LOCAL COIL WITH PILOT TONE TRANSMITTER

Description

[0001] The invention relates to a local coil for a magnetic resonance imaging device with a transmitter for generating the pilot tone, a transmitting antenna for emitting the pilot tone and a receiving antenna for receiving the pilot tone.

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

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

[0004] Depending on the pulse sequence used, image acquisition during magnetic resonance imaging requires several milliseconds to seconds. A longer acquisition time usually results in minimal noise artifacts. It is therefore advisable to start image acquisition at the beginning of a phase in which the body remains relatively still in order to avoid motion artifacts caused by movement during image acquisition. Unavoidable movements include breathing and the heartbeat. However, a phase with movement is often followed by a phase of relative rest, for example after exhalation or a contraction of the heart muscle. Image acquisition during this phase can be expected to be a relatively long period with little movement, so the best measurement results can be expected during this phase.

[0005] It is already known in principle to record movements, for example, using mechanical sensors or electrodes that measure the excitation potentials of the muscles.

[0006] The publication DE 10 2015 203 385 describes a basic method for detecting movement using a high-frequency signal. The signal is continuously recorded in a patient image of an MRI scanner, and signal changes caused by movement, for example, due to changing interference or attenuation, are evaluated. From certain patterns of this signal, a patient's movement caused by breathing or heartbeat can then be detected.

[0007] WO 2015 / 150953 A1 discloses a transmitter for transmitting a synchronization signal. Its two antennas are arranged at the ends of a patient feedthrough. A pilot tone can also be transmitted as a synchronization signal.

[0008] The publication DE 10 2015 224 158 describes a transmitter for pilot tone navigation in a magnetic resonance imaging scanner and a method for detecting patient movement. The transmitter has a power supply and an antenna. The transmitter is designed to transmit a pilot tone signal via the antenna. The transmitter also has a decoupling element to protect the transmitter output from signals that the antenna receives from excitation pulses of the magnetic resonance imaging scanner during magnetic resonance imaging. In the method, movement-dependent changes in the transmitter's pilot tone signal are detected by a controller of the magnetic resonance imaging scanner.

[0009] The document "PISA STEFANO ET AL: "A survey of radar systems for medical applications", IEEE AEROSPACE AND ELECTRONIC SYSTEMS MAGAZINE, IEEE SERVICE CENTER, PISCATAWAY, NJ, US, Vol. 31, No. 11" provides an overview of radar systems for medical applications, including cardiac and respiratory monitoring. Antennas, including planar antennas, are also described.

[0010] The document "THIEL F ET AL: "Non-contact detection of myocardium's mechanical activity by ultrawideband RF-radar and interpretation applying electrocardiography", REVIEW OF SCIENTIFIC INSTRUMENTS, AIP, MELVILLE, NY, US, Vol. 80, No. 11" describes the synergistic use of broadband radar in conjunction with magnetic resonance imaging to provide complementary information to improve diagnosis and imaging, particularly in cardiac magnetic resonance imaging as a non-contact sensor of cardiac contraction.

[0011] From the document US 2014 / 0239953 A1, a multi-channel antenna for a magnetic resonance imaging device is known, which has a first resonant antenna loop with a transmitting element and a second resonant antenna loop with a transmitting element, wherein the second resonant antenna loop is arranged next to the first resonant antenna loop.

[0012] Document WO 2014 / 096997 A1 discloses a decoupling circuit arranged between two adjacent radio-frequency receiving coils to automatically decouple these receiving coils. In one embodiment, the document proposes coupling a test signal into the first receiving coil to measure the coupling between the first receiving coil and the second receiving coil.

[0013] The document US 6008649 A describes a local coil device for a magnetic resonance system.

[0014] The local coil comprises a coil arrangement with a pair of elongated coils arranged parallel to each other and spaced apart, and a predetermined number of lateral coils arranged parallel to each other along the elongated coils and spaced apart from each other. First capacitors are positioned along the elongated coils, and a second capacitor is integrated into each of the lateral coils. A first coupling network operates the coil arrangement in a first mode to receive a B1 magnetic field at a first frequency determined by the first and second capacitors. Similarly, the antenna arrangement is operated in a second mode at a second frequency via a second coupling network.

