DE102021206487A1 - Device and method for transmitting clock signals from a magnetic resonance tomograph to a peripheral device - Google Patents

Abstract

The invention relates to a peripheral device for a magnetic resonance tomograph, a system made up of a magnetic resonance tomograph and a peripheral device, and an operating method. The peripheral device has a first sensor for receiving an electromagnetic data signal from the environment of the peripheral device. The peripheral device is designed to carry out signal processing as a function of the electromagnetic data signal and a frequency of the electromagnetic data signal is greater than a Larmor frequency of the magnetic resonance tomograph.

Description

The invention relates to a device for transmitting a clock signal and/or control signal from a magnetic resonance tomograph to a peripheral device.

Magnetic resonance tomographs are imaging devices which, in order to image an examination object, align nuclear spins of the examination object with a strong external magnetic field and excite them to precess around this alignment by means of an alternating magnetic field. The precession or return of the spins from this excited state to a lower-energy state in turn generates an alternating magnetic field in response, which is received by antennas.

With the help of magnetic gradient fields, a spatial coding is impressed on the signals, which subsequently enables the received signal to be assigned to a volume element. The signal received is then evaluated and a three-dimensional imaging representation of the examination object is provided. To receive the signal, local receiving antennas, so-called local coils, are preferably used, which are arranged directly on the examination object to achieve a better signal-to-noise ratio.

Peripheral or accessory devices that do not have a direct interface to the MRI system, such as communication devices, music/video entertainment devices, patient monitors for monitoring vital signs, cameras, motor-driven mobile devices are increasingly being operated within the shielded cabins of MRI systems Lie. The signal processing within the devices is usually clocked with signals from free-running oscillators. The clock signals do not run synchronously with the system clock of the MR system.

Since MRT systems have very sensitive reception systems, which, due to the use of multidimensional Fourier analysis techniques, make continuous interference signals particularly prominent, artefacts can potentially easily occur in the imaging. This interference can arise when harmonics of the clock signals fall spectrally either directly in the reception range of the MRI system or appear in a reception range or an alias band of the analog-digital conversion due to intermodulation with other signals that are at least temporarily continuously running. In addition, methods for generating movement information, e.g. pilot tone methods, can be disrupted by harmonics of the clock signals.

An additional problem is that the accessory devices cannot be controlled from outside the shielded cabin, for example via the user interface of the magnetic resonance tomograph.

It is therefore an object of the present invention to provide peripheral devices and a magnetic resonance tomograph that enable improved image acquisition.

This object is achieved by a peripheral device according to claim 1 and a magnetic resonance tomograph according to claim 8 and a method according to claim 14.

The peripheral device according to the invention has a first sensor for receiving an electromagnetic data signal from the environment of the magnetic resonance tomograph. Electromagnetic data signals are electromagnetic waves, including those in the wavelength range of visible light.

The environment is considered to be the free space surrounding the magnetic resonance tomograph, in particular in a position appropriate to the application, for example in the vicinity of the openings of the patient tunnel where the operator is working, or also in a patient tunnel with a patient. The distance in the vicinity is preferably less than 10 m, 5 m or 2 m from the magnetic resonance tomograph. The environment is within a screened cabin, if available. In particular, this does not mean a guided connection such as a cable or an optical waveguide between the peripheral device and the magnetic resonance tomograph, but rather an open-air transmission.

The first sensor converts the electromagnetic data signal into an electrical signal, which is further used in the peripheral device for signal processing dependent on the electromagnetic data signal. The electromagnetic data signal transmits information from the magnetic resonance tomograph to the peripheral device. The electromagnetic data signal can transmit a clock signal, for example, which is the basis of signal processing in the peripheral device. In this case, the electromagnetic data signal has a frequency that is greater than a Larmor frequency of the magnetic resonance tomograph for nuclear spins that are to be recorded with the magnetic resonance tomograph in an image recording. It is preferably the Larmor frequency of hydrogen nuclei in a static magnetic field B0 of the magnetic resonance tomograph. A frequency that is more than twice, three times or ten times the frequency of the frequency is preferably considered to be greater Larmor frequency is. The electromagnetic data signal can be, for example, visible light or infrared light or a radio wave, preferably in a license-free ISM band such as 2.4 GHz, 5.8 GHz, 24 GHz or 61 GHz as the transmission medium. The information is modulated onto the transmission medium, preferably by means of amplitude modulation. It is also conceivable that a carrier wave is modulated onto the transmission medium, onto which a control signal or other information to be transmitted is modulated. In a preferred embodiment, for example, a clock signal is modulated onto the transmission medium of light or radio waves, the clock signal in turn being modulated as a carrier wave with a control command or other information to be transmitted.

The sensor can have, for example, an antenna for radio signals or a photodiode, a phototransistor or another electronic element as a detector element for optical signals, or it can be a combination of electronic components, for example with an amplifier that converts the electromagnetic data signal into an electrical signal or sensor signal .

The peripheral device according to the invention advantageously allows transmission of information for controlling the peripheral device via the sensor without disrupting the magnetic resonance reception or cables interfering with handling. In particular, the frequency of the electromagnetic data signal being greater than the Larmor frequency ensures that no harmonic frequencies interfere with the reception of the magnetic resonance signal.

