Dual-resonant MRI local coil with integrated pilot tone signal frequency converter

Abstract

The invention relates to a local coil for a magnetic resonance imaging (MRI) scanner. The local coil is designed to receive a magnetic resonance signal and a pilot tone signal and transmit them to a receiver of the MRI scanner for evaluation. The pilot tone signal lies in a first frequency range, which is disjoint and has a significant frequency separation from a second frequency range containing the magnetic resonance signal. The local coil includes at least one frequency converter configured to convert the pilot tone signal and the magnetic resonance signal into a common frequency range.

Description

[0001] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

[0002] The invention relates to a local coil for receiving a pilot tone signal and a magnetic resonance imaging (MRI) scanner with a local coil according to the invention. The MRI system has a receiver configured to simultaneously receive and evaluate a pilot tone signal and a magnetic resonance signal supplied to it by the local coil.

[0003] Magnetic resonance imaging (MRI) scanners are imaging devices that use a strong external magnetic field to align the nuclear spins of a sample and then stimulate them to precess around this alignment using an alternating magnetic field. The precession, or return of the spins from this excited state to a lower-energy state, in turn generates an alternating magnetic field that is received by antennas.

[0004] 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 magnetic resonance signal is then evaluated and a two- or three-dimensional imaging representation of the object under investigation is provided.

[0005] Magnetic resonance signals are very weak. To achieve a sufficiently high signal-to-noise ratio, the signal must therefore be acquired over a long period, either in a single measurement or in repeated scans. Acquiring the magnetic resonance signals is slow compared to unavoidable patient movements such as heartbeat or breathing. These movements cause artifacts in the generated images.

[0006] However, one way to image the moving organs is to repeatedly perform a short image capture synchronized with the movement and average the captured data.

[0007] Synchronization can be achieved using dedicated sensors such as a breathing belt or ECG electrodes.

[0008] To avoid these additional sensors, it is already known from publication US 2015 / 0320342 A1 to couple a continuous, monofrequency alternating magnetic field originating from a small conductor loop, at least partially through the patient's body into the individual elements of a magnetic resonance local coil.

[0009] Since most biological tissues are almost completely transparent to magnetic fields, the generated magnetic field penetrates the patient's body almost unchanged. However, most tissues are (weakly) conductive, and therefore the continuous wave magnetic field induces eddy currents. These eddy currents then generate a magnetic field that is superimposed on the excitation field, leading to modulations in the received magnetic field at the receiver coil.

[0010] By evaluating this signal, conclusions can be drawn about a movement phase of the heart or breathing.

[0011] It reduces the hardware requirements if a signal with a frequency close to that of the magnetic resonance signal is used to detect movement, allowing it to be evaluated by the same receiver, preferably simultaneously. However, the degree of modulation decreases sharply with frequency, especially for cardiac movements, so that reliable heartbeat detection in this way is hardly possible with magnetic resonance systems operating at low static magnetic field strengths B0, for example, 0.5 T.

[0012] From an unpublished patent application of the applicant, a local coil is known which performs a frequency conversion of the pilot tone signal into a common frequency range with the magnetic resonance signal.

[0013] It is therefore an object of the present invention to improve the local coil.

[0014] The problem is solved by a local coil according to the invention as claimed in claim 1.

[0015] The local coil according to the invention is designed for use with a magnetic resonance imaging (MRI) scanner to record magnetic resonance signals from a patient or object in a static magnetic field B0 of the MRI scanner. The frequency of the magnetic resonance signal is defined by the magnetic field strength B0 and a magnetic moment of the nuclear spins to be detected and is referred to as the Larmor frequency. The term "local coil" includes the elements necessary for operating the coil, such as cables, plugs, and connectors or adapters in the lead to the MRI scanner.

