Pilot tone signal generator, magnetic resonance tomography apparatus and motion correction method

Abstract

The invention relates to a pilot tone signal generator, to a magnetic resonance tomography apparatus, to a method for transmitting a synchronization signal and to a computer program product. The pilot tone signal generator comprises a receiving unit for receiving a synchronization signal of a system control unit of a magnetic resonance tomography apparatus. Here, the synchronization signal comprises a clock signal and the pilot tone signal generator is designed to emit a pilot tone signal in dependence on the synchronization signal.

Description

Technical Field

[0001] This invention relates to a pilot tone signal generator, a magnetic resonance imaging device, a method for transmitting synchronization signals, and a computer program product. Background Technology

[0002] Magnetic resonance imaging (MRI) is an imaging device used to image a subject, such as a human or animal patient. It uses an external strong magnetic field (B0 field) to orient the nuclear spins of the subject and then excites these spins to precess around that orientation using an alternating magnetic field (B1 field). The precession or return of the nuclear spins from this excitation state to a lower energy state generates an alternating magnetic field in response, which can be received by a receiving coil. For receiving MRI signals, a local receiving coil, also known as a local coil, is preferably used. This local coil is positioned directly on the subject to achieve a better signal-to-noise ratio (SNR).

[0003] Using a gradient magnetic field, a positional encoding is applied to the signal, which then allows for the correlation between the received magnetic resonance signal and volume elements. The received magnetic resonance signal is then typically reconstructed to create one or more magnetic resonance images.

[0004] Document US 10222443 B2 discloses a method that enables the transmission of pilot tones during magnetic resonance imaging (MRI) examinations based on a sequence of physiological movements such as respiration and / or heartbeats. This avoids motion artifacts (prospective motion correction) and / or eliminates motion artifacts during digital post-processing (retrospective motion correction). It proposes transmitting a weak, particularly constant, high-frequency (HF) pilot tone signal during MRI examinations, such that the pilot tone signal can be received by one or more receiving coils without interfering with the reception of the MRI signal. Detection of patient motion occurs by assessing the temporal trajectory of the amplitude and / or phase of the received pilot tone signal. A pilot tone signal generator for transmitting the pilot tone signal is exemplarily described in document US 10393845 B2. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a pilot tone signal generator capable of achieving improved motion correction. In particular, it is desirable to achieve more accurate evaluation of the pilot tone signal.

[0006] The technical problem described herein is solved by a pilot tone signal generator, a magnetic resonance tomography device for examining an object, a method for motion correction of magnetic resonance measurements, and a computer program product. Advantageous designs are described in the specification.

[0007] Therefore, a pilot tone signal generator is proposed, wherein the pilot tone signal generator includes a receiving unit for receiving a synchronization signal from the system control unit of a magnetic resonance imaging (MRI) device. Here, the synchronization signal includes a clock signal, and the pilot tone signal generator is designed to transmit a pilot tone signal according to the synchronization signal.

[0008] The pilot tone signal generator is preferably designed to transmit the pilot tone signal as a high-frequency electromagnetic signal to the object being examined. The pilot tone signal receiver, particularly the local coil and / or the body coil permanently installed in the magnetic resonance imaging device, is preferably designed to receive the pilot tone signal and transmit the pilot tone signal to the evaluation unit in order to assess information about physiological processes in and / or during the movement of the object being examined.

[0009] The pilot tone signal generator can be, for example, part of a local coil, particularly integrated and / or mounted within the local coil. Advantageously, the pilot tone signal generator can therefore be positioned particularly close to the object being inspected, so that the pilot tone signal emitted by the pilot tone signal generator and then received, for example, by the receiving antenna of the local coil advantageously has a particularly high signal-to-noise ratio.

[0010] The generated pilot tone signal preferably has a first frequency band, wherein the pilot tone signal receiver, in particular, is designed to record a receiving frequency band including the first frequency band. Preferably, the magnetic resonance imaging apparatus includes a high-frequency unit designed to output high-frequency pulses having a second frequency band, wherein the magnetic resonance signal of the high-frequency pulses is detected by the pilot tone signal receiver, wherein the magnetic resonance signal has a third frequency band at least substantially outside the first frequency band. Advantageously, the first frequency band does not conflict with the actual measurement signal, i.e., the magnetic resonance signal, which is located in the third frequency band.

[0011] The clock signal is preferably the clock signal of the magnetic resonance imaging (MRI) apparatus, and especially the reference clock signal. The clock signal is preferably suitable for transmitting the reference clock of the MRI apparatus. Advantageously, the pilot signal is phase-synchronized with the reference clock of the MRI apparatus. Advantageously, uncontrolled fluctuations in the absolute phase of the pilot signal relative to the reference clock of the MRI apparatus can therefore be avoided. Therefore, the absolute phase can be evaluated. This overcomes the limitation that phase evaluation might be confined to the relative phase between receiving channels. Instead, phase information used as a reference for receiving channels or channel combinations can be evaluated. Furthermore, signal-to-noise ratio loss in the pilot tone phase can be avoided by subtracting multiple measurement signals.

[0012] Based on the detection of the pilot signal, the phase fluctuations in the low-pass filter are converted into amplitude fluctuations. Synchronization using a synchronization signal can reduce phase fluctuations, thereby making the amplitude of the received pilot tone signal more stable.

[0013] One embodiment of the pilot tone signal generator specifies that the synchronization signal is an optical data signal, wherein the receiving unit includes a first sensor for receiving the optical data signal from the surrounding environment of the pilot tone signal generator, and the receiving unit is designed to generate an electrical sensor signal from the optical data signal.

[0014] Preferably, the optical data signal includes not only signals within the wavelength range of 380nm to 750nm that are normally visible to the human eye, but also signals within adjacent wavelength ranges, such as ultraviolet light between 150nm and 380nm or near-infrared light between 750nm and 2000nm. In particular, the optical data signal has a wavelength between 780nm and 1000nm. Specifically, the energy of the photon is greater than 0.8eV.