The magnetic resonance tomograph according to the invention has a transmitter which is designed to wirelessly transmit a clock signal and/or control signal by means of the electromagnetic data signal to a peripheral device in the vicinity of the magnetic resonance tomograph. The transmitter can have, for example, an oscillator and an antenna for a radio signal and a light source such as an LED or semiconductor laser for an optical data signal. In this case, a frequency of the electromagnetic data signal is greater than a Larmor frequency of the magnetic resonance tomograph.

The information on the signals and frequencies given above for the peripheral device applies here in the same way.

The terms first modulation frequency and second modulation frequency are used below. In the case of analog modulation using a multiplier, for example, the carrier signal to be modulated or the carrier frequency to be modulated corresponds to the first modulation frequency, which preferably corresponds to the frequency of the clock signal to be transmitted. In a preferred embodiment explained below, frequency shift keying takes place between two widely spaced frequencies, which are therefore referred to as the first modulation frequency and the second modulation frequency and do not represent a carrier signal in the strict sense that is modulated. A signal with the respective frequency can also be referred to as the first or second modulation frequency.

In this way, a data signal can advantageously be transmitted from the magnetic resonance tomograph to the peripheral device without disrupting the magnetic resonance reception.

Further advantageous embodiments are explained in the dependent claims.

In one possible embodiment of the peripheral device according to the invention, the electromagnetic data signal is an optical data signal. An optical data signal is not only a signal in the wavelength range visible to the eye from 380 nm to 750 nm, but also in neighboring wavelength ranges such as ultraviolet between 150 nm and 380 nm or near infrared between 750 nm and 2000 nm. The energy of the light quanta is preferred greater than 0.8 eV.

Advantageously, optical signals are easy to generate and receive, and they are not subject to any regulation in terms of the intensities used, so that no additional approval criteria need to be taken into account.

In a conceivable embodiment of the peripheral device according to the invention, the peripheral device has a second sensor that is adjacent to the first sensor. A distance between the first sensor and the second sensor is considered to be adjacent where the electric and/or magnetic fields generated by the magnetic resonance tomograph have essentially the same strength, so that interference caused by these fields essentially occurs in the first and the second sensor are identical. The distance can be less than 2 mm, 5 mm, 1 cm or 5 cm, for example. The sensors are preferably arranged or aligned in such a way that an optical signal source in the vicinity of the peripheral device achieves essentially the same effect in a detector element of the first sensor and the second sensor, in particular with regard to strength or magnitude, for example an approximately equal effect Generate number of electron-hole pairs in the two sensors.

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 has an amplitude that is inverse to the second output signal, in other words the absolute value is essentially the same but the sign is different. The output signal is considered in relation to or relative to a rest level or offset, which the first sensor and the second sensor generate, for example, without the influence of an optical signal, or as an AC component of a signal generated by the sensors with frequency components greater than 1 Hz, 100 Hz, 1kHz, 100kHz or 1MHz. Opposite signs can be achieved, for example, by connecting a photodiode in the first sensor to the positive supply voltage and via a resistor to the negative supply voltage, while a photodiode in the second sensor is connected to the negative supply voltage and via a resistor to the positive supply voltage is connected. The output signal with different signs is then present at the connection points of the photodiodes with the resistors.

The peripheral device has an inverter that inverts the output signal of the first sensor, and a summing element that is designed to add the inverted output signal of the first sensor and the output signal of the second sensor to form a sensor signal. The signal can be inverted, for example, by an emitter circuit with a transistor.

Output signals generated by optical signals are advantageously added by the inverse sign and subsequent inversion with the same sign and thus amplified, while electrically and/or magnetically induced interference is ideally canceled out by the simple inversion during the summation, so that the interference component in the sensor signal is significantly reduced.

In one possible embodiment of the peripheral device according to the invention, the peripheral device has a filter which is designed to select a carrier signal frequency or a first modulation frequency of the sensor signal. In other words, the filter has a local or preferably also global minimum transmission loss for a predetermined carrier signal frequency. Preferably, the filter attenuates a sensor signal at a frequency one octave apart or at a frequency equal to half or twice the carrier signal frequency from attenuation at the modulation frequency by more than 24 dB, 30 dB or 36 dB. The filter can preferably be designed as a bandpass filter, but also, for example, as a lowpass filter, depending on the frequency spectrum of the optical signal.

Furthermore, the peripheral device has a narrow-band phase lock loop circuit (PLL circuit) which is designed to stabilize a stable oscillator, preferably a quartz oscillator, as a function of the carrier signal frequency. According to the invention, a narrow-band PLL circuit is a PLL circuit that locks in at a frequency deviation from the oscillator's natural frequency of less than 100 ppm, 10 ppm or 1 ppm.

A narrow-band PLL circuit is advantageously fail-safe and can provide an accurate clock signal even in the event of brief interruptions in the sensor signal.

In a conceivable embodiment of the peripheral device according to the invention, the peripheral device has an amplitude demodulator with a compensation circuit. The compensation circuit is designed to compensate for a signal component of the sensor signal that is low-frequency compared to a modulation frequency of the modulation signal. According to the invention, a low-frequency component is a spectral component of the signal demodulated by the amplitude demodulator, the frequency of which is less than 10%, 1% or one per thousand of the modulation frequency. In particular, low-frequency components are considered to be those caused by movement of the peripheral device during use or by external light sources and are in the range below 200 Hz, 120 Hz or 60 Hz. Possible embodiments are detailed in the following claims.

