DEVICE AND METHOD FOR DETECTING SPIKES IN A MAGNETIC RESONANCE MEASUREMENT

Description

[0001] The invention relates to a method and a magnetic resonance imaging scanner with a device for detecting an interference signal, in particular spikes, during a magnetic resonance measurement. The device comprises a receiver with a first signal connection for connecting a local coil and a second signal connection for connecting a reference antenna.

[0002] Magnetic resonance imaging scanners are imaging devices that, to create an image of a subject, align the nuclear spins of the subject with a strong external magnetic field and then excite them to precess around this alignment using an alternating magnetic field. The precession, or return, of the spins from this excited state to a lower-energy state, in turn generates an alternating magnetic field in response, also known as a magnetic resonance signal, which is received via antennas.

[0003] Using magnetic gradient fields, a spatial coding is imprinted on the signals, which subsequently allows the received signal to be assigned to a volume element. The received signal is then evaluated, and a three-dimensional imaging representation of the object under examination is provided. The generated representation indicates a spatial density distribution of the spins.

[0004] The received magnetic resonance signals are very weak, meaning in some cases they are just above the signal-to-noise level. At the same time, the excitation pulses have energies in the kilowatt range, and currents of several hundred amperes flow through the gradient fields. Due to the high fields and indirectly due to mechanical-electrical interactions, electrical potentials can form in the area of ​​the patient feedthroughs. Their equalization can lead to small discharges, which result in short, broadband interference pulses, so-called spikes. During subsequent image processing, individual spikes can be transformed into entire patterns of artifacts distributed across the image through Fourier transformation.

[0005] From the unpublished patent application DE 10 2016 223478, a magnetic resonance imaging scanner is known which records the patient's physiological data and aligns the image acquisition processes in such a way that the acquisition of the physiological signals is not disturbed.

[0006] Document WO 2014 / 167561 A2 discloses a method and system for real-time interference reduction during MR data acquisition. At least one antenna is arranged near or within a cavity of a conventional magnetic resonance device and is connected to a conventional signal acquisition device. Data acquisition from the antenna is synchronized with the magnetic resonance pulses. Any remaining signal is considered interference.

[0007] The document US 2017 / 0108569 A1 describes a magnetic resonance system which, in addition to the magnetic resonance signals, simultaneously acquires interference signals and calibration data without excitation pulses using an interference detection coil and uses these to determine calibration interference signals.

[0008] Document DE 195 24 573 A1 discloses an MRI system with a noise filter that receives each NMR signal acquired during a scan and detects short-term noise pulses by measuring the signal level in a frequency band outside the imaging bandwidth. A gating circuit suppresses the NMR signal during each time interval in which a noise pulse is detected to remove the noise pulse before image reconstruction.

[0009] The document US 2006 / 0181278 A1 relates to a magnetic resonance device with a local coil matrix, a pre-scanning device which carries out a pre-scan of at least part of the acquisition area before the magnetic resonance acquisition, and a test unit which detects deviating local coils based on the local coil signal of the pre-scan.

[0010] It is therefore an object of the present invention to improve the image quality of magnetic resonance imaging.

[0011] The object is achieved by the magnetic resonance imaging device according to the invention according to claim 1 and the method according to the invention according to claim 7.

[0012] The device of the magnetic resonance tomograph according to the invention for detecting a spike during a magnetic resonance measurement has a first receiver with a first signal connection for connecting a local coil and a second receiver with a second signal connection for connecting a reference antenna. The reference antenna is the body coil of the magnetic resonance tomograph. The device is designed to compare a first signal received by the first receiver via the first signal connection with a second signal received by the second receiver via the second signal connection. The received signal can be, for example, the analog and amplified antenna signal or a signal converted to an intermediate frequency. An antenna signal digitized directly or after filtering is also conceivable. The receivers are receivers orReceiving channels of the magnetic resonance imaging scanner's receiver, which are also used to prepare the magnetic resonance signals for image acquisition. The signals received by the receivers can be digitally processed.

[0013] The comparison can be a direct comparison of the size of the amplitudes or of a size derived therefrom, such as the energy. Averaging over a certain period of time is also conceivable. It can also be part of the comparison that the device is designed to compensate for different sensitivities of the local coil by scaling one or both signals with a factor or analogously using an amplifier. The comparison can be carried out analogously, for example, using a comparator or, in the case of digital signal processing, by a mathematical operation such as forming the difference or quotient. The device is designed to output a warning signal in the event of a predetermined deviation. The predetermined deviation can occur, for example, when the difference between the second signal and the first signal is greater than 10%, 20%, 50% or 100% of the size of the first signal.It would also be conceivable to calculate a quotient of the second signal to the first signal. Exceeding the predetermined deviation can occur, for example, if the quotient assumes a value greater than 1.1, 1.3, 1.5, or two.

