Magnetic resonance system with integrated pilot tone device and method for operating same

Abstract

The invention relates to a magnetic resonance system with a pilot tone device and a method for operating the magnetic resonance system. The magnetic resonance system has a receiver which is designed to simultaneously receive and evaluate a pilot tone signal and a magnetic resonance signal. The receiver also has a frequency-dependent attenuator which is designed to attenuate the received pilot tone signal relative to the received magnetic resonance signal.

Description

Technical Field

[0001] The invention also relates to a magnetic resonance system with a pilot sound device. The magnetic resonance system has a receiver which is designed to simultaneously receive and evaluate a pilot tone signal and a magnetic resonance signal. Background Art

[0002] A magnetic resonance tomography device is an imaging device that orients the nuclear spins of the subject of examination using an external strong magnetic field in order to image the subject of examination, and excites the nuclear spins of the subject of examination to precess around the orientation by an alternating magnetic field. The precession or return of the spins from this excited state to a lower energy state in response generates an alternating magnetic field, which is received by an antenna.

[0003] The signals are subjected to a spatial encoding by means of the gradient magnetic field, which subsequently enables the received signals to be associated with volume elements. The received magnetic resonance signals are then evaluated and a two-dimensional or three-dimensional image representation of the examination object is provided.

[0004] The magnetic resonance signal is very weak. Therefore, in order to achieve a sufficiently high signal-to-noise ratio, the signal must be acquired over a long period of time in one or repeated measurements. The acquisition of the magnetic resonance signal is slow compared to the inevitable movements of the patient, such as heartbeat or breathing movements. These movements cause artifacts in the resulting image.

[0005] However, it is still possible to image moving organs if brief image acquisitions are repeatedly carried out synchronously with the movement and the acquired data are averaged.

[0006] Synchronization can be achieved by dedicated sensors, such as a respiratory belt or ECG electrodes.

[0007] In order to avoid the use of these additional sensors, patent specification US 2015 / 0320342 A1 discloses coupling a continuous, single-frequency alternating magnetic field starting from a small conductor loop at least partially through the patient's body into the individual elements of the magnetic resonance coil.

[0008] Since most biological tissues are almost completely transparent to magnetic fields, the generated magnetic field penetrates the patient's body almost unchanged. However, most tissues have (weak) electrical conductivity and therefore the continuous wave magnetic field induces eddy currents. These eddy currents in turn generate a magnetic field superimposed on the excitation magnetic field, resulting in a modulation in the received magnetic field in the receiving coil.

[0009] By analyzing this signal, it is possible to infer the phase of the heart or breathing movement.

[0010] However, when using the pilot tone signal, it may happen that the received signal and thus the amplitude fluctuations caused by the movement are also very weak and can hardly be analyzed. This is particularly true when the pilot tone and the magnetic resonance signal are simultaneously received and evaluated by a common receiver, since the effective amplitude dynamic range of the receiving system should not be limited by the pilot tone signal, thereby limiting the maximum achievable pilot tone signal reception level. It may also happen that the proportion of the alternating magnetic field that penetrates the tissue moving due to the heartbeat is too low and in this case the demodulated received signal has a too low signal-to-noise ratio. Summary of the invention

[0011] Therefore, the technical problem to be solved by the present invention is to make the acquisition of patient movements better and more reliable.

[0012] This object is achieved by a magnetic resonance system according to claim 1 of the present invention.

[0013] The magnetic resonance system according to the invention has a pilot tone device. The pilot tone device decouples a high-frequency magnetic alternating field, which is also referred to as a pilot tone in the following. The alternating field interacts with the patient's body via induced currents and susceptibility transitions at organ boundaries. This interaction changes with the movement of the body and causes the pilot tone to be modulated into a pilot tone signal, which is received by a receiver of the pilot tone device and can thereby detect the movement of the patient's body or a movement within the body.

