阅读说明：本技术 一种双调谐射频线圈装置 (Double-tuning radio frequency coil device ) 是由 郑海荣 李烨 李楠 杜凤 陈巧燕 刘新 于 2020-12-18 设计创作，主要内容包括：本发明实施例公开了一种双调谐射频线圈装置,该双调谐射频线圈装置包括至少两个差模共模谐振结构,相邻的所述差模共模谐振结构之间设置有解耦回路；所述差模共模谐振结构包括共模线圈和差模线圈,所述共模线圈用于产生第一磁场,所述差模线圈用于产生第二磁场,所述第一磁场的主磁场方向与所述第二磁场的主磁场方向相互垂直；所述解耦回路用于对相邻的所述差模共模谐振结构进行解耦。本发明实施例提供的双调谐射频线圈装置通过解耦回路实现相邻差模共模谐振结构之间的解耦,提高了不同元素核磁共振信号的激发和接收效率,进而提高了基于信号生成的图像的信噪比。(The embodiment of the invention discloses a double-tuning radio frequency coil device, which comprises at least two differential mode common mode resonance structures, wherein a decoupling loop is arranged between the adjacent differential mode common mode resonance structures; the differential mode and common mode resonance structure comprises a common mode coil and a differential mode coil, the common mode coil is used for generating a first magnetic field, the differential mode coil is used for generating a second magnetic field, and the main magnetic field direction of the first magnetic field is perpendicular to the main magnetic field direction of the second magnetic field; the decoupling loop is used for decoupling the adjacent differential mode common mode resonance structures. According to the double-tuning radio frequency coil device, the decoupling between the adjacent differential mode common mode resonance structures is realized through the decoupling loop, the excitation and receiving efficiency of nuclear magnetic resonance signals of different elements is improved, and the signal-to-noise ratio of an image generated based on the signals is further improved.)

1. A double-tuned radio frequency coil apparatus, comprising: the decoupling circuit is arranged between the adjacent differential mode common mode resonance structures;

the differential mode and common mode resonance structure comprises a common mode coil and a differential mode coil, the common mode coil is used for generating a first magnetic field, the differential mode coil is used for generating a second magnetic field, and the main magnetic field direction of the first magnetic field is perpendicular to the main magnetic field direction of the second magnetic field;

the decoupling loop is used for decoupling the adjacent differential mode common mode resonance structures.

2. The double-tuned radio frequency coil device according to claim 1, wherein said differential mode coil is within said common mode coil.

3. The double-tuned radio frequency coil assembly according to claim 1, wherein the centerlines of said common mode coil and said differential mode coil are collinear.

4. The double-tuned radio frequency coil arrangement according to claim 1, wherein said differential mode coil and said common mode coil are of microstrip line construction.

5. The double-tuned radio frequency coil device according to claim 1, wherein said differential mode coil and/or said common mode coil is rectangular.

6. The dual-tuned rf coil device according to claim 1, wherein at least two tuning capacitors and at least one second tuning capacitor are disposed on the common mode coil, the first tuning capacitor is disposed at a driving port of the differential mode coil, the second tuning capacitor is disposed between a parallel copper strip on the differential mode coil and ground, and the first tuning capacitor and the second tuning capacitor are used for adjusting a resonant frequency of the differential mode coil.

7. The double-tuned radio frequency coil device according to claim 1, wherein said differential mode coil comprises a third tuning capacitor and a fourth tuning capacitor, said third tuning capacitor being disposed at a drive port of said differential mode coil, said fourth tuning capacitor being disposed at a parallel to-be-conductor connection of said differential mode coil.

8. The double-tuned radio frequency coil device according to claim 1, wherein said decoupling loop is a decoupling structure based on an induced current compensation method, said decoupling loop comprising at least two decoupling capacitors uniformly distributed.

9. The double-tuned radio frequency coil device according to claim 1, wherein said double-tuned radio frequency coil device is disposed on a substrate, said substrate being a body coil structure.

Technical Field

The embodiment of the invention relates to the field of medical equipment, in particular to a double-tuning radio frequency coil device.

