Superconducting local coil for a magnetic resonance imaging device and method for operation

Abstract

The invention relates to a local coil for a magnetic resonance imaging scanner. The local coil comprises an antenna coil with a superconductor that exhibits superconductivity that can be induced at room temperature. The local coil further comprises an irradiation device that induces the induced superconductivity by irradiating the superconductor with electromagnetic waves. The invention further relates to a magnetic resonance imaging scanner, a system comprising the local coil and the magnetic resonance imaging scanner, and a method for operating the device.

Description

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

[0002] The invention relates to a local coil for a magnetic resonance imaging scanner. The local coil comprises an antenna coil with a superconductor that exhibits superconductivity that can be induced at room temperature. The local coil further comprises an irradiation device that induces the induced superconductivity by irradiating the superconductor with electromagnetic waves. The invention further relates to a magnetic resonance imaging scanner, a system comprising the local coil and the magnetic resonance imaging scanner, and a method for operating the device.

[0003] 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.

[0004] Using magnetic gradient fields, a spatial coding is imprinted on the signals, which subsequently allows the received signal to be assigned to a volume element. The received signal is then evaluated, and a three-dimensional image of the object under examination is provided. Local receiving antennas, so-called local coils, are preferably used to receive the signal. These antennas are positioned directly on the object under examination to achieve a better signal-to-noise ratio.

[0005] The signal-to-noise ratio depends on the quality factor of the local coil, which increases with decreasing resistance of an induction loop used to detect the magnetic resonance signal. An induction loop made of a superconductor would be ideal. However, known superconductors require cooling to cryogenic temperatures.

[0006] Evidence of induced superconductivity at room temperature is reported in publication 1. Wang, E., Adelinia, JD, Chavez-Cervantes, M. et al. Superconducting nonlinear transport in optically driven high-temperature K3C60. Nature Communications 14, 7233 (2023) (https: / / doi.org / 10.1038 / s41467-023-42989-7).

[0007] It is an object of the invention to provide a local coil with improved reception behavior.

[0008] The object is achieved by a local coil according to claim 1, a system according to claim 6 or 7 and a method according to claim 8.

[0009] The local coil according to the invention is intended for a magnetic resonance imaging scanner for acquiring magnetic resonance signals from an examination subject. The local coil has an antenna coil. An antenna coil is an electrical conductor designed to convert alternating electrical and / or magnetic fields of the magnetic resonance signals into currents and / or voltages in the conductor. The antenna coil is preferably designed as an induction loop.

[0010] The antenna coil contains a superconductor. This means that the superconductor is part of the antenna coil in such a way that, upon receiving the magnetic resonance signal, its superconductivity improves the received signal, i.e., the signal-to-noise ratio (SNR) and / or the signal amplitude. In an induction loop, the superconductor can be applied to the surface of a conductor of the induction coil, for example. It would also be conceivable, however, for the superconductor to be applied to the surface of a non-conductor, such as a waveguide made of a metamaterial for microwaves or an optical conductor.

[0011] The superconductor exhibits inducible superconductivity, which occurs at room temperature. Induced superconductivity is defined as superconductivity induced by external influences, but not by lowering the temperature as in classical superconductors. Superconductivity is preferably induced by electromagnetic waves or photons of light, infrared light, terahertz radiation, or microwaves. Sources of this radiation can be, for example, lasers, masers, or microwave oscillators. Room temperature is defined as a temperature range such as that prevailing in treatment rooms or around the patient in these rooms, preferably between 15 and 30 degrees Celsius.In the broader sense of the invention, however, this also includes a temperature range that can be achieved without special cryogenic cooling and cryogenic insulation, for example by means of conventional cooling units or Peltier elements and cooling with a fluid, for example between 0 and 20 degrees Celsius or -10 and 10 degrees Celsius.

[0012] The local coil according to the invention comprises an irradiation device designed to induce superconductivity by irradiating the superconductor with electromagnetic waves or photons. The irradiation device comprises at least one device for distributing or applying the electromagnetic waves to the superconductor, as explained below in various embodiments in relation to the subclaims. It is also conceivable for the irradiation device to comprise the source or generator or oscillator for generating the electromagnetic waves. However, it is also conceivable for the electromagnetic waves or photons to be supplied to the local coil via a line or waveguide from the magnetic resonance imaging scanner.

[0013] Advantageously, the local coil according to the invention improves the reception of the magnetic resonance signal even at room temperature due to the induced superconductivity.