[0016] The local coil according to the invention is specifically designed to receive a magnetic resonance signal and a pilot tone signal and transmit them to a receiver of the magnetic resonance imaging scanner for evaluation. "Receiving" here refers in particular to the conversion of the magnetic or electromagnetic high-frequency alternating field of the nuclear spins and the pilot tone signal into an electrical signal by an antenna or induction loop. "Pilot tone" refers to a magnetic or electromagnetic high-frequency signal emitted by a pilot tone transmitter, preferably integrated into the local coil, which interacts with the patient, such that the patient's movements, such as breathing or heartbeat, modulate the pilot tone into a pilot tone signal. The receiving process can also include further signal processing steps such as amplification or filtering.

[0017] The pilot tone, and thus the pilot tone signal received by the antenna or induction loop, lies within a first frequency range. A frequency range is a spectral range that includes the frequency of the pilot tone and at least the sidebands caused by the modulation. The first frequency range can have a bandwidth of more than 10 Hz, 100 Hz, 1 kHz, 10 kHz, or even 100 kHz.

[0018] The magnetic resonance signal received by the antenna or induction loop lies in a second frequency range, which encompasses the already defined Larmor frequency of the magnetic resonance imaging (MRI) scanner for which the local coil is designed. The bandwidth of the second frequency range is essentially determined by the bandwidth of the magnetic resonance signals, which in turn depends on the slice thicknesses to be acquired and the magnetic field strengths of the gradients used for spatial encoding. The bandwidth of the second frequency range can exceed 100 kHz, 50 kHz, 1 MHz, or 5 MHz. According to the invention, the processed magnetic resonance signal, optionally also the frequency-converted magnetic resonance signal, is also referred to as such.

[0019] The first and second frequency bands are disjoint. A signal in the first frequency band is not part of the second, and vice versa. In particular, it is also conceivable that a frequency in the first frequency band is an integer multiple of a frequency in the second. The frequency bands have a frequency separation that is significantly larger than, for example, the center frequency or bandwidth of one of the frequency bands, such as an integer multiple with n>1.

[0020] The local coil according to the invention comprises a frequency converter. A frequency converter is a device that converts a signal from one frequency to another. The frequency converter is preferably implemented by a nonlinear element that converts a first signal with a first frequency to one or more other frequencies by mixing, e.g., multiplying or performing another nonlinear operation with a second signal.

[0021] According to the invention, the frequency converter of the local coil converts the recorded pilot tone signal into a common frequency range with the magnetic resonance signal.

[0022] One possibility is that a first frequency converter converts the pilot tone signal into the frequency range that also includes the magnetic resonance signal emitted by the nuclear spins, meaning that the common frequency range is the second frequency range.

[0023] However, it is also conceivable that the first frequency converter transforms the pilot tone signal into a common frequency range that is disjoint from the first and second frequency ranges. The magnetic resonance signal received by the antenna is then converted into this common frequency range by a second frequency converter or even by the first frequency converter. This could, for example, be an intermediate frequency that prevents feedback to the input signal, reduces attenuation during transmission to the MRI scanner, and / or serves as a frequency-division multiplexer.

[0024] In other words, the pilot tone signal is converted from the first frequency range into a common frequency range with the magnetic resonance signal, so that the converted pilot tone signal can then be processed and evaluated by a receiver for the magnetic resonance signal.

[0025] Preferably, the frequency conversion is performed such that the magnetic resonance signal and the converted pilot tone signal do not overlap in the second frequency range, but lie side by side. This can be achieved, for example, by appropriately selecting the frequency of a mixer or oscillator signal.

[0026] Advantageously, the frequency converter of the local coil according to the invention enables the pilot tone signal to have a significantly higher frequency than the magnetic resonance signal and yet still be processed by a receiver for the magnetic resonance signal. Particularly in magnetic resonance imaging scanners with a low static magnetic field, this allows for a cost-effective pilot tone solution with high motion sensitivity.