[0015] The task therefore arises to simplify the application of a pilot tone signal.

[0016] The object is achieved by a local coil according to the invention according to claim 1.

[0017] The local coil according to the invention has a transmitting antenna for emitting a pilot tone. The transmitting antenna can be, for example, a magnetic antenna in the form of a coil or loop, but an electrical antenna such as a dipole is also conceivable, depending on the wavelength of the pilot tone signal. The pilot tone can be supplied from a magnetic resonance imaging scanner via a signal connection, but it is also conceivable for the local coil to have a transmitter for generating a pilot tone and a transmitting antenna for emitting the pilot tone. The transmitter can be, for example, an oscillator or other signal generator that can generate a signal with a predetermined frequency. An amplifier or mixer that generates or derives the pilot tone signal from a signal supplied by the magnetic resonance imaging scanner is also conceivable.

[0018] Furthermore, the local coil has a receiving antenna for receiving the pilot tone. The receiving antenna can, for example, be an antenna coil that is also provided for receiving MR signals in the local coil. It is conceivable that the pilot tone signal lies in the same frequency range as the MR signal or that the receiving antenna is tuned to several frequencies, that of the pilot tone signal and that of the MR signal. However, it is also conceivable that a separate receiving antenna is provided for the pilot tone signal. Finally, the local coil according to the invention has a decoupling device for decoupling the receiving antenna from the transmitting antenna. Decoupling is defined as a reduction in the signal directly transmitted by the receiving antenna, ieThe reduction in the pilot tone signal received without interaction with the environment, in particular the patient, for example in free space, is considered to be 3 dB, 6 dB, 12 dB, or more, compared to a local coil without a decoupling device. Various embodiments of a decoupling device are specified below in the subclaims.

[0019] The decoupling device of the local coil according to the invention is designed to inductively decouple the transmitting antenna from the receiving antenna. According to the invention, inductive decoupling can be achieved with a magnetic receiving and transmitting antenna through geometric overlap, as described below. In embodiments not claimed, however, it is also conceivable for additional inductors or high-frequency transformers to provide decoupling.

[0020] The receiving antenna of the local coil according to the invention has a first antenna coil and the decoupling device has a first induction loop.

[0021] An antenna coil is a magnetic antenna whose effect is based on the induction of the magnetic component of an alternating electromagnetic field. For this purpose, the electrical conductor of the antenna coil encloses a surface in the interior of which a magnetic field is induced by a current flowing through the conductor. The enclosed surface is preferably essentially flat, i.e. deviations of the enclosed surface perpendicular to a plane passing through the surface amount to less than 30%, 20% or 10% of a maximum dimension of the enclosed surface parallel to the plane. The antenna coil can have one or more turns. In the sense of the invention, an induction loop is also a flat surface at least partially enclosed by a conductor. However, it is conceivable that the induction loop is not completely closed; for example, the conductors are not fed to the induction loop or connected to it in the immediate vicinity of one another.The induction loop can therefore also be U-shaped, for example. The conductor can also contain capacitive and / or inductive elements, meaning it can generally be a conductor with complex impedance.

[0022] In this case, a projection of a surface enclosed by the first induction loop onto a surface enclosed by the first antenna coil has a non-empty intersection along a surface normal of the enclosed surface. The surface normal is considered, for example, to be the surface normal of the previously described planes through the surfaces.

[0023] In other words, the antenna coil and the induction loop overlap, so that a current flowing through the induction loop creates a magnetic field that passes through the antenna coil and induces a current there.

[0024] The local coil according to the invention simplifies the application of the pilot tone by integrating the transmitting and receiving antennas. The local coil with decoupling device advantageously reduces portions of the pilot tone signal that are received without interaction with the patient and therefore do not contain any information about, for example, heart rate or respiration. Without these direct signal components, the degree of modulation of the received pilot tone signal increases, improving the signal-to-noise ratio and the detection of physiological processes. Advantageously, inductive decoupling without galvanic contact between the pilot tone transmitter and the receiving antenna can achieve decoupling and thus avoid additional interference and attenuation.

