High Frequency Antenna System

Abstract

A radio frequency antenna system for a magnetic resonance imaging device includes a radio frequency coil having a total coil impedance composed of the coil impedance and a patient-specific impedance of a patient, the preamplifier amplifying a signal received by the radio frequency coil to output an amplified output signal, the total coil impedance of the radio frequency coil being adjustable, the radio frequency coil including a first coil loop and at least one further coil loop, the controller connected to the radio frequency coil, the matching network, and the preamplifier, the controller controlling the use of the first coil loop and the at least one further coil loop and controlling the matching network to adapt the noise impedance matching between the radio frequency coil and the preamplifier as a function of a patient-specific parameter.

Description

[Technical Field]

[0001] The present invention relates to the field of magnetic resonance imaging, and in particular to the field of radio frequency antenna systems for magnetic resonance imaging devices. [Background technology]

[0002] Part of every magnetic resonance imaging device is a radio-frequency antenna system for receiving and converting magnetic resonance signals. The radio-frequency antenna system includes a radio-frequency coil, a matching network, a preamplifier, and an analog-to-digital converter. In magnetic resonance imaging, a system-integrated body coil generates an excitation field for the spin system, causing premature relaxation. Precession of the net magnetization induces a current in the radio-frequency coil via electromagnetic induction. The radio-frequency coil generally includes a conductor having an inductance and a coil resistance. Like any radio-frequency operating system, radio-frequency antenna systems are subject to noise. Therefore, the most important requirement is to achieve a minimum noise figure. The lowest noise figure is achieved when the source impedance is matched to the noise impedance of the preamplifier. Particularly for magnetic resonance CT in the low-field range (<1 T), the signal-to-noise ratio depends on the patient geometry, the size of the bore, and the relative position of the radio-frequency coil within the bore. This means that the signal-to-noise ratio cannot be maintained at a constant level because it depends on the patient, the coil used, and the position of the coil on the patient.

[0003] The paper "An Adjustable RF Coil Loading Device" by C.E. Hayes in Magnetic Resonance Imaging, 99 (1993) 81-86, discloses an adjustable loading device having two conductive end rings connected by an adjustable even number of uniformly spaced resistive straight segments or strips. This adjustable loading device can substitute for tissue loss in patients of various sizes in whole-body magnetic resonance imaging systems. The conductor structure does not change with the load. A birdcage structure is constructed with a fixed resistance. The entire setup mimics a liquid-loaded phantom within the transmit / receive coil.

[0004] Accurate and efficient impedance matching is important in magnetic resonance CT to achieve a minimum noise figure. However, most existing methods suggest radio frequency coils to be used for imaging entities that are not visible to the patient and for which device-dependent matching is not possible.

[0005] The paper "MEMS Switch Integrated Radio Frequency Coil and Array for Magnetic Resonance Computed Tomography" by SB Bulumulla et al. (Review of Scientific Instruments 88 (2017) 025003) discloses a configurable coil that can be switched between a small configuration and a large configuration. Summary of the Invention [Problem to be solved by the invention]

[0006] It is an object of the present invention to provide a radio frequency antenna system with improved impedance matching. [Means for solving the problem]

[0007] According to the present invention, this object is addressed by the subject matter of the independent claims. Preferred embodiments of the invention are set forth in the dependent claims.

