Dual resonant MRI local coil with integrated pilot audio signal frequency converter

Abstract

The invention relates to a local coil for a magnetic resonance tomography apparatus. The local coil is designed to record the magnetic resonance signal and the pilot tone signal and to transmit the magnetic resonance signal and the pilot tone signal to a receiver of the magnetic resonance tomography device for evaluation. The pilot audio signal is located within a first frequency range, and the magnetic resonance signal is located within a second frequency range.

Description

Technical Field

[0001] Regardless of the grammatical gender of a particular term, it includes people with both male and female gender identities.

[0002] The invention relates to a local coil for receiving a pilot tone signal and a magnetic resonance tomography apparatus having the local coil according to the invention. The magnetic resonance system has a receiver which is designed to simultaneously receive and evaluate the pilot tone signal and the magnetic resonance signal provided to the receiver by the local coil. Background Art

[0003] A magnetic resonance tomography device is an imaging device that aligns the nuclear spins of the subject to be examined with a strong external magnetic field in order to image the subject, and excites the spins to precess around the alignment by an alternating magnetic field. When the spins precess from the excited state or return to a state of lower energy, they generate an alternating magnetic field in response, which is received by an antenna.

[0004] The signals are spatially encoded by means of magnetic gradient fields, which subsequently enables the received signals to be associated with volume elements. The received magnetic resonance signals are then evaluated and provide a two-dimensional or three-dimensional representation of the examination object.

[0005] Magnetic resonance signals are very weak. Therefore, in order to achieve a sufficiently high signal-to-noise ratio, the signals must be acquired over a long period of time in one measurement or repeated measurements. In this case, the magnetic resonance signals are acquired slowly compared to the inevitable movements of the patient (e.g. heartbeat or breathing movements). These movements lead to artifacts in the generated images.

[0006] However, one possibility for imaging moving organs is to repeatedly carry out short image acquisitions synchronously with the movement and to average the acquired data.

[0007] Synchronization can be performed using dedicated sensors such as a respiratory belt or electrocardiogram electrodes.

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

[0009] Since most biological tissues are almost completely transparent to magnetic fields, the generated magnetic field passes through the patient's body almost unchanged. However, most tissues are (weakly) conductive, so the continuous wave magnetic field induces eddy currents. These eddy currents then in turn generate a magnetic field that is superimposed on the excitation field, which causes the magnetic field received in the receiving coil to be modulated.

[0010] By evaluating this signal, it is possible to infer the phase of cardiac or respiratory movement.

[0011] If a signal with a frequency close to that of the magnetic resonance signal is used to detect the motion, the hardware outlay is reduced and the evaluation can therefore be performed using the same receiver, preferably simultaneously. However, especially for cardiac motion, the modulation degree decreases sharply with frequency, making it almost impossible to reliably detect the heartbeat in this way in magnetic resonance systems with low static magnetic field strengths B0 (e.g. 0.5 T). Summary of the invention

[0012] The object of the present invention is therefore to detect patient movements in a low-field system in a better and more reliable manner.

[0013] The above-mentioned technical problem is solved by a local coil according to the invention and a magnetic resonance tomography system according to the invention.

[0014] The local coil according to the invention is provided for use with a magnetic resonance tomography apparatus in order to record magnetic resonance signals of a patient or object in a static magnetic field B0 of the magnetic resonance tomography apparatus. The frequency of the magnetic resonance signal is defined by the magnetic field strength B0 and the magnetic moment of the nuclear spins to be acquired and is referred to as the Larmor frequency.

[0015] The local coil according to the invention is designed in particular to record magnetic resonance signals and pilot tone signals and to transmit them to a receiver of a magnetic resonance tomography device for evaluation. Here, recording means in particular the conversion of a magnetic high-frequency alternating field or an electromagnetic high-frequency alternating field of nuclear spins and a pilot tone signal into an electrical signal by means of an antenna or an induction loop. The pilot tone means a magnetic high-frequency signal or an electromagnetic high-frequency signal which is emitted by a pilot tone transmitter which is preferably integrated into the local coil and interacts with the patient so that the patient's movements, such as breathing movements or heartbeats, modulate the pilot tone into a pilot tone signal. The recording may also include further signal processing steps, such as amplification or filtering.