[0014] The magnetic resonance system according to the present invention has a receiver for a magnetic resonance tomography instrument for receiving magnetic resonance signals to image a patient. In the magnetic resonance system according to the present invention, the receiver of the magnetic resonance tomography instrument is also used to receive the pilot tone signal while receiving the magnetic resonance signal. Therefore, the available signal processing resources are used in a dual manner. In this case, the pilot tone device must generate the pilot tone within a range that can also be detected by the receiver. In other words, the frequency range of the pilot tone is close to the Larmor frequency, and the Larmor frequency is predetermined by the nuclear spin to be detected and the static magnetic field B0 of the magnetic resonance tomography instrument. The frequency of the pilot tone deviates slightly from the Larmor frequency, for example, the deviation is less than 5%, 2% or 1%, and is not within the frequency band occupied by the magnetic resonance signal.

[0015] Preferably, further resources can also be used jointly for the pilot tone signal and the magnetic resonance signal. In particular, it is also conceivable that the magnetic resonance system can provide a common receiving path for the pilot tone signal and the magnetic resonance signal, as well as a common signal path or receiving path to the receiver, including an antenna coil and / or a preamplifier or a low noise amplifier (LNA).

[0016] The magnetic resonance system according to the present invention further comprises a frequency-dependent attenuator, which is configured to attenuate the received pilot tone signal relative to the received magnetic resonance signal. In other words, the frequency-dependent attenuator has a frequency-dependent characteristic curve, which does not or only slightly attenuates the magnetic resonance signal, but has a predetermined stronger attenuation of the pilot tone signal. The attenuation ratio of the magnetic resonance signal and the pilot tone signal is a predetermined value.

[0017] Here, the attenuator is arranged in the signal path upstream of the A / D converter of the receiver, in particular between the amplifier and the A / D converter, and is preferably implemented as an analog circuit. It is also conceivable that the attenuator is an integrated component of the preamplifier, for example as frequency-dependent negative feedback.

[0018] Here, the concepts of pilot tone and pilot tone signal are different. Pilot tone refers to a signal generated by a pilot tone device and transmitted to the patient's body. In contrast, a pilot tone signal refers to a signal synthesized by the pilot tone and evaluated in a receiver. On the one hand, the pilot tone signal has an expected fluctuation, which causes amplitude fluctuation or modulation of the pilot tone in the propagation and received by the local coil through the movement of the patient's body or the movement in the body. On the other hand, the pilot tone signal can also be further preprocessed, such as filtering through the frequency characteristics of the local coil or bandpass filter, preventing amplification through LNA, or frequency conversion to an intermediate frequency. However, these effects on the magnetic resonance signal received by the local coil and the modulated pilot tone are basically the same, such as the frequency separation and amplitude ratio remain basically unchanged or completely unchanged. Therefore, if necessary, the intermediate frequency signal is converted into a pilot tone signal in terms of the description of the Larmor frequency of the magnetic resonance signal and the frequency of the intermediate frequency signal.

[0019] In current MR receiving systems, the available amplitude dynamic range is determined by an analog-to-digital converter (ADC). By inserting an attenuator upstream of the ADC, the available amplitude dynamic range of the receiving system can be increased, but with an increase in the noise figure. Since the noise figure of the receiver and the background noise increase significantly less than with the attenuator used, the signal-to-noise ratio (SNR) of the received pilot tone signal is advantageously increased if the level of the transmitted pilot tone and the level of the pilot tone signal are increased to the same extent as the receiver gain factor reduced by means of the attenuator. The available amplitude dynamic range of the MR receiving path is not affected by this.

[0020] Further advantageous embodiments are given in the dependent claims.

[0021] In a conceivable embodiment of the magnetic resonance system according to the invention, the attenuation of the received pilot tone signal relative to the received magnetic resonance signal by the attenuator is greater than 3 dB, 6 dB or 9 dB. Preferably, however, the attenuation is not greater than 12 dB, 18 dB or 24 dB.

[0022] In order not to interfere with the reception of the magnetic resonance signal, the received pilot tone signal must have a significantly lower level, in particular at the input of the common A / D converter. In order to increase the signal-to-noise ratio (SNR) of the pilot tone signal, according to the invention, the level of the transmitted pilot tone is increased by a comparable level so that it can be reduced again by the attenuator. As a result, even if the level interval between the magnetic resonance signal and the pilot tone signal is the same at the input of the A / D converter downstream of the attenuator, the signal-to-noise ratio of the pilot tone signal can be increased in an advantageous manner by a value that roughly corresponds to the attenuation or increase of the transmitted pilot tone. This value is limited by the transmitted pilot tone and the limited amplitude dynamic range of the preamplifier signal control (such as preamplifier and / or mixer), SAR, regulatory restrictions and / or interference of the pilot tone to other units. However, in principle, the attenuator can also be arranged upstream of the receive path signal control, but the noise components (such as thermal noise) will also be amplified by the preamplifier.