[0025] Advantageously, an overlap between the areas of the induction loop and the antenna coil can achieve a targeted interaction, which can be adjusted via the area of ​​overlap, i.e., the area of ​​intersection. Depending on the current direction, the polarity of the magnetic field and thus also the polarity of the induced current can be changed.

[0026] In the local coil according to the invention, the decoupling device has a second induction loop that is aligned substantially parallel to the first induction loop. "Substantially parallel" means that the surface normals of the first induction loop and the second induction loop enclose an angle of less than 45 degrees, 30 degrees, or 10 degrees. When the pilot tone is emitted, the decoupling device is designed to generate a magnetic field in each of the first induction loop and the second induction loop, the magnetic field components of which have the same sign in the direction of the surface normal of the area enclosed by the first induction loop and / or second induction loop. In other words, the magnetic field generated by the first induction loop and the second induction loop is aligned substantially in the same direction.This can be achieved by having the same current flow through the first induction loop and the second induction loop in the same direction of rotation, for example both clockwise or both counterclockwise.

[0027] A projection of the area enclosed by the second induction loop onto an area enclosed by the first antenna coil along a surface normal of the enclosed area has an empty intersection.

[0028] With regard to the surfaces and their surface normals, the above applies. In other words, the surfaces of the first antenna coil and the second induction loop do not overlap, but are disjoint. As a result, the second induction loop is arranged with respect to the first antenna coil such that the surface enclosed by the first antenna coil lies outside the surface enclosed by the second induction loop. If, as previously stated, the magnetic field generated within the induction loops has the same sign, the magnetic field outside the induction loop has exactly the opposite sign. The effect of the first induction loop and the second induction loop on the first antenna coil thus reduce each other. With suitable dimensioning of the first induction loop and the second induction loop and suitable positioning relative to the first antenna loop, it is even possible for the effect to cancel each other out.

[0029] A transmitting antenna consisting of a first induction loop and a second induction loop is then advantageously decoupled from the receiving coil by the decoupling device in accordance with the invention.

[0030] Further advantageous embodiments are specified in the subclaims.

[0031] In one conceivable embodiment of the local coil according to the invention, the local coil has a second antenna coil. The intersection of a projection of the area enclosed by the first induction loop onto an area enclosed by the second antenna coil along a surface normal of the enclosed area is an empty intersection. The above statements apply to the areas and surface normals. In other words, the first induction loop and the second antenna coil are disjoint and have no overlapping areas. Furthermore, a projection of the area enclosed by the second induction loop onto an area enclosed by the second antenna coil along a surface normal of the enclosed area has a non-empty intersection. In other words, the second induction loop and the second antenna coil overlap, just as the first induction loop and the first antenna coil do.

[0032] As already explained, the first induction loop advantageously acts to decouple the second induction loop from the second antenna coil, just as the second induction loop does for the first antenna coil. At the same time, however, the first induction loop and the second induction loop act as transmitting antennas for the pilot tone signal due to their rectified magnetic fields.

[0033] In one possible embodiment of the local coil according to the invention, a projection of the area enclosed by the first antenna coil along the surface normal onto the area enclosed by the second antenna coil has a non-empty intersection. In other words, the first antenna coil and the second antenna coil overlap. It is conceivable that the overlapping area occupies a proportion of less than 50%, 30%, 20%, or 10% of the area enclosed by the first or second antenna coil, respectively.

[0034] Advantageously, the overlapping surface acts as an inductive decoupling between the first antenna coil and the second antenna coil in the same manner as described for the induction loops.

[0035] In one conceivable embodiment of the local coil according to the invention, the first induction loop has a bridging conductor. The bridging conductor electrically connects two points on the circumference of the first induction loop. It is also conceivable for the bridging conductor to comprise elements with a complex resistance, such as a capacitance. In this way, the bridging conductor divides the induction loop into two disjoint areas or regions. The two regions can have the same surface area, but it is also conceivable for the areas to differ in ratio, for example, 40:60 or 30:70.