[0014] However, it is also conceivable that more complex methods might be used, such as pattern matching or autocorrelation between the first signal and the second signal. If the second signal from the reference antenna is detected in the first signal from the local coil, the magnetic resonance signal is likely to be distorted.

[0015] The method according to the invention is defined in claim 7 .

[0016] In one step of the method, a first signal is received from the local coil via the first signal connection of the receiver. "Received" means that the signal is available for further processing, either through digital signal processing if the signal has already been digitized, or through analog processing in the device according to the invention. The signal is preferably temporally resolved such that at least the fundamental frequency of a spike (equal to the inverse of the spike duration) is detected, for example, by setting the sampling frequency to twice the fundamental frequency or by ensuring that the frequency response of the receiver includes at least the fundamental frequency.

[0017] In another step of the process, a second signal is received from the body coil via the receiver's second signal port. The same applies to reception and the fundamental frequency as described above.

[0018] In another step of the method according to the invention, the first signal and the second signal are compared by the device. The comparison is preferably performed by a comparison unit, for example, with an analog comparator or digitally by forming a difference or quotient, or with a digital comparator. It is conceivable that, as part of the comparison, one or both signals are initially amplified or scaled, for example, to compensate for different sensitivities of the local coil and the reference antenna.

[0019] In a further step of the method according to the invention, the device checks the result of the comparison for a predetermined deviation. For example, the result of the comparison can be evaluated using an analog or digital window discriminator. Comparison with stored values ​​in a table or logical combinations of predetermined inequalities using the comparison result are also conceivable. If a predetermined deviation is detected by the device, it issues a warning signal.

[0020] Further advantageous embodiments are specified in the subclaims.

[0021] In one conceivable embodiment of the magnetic resonance imaging system according to the invention, the device is designed to perform the comparison in real time. For example, the device can have corresponding elements for scaling, subtracting, and evaluating the first signal and the second signal, either as interconnected analog components or as digital signal processing resources with appropriate programming. In one embodiment, it can also be considered real time within the meaning of the invention if the comparison of the signals and the detection of a deviation between two excitation pulses of a sequence can take place.

[0022] Advantageously, real-time comparison and detection allows the pulse train currently disturbed by a spike to be repeated and, with minimal time expenditure, only the disturbed part of an image acquisition to be repeated instead of the entire image acquisition and possibly image analysis if the problem is only detected through artifacts in the image.

[0023] In one possible embodiment of the magnetic resonance imaging system according to the invention, the device is designed to perform the comparison only if the first and / or the second signal fulfill a predetermined precondition. For example, one or both signals can be monitored, and the comparison can be performed if a predetermined threshold is exceeded.

[0024] Advantageously, evaluating a precondition increases the probability of correctly detecting an interference signal while simultaneously reducing the processor load, since, for example, in a digital device, the comparison does not have to be performed continuously and the computing resources can be used for other purposes.

[0025] In a conceivable embodiment of the magnetic resonance imaging device according to the invention, the predetermined deviation comprises exceeding a predetermined limit value for an amplitude ratio of the second signal to the first signal.

[0026] The body coil is positioned to pick up the interference signal relative to background noise and / or to the magnetic resonance signal with a higher amplitude than the local coil, by placing the body coil closer to a wall of the patient tunnel, so that it is closer to a potential discharge source and further from the source of the magnetic resonance signals.

[0027] Advantageously, interference signals from spikes are characterized by a relatively higher amplitude in the second signal compared to the first signal and can thus be identified more easily.

[0028] In one possible embodiment of the magnetic resonance imaging system according to the invention, it has a controller in signal communication with the device. Upon receipt of the warning signal from the device, the controller is designed to repeat the last measurement cycle or pulse train, based on whose first and second signals the warning signal was generated.

[0029] A disrupted magnetic resonance signal in a pulse train during the scanning of one or more k-space points affects the entire image due to the algorithms used in image analysis. However, if the disruptive signal is detected during the acquisition of one or more points in k-space, this individual pulse train can be repeated, thus saving significant time compared to repeating the entire image acquisition.