Intensity changes due to movement or shadowing are slow signal changes which, due to this frequency difference, can advantageously be separated from a clock frequency or a data signal by a compensation circuit and suppressed.

In one possible embodiment of the peripheral device according to the invention, the compensation circuit has a differential amplifier wired up as a comparator with a reference voltage input which is connected in series with a low-pass filter in the first signal connection to the sensor signal via an attenuator, preferably in the form of a resistive voltage divider. In this case, a circuit is regarded as a comparator which compares two input signals and provides a comparison result in the form of a controlled output voltage even if there is only a small difference between the input signals. In this respect, the comparator corresponds to a differential amplifier with a high gain. It is conceivable, for example, for the reference voltage input to be non-inverting and for the sensor signal or a signal proportional thereto to be present at the inverting input of the differential amplifier.

Only the low-frequency components in the sensor signal are applied to the non-inverting input of the differential amplifier as a comparator reference signal via the low-pass filter. As a result, the comparator reference signal advantageously follows slow changes, such as those caused by movement or shadowing. An upstream voltage divider preferably reduces the voltage of the low-pass filtered reference signal so that it is below the sensor voltage without any control signal transmission and the comparator output signal reacts exclusively to the component in the sensor signal caused by the rapidly changing modulation signal.

In a conceivable embodiment of the peripheral device according to the invention, the first signal connection has a track and hold element that is actuated as a function of a differential voltage between the sensor signal and the reference voltage. For example, an electronic switch can be arranged between a source of the sensor signal and the low-pass filter.

The switch then advantageously separates the sensor signal path to the low-pass filter when the sensor signal voltage falls below the reference voltage due to the rapid modulation, whereby the reference voltage is kept constant until the sensor signal voltage exceeds the reference voltage again due to the modulation content.

The magnetic resonance tomograph according to the invention has an optical transmitter with an amplitude modulation. The optical transmitter is designed to transmit a clock signal and/or control signal to a peripheral device according to the invention in an environment or an examination area of the magnetic resonance tomograph by means of an optical free-air transmission of an optical signal. For example, a clock signal can be amplitude-modulated as a carrier signal with a multiplier as an amplitude modulator with a data signal and then amplified and applied to an LED, which then emits an optical signal modulated in its intensity with a clock and control signal as a data signal for the peripheral device. However, digital modulation by switching a light source and thus the optical signal on and off is also conceivable. The information given for the corresponding peripheral device applies to the optical signal in the same way.

Advantageously, optical signals are easy to generate and receive and they are not subject to any regulation in the intensities used, so that no additional approval criteria have to be taken into account.

In a possible embodiment of the magnetic resonance tomograph according to the invention, the optical transmitter has an amplitude modulator. The amplitude modulator is designed to synchronously perform a light intensity change for modulation of the optical signal for transmission of the clock signal and the control signal. For example, the amplitude modulator could have a sample and hold element or flip-flop that switches a change in the control signal to a multiplier of the amplitude modulator only with an edge of the clock signal. In particular, the amplitude modulator is understood to mean a device that is designed to also set intermediate stages in the optical signal between the states “off” without light emission and “on” with maximum intensity and thus differs from the switch described below in the context of the present invention to digital modulation.

Advantageously, synchronous modulation by the control signal means that the phase of the modulated clock signal is not changed and a PLL on the receiving side is not disturbed.

In a conceivable embodiment of the magnetic resonance tomograph according to the invention, the transmitter is designed to switch the optical signal on and off with a modulation frequency to transmit the control signal and the clock signal (amplitude shift keying or amplitude on/off keying), with the magnetic resonance tomograph being designed to transmit the control signal Changing the frequency of the modulation signal from a first modulation frequency to a second modulation frequency unequal to the first modulation frequency.

Since the intensity of the optical signal is switched sharply in time with the modulation frequency, the light sensor supplies a square-wave sensor signal whose fundamental frequency corresponds to the first or second modulation frequency. In conjunction with a bandpass filter as part of the signal path for the sensor signal or another filter with frequency-dependent attenuation, a spectral component (harmonic or fundamental frequency) of the square-wave signal is selected for further processing.

With non-linear light sources such as LEDs or semiconductor lasers, changing the modulation frequency can be implemented much more easily and precisely than direct control of the luminosity. Preferably, only the modulation frequency with which a switch turns the light source on and off is changed.

In a conceivable embodiment of the magnetic resonance tomograph according to the invention, a frequency of the clock signal is an odd multiple of the modulation frequency. For example, the clock signal can have a frequency of 10 MHz. The modulation frequency is then preferably switched between a first modulation frequency and a second modulation frequency, the first modulation frequency being 10 MHz in the example and the second modulation frequency being a third of this, i.e. the frequency of the clock signal being equal to the first modulation frequency multiplied by 1 and equal to the second modulation frequency multiplied by 3. The modulation frequency is switched over in a phase-neutral manner, ie in the time frame of the clock frequency or the first modulation frequency.