[0036] In one possible embodiment, the bridging conductor has a variable capacitance. This capacitance can, for example, be a trimming capacitor, which allows identical transmit coils to be inserted and adjusted into local coils with different geometries.

[0037] However, it is also conceivable to use capacitances that can be changed by the magnetic resonance imaging device using a voltage or a control signal and a control device, for example PIN diodes or switchable capacitances with MEMS switches.

[0038] The variable capacitance advantageously allows for the effective area and thus the inductive coupling of the induction loop to the antenna coils to be varied. This advantageously allows the decoupling of the transmitting antenna from the receiving antenna by the magnetic resonance imaging scanner to be adapted to different conditions, for example, due to interaction with the patient's body or a different geometry in a flexible local coil, and always achieves maximum sensitivity for the pilot tone signal coming from the patient's body.

[0039] In a non-claimed embodiment of the local coil according to the invention, the decoupling device comprises a capacitive decoupling network with a capacitance that electrically connects the transmitting antenna to the receiving antenna. For example, one or preferably two capacitances are possible, each of which electrically connects a tap on the transmitting antenna, e.g., on the first induction loop, to a tap on the receiving antenna, e.g., on the antenna coil.

[0040] In one conceivable embodiment of the magnetic resonance imaging system according to the invention, the magnetic resonance imaging system comprises a local coil according to the invention. The magnetic resonance imaging system is designed to provide a pilot tone signal, for example, in a radio-frequency unit, transmit it to the local coil via a signal connection, and transmit it via the transmitting antenna of the local coil.

[0041] The magnetic resonance imaging system is advantageously capable of providing signals with highly precise and stable frequencies, thus advantageously preventing the pilot tone signal from interfering with magnetic resonance imaging. For example, frequencies can be selected that lie just outside the frequency range of the magnetic resonance signal, for example, by less than 100 kHz, 500 kHz, or 1 MHz. This advantageously ensures that the antennas for the MR signal still have sufficient sensitivity for reception and can be used, while interference is avoided by precisely maintaining the frequency spacing.

[0042] In one conceivable embodiment of the magnetic resonance imaging system according to the invention, the magnetic resonance imaging system has a local coil with a variable capacitance in the first induction loop. The magnetic resonance imaging system further has a control unit configured to adjust the variable capacitance such that the receiving antenna is optimally decoupled from the transmitting antenna. Optimal decoupled is considered, for example, if the signal-to-noise ratio of the received pilot tone signal with physiological information has a local maximum. The control unit can achieve this local maximum, for example, using a numerical gradient optimization method or another optimization method while varying the variable capacitance.

[0043] The above-described properties, features and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more clearly understood in connection with the following description of the embodiments, which are explained in more detail in connection with the drawings.

[0044] They show: Fig. 1 shows a schematic representation of a magnetic resonance imaging scanner with a local coil according to the invention; Fig. 2 shows a schematic representation of a local coil according to the invention; Fig. 3 shows a schematic representation of a local coil according to the invention; Fig. 4 shows a schematic representation of an induction loop of an embodiment of a local coil according to the invention; Fig. 5 shows a schematic representation of a capacitive decoupling network of a local coil according to the invention.

[0045] Fig. 1 shows a schematic representation of an embodiment of a magnetic resonance imaging device 1 with a local coil 50 according to the invention.

[0046] The magnet unit 10 has a field magnet 11 that generates a static magnetic field B0 for aligning nuclear spins of samples or the patient 100 in a recording area. The recording area is characterized by an extremely homogeneous static magnetic field B0, whereby the homogeneity particularly affects the magnetic field strength or magnitude. The recording area is almost spherical and arranged in a patient tunnel 16 that extends in a longitudinal direction 2 through the magnet unit 10. A patient bed 30 is movable in the patient tunnel 16 by the traversing unit 36. The field magnet 11 is typically a superconducting magnet that can generate magnetic fields with a magnetic flux density of up to 3T, and even higher in the latest devices. However, permanent magnets or electromagnets with normally conducting coils can also be used for lower field strengths.