[0016] The pilot tone and thus the pilot tone signal recorded by the antenna or the induction loop is located in a first frequency range, wherein the frequency range denotes a spectral range including the frequency of the pilot tone and at least the sidebands caused by the modulation. The first frequency range may include a bandwidth greater than 10 Hz, 100 Hz, 1 kHz, 10 kHz or 100 kHz.

[0017] The magnetic resonance signals recorded by the antenna or the induction loop are in a second frequency range, which includes the already defined Larmor frequency of the magnetic resonance tomography device used with the local coil. The bandwidth of the second frequency range is essentially predetermined by the bandwidth of the magnetic resonance signal, which in turn depends on the slice thickness to be acquired and the magnetic field strength of the gradient used for the position encoding. The bandwidth of the second frequency range can include more than 100 kHz, 50 kHz, 1 MHz or 5 MHz. In the following, the processed magnetic resonance signal may also be referred to as such a frequency-converted magnetic resonance signal.

[0018] In this case, the first frequency range and the second frequency range do not intersect. A signal in the first frequency range is not part of the second frequency range and vice versa. As explained below with respect to the exemplary embodiments, it is also conceivable in particular that the frequencies in the first frequency range are integer multiples of the frequencies in the second frequency range. The frequency ranges have a frequency interval that is significant compared to, for example, a center frequency or a bandwidth of one of the frequency ranges, for example greater than an integer multiple of the center frequency or the bandwidth (where n>1).

[0019] The local coil according to the invention has a frequency converter. A frequency converter is a device that converts a signal from one frequency to another frequency. The frequency converter can be implemented, for example, by a mixer, which converts a first signal having a first frequency into one or more other frequencies by mixing (for example, multiplication or another nonlinear operation).

[0020] According to the invention, a frequency converter of the local coil converts the recorded pilot tone signal into a common frequency range with the magnetic resonance signal.

[0021] For this purpose, it is conceivable on the one hand that the first frequency converter converts the pilot tone signal into a frequency range in which the magnetic resonance signals emitted by the nuclear spins also fall, ie the common frequency range is the second frequency range.

[0022] However, it is also conceivable that the first frequency converter converts the pilot tone signal into a common frequency range that does not intersect the first frequency range and the second frequency range. The magnetic resonance signal acquired by the antenna is converted into this common frequency range by the second frequency converter or the first frequency converter. This can be, for example, an intermediate frequency that prevents reverse coupling of the input signal, reduces attenuation during transmission to the magnetic resonance tomography device and / or is used for frequency multiplexing.

[0023] 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 of the magnetic resonance signal.

[0024] The frequency conversion is preferably performed in such a way that the magnetic resonance signal and the converted pilot tone signal do not overlap in the second frequency range, but lie adjacent to each other. This can be achieved, for example, by suitable selection of the frequency of the mixer signal or oscillator signal.

[0025] The frequency converter of the local coil according to the invention advantageously enables the pilot tone signal to have a significantly higher frequency than the magnetic resonance signal and still be processed by the receiver for the magnetic resonance signal. In particular, in the case of magnetic resonance tomography systems with a low static magnetic field, a pilot tone solution with high motion sensitivity can be provided cost-effectively.

[0026] Further advantageous embodiments are explained in the examples.

[0027] In a conceivable embodiment, the local coil according to the invention has an input for an oscillator signal. An oscillator signal is understood to be a signal with a predetermined reference frequency, which is preferably highly constant. For example, the oscillator signal can be an analog sinusoidal signal that is transmitted electrically by current or voltage modulation. Modulation of the carrier wave is also conceivable, for example for wireless transmission by radio waves, optically or by induction of a modulated magnetic field. The input can be an electrical input or an optical input, an antenna or an induction loop. Digital transmission is also possible by means of a corresponding encoding on the carrier signal.

[0028] The local coil according to the invention is designed to generate the pilot tone for transmission or coupling from the oscillator signal. For example, it is conceivable to convert or multiply the oscillator signal to another frequency using a frequency converter. However, it is also conceivable that the oscillator signal is additionally amplified and / or filtered or is only amplified and / or filtered.