[0023] In a possible embodiment of the magnetic resonance system according to the present invention, the magnetic resonance system is configured to compensate for the attenuation of the attenuator by increasing the emission level of the pilot tone of the pilot tone device. When the attenuator reduces the pilot tone level by, for example, 6 dB, the pilot tone level transmitted by the pilot tone device is increased by 2 times or 6 dB accordingly. Here, the pilot tone signal of the system in the prior art can be used as a reference level, or the level of the maximum magnetic resonance signal to be evaluated can be used. In other words, by increasing the attenuator and the pilot tone level, the pilot tone signal level at the input of the A / D converter remains substantially unchanged compared to the magnetic resonance tomography apparatus in the prior art that does not have the attenuator according to the present invention. In particular, the level spacing between the magnetic resonance signal and the pilot tone signal downstream of the attenuator is maintained at the same value as the magnetic resonance system of the prior art.

[0024] As previously described, by increasing the transmit level of the pilot tone, an improved signal-to-noise ratio of the received pilot tone signal is achieved.

[0025] In a conceivable embodiment of the magnetic resonance system according to the invention as claimed in any of the preceding claims, the attenuator has a notch filter (band rejection filter). A notch filter is a filter that reduces the level of only narrowband signals while leaving other frequency domains essentially unchanged during the passage, i.e., for example, with a fluctuation of less than 0.5 dB or 0.1 dB. Such a signal is considered narrowband in this case, which has a bandwidth of the same order of magnitude as the bandwidth of the pilot tone signal, i.e., less than 500 Hz, 100 Hz, 50 Hz or 10 Hz. In addition, multiple pilot tone signals with similar spectra can also be attenuated by a common filter.

[0026] Advantageously, the notch filter attenuates only the pilot tone signal and allows the magnetic resonance signal to remain substantially unchanged.

[0027] In one possible embodiment of the magnetic resonance system according to the invention, the attenuator has a quartz resonator.

[0028] Advantageously, the quartz resonator has frequency stability.

[0029] In a possible embodiment of the magnetic resonance system according to the invention, the attenuator has a plurality of coupled resonators. Here, a resonator is a filter element having a resonant frequency (i.e. a preferred pass frequency or attenuation frequency). Coupling here refers to the interaction between the resonators in two directions. For example, the coupling can be inductive, capacitive and / or resistive.

[0030] Advantageously, the characteristics of the attenuator or bandpass filter, in particular the bandwidth of the passband, can be influenced by the interacting resonators. Thus, when selecting the bandwidth and attenuation, a compromise can be made between attenuation of the PR signal frequency, limited influence on the MR signal and the shortest possible transient oscillation time.

[0031] A high-pass filter or a low-pass filter can also be used as an attenuator. In this case, the edge of the filter is located between the frequency of the pilot tone and the frequency of the magnetic resonance signal, in particular the Larmor frequency of the magnetic resonance system, which is determined by the magnetic moment of the nuclear spin to be detected and the static magnetic field B0 of the magnetic resonance system. If the frequency of the pilot tone is higher than the Larmor frequency, a low-pass filter is used to attenuate the pilot tone signal, and correspondingly, if the frequency of the pilot tone is lower than the Larmor frequency, a high-pass filter is used. Here, the frequency selectivity of the resonant antenna coil can also be used to attenuate pilot tone signals that are spectrally distant.

[0032] However, a combination of a high-pass filter and a low-pass filter can also be used as an attenuator or notch filter.

[0033] Advantageously, this allows a particularly fast transient behavior and / or a low ripple within the passband to be achieved using high-pass and low-pass filters.

[0034] In a possible embodiment of the magnetic resonance system according to the invention, the magnetic resonance system has a plurality of receiving channels. The receiving channels can be used, for example, for parallel reception of magnetic resonance signals of a local coil matrix. According to the invention, the plurality of receiving channels each have a receiver and a frequency-dependent attenuator. The attenuator can be part of the receiver, but can also be part of a preamplifier or an LNA (for example), or be arranged between the LNA and the receiver. It is conceivable that the attenuators each have different attenuation values. The attenuation value preferably depends on the expected reception level of the pilot tone in operation in the specific case. The attenuation values ​​should be configured so that the pilot tone signal has essentially the same level in all receivers downstream of the attenuator.