When the optical signal is modulated by switching it on or off, a square-wave signal is generated which has harmonics at odd-numbered multiples of the modulation frequency. If the second modulation frequency is one third of the clock signal frequency, then the third harmonic is three times the modulation frequency or exactly at the frequency of the clock signal. A bandpass filter can be used to select the spectral component at the clock signal frequency and to suppress or attenuate all other harmonics, e.g. by more than 24 dB, 30 dB or 36 dB. A clock signal of, for example, 10 MHz is therefore provided both with the first modulation frequency and with the second modulation frequency. Due to the phase-neutral switching between the two modulation frequencies, the phase of the transmitted clock signal also remains unchanged, so that downstream PLL circuits, for example, supply a phase-stable output signal for the peripheral device. At the same time, however, in the case of a symmetrical square-wave signal with no direct component, the amplitude of the third harmonic is only a third as large as the amplitude of the fundamental wave (first harmonic), which results in an amplitude modulation of the bandpass-filtered received signal between a third and the full amplitude during frequency shift keying. With this amplitude modulation of the selected spectral component, a control signal can be transmitted, such as for activating the peripheral device or for issuing a message to an operator. With a digital code transmitted with this amplitude modulation, correspondingly different control commands can be transmitted and at the same time a phase-stable clock signal can be provided, with the modulation preferably being able to be provided by a simple switch.

However, it is also conceivable that the first sensor 51 is not an optical sensor but a receiver for radio waves for receiving the electromagnetic data signal. The radio waves assume the function of the light waves as a carrier signal for the first modulation frequency and the second modulation frequency. The output signal of the sensor can then be generated by rectifying the received radio wave or an intermediate frequency derived therefrom. The frequency of the electromagnetic data signal, the intermediate frequency or the radio wave is preferably significantly greater than the first modulation frequency and the second modulation frequency, for example by more than a factor of 10, 100 or 1000.

The method according to the invention is provided for the wireless transmission of a clock signal and a control signal with a system made up of a magnetic resonance tomograph according to the invention and a peripheral device according to claim 4. The method has the step of modulating an electromagnetic data signal with a first modulation frequency and transmitting the first modulated electromagnetic data signal . For example, an LED or a semiconductor laser can be switched on and off in time with the first modulation frequency by an electronic switch, as a result of which an optical signal with the first modulation frequency is generated and transmitted. However, it is also conceivable that a high-frequency signal with a frequency in the radio wave range is switched on and off in a transmitter or is switched between two amplitude values and is emitted as a radio wave via an antenna into an area surrounding the peripheral device.

In a further step, the electromagnetic data signal with the first modulation frequency is received with the first sensor and converted into an electrical signal.

The first sensor can be a photodiode, for example, which receives the optical signal and converts it into an electrical signal, which is also referred to below as the first output signal. It is also conceivable that the electrical signal is amplified and/or combined with electrical signals from other sensors.

However, the first sensor can also be a receiver for radio waves and a first off Output output signal, which depends, preferably proportionally, on the field strength of the radio signal.

In a further step, the first output signal from the first sensor is then filtered using the bandpass filter.

In the same way, in further steps, the electromagnetic data signal is modulated with a second modulation frequency that differs from the first modulation frequency, sent and received with the first sensor and filtered with the bandpass.

The output signal of the first sensor for the second modulation frequency is referred to as the third output signal, since the term second output signal is already used for the output signal of the second sensor. In general, the method is also conceivable with an output signal from the second sensor or with a combination according to the invention of the two output signals of both sensors for interference suppression.

A change between the first modulation frequency and the second modulation frequency preferably takes place in phase synchronism.

In a further step, a clock frequency of an oscillator is stabilized with the first output signal and the third output signal of the first sensor. In another step, a control signal is obtained from the first output signal and the third output signal by demodulation with an amplitude modulator.

An essential point for the steps is the property that a frequency of the clock signal is an odd multiple of the modulation frequency. For example, the first modulation frequency can correspond to the clock frequency, which corresponds to a simple multiple. The second modulation frequency could correspond to a third of the frequency of the clock signal, so that the frequency of the clock signal corresponds to three times the second modulation frequency. Modulation by switching on and off produces a square-wave signal that has harmonics with frequencies that correspond to an odd multiple of the fundamental frequency. A frequency component equal to the frequency of the clock signal thus arises both at a first modulation frequency equal to the frequency of the clock signal and at a second modulation frequency equal to one third of the frequency of the clock signal. The bandpass filter allows the frequency range of the clock signal to pass through and suppresses other harmonics or the lower fundamental frequency. A signal with a frequency of the clock signal is therefore always available as the output signal of the sensor in order to stabilize an oscillator, e.g. by means of a PLL.

At the same time, the output signal has a different amplitude depending on the frequency of the modulator signal, since the harmonics have a lower amplitude than the fundamental. By switching between the two modulation frequencies, an amplitude modulation can be applied to the selected spectral component, which can then be demodulated by an amplitude demodulator and then decoded. In the case of phase-synchronous shift keying between the modulation frequencies, the transmitted clock signal is undisturbed.

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

• 1 eine schematische Darstellung eines erfindungsgemäßen Magnetresonanztomographen;
• 2 eine schematische Darstellung einer beispielhaften Ausführungsform eines optischen Senders eines erfindungsgemäßen Magnetresonanztomographen;
• 3 eine schematische Darstellung von Komponenten zur Rückgewinnung eines Taktsignals eines erfindungsgemäßen Peripheriegeräts;
• 4 eine schematische Darstellung einer beispielhaften Ausführungsform eines optischen Senders eines erfindungsgemäßen Magnetresonanztomographen;
• 5 eine schematische Darstellung einer Kombination von Sensoren eines erfindungsgemäßen Peripheriegeräts;
• 6 eine schematische Darstellung eines Kompensationskreises einer möglichen Ausführungsform eines erfindungsgemäßen Peripheriegeräts;
• 7 eine schematische Darstellung eines Senders einer möglichen Ausführungsform des erfindungsgemäßen Magnetresonanztomographen;
• 8 ein schematisches Ablaufdiagramm für ein beispielhaftes erfindungsgemäßes Verfahren.

