Positioning of a device in an MRI system

Description

[0001] The present invention relates to a method for determining the position of a device within an imaging volume of an MRI system, wherein the imaging volume is surrounded by a field magnet for generating a static basic magnetic field along a longitudinal axis and by a gradient coil of the MRI system. The invention is further directed to a corresponding MRI system.

[0002] Magnetic resonance imaging (MRI) systems are imaging devices that use a strong external magnetic field to align the nuclear spins of an object under examination and, by applying an RF excitation pulse, excite them to precess around the corresponding alignment. The precession, or transition of the spins from this excited state to a lower-energy state, generates an alternating electromagnetic field in response, which can be detected as an MR signal via receiving antennas.

[0003] Using magnetic gradient fields, a position coding can be applied to the signals, which then allows the received signal to be assigned to a volume element of the object under examination. The received signal can then be evaluated, for example, to provide an image representation of the object under examination.

[0004] In many MRI applications, it is advantageous to know the location, in particular the position and / or orientation, of a device within the imaging volume defined by the gradient coil, for example, in relation to the patient's anatomy. The position of the patient in the imaging volume can be defined or determined, for example, by visual markers or the like, so it is desirable to determine the position of the device as accurately as possible. The device can, for example, be local MR receive coils that are arranged directly on the patient, such as head coils, knee coils, and so on. However, it can also be a device for medically treating the patient, such as a catheter, a surgical instrument, a biopsy needle, a robotic arm, and so on.

[0005] Patient movement during an MRI examination is a well-known problem, especially for MRI examinations that take longer. Patient movement can alter the measured signals and cause image artifacts that can obscure or hinder the recognition of key features, particularly the radiological findings. Determining the position of the device is particularly advantageous in this context.

[0006] One method for detecting the position of a receive coil uses a Hall sensor integrated into the coil electronics, which measures the local intensity of the static magnetic field. Outside the imaging volume, the static magnetic field is very inhomogeneous and exhibits strong static field gradients. The measurements from the Hall sensor can be used to determine the position of the receive coil when the receive coil and patient are moved from the patient table into the imaging volume. However, once the receive coil is within the imaging volume, which is characterized by a very homogeneous magnetic field, the Hall sensor signal remains essentially constant, even if the coil position within the imaging volume changes. This means that changes in the position of the receive coil due to patient movement may go undetected.

[0007] Other techniques use a camera to detect the position of the receive coil on the patient's body before the patient is moved into the imaging volume on the patient table. Another camera in the imaging volume can detect the movement of the receive coil during the examination. However, this requires a clear line of sight between the camera and the receive coil, which can be obstructed by, for example, blankets, other accessories, support elements, or dielectric pads to improve the RF environment, or the patient's limbs.

[0008] Document WO 00 / 13586 A1 discloses a method and device for determining the location and orientation of an object, such as a medical instrument, inside or outside a body while the body is being scanned by a magnetic resonance imaging scanner. In particular, the method and device enable the position of various devices, such as catheters, surgical instruments, biopsy needles, etc., to be determined by detecting voltages induced by time-varying magnetic fields in a set of miniature coils. Such time-varying fields are generated by the magnetic resonance imaging scanner during normal operation.

[0009] Document US 2021 / 212588 A1 describes a wireless signal acquisition device comprising three mutually orthogonal detection coils into which a voltage is induced by a time-varying and location-dependent magnetic field in a measurement chamber of a magnetic resonance scanner. A radio-frequency detection circuit detects radio-frequency pulses emitted by a magnetic resonance scanner, and a wireless transmission circuit transmits the data from the signal acquisition device. A 3-axis magnetometer is used to measure a magnetic flux in the measurement chamber of the magnetic resonance scanner. A processor uses the detected radio-frequency pulses to synchronize the measured values ​​acquired by the magnetometer and the detection coils with a time frame of the gradient control hardware, fitting the measured values ​​into a pulse sequence.The processor further combines measurements of voltages induced in the sensing coils and the magnetic flux with the pulse sequence trace to determine the instantaneous position and orientation of the signal acquisition device.

[0010] From the document JAMES A. SMITH ET AL: "A Novel Position and Orientation Sensor for MRI", PROCEEDINGS OF THE INTERNATIONAL SOCIETY FOR MAGNETIC RESONANCE IN MEDICINE, ISMRM, 25TH ANNUAL MEETING AND EXHIBITION, HONOLULU, HAWAII, USA, 22-27 APRIL 2017, No. 3930, 7 April 2017 (2017-04-07), page 3930, XP040691498, a device is known that can detect a head posture in and in relation to a scanner. By combining commercially available 3-axis MEMS accelerometers and an anisotropic magnetoresistive bridge, device alignment is monitored with a root mean square error of + / - 0.12 degrees, while translational motion is estimated with an accuracy of + / - 0.45 mm using voltage induced in a single coil by time-varying magnetic field gradients.

[0011] Document US 2003 / 184297 A1 describes a system for combining electromagnetic position and orientation tracking with a magnetic resonance scanner. One embodiment includes a magnetic resonance scanner that provides a reference coordinate system for scanning a target. A magnetic field source that generates a magnetic field is coupled to the magnetic resonance scanner. The magnetic field is detected by a magnetic field sensor that generates a signal proportional to the magnetic field. The magnetic field sensor has a position relative to the reference coordinate system. The position of the magnetic field sensor relative to the reference coordinate system of the magnetic resonance scanner is determined by a position tracking unit using at least one field line segment model of the magnetic field source and the signal from the magnetic field sensor.

[0012] From document US 2007 / 145978 A1, a magnetic resonance system for image acquisition in various ways, in which a patient table is moved, is known. The magnetic resonance system comprises a patient table having a table surface movable in a predetermined direction by a static magnetic field, as well as a plurality of radio-frequency receiving coils, which, for example, consist of a plurality of coil groups. The table surface is automatically moved longitudinally in the predetermined direction according to the length of each coil group. At each reached position, image acquisition is performed using a given pulse sequence. An echo signal is acquired by the plurality of radio-frequency coils and then forwarded by an input switching unit to a receiving system circuit. The echo signal is subjected to predetermined signal processing in the circuit to be converted into echo data.A host computer generates a magnetic resonance image from the echo data.

[0013] It is an object of the present invention to reliably determine the position of a device within the imaging volume of an MRI system.

[0014] This object is achieved by the respective subject matter of the independent claims. Advantageous further developments and preferred embodiments are the subject matter of the dependent claims.

