JP2006272004A - High-frequency coil and magnetic resonance imaging apparatus using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a plane high-frequency coil, suitable for an open type MRI apparatus, and having high irradiating/receiving efficiency and good irradiating/receiving uniformity. <P>SOLUTION: This high-frequency coil is a plane coil constructed so that a plurality of linear conductors 2 are radially connected to the inside of a ring-like conductor 1. A resonance capacity 3 inserted in the ring-like conductor 1 is adjusted to take wavelength matching in one wavelength of irradiation frequency. This plane coil generates a rotary magnetic field of a plane parallel to the coil surface when a high-frequency current is input, and an orthogonal coil can be obtained using one coil by inputting the current from two positions having 90-degree separation of the ring-like conductor 1. This type of plane coils are disposed opposite to each other on the upper and lower sides of an MRI apparatus to constitute an irradiation coil 10. Since the irradiation coil can be shaped like a plane not to obstruct opening of an imaging space, it is possible to provide the high-performance MRI apparatus, which can easily gain access to a subject and achieve wide-range clinical application. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a novel high-frequency coil used in a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) and an MRI apparatus including the same.

  In the MRI apparatus, the nuclear magnetic resonance phenomenon is used to measure the nuclear spin density distribution and relaxation time distribution at a desired examination site in the subject, and the cross section of the subject is displayed as an image from the measured data. An irradiation coil for irradiating a subject with a high frequency pulse and a receiving coil for receiving a nuclear magnetic resonance signal generated from the subject are provided.

  These irradiation coils and reception coils use high-frequency coils having various shapes and forms according to the direction of the static magnetic field. For example, in the case of a vertical magnetic field type MRI apparatus that applies a static magnetic field in the vertical direction of the subject, the high-frequency magnetic field is a horizontal rotating magnetic field with the vertical direction as the axis, and a solenoid type coil or a saddle type coil is used. Further, an orthogonal method is also used for the purpose of improving irradiation, reception efficiency and uniformity. This is a combination of two high-frequency coils (solenoid type coil and saddle type coil, etc.) arranged in an orthogonal state (90 degrees), and simultaneously irradiates and receives irradiation pulses or received signals whose phases differ by 90 degrees to generate a rotating magnetic field. Application and detection.

  On the other hand, in recent years, a technique called IVR is becoming widespread in the field of X-ray apparatuses, but in the field of MRI apparatuses, it is required to perform various clinical actions while imaging. For this purpose, it is important that the imaging space of the MRI apparatus is open. In other words, it is necessary that the static magnetic field generating magnet has a large opening area and the high-frequency coil that performs irradiation and reception does not cover the subject.

  As an MRI apparatus suitable for IVR, a vertical magnetic field type MRI apparatus adopting an open structure magnet having a wide opening area has been put into practical use. When the high frequency coil is applied, there is a problem that a part of the subject is covered. In particular, the irradiation coil requires a relatively large coil for the purpose of ensuring the uniformity of irradiation, and has a great influence on the open structure.

  On the other hand, a planar coil 7 as shown in FIG. 7A has been proposed as a planar coil suitable for a vertical magnetic field type MRI apparatus, and two planar coils are arranged opposite to each other above and below the magnetic field space for irradiation. An MRI apparatus used as a coil has been put into practical use. In this coil, left and right loops are formed so that currents flow in the same direction through two conductors located at the center. By flowing a high-frequency current i as shown in the figure, irradiation in the horizontal direction is performed above and below the coil. A magnetic field B is generated.

  However, in this planar irradiation coil, the horizontal magnetic field can be generated in the upper and lower parts of the central straight conductor, and the vertical magnetic field is generated in the central part of the left and right loops. There is a problem that it is narrow and the uniformity of the irradiation magnetic field and the irradiation efficiency are low.