[0047] Furthermore, the magnet unit 10 has gradient coils 12, which are designed to superimpose variable magnetic fields in three spatial directions on the magnetic field B0 for spatial differentiation of the acquired imaging regions in the examination volume. The gradient coils 12 are typically coils made of normally conducting wires that can generate mutually orthogonal fields in the examination volume.

[0048] The magnet unit 10 also has a body coil 14 which is designed to radiate a high-frequency signal supplied via a signal line into the examination volume and to receive resonance signals emitted by the patient 100 and to emit them via a signal line.

[0049] A control unit 20 supplies the magnet unit 10 with the various signals for the gradient coils 12 and the body coil 14 and evaluates the received signals.

[0050] Thus, the control unit 20 has a gradient control 21 which is designed to supply the gradient coils 12 with variable currents via supply lines, which provide the desired gradient fields in the examination volume in a time-coordinated manner.

[0051] Furthermore, the control unit 20 has a radio-frequency unit 22, which is designed to generate a radio-frequency pulse with a predetermined temporal profile, amplitude, and spectral power distribution for exciting a magnetic resonance of the 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 a local transmitting antenna.

[0052] A controller 23 communicates via a signal bus 25 with the gradient controller 21 and the high-frequency unit 22.

[0053] 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.

[0054] In Fig. 2 An embodiment of a local coil 50 according to the invention is shown. The local coil has a first antenna coil 51 as a receiving antenna for MRI signals. A conductor loop with a single turn is shown here as an example, but antenna coils with multiple turns are also conceivable. Additional capacitances in the antenna coil are also conceivable for tuning during reception or detuning during an excitation pulse. For reasons of clarity, further elements for processing the received signals in the local coil are not shown, such as low-noise preamplifiers, oscillators, and mixers for frequency conversion or analog-to-digital converters for digitization.

[0055] In one possible embodiment, the local coil 50 further comprises a pilot tone transmitter 60 which generates a pilot tone signal. The pilot tone transmitter 60 is electrically connected to a transmitting antenna which has a first induction loop 61 and a second induction loop 62. The first induction loop 61 is located within an area enclosed by the first antenna loop 51, while the second induction loop 62 is located outside this enclosed area. It is also possible for the first induction loop 61 to be spaced apart in the direction of a surface normal to the enclosed area, i.e., to be located above or below the antenna loop. The spacing is preferably smaller than a lateral dimension of the induction loop, e.g., the diameter of the induction loop.

[0056] In another embodiment, it is also conceivable for the pilot tone signal to be provided in the magnetic resonance imaging system 1, for example, in the radio-frequency unit 22, and fed to the local coil 50 via the local coil's connecting line 33. Mixed forms are also conceivable, for example, for the magnetic resonance imaging system to supply a signal from which the pilot tone signal is generated through amplification, frequency conversion, and / or modulation. This ensures, in particular, a highly precise and stable frequency that does not interfere with the actual magnetic resonance imaging.

[0057] The first antenna coil 51, the first induction loop 61 and the second induction loop 62 are aligned substantially parallel, ie a plane through the first antenna coil 51 and a plane through the first induction coil 61 or the second induction loop 62 enclose an angle that is less than 45 degrees, 30 degrees or 10 degrees.

[0058] A current generated by the pilot tone transmitter 60 and fed to the transmitting antenna flows through the first induction loop 61 and the second induction loop 62 in the same direction of rotation, so that it generates a magnetic field within the areas enclosed by the induction loops 61, 62 that is essentially parallel to each other or in the same direction. Outside the areas enclosed by the induction loops 61, 62, the magnetic field component changes its sign in the direction of the surface normal of the areas enclosed by the induction loops. For the first induction loop 61 in Fig. 2 a part of the magnetic field lines generated by it also runs outside the first antenna loop 51. The field generated by the first induction loop 61 inside the area enclosed by the first induction loop 61 is only partially compensated by the field generated in the outer space of the first induction loop 61 with respect to a current induced in the first antenna loop 51.

[0059] Only by means of the second induction loop 62 and a suitable dimensioning of the surfaces and lateral distances can compensation be achieved and the coupling between the transmitting antenna and the first antenna coil 51 reduced or completely eliminated.