[0015] The invention is based on the finding that a gradient coil generates a magnetic field inside the gradient coil and thus inside the imaging volume. This field always contains significant components perpendicular to the basic magnetic field in addition to components parallel to it. These components are detected by at least one conductor loop and used to determine the position of the conductor loop and thus of the device.

[0016] According to one aspect of the invention, a method for determining the position of a device inside an imaging volume of an MRI system is specified according to claim 1. The MRI system has a field magnet for generating a static basic magnetic field along a longitudinal axis of the MRI system and a gradient coil surrounding the imaging volume. The device has at least one first conductor loop running within a first loop plane. By means of the gradient coil, an alternating magnetic field is generated in the imaging volume, and by means of the at least one first conductor loop, at least one first measured value is determined, which depends on a first induction voltage induced in the at least one first conductor loop by a first component of the alternating field perpendicular to the longitudinal axis.A position of the device inside the imaging volume is at least partially determined depending on the at least one first measured value and a predetermined magnetic field model for the gradient coil.

[0017] The imaging volume is, in particular, an area within the MR scanner of the MRI system, in particular within the so-called patient tunnel, also referred to as the bore, which is essentially defined by the gradient coil and, if applicable, an RF transmit coil arranged radially within the gradient coil for transmitting high-frequency alternating fields. The imaging volume is therefore to be understood, in particular, as a volume range within which an object can in principle be imaged, for example, if the RF transmit coil is also used as a receive coil. If a local MR receive coil is used, it is located, in particular, within the imaging volume. This means that the local MR receive coil does not define the imaging volume, but rather, if applicable, another imaging volume inside the local MR receive coil.

[0018] The at least one conductor loop is part of the local MR receiving coil; in other words, the device corresponds to the local MR receiving coil. By determining the position of the device, the position of other objects, such as the patient or a body part of the patient or a medical tool, can be deduced if its relative position to the device is correspondingly predetermined or fixed.

[0019] In order to at least partially determine the position of the device, a position of the at least one conductor loop is at least partially determined depending on the at least one measured value and the magnetic field model. The position of the at least one conductor loop can correspond directly to the position of the device, or the position of the device can be derived from the position of the at least one conductor loop.

[0020] The at least one conductor loop can also have a finite extension in a direction perpendicular to the first loop plane. The fact that the at least one conductor loop extends within the first loop plane can be understood as meaning that all conductor loops of the at least one first conductor loop are parallel to each other and parallel to the loop plane.

[0021] The position of the at least one conductor loop and, accordingly, the position of the device can be determined in a predetermined coordinate system, for example, a stationary coordinate system relative to the gradient coil. The longitudinal axis of the MRI system can, for example, be understood as the z-direction of this coordinate system, and corresponding directions perpendicular to the z-direction as the x- and y-axes of the coordinate system. However, other reference systems can also be selected if they are advantageous for further use. The term "position" can be understood, in particular, as the combination of the three-dimensional position and three-dimensional orientation in the corresponding reference system. In other words, the position can also be referred to as a pose. For example, the position can be given by three-dimensional coordinates of a reference point of the device and three orientation angles of a specific reference direction of the device.In order to fully determine the position, the three-dimensional position and the three-dimensional orientation of the device would have to be determined.

[0022] The fact that the position of the device is at least partially determined in the method according to the invention can be understood to mean that the position is determined either completely or incompletely. An incomplete determination of the position can be understood to mean that, for example, only individual coordinates of the three-dimensional position or individual angles of the three-dimensional orientation are determined, but not all three coordinates of the position and all three angles of the orientation. Alternatively, the partial determination of the position of the device can consist in one or more restrictions of the six degrees of freedom, i.e. the three coordinates and the three orientation means, being determined explicitly or implicitly, wherein the restriction goes beyond the arrangement of the at least one first conductor loop within the imaging volume.

[0023] For example, by at least partially determining the position, it is possible to determine the distance from the center of the imaging volume, i.e., the longitudinal axis, to which the reference point of the device is located, the angular range in which the corresponding orientation means are located, and so on. Whether the position is determined completely or only partially, or how much information regarding the position of the device is determined, depends on whether and which additional information is available for determining the position in addition to the at least one measured value and the magnetic field model. However, it should be noted that even without such additional information, a partial determination of the position is possible based solely on the at least one measured value and the magnetic field model.However, a complete determination of the position usually requires additional information, which may be provided, for example, due to structural limitations or can be determined by additional sensor systems, such as Hall sensors, additional conductor loops, cameras, and so on.

[0024] The magnetic field model can, for example, contain the magnetic field generated by the gradient coil inside the imaging volume in spatially resolved form and in all three spatial directions or at least in three spatial dimensions, for example also in spatially resolved and time-resolved form. The magnetic field model therefore depends in particular on the geometric and electrical properties of the gradient coil and, if applicable, on the control of the gradient coil. The magnetic field model can be determined, for example, by measuring the magnetic field inside the imaging volume and / or by simulations and / or other calculations and can be stored on the MRI system, in particular on at least one evaluation unit of the MRI system.

[0025] The gradient coil of an MRI system is designed to generate a magnetic field gradient within the imaging volume, primarily along the longitudinal axis. The magnetic field gradient corresponds to a change in the magnetic field strength in one of the three spatial directions. An MRI system typically has three such gradient coils, each of which changes its magnetic field strength along a different spatial direction. However, a magnetic field oriented exclusively along the longitudinal axis is only theoretically possible, for example, for infinitely large coils. In every actual implementation of a gradient coil, the magnetic field generated by it within the imaging volume always has components in all three spatial directions. An exception may be exceptional points of symmetry in the exact center of the imaging volume, although these are also only of theoretical significance.This finding and the use of this finding to determine the position of the device in the imaging volume of an MRI system are the basis of the present invention.

[0026] In particular, due to the circumstances outlined above, it is always possible to measure a corresponding induced voltage or current regardless of the orientation of the loop plane relative to the longitudinal axis and, with simultaneous knowledge of the magnetic field model, to draw conclusions about the position of at least one conductor loop. In this way, the complexity of the means for determining the position of the device, and in particular the complexity of the device itself, can be reduced.

