Local coil transmitter device for magnetic resonance imaging scanners with high magnetic fields

Abstract

The invention relates to a local coil transmitter device for a high-field magnetic resonance imaging (MRI) scanner. The local coil transmitter device comprises a plurality of mechanically separate antenna coils arranged at an angle relative to each other.

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 transmitter device for generating an alternating electromagnetic field for image acquisition using magnetic resonance tomography with a plurality of transmitting coils in a high-field magnetic resonance tomograph.

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

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

[0005] As the strength of the static magnetic field increases, the energy gap between the states of the nuclear spins, and thus also the signal-to-noise ratio of the nuclear spins, also increases. This makes it possible to detect corresponding nuclear spins at lower concentrations or to accelerate image acquisition in order to capture rapid physiological processes such as the heartbeat.

[0006] With increasing energy separation, the wavelength of the alternating electromagnetic field also decreases and approaches the dimensions of the patient's body, particularly in the interior of the body with its high relative permittivity. This can lead to the formation of standing waves and an inhomogeneity in the alternating magnetic field within the patient, resulting in corresponding inhomogeneities in the imaging. To counteract this, multiple antennas with different signals are used to homogenize the field strength.

[0007] The antennas interact with each other, which complicates signal optimization. To reduce this interaction, overlapping coils or decoupling elements such as capacitors are used, for example.

[0008] It is therefore an object of the invention to provide a local coil transmitter device with which image acquisition with a high-field magnetic resonance tomograph can be improved.

[0009] The local coil transmitter device according to the invention is intended for use with a high-field magnetic resonance imaging (MRI) scanner. A high-field MRI scanner is defined as a MRI scanner with a static magnetic field B0 of at least 3 T, 5 T, or 7 T. The local coil transmitter device comprises a plurality of mechanically separate antenna coils, in particular more than two antenna coils.

[0010] Mechanically separate antenna coils are understood to be antenna coils that have no direct mechanical connection, in particular no common conductor elements that directly contribute to the generation of the magnetic fields. This is particularly important to distinguish them from a body coil or bird-cage antenna, which can also be considered as adjacent antenna loops that share longitudinal rods or rungs. As a result, the antenna coils of the local coil transmitting device according to the invention are preferably also electrically separated, at least apart from remaining interactions via alternating electromagnetic fields.

[0011] The antenna coils are designed to be driven with a high-frequency alternating current at a Larmor frequency corresponding to the nuclear spins to be detected within the magnetic field B0, in order to generate an alternating magnetic field B1 for exciting nuclear spins in the patient. In particular, the antenna coils are designed to be driven with a high-frequency power sufficient to excite the nuclear spins, for example, more than 5 watts, 20 watts, or 100 watts. Antenna coils are defined, in particular, as conductors that enclose an area such that a current flowing through the conductor generates a magnetic field through the enclosed area.

[0012] Adjacent antenna coils are arranged at an angle to each other. Adjacent antenna coils are defined as directly adjacent antenna coils; that is, no other antenna coil is located between two adjacent antenna coils. "Arranged at an angle to each other" means that the normal vectors to the surfaces enclosed by the antenna coils of adjacent antenna coils form an angle greater than 10 degrees. It is also conceivable that the antenna coils are not arranged around a rotationally symmetric body, but rather conform to the patient along an axis. For example, in the case of a head coil, the coils might converge at the top of the skull towards the vertex or diverge in the shoulder region. The angle is then defined as the angle formed by the projections of the normal vectors of the respective enclosed surfaces onto a plane perpendicular to the axis.The angle can also be defined as the angle formed by perpendiculars on a line connecting the points where the conductors of the antenna coils intersect the plane within the plane. In particular, adjacent antenna coils are not aligned parallel to each other; that is, normal vectors on surfaces enclosed by the antenna coils are not parallel.