[0060] The pilot tone signal can be sent to the transmitting antenna or the induction loop directly via connections as shown in Fig. 2 bis 5 As indicated, they can be fed in and lead to a current flow in the induction loop. However, it is also possible, for example, to achieve a current flow in the induction loop through inductive coupling, as is often the case with magnetic antennas. Other matching networks with inductive and / or capacitive elements are also conceivable. The same applies to the decoupling of a received pilot tone signal from the receiving antennas.

[0061] In principle, however, it would also be conceivable for the first induction loop 61 and the second induction loop 62 together to form a coil with, for example, one turn and a continuous enclosed area, which lies partly above the first antenna coil 51 and partly outside it.

[0062] In Fig. 3 An embodiment of the local coil with a first antenna coil 51 and a second antenna coil 52 is shown. The first antenna coil 51 and the second antenna coil 52 are arranged next to one another in order to be able to detect different areas of the patient 100 independently of one another, possibly even simultaneously. A signal from the neighboring coil is considered an interference signal. The magnetic fields extending into the space of the other coil also lead to interactions such as a detuning of the resonant frequency of the antenna coils. It is therefore desirable that the first antenna coil 51 and the second antenna coil 52 are decoupled from one another in the sense that the interactions are reduced. As already mentioned Fig. 2 As explained with reference to the first antenna coil 51 and the transmitting coil with the first induction loop 61 and the second induction loop 62, this can be achieved by overlapping enclosed areas. Fig. 3 Therefore, an area is provided between the first antenna coil 51 and the second antenna coil 52, which is enclosed by both antenna coils or in which the two antenna coils overlap. With a suitable selection of the overlapping surface areas and the non-overlapping surface areas, complete decoupling of the first antenna coil 51 from the second antenna coil 52 can be achieved.

[0063] An arrangement of the first induction loop 61 or second induction loop 62 in this overlapping area of ​​the two antenna coils 51, 52 reduces the degrees of freedom for decoupling the transmitting coil from the two antenna coils 51 and 52. These degrees of freedom can only be regained if the first induction loop 61 and the second induction loop 62 are connected by conductors that limit the smallest possible effective area, for example, by these connecting conductors being closely arranged parallel to one another or twisted together. Fig. 3 This is indicated by a "bone-shaped" outline of the first induction loop 61, the second induction loop 62, and the connecting conductor between them. By reducing the effective area of ​​the connection between the first induction loop 61 and the second induction loop 62 for induction by the first antenna coil and / or the second antenna coil to zero, the length of the connection can be varied, and thus the distance or position relative to the first antenna coil 51 and the second antenna coil 52 can be used to improve or optimize the decoupling of the transmitting coil from the antenna coils 51, 52 simultaneously with the mutual decoupling of the first antenna coil 51 and the second antenna coil 52.

[0064] In Fig. 4 shows a possibility of how the effective area of ​​the first induction loop 61 and / or the second induction loop 62 can be adjusted without mechanically changing the conductor loop. Fig. 4 shows the first induction loop 61 as an example. In the embodiment shown, this is provided with a compensating capacitance 65 to equalize the current distribution in the first induction loop 61 or the transmitting antenna consisting of the first induction loop 61 and the second induction loop 62 between the areas divided by a bridging conductor 63 explained below. Capacitances for tuning a resonance would also be conceivable.

[0065] Secondly, the first induction loop 62 also has two taps or contact points along the loop conductor, which are electrically connected to one another by the bridging conductor 63. The bridging conductor 63 divides the area enclosed by the first induction loop into two disjoint sub-areas. The area ratio of the sub-areas can be, for example, 50:50, but also 40:60 or less. The bridging conductor 63 can have one or more capacitors connected in series in the bridging conductor. The capacitance can have a constant value and, when equipping the local coil 50, can adapt the transmitting antenna or the first induction loop 61 to different geometries, so that the same conductor loop can be used for different induction loops in different local coils. A trimming capacitor is also conceivable, which also allows adjustment during installation.