Background

Magnetic resonance imaging is an imaging technique that utilizes the signals generated by the resonance of atomic nuclei within a strong magnetic field for image reconstruction. The basic principle is as follows: some atoms containing single protons in human tissues, such as hydrogen atoms, can perform spinning motion and generate magnetic moments, in a normal state, the spinning directions of the small magnets are arranged irregularly, but can generate directional arrangement under the action of a fixed static magnetic field, at the moment, when a radio-frequency pulse with the same frequency as the static magnetic field is applied, the hydrogen atoms absorb certain energy to generate resonance, the spinning directions deflect under the action of the radio-frequency pulse and are regularly arranged, namely, the magnetic resonance phenomenon occurs, after the radio-frequency pulse disappears, the hydrogen atoms are restored to the original state, in the recovery process, the energy is released and the spinning directions are changed, at the moment, signals generated by the hydrogen atoms are sampled, and then the acquired signals are utilized to perform image reconstruction, so that an image of the human tissues can be obtained. In the above process, the rf coil is mainly used to transmit rf pulses and to acquire mr rf signals. With the rapid development of multi-nuclear magnetic resonance imaging technology, the design of the double-tuned coil becomes a technical problem to be solved urgently.

In a double-tuned coil, shimming is usually performed using the H-channel, which requires that the magnetic field distributions of the two frequency channels are similar. There are several strategies currently available for double-tuned rf coils: firstly, using a single-frequency adjustable coil; secondly, switching the resonance frequency of the automatic tuning coil with the help of an external computer program; and thirdly, realizing double resonance by rearranging the orthogonal mode of the birdcage coil into two independent linear modes and adjusting the two independent linear modes to 1H/19F frequency. In the above strategy, the target signal must be acquired by manually adjusting to different frequencies using a single-frequency adjustable coil, but the F signal is weak, and the acquisition time is usually increased to maximally improve the signal-to-noise ratio (SNR), which results in additional time cost and movement of the imaging position, and results in signal attenuation caused by 1H/19F misregistration, which blurs the image; the auto-tuned coil has high complexity and cost for switching the resonant frequency with the help of an external computer program; both methods show some limitations in terms of sensitivity and inaccuracy of the co-matched 1H/19F signal; achieving dual resonance by rearranging the quadrature mode of the birdcage coil into two separate linear modes in the birdcage structure design results in a loss of signal-to-noise ratio (SNR) of the two nuclei of about 30%, reduces the radio frequency transmit power efficiency, reduces the B1 field uniformity, generally requires greater frequency separation for coil structures with notch circuits, and is not easily achieved with relatively close 1H/19F frequencies. Therefore, the current double-tuned coil has low signal excitation and receiving efficiency, and the signal-to-noise ratio of the image obtained based on the signal is also low.

Disclosure of Invention

The embodiment of the invention provides a double-tuning radio frequency coil device, which is used for improving the efficiency of signals excited and received by a radio frequency coil, so that the signal-to-noise ratio of an image obtained based on the signals is improved.

The embodiment of the invention provides a double-tuning radio frequency coil device, which comprises at least two differential mode common mode resonance structures, wherein a decoupling loop is arranged between the adjacent differential mode common mode resonance structures;

the differential mode and common mode resonance structure comprises a common mode coil and a differential mode coil, the common mode coil is used for generating a first magnetic field, the differential mode coil is used for generating a second magnetic field, and the main magnetic field direction of the first magnetic field is perpendicular to the main magnetic field direction of the second magnetic field;

the decoupling loop is used for decoupling the adjacent differential mode common mode resonance structures.

Optionally, on the basis of the above scheme, the differential mode coil is in the common mode coil.

Optionally, on the basis of the above scheme, the center lines of the common mode coil and the differential mode coil are on the same straight line.

Optionally, on the basis of the above scheme, the differential mode coil and the common mode coil are of a microstrip line structure.

Optionally, on the basis of the above scheme, the differential mode coil and/or the common mode coil are rectangular.

Optionally, on the basis of the above scheme, the common mode coil is provided with at least two tuning capacitors and at least one second coordination capacitor, the first tuning capacitor is disposed at a driving port of the differential mode coil, the second coordination capacitor is disposed between a parallel copper strip on the differential mode coil and ground, and the first coordination capacitor and the second coordination capacitor are used to adjust a resonant frequency of the differential mode coil.

Optionally, on the basis of the above scheme, the differential mode coil includes a third coordination capacitor and a fourth coordination capacitor, the third coordination capacitor is disposed at a driving port of the differential mode coil, and the fourth coordination capacitor is disposed at a connection position of parallel conductors to be connected of the differential mode coil.