Show it:

• 1 a schematic representation of a magnetic resonance tomograph according to the invention;
• 2 a schematic representation of an exemplary embodiment of an optical transmitter of a magnetic resonance tomograph according to the invention;
• 3 a schematic representation of components for recovering a clock signal of a peripheral device according to the invention;
• 4 a schematic representation of an exemplary embodiment of an optical transmitter of a magnetic resonance tomograph according to the invention;
• 5 a schematic representation of a combination of sensors of a peripheral device according to the invention;
• 6 a schematic representation of a compensation circuit of a possible embodiment of a peripheral device according to the invention;
• 7 a schematic representation of a transmitter of a possible embodiment of the magnetic resonance tomograph according to the invention;
• 8th a schematic flowchart for an exemplary method according to the invention.

1 shows a schematic representation of an embodiment of a magnetic resonance tomograph 1 according to the invention.

The magnet unit 10 has a field magnet 11 that generates a static magnetic field B0 for aligning nuclear spins of samples or of the patient 100 in a recording area. The recording area is characterized by an extremely homogeneous static magnetic field B0, with the homogeneity particularly affecting the magnetic field strength or the amount. The recording area is almost spherical and is arranged in a patient tunnel 16 which extends through the magnet unit 10 in a longitudinal direction 2 . A patient couch 30 can be moved in the patient tunnel 16 by the traversing unit 36 . The field magnet 11 is usually a superconducting magnet that can provide magnetic fields with a magnetic flux density of up to 3T, and even more in the case of the latest devices. For lower magnetic field strengths, however, permanent magnets or electromagnets with normally conducting coils can also be used.

Furthermore, the magnet unit 10 has gradient coils 12 which are designed to superimpose temporally and spatially variable magnetic fields in three spatial directions on the magnetic field B0 for spatial differentiation of the recorded imaging regions in the examination volume. The gradient coils 12 are usually coils made from normally conducting wires, which can generate mutually orthogonal fields in the examination volume.

The magnet unit 10 also has a body coil 14 which is designed to emit a high-frequency signal supplied via a signal line into the examination volume and to receive resonance signals emitted by the patient 100 and emit them via a signal line.

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

Thus, the control unit 20 has a gradient control 21, which is designed to supply the gradient coils 12 with variable currents via supply lines, which provide the desired gradient fields in the examination volume in a time-coordinated manner.

Furthermore, the control unit 20 has a high-frequency unit 22 which is designed to generate a high-frequency pulse with a predetermined time profile, amplitude and spectral power distribution for exciting a magnetic resonance of the nuclear spins in the patient 100 . Pulse powers in the kilowatt range can be achieved. The excitation signals can be radiated into the patient 100 via the body coil 14 or also via a local transmitting antenna.

A controller 23 communicates with the gradient controller 21 and the radio-frequency unit 22 via a signal bus 25.

In the vicinity of the magnetic resonance tomograph 1 there is a large number of peripheral devices 80 or accessory devices which can be involved in the acquisition of a magnetic resonance recording. These can be operating elements, devices for communicating with or entertaining the patient, or also sensors that record physiological parameters of the patient 100 . A tablet for the user interface is shown here as an example of peripheral device 80 . It is also conceivable that the peripheral device 80 is the local coil 50 .

Many of these peripheral devices 80 contain digital circuits or processors whose clock signals and signals derived therefrom can generate interference. In particular, frequencies that are below the clock frequency are also generated during signal processing, usually with frequencies that are integer fractions of the clock frequency, and in turn have harmonics. With a suitable choice of the clock frequency, however, it is possible to ensure that these interference signals lie between the useful frequencies of the signal or a subsequent signal processing path and do not interfere. However, the clock frequencies must be sufficiently stable so that they do not migrate into disruptive frequency ranges, for example due to temperature changes. The magnetic resonance tomograph 1 according to the invention therefore has a transmission device for the wireless transmission of a clock signal, for example the optical transmitter 70 in 1 or a transmitter 73 for radio waves in 6 . The clock signal is preferably derived from a stable master clock of the magnetic resonance tomograph 1, which the control unit 20 provides to the optical transmitter 70 or the transmitter 73.

For transmission to the peripheral device 80, the control unit 20 has the optical transmitter 70, the light emitters 71, which emit the light, being arranged in the vicinity of the magnetic resonance tomograph in such a way that they directly or indirectly illuminate the peripheral device 80. The magnetic resonance tomograph preferably has a plurality of light emitters 71 that are spatially distributed or illuminate a light-scattering surface, so that shadowing of an optical signal emitted by the light emitters 71 is avoided in the peripheral device 80 . The light emitters 71 can be LEDs or semiconductor lasers, which receive an electrical signal from the optical transmitter 70 and convert this into the optical signal and emit it into the environment. However, it would also be conceivable for an LED emitter or semiconductor laser in the optical transmitter 70 to already convert it into light and for this to be guided to emission points in the area via glass fibers and optionally optical splitters.