[0066] Finally, it is also conceivable for the capacitance to be designed as a variable capacitance 64, so that it can be changed without mechanical access by the magnetic resonance imaging device 1. A PIN diode is conceivable as the variable capacitance 64, in which the pilot tone signal is superimposed with a DC voltage as a control voltage by the control unit 20 or the radio-frequency unit 22. A control in the local coil 50 is also conceivable, which applies a voltage to the PIN diode in response to a signal from the control unit 20 of the magnetic resonance imaging device. Capacitances that can be switched on by the control of the local coil via switches are also conceivable.

[0067] Since the magnetic resonance imaging device 1 is designed to adapt the effective area of ​​the first induction loop 61 and, in an embodiment not shown, also of the second induction loop 62 via the variable capacitance 64 in the bridging conductor 63, the decoupling can also be automatically adapted to changed conditions, for example to other patients, and an optimal signal-to-noise ratio for the pilot tone signal can always be achieved with the local coil according to the invention.

[0068] In Fig. 5An unclaimed embodiment of the local coil 50 is shown, in which the decoupling of the first antenna coil 51 from the first induction loop 61 is achieved by capacitive decoupling using a decoupling network. In the simplest case, the decoupling network is provided by two decoupling capacitors 53 that connect the first antenna coil 51 to the first induction loop 61 at different taps.

[0069] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited to the disclosed examples and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

1. Local coil for a magnetic resonance tomography device (1), wherein the local coil (50) has a transmit antenna for radiating a pilot tone and a receive antenna for receiving the pilot tone, wherein the local coil further has a decoupling apparatus for decoupling the receive antenna from the transmit antenna, wherein the decoupling apparatus is designed to decouple the transmit antenna inductively from the receive antenna, wherein the receive antenna has a first antenna coil (51) and the decoupling apparatus has a first induction loop (61), wherein a projection of a surface enclosed by the first induction loop (61) onto a surface enclosed by the first antenna coil (51) along a surface normal of the enclosed surface has a non-empty intersection, characterised in that the decoupling apparatus has a second induction loop (62), which is aligned essentially parallel to the first induction loop (61), wherein the decoupling apparatus is designed to generate in each case a magnetic field in the first induction loop (61) and the second induction loop (62) when the pilot tone is emitted, the magnetic field components of which have the same signs in the direction of the surface normal of the surface enclosed by the first induction loop (61) and / or second induction loop (62), wherein a projection of the surface enclosed by the second induction loop (61) onto a surface enclosed by the first antenna coil (51) along a surface normal of the enclosed surface has an empty intersection, wherein the transmit antenna has the first induction loop (61) and the second induction loop (62).

2. Local coil according to claim 1, wherein the local coil (50) has a pilot tone transmitter (60) for generating the pilot tone.

3. Local coil according to claim 1 or 2, wherein the local coil (50) has a second antenna coil (52), wherein a projection of the surface enclosed by the first induction loop (61) onto a surface enclosed by the second antenna coil (51) along a surface normal of the enclosed surface has an empty intersection and wherein a projection of the surface enclosed by the second induction loop (61) onto a surface enclosed by the second antenna coil (51) along a surface normal of the enclosed surface has a non-empty intersection.

4. Local coil according to claim 3, wherein a projection of the surface enclosed by the first antenna coil (51) along the surface normal onto the surface enclosed by the second antenna coil (51) has a non-empty intersection.

5. Local coil according to one of claims 1 to 4, wherein the first induction loop (61) has a jumper conductor (63), which connects two points of the first induction loop (61) electrically with one another and which divides the surface enclosed by the first induction loop (61) into two disjunct regions.

6. Local coil according to claim 5, wherein the jumper conductor (63) has a changeable capacitance (64).

7. Magnetic resonance tomography device with a local coil (50) according to claim 1, wherein the magnetic resonance tomography device (1) is designed to provide a pilot tone signal, to transmit the same via a signal connection to the local coil (50) and to emit the same via the transmit antenna of the local coil (50).

8. Magnetic resonance tomography device with a local coil (50) according to claim 6, wherein the control unit (20) of the magnetic resonance tomography device (1) is designed to adjust the changeable capacitance (64) in such a way that the receive antenna is decoupled optimally from the transmit antenna.