[0029] The local coil according to the invention is also designed to generate a mixer signal from an oscillator signal or to derive a mixer signal from an oscillator signal for frequency conversion of the pilot tone signal by means of a frequency converter. If the pilot tone is generated by frequency multiplication, the mixer signal can also be generated by frequency multiplication from the oscillator signal. However, it is also conceivable that the oscillator signal has a high frequency, for example higher than the mixer signal and / or the pilot tone, so that the mixer signal and / or the pilot tone are generated by frequency division. Some particularly advantageous configurations are explained in more detail below. In this case, frequency multiplication is regarded as multiplication by integer factors, such as is achieved by higher harmonics or harmonics, but also as multiplication by rational factors, such as can be achieved by a synthesizer or PLL.

[0030] The generation of the pilot tone and the mixer signal from a common provided oscillator signal advantageously results in a high frequency stability and phase synchronization of the pilot tone signal and thus also avoids interference with the magnetic resonance imaging.

[0031] In one possible embodiment of the local coil according to the invention, the local coil has a first frequency multiplier with a first multiplication factor m for generating a pilot tone from an oscillator signal and a second frequency multiplier with a second multiplication factor n for generating a mixer signal from the oscillator signal. In other words, the frequency of the pilot tone is m times the frequency of the oscillator signal, and the frequency of the mixer signal is n times the frequency of the oscillator signal.

[0032] Advantageously, both the pilot tone and the mixer signal for down-mixing the pilot tone signal are generated from a single oscillator signal, which simplifies the frequency scheme and reduces possible sources of interference for the magnetic resonance signal.

[0033] In a preferred embodiment of the local coil according to the invention, the first multiplication factor m and the second multiplication factor n differ by 1. In other words, the magnitude of the difference (nm) is equal to 1. Preferably, m and n are natural numbers greater than 1, but it is also conceivable that one of m and n has the value 1.

[0034] Advantageously, frequency multiplication with the mentioned value ratio has the result that at least one of the mixing products has the frequency of the oscillator signal again in a stable manner, which avoids image artifacts caused by interference signals in the form of mixing products with other frequency values ​​and enables processing by a receiver for magnetic resonance signals.

[0035] In a conceivable embodiment of the local coil according to the invention, the mixer signal has the same frequency as the oscillator signal. In other words, the oscillator signal is not frequency-changed by frequency division, frequency multiplication or frequency mixing before being used as the mixer signal. The frequency converter to which the mixer signal is supplied is a switching mixer. A switching mixer is understood here to be a mixer which does not perform an analog multiplication of the two signals to be mixed, but rather switches the polarity of the frequency to be mixed in a controlled manner via the mixer frequency. It is conceivable that the polarity switching is replaced by switching on and off. The mixer signal is preferably a symmetrical square wave signal, i.e., has a period duration of the same length for both states.

[0036] Due to the Fourier equivalence, the switching mixer advantageously operates like an analog mixer with a mixer signal consisting of a Fourier series with odd multiples of the fundamental frequency as components. A separate frequency converter can thus be omitted for generating the mixer signal from the oscillator signal. The reduced mixer conversion gain for the pilot tone signal due to the lower amplitude of the mixer signal harmonics can be compensated by a corresponding gain or frequency dependency of the subsequent signal processing.

[0037] In one possible embodiment of the local coil according to the invention, the local coil has an antenna coil for receiving the pilot tone signal and the magnetic resonance signal. In other words, the local coil is designed to simultaneously record the pilot tone signal and the magnetic resonance signal with the antenna coil at a signal level that allows evaluation in the form of imaging and motion acquisition. For this purpose, the antenna coil has resonance in the first frequency range and the second frequency range at the same time, i.e. for the pilot tone signal and the magnetic resonance signal.

[0038] Advantageously, therefore, no separate antenna coils are required for the magnetic resonance signal and the pilot tone signal having significantly different frequencies, which correspondingly reduces the space and expense of detuning.