[0008] Therefore, according to the present invention, there is provided a radio frequency antenna system for a magnetic resonance imaging apparatus, the radio frequency antenna system comprising: a radio frequency coil having a total coil impedance composed of a coil impedance and a patient-specific impedance when attached to a patient; a preamplifier connected to the radio frequency coil, wherein a signal representative of magnetic flux captured by the radio frequency coil is amplified and an amplified output signal can be output; a matching network interconnected between the radio frequency coil and the preamplifier, wherein the total coil impedance of the radio frequency coil is adjustable; and a controller connected to the radio frequency coil, the matching network, and the preamplifier, the controller applying noise impedance matching between the radio frequency coil and the preamplifier as a function of patient-specific parameters by controlling the use of the first coil loop and the at least one further coil loop and by controlling the matching network. Signals received by the radio frequency coil, representative of magnetic flux due to magnetic resonance signals, are captured by the first coil loop and / or the further coil loop. The first and further coil loops are configured to cover substantially equal areas through which substantially equal magnetic flux penetrates. Thus, substantially equal magnetic flux from substantially equal fields of view is picked up by the radio frequency coil regardless of whether the first coil loop and / or the further coil loop is used. When both the first coil loop and the further coil loop are used to pick up magnetic flux from substantially the same field of view.

[0009] Here, the term "attached to the patient" means that the radio frequency coil is positioned relative to the patient within the bore of the magnetic resonance imaging device. Advantageously, the radio frequency coil may be positioned on the surface of the patient's body to reduce signal transmission through the air. However, as is well known to those skilled in the art, this is not necessarily the case. In some cases, such as birdcage coils, the radio frequency coil cannot be positioned on the surface of the patient's body; rather, in this case, the radio frequency coil is close to the patient, but still within the bore of the magnetic resonance imaging device. The bore is the opening of the magnetic resonance imaging device designed to accommodate the treatment couch with the patient and the radio frequency coil.

[0010] As previously mentioned, a radio frequency coil typically comprises a piece of conductive wire that has an inductance and coil resistance that, when loaded with a patient, changes to an overall coil impedance that is a combination of the radio frequency coil impedance and the patient impedance.

[0011] This must be taken into account when the received radio signal is sent to the preamplifier, which has an impedance different from the total coil impedance. Therefore, for low-noise signal transmission, patient-specific impedance matching is recommended to match the total coil impedance to the impedance of the preamplifier. The goal is to achieve a minimum noise figure, defined as the ratio of the input signal-to-noise ratio to the output signal-to-noise ratio. The output signal-to-noise ratio corresponds to the output signal-to-noise ratio of the preamplifier.

[0012] In particular, low-field magnetic resonance CT systems below 1 T require reconsideration of radio-frequency coil topology. While higher-field-strength radio-frequency coils primarily burden the patient, lower-field-strength radio-frequency coils can easily exhibit additional electronic noise contributions if the coil is not adequately designed. Furthermore, in clinical applications, radio-frequency coils are placed directly on the patient's body, or dedicated coils that conform to the shape of the patient's body are used. However, there are clinical situations where the optimal coil-to-tissue distance cannot be applied, such as for pediatric imaging, magnetic resonance imaging used in linear accelerators, and interventional applications.

[0013] Therefore, for low-field magnetic resonance CT, the total coil impedance, and therefore its noise behavior, may be governed by the radio frequency coil design itself, rather than by the inductive load of the patient's body. At the same time, due to variations in the geometry of the radio frequency coil, additional total coil impedance adjustments must be considered for stretchable or flexible radio frequency coils.

[0014] This is illustrated by having a controller connected to both the radio frequency coil and the matching network. Based on patient-specific parameters, the controller is configured to execute one or more rules to generate the state of the matching network and the number of loops switched on by turning individual coil loops on or off and / or using the matching network to perform additional impedance adjustments so that the overall coil impedance can be matched to the impedance of the preamplifier and the signal can be amplified by the preamplifier.

[0015] Generally, radio frequency antennas can be operated in different ways by a controller. However, according to a preferred embodiment of the present invention, the radio frequency coil comprises a switch controllable by the controller, which makes it possible to switch on and off each individual coil loop, and a further switch controllable by the controller and located between the matching network and the preamplifier. This allows switching options with low insertion loss to be established.

[0016] Different coil loop configurations are possible. However, according to a preferred embodiment of the present invention, at least one further coil loop is formed as an additional series- or parallel-connected winding within the first coil loop. Increasing the number of windings results in a higher impedance at the input of the loop, thus reducing losses in the matching network. By switching the number of loops and / or the serial-parallel winding pattern, the matching transformation ratio can be matched to the matching network, and thereby to the preamplifier.