  In order to improve this, as shown in FIG. 7B, the planar coils 7a and 7b are combined in an orthogonal state and orthogonal irradiation is performed. In this case as well, in order to sufficiently solve the unevenness of irradiation. In this case, it is necessary to enlarge the planar coil 7, which is difficult due to restrictions on the apparatus. In addition, in order to configure such an orthogonal coil, the thickness of the irradiation coil increases, and a new problem of narrowing the imaging space has occurred. Furthermore, since the frequency of the irradiation signal is generally increased and the wavelength is shortened in the high magnetic field apparatus, when such a planar coil is applied, an unnecessary current distribution is generated on the coil conductor, and the operation as an irradiation coil is good. Cannot be expected. Of course, the above problem is not only applied to the irradiation coil but also to this type of receiving coil.

  An object of the present invention is to provide a planar high-frequency coil that has a structure that does not hinder the opening of a subject in an open-structure MRI apparatus, has high irradiation efficiency, reception sensitivity, and uniformity, and can be applied to a high magnetic field apparatus. And It is another object of the present invention to provide an MRI apparatus having an open structure suitable for IVR and capable of obtaining a good image.

  The high-frequency coil of the present invention that achieves the above object includes a ring-shaped conductor, a plurality of linear conductors radially connecting the inside of the ring-shaped conductor, and a plurality of capacitances connected or distributed to the ring-shaped conductor or the linear conductor. It has.

  In the high-frequency coil of the present invention, the capacitance connected or distributed to the ring-shaped conductor or the straight conductor is a concept including not only the capacitance due to an element such as a capacitor but also the capacitance of the conductor itself, and includes the capacitance of the conductor itself. It is assumed that the capacitance is adjusted to be wavelength-matched with one wavelength of the high-frequency current flowing through the ring-shaped conductor. Therefore, when wavelength matching is performed only by the capacitance of the conductor itself, it is not always necessary to insert an element. When connecting the capacitance element, it may be a ring-shaped conductor, a straight conductor, or both, but it is preferable to connect to the ring-shaped conductor from the viewpoint of the uniformity of the generated magnetic field.

  In this high frequency coil, a transverse magnetic field is generated by the current flowing through the straight conductor. In this case, the ring-shaped conductor is wavelength-matched with the high-frequency current flowing therethrough, so that a current distribution corresponding to one wavelength is generated in the ring-shaped conductor, and a current in one direction flows at a certain moment. Since this current distribution changes as the phase of the high-frequency current changes, this causes current to flow sequentially through the radial linear conductor. Therefore, a rotating magnetic field parallel to the surface of the high frequency coil is generated. Similarly, a rotating magnetic field parallel to the coil surface can be detected. Therefore, in an MRI apparatus having an open structure and a vertical magnetic field method, the MRI apparatus is arranged without hindering the opening of the subject, and a horizontal rotating magnetic field can be generated or detected.

  The high-frequency coil of the present invention preferably has input and / or output terminals at at least two locations of the ring-shaped conductor, and one coil constitutes an orthogonal irradiation coil or an orthogonal reception coil. Thereby, it can orthogonalize without increasing the thickness of a coil, and can improve irradiation efficiency or detection efficiency.

  The high-frequency coil of the present invention can be used as an irradiation coil of an MRI apparatus, as a reception coil or both, and as an irradiation and reception coil.

  The MRI apparatus may be either a vertical magnetic field system or a horizontal magnetic field system, and in any case, the MRI apparatus is disposed on a plane perpendicular to the static magnetic field direction. When the high-frequency coil according to the present invention is provided as an irradiation coil or an irradiation / reception coil, the two coils are positioned opposite to each other with the subject interposed therebetween, and are arranged so that the magnetic field directions formed by both coils coincide. It is suitable to do. In particular, in an open structure vertical magnetic field type MRI apparatus in which a magnet and a planar gradient magnetic field coil are disposed above and below, it is preferable to dispose the magnetic field coil close to the gradient magnetic field coil.