[0030] The magnetic resonance imaging scanner according to the invention has a body coil. The body coil is connected to the second signal port of the device as a reference antenna. In other words, the magnetic resonance imaging scanner is designed to forward a signal received by the body coil as a second signal to the device according to the invention, where it can be compared with the first signal or to perform the other processes described in connection with the second signal.

[0031] The body coil is typically located at a greater distance from the patient's body than a local coil, while the distance from the wall of the patient tunnel is smaller. A discharge in the structure of the patient tunnel, or more frequently in neighboring elements such as an RF shield or gradient coil, therefore advantageously results in a larger signal amplitude in the body coil relative to a magnetic resonance signal from the patient's body and can therefore be used to identify an interference pulse, especially when compared to the first signal from the local coil.

[0032] In one possible embodiment of the magnetic resonance tomograph according to the invention, the latter is designed to detune the body coil and to compare a second signal at a frequency unequal to the Larmor frequency of the magnetic resonance tomograph, preferably in an adjacent frequency range with a frequency deviation of at least 1%, 5%, 10% or more of the Larmor frequency in the device according to the invention.

[0033] While magnetic resonance signals are relatively narrowband in the Larmor frequency range, interference pulses from discharges are relatively broadband. If the reference antenna is detuned, i.e., tuned to a resonant frequency different from the Larmor frequency, the magnetic resonance signal is largely suppressed in the second signal, while the envelope of an interference pulse at this frequency is largely unchanged compared to an envelope at the Larmor frequency. Comparing the envelope of the first signal at the Larmor frequency with the envelope of a second signal at the detuned frequency thus advantageously further distinguishes a potential interference signal from the magnetic resonance signal.However, due to the higher sensitivity of the body coil to E-fields, it is also conceivable to leave the receiver tuned to the Larmor frequency despite the detuned body coil. In this case, the magnetic resonance signals from the patient's body are attenuated due to the predominant B-field component compared to the spike with its predominantly E-field component. In this case, a receiver for imaging can also be used for interference detection without modification.

[0034] In one conceivable embodiment of the method according to the invention, the method further comprises the step of emitting a test pulse. It is conceivable that the test pulse is an excitation pulse and is designed to excite nuclear spins of a sample or a patient. The power can correspond to the excitation power of a pulse from an image acquisition sequence. Preferably, the power can be lower to reduce SAR exposure, for example to a value by a factor of 2, 5, 10 or less. It is only necessary that the excitation pulse, at any time after the excitation pulse, generates a magnetic resonance signal of the spins in the local coil and the reference antenna that is above a noise level.

[0035] It is also conceivable, however, that the test pulse is not intended to excite nuclear spins, but rather to be received directly by the local coil and / or the reference antenna. A separate transmitting antenna can be provided, preferably in an area where spikes occur. In this way, in addition to the local coil, the body coil also receives a test signal generated by the test pulse. It is also conceivable, however, that the test pulse is emitted via the body coil as a transmitting antenna. Since the geometry and properties of the body coil hardly change during operation, a predetermined reference value for the test signal of the body coil can be assumed, which is obtained through calculation or previous measurements. The local coil, on the other hand, can be replaced and is measured using the test pulse and the received test signal.Advantageously, by directly receiving the test pulse as a test signal, the power for the test pulse can be even lower in a range of a few watts, milliwatts or less and the SAR exposure can be further reduced.

[0036] In a further step, a test signal is received via the local coil and the device's first signal port. If the test pulse was an excitation pulse and generated a magnetic resonance signal, or if the test pulse was transmitted via a separate transmitting antenna, the test signal is received via the body coil and the device's second signal port in another step.

[0037] In another step, a relationship between the test signal received via the first signal connection and the test signal received via the second signal connection is determined. This determined relationship is particularly suitable for detecting differences in the reception paths in order to take these into account during comparison or to compensate for them beforehand. If the test pulse was transmitted via the body coil and received directly as a test signal by the local coil, a predetermined value is used for the test signal received via the second signal connection, which value was determined, for example, by a measurement performed at an earlier time with a separate transmitting antenna.

[0038] The predetermined deviation, which is determined and evaluated when comparing the first signal with the second signal, is a significant deviation of the dependency of the first signal to the second signal from the previously determined dependency of the test signals. For example, if the test signal determined that a magnetic resonance signal from the body in the body coil always only has a signal amplitude of 20% compared to the signal from the local coil, an interference signal in the body coil with an amplitude of 50% or more compared to the signal generated in the local coil can be considered a predetermined deviation in that it deviates significantly from the usual amplitude ratio of 20:100 with a ratio of 50:100.