[0027] In particular, the invention enables a local MR receiving coil itself to be used to determine the position of the MR receiving coil, even though the conductor loops of a local MR receiving coil are generally parallel or at least not perpendicular to the basic magnetic field. This enables a particularly advantageous synergistic combination of the basic functionality of the local MR receiving coil, namely the detection of MR signals from the object to be examined, with the additional functionality of position determination. It should be noted that the receiving coil can detect not only high-frequency alternating electromagnetic fields of the transmitting coil or the nuclear magnetic resonance signals in response to these, but also the signals of the gradient coil, which may have a significantly lower frequency and which must be generated anyway during the MRI examination to generate the magnetic field gradients.It is therefore not absolutely necessary for the gradient coil's alternating magnetic field to be generated specifically for position determination. While this is possible, the gradient pulses already generated can be used to advantage.

[0028] If the position of the device is known or at least limited, a user of the MRI system can, for example, be automatically informed whether the position deviates from a desired or optimal position. In addition, an algorithm can be used that automatically selects one or more scan parameters, such as an acceleration factor R, a phase encoding direction, and so on, depending on the at least partially determined position of the device in such a way that the given position or, for example, of the device relative to the patient can be optimally utilized. Alternatively or additionally, image reconstruction algorithms can also use the at least partially determined position to improve the resulting image quality, for example by more precisely estimating the coil sensitivity of the local MR receive coil.

[0029] According to the method according to the invention, an MR image is generated in the imaging volume depending on an MR signal from an object to be examined.

[0030] When generating the MR image, the at least partially determined position of the device can be taken into account automatically or manually. For example, an MR acquisition can be repeated or partially repeated if the position does not meet a specified requirement or expectation. Furthermore, motion compensation algorithms can be automatically executed depending on the at least partially determined position to generate the MR image.

[0031] To generate the MR signal, a known MR sequence can be applied. For example, a global RF transmit coil surrounding the imaging volume radiates corresponding RF pulses into the imaging volume, and a sequence of magnetic field gradients is generated in the imaging volume using the gradient coil. This induces nuclear magnetic resonance in the object to be examined, and the resulting high-frequency MR signals can then be detected using the RF transmit coil, if it is also used as a receive coil, and / or one or more additional local MR receive coils.

[0032] According to the method according to the invention, the MR signal from the object to be examined in the imaging volume is detected by means of the at least one first conductor loop and the MR image is generated depending on the MR signal.

[0033] In other words, the at least one first conductor loop is used not only to generate the at least one measured value and thus to at least partially determine the position of the device, but also as a regular local MR receiver coil. For this purpose, the device or the at least one conductor loop is, in particular, part of a local MR receiver coil.

[0034] In principle, the MR signal is also detected via a corresponding induction voltage in the at least one conductor loop. The induction voltages due to the MR signal and due to the alternating magnetic field of the gradient coil, which is used to determine the at least one measured value, can be separated from one another in time or in another way. For example, it can be exploited that the frequency of the signals generated by the transmit coil and, accordingly, the frequency of the MR signals is many times higher than a frequency of the alternating magnetic fields generated by the gradient coil. Thus, in particular, frequency filtering can be performed to separate the detection of the MR signal from the detection of the alternating magnetic field generated by the gradient coil. The frequency of the MR signal corresponds in particular to the corresponding Larmor frequency of the atomic nuclei used for imaging.This frequency, for example, lies in a range of 1 MHz to 500 MHz, depending on the basic magnetic field strength of the MRI system's field magnet. The pulsed gradient fields of the gradient coil, i.e., the alternating magnetic field generated by the gradient coil, have a frequency in the range of a few kHz to a few tens of kHz.

[0035] According to at least one embodiment in which the MR signal is detected according to the invention by means of the at least one first conductor loop, the MR signal or the further induction voltage resulting from the MR signal or a corresponding signal is suppressed, in particular by means of a filter circuit, in order to determine the at least one first measured value.

[0036] This ensures that at least one measured value reflects only the alternating magnetic field of the gradient coil, but not the MR signal.

[0037] On the other hand, to determine or detect the MR signal, the induction voltage due to the alternating magnetic field of the gradient coil can be suppressed, for example by means of the filter circuit or another filter circuit.

[0038] This ensures that the detection of the MR signal is not influenced by the alternating magnetic field of the gradient coil.

[0039] For example, the at least one first measured value and the MR signal can be detected by different measuring channels or receiving channels, wherein the filter circuit and / or the further filter circuit is implemented in the corresponding measuring channels.

[0040] According to at least one embodiment, the device is positioned in the imaging volume such that the loop plane is parallel to the longitudinal axis, in particular at least approximately parallel to the longitudinal axis.

[0041] The loop plane can, for example, be considered to be at least approximately parallel to the longitudinal axis if an angle between a normal direction perpendicular to the loop plane and the longitudinal axis is at least approximately equal to 90°, that is to say, for example, greater than 60 degrees and less than 120 degrees or, for example, greater than 70° and less than 110° or, for example, greater than 80° and less than 100°.

[0042] This may be the case in particular if the device corresponds to or is part of a local MR receiving coil.

[0043] According to at least one embodiment, the device has at least one second conductor loop extending within a second loop plane that is different from the first loop plane. The at least one second conductor loop is used to determine at least one second measured value that depends on a second induction voltage induced in the at least one second conductor loop by a second component of the alternating field perpendicular to the longitudinal axis. The position of the device is at least partially determined depending on the at least one measured value, the at least one second measured value, and the magnetic field model for the gradient coil.

[0044] For example, in such embodiments, the at least one first and the at least one second conductor loop may be part of a local MR receiving coil.

[0045] In particular, the device is positioned in the imaging volume such that the second loop plane is at least approximately parallel to the longitudinal axis and the first loop plane is in particular also at least approximately parallel to the longitudinal axis.

[0046] For example, the relative position of the at least one first conductor loop to the at least one second conductor loop can be known or fixedly predetermined. To at least partially determine the position of the device, the position of the at least one first conductor loop and the position of the at least one second conductor loop can be determined as described. The predetermined or known relative positions to one another thus allow the position of the device to be determined more precisely or restricted more strongly.

[0047] In corresponding developments, a plurality of further conductor loops can be used analogously to the at least one first and the at least one second conductor loop in order to enable the position of the device to be determined as accurately or completely as possible, wherein the further conductor loops can also be part of the local MR receiving coil, for example.