[0039] In one conceivable embodiment of the local coil according to the invention, the local coil has an attenuator. An attenuator is considered to be an electronic component or circuit having a signal input and a signal output, wherein the attenuator outputs at the signal output a signal applied to the signal input at at least one predetermined frequency, the signal being reduced in level by a predetermined attenuation factor. The attenuator is designed to attenuate the pilot tone signal to a level comparable to or lower than the magnetic resonance signal to be received before the pilot tone signal is passed to the magnetic resonance tomography device for joint evaluation together with the magnetic resonance signal. "Comparable" is understood to mean that after the attenuator, the level of the pilot tone signal is equal to or lower than the level of the received magnetic resonance signal, in particular the level of the maximum magnetic resonance signal to be received, which is defined by the upper limit of the linear amplification range of the input amplifier of the receiver of the magnetic resonance tomography device. The attenuation of the received pilot tone signal by the attenuator is preferably greater than 3 dB, 6 dB or 9 dB relative to the received magnetic resonance signal. However, the attenuation is preferably not greater than 12 dB, 18 dB or 24 dB.

[0040] Here, the attenuator for the pilot tone signal is preferably arranged in the signal path of the pilot tone signal before the frequency conversion, but may also be arranged in the signal path of the pilot tone signal after the frequency conversion or may also be part of the frequency conversion.

[0041] In one embodiment of the local coil according to the invention, a plurality of antenna coils are present in an antenna matrix, and it is conceivable that, due to the different distances of the antenna coils from a transmitting antenna or an induction loop for the pilot tones integrated in the local coil, the attenuation of the attenuator for the pilot tones is designed differently for the individual signal paths associated with the antenna coils. Preferably, due to the expected lower levels of the pilot tone signals, the attenuation will decrease with increasing distance between the antenna coil and the transmitting antenna. It is also conceivable that the attenuator can be adjusted by the control of the magnetic resonance tomography device, in order, for example, to be able to take into account different arrangements in the flexible local coil matrix.

[0042] Advantageously, the attenuator, in combination with the large frequency interval between the pilot tone signal and the Larmor frequency, allows only the pilot tone signal to be attenuated compared to the magnetic resonance signal. The pilot tone signal can thus be emitted at a significantly higher level and the SNR (signal-to-noise ratio) for evaluating the pilot tone signal can be improved. As a result, the recognition of movements such as heartbeats or respiratory movements is more stable and reliable.

[0043] In one possible embodiment of the local coil according to the invention, the local coil is designed to transmit the pilot tone at a higher level than the magnetic resonance signal to be received. In other words, due to the frequency interval between the Larmor frequency and the pilot tone, the level of the pilot tone can advantageously be greater than the level of the magnetic resonance signal to be received without interfering with the reception of the magnetic resonance signal.

[0044] The SNR of the pilot tone signal can thus be improved and the movement recognition with the aid of the pilot tone can be improved, in particular made more stable and reliable.

[0045] In a conceivable embodiment of a magnetic resonance tomography device according to the invention, the magnetic resonance tomography device has a local coil according to the invention, and also has an oscillator and a receiver. The magnetic resonance tomography device is designed to provide an oscillator signal for the local coil using an oscillator. This is particularly related to the selection of the frequency and its stability. On the one hand, the oscillator signal and the mixer signal derived therefrom must be suitable for converting the magnetic resonance signal and / or the pilot tone signal into the desired frequency or intermediate frequency, and at the same time be selected so that the corresponding mixing products and harmonics do not interfere with the received magnetic resonance signal and the intermediate frequency. In addition, the receiver is designed to evaluate the pilot tone signal and the magnetic resonance signal received from the local coil according to the invention for image acquisition and motion detection. The oscillator signal as well as the frequency converter and the mixer are particularly designed to fall within a common frequency range that can be evaluated by the receiver. A detailed example for a suitable frequency scheme is explained below with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The characteristics, features and advantages of the present invention described above and the manner of implementation thereof will be more clearly understood with reference to the following description of the embodiments explained in more detail with reference to the accompanying drawings.

[0047] In the attached picture:

[0048] Figure 1 A schematic diagram of a magnetic resonance system according to the present invention is shown;

[0049] Figure 2 A schematic diagram showing an exemplary embodiment of a local coil according to the present invention;

[0050] Figure 3A schematic diagram showing a portion of an exemplary embodiment of a local coil according to the present invention;

[0051] Figure 4 A schematic diagram shows an exemplary embodiment of a local coil according to the invention. DETAILED DESCRIPTION

[0052] Figure 1 A schematic diagram of an embodiment of a magnetic resonance system 1 according to the invention is shown.