[0014] A system according to the invention comprises a corresponding local coil and a magnetic resonance imaging scanner. The irradiation device of the local coil does not itself comprise the source, generator, or oscillator for the electromagnetic waves. Instead, the magnetic resonance imaging scanner is designed to supply the electromagnetic waves for inducing superconductivity to the irradiation device of the local coil. The generator or oscillator is part of the magnetic resonance imaging scanner, and the electromagnetic waves are supplied to the local coil, for example, via a cable, a waveguide, or an optical waveguide.

[0015] In this way, the local coil can be designed to be smaller, with less electrical power consumption and waste heat.

[0016] The method according to the invention is intended for acquiring an image by means of the system comprising a local coil and a magnetic resonance imaging scanner.

[0017] In one step of the procedure, nuclear spins of an object under examination are excited by a high-frequency pulse using a high-frequency transmitter and a transmitting antenna in a static magnetic field of the magnetic resonance imaging scanner.

[0018] Subsequently, magnetic resonance signals of the nuclear spins are recorded using the superconducting antenna coil. Inducing superconductivity requires a high intensity of the electromagnetic waves or photon density, and thus power from the source or generator. To limit the power requirement, a clock generator for the irradiation device is designed to induce superconductivity using the irradiation device at a predetermined mixing frequency, i.e., at a predetermined repetition rate. The duty cycle is not necessarily symmetrical; i.e., the phases of superconductivity are preferably significantly shorter than those without superconductivity, further reducing the average power required to induce superconductivity.

[0019] In this way, the antenna coil also becomes superconducting for only a short time. If this occurs at a fixed, predetermined frequency and phase position, the superconductivity for the magnetic resonance signal acquired by the antenna coil acts similarly to a sample and hold (S&H) element for an analog-to-digital converter (ADC). Preferably, the S&H element of the ADC is also clocked synchronously with the phases of superconductivity, so that the ADC acquires the signal during or shortly before the end of superconductivity.

[0020] If the sampling rate is significantly lower than the frequency of the magnetic resonance signal, and the duration of the superconductivity and S&H duration are significantly shorter than the duration of a half-wave of the magnetic resonance signal, the magnetic resonance signal is acquired using subsampling. The digitized signal then corresponds to a digitized magnetic resonance signal converted to an intermediate frequency using subsampling.

[0021] In this way, the power requirement can be further reduced in an advantageous manner or, if the continuous power of the excitation source is limited, scanning with an antenna coil operating through induced superconductivity can be made possible in the first place.

[0022] The further course of image acquisition then corresponds to further digital signal processing with an intermediate frequency, which differs depending on the sequence used.

[0023] Image reconstruction based on the acquired data also follows the established procedures. Finally, the reconstructed image is output to a user.

[0024] In one embodiment of the system according to the invention, the magnetic resonance tomograph is designed to synchronize the clock generator of the local coil with a system clock of the magnetic resonance tomograph according to the method and to reconstruct an image from a magnetic resonance signal induced in the superconducting antenna coil.

[0025] Alternatively, it is also conceivable for the magnetic resonance imaging scanner to generate the electromagnetic wave field with the appropriate temporal position and duration, or to supply corresponding alternating currents for transmission, for example, via a cable or waveguide. In this way, the local coil itself can be implemented more simply, lighter, and with less heat loss.

[0026] Further advantageous embodiments are specified in the subclaims.

[0027] In one conceivable embodiment of the local coil according to the invention, the irradiation device comprises a waveguide. The waveguide is designed to guide the electromagnetic waves along a predetermined path. The path is designed to guide the electromagnetic waves to the superconductor so that they can act on the superconductor and induce induced superconductivity. In the simplest case, for example, it is possible for the superconductor to be applied to the waveguide. The waveguide then acts as the core of the antenna loop, but does not itself form the induction loop because it is poorly or non-conductive. The surface of the waveguide is designed to feed the electromagnetic waves or photons to the superconductor to induce induced superconductivity. In optical waveguides, this can be achieved by a rough surface or a strong curvature.For electrical waveguides or hollow conductors, slots or openings can be provided in the conductive shell. For metamaterials, corresponding coupling elements along the length are conceivable.

[0028] It is also conceivable that the waveguide itself is not the core or carrier for the superconductor, but rather a different material, or that the waveguide is self-supporting. The waveguide is then positioned on or along the superconductor, so that the radiation is directed accordingly. A resonator could also be conceivable as a waveguide for the alternating electromagnetic field, with the superconductor located inside it. This could particularly be a dielectric resonator, since it interacts less with the magnetic resonance measurement.

[0029] Advantageously, the waveguide enables a predetermined and homogeneous irradiation of the superconductor to induce superconductivity.