Optionally, on the basis of the above scheme, the decoupling loop is a decoupling structure based on an induced current compensation method, and the decoupling loop includes at least two decoupling capacitors that are uniformly distributed.

Optionally, on the basis of the above scheme, the double-tuned radio frequency coil device is disposed on a substrate, and the substrate is of a body coil structure.

The double-tuning radio frequency coil device provided by the embodiment of the invention comprises at least two differential mode common mode resonance structures, wherein a decoupling loop is arranged between the adjacent differential mode common mode resonance structures; the differential mode and common mode resonance structure comprises a common mode coil and a differential mode coil, the common mode coil is used for generating a first magnetic field, the differential mode coil is used for generating a second magnetic field, and the main magnetic field direction of the first magnetic field is perpendicular to the main magnetic field direction of the second magnetic field; the decoupling loop is used for decoupling the adjacent differential mode common mode resonance structures, decoupling between the adjacent differential mode common mode resonance structures is achieved through the decoupling loop, excitation and receiving efficiency of nuclear magnetic resonance signals of different elements is improved, and then signal-to-noise ratio of an image generated based on the signals is improved.

Drawings

Fig. 1 is a schematic structural diagram of a double-tuned rf coil device according to an embodiment of the present invention;

FIG. 2 is a schematic structural diagram of another dual-tuned RF coil apparatus according to an embodiment of the present invention;

fig. 3 is a schematic structural diagram of another dual core rf array coil apparatus according to an embodiment of the present invention.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and examples. It is to be understood that the specific embodiments described herein are merely illustrative of the invention and are not limiting of the invention. It should be further noted that, for the convenience of description, only some of the structures related to the present invention are shown in the drawings, not all of the structures.

Aiming at the technical problem that the excitation and receiving efficiency of the resonance signals of the existing double-tuned radio frequency coil is low, the common mode and differential mode coils realized by the microstrip transmission line and the decoupling structure based on the induced current compensation method are utilized in the embodiment of the invention to respectively realize the decoupling between the common mode and differential mode coils and the decoupling between the adjacent common mode and differential mode structures, so that the excitation and receiving efficiency of nuclear magnetic resonance signals of different elements is improved, and the signal-to-noise ratio of the image generated based on the signals is further improved.

Example one

Fig. 1 is a schematic structural diagram of a dual-tuned rf coil device according to an embodiment of the present invention, where the dual-core rf coil device provided in this embodiment is applicable to a multi-core magnetic resonance imaging system. As shown in fig. 1, the double-tuned radio frequency coil apparatus includes: at least two differential mode common mode resonant structures 110, and a decoupling loop 120 is arranged between the adjacent differential mode common mode resonant structures 110.

The differential mode and common mode resonant structure 110 comprises a common mode coil 111 and a differential mode coil 112, wherein the common mode coil 111 is used for generating a first magnetic field, the differential mode coil 112 is used for generating a second magnetic field, and the main magnetic field direction of the first magnetic field is perpendicular to the main magnetic field direction of the second magnetic field;

the decoupling loops 120 are used for decoupling the adjacent differential mode common mode resonant structures 110.

Specifically, the differential mode and common mode resonance structure generates a first magnetic field and a second magnetic field which are perpendicular to each other in the main magnetic field directions through the differential mode coil and the common mode coil, and the decoupling of the differential mode coil and the common mode coil in the differential mode and common mode resonance structure is realized through the first magnetic field and the second magnetic field which are orthogonal to each other, and the shimming efficiency is improved. And the differential mode and common mode resonance structure can independently coordinate the required working frequency under each mode. It will be appreciated that the differential mode coil and the common mode coil are in a loop configuration for generating the magnetic field. Wherein the required working frequency is the larmor frequency of the corresponding element under the magnetic resonance imaging system. Taking 1H and 31P in 7T magnetic resonance imaging as examples, wherein the larmor frequency of 1H under 7T magnetic resonance imaging system is 300Mhz, and the larmor frequency of 31P under 7T magnetic resonance imaging system is 121Mhz, assuming that the radio frequency coil corresponding to 1H channel is differential mode coil, and the radio frequency coil corresponding to 31P channel is common mode coil, the operating frequency of the differential mode coil is 300Mhz, and the operating frequency of the common mode coil is 121Mhz, so that when the corresponding nuclei are imaged by using the differential mode coil and the common mode coil respectively, high quality images can be generated.