[0039] By determining or ascertaining the dependence of signals received via the body coil and the local coil using an excitation pulse in a calibration process, the method according to the invention can take into account different geometries, for example in the case of a changed position or type of the local coil, and in this way detect disturbances more reliably and with greater sensitivity and prevent their consequences in the image display.

[0040] In one possible embodiment of the method according to the invention, the dependency determined in the calibration process described above is a measure of the ratio of the amplitudes of the magnetic resonance signal received via the second signal connection to the magnetic resonance signal received via the first signal connection.

[0041] A significant deviation within the meaning of the invention occurs when the predetermined ratio is exceeded by more than 10%, 20%, 50% or 100%.

[0042] The calibration process described above determines how and with what amplitude a test signal or a desired magnetic resonance signal from the patient's body is received by the local coil and the body coil. If a signal received via the local coil and the body coil deviates from this amplitude ratio, this is an indication that the signal is not coming from the patient's body and is therefore not a useful signal but an interference signal. It is conceivable that the amplitude ratio determined during the calibration process serves as the benchmark for the comparison, or that the signals are standardized to a 1:1 ratio by scaling them according to the amplitudes determined during calibration.

[0043] In one conceivable embodiment of the method according to the invention, the reference antenna is a body coil. The body coil is detuned in the receiving step via the second signal connection. For example, a control system of the magnetic resonance imaging scanner can connect a capacitor to the body coil via a switch, thus shifting the resonant frequency of the body coil relative to the Larmor frequency, for example, by 2%, 5%, 10%, or more. Variable capacitors such as PIN diodes are also conceivable, where the degree of detuning can be controlled via an applied voltage.

[0044] Due to its proximity to the wall of the patient tunnel and the gradient coil, the body coil is particularly sensitive to interference in this area, allowing for better detection of interference signals when receiving signals simultaneously via the body coil and local coil. Suppressing the magnetic resonance signal by detuning also advantageously facilitates differentiation from the magnetic resonance signal.

[0045] In one possible embodiment of the method according to the invention, the method further comprises the step of repeating the last magnetic resonance measurement by the magnetic resonance imaging scanner when the warning signal is received from the device. The last magnetic resonance measurement is considered to be, in particular, the measurement sequence after the last radiofrequency excitation pulse for the nuclear spins, so that the disturbed data can be replaced by an identical measurement without interference.

[0046] Advantageously, the method according to the invention makes it possible to detect a disturbance in real time, so that only the disturbed part of a measurement needs to be repeated in order to replace the disturbed data with undisturbed data and thus avoid errors in image processing.

[0047] In one conceivable embodiment of the method according to the invention, the method further comprises the step of removing the interference signal from the last magnetic resonance measurement by the magnetic resonance imaging scanner when the warning signal is received from the device. For example, it is conceivable that the data acquired immediately during the interference is deleted or replaced by a constant value. Interpolation is also conceivable. The warning signal preferably contains information about the beginning and end of the interference.

[0048] However, it would also be conceivable that by correlating the first signal and the second signal, a portion of the interference signal in the first signal is determined and removed from the first signal, for example by scaling and subtracting the interference signal accordingly by the receiver and / or the control of the magnetic resonance imaging device.

[0049] In an advantageous way, reception with a local coil and a body coil makes it possible to distinguish and separate the interference signal from the magnetic resonance signal, so that the data can be corrected and artifacts can be reduced accordingly even without additional measurement.

[0050] The above-described properties, features and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more clearly understood in connection with the following description of the embodiments, which are explained in more detail in connection with the drawings.

[0051] They show: Fig. 1 an exemplary schematic representation of a magnetic resonance imaging device according to the invention Fig. 2 a schematic representation of the device for detecting spikes of the magnetic resonance imaging device according to the invention; Fig. 3 a schematic flow chart of a method according to the invention for detecting spikes, Fig. 1 shows a schematic representation of an embodiment of a magnetic resonance imaging device 1 according to the invention with a device 60 for detecting spikes.