[0015] Here, the surrounding environment refers to the free space around the pilot tone signal generator, especially in a location suitable for the application, such as free space in a patient access corridor. Specifically, the environment is not understood to refer to the guiding optical connection, such as an optical waveguide, between the pilot tone signal generator and the magnetic resonance imaging equipment.

[0016] Preferably, the first sensor converts the optical data signal into an electrical signal, which is further used in a pilot tone signal generator to generate a pilot tone signal. The optical data signal here preferably transmits information from the magnetic resonance imaging device to the pilot tone signal generator. The sensor may, for example, have a photodiode, phototransistor, or other electronic component as a detector element, or may be a combination of electronic components, such as an amplifier, which converts the optical signal into an electrical signal or a sensor signal.

[0017] The wireless pilot tone signal generator advantageously allows information used to control the pilot tone signal generator to be transmitted without interference from the magnetic resonance receiver or in the event of interference with the operation of the magnetic resonance tomography equipment via optical sensors.

[0018] Another embodiment of the pilot tone signal generator specifies that the receiving unit includes a second sensor adjacent to the first sensor, wherein the first sensor is designed to generate a first output signal from the optical data signal, and the second sensor is designed to generate a second output signal from the same optical data signal, wherein the first output signal has an amplitude opposite to that of the second output signal, and wherein the pilot tone signal generator includes an inverter that inverts the output signal of the first sensor and an adder element, the adder element being designed to add the inverted output signal of the first sensor to the output signal of the second sensor to form the sensor signal.

[0019] Here, "adjacent" can specifically refer to the distance between the first and second sensors such that the electric and / or magnetic fields generated by the magnetic resonance imaging equipment have substantially the same intensity, thereby causing substantially the same interference in the first and second sensors. For example, this distance can be less than 2 mm, 5 mm, 1 cm, or 5 cm. Preferably, these sensors are arranged or oriented such that the optical signal source in the environment of the pilot tone signal generator achieves substantially the same effect, particularly in terms of intensity or quantity, in the detector elements of the first and second sensors, for example, generating approximately the same number of electron-hole pairs in both sensors.

[0020] Preferably, the first sensor is designed to generate a first output signal from the optical data signal, and the second sensor is designed to generate a second output signal from the same optical data signal. The first output signal preferably has an amplitude opposite to the second output signal; in other words, they are substantially the same in magnitude but different in sign. Here, the output signal is viewed with respect to, or relative to, an idle level or offset, which the first and second sensors generate, for example, without being affected by the optical signal, or the output signal is considered as an AC component of the signal generated by the sensor, having a frequency component greater than 1 Hz, 100 Hz, 1 kHz, 100 kHz, or 1 MHz. The opposite sign can be achieved, for example, by connecting the photodiode in the first sensor to a positive power supply voltage and then to a negative power supply voltage via a resistor, while connecting the photodiode in the second sensor to a negative power supply voltage and then to a positive power supply voltage via a resistor. In this case, output signals with different signs are applied to the connection point of the photodiode and the resistor, respectively.

[0021] Preferably, the pilot tone signal generator includes an inverter that inverts the output signal of the first sensor and an adder element, the adder element being used to add the inverted output signal of the first sensor to the output signal of the second sensor to form the sensor signal. The signal inversion can be achieved, for example, through an emitter circuit with a transistor.

[0022] The output signal generated by the optical signal is advantageously amplified by adding it with the same sign through opposite signs and subsequent inversion, while interferences from induction and / or magnetic induction are ideally canceled out by simple inversion during summation, thus greatly reducing interference components in the sensor signal.

[0023] Preferably, the synchronization signal, especially the clock signal, has amplitude modulation, wherein the pilot tone signal generator is designed to demodulate the optical data signal.

[0024] Preferably, the pilot tone signal generator includes a filter designed to select the carrier signal frequency or a first modulation frequency of the sensor signal. In other words, the filter has a local or preferably global minimum passband attenuation for a predetermined carrier signal frequency. Preferably, relative to the attenuation at the modulation frequency, the filter will attenuate the sensor signal at frequencies a distance of one octave, or frequencies equal to half or twice the carrier signal frequency, by more than 24 dB, 30 dB, or 36 dB. Depending on the spectrum of the modulation of the optical signal, the filter can preferably be designed as a bandpass filter, but can also be designed, for example, as a low-pass filter.

[0025] Another embodiment of the pilot tone signal generator specifies that the pilot tone signal generator includes a particularly narrow-band phase-locked loop (PLL) circuit for synchronizing the pilot tone signal, wherein the signal derived from the sensor signal is used as a reference signal for the PLL circuit.

[0026] Another embodiment of the pilot tone signal generator specifies that the pilot tone signal generator has a filter designed for selecting the modulation frequency of the sensor signal, wherein, in particular, the narrowband phase-locked loop circuit is designed to stabilize the oscillator according to the modulation frequency and preferably according to the modulation phase.

[0027] In particular, the narrowband phase-locked loop (PLL) circuit is designed to stabilize an oscillator, preferably a quartz oscillator and / or a voltage-controlled oscillator (VCO), based on the carrier signal frequency. The quartz oscillator can especially be a voltage-controlled crystal oscillator (VCXO). In the case of a VCO, the oscillator frequency can advantageously be changed by applying a voltage. The narrowband PLL circuit is particularly effective when the frequency deviation from the oscillator's inherent frequency is less than 100 ppm, 10 ppm, or 1 ppm. Advantageously, the narrowband PLL circuit is interference-resistant and can provide an accurate clock signal even with brief interruptions in the sensor signal.