In 2 an exemplary embodiment for an optical transmitter 70 of a magnetic resonance tomograph 1 according to the invention is shown.

The optical transmitter 70 has an amplitude modulator 72 which modulates a high-frequency clock signal with a low-frequency control signal. For example, the amplitude modulator can have a multiplier that multiplies the clock signal by a low-frequency control signal. The clock signal is preferably modulated in phase synchronism, i.e. with a rising or falling edge of the clock signal. This can be achieved, for example, by switching the control signal through to the multiplier via a sample and hold element, the sample and hold element being controlled by the clock signal, e.g. switching through when the clock signal is at a low level.

An output signal from the amplitude modulator is output directly or via an output stage to one or more LEDs or LED emitters or semiconductor lasers for conversion into an optical signal.

In 3 In an exemplary embodiment, components of the peripheral device 80 according to the invention are shown which are involved in transmission or recovery of the clock signal and control signal. Other elements of peripheral device 80 are not shown for the sake of clarity

A first optical sensor 51, for example a photodiode with a preamplifier, converts the optical signal into an electrical signal. A filter 53, for example a bandpass filter or low-pass filter, preferably lets through a signal with a frequency of the clock signal and attenuates signals with other frequencies, for example by more than 24 dB, 30 dB, 36 dB.

The filtered signal is fed to an amplitude demodulator 54 in a demodulation branch in order to recover the control signal modulated onto the clock signal. In the simplest case, the amplitude demodulator 54 can consist of a diode as a rectifier and a low-pass filter or buffer capacitor.

The demodulated signal still shows fluctuations, which are caused, for example, by changing shadows on the propagation path. These fluctuations are compensated with a compensation circuit 55, which leads to a subsequent 6 is explained.

The filtered signal is also fed in parallel to a branch of the circuit that is used to generate a stable master clock for the peripheral device 80 . The filtered signal of the first sensor 51 is first amplified in a limiting amplifier 56 in such a way that the amplitude fluctuations are eliminated by the amplitude limitation and only the phase information of the carrier wave remains. In a PLL circuit (phased-lock-loop control circuit), the frequency and the phase of a VCO 58 (voltage-controlled oscillator) are thus stabilized, which is preferably a quartz-stabilized VCO. The output of the VCO 58, or a signal derived from it, is the master clock for the peripheral device 80.

In 4 another conceivable embodiment for the optical transmitter 70 is shown. This embodiment is based on the idea that square-wave signals have harmonic components whose frequency corresponds to an odd multiple of the fundamental frequency. A square-wave signal with a high frequency, also known as the first modulation frequency, for example 10 MHz, and a square-wave signal with a lower frequency or second modulation frequency of (10/3) MHz, i.e. approximately 3.33 MHz, are shown here as input signals of the optical transmitter . The control unit 20 provides these signals with high accuracy derived from a stable master clock. Even if the optical transmitter 70 emits an optical signal with the second modulation frequency, this then has frequency components with the first modulation frequency of 10 MHz.

Clocked by the first modulation signal or the second modulation signal, the optical transmitter switches the power supply for the LED or a semiconductor laser as light emitter 71 or light source on and off by means of an electronic switch and thus generates an optical signal modulated with the square-wave signal. A large number of light emitters 71 are preferably switched on at the same time in order to avoid shadowing of the sensor 51 . Alternatively, a wall or a reflector on the magnetic resonance tomograph 1 can advantageously be designed to scatter light and be illuminated by one or more light emitters 71 . Advantageously, modulation with a square-wave signal can be implemented easily using a switch and is more efficient than linear intensity modulation.

Due to the change between the first modulation frequency and the second modulation frequency, a signal component with the first modulation frequency is always available, also because of the harmonics, which is selected by the filter 53 in the peripheral device 80 . Since the harmonic components always have a lower amplitude than the fundamental wave, the change between the modulation frequencies leads to an amplitude modulation in a spectral range of the first modulation frequency, which can be carried out, for example, with the peripheral device 80 according to the invention 3 can be evaluated. If the signals of the first modulation frequency and second modulation frequency are in a phase-stable relationship, preferably in that the edges of the signal with the second modulation frequency are synchronous with edges of the signal with the first modulation frequency, the clock signal generated via a PLL is not disturbed by the frequency shift keying can be achieved, for example, by obtaining the second modulation frequency from the first modulation frequency by phase-synchronous frequency division.

5 12 schematically shows an advantageous combination of a first sensor 51 and a second sensor 52 for detecting the optical signal. In particular, the combination reduces electromagnetic interference on the reception of the optical signal, which is caused by the operation of the magnetic resonance tomograph.

For this purpose, the first sensor 51 and the second sensor 52 are arranged adjacent to one another in order to minimize induction in connecting lines and to expose both sensors 51, 52 to the same electromagnetic fields as possible. The distance is preferably less than 2 cm, 1 cm or 5 mm.

The first sensor 51, here a photodiode, is in 5 connected directly to the positive supply voltage in reverse bias, while the connection to ground potential is made via a resistor.

For the second sensor 52, the roles of resistor and sensor are reversed, ie the resistor is connected directly to the positive supply voltage and the second sensor 52 to ground potential. By interchanging the arrangement, an identical optical signal in the two sensors 51, 52 generates an electrical signal with a comparable amplitude but opposite sign.