[0052] The magnet unit 10 has a field magnet 11 that generates a static magnetic field B0 for aligning nuclear spins of samples or patients 100 in a recording area. The recording area is arranged in a patient tunnel 16 that extends in a longitudinal direction 2 through the magnet unit 10. A patient 100 can be moved into the recording area by means of the patient bed 30 and the movement unit 36 ​​of the patient bed 30. The field magnet 11 is typically a superconducting magnet that can generate magnetic fields with a magnetic flux density of up to 3T, and even higher in the latest devices. However, permanent magnets or electromagnets with normally conducting coils can also be used for lower field strengths.

[0053] Furthermore, the magnet unit 10 has gradient coils 12, which are designed to superimpose variable magnetic fields in three spatial directions on the magnetic field B0 for the spatial differentiation of the acquired imaging regions in the examination volume. The gradient coils 12 are typically coils made of normally conducting wires that can generate mutually orthogonal fields in the examination volume. The magnet unit 10 also has a body coil 14, which is designed to radiate a radio-frequency signal supplied via a signal line 33 into the examination volume and to receive resonance signals emitted by the patient 100 and output them via a signal line. Preferably, however, the body coil 14 for transmitting and / or receiving the radio-frequency signal is replaced by local coils 50, which are arranged in the patient tunnel 16 close to the patient 100.However, it is also conceivable that the local coil 50 is designed for transmitting and receiving.

[0054] 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. A magnetic resonance imaging controller 23 coordinates the subunits.

[0055] 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.

[0056] Furthermore, the control unit 20 has a radio-frequency unit 22, which is designed to generate a radio-frequency pulse with a predetermined temporal profile, amplitude, and spectral power distribution for exciting magnetic resonance of the nuclear spins in the patient 100. Pulse powers in the kilowatt range can be achieved. The individual units are interconnected via a signal bus 25.

[0057] The radio-frequency signal generated by the radio-frequency unit 22 is fed to the body coil 14 via a signal connection and transmitted into the body of the patient 100 to excite the nuclear spins there. However, it is also conceivable to transmit the radio-frequency signal via one or more local coils 50. The local coil 50 then preferably receives a magnetic resonance signal from the body of the patient 100, because due to the short distance, the signal-to-noise ratio (SNR) of the local coil 50 is better than when received by the body coil 14. The MR signal received by the local coil 50 is processed in the local coil 50 and forwarded to the radio-frequency unit 22 of the magnetic resonance imaging scanner 1 for evaluation and image acquisition. Preferably, the signal connection 33 is used for this purpose, but wireless transmission is also conceivable, for example.

[0058] The device 60 for detecting spikes is subsequently Fig. 2 described in more detail. The Fig. 2 The device 60 shown has a first signal terminal 62, which is in signal communication with the local coil 50 and receives a magnetic resonance signal from the local coil 50. The magnetic resonance signal can be analog or digital. Furthermore, the device 60 has a second signal terminal 63, which is in signal communication with the body coil 14 as a reference antenna.

[0059] The signal of the reference antenna, like the signal of the local coil 50, can be analogue or digital and can be pre-processed by signal processing, for example converted into a different frequency range or filtered.

[0060] The antenna signals at the first signal connection 62 and the second signal connection 63 are processed by a first receiver 64 and a second receiver 65. If the antenna signals are analog, for example, the receiver 64, 65 can have an amplifier and a filter stage to amplify the signals and remove interference outside the frequency of the magnetic resonance signal. It is also conceivable for the signals to be converted to an intermediate frequency and / or demodulated and / or digitized. If the antenna signals are already digitized, digital signal processing can be performed using appropriate filters or functions. In principle, the receivers 64, 65 can be provided exclusively for the device 60. Preferably, however, receivers or receiving channels of the radio-frequency unit 22 that are also used for image acquisition are used.In particular, for the signals of the local coil 50, the same signals of the first receiver 64 in signal connection with the local coil 50 can be used both for detecting a spike and for image acquisition.

[0061] Furthermore, the device 60 has a comparison unit 61 which is designed to relate the signals received via the first signal terminal 62 and those received via the second signal terminal 63 to one another.

[0062] Preferably, the device 60 is designed to perform the comparison in real time. This can be done, for example, using an analog comparator circuit or digital signal processing.

[0063] Digital signal processing, in particular, is capable of evaluating signals according to complex criteria, for example, whether they correspond to a spike in a specific pattern such as slew rate, progression, duration, frequency distribution, or similar. Digital signal processing can store such patterns, but an analog high-pass or bandpass filter is also capable of distinguishing a slew rate or frequency distribution.