[0035] For example, the reception level will be affected by the distance and geometric arrangement of the connected antenna coil relative to the pilot tone transmitting antenna.

[0036] It is also contemplated that the attenuator or attenuators have an attenuation amount that is adjustable by the magnetic resonance system controller, such that the controller can adjust the attenuation amount to reduce the pilot tone signal to a predetermined level at the receiver.

[0037] Due to the geometrical arrangement and the different attenuations achieved by the patient, the received level of the pilot tone varies in different antenna coils and receiving channels. In this case, the antenna coil with the highest level represents the limit for the maximum level of the transmitted pilot tone. If there are individual attenuators in most or all receiving channels, in particular if the attenuation values ​​of these attenuators are such that the level differences between the pilot tone signal between the receivers and the received signal of the antenna coil are reduced, the pilot tone signal can be transmitted at a higher level, thereby further improving the signal-to-noise ratio of the pilot tone signal.

[0038] In a possible embodiment of the magnetic resonance system according to the invention, the magnetic resonance system has a controller which is configured to adjust the attenuation of the frequency-dependent attenuator. For example, the adjustable digital-to-analog converter can be varied as a variable resistor, or a variable capacitor in the attenuator circuit can be changed in the form of a PIN diode via an output voltage, as shown in the following figure.

[0039] The controller is designed to adjust the attenuator so as to reduce the signal level difference between the pilot tone signals in the receivers of the plurality of receive channels. This can be achieved, for example, by the controller executing the procedure described below according to the method for operating a magnetic resonance system according to the present invention.

[0040] In the method according to the invention, the controller determines the signal level of the pilot tone signal in a plurality of receivers in one step. It is possible, for example, for the digital receivers to filter out the pilot tone signal by means of a Fourier transform and to determine its level. However, other filters are also conceivable, for example a bandpass filter, which separates out the pilot tone signal. However, it is also possible to determine the level of the receivers as part of a calibration process without magnetic resonance signals and active pilot tones, in which case essentially only the pilot tone signal is detected and preferably expensive filtering measures can be omitted.

[0041] In a subsequent step, the controller adjusts the attenuation of the frequency-dependent attenuator to reduce the signal level difference between the pilot tone signals in the receivers of the multiple receiving channels. In other words, by adjusting the attenuator, the level of the pilot tone signal in the receiving channel is compensated.

[0042] For this purpose, it is conceivable, for example, to first determine the receiving channel with the lowest signal level for the pilot tone signal. For the other receiving channels, the controller then determines the level difference with respect to this receiving channel. Subsequently, the controller adjusts the respective attenuators of the other receiving channels so that their signal levels are reduced, preferably substantially the same as the signal level of the receiving channel with the lowest signal level. If the signal level of a receiving channel is 6 dB higher than the signal level of the weakest receiving channel, the attenuation of the attenuator can be increased by 6 dB, or by 7 dB or 8 dB or more.

[0043] In a subsequent step, the transmission level of the pilot tone is adjusted so that the signal level of the receiving channel does not exceed a predetermined signal level upstream of the A / D converter. In this case, the predetermined signal level can be determined by the expected signal level of the magnetic resonance signal and / or the input level of the A / D converter.

[0044] For example, if the expected signal level of the magnetic resonance signal is 0 dBFS, i.e., the maximum input level processed by the A / D converter, and the signal level of the pilot tone signal of the receiving channel with the lowest signal level is -18 dBFS, the controller will adjust the attenuators of the other receiving channels so that their signal levels do not exceed -18 dBFS. Then, if a level interval of -6 dB between the maximum expected magnetic resonance signal and the pilot tone signal is desired, the signal level of the pilot tone can be increased by 12 dB.

[0045] In this case, the specified level in the example is only exemplary, and other values ​​may also be used. For example, it may also be determined which receiving channel has the highest pilot tone signal level, and the attenuation of the attenuators of other receiving channels may be reduced accordingly to obtain substantially the same pilot tone signal level, and then the transmission level of the pilot tone may be reduced or increased.