The electrical signal generated by the sensors 51, 52 is amplified and fed to a differential amplifier 49 at the inverting or non-inverting input. In an advantageous manner, the electrical signals caused by the optical signal at sensors 51, 52 add up at the output of differential amplifier 59 due to the different sign. Electromagnetic interference, on the other hand, preferably induces interference signals with the same sign in both branches, so that these are reflected in the differential amplifier in essential nullification. At the same time, the signal-to-noise ratio of the sum signal is 3 dB higher than that of the individual signals, since the noise contributions of the electrical components are uncorrelated and thus add up in terms of power, while the correlated received signals add up in terms of voltage.

In 6 a possible embodiment of a compensation circuit 55 is shown schematically. The filtered and demodulated sensor signal is fed to the inverting input of a comparator 60 . At the same time, the sensor signal is reduced by a resistive voltage divider, low-pass filtered via an RC element and fed to the non-inverting input of the comparator 60. The capacitance of the RC element is also used as the charging capacitance of a track-and-hold element whose electrical switch is designed here as a MOS-FET. The track and hold element is controlled by the output of the comparator 60, so that the switch of the track and hold element is opened when the sensor signal is smaller than the reference signal.

In 7 an exemplary magnetic resonance tomograph 1 and peripheral device 80 according to the invention are shown, which use a radio wave for transmission of the electromagnetic data signal or the first modulation frequency instead of an optical signal. For this purpose, the control unit 20 has a transmitter 73 for emitting a radio wave. An oscillator generates a high-frequency signal with a frequency that is higher than the Larmor frequency in an imaging of nuclear spins to be detected with the magnetic resonance tomograph 1 in the static magnetic field of the magnetic resonance tomograph 1 . For example, the frequency of the radio frequency signal may be greater than two, five, or ten times the Larmor frequency. The frequency is preferably less than 100 GHz, 50 GHz or 10 GHz.

The transmitter 73 modulates the high-frequency signal with a clock signal and/or control signal for the peripheral device and emits it via an antenna as a radio wave in an area surrounding the magnetic resonance tomograph 1 in which the peripheral devices 80 are located. It is also conceivable that the frequency corresponds to a multiple of the clock frequency to be transmitted, which is obtained from the clock signal by a frequency multiplier.

The peripheral device receives the radio wave with a receiver 81. The clock signal and/or control signal to be transmitted can be transmitted by the peripheral device 80 by demodulating the radio wave, as already done for the output signal of the sensor 51, 52 2 and 3 described. If the frequency of the radio wave or the high-frequency signal is a multiple of the clock signal to be transmitted, it is also conceivable for the clock signal to be recovered by frequency division.

The peripheral device 80 according to the invention uses the recovered clock signal as a master clock for internal signal processing. This also ensures that, as already described, interference frequencies generated by the signal processing and their harmonics are not on signal frequencies of the magnetic resonance signals.

In 8th a method according to the invention for the wireless transmission of a clock signal and a control signal with a magnetic resonance tomograph is shown schematically in a flow chart.

In a step S10, an electromagnetic data signal is modulated with a first modulation frequency. For example, an electronic switch that connects one or more LEDs to a supply voltage can be switched on and off by the control unit 20 with the first modulation frequency. The LED then emits an optical signal modulated with the first modulation frequency. However, it is also conceivable for a high-frequency signal with a frequency in the radio wave range to be switched on and off in a transmitter 73 or switched between two amplitude values and emitted as a radio wave via an antenna into the surroundings of the peripheral device 80 .

In a further step S20, a first sensor 51, for example a photodiode, receives the optical signal and converts it into an electrical signal, also referred to as the first output signal. It is also conceivable that the electrical signal is amplified and/or combined with electrical signals from other sensors. However, the first sensor can also be a receiver for radio waves and output a first output signal which is preferably proportional to the field strength of the radio signal. This can be achieved, for example, by rectifying with a diode and filtering the received radio signal or an intermediate frequency generated from it with a buffer filter. The buffer filter, for example an RC element, has a frequency response which is designed to keep the output voltage essentially constant between two half-waves of the high-frequency signal or the intermediate-frequency signal and has essentially the same transmission loss for both of the modulation frequencies mentioned below, e.g Difference in attenuation less than 1 dB.

In another step S30, the first output signal from the first sensor is filtered using the bandpass filter. The bandpass filter preferably has a minimum insertion loss at a frequency of the clock signal.

In another step S40, the electromagnetic data signal is modulated as in step S10, but with a second modulation frequency that differs from the first modulation frequency. A frequency of the clock signal is an odd multiple of the first and the second modulation frequency, the multipliers differing at the first and second modulation frequency. For example, the multiplier is equal to 1 at the first modulation frequency, so that the first modulation frequency corresponds to the frequency of the clock signal. The multiplier of the second modulation frequency can be 3, for example, so the second modulation frequency is one third of the frequency of the clock signal. The electromagnetic data signal with the second modulation frequency is also sent out by the transmitter 73 as described.

In a step S50, the electromagnetic data signal with the second modulation frequency is received with the first sensor 51, as already described in step S20, and converted into a third electrical output signal. The term second output signal is already used and preassigned for the output signal of the second sensor 52 . In principle, however, it is also conceivable to use the method with a previously described combination of first sensor 51 and second sensor 52 for interference suppression, with the term first output signal being applied accordingly to the combined output signal of the sensors at the first modulation frequency, analogously for the third Output signal at the second modulation frequency.