  In a preferred aspect of the present invention, the planar high-frequency coil is provided with terminals at two locations, and one coil constitutes an orthogonal irradiation coil or an orthogonal reception coil. Since the orthogonal coil can be configured by one coil, the orthogonal coil can be provided without increasing the thickness, that is, providing a wide space and having high irradiation efficiency or detection efficiency.

  Embodiments of a high frequency coil according to the present invention will be described with reference to the drawings.

  FIG. 1 is a view showing an embodiment of an irradiation coil for an MRI apparatus. This irradiation coil 10 is a planar coil comprising a ring-shaped conductor 1 and eight linear conductors 2 connected radially inside the ring-shaped conductor 1. Thus, eight resonance capacitors (capacitances) 3 are inserted so that the ring-shaped conductor 1 is divided into eight, and the input terminals 4 are connected to both ends of the resonance capacitor 3 at one location.

  The principle of generating a horizontal rotating magnetic flux in such an irradiation coil will be described with reference to FIG. In the figure, a to h indicate eight junction points of the ring-shaped conductor 1 and the straight conductor 2. FIG. 2A is a circuit expressed in a state where the ring-shaped conductor 1 of the planar irradiation coil 10 of FIG. 1 is separated and opened, and the resonance capacitor 3 and the inductance of the linear conductor 2 constitute a ladder-type circuit. Yes. The high-frequency current input from the input terminal 4 shows a specific current distribution due to the delay characteristic of the ring-shaped conductor 1. By adjusting the resonance capacitance 3, this current distribution can be formed so that a current distribution corresponding to one wavelength (FIG. 5B) is formed in the ring-shaped conductor 1, and this is the case with the irradiation coil of the present invention. Adjustments have been made.

  The current distribution changes momentarily according to the input high-frequency current, causing a change in current density on the ring-shaped conductor 1. Considering the moment when the current distribution as shown in the figure occurs, since the current density at the junction point a and the junction point e is the highest and has a reverse polarity, a large current i flows through the straight conductor 2 between a and e. This is understood ((c) in the figure). Further, since the junction point b and the junction point d have the same current density and the opposite polarity, and similarly, the junction point h and the junction point f have the same current density and the opposite polarity, and therefore, between the points b-d and h-f. Although current i flows, no current flows because there is no current density at junctions c and g.

  Therefore, when viewed as the entire planar coil, current i flows from point a to point e as shown in FIG. 4D, and horizontal magnetic flux B is generated above and below the planar irradiation coil. The direction of the current i changes according to the change in the current density of the ring-shaped conductor 1, and as a result, the horizontal magnetic flux B rotates.

  Although the operation as the irradiation coil has been described above, the operation as the reception coil is the same by the reciprocity theorem, and the horizontal magnetic flux B is detected when the current i is induced in the linear conductor by the horizontal magnetic flux B. A rotating magnetic field can be detected by changing the direction of the current.

  The high-frequency coil having such a configuration can be suitably used for a high magnetic field device. For example, in a high magnetic field device having a static magnetic field strength of 1.5 Tesla, a resonant frequency of about 64 MHz, a wavelength of about 4.7 m, and a practical diameter having a diameter of about 1 m. A high-frequency coil can be configured.

  When this high-frequency coil is applied to a low magnetic field device, the frequency used is low, and it is difficult to form a current distribution for one wavelength in the ring-shaped conductor 1 depending on the size of the coil. In that case, it is possible to configure a practically sized coil by inserting inductance into the ring-shaped conductor 1 or the straight conductor 2 and adjusting the delay amount.

  The high-frequency coil of the present invention can be variously modified depending on the performance and application of the applied MRI apparatus. For example, in the illustrated example, the resonance capacitor 3 is inserted into the ring-shaped conductor 1, but this is a straight line. It may be inserted into the conductor 2. Further, the number of linear conductors is not limited to eight as shown in the figure, but may be more or less, and the uniformity of irradiation can be improved by increasing the number of linear conductors.