[0028] Another embodiment of the pilot tone signal generator specifies that the pilot tone signal generator has a filter designed to select the odd-order harmonics of the output signal of the phase-locked loop circuit. For example, if the output signal of the phase-locked loop circuit has a frequency of 12.5 MHz, the selected harmonic can be, in particular, the 5th harmonic, which has a frequency of 62.5 MHz (equal to 5 times 12.5 MHz).

[0029] In particular, the oscillator, especially the voltage-controlled quartz oscillator, has a rectangular time signature. Therefore, the signal spectrum particularly contains odd-order harmonics. Thus, such an oscillator is especially well-suited for selecting odd-order harmonics. Odd-order harmonics, such as the fifth harmonic, can be selected, for example, by a bandpass filter and input into the transmitting antenna of a pilot tone signal generator, particularly a small conductor loop, for transmission. The alternating magnetic field generated during this process is coupled, for example, to the receiving antenna of a magnetic resonance imaging (MRI) device, for example, to a receiving antenna arranged in a local coil.

[0030] In one possible implementation of the pilot tone signal generator, the pilot tone signal generator includes an amplitude demodulator with compensation circuitry. The compensation circuitry is designed to compensate for signal components of the sensor signal that are lower in frequency than the modulation frequency of the modulating signal. In the context of this invention, low-frequency components can refer to spectral components of the signal demodulated by the amplitude demodulator whose frequencies are less than 10%, 1%, or one-thousandth of the modulation frequency. In other words, the frequency of this frequency component is less than 10%, 1%, or one-thousandth of the frequency of the useful signal to be transmitted, such as a synchronization signal, especially a clock signal and / or a control signal. In particular, components that are caused by movement of the pilot tone signal generator (e.g., by movement of a local coil in which the pilot tone signal generator is arranged) or by an external light source and are in the range below 200Hz, 120Hz, or 60Hz can be considered low-frequency.

[0031] Intensity changes caused by motion or shielding are usually slow signal changes, and due to this frequency difference, these signal changes can be advantageously separated from and / or suppressed by compensation circuitry from the clock frequency or data signal.

[0032] In one possible implementation of the pilot tone signal generator, the compensation circuit includes a comparator with a reference voltage input, which is connected to the sensor signal via a low-pass filter to form a first signal connection. Specifically, the compensation circuit has a differential amplifier connected as a comparator, also with a reference voltage input, which is connected to the sensor signal via an attenuator, preferably in the form of a resistive voltage divider, connected in series with the low-pass filter to form a first signal connection. Here, such a circuit can be considered as a comparator that compares two input signals and provides a comparison result in the form of a conduction-enabled output voltage when the difference between the two input signals is small. In this respect, the comparator is equivalent to a differential amplifier with high gain. For example, it can be considered that the reference voltage input is non-inverting and the sensor signal or a signal proportional to it is applied to the inverting input of the differential amplifier.

[0033] Thus, by using a low-pass filter, only the low-frequency components of the sensor signal are applied as the comparator reference signal to the non-inverting input of the differential amplifier. Therefore, the comparator reference signal advantageously follows slow changes, such as those caused by motion or obstruction. Preferably, a pre-divider reduces the voltage of the low-pass filtered reference signal so that this voltage is lower than the sensor voltage in the absence of control signal transmission, and the comparator output signal responds only to the components of the sensor signal caused by rapidly changing modulation signals.

[0034] In a possible implementation of the pilot tone signal generator, the first signal connection has a tracking and holding element that is manipulated based on the differential voltage between the sensor signal and a reference voltage. For example, an electronic switch can be arranged between the sensor signal source and a low-pass filter.

[0035] In this situation, when the sensor signal voltage falls below the reference voltage due to rapid modulation, the switch advantageously disconnects the sensor signal path to the low-pass filter, thereby keeping the reference voltage constant until the sensor signal voltage exceeds the reference voltage again due to the modulation content.

[0036] One possible implementation of the pilot tone signal generator specifies that the synchronization signal includes a control signal, wherein the pilot tone signal generator includes a control component adapted to control the amplitude and / or phase of the pilot tone signal by means of the control signal. The control signal is advantageously suited to transmit control commands for generating the pilot tone signal. For example, a high amplitude can be emitted to detect rapid motion, such as cardiac activity, and a low amplitude can be emitted to detect slow motion, such as respiratory activity, and the pilot tone signal can be turned off in measurements insensitive to motion.

[0037] One possible implementation of the pilot tone signal generator specifies that the control components of the pilot tone signal generator are also adapted to determine generator-specific information for the pilot tone signal generator by means of the control signal, so as to specifically control the pilot tone signal generator (especially relative to other pilot tone signal generators that may be present, arranged above, in, and / or around the magnetic resonance imaging device). In particular, specific phases and / or amplitudes can be applied to the corresponding pilot tone signals. The subsequently received pilot tone signals can then be advantageously separated by means of these applied characteristics, for example, by the evaluation unit of the magnetic resonance imaging device.

[0038] One possible implementation of the pilot tone signal generator specifies that the pilot tone signal generator includes a transmitting antenna and is designed to transmit a pilot tone signal through the transmitting antenna. In particular, the pilot tone signal generator may include decoupling elements for decoupling the transmitter output from the signal received by the antenna in the magnetic resonance imaging (MRI) apparatus via the excitation pulse of the MRI apparatus. For example, the decoupling elements include diodes or other components with nonlinear characteristics and / or components with frequency-dependent characteristics.

[0039] In the imaging area of ​​the subject during magnetic resonance imaging (MRI) measurements, the high-frequency field generated to excite nuclear spins, typically in the kilowatt range, can damage unprotected electronics of the pilot tone generator, especially when these electronics are unavoidably connected to an antenna exposed to the external high-frequency field. Decoupling elements advantageously ensure that the output of the pilot tone generator is decoupled from the excitation pulses incident through the antenna and is not damaged by these pulses.