[0027] The local coil according to the invention includes an amplifier unit for amplifying the magnetic resonance signal. This refers in particular to a low-noise preamplifier, which is arranged directly behind the antenna coil for amplifying the magnetic resonance signal and is also referred to as an LNA (low noise amplifier).

[0028] The frequency converter is formed by the amplifier unit. This means that the nonlinear operation is performed by an active element of the amplifier unit, which also amplifies the magnetic resonance signal in accordance with the LNA (Linear Numerical Analysis). Preferably, the amplifier element is a transistor, for example, a bipolar transistor or a field-effect transistor.

[0029] The use of the LNA as a mixer advantageously reduces the complexity and thus, in particular, the power consumption and waste heat in the local coil according to the invention.

[0030] In a preferred embodiment of the local coil according to the invention, the amplifier unit is a transistor. The input signal, in particular the pilot tone signal, is converted into a different frequency range by the frequency converter by mixing it with a mixer signal or oscillator signal. For this purpose, the mixer signal must be supplied to the frequency mixer. If, as here, the mixer is implemented as an amplifier element of an amplifier circuit using a transistor, one possibility is to utilize the coupling through the parasitic capacitances between the transistor control input (e.g., base) and the transistor output (e.g., collector) to couple the mixer signal. Since the mixer signal is available with a high amplitude, even the small coupling through the parasitic capacitance is sufficient. It is also conceivable to utilize other parasitic couplings from output terminals to the input, for example, from the emitter or drain or source in the case of a FET transistor.

[0031] In one possible embodiment of the local coil according to the invention, the mixed signal is fed to the output pole via a filter. Filters are considered to be, in particular, elements that have a frequency-dependent transfer function, for example, high-pass, low-pass, or band-pass filters. Advantageously, depending on the frequency of the mixed signal, pilot tone signal, and magnetic resonance signal, these signals can be decoupled from each other.

[0032] In one conceivable embodiment of the local coil according to the invention, the local coil comprises a plurality of antenna coils. The amplifier unit has at least two cascaded stages. Cascaded stages are defined here as amplifier elements that are directly connected one after the other, i.e., connected, for example, only by passive components and located in close proximity, i.e., in particular, arranged within the local coil. It is particularly conceivable that in a first stage, each antenna coil is assigned a first amplifier element, for example, to decouple the antenna elements. In a second stage, at least the signals from two first stages are combined and amplified in an amplifier element of the second amplifier stage. The frequency converter is implemented by an amplifier element of the second stage.As already explained, the amplifier element with mixer function differs from a pure mixer in its amplification of the magnetic resonance signal.

[0033] Advantageously, in a cascaded amplifier unit, the frequency conversion can be combined in the second stage, thus reducing the number of components, while the antenna coils remain decoupled from the frequency mixing and the mixing signal.

[0034] In one possible embodiment of the local coil according to the invention, the mixing gain of the amplifier unit is set so low that a noise component caused by the mixing process is negligible in a frequency range of the magnetic resonance signal. The mixing gain is defined as the ratio of the amplitude of the signal to be mixed at the input of the mixer, here the pilot tone, to the amplitude of the mixing product provided at the output of the mixer, here the pilot tone signal converted to a different frequency. The mixing gain depends, firstly, on the classical gain of the mixer itself, i.e., for example, in the case of a transistor as the mixer, on its intrinsic current gain and the circuit configuration. Furthermore, the amplitude of the mixing product depends on the amplitude of the oscillator signal with which the input signal is mixed in the mixer.The mixing gain can therefore be influenced independently of the gain of the magnetic resonance signal by raising the level of the oscillator signal, for example by the magnetic resonance imaging scanner raising its level.

[0035] The noise component is preferably considered negligible if the additional noise from mixing is 12 dB or 20 dB lower than the noise contribution of the MR signal path. This additional noise component is caused, for example, by thermal noise in a frequency range that is converted to the frequency of the magnetic resonance signal by the mixing process. The mixing gain can advantageously be adjusted via the level of the local intermodulation (LO) signal. This can also be sequence-dependent, for example, depending on the expected amplitude of the magnetic resonance signal. Since the mixing process is a second-order intermodulation, the level of the mixing product is proportional to the LO signal level.