[0013] Advantageously, the angled arrangement of the antenna coils leads to reduced interaction. For two coils positioned perpendicular to each other and arranged along a common axis of symmetry in free space, the interaction is minimal. Depending on their relative arrangement to each other and to a patient, the optimal angle for low interaction, even under the boundary conditions described in the dependent claims, is less than 90 degrees.

[0014] Further advantageous embodiments are described in relation to the dependent claims.

[0015] In one possible embodiment of the local coil transmitting device according to the invention, adjacent antenna coils are arranged at an angle between 20 degrees and 70 degrees or between 30 and 60 degrees.

[0016] An angle of less than 90 degrees in an antenna coil array can advantageously improve decoupling. Furthermore, such an angle can lead to a reduced height or thickness of the local coil transmitter. Additionally, the homogeneity of the generated B1 field can be improved, as patient asymmetries, due to the dielectric constant and susceptibility within the body, lead to asymmetries in the field strength distribution of the respective antenna coils, which can be compensated for by tilting the coils.

[0017] In one conceivable embodiment of the local coil transmitting device according to the invention, adjacent antenna coils are spaced apart from one another. This means that the spacing of the antenna coils, which is considered to be the distance between the nearest opposite points of the adjacent antenna coils, is greater than 10%, 50%, 100%, or 200% of the dimension of the antenna coil in the direction of the spacing. This can also mean that the spacing is greater than 10%, 50%, 100%, or 200% of the smaller dimension of the area.

[0018] In one possible embodiment of the local coil transmitting device according to the invention, the antenna coils are arranged essentially in one plane. This does not mean that all antenna coils are arranged flat in the plane so that their normal vectors are parallel. Rather, the antenna coils are aligned relative to each other at respective angles, with the antenna coils being arranged between two parallel planes whose distance from each other corresponds at most to the smaller of the dimensions of the antenna coils, length and width, i.e., the width.

[0019] Advantageously, a local coil transmitter device according to the invention can be arranged so flat, for example as a transmitter coil for spinal imaging, under the patient.

[0020] In a conceivable local coil transmitter device, the majority of the antenna coils are arranged in a support structure, allowing them to be positioned around the patient's body curvature in a suitable application position. For example, rigid support structures shaped to the patient's body parts to be imaged are conceivable, possibly even in two parts as connectable half-shells. Examples of this are head coils or knee coils. However, flexible support structures are also conceivable, in which the spacing and orientation of adjacent antenna coils are defined such that, when positioned on a patient, they change only slightly, for example, by less than 20% or 10% of the values ​​predetermined for imaging, or are achieved with a smaller deviation.

[0021] In this advantageous way, decoupling according to the invention can also be achieved for a curved local coil transmitter device, even if it is designed as a flexible local coil, e.g. for the abdomen.

[0022] In one possible embodiment of the local coil transmitter device according to the invention, two adjacent antenna coils are not simultaneously arranged tangentially to the curvature of the body in the position required for application on the patient. Preferably, both adjacent antenna coils are not arranged tangentially. For example, the antenna coil forms an angle between 10 degrees and 60 degrees or between 20 degrees and 50 degrees with the tangent.

[0023] An angled arrangement of both antenna coils advantageously results in an overall reduction in the height or thickness of the local coil transmitter. Furthermore, asymmetries of the body, in conjunction with the dielectric constant or susceptibility, lead to a deformation of the antenna coil's transmission profile. The lobe of equal field strength is deflected away from the body, which can be compensated for by rotating the antenna coil in the opposite direction. The non-tangential arrangement of at least one of the two antenna coils can also serve as a disclaimer to distinguish this design from any prior art.

[0024] In one conceivable embodiment of the local coil transmitter according to the invention, the width of the antenna coils is smaller than the length of the antenna coils. The width can, for example, be less than 50%, 20%, or 10% of the length. In the case of a curved local coil transmitter, the width of the antenna coil is preferably the dimension along the curvature of the body, and the length is preferably the dimension of the antenna coil perpendicular to the width and the normal direction of the antenna coils.