  FIG. 3 shows another embodiment of the high frequency coil of the present invention. FIG. 3 (a) shows an embodiment in which the high frequency coil of FIG. 1 is configured as an orthogonal coil 10 '. The high frequency coil 10' has two terminals of the ring-shaped conductor 1 at positions of 90 degrees with input terminals 4a and 4b. Is provided. When this high-frequency coil is used as an irradiation coil, irradiation pulses (high-frequency currents i and i ') having a phase difference of 90 degrees are input from these input terminals 4a and 4b. Since the ring-shaped conductor 1 is matched to one wavelength, when viewed from the input terminal 4a, no current distribution is generated at the 90 ° position (FIG. 2, junction point c). Therefore, even if an irradiation pulse is input from this position, the two inputs do not interfere with each other, and can operate as if they were two irradiation coils.

  By using two irradiation coils arranged in an orthogonal state in this way, the output of the power amplifier can be halved to obtain an irradiation pulse with a required intensity. Further, in contrast to the conventional orthogonal irradiation method that requires two irradiation coils, since the orthogonal irradiation coil can be configured by one coil, there is an advantage that the thickness of the irradiation coil is suppressed and the imaging space is not compressed.

  This orthogonal coil can also be applied as a receiving coil of an MRI apparatus by using the terminals 4a and 4b as output terminals, and the detection efficiency can be improved.

  In addition, when using the high frequency coil of this invention as an orthogonal coil in this way, it is necessary to make the number of straight conductors into a predetermined number corresponding to the angle of two sets of terminals. For example, when two input terminals are provided at an angle of 90 degrees, a multiple of 4, for example, 8 or 12 is appropriate.

  FIG. 3 (b) is a diagram showing another embodiment of the high-frequency coil 10 ″ of the present invention, in which this high-frequency coil has a ring 5 inserted in the center. As already described, the number of the linear conductors 2 is as follows. It is considered that the uniformity of the irradiation pulse increases as the number increases, but the intensity of the irradiation magnetic field increases near the center of the planar irradiation coil where the linear conductors gather. In this high frequency coil 10 ″, by inserting the center ring 5, It can reduce that magnetic field intensity becomes high. Even when the center ring 5 is inserted, the operating principle of the coil is exactly the same as that of the high-frequency coil of FIG.

Next, the MRI apparatus of the present invention incorporating the above-described high frequency coil will be described.
FIG. 4 is a block diagram showing an overall configuration outline of a vertical magnetic field type MRI apparatus. This MRI apparatus is superposed on a magnetic field generator 11 that generates a static magnetic field mainly in a measurement space where a subject 14 is placed, and the static magnetic field. Gradient magnetic field coil 12 that generates a gradient magnetic field, irradiation coil 10 that irradiates a subject 14 with a high-frequency magnetic field, receiving coil 13 that receives a nuclear magnetic resonance (NMR) signal generated from the subject 14, and subject 14 that are laid down It comprises a bed 15 for transporting to a space, an MRI unit 16 for controlling these, and a display device 17.

  The MRI unit 15 includes a control device 18 for controlling various pulse sequences in imaging, a high-frequency device 19 for irradiating a subject 14 with a high-frequency pulse for spin excitation by an irradiation coil 10 according to the control of the control device 18, a gradient magnetic field power supply 20 And an image data calculation device 21 that performs image reconstruction calculation and the like on the signal data detected by the receiving coil 17 and collected by the high frequency device 19. The display device 17 displays the MRI image obtained by the arithmetic device 21.

  The magnetic field generator 11 generates a strong and uniform static magnetic field perpendicular to a space around the subject 14, and includes a magnetic field generating means such as a permanent magnet system or a superconducting system. It has an open structure so that a person can perform a procedure such as biopsy near the subject.

  The gradient magnetic field coils 12 are obtained by combining three coils in the X, Y, and Z directions in a flat plate shape, and three sets of gradient magnetic field coils 12 are arranged close to the upper and lower magnetic field generating means 11. These gradient magnetic field coils 12 form a necessary gradient magnetic field space around the subject 14 by the output current of the gradient magnetic field power source 20 controlled by the control device 18, and give position information to the NMR signal.