In this embodiment, the double-tuned radio frequency coil device in this embodiment may be a differential mode and common mode resonant structure and a decoupling loop arranged in an array, and may also be a differential mode and common mode resonant structure and a decoupling loop arranged in a body coil structure, which is not limited herein.

On the basis of the scheme, an insulating layer can be arranged between the differential mode coil and the common mode coil to eliminate the condition that the magnetic field is unstable due to the fact that the differential mode coil and the common mode coil are in error contact, the stability of the magnetic field is guaranteed, and excitation and receiving of magnetic resonance signals are stable.

The double-tuning radio frequency coil device provided by the embodiment of the invention comprises a common mode coil and a differential mode coil, wherein the common mode coil is used for generating a first magnetic field, the differential mode coil is used for generating a second magnetic field, and the main magnetic field direction of the first magnetic field is vertical to the main magnetic field direction of the second magnetic field; the decoupling loop is used for decoupling the adjacent differential mode and common mode resonance structures, wherein the differential mode and common mode resonance structures generate a first magnetic field and a second magnetic field which are perpendicular to each other in the direction of the main magnetic field, so that the structures of the differential mode and the common mode are decoupled due to the existence of the orthogonal first magnetic field and the orthogonal second magnetic field, the decoupling between the adjacent differential mode and common mode resonance structures is realized through the decoupling loop, the excitation and receiving efficiency of nuclear magnetic resonance signals of different elements is improved, and the signal-to-noise ratio of the image generated based on the signals is further improved.

Optionally, the position relationship between the differential mode coil and the common mode coil is not limited. Preferably, the differential mode coil is within the common mode coil. To simplify the complexity of the radio frequency coil, the differential mode coil may be arranged within the common mode coil. I.e. the size of the common mode coil is larger than the size of the differential mode coil.

In the present embodiment, the shapes of the common mode coil and the differential mode coil are not limited. However, in addition to the above-described configuration, to improve the stability of the magnetic field, the center lines of the common mode coil and the differential mode coil may be aligned on the same straight line. Preferably, the common mode coil and the differential mode coil are regular in shape. For example, the common mode coil and the differential mode coil can be circular, the common mode coil and the differential mode coil can also be rectangular, and the common mode coil and the differential mode coil can also be square. When the common mode coil and the differential mode coil are prototypes, the center line of the common mode coil is a straight line passing through the center of the common mode coil, and the center line of the differential mode coil is a straight line passing through the center of the differential mode coil; when the common mode coil and the differential mode coil are rectangular or square, a center line of the common mode coil may be a straight line parallel to a side of the common mode coil and passing through a center of the common mode coil, and a center line of the differential mode coil may be a straight line parallel to a side of the differential mode coil and passing through a center of the differential mode coil. Preferably, the differential mode coil and/or the common mode coil are rectangular, the rectangular shape is regular, the structure is simple, the structure of the double-tuning radio frequency coil device is regular and simple, and the robustness of the double-tuning radio frequency coil device is improved.

In one embodiment, the differential mode coil and the common mode coil are microstrip line structures. Optionally, a microstrip line structure and a common mode differential mode technology may be used to generate a resonant structure with two resonant frequencies. The common mode is a second harmonic mode of the microstrip resonator and is driven by a coaxial coil at the center of the microstrip, and the current directions of two parallel copper strip conductors of the resonator are the same in the working mode; the differential mode is the primary resonant mode of the microstrip resonator, driven by a square inductive loop. In this mode the currents on the two parallel strip conductors of the microstrip resonator are in opposite directions. Optionally, the two modes of the differential mode coil and the common mode coil are respectively driven by current sources with equal amplitude and 90 phase difference, the input power is set to be 1w, and the electromagnetic field distribution of each channel under each nuclide can be obtained.

Optionally, at least two tuning capacitors and at least one second coordination capacitor are disposed on the common-mode coil, the first tuning capacitor is disposed at a driving port of the differential-mode coil, the second coordination capacitor is disposed between a parallel copper strip on the differential-mode coil and ground, and the first coordination capacitor and the second coordination capacitor are used for adjusting a resonant frequency of the differential-mode coil. Fig. 2 is a schematic structural diagram of another dual-tuned rf coil device according to an embodiment of the present invention. As shown in fig. 2, the first tuning capacitor Ccm-t1 is located at the drive port, and the second tuning capacitor Ccm-t2 is an auxiliary tuning capacitor and is located between the parallel copper strips and the ground plane. The first tuning capacitor and the second tuning capacitor cooperate to tune the CM operating frequency to a desired operating frequency.