[0048] According to at least one embodiment, the device has at least one third conductor loop running within a third loop plane. By means of the at least one third conductor loop, at least one third measured value is determined, which depends on a third induction voltage induced in the at least one third conductor loop by a third component of the alternating field perpendicular to the longitudinal axis. A first position of the at least one first conductor loop inside the imaging volume is at least partially determined depending on the at least one first measured value and the magnetic field model, and a third position of the at least one third conductor loop inside the imaging volume is at least partially determined depending on the at least one third measured value and the magnetic field model.A relative position of the at least one third conductor loop with respect to the at least one first conductor loop is determined depending on the first position and depending on the third position.

[0049] In such embodiments, the relative position of the third and first conductor loops is therefore not known in advance or not precisely known, but can be determined or approximately determined in the manner described. Such embodiments are also particularly advantageous when the local MR receiving coil includes the at least one first conductor loop and the at least one third conductor loop and is designed, for example, as a flexible surface coil. Such flexible surface coils adapt, if necessary, to the surface of the patient or the like, so that the individual conductor loops have an orientation relative to one another that is not known per se. In the manner mentioned, the position or shape of the surface of the flexible surface coil can be determined.

[0050] According to a further aspect of the invention, an MRI system according to claim 7 is also specified, which has a field magnet for generating a static basic magnetic field along a longitudinal axis and a gradient coil, wherein the field magnet and the gradient coil surround an imaging volume of the MRI system. The MRI system has a device with at least one first conductor loop, wherein the at least one first conductor loop runs within a first loop plane. The MRI system has a control unit configured to control the gradient coil to generate an alternating magnetic field in the imaging volume. The MRI system has a measuring unit connected to the at least one first conductor loop. The measuring unit is configured to, depending on a first induction voltage generated by a component of the

[0051] Alternating field perpendicular to the longitudinal axis in which at least one first conductor loop is induced, to determine at least one first measured value. The MRI system has at least one evaluation unit configured to at least partially determine a position of the device within the imaging volume depending on the at least one first measured value and a predefined magnetic field model for the gradient coil.

[0052] The control unit, the measuring unit and / or the at least one evaluation unit can be provided separately from one another in various embodiments or can also be partially or completely combined.

[0053] The device according to the invention comprises a tuning capacitance, in particular a tuning capacitor, which is arranged between a first terminal of the at least one first conductor loop and a second terminal of the at least one first conductor loop. The device comprises an inductive component arranged electrically parallel to the tuning capacitance.

[0054] Such an embodiment of the device is particularly advantageous if the device is part of the local MR receiving coil arrangement or is equal to the local MR receiving coil arrangement.

[0055] According to at least one embodiment, the MRI system comprises a local MR receiving coil arrangement containing the device, wherein the local receiving coil arrangement is configured as a flexible surface coil array.

[0056] The at least one first conductor loop then corresponds to a surface coil of the surface coil array.

[0057] According to at least one embodiment, the MRI system comprises a device for medically treating a patient, wherein the at least one first conductor loop and the device have a predetermined spatial position relative to one another.

[0058] The device can be, for example, a biopsy needle, a catheter, a surgical instrument, a robotic arm, and so on.

[0059] The induction voltage is present in particular between the first and the second terminal of the at least one conductor loop.

[0060] The tuning capacitance can be implemented as a tuning capacitor, i.e. as a corresponding electronic component, or as a parasitic capacitance between conductor segments of the at least one first conductor loop.

[0061] For example, the device may comprise a plurality of tuning capacitors arranged between the first and second terminals. In this case, the device comprises, in particular, for each tuning capacitor a corresponding associated inductive component that is electrically connected in parallel with the tuning capacitor.

[0062] Without the inductive component, the at least one conductor loop would be non-conductive due to the tuning capacitance for direct current, or would have a very high impedance with respect to low-frequency alternating currents. The inductive component increases conductivity at low frequencies. With respect to high-frequency MR signals, however, the inductance of the inductive component effectively acts as a resistor, so that the parallel connection to the tuning capacitance does not significantly influence the detection of the MR signal.

[0063] In various embodiments, the device can also comprise a detuning capacitance, in particular a detuning capacitor, arranged between the first and second terminals. In this case, the device comprises a further inductive component electrically connected in parallel with the detuning capacitance. Thus, the detuning capacitance has little or no effect at low frequencies, whereas the further inductive component has little or no effect at high frequencies.

[0064] According to at least one embodiment, the measuring unit has an amplifier which is connected to the first terminal and the second terminal, in particular directly or indirectly, and is configured to provide the at least one measured value at an output of the amplifier which is connected to the at least one evaluation unit.

[0065] In particular, the amplifier has a first and a second input, the first input being connected to the first terminal and the second input being connected to the second terminal.

[0066] According to at least one embodiment, the measuring unit or the device comprises a further amplifier, which is also connected to the first and second terminals and is configured to provide the MR signal or a measurement signal dependent on the MR signal at an output of the further amplifier, which is also connected to the at least one evaluation unit. The amplifier and the further amplifier can thus, in particular, be corresponding parts of a first and a second measurement channel, which detect different currents detected by the at least one conductor loop.

[0067] According to at least one embodiment, the measuring unit comprises a filter circuit arranged between the first terminal and a first input of the amplifier, as well as between the second terminal and a second input of the amplifier. The filter circuit is designed to suppress an MR signal detected by means of the at least one conductor loop.

[0068] The filter circuit has, in particular, a first input connected to the first terminal and a first output connected to the first input of the amplifier. Furthermore, the filter circuit has, in particular, a second input connected to the second terminal and a second output connected to the second input of the amplifier.

[0069] The filter circuit can be configured, for example, as a low-pass filter or a band-pass filter. In any case, the filter circuit is tuned to the gradient coil, the basic magnetic field, or the control of the gradient coil in such a way that it essentially allows frequencies corresponding to the alternating magnetic field of the gradient coil to pass, while essentially suppressing frequencies corresponding to the MR signal.

[0070] For example, the device may comprise a further filter circuit arranged between the first terminal and a first input of the further amplifier, as well as between the second terminal and a second input of the further amplifier. The further filter circuit is designed to suppress the signal generated by the alternating magnetic field of the gradient coil in the at least one first conductor loop.

[0071] The second filter circuit can be configured, for example, as a high-pass filter or as a further bandpass filter. The further filter circuit is therefore designed, in particular, to be complementary to the filter circuit. In particular, the first terminal of the at least one conductor loop can be connected to a first input of the further filter circuit, and the second terminal of the at least one conductor loop can be connected to a second input of the further filter circuit. A first output of the further filter circuit is connected, for example, to a first input of the further amplifier, and a second output of the further filter circuit is connected to a second input of the further amplifier.