[0040] Furthermore, a magnetic resonance imaging (MRI) device for examining a subject is proposed. The MRI device includes an optical transmitter designed to transmit optical data signals to at least one pilot tone signal generator via open-air optical transmission. Preferably, the optical transmitter is designed to radiate the optical data signals into a patient channel and / or distribute the optical data signals by scattering them across the surface of the patient channel.

[0041] Optical data signals can be, for example, synchronization signals. Synchronization signals can, in particular, include clock signals and / or control signals.

[0042] Preferably, the magnetic resonance imaging apparatus includes at least one receiving coil, wherein the at least one receiving coil is designed to receive a pilot tone signal emitted by at least one pilot tone signal generator. Furthermore, advantageously, the at least one receiving coil is also designed to receive magnetic resonance signals, thus requiring only one receiving coil to receive both the pilot tone signal and the magnetic resonance signal.

[0043] Preferably, the magnetic resonance imaging apparatus includes at least one evaluation unit for evaluating the pilot tone signal received by the receiving coil, wherein the evaluation unit is designed to determine the motion of the object being examined by means of the pilot tone signal received by the receiving coil. Motion correction can thus be advantageously implemented, resulting in better quality of the possible magnetic resonance images.

[0044] Preferably, the evaluation unit is designed to separate the pilot tone signals from multiple received pilot tone signal generators and, in particular, to allocate all or part of them to one of the multiple pilot tone signal generators that transmit the pilot tone signals. Advantageously, this allows for individual control of different pilot tone signal generators.

[0045] In one possible implementation of a magnetic resonance imaging (MRI) apparatus, the light emitter has an amplitude modulator. The amplitude modulator is preferably designed to change the light intensity synchronously with the modulation of the optical signal in order to transmit the optical signal, particularly the synchronization signal. For example, the amplitude modulator may have a sample-and-hold element or a trigger that switches the control signal to the multiplier of the amplitude modulator only by the edge of a clock signal. In particular, the amplitude modulator is understood herein as a device designed to also modulate the optical signal, forming an intermediate stage between a state of "off" with no light emission and a state of "on" with maximum intensity, and thus distinguishing it from the switches used for digital modulation described later.

[0046] Advantageously, by controlling the synchronous modulation of the signal, the phase of the modulated synchronization signal, especially the clock signal, remains unchanged and the phase-locked loop is not disturbed on the receiving side.

[0047] In a possible embodiment of the magnetic resonance tomography apparatus according to the invention, the transmitter is designed to, for example, turn the optical data signal on and off at a modulation frequency via amplitude-shift keying (ASK), and in particular on-off keying (OOK), in order to transmit a synchronization signal, wherein the magnetic resonance tomography apparatus is designed to change the frequency of the modulation signal from a first modulation frequency to a second modulation frequency in order to transmit a control signal, the second modulation frequency being not equal to the first modulation frequency.

[0048] Because the intensity of the optical signal switches abruptly in sync with the modulation frequency, the optical sensor provides a square wave sensor signal whose fundamental frequency is equal to the first or second modulation frequency. In conjunction with a bandpass filter, as previously described, which is part of the signal path of the sensor signal, or other filters with frequency-dependent attenuation, the spectral components of the square wave signal (e.g., harmonics, especially the fundamental frequency) are advantageously selected for further processing.

[0049] In the case of nonlinear light sources such as LEDs or semiconductor lasers, changing the modulation frequency can generally be achieved much more easily and precisely than directly controlling the brightness. Preferably, only the modulation frequency is changed, through which the switch turns the light source on and off.

[0050] In a conceivable embodiment of the magnetic resonance imaging apparatus according to the invention, the frequency of the clock signal is an odd multiple of the modulation frequency. For example, the clock signal may have a frequency of 12.5 MHz. In this case, the modulation frequency is preferably switched between a first modulation frequency and a second modulation frequency, wherein, in this example, the first modulation frequency is 12.5 MHz and the second modulation frequency is one-third of it, i.e., the frequency of the clock signal is equal to the first modulation frequency multiplied by 1 and equal to the second modulation frequency multiplied by 3. The switching of the modulation frequency is advantageously performed in a phasenneutral manner, i.e., within a time interval (Zeitraster) of either the clock frequency or the first modulation frequency.

[0051] When an optical signal is modulated by switching it on or off, a square wave signal is generated. This square wave signal has harmonics that are odd multiples of the modulation frequency. Therefore, if the second modulation frequency is one-third of the clock signal frequency, the third harmonic is three times the modulation frequency or exactly the clock signal frequency. Here, a bandpass filter can be used to select the spectral component at the clock signal frequency and can suppress or attenuate all other harmonics, for example, by more than 24dB, 30dB, or 36dB. Thus, a clock signal of, for example, 12.5MHz is provided by both the first and second modulation frequencies. Due to the phase-neutral switching between the two modulation frequencies, the phase of the transmitted clock signal remains unchanged, thus providing a phase-stable output signal for the pilot tone signal generator, for example, through a subsequent phase-locked loop circuit. However, for a symmetrical square wave signal without a DC component, the amplitude of the third harmonic is only one-third of the amplitude of the fundamental wave (first harmonic), thereby generating amplitude modulation of the bandpass-filtered received signal between one-third and full amplitude via frequency shift keying. In particular, control signals can be transmitted by amplitude modulation of the selected spectral components. These control signals are used, for example, to control the generation of pilot tone signals. In the case of digital encoding transmitted via this amplitude modulation, different control commands can be transmitted accordingly, and a phase-stable clock signal can be provided simultaneously. The modulation can preferably be provided by a simple switch.