[0046] By adaptively adjusting the attenuator and the pilot tone, in particular for a plurality of receiving channels, a more reliable detection of patient movements or movements in the body can be ensured. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] The above characteristics, features and advantages of the present invention and their implementation methods will be clearer and easier to understand with reference to the following description of the embodiments, which are described in detail with reference to the accompanying drawings.

[0048] In the attached picture:

[0049] Figure 1 A schematic diagram of a magnetic resonance system according to the present invention is shown;

[0050] Figure 2 An exemplary embodiment of a receive path of a magnetic resonance system according to the invention is shown;

[0051] Figure 3 An exemplary embodiment of an attenuator of a magnetic resonance system according to the present invention is shown;

[0052] Figure 4 An exemplary embodiment of an attenuator of a magnetic resonance system according to the invention is shown. DETAILED DESCRIPTION

[0053] Figure 1 A schematic diagram of an embodiment of a magnetic resonance system 1 according to the invention is shown.

[0054] The magnet unit 10 has a field magnet 11 which generates a static magnetic field B0 for orienting the nuclear spins of a sample or patient 100 in an acquisition region. The acquisition region is characterized by an extremely uniform static magnetic field B0, wherein the uniformity is particularly related to the magnetic field strength or direction and value. The acquisition region is almost spherical and is arranged in a patient channel 16 which extends through the magnet unit 10 in a longitudinal direction 2. The patient bed 30 can be moved in the patient channel 16 by a movement unit 36. The field magnet 11 is usually a superconducting magnet which can provide a magnetic field with a magnetic flux density of up to 3 T, which is even higher in the case of the latest devices. However, for smaller field strengths, permanent magnets or electromagnets with normally conductive coils can also be used.

[0055] Furthermore, the magnet unit 10 has gradient coils 12 which are designed to superimpose a magnetic field B0 with a variable magnetic field in three spatial directions for spatial differentiation of the acquired imaging region in the examination volume. The gradient coils 12 are usually coils made of normally conducting wires which can generate mutually orthogonal fields in the examination volume.

[0056] The magnet unit 10 also has a body coil 14 which is designed to radiate high-frequency signals conveyed via signal lines into the examination volume and to receive resonance signals emitted by the patient 100 and to output the resonance signals via the signal lines.

[0057] The control unit 20 provides different signals for the gradient coil 12 and the body coil 14 to the magnet unit 10 , and analyzes the received signals.

[0058] The control unit 20 therefore has a gradient controller 21 which is designed to supply the gradient coils 12 with variable currents via supply lines, which provide the desired gradient fields in a temporally coordinated manner in the examination volume.

[0059] The control unit 20 also has a radio-frequency unit 22, which is designed to generate radio-frequency pulses with a predetermined time course, amplitude and spectral power distribution for exciting magnetic resonance of nuclear spins in the patient 100. Pulse powers in the kilowatt range can be achieved. The excitation pulses can be radiated into the patient 100 via the body coil 14 or also via local transmission antennas.

[0060] The high-frequency unit 22 also has a receiver 40 for receiving and / or processing magnetic resonance signals from the patient 100, which are received by the local coil 50 and transmitted via a signal connection to the receiver 40. The receiver 40 is also designed to receive and evaluate pilot tones.

[0061] The high-frequency unit 22 can also have a pilot tone transmitter 60 which transmits the pilot tone via a signal connection to a pilot tone transmitting antenna and thus optionally to the local coil 50 for decoupling. However, a separate pilot tone transmitter 60 which reacts independently of the magnetic resonance system 1 is also conceivable.

[0062] The controller 23 communicates with the gradient controller 21 and the high-frequency unit 22 via a signal bus 25 .

[0063] A local coil 50 is arranged on the patient 100 and is connected to the high-frequency unit 22 and its receiver via a connecting line 33 .

[0064] Preferably, the local coil 50 also has an antenna, with which the pilot tone can be transmitted or induced into the patient 100 .

[0065] Figure 2 An exemplary embodiment of a reception path of a magnetic resonance system 1 according to the invention is shown.