[0017] In principle, different algorithms can be implemented in the controller. However, according to a preferred embodiment of the present invention, a trained artificial neural network is implemented in the controller. The use of an artificial intelligence system such as an artificial neural network allows dynamic adaptation to k-space amplitudes. Even complex decision-making tasks can be achieved for which manually determined logic or rules for threshold and k-space application may not be appropriate. Furthermore, the output of the artificial neural network allows pre-scans to be omitted. K-space is defined as an array of numbers representing spatial frequencies in a magnetic resonance image. The configuration of the artificial neural network can depend on many parameters, such as the coil position and shape, the distance to the tissue and bore wall, the region of interest, the magnetic field strength, and the type of coil typically used.

[0018] Data for training the artificial neural network can be acquired using a prototype coil by scanning several phantom anatomical structures with different matching network settings. The raw data is assumed to be the resulting image and artificial neural network settings for each coil. Additionally, data from typically applied RF calibration procedures, such as noise correlation matrix measurements and frequency response curves, are used. Furthermore, electromagnetic simulation techniques can be used to calculate sensitivity profiles that can be modified to resemble image data, thereby eliminating the need for experimental trials.

[0019] These elements are input data that can be used in combination with output data generated by offline reconstruction to train an artificial neural network. In offline reconstruction, output data is obtained by combining images of the coils of the array, including the calculation of the best-fit network configuration. The trained artificial neural network can then generate output data based on a complete or incomplete input data set.

[0020] The controller can be a field programmable gate array or some other processor; due to bandwidth limitations, the field programmable gate array rule-based machine communicates directly with the magnetic resonance CT sequencer. In case of patient table motion, start and stop interrupts can be activated so that the dynamic alignment process does not produce image artifacts.

[0021] Generally, a variety of patient-specific parameters may be used. However, in accordance with a preferred embodiment of the present invention, the patient-specific parameters include the distance and position of the radio frequency coil relative to the patient, and the total coil impedance. These patient-specific parameters are determined while the patient is lying on the patient table in the bore immediately prior to image acquisition.

[0022] It is possible to determine the total coil impedance in different ways, however, according to a preferred embodiment of the present invention, the radio frequency coil has an impedance sensor that can measure the total coil impedance, thereby providing the possibility to measure the individual total coil impedance.

[0023] In principle, various methods can be selected to determine the noise of the received signal. However, according to a preferred embodiment of the invention, the controller is configured to determine the coil loop to be used and to control the matching network by noise measurements, which are performed in the form of a pre-scan prior to image acquisition.

[0024] Generally, it is possible to use an internal signal source for noise measurements. However, according to a preferred embodiment of the present invention, an external signal source is provided to support noise measurements. The external signal source is part of the system and is electrically isolated from the controller. Furthermore, the external signal source is known and remotely controllable, thus allowing the measurement system to be tuned over the entire frequency range of interest. Because the signal source is separated from the high-frequency coil, it can be placed directly next to it. This reduces cable attenuation and improves the signal-to-noise ratio of the measurement.

[0025] It is possible for the preamplifier to be located outside the radio frequency coil, however, according to a preferred embodiment of the invention the preamplifier is integrated into the radio frequency coil housing, which means that each radio frequency coil has its own preamplifier.

[0026] In principle, the radio frequency coil may be analog. However, according to a preferred embodiment of the present invention, the radio frequency coil is a digital coil, the controller is located in the radio frequency coil housing, and a digital preamplifier is used for signal amplification. The use of a digital coil allows for digital signal acquisition and processing directly in the patient. The signal is digitized directly in the radio frequency coil closest to the patient and then transferred and processed throughout the entire imaging chain, generally providing a better signal-to-noise ratio.