[0030] In one possible embodiment of the local coil according to the invention, the local coil has a clock generator in signal communication with the irradiation device. A clock generator is referred to here as a device that, at predetermined time intervals, preferably synchronized with a magnetic resonance system used to acquire images through the local coil according to the invention, induces the induced superconductivity in the superconductor by means of the irradiation device. The time intervals are preferably periodic and short compared to a period of the Larmor frequency, as already explained with regard to the method according to the invention. The irradiation device induces the induced superconductivity during the predetermined time intervals by radiating the alternating electromagnetic field or the photons into the superconductor.

[0031] The irradiation device is designed to temporarily induce superconductivity in the superconductor in response to a signal from the clock generator. Preferably, the local coil also includes the source or oscillator for the alternating electromagnetic field or the photons, in addition to the clock generator.

[0032] Advantageously, the local coil with the clock generator and, in particular, the source is more independent of the magnetic resonance imaging system because no supply of the alternating field or photons via a separate waveguide is required. The local coil according to the invention can thus also be retrofitted transparently to an existing magnetic resonance system.

[0033] 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.

[0034] They show: Fig. 1 a schematic representation of an embodiment of a magnetic resonance imaging apparatus with a local coil according to the invention; Fig. 2 a schematic representation of an embodiment of a local coil according to the invention; Fig. 3 a schematic representation of an embodiment of a local coil according to the invention; Fig. 4 a cross-section through an antenna loop of a local coil according to the invention; Fig. 5 is a schematic cross-sectional view of an embodiment of a local coil according to the invention; Fig. 6 a schematic representation in lateral cross section of an embodiment of a local coil according to the invention; Fig. 7 a schematic representation of the temporal relationships of the signals of an embodiment of the method according to the invention; Fig. 8 a schematic representation of the temporal relationships of the signals of an embodiment of the method according to the invention.

[0035] Fig. 1 shows a schematic representation of an embodiment of a magnetic resonance imaging device 1 for image acquisition with the local coil 50 according to the invention.

[0036] 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, with the homogeneity particularly affecting the magnetic field strength or magnitude. The recording area is nearly spherical and arranged in a patient tunnel 16 that extends in a longitudinal direction 2 through the magnet unit 10.

[0037] A patient bed 30 is movable in the patient tunnel 16 by the movement unit 36.

[0038] Typically, the field magnet 11 is a superconducting magnet capable of generating 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.

[0039] Furthermore, the magnet unit 10 has gradient coils 12, which are configured 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 gradients of the static magnetic field B0 in the examination volume.

[0040] The magnet unit 10 has a body coil 14 configured to radiate a high-frequency signal supplied via a signal line into the examination volume. For some image acquisitions, a local coil 50 may also be provided as a transmitting and receiving antenna.

[0041] 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.

[0042] Thus, the control unit 20 has a gradient control 21 which is configured 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.

[0043] Furthermore, the control unit 20 has a radio-frequency unit 22 configured to generate a radio-frequency pulse with a predetermined temporal profile, amplitude, and spectral power distribution for exciting magnetic resonance of the nuclear spins in the patient 100. Pulse powers in the kilowatt range can be achieved. The excitation signals are radiated into the patient 100 via the body coil 14.

[0044] The local coil 50 detects the magnetic resonance signals emitted from the patient 100 and transmits them to the magnetic resonance tomograph 1.

[0045] A controller 23 communicates with the gradient controller 21 and the high-frequency unit 22 via a signal bus 25. A user can make inputs to the control unit 20 via an operating terminal 60.

[0046] Fig. Figure 2 shows a schematic representation of a local coil 50 according to the invention. The local coil 50 has an antenna coil 51, which is provided by a superconductor with inducible superconductivity. Since the induced superconductivity is caused by the irradiation of photons with predetermined energy, the superconductor is preferably applied as a thin layer on a carrier so that the photons can penetrate the layer. It is conceivable that the carrier material itself is a good normal conductor such as copper or silver in order to bridge possible microscopic gaps in the superconductor with the lowest possible loss. It is also conceivable that the normal conductor itself provides the resonant circuit, which is only supported by the superconductivity for a short time in order to temporarily reduce the resistance of the resonant circuit and thus reduce the noise component induced by the resistance, which improves the SNR.