[0072] Further embodiments of the MRI system according to the invention follow directly from the various embodiments of the method according to the invention, and vice versa. In particular, individual features and corresponding explanations relating to the various embodiments of the method according to the invention can be transferred analogously to corresponding embodiments of the MRI system according to the invention, and vice versa. In particular, the MRI system according to the invention is designed or programmed to carry out a method according to the invention. In particular, the MRI system according to the invention carries out the method according to the invention.

[0073] According to a further aspect, which does not fall within the scope of the invention defined by the claims, a device for determining the position of an MRI system is also provided. The device has at least one first conductor loop running within a first loop plane. The device also has a measuring unit connected to the at least one conductor loop and configured to determine at least one first measured value depending on a first induction voltage induced in the at least one first conductor loop.

[0074] According to at least one embodiment of the device, it comprises the tuning capacitance and the inductive component as described above.

[0075] Further embodiments of the device follow from the various embodiments of the MRI system according to the invention and the method according to the invention.

[0076] A computing unit can be understood, in particular, as a data processing device that contains a processing circuit. The computing unit can therefore, in particular, process data to perform computing operations. This may also include operations for performing indexed access to a data structure, for example, a look-up table (LUT).

[0077] The computing unit may, in particular, contain one or more computers, one or more microcontrollers, and / or one or more integrated circuits, for example, one or more application-specific integrated circuits (ASICs), one or more field-programmable gate arrays (FPGAs), and / or one or more single-chip systems (SoCs). The computing unit may also contain one or more processors, for example, one or more microprocessors, one or more central processing units (CPUs), one or more graphics processing units (GPUs), and / or one or more signal processors, in particular one or more digital signal processors (DSPs). The computing unit may also include a physical or virtual network of computers or other of the aforementioned units.

[0078] In various embodiments, the computing unit includes one or more hardware and / or software interfaces and / or one or more memory units.

[0079] A memory unit can be a volatile data storage device, such as dynamic random access memory (DRAM) or static random access memory (SRAM), or a non-volatile data storage device, such as read-only memory (ROM), programmable read-only memory (PROM), erasable read-only memory (EPROM), electrically erasable read-only memory (EEPROM), flash memory (FRAM), ferroelectric random access memory (FRAM), magnetoresistive random access memory (MRAM), or phase-change random access memory (PROM).PCRAM (phase-change random access memory),

[0080] The at least one evaluation unit, the control unit and / or the measuring unit of the MRI system according to the invention can include one or more computing units according to this understanding or one or more computing units of the MRI system can include the at least one evaluation unit, the control unit and / or the measuring unit.

[0081] If, within the scope of the present disclosure, it is stated that a component of the MRI system according to the invention, in particular the control unit, the measuring unit or at least one evaluation unit of the MRI system, is set up, designed, configured or the like to carry out or implement a specific function, to achieve a specific effect or to serve a specific purpose, this can be understood to mean that the component, beyond the fundamental or theoretical usability or suitability of the component for this function, effect or purpose, is concretely and actually capable of carrying out or implementing the function, achieving the effect or serving the purpose through appropriate adaptation, programming, physical design and so on.

[0082] Unless expressly stated otherwise, a connection between two electrical or electronic components can be understood to mean that an electrical connection exists between the components or can be established by actuating one or more switching elements.

[0083] In particular, the components can be connected to one another directly or indirectly, unless otherwise stated. A direct connection can be understood as meaning that, apart from the optional one or more switching elements, no further electrical or electronic components are arranged between the components, while an indirect connection can be understood as meaning that, in addition to the optional one or more switching elements, one or more further electrical or electronic components, such as resistors, capacitors, coils, and so on, are arranged between the components.

[0084] Further features of the invention emerge from the claims, the figures and the description of the figures. The features and combinations of features mentioned above in the description as well as the features and combinations of features mentioned below in the description of the figures and / or shown in the figures can be encompassed by the invention not only in the respectively specified combination, but also in other combinations. In particular, the invention can also encompass embodiments and combinations of features that do not have all the features of an originally formulated claim. Furthermore, the invention can encompass embodiments and combinations of features that go beyond the combinations of features set out in the backreferences to the claims or deviate from them.

[0085] However, the scope of the invention is generally determined by the claims.

[0086] The invention is explained in more detail below using specific embodiments and associated schematic drawings. In the figures, identical or functionally equivalent elements may be provided with the same reference numerals. The description of identical or functionally equivalent elements may not necessarily be repeated for different figures. The figures show: FIG 1 shows a schematic representation of an exemplary embodiment of an MRI system according to the invention; FIG 2 shows a schematic representation of a magnetic field; FIG 3 shows a schematic representation of a device of a further exemplary embodiment of an MRI system according to the invention; FIG 4 shows a schematic representation of a device of a further exemplary embodiment of an MRI system according to the invention; FIG 5 shows a schematic representation of a device of a further exemplary embodiment of an MRI system according to the invention; FIG 6 shows a schematic representation of a device of a further exemplary embodiment of an MRI system according to the invention; FIG 7 shows a schematic representation of a device of a further exemplary embodiment of an MRI system according to the invention; FIG 8 shows a schematic representation of a device of a further exemplary embodiment of an MRI system according to the invention;FIG. 9 shows a schematic representation of a device of a further exemplary embodiment of an MRI system according to the invention; and FIG. 10 shows a schematic representation of possible basic shapes of a local MR receiving coil.

[0087] In FIG 1 An exemplary embodiment of an MRI system 1 according to the invention is shown schematically.

[0088] The MRI system 1 includes a field magnet (not shown) that generates a static magnetic field for aligning nuclear spins of a sample, for example, a patient, in an imaging volume 3 in the z-direction, which can be referred to as the longitudinal axis of the MRI system. The imaging volume 3 is characterized by a very homogeneous static magnetic field in the z-direction. The field magnet can, for example, be a superconducting magnet that can provide magnetic fields with a magnetic flux density of up to 3 T or more. However, permanent magnets or electromagnets with normally conducting coils can also be used for lower field strengths.

[0089] Furthermore, the MRI system 1 comprises a gradient coil 2 and a control unit 5 for controlling the gradient coil 2. These control units are designed to superimpose magnetic fields on the static magnetic field for spatial differentiation of the acquired imaging regions in the imaging volume 3. The magnitude of these magnetic fields can vary depending on the location along all three spatial directions x, y, and z. The gradient coil 2 can be configured, for example, as a coil made of normally conducting wires.