[0064] Based on the evaluation according to different criteria, the device 60 can then also be designed to perform a comparison only if the first and / or the second signal fulfill a predetermined precondition. For example, discriminators in the comparison unit 61 can only pass signals for the actual comparison that exceed a certain minimum amplitude in order to avoid false triggering due to noise. However, the discriminators could also additionally or alternatively have a filter function to allow only predetermined specific frequency distributions to pass through.

[0065] A preferred comparison criterion for the comparison unit 61 can, for example, be an amplitude ratio of a second signal at the second signal connection 63 of the body coil 14 to a first signal at the first signal connection 62 of the local coil 50 that must be achieved at least to detect a spike, because for a spike in the vicinity of the body coil 14, a significantly stronger signal is to be expected via the second signal connection 63 connected to the body coil 14 than from a more distant local coil 50 at the first signal connection 62.

[0066] In Fig. 3 a schematic flow diagram for a possible embodiment of a method according to the invention for detecting a spike in a magnetic resonance measurement with a magnetic resonance tomograph 1 is shown.

[0067] In a step S20, a first signal is received from the local coil 50 via the first signal terminal 62 of the device via the first receiver 64. The reception may, for example, also include filtering, conversion to an intermediate frequency, or digitization.

[0068] In step S21, a second signal is received from the body coil as a reference antenna via the device's second signal port 63. Preferably, steps S2C and S21 are performed simultaneously and also in real time. The statements regarding reception under S20 also apply to step S21.

[0069] In a step S30, the first signal and the second signal are compared by the device, preferably the comparison unit 61. The comparison may, for example, involve forming a difference and / or quotient between the first signal and the second signal, but other logical or arithmetic functions are also conceivable.

[0070] In a step S31, a warning signal is output by the device to the magnetic resonance imaging scanner if a predetermined deviation is detected during the comparison. The value obtained during the comparison can, for example, be compared with a predetermined threshold value, and the warning signal can be output if this threshold is exceeded. For example, a quotient or a difference can be formed during the comparison between the output signal of the second receiver in signal connection with the reference antenna and the first receiver in connection with the local coil 50. If the quotient for magnetic resonance signals has the value 0.5, for example, due to the geometry and different sensitivities for magnetic resonance signals from the body, a threshold value of 0.7 can be provided.If the quotient then assumes the value 0.8 or greater because a spike in the body coil 14 as a reference antenna generates a stronger signal than in the local coil 50, the predetermined threshold value is exceeded and a deviation is detected.

[0071] In one possible embodiment of the method according to the invention, it is conceivable that the device 60 is also calibrated in order to compensate for different sensitivities, in particular when using different local coils 50.

[0072] For this purpose, a test pulse is transmitted in step S10. There are two different embodiments of the method according to the invention.

[0073] In one embodiment, the test pulse is an excitation pulse for nuclear spins of a sample or of the patient 100 in the patient tunnel 16. The nuclear spins, in turn, generate a magnetic resonance signal, which is received in a step S11 via the local coil 50 and the first signal connection 62 and in a step S12 via the reference antenna and the second signal connection 63 in order to determine an amplitude ratio or a relative sensitivity of the local coil 50 to the reference antenna. Since the excitation and reception of the magnetic resonance signal as a test signal occur with a time offset, the body coil 14 itself can serve as the transmitting antenna for the test pulse and as the reference antenna for the test signal in the form of the magnetic resonance signal. The different and variable distance between the patient 100 and the local coil 50 and the body coil 14 is also directly taken into account here, as is subsequently also the case with the magnetic resonance signals for imaging.However, much higher powers are required to excite the nuclear spins, so the test pulse places a greater burden on the SAR budget, even if it can be weaker than for imaging due to the lower required signal-to-noise ratio, for example with a power of less than 1000 W, 500 W, 200 W or 100 W.

[0074] In another embodiment, the test pulse is received directly and simultaneously as a test signal. For this purpose, it is conceivable that the test pulse is emitted via a separate transmitting antenna, so that the local coil 50 can receive this test pulse directly and simultaneously as a test signal in a step S11 via the first signal connection 62, or the body coil 14 can receive this test pulse directly and simultaneously as a test signal in a step S12 via the second signal connection 63. The position of the transmitting antenna must then be taken into account in the subsequent determination of the dependency. If the transmitting antenna is close to the location where spikes occur, close to the wall of the patient tunnel and to the gradient coil, the received signal strengths of the test signal correspond more closely to the ratio of the signal strengths for a spike, whereas if it is arranged closer to the position of the patient 100, they correspond more closely to the ratio of the signal strengths for the magnetic resonance signal.For the latter case, it is conceivable that the local coil 50 has a transmitting antenna.