[0047] The photons for inducing superconductivity are introduced into the superconductor by an irradiation device. The irradiation device comprises a radiation source 54 and a waveguide 53. In the embodiments of the Fig. 2 and Fig. 3, the radiation source is arranged in the magnetic resonance imaging scanner 1, for example, in the magnet unit 10. This simplifies the energy supply to the radiation source 54 and the dissipation of any resulting waste heat. The volume and weight of the local coil 50 are also reduced. However, with suitable radiation sources 54 with high efficiency and a small volume, an arrangement in the local coil 50 is also conceivable. Radiation sources in the optical and infrared range are preferably lasers, in particular semiconductor lasers or solid-state lasers pumped by semiconductor lasers. A radiation source 54 in the terahertz or microwave range, which can be provided, for example, by semiconductor oscillators, is also conceivable.

[0048] A waveguide 53 guides the photons from the radiation source 54 to the superconductor of the antenna coil 51. Depending on the energy of the photons or wavelength, the waveguide 53 can be a glass fiber, a waveguide made of a metamaterial, a hollow guide, or even a type of coaxial cable.

[0049] Preferably, the photons are radiated as homogeneously as possible into the superconductor of the antenna coil 51, but at least along the extension with a minimum intensity in order to cause the induced superconductivity continuously along the extension. In the embodiments of the Fig. 2 and Fig. 3, the waveguide 54 is guided along or parallel to the antenna loop 51 with the superconductor. Along its length, the photons or alternating electromagnetic fields are coupled out of the waveguide 53. Depending on the type of waveguide 53, different paths are conceivable for this. In the case of hollow guides or coaxial cables, openings in the waveguide or its cladding are conceivable in order to couple out energy and couple it into the immediately adjacent superconductor. In the case of metamaterial waveguides, coupling can be achieved simply by the proximity to the superconductor and / or by structures on the waveguide 53. In the case of optical waveguides, it is conceivable to achieve outcoupling by means of a strong curvature on the outside and / or to enhance it by means of suitable surface structures.

[0050] An electronics module 52 provides further processing of the received magnetic resonance signals, including impedance matching, amplification, and / or conversion to a different frequency range. A detuning device may also be part of the electronics module 52.

[0051] However, it is also conceivable that the superconductor is applied to a non-conductor or dielectric. Advantageously, the antenna loop 51 is poorly or non-conductive without induced superconductivity, so that additional detuning could be omitted. A corresponding embodiment with a loop made of a dielectric as a carrier with a coating of the superconductor as the antenna coil 51 is described in Fig. 3. In Fig. Figure 4 shows a cross-section through such an antenna coil 51. A waveguide 53 is provided as a carrier, on which the superconductor is applied as an antenna coil 51. If the waveguide 53 is, for example, a metamaterial or a glass fiber with a suitable surface structure, photons can be coupled directly into the superconductor in an advantageous manner.

[0052] In Fig. Figure 5 shows another conceivable embodiment of a local coil according to the invention. The radiation source 54 is arranged in the local coil 50. The antenna coil 51 with the superconductor and the radiation source are surrounded by a cavity resonator. The radiation source 54 emits the alternating electromagnetic field or the photons, which, due to the shape of the cavity resonator 55, the position and shape of the radiation source 54, and the antenna coil 51, generate a sufficient field strength or intensity of the alternating electromagnetic field or the photons at the location of the superconductor of the antenna coil 51. For example, a cavity resonator 54 with a circular cross-section in plan view is conceivable. The radiation source 54 is arranged in the center, while the antenna coil 51 is located at a predetermined distance from a concave outer wall of the cavity resonator 55.A cross section in a plane perpendicular to the plane of representation through the center is in . Fig. 6. The cavity resonator 55 can, for example, have the shape of two intersecting ellipses, with the radiation source 54 located at a common focal point of both ellipses and the antenna coil 51 at a second focal point of the ellipses. Photons or waves from the radiation source 54 are thus advantageously focused onto the antenna coil 51, particularly when the dimensions of the cavity resonator are significantly larger than the wavelength of the photons or alternating electromagnetic fields used to induce superconductivity.

[0053] Induced superconductivity requires a high photon density and, without photons, lasts only briefly. The idea of ​​the invention is to utilize the short period of superconductivity relative to the period of the magnetic resonance signal to simultaneously sample the slower magnetic resonance signal. Since the magnetic resonance signal is periodic to a first approximation, the magnetic resonance signal can be sampled with a repetition frequency of the radiation source pulses that induce the induced superconductivity that deviates slightly from the Larmor frequency and simultaneously downconverted to a lower frequency range according to the method.

[0054] Fig. Figure 7 schematically shows the time sequences. To the right, time is plotted in arbitrary units.