[0052] Furthermore, a method for motion correction of magnetic resonance measurements of an object under examination is proposed, the method comprising:

[0053] - Transmit the synchronization signal of the magnetic resonance imaging equipment to at least one pilot tone signal generator, and in particular the receiving unit of the at least one pilot tone signal generator;

[0054] - At least one pilot tone signal is generated and emitted by the at least one pilot tone signal generator according to the synchronization signal;

[0055] - The pilot tone signal is received by at least one receiving coil of the magnetic resonance imaging device;

[0056] - Motion correction triggering is performed based on the pilot tone signal.

[0057] The advantages of the method for motion correction of magnetic resonance measurements are essentially consistent with those of pilot tone signal generators and magnetic resonance tomography equipment, which have been described in detail above. The features, advantages, or alternative embodiments mentioned herein can also be applied to other claimed technical solutions, and vice versa.

[0058] In other words, the pilot tone signal generator and the magnetic resonance imaging equipment can also be extended in design using the features described or claimed in relation to the method. The corresponding functional features of the method are formed here by the corresponding specific modules, especially hardware modules.

[0059] The transmission of the synchronization signal, the transmission of the pilot tone signal, and the reception of the pilot tone signal occur during the magnetic resonance measurement. That is, in addition to the pilot tone signal, the magnetic resonance signal is also measured.

[0060] Preferably, the pilot tone signal generator is positioned close to the heart or lungs of the subject being examined. This advantageously allows for a high signal-to-noise ratio of the received pilot tone signal.

[0061] The synchronization signal of the magnetic resonance imaging (MRI) device is preferably transmitted wirelessly to the receiving unit of the pilot tone signal generator; that is, the synchronization signal is an optical data signal. Preferably, the synchronization signal is received by the sensor of the receiving unit, especially the first sensor, and output as an output signal.

[0062] Preferably, the pilot tone signal is generated by the pilot tone signal generator using the output signal, which includes one or more of the following aspects:

[0063] - Use a filter to filter the output signal or the signal derived from it;

[0064] - Stabilize the oscillator's clock frequency using the output signal or a signal derived from it;

[0065] - Select the odd harmonics of the output signal or the signal derived from it.

[0066] Preferably, the synchronization signal of the magnetic resonance imaging (MRI) device is transmitted to multiple pilot tone signal generators. Preferably, specific information for each pilot tone signal generator is transmitted using a time-division multiplexing method.

[0067] In particular, generator-specific information of the synchronization signal is identified, and corresponding pilot tone signals are generated by multiple pilot tone signal generators based on this generator-specific information.

[0068] Preferably, the contribution of the pilot tone signal received by at least one receiving coil of the magnetic resonance imaging apparatus is separated. Advantageously, this can significantly improve the information density of the pilot tone data, thereby achieving improved motion correction.

[0069] In order to implement motion correction using pilot tone signals, signal analysis in the evaluation unit of a magnetic resonance imaging (MRI) device is used to identify motion-related changes in the received pilot tone signals. Attached Figure Description

[0070] Other advantages, features, and details of the invention will become apparent from the embodiments described below and with reference to the accompanying drawings. Corresponding parts are given the same reference numerals in all the drawings.

[0071] In the attached diagram:

[0072] Figure 1 A magnetic resonance imaging device with a pilot tone signal generator is shown.

[0073] Figure 2 An optical transmitter according to the first embodiment is shown;

[0074] Figure 3 A pilot tone signal generator suitable for processing clock signals is shown;

[0075] Figure 4 An optical transmitter according to a second embodiment is shown;

[0076] Figure 5 A pilot tone signal generator suitable for processing clock and control signals is shown;

[0077] Figure 6 The receiving unit of the pilot tone signal generator is shown;

[0078] Figure 7 The compensation circuit of the pilot tone signal generator is shown;

[0079] Figure 8 This paper demonstrates a method for motion correction of magnetic resonance measurements using a pilot tone signal generator. Detailed Implementation

[0080] Figure 1A schematic diagram illustrating an embodiment of a magnetic resonance imaging (MRI) device 1 is shown. The MRI device 1 includes a magnet unit 10 comprising a field magnet 11 that generates a static magnetic field B0 for orienting the nuclear spin of the subject, in this case, patient 100, within the imaging region of the MRI device 1. The imaging region is characterized by a highly uniform static magnetic field B0, wherein this uniformity is particularly related to the strength or quantity of the magnetic field. The imaging region is nearly spherical and is arranged within a patient passage 16 extending longitudinally 2 through the magnet unit 10. A patient bed 30 is movable within the patient passage 16 by a movement unit 36. The field magnet 11 is typically a superconducting magnet, which can provide magnetic fields with flux densities up to 3 T, and even higher in the case of state-of-the-art equipment. However, permanent magnets or electromagnets with normally conductive coils can also be used for lower magnetic field strengths.

[0081] Furthermore, the magnet unit 10 has a gradient coil 12, which is designed to superimpose a magnetic field B0 with a temporally and spatially variable magnetic field in three spatial directions for spatial differentiation of the detected imaging region in the examination volume. The gradient coil 12 is typically a coil made of normally conductive wire, which can generate mutually orthogonal fields in the examination volume.

[0082] The magnet unit 10 also has a body coil 14, which is designed to radiate high-frequency signals input via signal lines into the examination volume. Furthermore, it is conceivable that the body coil 14 can receive magnetic resonance signals emitted by the patient 100 and transmit them via signal lines; in this case, the body coil 14 functions as a receiving coil.

[0083] The system control unit 20 provides different signals to the magnet unit 10 for the gradient coil 12 and the body coil 14 and analyzes the received signals.

[0084] Therefore, the system control unit 20 has a gradient manipulator 21, which is designed to provide variable currents to the gradient coil 12 via feed lines, which provide the desired gradient field in a time-coordinated manner within the inspection volume.

[0085] Furthermore, the system control unit 20 has a high-frequency unit 22 designed to generate high-frequency pulses with a predetermined time course, amplitude, and spectral power distribution to excite the magnetic resonance of nuclear spins in the patient 100. Pulse power in the kilowatt range can be achieved here. The excitation signal can be radiated into the patient 100 via the body coil 14 or via a local transmitting antenna.