[0066] In the local coil 50, the antenna coil 51 receives the pilot tone signal and the magnetic resonance signal from the patient 100. The received signal is amplified by the low noise amplifier LNA 52. Then, the notch filter 53 selectively attenuates the pilot tone signal on the receiving path by a predetermined value. The details of the notch filter 53 are described in Figure 3 Given in.

[0067] However, other forms of attenuators can also be considered. For example, a narrow-band notch filter can also be implemented by a quartz filter, which has the advantage of high frequency stability.

[0068] The bandwidth of the notch filter 53 or the narrow bandpass filter can also be influenced and adjusted by means of two coupled resonant filters (e.g. parallel or series resonant circuits). The attenuator can also be implemented as a high-pass or low-pass filter, the edge of which is located between the magnetic resonance signal and the pilot tone signal and the required amplitude ratio is adjusted by attenuation. In this case, noise components outside the frequency range of the magnetic resonance signal and the pilot tone signal can be suppressed by further filters. For example, the antenna coil 51, which is usually tuned to the resonant frequency of the magnetic resonance signal during reception, already acts as a bandpass filter and can reduce such external frequency components. Frequency components located outside can also be suppressed by further bandpass or complementary high-pass or low-pass filters.

[0069] The A / D converter in the receiver 40 then digitizes and evaluates the magnetic resonance signal and the pilot tone signal. Preferably, the pilot tone signal and the magnetic resonance signal are separated into different signal currents, which are respectively provided to different processing processes. In this case, the amplitude changes of the pilot tone signal are evaluated in particular as a function of characteristic changes such as respiratory movements and heartbeats. In contrast, image data are obtained from the magnetic resonance signal by image reconstruction. Correlations in the processing process do still occur here, for example, the magnetic resonance signal is only processed further if no movement or a predetermined movement phase is detected in the pilot tone signal.

[0070] Figure 2 The distribution of the functional groups in the embodiment is only an example of an embodiment in this example. According to the present invention, there may be other variants. For example, the attenuator 53 may also be a component of the local coil 50 or the receiver 40. It is also possible to change the sequence or combination of functions in the receiving path, for example, to integrate the filter function in the LNA.

[0071] Figure 3 An exemplary embodiment of an attenuator for a magnetic resonance system according to the invention is shown. This embodiment involves a notch filter using quartz, here for example with an exemplary frequency of 22.5 MHz. The input and output impedance of the proposed attenuator are both 50 ohms, so that it can be incorporated into the high-frequency path of a common receiver circuit.

[0072] A simple notch filter can be made of quartz, which is connected to the signal line and connected to the signal ground in series with a capacitor C and a resistor R. The boundary condition here is the impedance of the signal line. Here, the maximum attenuation can be selected by the value of the series resistor R. The center frequency and bandwidth of the notch filter are predetermined by the characteristics of the quartz and the values ​​of C and R, taking into account the impedance of the signal line.

[0073] exist Figure 3 A possible way of enlarging the bandwidth of a notch filter using quartz is shown in FIG.

[0074] When resonance occurs, the impedance of the quartz is mainly determined by the internal dissipation resistance. With the transformer 54 having a winding ratio of 1:n, the resistance composed of the resistor R and the internal dissipation resistance of the quartz in series is connected to the signal line at 1 / (1+n) 2 The coefficient of the voltage is reduced. To this end, the primary side of the transformer and the series circuit consisting of the quartz Q, capacitor C and resistor R and the secondary side of the transformer 54 are arranged in anti-parallel between the signal line and the signal ground. Here, the winding ratio of the primary side winding to the secondary side winding is 1:n. Therefore, the series resistance value of the resistor R and the internal dissipation resistance of the quartz can be increased by increasing the resistance value of R (1+n) 2 The impedance of the signal line remains unchanged. At the same time, the bandwidth of the attenuator can be increased by (1+n) by using transformer 54. 2 Advantageously, an additional degree of freedom is gained in the design of the attenuator.

[0075] Figure 4 Another exemplary embodiment of an attenuator is shown. Figure 3 In contrast, a further widening of the bandwidth is achieved here by coupling two resonant elements, here quartz resonators, of different resonant frequencies to one another. The coupling is achieved here by a parallel connection, wherein the degree of coupling is primarily influenced by the value of the resistor R. Since the quartz Q generally has a certain standard frequency, different resonant frequencies can be achieved by connecting different reaction elements in series to the two quartzes. Thus, despite the use of two identical quartzes, the two resonant elements can also achieve different resonant frequencies.