[0053] The magnet unit 10 has a field magnet 11 which generates a static magnetic field B0 for aligning the nuclear spins of a sample or patient 100 in a recording region. The recording region is characterized by an extremely uniform static magnetic field B0, wherein the uniformity relates in particular to the magnetic field strength or orientation and size. The recording region is almost spherical and is arranged in a patient channel 16 which extends through the magnet unit 10 in a longitudinal direction 2. The patient couch 30 can be moved in the patient channel 16 by a travel unit 36. The field magnet 11 is usually a superconducting magnet which can provide a magnetic field with a flux density of up to 3 T, and even higher in the latest devices. However, for lower field strengths, permanent magnets or electromagnets with constant conducting coils can also be used.

[0054] In addition, the magnet unit 10 has gradient coils 12, which are designed to superimpose variable magnetic fields on the magnetic field B0 in three spatial directions in order to spatially differentiate acquired imaging regions in the examination volume. The gradient coils 12 are usually coils made of normally conducting wires, which can generate mutually orthogonal fields in the examination volume.

[0055] The magnet unit 10 furthermore has a body coil 14 which is designed to radiate high-frequency signals provided via signal lines into the examination volume and to receive resonance signals emitted by the patient 100 and to emit them via signal lines.

[0056] The control unit 20 supplies the magnet unit 10 with the different signals for the gradient coils 12 and the body coil 14 and evaluates the received signals.

[0057] The control unit 20 has a gradient control device 21 which is designed to supply the gradient coils 12 via supply lines with a variable current which provides the desired gradient fields in the examination volume in a time-coordinated manner.

[0058] The control unit 20 also has a radio-frequency unit 22, which is designed to generate radio-frequency pulses with a predetermined time profile, amplitude and spectral power distribution for exciting magnetic resonance of nuclear spins in the patient 100. Pulse powers in the kilowatt range can be achieved. The excitation pulses can be transmitted into the patient 100 via the body coil 14 or via local transmission antennas.

[0059] The high-frequency unit 22 also has a receiver 40 for receiving or processing magnetic resonance signals from the patient 100, which are recorded by the local coil 50 and transmitted via a signal connection to the receiver 40. The receiver 40 is also designed to receive and evaluate pilot tones.

[0060] In one embodiment, the high-frequency unit 22 also has a pilot tone transmitter 70, which transmits the pilot tone to a pilot tone transmission antenna or a coupling loop (possibly also in the local coil 50) for coupling out via a signal connection. However, a separate pilot tone transmitter 70 operating independently of the magnetic resonance system 1 is also conceivable.

[0061] The control device 23 communicates with the gradient control device 21 and the high-frequency unit 22 via a signal bus 25 .

[0062] A local coil 50 is arranged on the patient 100 and is connected to the high-frequency unit 22 and its receiver via a connecting line 33 .

[0063] The local coil 50 preferably also has a pilot tone transmitting antenna or a coupling loop, with which the pilot tone can be transmitted or induced into the body of the patient 100. The pilot tone is preferably generated by the coupling loop in the form of an alternating magnetic field, which at least partially penetrates the body of the patient 100 and is coupled into the antenna coil 51 of the magnetic resonance tomography apparatus 1.

[0064] Figure 2 A schematic diagram of an exemplary embodiment of a local coil 50 according to the present invention is shown.

[0065] The local coil 50 has a plurality of antenna coils 51. The antenna coils 51 of this embodiment are designed to receive magnetic resonance signals and pilot tone signals at substantially different frequencies. In order to achieve the highest possible sensitivity at the two frequencies, the antenna coils 51 are double resonant at these frequencies. For example, Figure 2 As shown, this can be achieved by a parallel resonant circuit connected to the loop of the antenna coil 51.

[0066] The antenna coil 51 is also connected to a detuning device 52 which detunes the antenna coil 51 in order to protect the antenna coil during the excitation pulses.