  In this embodiment, the receiving coil 13 is a coil having a shape corresponding to the examination site (here, the head) and is attached to the subject 14 during examination.

  The irradiation coil 10 is a planar coil composed of a ring-shaped conductor and a radial linear conductor as shown in FIG. 1, and two planar irradiation coils 10a and 10b are connected to a magnetic field space in the magnetic field generator 11 as shown in FIG. The irradiation coil 10 is configured so as to face the upper and lower sides. At this time, the input terminals 4 of the upper and lower planar irradiation coils 10a and 10b are reversely connected to each other, or the phase of the input high frequency pulse is shifted by 180 degrees. As a result, currents i of the same magnitude and opposite directions flow in the corresponding linear conductors 2 of the upper and lower coils, and magnetic fluxes B in the same direction are generated near the center of the irradiation coil 10 by the respective currents. As described above, the direction of the generated magnetic flux B changes along with the change of the linear conductor 2 through which the current flows one after another by the input high-frequency current, and the horizontal rotation necessary to excite the spin in the magnetic field space. A magnetic field can be obtained.

  Although not shown, a decoupling circuit is added to the irradiation coil so that the coil loop is open during the receiving operation in order to avoid interference with the receiving coil. The decoupling circuit can be realized by connecting a high-frequency switching element such as a pin diode in series with the coil loop and controlling the switching current. In this case, it is necessary to add this decoupling circuit to a loop formed by all ring-shaped conductors and straight conductors.

  As described above, by using the planar coil of the present invention as the irradiation coil of the vertical magnetic field type MRI apparatus, an open structure MRI apparatus can be obtained, and high-frequency pulse irradiation can be performed with high efficiency and uniformity. Can be obtained.

  As the irradiation coil, in addition to the high-frequency coil provided with one input terminal 4 shown in FIG. 1, various modifications of the high-frequency coil of the present invention can be adopted. For example, the orthogonal irradiation method is effective for improving the irradiation efficiency of the irradiation coil, and for this purpose, as shown in FIG. 3A, a high-frequency coil provided with input terminals 4a and 4b at two locations on the ring-shaped conductor 1 Are used to input irradiation pulses (high-frequency current) having a phase difference of 90 degrees from these input terminals 4a and 4b. Accordingly, the orthogonal irradiation method can be realized with one coil, and the rotating magnetic field can be efficiently irradiated with a small output of the irradiation power amplifier.

  In order to improve the irradiation uniformity, a high frequency coil having a large number of linear conductors may be employed. In this case, in order to reduce the intensity of the irradiation magnetic field at the center of the coil, it is effective to use an irradiation coil in which the center ring 5 is inserted as shown in FIG.

  Further, in the case of a low magnetic field device, a high-frequency coil is used in which wavelength matching is achieved by adding not only a resonance capacity but also an inductance to a ring-shaped conductor or a straight conductor. Thereby, even a low magnetic field device having a relatively low frequency can incorporate a high-frequency coil having a practical size, and the same effect as the above-described embodiment can be obtained.

  The configuration of the planar coil according to the present invention and the MRI apparatus incorporating the planar coil according to the present invention have been described above. However, the planar coil according to the present invention can also be applied as a receiving coil of the MRI apparatus. An example of this application is shown in FIG.

  According to the reciprocity theorem, the planar coil of the present invention detects the magnetic field B when the current i is induced in a linear conductor orthogonal to the magnetic field B shown in FIG. Since the NMR signal is detected as a rotating magnetic field in a plane perpendicular to the static magnetic field, when the planar coil of the present invention is used as a receiving coil, it is installed in the vicinity of the subject and is parallel to the signal rotating surface. For example, FIG. 6A shows a case of a vertical magnetic field type MRI apparatus. In this case, the planar receiving coil 6 is installed horizontally below or above the region of interest (here, the head) of the subject. FIG. 6B shows a horizontal magnetic field type MRI apparatus. In this case, since the signal rotation plane is vertical, it is installed perpendicularly to the position of the top of the head or the like. FIG. 6C shows a few examples of the apparatus, but in the case of a transverse magnetic field type MRI apparatus, it can be used at the left and right positions of the subject 14.

