermined location of attenuation is the location of the sensor (71). [2] Magnetic resonance imaging scanner according to one of the preceding claims, wherein the predetermined location is specified by a regulatory predetermined distance for an EMC limit value. [3] Magnetic resonance imaging scanner according to claim 1 or 2, wherein the transmit interference control (72) has a signal connection with the transmitter which is designed to receive information about the excitation signal and is designed to determine the transmit interference signal depending on the information received. [4] Magnetic resonance imaging scanner according to one of claims 1 to 3, wherein the information about the excitation signal specifies an amplitude of the excitation signal. [5] Magnetic resonance imaging scanner according to one of the preceding claims, wherein the transmission interference suppression device (70) has a plurality of sensors (71) and / or transmission interference suppression antennas (60) at different locations and the transmission interference suppression device (70) is designed to determine a transmission interference suppression signal depending on the excitation signal of the transmitter received via the plurality of sensors (71) and to transmit it via the transmission interference suppression antennas (60), so that the excitation signal transmitted by the transmitter via the transmitting antenna is attenuated at the locations of the sensors (71). [6] Magnetic resonance imaging scanner according to claim 5, wherein the transmission interference suppression signal has a plurality of components, wherein the transmission interference suppression device (70) is designed to transmit one component at a time via one of the plurality of transmission interference suppression antennas (60). [7] Method for operating a magnetic resonance imaging scanner (1) according to any one of the preceding claims, wherein the method comprises the steps: (S50) Receiving information about the excitation signal; (S60) Determining a transmission interference suppression signal using the transmission interference suppression control (70) as a function of the information such that when the transmission interference suppression signal is transmitted via the transmission interference suppression antenna (60), the field strength of the excitation signal is reduced at a predetermined location; (S70) Emitting the transmit interference suppression signal via the transmit interference suppression antenna (60). [8] The method of claim 7, wherein the method further comprises the steps of: (S10) Emitting a test pulse with the transmitter via the transmitting antenna; (S20) Detection of field strength caused by the test pulse by the sensor (71); (S30) Determining a transfer function between transmitting antenna and sensor (71) by the transmit interference suppression device (70), wherein the step (S60) of determining the transmit interference suppression signal is performed depending on the transfer function. [9] Method according to claim 8, wherein the transmit interference suppression device (70) has a plurality of sensors and transmit interference suppression antennas (60) and in step (S20) detect the plurality of sensors (71) detects a field strength of the test pulse and in step (S60) determine a plurality of transfer functions for the plurality of sensors (71), wherein step (S60) of determining the transmit interference suppression signal is performed depending on the plurality of transfer functions. [10] Method according to one of claims 7 to 9, wherein the method comprises the step (S40) of detecting a field strength of the excitation signal with the sensor (71) and in the step (S60) of determining the transmit interference suppression signal an optimization algorithm minimizes the energy of the excitation signal received with the sensor (71) as a function of the detected field strength.

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