[0067] The subsequent matching circuit 53 is responsible for matching the impedance of the antenna coil 51 for subsequent signal processing. Figure 2 As shown, the adaptation circuit 53 can have the function of an unbalancer, ie a transition from a symmetrical antenna signal to a symmetrical amplifier input.

[0068] In the signal separator 54, the pilot tone signal and the magnetic resonance signal are separated due to the different frequencies and then amplified in the preamplifier or LNA 55. Subsequently, the pilot tone signal is filtered in the first filter 56 in order to avoid undesired frequency components in the subsequent downmixing in the mixer 57. Subsequently, the mixing products are limited to the frequency band of the magnetic resonance signal in the second filter 58 so that they can be transmitted together with the magnetic resonance signal and evaluated by the receiver 40. The generation of the mixing frequency and an exemplary frequency scheme are shown below.

[0069] Sometimes it is also possible to exchange the order of the individual processing steps, in particular for linear processing steps. It is also conceivable to combine functions such as adaptation and filtering.

[0070] In particular, it is also conceivable to provide an attenuator in a separate signal path of the pilot tone signal, with which the level of the pilot tone signal is reduced to a level that is less than or equal to the level of the received magnetic resonance signal. The attenuator can in principle be arranged in the entire signal path of the pilot tone signal before the pilot tone signal is combined with the magnetic resonance signal for further use. Preferably, the attenuator is arranged before the frequency conversion of the pilot tone signal into the frequency range of the magnetic resonance signal, so that no high signal levels that could interfere occur in the frequency range of the magnetic resonance signal. It is also conceivable that the attenuator is a component of the functional group already described, for example the LNA 55 or the first filter 56 in the signal path.

[0071] Figure 2 The local coil 50 also has a pilot tone transmitter 70. For this purpose, the local coil 50 receives an oscillator signal from the magnetic resonance tomography device 1 via a signal input, from which the pilot tone and the mixing frequency are derived. The pilot tone is derived from the oscillator signal or generated from the oscillator signal by the pilot tone transmitter 70 and is coupled out to the environment as an alternating magnetic field via a pilot tone transmission antenna or a coupling loop.

[0072] An exemplary frequency scheme is explained below. The determining element of this frequency scheme is the field magnet 10, which determines the frequency of the magnetic resonance signal, the so-called Larmor frequency, based on the type of nuclear spin to be detected. For the example calculation, a value of 23.6 MHz is assumed here for a low-field system. The oscillator signal has a nearby frequency, but is no longer within the frequency range or bandwidth of the magnetic resonance signal and does not interfere with the image acquisition. As an example value, a frequency of 22.5 MHz is assumed here.

[0073] In the first frequency multiplier 59, a pilot tone signal is generated, which is coupled out into the patient via the induction coil. The multiplication factor can be, for example, 5, so that the pilot tone signal has a frequency of 112.5 MHz. The mixing frequency is generated from the oscillator signal via a second frequency multiplier 60, with a different factor. For example, the factor here can be 4, so that the mixing frequency is 4*22.5 MHz=90 MHz.

[0074] For example, multiplication by a factor of 4 may be achieved using a full-wave rectifier and subsequent harmonic filtering, while odd harmonics may be generated by limiting / square-wave shaping.

[0075] If the pilot tone signal and the mixing signal are mixed in the mixer 57, among other things, a mixing product is obtained with a frequency corresponding to the difference between the frequencies of the mixing signal and the pilot tone signal, i.e. (5-4)*22.5 MHz = 22.5 MHz, which is again the frequency of the oscillator signal. It is also conceivable that the frequency of the mixing signal is multiplied by a factor higher than 1. However, other solutions are also possible, such as an oscillator signal with a smaller frequency, which corresponds to half the frequency equal to 11.25 MHz. It is then possible to choose a multiplication factor that differs by 2, which allows the use of a simple frequency multiplier with even harmonics for both signals.