[0086] The controller 23 communicates with the gradient controller 21 and the high-frequency unit 22 via the signal bus 25.

[0087] A local coil 80 with a pilot tone signal generator 50 is arranged on the patient 100. This local coil receives and transmits magnetic resonance signals from the patient 100's body; it also functions as a receiving coil. This transmission can be wireless, preferably via a radio connection.

[0088] Advantageously, the receiving coil has a sufficiently wide bandwidth, so that the receiving coil can receive not only magnetic resonance signals, but also pilot tone signals with different frequency bands than magnetic resonance signals.

[0089] The magnetic resonance signal and pilot tone signal received by the local coil 80 are transmitted to the evaluation unit 24 of the system control unit 20, where the pilot tone signal is evaluated in particular. If the patient 100 moves during the magnetic resonance measurement, the pilot tone signal is affected or altered by the movement. The movement of the patient 100 can be inferred from the signal change. For example, information about the patient's breathing or heartbeat can be derived from the pilot tone signal. By evaluating the pilot tone signal, motion information can thus be determined, which can be used for motion correction and / or for sequence triggering.

[0090] The evaluation of the pilot tone signal can be improved by evaluating not only its amplitude but also its phase. The phase can be defined, in particular, relative to the reference clock of the magnetic resonance imaging (MRI) apparatus 1. The system control unit 20 of the MRI apparatus 1 has a corresponding clock signal internally. For transmission to the pilot tone signal generator 50, the system control unit 20 also has an optical transmitter 70, wherein a plurality of optical transmitters 71 emitting light are arranged in the patient channel 16. The MRI apparatus preferably has a plurality of light transmitters 71 distributed on the inner surface of the patient channel 16 or on the inner surface from which scattered light is irradiated, thereby minimizing the possibility of the optical signal emitted by the light transmitters 71 being blocked at the pilot tone signal generator 50. The light transmitters 71 can be LEDs or semiconductor lasers, which receive electrical signals input from the optical transmitters 70 and convert these electrical signals into optical signals and transmit them into the patient channel 16. Alternatively, the LED radiator or semiconductor laser in the optical emitter 70 may have been converted into light and the light is passed through a glass fiber and optionally guided to the patient channel 16 via a beam splitter, wherein the glass fiber end is arranged in the patient channel 16 as a light emitter 71.

[0091] exist Figure 2 An exemplary embodiment of the optical transmitter 70 of the magnetic resonance imaging apparatus according to the present invention is shown. The optical transmitter 70 has an amplitude modulator 72. In the simplest case, the amplitude of the signal is modulated only by the reference clock of the reference clock of the magnetic resonance imaging apparatus 1.

[0092] Optionally, a high-frequency clock signal can be modulated using a low-frequency control signal. For example, amplitude modulator 72 may include a multiplier that multiplies the clock signal with the low-frequency control signal. Modulation of the clock signal is preferably phase-synchronized, i.e., performed via the rising or falling edge of the clock signal. This can be achieved, for example, by switching the control signal to the multiplier via a sample-and-hold element controlled by the clock signal, for example, switching when the clock signal is low. The output signal of amplitude modulator 72 is output directly or via an output stage to one or more LEDs or LED radiators or semiconductor lasers for conversion into an optical signal.

[0093] exist Figure 3 The exemplary embodiment shown illustrates components of the pilot tone signal generator 50, which are involved in the transmission or recovery of clock and control signals. Other components of the pilot tone signal generator 50 are not shown for clarity.

[0094] The first optical sensor 51, such as a photodiode with a preamplifier, converts the optical signal into an electrical signal. The filter 53, such as a bandpass filter or low-pass filter, preferably allows signals with a clock signal frequency to pass through and attenuates signals with other frequencies, for example, attenuating them by more than 24 dB, 30 dB, or 36 dB.

[0095] The filtered signal is input to other components to generate a stable master clock for the pilot tone signal generator 50. Here, the filtered signal from the first sensor 51 is first amplified in the limiting amplifier 56, so that amplitude fluctuations are eliminated by amplitude limiting and only the phase information of the carrier is retained. The frequency and phase of the oscillator 58, especially the voltage-controlled quartz oscillator, are thus stabilized in the PLL adjustment circuit. The output signal of the oscillator 58, or the signal derived therefrom, is the master clock of the pilot tone signal generator 50.

[0096] Filter 62 selects the odd-order harmonics of the output signal of phase-locked loop circuit 57. For example, it selects the 62.5MHz fifth harmonic of the 12.5MHz fundamental frequency.

[0097] The pilot tone signal generator 50 also includes a transmitting antenna 64 for transmitting a pilot tone signal. In this example, the transmitting antenna 64 and the decoupling element 63 form a resonant circuit. The decoupling element 63 here includes two diodes connected in anti-parallel, which are grounded before the transmitting antenna 64. This ensures that the voltage induced in the transmitting antenna 64 by the excitation pulse of the magnetic resonance imaging device 1 is limited to below the positive voltage.

[0098] exist Figure 4Another possible implementation of the optical transmitter 70 is shown. This implementation is based on the idea that a square wave signal has harmonic components with a frequency equal to an odd multiple of the fundamental frequency. An exemplary example shown here is a square wave signal with a high frequency (also referred to as the first modulation frequency), for example 12.5 MHz, and a square wave signal with a lower frequency, or second modulation frequency (12.5 / 3) MHz, as the input signal to the optical transmitter. The system controller 20 provides these high-precision signals derived from a stable master clock. Therefore, even if the optical transmitter 70 transmits an optical signal with the second modulation frequency, this optical signal still has a frequency component with the first modulation frequency of 12.5 MHz.