[0027] The present invention further provides a method for operating a radio frequency antenna system for a magnetic resonance imaging device, comprising the steps of: attaching a radio frequency coil to a patient to be examined by the magnetic resonance imaging device, the radio frequency coil having a total coil impedance composed of a coil impedance and a patient-specific impedance, the radio frequency coil comprising a first switchable coil loop and at least one further switchable coil loop; analyzing patient-specific parameters to determine which coil loops are switched on and off, respectively; controlling a matching network based on the analyzed patient-specific parameters; receiving signals through the switched coil loops of the radio frequency coil; impedance-matching the signals received by the matching network to the impedances of the preamplifiers; and amplifying the signals received by the preamplifiers. In this context, matching an impedance to one of the preamplifiers means "approximately," i.e., ensuring that the difference between the impedances is small, preferably below a predetermined threshold. Preferably, the radio frequency coil comprises multiple receive channels with their own amplifiers and matching networks, and the method is performed for all of these receive channels.

[0028] According to a preferred embodiment of the present invention, the method further comprises a trained artificial neural network located on a controller that decides to switch the coil loops on and off, respectively, and controls the matching network, which is performed digitally so that the user does not need to have any specific knowledge of the implementation.

[0029] According to a preferred embodiment of the present invention, noise measurements are used to determine which coil loops are switched on and off, respectively, and to control the matching network.

[0030] Further, according to the present invention, there is provided a computer program for noise impedance matching for a magnetic resonance imaging apparatus, the computer program comprising instructions that, when the program is run by a computer, cause the computer to carry out the method according to any one of claims 1 to 14 (first method claim to last method claim).

[0031] Typically, the radio frequency coil can be used exclusively for signal reception. However, according to a preferred embodiment of the present invention, the radio frequency coil switch is used as a detuning circuit. In some configurations, the coil elements may be positioned outside the examination region and may not contribute much to the image. However, the preamplifier still generates noise. In this case, it is ideal to mismatch the radio frequency coil and the preamplifier so that a noise trap is created.

[0032] Different types of switches can be used. However, according to a preferred embodiment of the present invention, the switches are gallium nitride field effect transistors (GaN-FETs) or single-pole double-throw (SPDT) switches, which have low insertion loss. GaN-FETs or SPDT switches have lower switching losses compared to silicon field effect transistor switches due to their high-speed switching capabilities.

[0033] There are several ways to detect the position of the radio frequency coil on the surface of the patient within the bore. However, according to a preferred embodiment of the present invention, the radio frequency coil is provided with markings that can be detected by a camera. For this purpose, special infrared markers can be provided on the coil, which can be detected by an infrared-sensitive camera and do not distort the magnetic field.

[0034] In principle, a rigid radiofrequency coil could also be provided, however, according to a preferred embodiment of the invention, the radiofrequency coil is designed to be flexible and stretchable, which allows for the most conformal placement of the radiofrequency coil on the patient surface.

[0035] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter, but such embodiments do not necessarily represent the full scope of the invention, and reference should therefore be made to the claims and this specification for interpreting the scope of the invention. [Brief explanation of the drawings]