[0025] Elongated antenna coils, for example in the shape of an ellipse, an oval, or an elongated rectangle, advantageously allow for a variation of the field distributions and thus the possibility of achieving a more homogeneous B1 field distribution. In particular, sidelobes are suppressed, which simplifies the optimization of the transmitted signals for a homogeneous field distribution, since the system of equations that describes the relationship between transmitted signals and field strengths in the enclosed volume is more diagonalized and therefore easier to solve.

[0026] In one possible embodiment of the local coil transmitter according to the invention, the local coil transmitter is designed such that, in a typical application on the patient, the antenna coils are arranged at a distance from the patient. For example, a support device of the local coil transmitter can fix the antenna coils at a predetermined distance in a typical application position relative to a surface facing the patient. The predetermined distance can, for example, be greater than 10%, 50%, 100%, or 200% of the width of an antenna coil.

[0027] Advantageously, a spaced arrangement of the antenna coils enables better homogeneity of the generated B1 field, even when the number of simultaneously active antenna coils is limited by the number of transmitters.

[0028] In one conceivable embodiment of the local coil transmitter according to the invention, the local coil transmitter has at least one receiving antenna coil for receiving a magnetic resonance signal. A receiving antenna coil is configured to receive a magnetic resonance signal with a preferably optimal signal-to-noise ratio (SNR). The receiving antenna coil is arranged in the local coil transmitter such that, in a typical application arrangement of the local coil transmitter at the patient, it is positioned between the transmitting antenna coils and the patient. Preferably, the arrangement is such that the receiving antenna coil is located as close as possible to the patient.

[0029] The proximity of the receiving antenna coil advantageously results in the best possible SNR and high signal strength of the magnetic resonance signal.

[0030] The properties, features and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more easily understood in connection with the following description of the exemplary embodiments, which are explained in more detail in conjunction with the drawings.

[0031] They show: Fig. 1 a schematic representation of an embodiment of a magnetic resonance tomograph with a local coil transmitter device according to the invention; Fig. 2 a schematic transverse cross-section through an exemplary local coil transmitter device; Fig. 3 a line of equal field strength for an antenna coil of an embodiment of a local coil transmitting device according to the invention at the head; Fig. 4 a schematic longitudinal section of an embodiment of a local coil transmitter device according to the invention; Fig. 5 a cross-section through an embodiment of a local coil transmitter device according to the invention.

[0032] Fig. Figure 1 shows a schematic representation of an embodiment of a magnetic resonance tomograph 1 for image acquisition with the local coil transmitter device 50 according to the invention.

[0033] The magnet unit 10 includes a field magnet 11 that generates a static magnetic field B0 for aligning the nuclear spins of samples or the patient 100 within a recording area. The recording area is characterized by an extremely homogeneous static magnetic field B0, with the homogeneity relating in particular to the magnetic field strength or magnitude. The recording area is nearly spherical and arranged within a patient tunnel 16 that extends longitudinally 2 through the magnet unit 10.

[0034] A patient bed 30 can be moved in the patient tunnel 16 by the transport unit 36.

[0035] The field magnet 11 is typically a superconducting magnet capable of generating magnetic fields with a magnetic flux density of up to 3 T, and even higher in the latest devices. However, for lower field strengths, permanent magnets or electromagnets with normally conducting coils can also be used.

[0036] Furthermore, the magnet unit 10 includes gradient coils 12, which are configured to generate variable magnetic field gradients in three spatial directions within the magnetic field B0 for the spatial differentiation of the captured imaging areas in the investigation volume. The gradient coils 12 are typically coils made of normally conducting wires that can generate mutually orthogonal gradients of the static magnetic field B0 in the investigation volume.