Optionally, the differential mode coil includes a third coordination capacitor and a fourth coordination capacitor, the third coordination capacitor is disposed at the driving port of the differential mode coil, and the fourth coordination capacitor is disposed at the connection position of the parallel conductors to be connected of the differential mode coil. As shown in fig. 2, a fourth tuning capacitor Cdm-t is located at the parallel strip conductor junction, and a third tuning capacitor Cdm is at the drive port, tuning the DM operating frequency to the desired operating frequency. In this mode the currents on the two parallel strip conductors of the microstrip resonator are in opposite directions.

In one embodiment of the present invention, the double-tuned radio frequency coil device is disposed on a substrate, and the substrate is a body coil structure. Fig. 3 is a schematic structural diagram of another dual core rf array coil apparatus according to an embodiment of the present invention. The structure of the dual coordination radio frequency coil of the body coil structure is schematically shown in fig. 3 with 16 channels as an example. As shown in fig. 3, differential mode and common mode resonant structures are uniformly distributed on the inner side of the body coil structure substrate, and a decoupling loop is arranged between adjacent differential mode and common mode resonant structures. The labels in fig. 3, except for the labels 110 and 120, are tuning capacitors. When the number of channels is changed, the excitation amplitude and the phase of each channel can be adjusted, so that the excitation phase difference between adjacent units is 45 degrees, and orthogonal excitation is realized. The differential mode and common mode resonant structure and the decoupling loop may be implemented by referring to the above embodiments, which are not described herein again. By optimizing the body coil structure, such as substrate thickness and coil size, the signal-to-noise gain of the image obtained based on the acquired signals can be further improved.

Example two

The present embodiment provides a preferred embodiment based on the above-described embodiments. The embodiment of the invention embodies the double-tuning radio frequency coil device into an HF double-tuning radio frequency coil system, and explains the double-tuning radio frequency coil device.

The HF double-tuned radio frequency coil system provided in the embodiments of the present invention includes a common-mode coil unit and a differential-mode coil unit (corresponding to the differential-mode common-mode resonant structure in the above embodiments) implemented by using microstrip transmission lines, and a decoupling structure (corresponding to the decoupling loop in the above embodiments) based on an induced current compensation (ICE) method.

The common mode and differential mode coil units in this embodiment generate a resonant structure with two resonant frequencies by using a microstrip line structure and a Common Mode (CM) and Differential Mode (DM) technology. That is to say, the common mode and differential mode coil units can independently tune the operating frequencies required by the common mode and differential mode, that is, the operating frequencies required by the modes can be independently tuned under the coil array structure. Meanwhile, the common-mode coil unit and the differential-mode coil unit can generate magnetic fields which are orthogonal to each other and similar in distribution essentially, so that excellent nuclide decoupling is realized, and the shimming efficiency is improved. Meanwhile, an ICE decoupling structure arranged between adjacent units of the coil array realizes excellent electromagnetic decoupling between channels. Compared with the prior art, the array has simple rectangular geometry, so that a simple and robust implementation method is provided for the HF double-tuned radio frequency coil.

In this embodiment, the CM structure is tuned to the H element frequency and the DM structure is tuned to the F element frequency. According to the embodiment, in the double-tuned radio frequency coil device, the common mode structure and the differential mode structure are decoupled due to the existence of the orthogonal magnetic field, so that the electromagnetic fields of the two HF nuclides are orthogonal and do not influence each other; the above characteristics are key to improving the proton and non-hydrogen magnetic resonance signal excitation and reception efficiency.

Meanwhile, based on the characteristics of the double-tuning structure unit, in a double-tuning coil array or a double-tuning transceiver array for parallel imaging or parallel excitation application, an induced current compensation (ICE) method suitable for design of a non-overlapping coil array is adopted to solve the coupling problem among CMDM arrays, namely, a single ICE decoupling unit is used for simultaneously realizing decoupling among units of HF nuclides. Because the array system has a regular and simple rectangular geometric shape and the magnetic fields are orthogonal, the decoupling is realized between the adjacent channels, and the excitation and receiving efficiency of the 1H proton and 19F nuclear magnetic resonance signals is improved.