[0075] It is also conceivable that the test pulse is transmitted via the body coil 14, in which case simultaneous reception via the body coil 14 is not possible. It is therefore necessary to use a predetermined value, for example, received during a previous measurement in step S12 via the reference antenna and the second signal terminal 63 or obtained by calculation, as the reference antenna for the test signal of the body coil 14.

[0076] In both cases, the power of the test pulse can advantageously be reduced to less than 0.01 watts, 0.1 watts, 1 watt or 10 watts, so that the test pulse does not need to be considered in the SAR budget.

[0077] In step S13, a relationship is determined between the test signal received via the first signal connection and the test signal received via the second signal connection. This relationship can be, for example, a ratio or a difference between the amplitudes or power of the test signals, absolute or averaged across the signals. If calibration is performed using a test pulse with a source near the reference antenna, a relationship can also be formed, for example, by an inverse of the ratio or by a threshold value corresponding to an average of the two signals.

[0078] The predetermined deviation in step S31 is then preferably a significant deviation of a dependency of the first signal to the second signal from the dependency determined in step S13. The dependency is preferably related to a magnetic resonance signal, so that, for example, a signal whose amplitude ratio deviates from it, in that the received signal for the body coil 14, with reference to or weighting with the determined dependency, is stronger than the signal received by the local coil 50, is detected as a spike.

[0079] In one embodiment, the dependency can be a measure of the ratio of the amplitudes of the test signal received via the second signal terminal 63 to the test signal received via the first signal terminal 62. A significant deviation within the meaning of the invention exists, for example, if the predetermined ratio is exceeded by more than 10%, 20%, 50%, or 100%.

[0080] It can be provided in step S21 and / or S12 of receiving via the second signal connection, in this case, ie during the reception of the signal or

[0081] test signal, to detune the body coil 14. An interference signal or spike typically has a frequency spectrum that is broader than a magnetic resonance signal. By detuning the body coil 14 by more than 10 kHz, 100 kHz, or 1 MHz relative to the Larmor frequency for receiving an interference signal and / or during calibration in step S12, the interference signal can be emphasized relative to a magnetic resonance signal and more easily detected. This also applies if the second receiver 65 itself remains tuned to the Larmor frequency, because the body coil 14 has an increased sensitivity to E fields, which predominate in the near range during a spike and emphasize it relative to the B1 fields of the MR signals from the body of the patient 100 from the body coil 14.

[0082] In a step S40 of one embodiment of the method according to the invention, the last magnetic resonance measurement is repeated by the magnetic resonance tomograph 1 when the warning signal is received from the device 60. For example, the device 60 is connected to the control unit 20 of the magnetic resonance tomograph 1 via a signal connection, via which the warning signal can be transmitted. If the control unit 20 receives this warning signal, it preferably repeats the last pulse train, i.e., the image acquisition sequence after the last excitation pulse, in order to obtain interference-free measurement data for image reconstruction without artifacts caused by spikes.

[0083] In another possible embodiment, the control unit 20 is configured to remove the interference signal from the last magnetic resonance measurement when the warning signal is received from the device 60. For example, it is conceivable that the control unit 20 simulates measurement data for the duration of the interference by interpolation. It would also be conceivable for the interference signal to be extracted from the magnetic resonance signal of the reference antenna and essentially removed from the measurement data, for example, by scaling and subtraction or more complex methods.

[0084] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited to the disclosed examples and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention as defined by the appended claims.

Claims

1. Magnetic resonance tomograph having a device for detecting a spike during a magnetic resonance scan, wherein the device comprises: a first receiver (64) with a first signal connection (62) for connecting a local coil (50) and a second receiver (65) with a second signal connection (63) for connecting a reference antenna, wherein the device (60) is configured to compare a first signal received by the first receiver (64) via the first signal connection (62) with a second signal received by the second receiver (65) via the second signal connection (63), by means of a comparator unit (61) and to output a warning signal in the event of a predetermined deviation, wherein the magnetic resonance tomograph comprises a body coil and a local coil, and the local coil is connected to the first signal connection, characterised in that the body coil is connected as a reference antenna to the second signal connection of the device.

2. Magnetic resonance tomograph according to claim 1, wherein the device (60) is designed to carry out the comparison in real time.