[0037] The magnet unit 10 includes a local coil transmitter 50, which is configured to radiate a high-frequency signal supplied via a signal line into the examination volume. It is also conceivable that the local coil transmitter 50 may furthermore include antennas or antenna coils for receiving a magnetic resonance signal. In the Fig. In the example shown, the local coil transmitter 50 is a head coil. However, the local coil transmitter 50 could also be, for example, a knee coil or a local coil for the abdomen. A planar embodiment, which would lie under the patient as a spine coil, is also conceivable. For some image acquisitions, a body coil 14 can also be provided as a transmitting and / or receiving antenna.

[0038] A control unit 20 supplies the magnet unit 10 with the various signals for the gradient coils 12 and the local coil transmitter 50 and evaluates the received signals.

[0039] The control unit 20 thus has a gradient control 21 which is set up to supply the gradient coils 12 with variable currents via supply lines, which provide the desired field gradients in the investigation volume in a time-coordinated manner.

[0040] Furthermore, the control unit 20 includes a high-frequency unit 22, which is configured to generate a high-frequency pulse with a predefined temporal profile, amplitude, and spectral power distribution to excite magnetic resonance of the nuclear spins in the patient 100. Pulse powers in the kilowatt range can be achieved. The excitation signals are emitted into the patient 100 via the local coil transmitter 50.

[0041] A controller 23 communicates with the gradient controller 21 and the high-frequency unit 22 via a signal bus 25. A user can input data to the controller 20 via an operating terminal 60.

[0042] The local coil transmitter 50 is arranged on the patient 100, in the illustrated exemplary embodiment as a head coil which is connected to the high-frequency unit 22 and its transmitter via a connecting line 33. The local coil transmitter 50 is designed to transmit signals from several transmitters or transmission channels independently of one another via a plurality of separate antenna coils 51. The signals can differ from one another in amplitude, phase, and also frequency spectrum, preferably in such a way that the most homogeneous excitation possible can be achieved in a volume to be imaged. The antenna coils 51 are mechanically and spatially separated, i.e., they do not share any conductor elements that contribute to their effect as radiators. Only a support device 53 connects and holds the antenna coils 51 in a predetermined relative position to one another, but itself has no direct effect on the generated fields.Ultimately, the antenna coils 51 are indeed electrically connected to each other via connecting lines and high-frequency power output stages and the power supply, but the power output stages in particular are designed to prevent an interaction of the antenna coils 51 via this path.

[0043] In Fig. Figure 2 shows a schematic transverse cross-section through an exemplary local coil transmitter 50, here a head coil. The local coil transmitter 50 has a support device 53, which is arranged around the head of the patient 100, similar to a helmet. The support device 53 can consist of several shells arranged concentrically around the head and held in a predetermined relative position to each other by spacers at predetermined distances. The position and orientation of the antenna coils 51 and receiving antenna coils 52 can be defined by fixing elements connected to the support device 53. The fixing element or spacer can also be a foam that fills the space between the shells. Preferably, the support device 53 has openings for the nose, mouth, and / or eyes.

[0044] The antenna coils 51 are not arranged tangentially to the head or the curvature of the support device 53 at the location of the antenna coil 51, but at an angle alpha which is in the Fig. 2 in a plane perpendicular to the longitudinal axis of the body or z-direction by a radial vector originating from the center of the head of the patient 100 or the local coil transmitter device 50, and a normal vector on the tendon connecting the two conductors of the antenna coil 51.

[0045] The receiving antenna coils 52 are positioned closer to the patient 100 to improve the signal-to-noise ratio. The in Fig. The two depicted receiving antenna coils 51 form a matrix, with the receiving antenna coils 52 overlapping for decoupling. However, it is also conceivable that the receiving antenna coils 52 are designed similarly to the transmitting antenna coils 51 and / or arranged at an angle to each other. Likewise, it would be conceivable that the antenna coils 51 are used for both transmitting and receiving, thus eliminating the need for separate receiving antenna coils 52.

[0046] In Fig. Figure 3 shows a line of equal field strength 54 for a previously described antenna coil 51 in the transmitting position at the head. The asymmetry caused by the head is evident and is used by tilting to generate a field strength maximum at the head.