[0036] [Figure 1] 1 illustrates schematically a radio frequency antenna system according to a preferred embodiment of the present invention; [Figure 2] 2 illustrates schematically a second radio frequency antenna system according to a preferred embodiment of the present invention; [Figure 3] 10 shows a schematic diagram of a third radio frequency antenna system according to a preferred embodiment of the present invention; [Figure 4] 10 illustrates schematically a fourth radio frequency antenna system according to a preferred embodiment of the present invention. [Figure 5] 1 shows a schematic diagram of a method according to a preferred embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0037] FIG. 1 schematically illustrates a radio frequency antenna system 1 according to a preferred embodiment of the present invention. The radio frequency antenna system 1 includes a radio frequency coil 2, which has a coil impedance and a patient-specific impedance when positioned next to a patient in the bore of a magnetic resonance imaging device. The total coil impedance is composed of the coil impedance and the patient-specific impedance. The second part of the radio frequency antenna system 1 is a matching network 4. The matching network 4 is interconnected with the radio frequency coil 2 via a switch 8 and with a preamplifier 3 via another switch 9. The switches 8 and 9 are low-insertion-loss gallium nitride field-effect transistor (GaN-FET) switches. The radio frequency coil 2 includes a first coil loop 6 and an additional coil loop 7. The first coil loop 6 and the additional coil loop 7 are arranged parallel to each other and connected to the switch 8. The switch 8, which can be controlled by a controller 5, enables the opening and closing of the individual coil loops 6 and 7. The first and additional coil loops are geometrically arranged close to each other. The first coil loop and the further coil loop may be positioned radially adjacent to each other or on top of each other orthogonal to the area of ​​the first coil loop and the further coil loop. Thus, the geometric size and shape of the first coil loop and the further coil loop may be substantially equal and may differ by a margin of up to 2-5% of their linear dimensions. The controller 5 is a field programmable gate array rule-based unit including a trained artificial neural network 10, which, in addition to the switch 8, is connected to the matching network 4, the further switch 9, the preamplifier 3, and the impedance sensor 11. The impedance sensor 11 is also mounted within the bore of the magnetic resonance imaging device so that the measured impedance corresponds to the total coil impedance.This measured impedance is transferred as an input parameter to the trained artificial neural network 10, which determines the number of coil loops 6, 7 to be switched on as well as the matching impedance adjustment to be made in the matching network 4, thereby ensuring a noise impedance adjustment to the impedance of the preamplifier 3 and thus enabling a more favorable signal transmission.

[0038] 2 shows a schematic diagram of a second radio frequency antenna system 1 according to a preferred embodiment of the present invention. A further coil loop 7 is provided as a further winding arranged in series with the first coil loop 6, and the controller 5 is adapted to determine the first and second coil loops 6 and 7 of the radio frequency coil 2 to be used and to control the matching network 4 by noise measurement. This noise measurement can be performed patient-specifically before a diagnostic imaging sequence, or alternatively, a phantom can be used as part of a quality assurance process, with the phantom serving as a simulated patient. For this purpose, an external signal source 12 is provided that is part of the radio frequency antenna system 1 but is electrically isolated from this system.

[0039] FIG. 3 shows schematically a third radio frequency antenna system 1 according to a preferred embodiment of the present invention, in which the preamplifiers 3 are integrated into the radio frequency coil housing 14 so that each radio frequency coil 2 has its own dedicated preamplifier 3.

[0040] FIG. 4 shows schematically a fourth radio frequency antenna system 1 according to a preferred embodiment of the present invention, in which the radio frequency coil 2 is a digital coil, the controller 5 is located in the radio frequency coil housing 14, and a digital preamplifier 13 is used for signal amplification.

[0041] 5 shows a schematic diagram of a method according to a preferred embodiment of the present invention, in which in a first step a radio frequency coil 2 is attached to a patient. The patient is examined by a magnetic resonance imaging device. The radio frequency coil 2 has a total coil impedance consisting of a coil impedance and a patient-specific impedance, and comprises a first switchable coil loop 6 and at least one further switchable coil loop 7.

[0042] Second, patient-specific parameters are analyzed.

[0043] Third, the coil loops 6, 7 to be switched on or off are determined, and further control of the matching network 4 is performed based on the analyzed patient-specific parameters to achieve a noise impedance match to the preamplifier.

[0044] Fourth, the signal is received by the switched coil loops 6, 7 of the radio frequency coil 2, and fifth, the received signal is impedance matched to the impedance of the preamplifier 6 by the matching network 4.

[0045] Sixth, the received signal is amplified by the preamplifier 3 .