[0036] The amplifier unit can advantageously provide signal mixing without any noticeable deterioration of the image.

[0037] A system according to the invention comprises a local coil and a magnetic resonance imaging (MRI) scanner. The MRI scanner provides a signal that is coupled into the patient via an induction loop and interacts with the patient. The response signal generated by induced eddy currents is superimposed on the excitation field to form a summed signal, which is preferably received and evaluated by an antenna coil together with the MRI signals as a pilot tone signal. The MRI scanner is configured to provide the pilot tone signal supplied to the coupling induction loop at a level high enough that the received pilot tone signal has a sufficient signal-to-noise ratio (SNR) for evaluating patient movement. In particular, this compensates for the previously described low mixing gain of the amplifier unit.It is also conceivable that the magnetic resonance imaging scanner provides an automatic and / or dynamic compensation function, i.e., for example, raises the level of the oscillator signal or mixed signal if the downmixed pilot tone signal provided for evaluation has too low an amplitude or too poor a signal-to-noise ratio (SNR).

[0038] In this advantageous way, the system can ensure a consistent quality of the pilot tone signal and the motion detection provided with it.

[0039] The properties, features and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more easily understood in connection with the following description of the exemplary embodiments, which are explained in more detail in conjunction with the drawings.

[0040] They show: Fig. 1 a schematic representation of a magnetic resonance system with a local coil according to the invention; Fig. 2 a schematic representation of an exemplary embodiment of a local coil according to the invention; Fig. 3 a schematic representation of an exemplary embodiment of a local coil according to the invention; Fig. 4 a schematic representation of an exemplary embodiment of a local coil according to the invention in conjunction with a magnetic resonance tomograph.

[0041] Fig. Figure 1 shows a schematic representation of an embodiment of a magnetic resonance system 1 with a local coil 50 according to the invention.

[0042] The magnet unit 10 includes a field magnet 11 that generates a static magnetic field B0 for aligning the nuclear spins of samples or the patient 100 within a recording area. The recording area is characterized by an extremely homogeneous static magnetic field B0, with the homogeneity relating in particular to the magnetic field strength, orientation, and magnitude. The recording area is nearly spherical and arranged within a patient tunnel 16 that extends longitudinally 2 through the magnet unit 10. A patient bed 30 can be moved within the patient tunnel 16 by the traversing unit 36. The field magnet 11 is typically a superconducting magnet capable of generating magnetic fields with a magnetic flux density of up to 3 T, and even higher in the latest devices. However, for lower field strengths, permanent magnets or electromagnets with normal-conducting coils can also be used.

[0043] Furthermore, the magnetic unit 10 includes gradient coils 12, which are designed to superimpose variable magnetic fields in three spatial directions onto the magnetic field B0 in order to spatially differentiate the captured imaging areas in the investigation volume. The gradient coils 12 are typically coils made of normally conducting wires that can generate mutually orthogonal fields in the investigation volume.

[0044] The magnet unit 10 also has a body coil 14, which is designed to couple a high-frequency signal supplied via a signal line into the examination volume and to receive 100 resonance signals emitted by the patient and to transmit them via a signal line.

[0045] 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.

[0046] The control unit 20 thus has a gradient control 21 which is designed to supply the gradient coils 12 with variable currents via supply lines, which provide the desired gradient fields in the investigation volume in a time-coordinated manner.

[0047] Furthermore, the control unit 20 includes a high-frequency unit 22 designed to generate a high-frequency magnetic field pulse with a predefined temporal profile, amplitude, and spectral power distribution to excite magnetic resonance of the nuclear spins in the patient 100. Pulse powers in the kilowatt range can be achieved. The excitation pulses can be coupled into the patient 100 via the body coil 14 or via a local transmitting antenna.