[0099] Driven by a first or second modulation signal clock, the optical transmitter electronically switches on and off the power supply to the LED or semiconductor laser, which serves as the light emitter 71 or light source, and thus generates an optical signal modulated with a square wave signal. Preferably, multiple LEDs distributed on the patient channel 16 are simultaneously turned on to avoid obscuring the sensor 51. Alternatively, the inner wall of the channel can be advantageously designed to scatter light and be illuminated from the outside of the channel. Advantageously, modulation with a square wave signal can be easily achieved by switching and is more efficient than linear intensity modulation.

[0100] Due to the transition between the first and second modulation frequencies, there is always a signal component with the first modulation frequency present due to harmonics, which is selected by filter 53 in the pilot tone signal generator. Since the harmonic component always has a smaller amplitude than the fundamental frequency, the transition between modulation frequencies results in amplitude modulation within the spectral range of the first modulation frequency, which can be, for example, used in... Figure 5 The pilot tone signal generator 50 in the PLL is evaluated. If the signals with the first modulation frequency and the second modulation frequency are in a phase-stable relationship, preferably with the edge of the signal with the second modulation frequency synchronized with the edge of the signal with the first modulation frequency, then the clock signal generated by the PLL is not affected by frequency shift keying. This can be achieved, for example, by obtaining the second modulation frequency from the first modulation frequency through phase synchronization frequency division.

[0101] exist Figure 5 The diagram shows the pilot tone signal generator and... Figure 3 Compared to the extended implementation, here the filtered signal is input to the amplitude demodulator 54 in the demodulation branch to recover the control signal modulated onto the clock signal. In the simplest case, the amplitude demodulator 54 can be composed of a diode as a rectifier and a low-pass filter or a buffer capacitor.

[0102] Typically, the demodulated signal still exhibits fluctuations, which may be caused by masking effects, for example, due to changes in the propagation path within the patient channel. These fluctuations can be compensated for using compensation circuit 55, which is related to subsequent... Figure 7 To be explained.

[0103] The phase control element 66 and / or the amplitude control element 67 can be controlled by the control signal of the synchronization signal via the control component 65. Therefore, the generation of the pilot tone signal can be controlled by the control signal. In particular, the amplitude can be controlled by the amplitude control element 67 and the phase by the phase control element 66. For example, in the detection of rapid motion, such as cardiac motion, a high pilot tone amplitude can be emitted, while in the detection of slow motion, such as breathing, a low pilot tone amplitude can be emitted; in motion-insensitive magnetic resonance measurements, the pilot tone signal can be turned off.

[0104] Optionally, the control unit 65 includes an ID decoder designed to determine, in particular, identification information specific to the pilot tone signal generator 50 from the control signals. This is especially advantageous when multiple pilot tone signal generators are used in magnetic resonance measurements.

[0105] By utilizing generator-specific information for the pilot tone signal generator 50, it is possible to specifically adjust the relative phase and amplitude to, for example, optimize the "illumination" of the patient 100 with the pilot tone signal. Furthermore, the amplitude and phase can switch on a timescale faster than the motion to be detected. Through the time-division multiplexing method known in the evaluation unit 24, particularly "time / phase multiplexing," the contribution of each pilot tone signal generator to the overall signal can be separated, and the information density of the pilot tone signal can be significantly improved.

[0106] Figure 6 An advantageous combination of a first sensor 51 and a second sensor 52 for detecting optical data signals is schematically shown. This combination, in particular, reduces electromagnetic interference to the reception of optical data signals caused by the operation of the magnetic resonance tomography apparatus 1.

[0107] For this purpose, the first sensor 51 and the second sensor 52 are arranged adjacent to each other to minimize induction in the connecting line and to expose the two sensors 51, 52 to the same electromagnetic field as much as possible. This distance is preferably less than 2 cm, 1 cm or 5 mm.

[0108] Here Figure 6 In this configuration, the first sensor 51 (here, a photodiode) is directly connected to the positive power supply voltage along the cutoff direction, while it is connected to the ground potential through a resistor.

[0109] For the second sensor 52, the resistor and sensor are interchanged; that is, the resistor is directly connected to the positive power supply voltage, while the second sensor 52 is connected to ground potential. Through this interchanged arrangement, the same optical signals from the two sensors 51 and 52 generate electrical signals with comparable amplitudes but opposite signs.

[0110] The electrical signals generated by sensors 51 and 52 are amplified and input to differential amplifier 59 at either inverting or non-inverting input terminals. Advantageously, the electrical signals induced by the optical signals at sensors 51 and 52 are added at the output of differential amplifier 59 due to their different signs. Conversely, electromagnetic interference preferably induces interference signals with the same sign in both branches, so that these interference signals substantially cancel each other out in differential amplifier 59. Simultaneously, the signal-to-noise ratio (SNR) of the combined signal is 3 dB higher than that of the individual signals because the noise contributions from these electrical components are uncorrelated and therefore add in terms of power, while the correlated received signals add in terms of voltage.

[0111] exist Figure 7 The diagram schematically illustrates a possible implementation of the compensation circuit 55. The filtered and demodulated sensor signal is input to the inverting input of comparator 60. Simultaneously, the sensor signal is down-divided by a resistor divider, low-pass filtered by an RC element, and input to the non-inverting input of comparator 60. The capacitance of the RC element also serves as the charging capacitor for a track-and-hold element, whose electrical switch is here designed as a MOS-FET. The track-and-hold element is controlled by the output of comparator 60, so that the switch of the track-and-hold element opens when the sensor signal becomes less than the reference signal. The comparator can be implemented, for example, as a differential amplifier with high gain or other equivalent circuitry.

[0112] exist Figure 8 The diagram illustrates a method for motion correction of magnetic resonance measurements of an examination subject. In S10, the examination subject, particularly patient 100, is positioned within the magnetic resonance imaging device. In S20, a synchronization signal from the magnetic resonance imaging device 1 is transmitted to at least one pilot tone signal generator 50, particularly the receiving unit of the at least one pilot tone signal generator.