[0046] While the invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered exemplary or illustrative and not restrictive, and the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims are not to be construed as limiting the scope. Moreover, for the sake of clarity, not all elements in the drawings have been labeled with reference signs. [Explanation of symbols]

[0047] High Frequency Antenna System 1 High frequency coil 2 Preamplifier 3 Matching Network 4 Controller 5 1st coil loop 6 7 additional coil loops Switch 8 Additional Switch 9 Artificial Neural Networks 10 Impedance Sensor 11 External signal source 12 Digital Preamplifier13 High frequency coil housing 14

Claims

1. 1. A radio frequency antenna system for a magnetic resonance imaging device, comprising: a radio frequency coil having a total coil impedance when attached to a patient, the total coil impedance being comprised of the coil impedance and a patient-specific impedance; a preamplifier connected to the radio frequency coil, wherein a signal representative of the magnetic flux captured by the radio frequency coil is amplified and an amplified output signal can be output; a matching network interconnected between the radio frequency coil and the preamplifier, wherein the total coil impedance of the radio frequency coil is adjustable; a controller, the radio frequency coil comprises a first coil loop and at least one further coil loop, the first and second coil loops covering substantially equal fields of view; the controller is connected to the radio frequency coil, the matching network, and the preamplifier; the controller applies a noise impedance match between the radio frequency coil and the preamplifier as a function of patient-specific parameters by controlling the use of the first coil loop and the at least one further coil loop and by controlling the matching network. Controller and A radio frequency antenna system comprising:

2. 2. The radio frequency antenna system of claim 1, wherein the radio frequency coil comprises a switch controllable by the controller that enables each of the individual coil loops to be switched on and off, and a further switch controllable by the controller and disposed between the matching network and the preamplifier.

3. 3. A radio frequency antenna system according to claim 1, wherein the at least one further coil loop is formed as an additional series or parallel connected winding within the first coil loop.

4. 4. A radio frequency antenna system according to claim 1, wherein a trained artificial neural network is implemented in the controller.

5. 5. The radio frequency antenna system according to claim 1, wherein the patient-specific parameters include the total coil impedance and the distance and position of the radio frequency coil relative to the patient.

6. 6. A radio frequency antenna system according to claim 1, wherein the radio frequency coil comprises an impedance sensor by means of which the total coil impedance can be measured.

7. 7. A radio frequency antenna system according to claim 1, wherein the controller is configured to determine the coil loop to be used and to control the matching network according to noise measurements.

8. 8. The radio frequency antenna system of claim 7, wherein an external signal source is provided to support the noise measurement.

9. 9. A radio frequency antenna system according to claim 1, wherein the preamplifier is integrated into the radio frequency coil housing.

10. 10. The radio frequency antenna system according to claim 1, wherein the radio frequency coil is a digital coil, the controller is located within the radio frequency coil housing, and a digital preamplifier is used for signal amplification.

11. 10. A method for operating a radio frequency antenna system according to claim 1 for a magnetic resonance imaging device, said method comprising: attaching a radio frequency coil to a patient being examined by the magnetic resonance imaging device, the radio frequency coil having a total coil impedance comprised of a coil impedance and a patient-specific impedance, the radio frequency coil comprising a first switchable coil loop and at least one further switchable coil loop; Analyzing patient-specific parameters; determining whether each of the coil loops is switched on or off and controlling a matching network based on the analyzed patient-specific parameters; receiving a signal by the switched coil loop of the radio frequency coil; impedance matching the received signal by the matching network to an impedance of the preamplifier; amplifying the received signal with the preamplifier; A method comprising:

12. 12. The method of claim 11, wherein a trained artificial neural network is installed on a controller that determines which of the coil loops are switched on and off and controls the matching network.

13. 12. The method of claim 11, wherein noise measurements are used to determine which of the coil loops are switched on and off, respectively, and to control the matching network.

14. A computer program for noise impedance matching for a magnetic resonance imaging device, comprising instructions which, when executed by a computer, cause the computer to carry out the method according to any one of claims 11 to 13.

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