[0076] The signal-to-noise ratio (SNR) of the MR signal (magnetic resonance signal) must not be significantly reduced by the combination with the PT reception signal (pilot tone reception signal), for example, the reduction is less than 50mdB. Therefore, from the antenna coil 51 until the magnetic resonance signal is combined with the pilot tone signal for transmission to the magnetic resonance tomography device, by appropriately designing the pilot tone signal level, it is ensured that the background noise of the PT reception signal is at least 20dB lower than the background noise of the MR reception signal during the combination. This can be achieved by appropriately designing the size of the preamplifier and / or by an asymmetric design in the combination, such as the resistance value of the summing element, which assumes the function of an attenuator. The noise figure increased thereby in the pilot tone signal reception path compared to the magnetic resonance signal reception path can be compensated by a correspondingly higher pilot tone amplitude.

[0077] Figure 3 A schematic diagram of a portion of an exemplary embodiment of a local coil 50 according to the present invention is shown. Figure 3 The local coil 50 and Figure 2 The embodiment of FIG. 1 is different in that the dual-resonance antenna coil 51 is not used jointly for receiving the pilot tone signal and the magnetic resonance signal, but separate antenna coils 51 are used for each. Figure 3 Only the Figure 2 Different parts.

[0078] Figure 3 A total of four antenna coils 51 are shown in FIG. 1 , each pair of antenna coils includes an antenna coil 51 for a pilot tone signal and an antenna coil 51 for a magnetic resonance signal, and the output signals thereof are provided together to a receiver 40 of a magnetic resonance tomography apparatus 1. Figure 3 In the embodiment, the paired antenna coils 51 are arranged concentrically, but this is not absolutely necessary, but the space requirement can be reduced. The antenna coil 51 for the pilot tone signal can also be designed to be significantly smaller than the antenna coil 51 for the magnetic resonance signal. The resulting reduction in sensitivity can be compensated by increasing the amplitude of the corresponding pilot tone.

[0079] exist Figure 3 In an embodiment, only one detuning device 52 is provided for each antenna coil 51 that is to receive the magnetic resonance signal. In contrast, the antenna coil 51 for the pilot tone signal can be permanently tuned to the significantly higher frequency of the pilot tone signal, so that it is only slightly and harmlessly excited by the excitation pulse. Since the signals are already physically separated, signal separation by a signal separator 54 can be omitted. Further processing of the signals is carried out as follows Figure 3 Proceed as shown.

[0080] Figure 4 A schematic diagram of an exemplary embodiment of a local coil 50 according to the present invention is shown. Figure 4 The local coil 50 and Figure 2 The embodiments of FIG. 5 differ in the type of mixer 57 and the processing of the pilot tone signal and the mixer frequency.

[0081] Figure 4 The mixer 57 provided in is a switching mixer, i.e. the signal to be mixed is not multiplied analogously with the preferably sinusoidal mixing signal, but is digitally switched in polarity or switched on and off at the mixer frequency. Due to the Fourier decomposition, this corresponds to mixing the signal to be mixed with a sinusoidal signal beam of the mixer frequency, which has odd harmonics of the original digital mixer signal, using an analog or multiplication mixer.

[0082] like Figure 2As shown, the magnetic resonance signal and the pilot tone signal are first recorded using a dual-resonance antenna coil 51. Detuning by a detuning device 52, adaptation by an adaptation circuit 53, and preamplification by an LNA 55 are performed together for both signals. Then, a first filter 56 and a second filter 58 separate the signals based on their frequencies. The amplitude of the pilot tone signal can be adapted by an attenuator 62 to ensure the best SNR for the magnetic resonance signal as described above. The first filter 56 and the second filter 58 also ensure that undesirable side frequencies are not included, which produce interfering mixing products with interfering frequencies in the subsequent mixer.

[0083] The mixer 57 as a switching mixer causes the input signal to be mixed at different frequencies at the same time and accordingly generates a large number of mixing products. From the mixing products, the third filter 63 is used in accordance with the following explanation. Figure 4 The frequency scheme of the embodiment of the present invention is used to select mixing products of the pilot tone signal and the magnetic resonance signal, which are located in a common frequency range and can be evaluated jointly by the subsequent receiver 40 .