In a step S60, a third output signal from the first sensor is filtered using the bandpass filter. Because the bandpass filter has a minimum of attenuation at the frequency of the clock signal, the bandpass filter preferably passes a harmonic of the second modulation signal that is at the frequency of the clock signal.

In a step S70, a receiver 81 of the peripheral device 80 stabilizes, for example by means of a PLL circuit, a clock frequency of an oscillator with the first output signal and the third output signal of the first sensor. This is made possible by the fact that the first modulation signal also has signal components which are at the frequency of the clock signal and are preferably passed on by the bandpass filter.

In another step S80, a control signal is demodulated from the first output signal and the third output signal with an amplitude demodulator. The amplitude differences of the first and the third electrical output signal result from the fact that the signal components of harmonics decrease with increasing order.

Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited by the disclosed examples and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims (14)

Peripheral device for a magnetic resonance tomograph (1), the peripheral device (80) having a first sensor (51) for receiving an electromagnetic data signal from the environment of the peripheral device (80), the peripheral device (80) being designed to process signals as a function of the perform electromagnetic data signal and a frequency of the electromagnetic data signal is greater than a Larmor frequency of the magnetic resonance tomograph (1). peripheral device claim 1 , the first sensor (51) for receiving an optical data signal from the environment of the peripheral device (80) is provided. peripheral device claim 2 , wherein the peripheral device (80) has a second sensor (52) adjacent to the first sensor (51), the first sensor (51) being designed to generate a first output signal from the optical data signal and the second sensor (52) being designed is to generate a second output signal from the same optical data signal, the first output signal having an amplitude inverse to the second output signal, the peripheral device (80) having an inverter which inverts the output signal of the first sensor (51) and a summing element , which is designed to add the inverted output signal of the first sensor (51) and the output signal of the second sensor (52) to form a sensor signal. Peripheral device according to one of the preceding claims, wherein the peripheral device (80) has a filter (53) which is designed to select a modulation frequency of the sensor signal, and a narrow-band phase-lock loop circuit (57) which is designed to have a To stabilize the oscillator depending on the modulation frequency and preferably the modulation phase. Peripheral device according to one of the preceding claims, wherein the peripheral device (80) has an amplitude demodulator (54) with a compensation circuit (55) which is designed to compensate for a low-frequency signal component of the sensor signal compared to a modulation frequency of the modulation signal. peripheral device claim 5 , wherein the compensation circuit (55) has a comparator (60) with a reference voltage input which is in first signal connection with the sensor signal via a low-pass filter. peripheral device claim 6 , The first signal connection having a track and hold element which is actuated as a function of a differential voltage between the sensor signal and the reference voltage. Magnetic resonance tomograph, wherein the magnetic resonance tomograph (1) has a transmitter (73), wherein the transmitter (73) is designed to transmit a clock signal and/or control signal wirelessly by means of an electromagnetic data signal to a peripheral device (80) in the vicinity of the magnetic resonance tomograph, wherein a frequency of the electromagnetic data signal is greater than a Larmor frequency of the magnetic resonance tomograph (1). magnetic resonance tomography claim 8 wherein the transmitter is an optical transmitter (70) adapted to transmit a clock signal and/or control signal to the peripheral device (80) by means of an optical over-the-air transmission of an optical signal. magnetic resonance tomography claim 9 , wherein the optical transmitter (70) has an amplitude modulator (72) which is designed to synchronously carry out an amplitude change for modulating the optical signal for the transmission of the clock signal and the control signal. magnetic resonance tomography claim 9 with the optical transmitter (70) for transmitting the clock signal and the control signal, the optical transmitter (70) being designed to modulate the optical signal with a modulation frequency for transmitting the control signal and the clock signal, the magnetic resonance tomograph (1) being designed, to transmit the control signal Change modulation frequency from a first modulation frequency to a second modulation frequency unequal to the first modulation frequency. magnetic resonance tomography claim 11 , wherein a frequency of the clock signal is an odd multiple of the modulation frequency. magnetic resonance tomography claim 9 , wherein the optical transmitter (70) is designed to radiate the optical signal into an area surrounding the magnetic resonance tomograph (70) and to distribute it on a surface by scattering. Method for the wireless transmission of a clock signal and a control signal with a magnetic resonance tomograph (1) according to one of Claims 8 until 13 and a peripheral device (80). claim 4 , wherein the method comprises the steps of: (S10) modulating an electromagnetic data signal with a first modulation frequency and transmitting the first modulated electromagnetic data signal with the transmitter (73); (S20) receiving the electromagnetic data signal with the first sensor (51); (S30) filtering the first output signal of the first sensor (51) with the filter (53); (S40) modulating the electromagnetic data signal with a second modulation frequency and transmitting the second modulated electromagnetic data signal with the transmitter (73); (S50) receiving the optical signal with the first sensor (51); (S60) filtering a third output signal of the first sensor (51) with the filter (53); (S70) stabilizing a clock frequency of an oscillator with the first output signal and the third output signal of the first sensor (51); (S80) demodulating a control signal from the first output signal and the third output signal with an amplitude demodulator (54), wherein a frequency of the clock signal is an odd multiple of the first and second modulation frequencies.

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