[0090] The MRI system 1 can have, for example, a body coil 30 as a transmitting antenna, which is designed to radiate a radio-frequency signal supplied via a signal line into the imaging volume 3.

[0091] The control unit 5 can supply the gradient coil 2 and the body coil 30 with various signals. The control unit 5 can, for example, have a gradient controller configured to supply the gradient coil 2 with variable currents via supply lines, which can provide the desired gradient fields in the imaging volume 3 in a time-coordinated manner.

[0092] The control unit 5 can also comprise a radio-frequency unit designed to generate radio-frequency pulses or excitation pulses with predetermined temporal profiles, amplitudes, and spectral power distribution for exciting a magnetic resonance of the nuclear spins in the patient. Pulse powers in the kilowatt range can be used. The excitation pulses can be radiated into the patient via the body coil 30 or via one or more local transmitting antennas. The control unit 5 can also contain a controller that can communicate with the gradient controller and the radio-frequency unit via a signal bus.

[0093] In some embodiments, the body coil 30 can also be used to receive resonance signals emitted by the patient, also referred to as MR signals, and transmit them via a signal line. In such embodiments, the body coil 30 can thus serve as both a receiving antenna and a transmitting antenna.

[0094] Optionally, a local MR receiving coil (not shown), also referred to as a local coil, of the MRI system 1 can be arranged in the immediate vicinity of the patient. This coil can be connected to a measuring unit 6 via a connecting cable. The measuring unit 6 can also be part of the control unit 5. Depending on the embodiment, the local coil can serve as a receiving antenna alternatively or in addition to the body coil 30.

[0095] The MRI system 1 also has an evaluation unit 7, which is connected to the control unit 5, in particular to the radiofrequency unit. The evaluation unit 7 can evaluate the MR signals and, based on them, reconstruct an MR image using known methods. The control unit 5 can also be part of the evaluation unit 7.

[0096] The MRI system 1 comprises a device with at least one conductor loop 4, which runs within a loop plane and is arranged in particular in the imaging volume 3 such that the loop plane is oriented substantially parallel to the z-direction.

[0097] The gradient coil 2 generates, as described, controlled by the control unit 5, an alternating magnetic field in the imaging volume 3. This alternating magnetic field generally has magnetic field components in all three spatial directions x,y, z. Consequently, an induced voltage is generated in the at least one conductor loop 4, even if the loop plane is oriented substantially parallel to the z-direction.

[0098] The measuring unit 6 is connected to the at least one conductor loop 4 and is configured to determine at least one measured value depending on the induction voltage. The evaluation unit 7 is configured to at least partially determine a position of the device within an imaging volume 3 depending on the at least one measured value and a predefined magnetic field model for the gradient coil 2.

[0099] In embodiments with a local MR receiving coil, this can include the device or at least one conductor loop 4. The position of the device then corresponds to the position of the local MR receiving coil.

[0100] An example of the magnetic field model is in FIG 2 shown schematically. Contrary to the usual simplifying assumption, the gradient fields generated by the gradient coil 2 in the imaging volume 3 are not aligned exactly parallel to the direction of the static magnetic field, i.e., the z-direction. Instead, the gradient coil 2 generates additional field components that are orthogonal to z, i.e., aligned along the x- or y-axis, and whose amplitude is also comparable to the z-component. Since the components of the alternating field along the x- or y-axis are much smaller than the static basic magnetic field, these components can be neglected for regular MRI imaging. For this reason, the components of the alternating field along the x- and y-axes have not been considered for other applications in the past.However, since they are time-dependent, in contrast to the static basic magnetic field, they contribute significantly to the induction in the conductor loop 4 and can therefore be used for position determination according to the invention.

[0101] FIG 2 shows, by way of example, the Cartesian components of the magnetic field as they are detected inside the gradient coil 2, which is operated in static mode, i.e., with a constant current, with no basic magnetic field of the field magnet present. At each sampling point, three field values ​​corresponding to the three orthogonal field components Bx, By, and Bz were measured using a vector magnetometer attached to a robot arm and positioned sequentially at 480 spatial positions distributed over the surface of a sphere. Based on these measured values, the magnetic field at any location within the imaging volume 3 can be calculated using a calibrated magnetic field model. The at least one conductor loop 4 located in the imaging volume 3 of the MRI system 1 thus detects a signal induced by pulsed gradient fields, even if the loop plane is oriented substantially parallel to the z-direction.

[0102] Thanks to the invention, it is no longer necessary to use additional sensors to determine the position of a local MR receiver coil. Instead, the existing conductor loops of the local MR receiver coil can be used both to detect the weak high-frequency MRI signals and to detect the signals induced by the gradient pulses in the low-frequency range.

[0103] The voltage induced in a conductor loop when the magnetic flux changes through the area enclosed by the conductor loop is obtained by integrating the change in the magnetic vector field B over the area A enclosed by the loop, i.e. by applying Faraday's law of induction: U = ∫ d B → dt ⋅ d s →

[0104] In FIG 3 bis FIG 5 are schematic implementations of the device with the at least one conductor loop 4 for various exemplary embodiments of the MRI system 1 according to the invention, for example the MRI system 1 from FIG 1 , shown.

[0105] Through these devices, the MRI system 1 is able to simultaneously receive the high-frequency MR signals and the low-frequency signals induced by pulsating gradient fields for position determination.

[0106] A multi-channel MR receiver coil can, for example, be designed as a two-dimensional flexible array consisting of several receiver elements, such as 2 to 32 or even 64 receiver elements. FIG 3 bis FIG 5 Such a receiving element is shown. The receiving element has at least one conductor loop 4, for example, embodied as at least one copper loop, as well as tuning means, for example, tuning capacitors 9a, 9b, which are arranged between a first terminal 21a and a second terminal 21b of the at least one conductor loop 4. Detuning means 10 can also be provided, which, for example, contains a detuning capacitor 12 and a series circuit arranged in parallel thereto with a detuning inductance 13 and a diode 14.

[0107] Furthermore, a preamplifier circuit 18 can be provided, which is connected to the terminals 21a, 21b via an adaptation circuit 16 on the input side and to an analog-to-digital converter 19 on the output side, which can be connected to the evaluation unit 7 or a computer via a data bus 20. The tuning capacitors 9a, 9b are distributed, for example, along the at least one conductor loop 4 in order to reduce the electric fields that otherwise arise over long conductor segments and can possibly lead to dielectric losses and thus a reduced signal-to-noise ratio. The capacitances of the tuning capacitors 9a, 9b are tuned in particular such that they resonate with the inductance of the at least one conductor loop 4 at the Larmor resonance frequency of the MRI system 1, which, depending on the field strength, can be a high frequency, for example in the range of 1 MHz to 500 MHz.