3. Magnetic resonance tomograph according to claim 1 or 2, wherein the comparison comprises a pattern comparison between the first signal and the second signal.

4. Magnetic resonance tomograph according to one of the preceding claims, wherein the device (60) is configured only to carry out the comparison if the first and / or second signal fulfil a predetermined prior condition.

5. Magnetic resonance tomograph according to one of the preceding claims, wherein the predetermined deviation comprises an exceeding of a predetermined limit value for an amplitude ratio of the second signal to the first signal.

6. Magnetic resonance tomograph according to one of the preceding claims, having a control unit (20) in signal communication with the device, wherein the control unit (20) is designed, on arrival of the warning signal from the device (60), to repeat the sequence of an image acquisition after a last excitation pulse, on the basis of the first signals and second signals of which the warning signal was generated.

7. Method for recognizing a spike in a magnetic resonance scan with a magnetic resonance tomograph (1), wherein the magnetic resonance tomograph (1) comprises a local coil, a reference antenna and a device (60) for detecting a spike during a magnetic resonance scan with a first receiver (64) having a first signal connection (62) in signal communication with the local coil (50) and a second signal connection (63) in signal communication with the reference antenna, wherein the magnetic resonance tomograph has a body coil, wherein the body coil is connected as a reference antenna to the second signal connection of the device, wherein the method comprises the steps: (S20) during a magnetic resonance scan, receiving a first signal from the local coil (50) via the first signal connection (62) of the first receiver (64); (S21) receiving a second signal from the body coil via the second signal connection (63) of the second receiver (65); (S30) comparing the first signal and the second signal by means of the device (60); (S31) emission of a warning signal by the device (60) to the magnetic resonance tomograph (1) if during the comparison a predetermined deviation is detected.

8. Method according to claim 7, wherein the method further comprises the steps for calibration, wherein the steps for calibration are carried out before the steps in claim 7: (S10) emission of a test pulse; (S11) receiving a test signal via the local coil (50) and the first signal connection (62) of the device (60); (S12) receiving the test signal via the reference antenna and the second signal connection (63) of the device (60); (S13) determining a dependency of the test signal received via the first signal connection (62) on the test signal received via the second signal connection (63); wherein the predetermined deviation in step (S31) is a substantial deviation of a dependency of the first signal on the second signal from the dependency determined in step (S13).

9. Method according to claim 8, wherein the dependency determined is a measure for the ratio of the amplitudes of the test signal received via the second signal connection (63) to the test signal received via the first signal connection (62) and a substantial deviation has taken place if the predetermined ratio is exceeded by more than 10%, 20%, 50% or 100%.

10. Method according to claim 8 or 9, wherein the step of receiving via the second signal connection (63) comprises the substep of detuning the body coil (14).

11. Method according to one of claims 7 to 10, wherein the method further comprises the steps: during the magnetic resonance scan, generating multiple pulse trains by emitting multiple excitation pulses for exciting nuclear spins in a patient and emitting via the body coil (14); (S40) repeating the last pulse train of the magnetic resonance scan, on the basis of the first signals and second signals of which the warning signal was generated, by way of the magnetic resonance tomograph (1) if the warning signal arrives from the device (60), wherein, in step (S20) receiving a first signal from the local coil (50) via the first signal connection (62) of the first receiver (64), the first signal is a magnetic resonance signal from the body of the patient.

12. Method according to one of claims 7 to 10, wherein the method further comprises the steps: during the magnetic resonance scan, generating multiple pulse trains by emitting multiple excitation pulses for exciting nuclear spins in a patient and emitting via the body coil (14); removing the interference signal, which corresponds to the spike, of the last pulse train of the magnetic resonance scan, on the basis of the first signals and second signals of which the warning signal was generated, by way of the magnetic resonance tomograph (1) if the warning signal arrives from the device (60), wherein, in step (S20) receiving a first signal from the local coil (50) via the first signal connection (62) of the first receiver (64), the first signal is a magnetic resonance signal from the body of the patient.

13. Computer program product which can be loaded directly into a processor of a control system (20) of the magnetic resonance tomograph according to claim 1, having program code means in order to carry out all the steps of a method according to one of claims 7 to 12 when the program product is executed on the control unit (20).

14. Computer readable storage medium with items of electronically readable control information stored thereon, which are configured in order, on use of the storage medium in the control unit (20) of the magnetic resonance tomograph (1) according to claim 1, to carry out the method according to one of claims 7 to 12.