[0048] The high-frequency unit 22 also includes a receiver 40 for receiving and processing a magnetic resonance signal from the patient 100, which is picked up by the local coil 50 and transmitted to the receiver 40 via a signal connection. The receiver 40 is also designed to receive and evaluate a pilot tone.

[0049] In one embodiment, the high-frequency unit 22 also includes a pilot tone transmitter 70, which transmits a pilot tone to a pilot tone transmitting antenna or coupling loop, possibly also arranged in the local coil 50, for coupling via a signal connection. However, a separate pilot tone transmitter 70, operating independently of the magnetic resonance system 1, is also conceivable.

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

[0051] A local coil 50 is arranged on the patient 100, which is connected to the high-frequency unit 22 and its receiver via a connecting line 33.

[0052] Preferably, the local coil 50 further comprises a pilot tone transmitting antenna or coupling loop with which a pilot tone can be transmitted or induced into the body of the patient 100. Preferably, the pilot tone is generated by the coupling loop in the form of an alternating magnetic field that at least partially penetrates the body of the patient 100 and couples into the antenna coils 51 of the magnetic resonance imaging scanner 1.

[0053] Fig. Figure 2 shows a schematic representation of an exemplary embodiment of an amplifier unit 60 of a local coil 50 according to the invention.

[0054] The amplifier unit 60 comprises a transistor 65 as its amplifier element. The transistor has a parasitic capacitance between its base and collector, which is indicated by the capacitor within the circuit symbol. For an exemplary transistor BFR182W, 0.34 pF is typical. A pilot tone signal and a magnetic resonance signal from receiver loops, such as the antenna coils 51, are fed to the base of the transistor 65. The pilot tone signal P and the magnetic resonance signal MR are preferably filtered beforehand by resonant elements such as the first filter 61 and the second filter 62. The circuit shown, consisting of the first filter 61 and the second filter 62, forms a so-called diplex filter, with which the two signals with different center frequencies can be decoupled from each other and combined with correct impedance matching. In an exemplary magnetic resonance imaging scanner 1 with a static magnetic field B0 of 0.With transistor 55T, the center frequency of the magnetic resonance signal is 23.6 MHz. The pilot tone has a frequency of 112.5 MHz. A mixed signal LO is fed via filter 63 and the collector, utilizing the paracitic collector-base coupling capacitance. To convert the pilot tone into the frequency range of the magnetic resonance signal, the frequency of the mixed signal is 90 MHz in this example, resulting in a frequency difference of 22.5 MHz between the pilot tone and the mixed signal. Due to the small spectral difference between the MR signal (MR) and the frequency-converted pilot tone signal (IF), both signals can be simultaneously detected and processed by the receiver. The operating point of transistor 65, and thus the non-linearity of the characteristic curve, can be influenced by the base or bias voltage.Furthermore, the amplitude of the attenuated pilot tone signal can also be changed via the amplitude of the mixed signal. At the output, this signal, along with the magnetic resonance signal, is filtered by a fourth filter 64. Filter 64 is designed such that both the MR signal and the IF signal can pass through essentially undamped, while interfering spectral components, in particular higher-order signal harmonics and the LO signal, are suppressed.

[0055] In Fig. Figure 3 shows a variant of the amplifier unit 60, which in a two-stage version can provide higher amplification and / or better energy efficiency.

[0056] The signal induced in the antenna coils 51 can be filtered by filters 61 and 62. However, it is also conceivable that the antenna coils 51 themselves are doubly resonant at the frequency of the pilot tone and the magnetic resonance signal. The first filter 61 and the second filter 62 can also function as matching elements to adapt the impedance of the antenna coil to the input of the amplifier unit 60.