[0108] An inductive component 11a, 11b, 11c is connected in parallel to the tuning capacitors 9a, 9b and to the detuning capacitor 12, so that the low-frequency signals induced by the pulsed gradient fields in the range of several kHz can also be detected. The inductance of the inductive components 11a, 11b, 11c is selected such that they have a high impedance for the induced high-frequency MR signals and practically correspond to an open circuit. In contrast, the electrical impedance of the inductive components 11a, 11b, 11c at low frequencies is essentially equivalent to a short circuit, which closes at least one conductor loop 4 for the signals induced by the pulsating gradient fields. The values ​​of these inductances can, for example, range from a few hundred µH to many mH, depending on the Larmor frequency.

[0109] In some embodiments, a signal preamplifier 17 can be connected on the input side via a filter circuit 15, which can be embodied, for example, as a low-pass filter, to the terminals 21a, 21b, and on the output side to another input of the analog-to-digital converter 19 or to another analog-to-digital converter (not shown). The signals induced by the pulsed gradients and detected by the at least one conductor loop 4 can then be read out via the data bus 20 and further used by signal processing algorithms to extract the information about the position of the at least one conductor loop 4. The position of additional receiving elements can also be determined analogously, and thus the shape of the flexible multi-channel MR receiving coil can be described.

[0110] FIG 4 schematically shows a receiving element of a device in a further embodiment of the inventive MRI system 1 for a newer type of MR receiving coils, which uses distributed tuning capacitances 9 instead of discrete capacitors formed by parasitic capacitances between conductor segments of at least one conductor loop 4. The separate conductor segments are effectively short-circuited by the inductive components 11a, 11b, 11c for low frequencies, so that the conductor segments form a double loop. FIG 4 a supply voltage 8 for the preamplifier circuit 18 is also indicated.

[0111] FIG 5 shows schematically a receiving element of a device in a further embodiment of the MRI system 1 according to the invention. The receiving element is based on the FIG 3 shown receiving element.

[0112] The receiving element of the FIG 5 In corresponding embodiments, it fulfills a function referred to as local shimming. For this purpose, the evaluation unit 7 or another computer connected to the receiving element can, for example, adjust a direct current through the at least one conductor loop 4 in order to compensate for local inhomogeneities in the static magnetic field. The desired digital value of the direct current is transmitted, for example, via a further data bus 25 to a further digital-to-analog converter 24, the output of which outputs a corresponding signal to a constant current driver 23. The constant current driver 23 can transmit this signal to the at least one conductor loop 4, for example, via a further low-pass filter 22.

[0113] The further low-pass filter 22 is designed in particular to transmit the direct current value from the constant current driver 23 to the at least one conductor loop 4 and, in doing so, to block the low-frequency alternating current signals induced by the pulsating gradient fields as well as the high-frequency MR signals. The filter circuit 15 can then be designed, for example, as a band-pass filter, which is designed to allow the alternating current signals induced by the pulsating gradient fields to pass through and to suppress the direct current component as well as the high-frequency MR signals. In an analogous manner, a receiving element with distributed tuning capacitances 9 as in FIG 4 be adjusted.

[0114] The spatial position of objects within the imaging volume 3 can be determined, for example, by processing the signals from orthogonal coils attached to the object as a function of the voltages induced by the pulsating gradient fields. One method can start with an initial estimate and then iteratively adjust the object position and orientation until certain convergence criteria are met. In another method, a translation matrix is ​​calibrated in a pre-training step in which a test object moves step by step, an image volume is acquired simultaneously for each step, and the gradient activity is measured. These methods can also be combined with the aid of the inventive method, the inventive MRI system 1, or the device of the inventive MRI system 1 to achieve the explained advantages of the invention.

[0115] In addition, another method for detecting the shape and position of local flexible MR receiving coils 28, as in FIG 6 bis FIG 9 schematically represented and used to detect patient movement. The previously explained advantages of the invention can be utilized.

[0116] Flexible MR receiving coils 28, for example, have a relatively large number of receiving elements with corresponding conductor loops, when attached to the body of the patient 29, can change their shape, as in FIG 6 bis FIG 9 shown to follow the body contour of the patient 29 and may also move due to the breathing movement or heartbeat of the patient 29.

[0117] In FIG 6 a flexible MR receiving coil 28 with a plurality of receiving elements having corresponding conductor loops 4 is shown, wherein the receiving elements can be embedded, for example, in a flexible plastic material 26 and can be connected to a controller 27. FIG 7 shows schematically a flexible MR receiving coil 28 for imaging the head of the patient 29, FIG 8 a flexible MR receiving coil 28 for imaging the patient’s knee 29 and FIG 9 a flexible MR receiving coil 28 for imaging the abdomen of the patient 29.

[0118] To mathematically describe the shape of a flexible MR receiver coil 28 with high accuracy, mathematical models for quadratic surfaces can be used. Quadratic surfaces include spheres, ellipsoids, cylinders, in particular circular cylinders or elliptic cylinders, elliptic paraboloids, parabolic cylinders, cones, hyperbolic cylinders, two-layer hyperboloids, hyperbolic paraboloids, single-layer hyperboloids, single- or two-layer hyperboloids, and so on, as schematically shown in FIG 10 shown in order from top left to bottom right. Mathematically, a quadratic surface is the graph of a second-degree equation in the three variables x, y, and z. The most general equation form is: A * x 2 + B * y 2 + C * z 2 + D * x * y + E * y * z + F * x * z + G * x + H * y + I * z + J = 0 , where A to J represent 10 coefficients that can be varied to adapt the coil shape. Based on this observation, the shape and position of the flexible MR receiver coil 28 can be determined from the measured voltages induced in the receiver elements by time-varying gradient fields. In particular, the following steps can be performed: a) Initialization of the quadratic area to an initial estimated shape by assigning initial values ​​to the coefficients A to J. b) Initialization of the offset (x 0 , y 0 ) of the coil and the angle of rotation of the coil with respect to the x-axis c) Adaptation of the arrangement of the receiving elements, i.e. the conductor loops 4, to the quadratic area. d) Calculation of the voltages induced in the conductor loops 4 taking into account the current coil shape and the gradient strengths as described above. e) Use of a gradient descent method to adjust the values ​​of the coefficients A to J, the offset (x 0 , y 0 ) and the angle of rotation such that the mean square error between the voltages calculated in step d) and the measured voltages is reduced f) Iterative repetition of steps c), d) and e) until the mean square error drops below a certain threshold.