[0076] In a magnetic resonance system (1) with a plurality of receivers (40) for pilot tone signals, it is preferably provided that all receivers (40) have an attenuator according to the invention. This is the case, for example, when the local coil (50) has a matrix of antenna coils. By receiving the pilot tone signal in a plurality of channels, the reliability of the pilot tone can be improved. The attenuation of the individual attenuators is preferably adjusted so that the audio signals downstream of the attenuators in the receivers (40) have approximately the same electrical frequency, i.e., have a difference of no more than 6 dB, 3 dB or 1 dB.

[0077] It is also conceivable that one or more attenuators are regulated in their attenuation by the controller 23. This can be achieved, for example, by replacing the capacitor C by a voltage-controlled PIN diode and / or replacing the resistor R by a voltage-controlled resistor, which is driven by a D / A converter. It is also possible to provide controlled switching between attenuators with different attenuation values ​​in the receiving channel. In this way, the controller can adapt the pilot tone device to different reception conditions caused by different arrangements and / or patients. Possible ways of the method according to the invention will be explained in more detail.

[0078] If not all receivers (40) are used for pilot tone processing, it is also conceivable that the receivers (40) without pilot tone function only allow the magnetic resonance signal to pass through by bandpass, while suppressing the pilot tone signal.

[0079] Although the details of the present invention have been illustrated and described in detail through the preferred embodiments, the present invention is not limited to the disclosed examples, and those skilled in the art may derive other different variations therefrom without departing from the scope of protection of the present invention.

Claims

1. A magnetic resonance system comprising a pilot tone device, wherein: The magnetic resonance system (1) has a receiver (40) which is designed to simultaneously receive and evaluate a pilot tone signal and a magnetic resonance signal. It is characterized in that The magnetic resonance system (1) has a frequency-dependent attenuator which is designed to attenuate a received pilot tone signal relative to a received magnetic resonance signal.

2. The magnetic resonance system according to claim 1, wherein: The attenuator is configured to attenuate the pilot tone signal relative to the magnetic resonance signal by more than 3 dB, 6 dB or 12 dB.

3. The magnetic resonance system according to claim 2, wherein: The magnetic resonance system (1) is designed to compensate for the attenuation of the attenuator by increasing the emission level of the pilot tone of the pilot tone device.

4. A magnetic resonance system according to any one of the preceding claims, wherein: The magnetic resonance system (1) has a preamplifier for a received pilot tone signal, and the attenuator is arranged in the receiver as a component of the preamplifier or between the preamplifier and the analog-digital converter.

5. A magnetic resonance system according to any one of the preceding claims, wherein: The attenuator has a notch filter (53).

6. A magnetic resonance system according to any one of the preceding claims, wherein: The attenuator has a quartz resonator.

7. A magnetic resonance system according to any one of the preceding claims, wherein: The attenuator has a plurality of coupled resonators.

8. The magnetic resonance system according to any one of claims 5 to 7, wherein: The attenuator has a transformer, the primary side and the secondary side of which are arranged in anti-parallel between a signal path and a signal ground, wherein a resonant element is arranged between the signal path and the signal ground in series with the secondary side.

9. A magnetic resonance system according to any one of the preceding claims, wherein: The magnetic resonance system has a plurality of acquisition channels, wherein each of the plurality of acquisition channels has a frequency-dependent attenuator.

10. The magnetic resonance system according to claim 9, wherein: The attenuation of the frequency-dependent attenuator is adjustable, and the magnetic resonance system has a controller (23) configured to adjust the attenuation of the frequency-dependent attenuator so as to reduce the signal level difference between the pilot tone signals in the receivers of the plurality of receive channels.

11. A method for operating a magnetic resonance system according to claim 10, wherein: The method comprises the following steps: detecting a signal level of a pilot tone signal in a plurality of receivers; adjusting the attenuation of the frequency-dependent attenuator by a controller (23) so as to reduce the signal level difference between the pilot tone signals in the receivers of the plurality of receiving channels; Adjusts the signal level of the pilot tone.

12. A computer program product having instructions which, when executed by a magnetic resonance system (1) according to claim 10, cause the magnetic resonance system to perform the method according to claim 11.

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