[0047] Furthermore, the field strength maximum thus generated is narrow, so that the excitation patterns of neighboring antenna coils 51 hardly overlap and the excitation at a location is essentially defined by a single antenna coil 51, which greatly simplifies the determination of the excitation signals.

[0048] Fig. Figure 4 shows a schematic longitudinal section in which, for the sake of clarity, only the head of patient 100 and two antenna coils 51 are depicted. The antenna coils 51 are tilted in the transverse plane, as shown in Fig. Figure 2 illustrates and explains. Along the body axis, however, the antenna coil 51 is not planar, but rather follows the contour of the body or head. Consequently, the sections of the antenna coils 51 with the transverse plane at different positions along the z-axis or body axis are not identical, but essentially parallel and of the same length.

[0049] Fig. Figure 5 shows a cross-section through another embodiment of a local coil transmitter 50 according to the invention. The essential difference to the previous embodiments is that the local coil transmitter 50 of the Fig. 5 in its intended application arrangement on the patient is essentially planar. This means that the support device 53 is bounded by two surfaces that are essentially parallel planes and whose distance is less than the width of the antenna coils. In other words, the local coil transmitter device is flat and can be positioned under the patient, for example, to excite nuclear spins along a spine for image acquisition as a spine coil. The surface of the support device 53 facing the patient can also be shaped to conform to the body contour, for example, having an upward curve between the pelvis and thorax or laterally for support.

[0050] Preferably, within the local coil transmitting device, the antenna coils 51 are aligned such that the length of the antenna coils 51 lies in the transverse direction or perpendicular to the longitudinal axis of the body in the application-specific arrangement.

[0051] Adjacent antenna coils 51 are each tilted relative to one another. The angle alpha is preferably defined by the angle formed by two surface normals of adjacent antenna coils 51. The previously stated characteristics of the angle alpha are also relevant.

[0052] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited by the disclosed examples and other variations can be derived by the person skilled in the art without leaving the scope of protection of the invention.

Claims

[1] Local coil transmitter device for a high-field magnetic resonance imaging scanner, in particular a high-field system, wherein the local coil transmitter device (50) comprises a plurality of mechanically separate antenna coils (51), characterized by , that adjacent antenna coils (51) are arranged at an angle relative to each other, so that the antenna coils (51) are decoupled from each other. [2] Local coil transmitting device according to claim 1, wherein adjacent antenna coils (51) are arranged at an angle between 20 degrees and 70 degrees to each other. [3] Local coil transmitting device according to one of the preceding claims, wherein adjacent antenna coils (51) are spaced apart from each other. [4] Local coil transmitting device according to one of the preceding claims, wherein the antenna coils (51) are arranged substantially in one plane. [5] Local coil according to one of claims 1 to 3, wherein the majority of the antenna coils (51) are arranged in a support device (53) so that they can be arranged in an application-appropriate position on the patient (100) circumferentially around a body curvature. [6] Local coil transmitting device according to claim 5, wherein at least one antenna coil (51) of two adjacent antenna coils (51) is arranged non-tangentially to the curvature of the body. [7] Local coil transmitting device according to one of the preceding claims, wherein a width of the antenna coils (51) along the curvature of the body is smaller than a length of the antenna coil (51) perpendicular to the width and normal direction of the antenna coil (51). [8] Local coil transmitter device according to one of the preceding claims, wherein the local coil transmitter device (50) is configured to arrange the antenna coils (51) at a distance from the patient (100) in an application-oriented arrangement on the patient (100). [9] Local coil transmitter device according to one of the preceding claims, wherein the local coil transmitter device (50) has a receiving antenna coil (52) for receiving a magnetic resonance signal, wherein the receiving antenna coil (52) is arranged in the local coil transmitter device (50) such that, in an application-specific arrangement of the local coil transmitter device (50) on the patient (100), it is positioned between the transmitting antenna coils (51) and the patient (100).

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