[0084] exist Figure 4 In an embodiment of the invention, the magnetic resonance signal does not vary, for example, in a frequency range of about 23.6 MHz. In this example, a frequency of 35 MHz is set for the oscillator signal. From this oscillator signal, a square wave signal of 35 MHz is generated as the mixer frequency for the switching mixer via a signal shaper 64 (e.g. a Schmitt trigger). A first frequency multiplier 59 (here a synthesizer) generates a pilot tone signal of 117.5 MHz from the oscillator signal. A PLL with a 117.5 MHz VCXO is conceivable. The comparison frequency is selected to be 2.5 MHz. To this end, the VCXO signal must be divided by 47 and the 35 MHz reference signal must be divided by 14.

[0085] The switching mixer controlled by the 35 MHz mixer frequency utilizes the 23.6 MHz magnetic resonance signal to generate a 35 MHz -23.6 MHz = 11.4 MHz signal.

[0086] For the pilot tone, consider the third harmonic of the mixing signal’s frequency: 117.5MHz – 3*35MHz = 12.5MHz.

[0087] After a third filter, which removes further unwanted mixing products outside the common frequency range of, for example, 11 MHz to 13 MHz, the mixed signal is provided to a receiver 40 for further processing.

[0088] In addition to reducing the number of mixers, which is also easier to implement as a switching mixer, a conversion to a lower intermediate frequency is advantageously carried out simultaneously, which reduces attenuation in the transmission path and decouples the primary magnetic resonance signal from the output signal in frequency space.

[0089] Of course, in Figure 2 and Figure 3 Downstream conversion of one or more intermediate frequencies for frequency division multiplexing is also contemplated in embodiments.

[0090] Although the present invention has been illustrated and described in detail by means of preferred embodiments, the present 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 present invention.

Claims

1. A local coil for a magnetic resonance tomography device (1), wherein: The local coil (50) is designed to record magnetic resonance signals and pilot tone signals and to transmit the magnetic resonance signals and the pilot tone signals to a receiver (40) of the magnetic resonance tomography device (1) for evaluation, wherein the pilot tone signal is located in a first frequency range, which does not intersect with a second frequency range of the magnetic resonance signals and has a significant frequency interval with the second frequency range, and the local coil (50) has at least one frequency converter, which is designed to convert the pilot tone signal and the magnetic resonance signal into a common frequency range.

2. The local coil according to claim 1, wherein: The local coil (50) has an input for an oscillator signal and is designed to generate, from the oscillator signal, a pilot tone for transmission and a mixer signal for frequency conversion of the pilot tone signal using the frequency converter.

3. The local coil according to claim 2, wherein: The local coil (50) has a first frequency multiplier (59) with a first multiplication factor for generating the pilot tone from the oscillator signal and a second frequency multiplier (60) with a second multiplication factor for generating the mixer signal from the oscillator signal. The first multiplication factor and the second multiplication factor are different.

4. The local coil according to claim 3, wherein: The first multiplication factor and the second multiplication factor differ by 1.

5. The local coil according to claim 2, wherein: The mixer signal has the same frequency as the oscillator signal, and the frequency converter is a switching mixer.

6. The local coil according to claim 1, wherein: The local coil (50) has an antenna coil (51) for receiving the pilot tone signal and the magnetic resonance signal, wherein the antenna coil (51) has resonance in the first frequency range and the second frequency range at the same time.

7. The local coil according to claim 1, wherein: The local coil (50) has an attenuator (62) which is designed to attenuate the pilot tone signal to a level comparable to or lower than that of the magnetic resonance signal to be received before the pilot tone signal is transmitted to the magnetic resonance tomography apparatus (1) for joint evaluation together with the magnetic resonance signal.

8. The local coil according to any one of claims 2 to 7, wherein: The local coil (50) is designed to transmit the pilot tone at a level that causes the level of the received pilot tone signal to be higher than the level of the magnetic resonance signal to be received.

9. The local coil according to claim 7 or 8, wherein: The local coil (50) has a plurality of antenna coils (51) and a plurality of attenuators (62) associated with the antenna coils (51), wherein attenuation values ​​of at least two attenuators are different.

10. A magnetic resonance tomography system having a local coil (50), an oscillator and a receiver (40) according to any one of the preceding claims, wherein: The magnetic resonance tomography system (1) is designed to provide an oscillator signal to the local coil (50) by means of the oscillator and to evaluate magnetic resonance signals and pilot tone signals by means of the receiver (40).

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