[0119] At this point, it should be noted that various steps of this method can be further optimized. In the case of a flexible MR receiver coil 28, such as that used in FIG 6 For example, as shown, the initial shape could be constrained to correspond to the surface of a cylinder or a parabolic cylinder oriented along the z-axis. A parabolic cylinder that is symmetric along the x-axis is mathematically described by a very simple equation: A*y 2< = 0.

[0120] For other MR receiver coils 28, the shape of one or more hyperbolic paraboloids may be more suitable. The simplified equation applies: A*x 2< - B*y 2< + z = 0.

[0121] This type of pre-optimization accelerates the conversion speed of the iterative algorithm by reducing the number of coefficients A through J and setting a starting point closer to the final solution. The same consideration applies to coil offset and coil rotation. Here, the numerical range within which these parameters are allowed to change can be restricted, thus forcing the iterative algorithm to stay close to the final solution.

[0122] The invention can also be advantageously applied to wireless coils that combine an analog-to-digital converter on the coil with a wireless transmission of digital signals.

[0123] The methods described above can be variably combined with already known methods for detecting patient movement, such as Hall sensors, 2D or 3D video cameras or MR motion navigators, to further refine and improve the results.

Claims

1. Method for determining the location of an apparatus inside an imaging volume (3) of an MRT system (1), wherein - the imaging volume (3) is surrounded by a field magnet for creating a static basic magnetic field along a longitudinal axis (z) and by a gradient coil (2) of the MRT system (1); - the apparatus has at least one first conductor loop (4), which runs within a first loop plane; - by means of the gradient coil (2) a magnetic alternating field is created in the imaging volume (3); - by means of the at least one first conductor loop (4) at least one first measured value is determined, which depends on a first induction voltage, which is induced by a first component of the alternating field at right angles to the longitudinal axis (z) in the at least one first conductor loop (4); - a location of the apparatus inside the imaging volume (3) is determined at least partly as a function of at least one first measured value and a predetermined magnetic field model for the gradient coil (2), characterised in that an MR signal from an object to be examined in the imaging volume (3) is detected by means of the at least one first conductor loop (4) and an MR image is created as a function of the MR signal.

2. Method according to claim 1 wherein, to determine the at least one first measured value, the MR signal is suppressed.

3. Method according to one of the preceding claims, wherein the apparatus is positioned in the imaging volume (3) in such a way that the first loop plane is at least approximately parallel to the longitudinal axis (z).

4. Method according to one of the preceding claims, - wherein the apparatus has at least one second conductor loop that runs within a second loop plane; - by means of the at least one second conductor loop at least one second measured value is determined, which depends on a second induction voltage that is induced by a second component of the alternating field at right angles to the longitudinal axis (z) in the at least one second conductor loop; and - the location of the apparatus is determined at least partly as a function of at least first measured value, the at least one second measured value and the magnetic field model for the gradient coil (2).

5. Method according to claim 4, wherein the apparatus is positioned in the imaging volume (3) in such a way that the second loop plane is at least approximately parallel to the longitudinal axis (z).

6. Method according to one of the preceding claims, - wherein the apparatus has at least one third conductor loop, which runs within a third loop plane; - by means of the at least one third conductor loop at least one third measured value is determined, which depends on a third induction voltage that is induced by a third component of the alternating field at right angles to the longitudinal axis (z) in the at least one third conductor loop; and - a first location of the at least one first conductor loop (4) inside the imaging volume (3) is determined at least partly as a function of the at least one first measured value and the magnetic field model; - a third location of the at least one third conductor loop inside the imaging volume (3) is determined at least partly as a function of the at least one third measured value and the magnetic field model; and - a relative location of the at least one third conductor loop with regard to the at least one first conductor loop (4) is determined as a function of the first location and the third location.

7. MRT system (1) having - a field magnet for creating a static basic magnetic field along a longitudinal axis (z) and a gradient coil (2), wherein the field magnet and the gradient coil (2) surround an imaging volume (3) of the MRT system (1); - an apparatus with at least one first conductor loop (4), which runs within a first loop plane; - a control unit (5), which is configured to activate the gradient coil (2) to create a magnetic alternating field in the imaging volume (3); - a measurement unit (6), which is connected to the at least one first conductor loop (4) and is configured, as a function of a first induction voltage that is induced by a component of the alternating field at right angles to the longitudinal axis (z) in the at least one first conductor loop (4), to determine at least one first measured value; and - at least one evaluation unit (7), which is configured to determine a location of the apparatus inside an imaging volume (3) at least partly as a function of at least one first measured value and a predetermined magnetic field model for the gradient coil (2), characterised in that the apparatus - has a tuning capacitance (9, 9a, 9b), which is arranged between a first terminal (21a) of the at least one first conductor loop (4) and a second terminal (21b) of the at least one first conductor loop (4); and - has an inductive component (11a, 11b, 11c), which is arranged electrically in parallel to the tuning capacitance (9, 9a, 9b), wherein the first conductor loop is designed to detect an MR signal from the object to be examined in the imaging volume by means of the at least one first conductor loop and the evaluation unit is configured, depending on the MR signal, to create an MR image.

8. MRT system (1) according to claim 7, having a local MR receive coil arrangement (28), which contains the apparatus, wherein the local MR receive coil arrangement (28) is embodied as a flexible surface coil array.

9. MRT system (1) as claimed in claim 7, having a device for medical treatment of a patient (29), wherein the at least one first conductor loop (4) and the device have a predetermined spatial location in relation to one another.

10. MRT system (1) according to claim 7, wherein the measurement unit (6) has an amplifier (17), which is connected to the first terminal (21a) and the second terminal (21b) and is configured to provide the at least one measured value at an output of the amplifier (17), which is connected to the at least one evaluation unit (7).

11. MRT system (1) according to in claim 10, wherein the measurement unit (6) has a filter circuit (15), which is arranged between the first terminal (21a) and a first input of the amplifier (17) as well as between the second terminal (21b) and a second input of the amplifier (17) and which is designed to suppress an MR signal acquired by means of the at least one conductor loop.