[0055] The top diagram shows the pulses from radiation source 54 that cause the induced superconductivity. The excitation pulses from the radiation source have a duration of, for example, one or a few nanoseconds. The repetition rate 1 / Ts is the frequency at which the magnetic resonance signal is subsampled or mixed. For a magnetic resonance imaging scanner with a low field strength of, for example, 0.1 T, a repetition rate or sampling frequency could be 500 kHz, and accordingly, the time Ts between two excitation pulses for superconductivity could be 2 microseconds.

[0056] The diagram below shows the state of induced superconductivity. Immediately after the start of the excitation pulse, induced superconductivity occurs and lasts for a period ts, until the states required for superconductivity decay or fall below a critical threshold.

[0057] The bottom diagram shows the control of the sample and hold (S&H) circuit at the input of the analog-to-digital converter. The S&H is preferably activated at the end of the superconducting phase, when the electrical signal in the antenna coil has stabilized in the superconducting state, and then maintains it constant for the duration of the conversion.

[0058] According to the Nyquist theorem, the sampling rate must be twice the frequency of the desired signal to be acquired. However, with a magnetic resonance signal, the actual useful bandwidth of the magnetic resonance signal carrying image information is significantly smaller than the Larmor frequency. In this case, it is possible to completely sample the magnetic resonance signal by subsampling with a sampling rate greater than twice the bandwidth of the desired signal. Using filtering, frequency conversion, and / or decimation, the acquired signal can be converted to baseband to reduce the amount of data to be processed.

[0059] The synchronization of the signals as in Fig. 7 can be achieved by having the device controller 23 centrally control the individual units according to the predetermined timing scheme. However, it is also conceivable for the local coil 50 to have a clock generator for the excitation pulses of the radiation source 54, which is synchronized with a central clock or clock generator of the magnetic resonance imaging scanner.

[0060] In Fig. Figure 8 shows a time sequence for a further embodiment of the method according to the invention. The individual diagrams essentially correspond to the Fig. 7 explained.

[0061] However, the excitation shown in the top diagram is different. Instead of a single pulse, a sequence of short excitation pulses is generated as a burst. These bursts then repeat at the same repetition rate or at the time interval Ts as in Fig. 7.

[0062] Such a burst mode is advantageous, for example, in the operation of lasers or other oscillators such as gun diodes, which can only be operated for short periods of time due to thermal load limits, but can deliver high power during the short excitation pulses, particularly at higher efficiency, so that the average power consumption and also the waste heat are reduced.

[0063] The induced superconductivity can thus be maintained longer due to the longer and overall stronger excitation, which extends the sampling time ts. This, in turn, allows the superconducting receiver circuit to settle for a longer time, and the SNR can also be improved through the longer averaging.

[0064] 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 imaging apparatus (1), wherein the local coil (50) has an antenna coil (51), wherein the antenna coil (51) has a superconductor which has a superconductivity which can be induced at room temperature, and an irradiation device which is designed to bring about the induced superconductivity by irradiating the superconductor with electromagnetic waves. [2] Local coil according to claim 1, wherein the irradiation device comprises a waveguide (53) which is designed to supply the electromagnetic waves for inducing the induced superconductivity to the superconductor. [3] Local coil according to claim 2, wherein the waveguide (53) is guided parallel to the antenna coil (51) and is designed to couple the electromagnetic waves into the superconductor along the extension of the antenna coil (51). [4] Local coil according to one of the preceding claims, wherein the local coil (50) has a clock generator in signal connection with the irradiation device and the irradiation device is designed to temporarily induce superconductivity in the superconductor in dependence on a signal from the clock generator. [5] Local coil according to one of the preceding claims, wherein the local coil (50) comprises a plurality of antenna coils (51) arranged in a matrix. [6] System comprising a local coil according to claim 2 and a magnetic resonance tomograph (1), wherein the magnetic resonance tomograph (1) is designed to supply the electromagnetic waves for inducing superconductivity to the irradiation device of the local coil (50). [7] System with a local coil (50) according to claim 4 or 5 and a magnetic resonance tomograph (1), wherein the magnetic resonance tomograph (1) is designed to synchronize the clock generator of the local coil (50) with a system clock of the magnetic resonance tomograph (1) and to reconstruct an image from a magnetic resonance signal induced in the superconducting antenna coil (51). [8] A method of capturing an image using a system according to claim 7, the method comprising the steps of: - Excitation of nuclear spins of an examination object in the magnetic resonance imaging device (1) by means of a high-frequency pulse; - detecting a magnetic resonance signal of the nuclear spins by means of the superconducting antenna coil (51), wherein the clock generator is designed to induce the superconductivity with a predetermined mixing frequency by means of the irradiation device, whereby the magnetic resonance signal is detected with a subsampling.

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