  As can be seen from FIG. 6, the planar receiving coil 6 of the present invention has a shape that does not cover the region of interest of the subject, so that it is possible to easily perform treatment in IVR or the like without giving the subject a feeling of blockage. it can.

  In addition, when the planar coil of the present invention is used as a receiving coil, the use of two output terminals provided at a 90-degree position makes it possible to increase the detection efficiency because an orthogonal receiving coil can be configured with one coil. Further, unlike the conventional coil, it is not necessary to arrange the two coils with their detection directions orthogonal to each other, so that a wide imaging space can be secured.

  As described above, according to the present invention, since a high-frequency coil having a planar shape and high irradiation / reception efficiency can be configured, it is easy to access a wide range of subjects without obstructing the opening of the imaging space. A high-performance MRI apparatus capable of clinical application can be provided.

The block diagram which shows one Example of the high frequency coil by this invention. The figure explaining the principle of the irradiation coil by this invention. (A) And (b) is a block diagram which shows one Example of the high frequency coil by this invention, respectively. The block diagram of the MRI apparatus with which this invention is applied. The figure explaining arrangement | positioning of the irradiation coil in an MRI apparatus. The figure explaining arrangement | positioning of the receiving coil by this invention. (A) is a figure which shows the conventional planar coil and its sensitivity distribution, (b) is a figure which shows the case where the conventional planar coil is used as an orthogonal coil.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Ring-shaped conductor 2 ... Linear conductor 3 ... Resonance capacity 4, 4a, 4b ... Input terminal 5 ... Center ring 6 ・ ・ ・ ・ ・ ・ Planar receiving coil
10, 10a, 10b ... Irradiation coil
10 ', 10 "... irradiation coil
11 .... Magnetic field generator (magnetic circuit)
12 ・ ・ ・ ・ ・ ・ Gradient magnetic field coil
21 ・ ・ ・ ・ ・ ・ Calculation device (Image reconstruction means)

Claims (6)

A high frequency coil that irradiates a subject with a high frequency magnetic field and / or detects a magnetic resonance signal generated from the subject in a magnetic resonance imaging apparatus,
A planar high-frequency device comprising: a ring-shaped conductor; a plurality of linear conductors radially connected to the inside of the ring-shaped conductor; and a plurality of capacitances connected to or distributed on the ring conductor or the linear conductor. coil.   2. The high frequency coil according to claim 1, wherein the capacitance is adjusted so as to match one wavelength of a high frequency current flowing through the ring-shaped conductor.   2. The high frequency coil according to claim 1, further comprising signal input means and / or signal output means in at least two places of the ring-shaped conductor. A magnetic circuit for applying a static magnetic field to the subject, a gradient magnetic field coil for applying a gradient magnetic field to the subject, an irradiation coil for applying a high-frequency pulse for causing magnetic resonance in atomic nuclei constituting the subject, In a magnetic resonance imaging apparatus comprising a receiving coil for detecting a magnetic resonance signal generated from a subject, and an image reconstruction means for reconstructing an image using the detected magnetic resonance signal,
The said irradiation coil and / or receiving coil are the planar high frequency coils of any one of Claim 1 thru | or 3, Comprising: It is arrange | positioned on the surface perpendicular | vertical to the magnetic field which the said magnetic circuit forms, It is characterized by the above-mentioned. Magnetic resonance imaging device.   The magnetic resonance imaging apparatus according to claim 4, wherein the two planar high-frequency coils are arranged at positions facing each other with the subject interposed therebetween.   6. The magnetic resonance imaging apparatus according to claim 4, wherein signal input / output means are provided at two locations of the planar high-frequency coil, and an orthogonal irradiation coil or an orthogonal reception coil is constituted by one coil.

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