[0113] In S30, a pilot tone signal is generated and transmitted by the at least one pilot tone signal generator 50 using a synchronization signal. When a pilot tone signal is generated, in S31, the output signal and / or the signal derived from the output signal are filtered using a filter. In S32, generator-specific information of the synchronization signal is identified, and corresponding pilot tone signals are generated by the multiple pilot tone signal generators based on this generator-specific information. In S33, the clock frequency of the oscillator is stabilized using the output signal or the signal derived therefrom. In S34, the odd-order harmonics of the output signal or the signal derived therefrom are selected.

[0114] It is conceivable that steps S31-S34 can also be performed in parallel and / or repeatedly in other different orders.

[0115] In S40, a pilot tone signal is received by at least one receiving coil of the magnetic resonance imaging device. In S50, motion correction and / or sequence triggering are performed using the pilot tone signal.

[0116] Finally, it should be reiterated that the methods described in detail above, as well as the pilot tone signal generator and magnetic resonance imaging device shown, are merely embodiments. Those skilled in the art can modify these embodiments in various ways without departing from the scope of the invention. Furthermore, the indefinite article "a" does not preclude the possibility that multiple related features may exist. Similarly, the term "unit" does not preclude that a related component may consist of multiple sub-components that work together and, if necessary, can be spatially distributed.

Claims

1. A pilot tone signal generator, in, The pilot tone signal generator (50) includes a receiving unit, which is used to receive the synchronization signal from the system control unit (20) of the magnetic resonance imaging device (1). The synchronization signal includes a clock signal. The pilot tone signal generator (50) is designed to transmit pilot tone signals according to the synchronization signal. The synchronization signal is an optical data signal. The receiving unit includes a first sensor (51) for receiving optical data signals from the surrounding environment of the pilot tone signal generator (50), and the receiving unit is designed to generate electrical sensor signals from the optical data signals. The receiving unit includes a second sensor (52) adjacent to the first sensor (51). The first sensor (51) is designed to generate a first output signal from the optical data signal, and the second sensor (52) is designed to generate a second output signal from the same optical data signal. Wherein, the first output signal has an amplitude opposite to that of the second output signal. The pilot tone signal generator (50) includes an inverter that inverts the output signal of the first sensor (51) and an adder element. The adder element is designed to add the inverted output signal of the first sensor to the output signal of the second sensor (52) to form the sensor signal.

2. The pilot tone signal generator according to claim 1, in, The pilot tone signal generator (50) includes a phase-locked loop circuit (57) for synchronizing the pilot tone signal. The signal derived from the sensor signal is used as a reference signal for the phase-locked loop circuit (57).

3. The pilot tone signal generator according to claim 2, in, The pilot tone signal generator (50) has a filter (53) designed to select the modulation frequency of the sensor signal. The phase-locked loop circuit (57) is designed to stabilize the oscillator (58) according to the modulation frequency and the modulation phase.

4. The pilot tone signal generator according to claim 2, wherein, The pilot tone signal generator (50) has a filter (62) designed to select the odd harmonics of the output signal of the phase-locked loop circuit (57).

5. The pilot tone signal generator according to claim 1, wherein, The pilot tone signal generator (50) includes an amplitude demodulator (54) with a compensation circuit (55) designed to compensate for low-frequency signal components of the sensor signal compared to the modulation frequency of the sensor signal.

6. The pilot tone signal generator according to claim 1, in, The synchronization signal includes a control signal. The pilot tone signal generator (50) includes a control component (65) adapted to control the amplitude and / or phase of the pilot tone signal by means of the control signal.

7. The pilot tone signal generator according to claim 6, wherein, The control unit (65) of the pilot tone signal generator (50) is adapted to determine generator-specific information for the pilot tone signal generator (50) by means of the control signal, so as to specifically control the pilot tone signal generator.

8. A magnetic resonance tomography (MRI) device (100) for examining an object to be examined. in, The magnetic resonance tomography device (1) includes an optical transmitter (70), The optical transmitter (70) is designed to transmit optical data signals to at least one pilot tone signal generator (50) according to any one of claims 1 to 7 via open-air optical transmission.

9. The magnetic resonance tomography device according to claim 8, in, The optical transmitter (70) is designed to transmit a synchronization signal to at least one pilot tone signal generator (50) via optical open-air transmission.

10. The magnetic resonance tomography device according to claim 8, in, The magnetic resonance imaging device (1) includes at least one receiving coil (80). The at least one receiving coil (80) is designed to receive a pilot tone signal emitted by at least one pilot tone signal generator (50).

11. The magnetic resonance tomography apparatus according to claim 10, wherein, The evaluation unit (24) is designed to separate the pilot tone signals from multiple received pilot tone signal generators.

12. The magnetic resonance tomography device according to claim 8, wherein, The optical transmitter (70) is designed to radiate the optical data signal into the patient channel and / or distribute the optical data signal by scattering it on the surface of the patient channel.

13. A method for motion correction of magnetic resonance measurements, the method comprising: - Transmit the synchronization signal of the magnetic resonance imaging device (S20) to the receiving unit of the pilot tone signal generator according to any one of claims 1 to 7; - At least one pilot tone signal is generated and transmitted by the pilot tone signal generator according to the synchronization signal (S30); - The pilot tone signal is received (S40) by at least one receiving coil of the magnetic resonance tomography apparatus; - Perform motion correction and / or sequence triggering based on the pilot tone signal (S50).

14. A computer program product comprising a program and capable of being directly loaded into the memory of a programmable system control unit (20) of a computed tomography apparatus (1), the computer program product having program elements for implementing the method according to claim 13 when the program is run in the system control unit (20) of the computed tomography apparatus (1).

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