For example, a CMDM orthogonal microstrip line array may be disposed on a teflon substrate (dielectric constant 2.1, permeability 1) with H1.27 cm thick, the substrate size is 34.5 cm × 12 cm (length 34.5 cm, width 12 cm), and the other side of the substrate is covered with a copper foil as a ground plane with a thickness of 0.1 mm. CMDM resonant cells were made using 5 mm wide copper tape, CM structural cells were rectangles of 4.4 CM x 9.6 CM size, DM structural cells were rectangles of 2.6 CM x 39 CM size. The decoupling between adjacent CMDM units is realized by a resonance unit ICE structure, the size of the resonance unit ICE structure is 1.6 cm by 9.4 cm, and the resonance unit ICE structure is spaced 0.3 cm from the adjacent cm DM resonance unit.

The microstrip line double-resonance coil structure provided by the embodiment of the invention has 8 channels in total, and comprises a 4-channel CM structural unit and a 4-channel DM structural unit, wherein the CM structural unit is used for realizing the acquisition of 1H signals, and the DM structural unit is used for realizing the acquisition of 19F signals. The CM common mode is a second harmonic mode of the microstrip resonator and is driven by a coaxial coil at the center of the microstrip, and the current directions of two parallel copper strip conductors of the resonator are the same in the working mode. Referring to fig. 2, a tuning capacitor Ccm-T1 is located at the drive port of the CM structural unit, and another auxiliary tuning capacitor Ccm-T2 is located between the parallel copper strips and the ground plane, which act together to tune the CM operating frequency to 298MHz, the resonance frequency of H under a 7T system. The matching capacitor Ccm-m is connected to the driving port in series, and the coaxial cable shielding layer and the copper grounding surface are both grounded. The DM differential mode is a main resonance mode of the microstrip resonator, and is driven by a square inductance loop, referring to fig. 2, a tuning capacitor Cdm-T is located at a connection position of parallel strip conductors, and tunes a DM operating frequency to 282MHz of a 19F resonance frequency under a 7T system, and a matching capacitor Cdm-m is located at a driving port of the DM structural unit, and current directions on two parallel strip conductors of the microstrip resonator are opposite in the mode. The two modes are respectively driven by current sources with equal amplitude and 90 phase difference, and the input power can be set according to actual requirements.

Referring to fig. 2, six capacitors with equal capacitance values are uniformly arranged in the loop of the ICE decoupling structure, so that uniform current distribution is generated in the coil loop. An independent coil type ICE decoupling structure loop is used as a decoupling element, the main function is to compensate or eliminate induced current generated by mutual coupling between adjacent CDMD microstrip line elements, and the induced current in the decoupling coil loop is changed by adjusting the capacitance value in the independent coil loop, so that effective decoupling between adjacent CMDM units is realized.

The HF double-tuned radio frequency coil system provided by the embodiment of the invention is used for 1H/19F nuclide imaging under magnetic resonance, Common Mode and Differential Mode (CMDM) technologies are utilized to enable a microstrip transmission line to simultaneously generate H, F nuclide double resonance with approximate frequency, the two-core orthogonal field distribution generation shows the intrinsic decoupling capacity between the common mode and the differential mode, and the decoupling structure of an induced current compensation (ICE) method is utilized to realize the simultaneous decoupling of the common mode and the differential mode structures. The array system has a regular and simple rectangular geometric shape, magnetic fields are orthogonal, decoupling is realized between adjacent channels, the excitation and receiving efficiency of 1H proton and 19F nuclear magnetic resonance signals is improved, the sensitivity of 1H/19F signal detection is improved, and the robustness of the double-tuned radio frequency array coil is improved.

It is to be noted that the foregoing is only illustrative of the preferred embodiments of the present invention and the technical principles employed. It will be understood by those skilled in the art that the present invention is not limited to the particular embodiments described herein, but is capable of various obvious changes, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, although the present invention has been described in greater detail by the above embodiments, the present invention is not limited to the above embodiments, and may include other equivalent embodiments without departing from the spirit of the present invention, and the scope of the present invention is determined by the scope of the appended claims.