[0057] The first amplifier stage is formed by transistor 65, as already described in the description. Fig. As can be seen in Figure 2. However, the way in which the mixed signal is fed to transistor 65 differs. Here, the local coil 50 receives both the mixed signal and the frequency-converted pilot tone signal and magnetic resonance signal via a common signal path to the magnetic resonance imaging (MRI) scanner 1. In order to amplify the MRI signal in the second amplifier stage 66, the mixed signal and the MRI signal are separated by diplexers 54 before the second amplifier stage and then recombined. Since the mixed signal preferably has a significantly higher frequency than the MRI signal, the diplexers 54 can each be implemented as a high-pass and a low-pass filter.

[0058] Advantageously, the second amplifier 66 can be designed as a narrowband amplifier in this case, since it only needs to amplify the common narrow frequency range of the magnetic resonance signal and the converted pilot tone signal. Thus, the same gain can be achieved with lower energy consumption compared to a single-stage, broadband amplifier stage with the same gain.

[0059] Fig. Figure 4 shows another possible embodiment of a local coil 50 according to the invention with a magnetic resonance tomograph 1. The pilot tone is supplied by a high-frequency unit 22 of the magnetic resonance tomograph 1 via a signal line to an induction loop 67 and coupled as a high-frequency alternating magnetic field into a patient 100.

[0060] The alternating magnetic field is received by the antenna loops 51, which are tuned to the frequency of the pilot tone and the magnetic resonance signal in a dual resonant configuration. The antenna loops 51 are further coupled to a detuning device 52, which, during the transmission of the excitation pulse, renders the antenna loops non-resonant at the frequency of the excitation pulse. The signal from the antenna coils 51 is passed via a matching circuit 53 to an amplifier unit 60 according to the invention, as already described in [reference to be added]. Fig. 2 was explained.

[0061] The local coil 50 is connected to the magnetic resonance imaging (MRI) scanner 1 via a signal line. The MRI signal and the pilot tone signal are fed to the local coil 50 in a common frequency range, so that a frequency diplexer 54 can multiplex the mixed signal fed to the local coil 50 and the pilot tone signal and MRI signal coming from the local coil 50 on the signal line. QUOTES INCLUDED IN THE DESCRIPTION

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

[0000] US 2015 / 0320342 A1

[0008]

Claims

[1] Local coil for a magnetic resonance imaging (MRI) scanner (1), wherein the local coil (50) is configured to receive a magnetic resonance signal and a pilot tone signal and to transmit them to a receiver (40) of the MRI scanner (1) for evaluation, wherein the pilot tone signal is in a first frequency range which is disjoint and has a substantial frequency separation from a second frequency range in which the magnetic resonance signal is located, and the local coil (50) has at least one frequency converter configured to convert the pilot tone signal and the magnetic resonance signal into a common frequency range, wherein the local coil (50) has an amplifier unit for amplifying the magnetic resonance signal, characterized by , that the frequency converter is formed by the amplifier unit. [2] Local coil according to claim 1, wherein the amplifier unit (60) comprises a transistor (65). [3] Local coil according to claim 2, wherein a sum signal of magnetic resonance signal and pilot tone signal is coupled into an input pole of the transistor (65). [4] Local coil according to claim 3, wherein a mixed signal is coupled to the frequency converter via an output pole of the transistor (65). [5] Local coil according to claim 4, wherein an output signal consisting of an amplified magnetic resonance signal and a frequency-converted pilot tone signal is coupled out from the output pole of the transistor (65). [6] Local coil according to claim 4, wherein the mixed signal is coupled into the output pole. [7] Local coil according to one of the preceding claims, wherein a mixing gain of the amplifier unit (60) is set so low that a noise component caused by mixing is negligible in a frequency range of the magnetic resonance signal. [8] System comprising a local coil according to one of the preceding claims and a magnetic resonance imaging scanner (1), wherein the magnetic resonance imaging scanner (1) is configured to compensate for a deteriorated signal-to-noise ratio of the pilot tone signal by boosting a mixed signal and / or emitted pilot tone signal.

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