JP4350889B2 - High frequency coil and magnetic resonance imaging apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus for imaging a desired portion of a subject using magnetic resonance.
[0002]
[Prior art]
A magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) uses a nuclear magnetic resonance phenomenon to measure the density distribution and relaxation time distribution of nuclear spins at a desired examination site in a subject, and uses the measurement data to measure the subject. The image of the cross section of the image is displayed.
[0003]
A subject's nuclear spin placed in a uniform and strong magnetic field generator precesses around the direction of the magnetic field at a frequency (Larmor frequency) determined by the strength of the magnetic field.
[0004]
Therefore, when the subject is irradiated with a high-frequency pulse having a frequency equal to the Larmor frequency from the outside, the spin is excited and transitions to a high energy state (nuclear magnetic resonance phenomenon).
[0005]
When the irradiation is terminated, the spin returns to a low energy state with a time constant corresponding to each state, and at this time, an electromagnetic wave (NMR signal) is emitted from the subject to the outside. This is detected by a high frequency receiving coil tuned to that frequency.
[0006]
At this time, for the purpose of adding position information in the space, a three-axis gradient magnetic field of the X axis, the Y axis, and the Z axis is applied to the magnetic field space.
[0007]
As a result, position information in the space can be captured as frequency information.
[0008]
For irradiation of the high frequency pulse, an irradiation coil that generates a high frequency magnetic field in a direction perpendicular to the direction of the static magnetic field is used. This irradiation coil has been studied and improved for improving irradiation uniformity in a wide area of the magnetic field space, and various coils are used.
[0009]
FIG. 4 is a diagram illustrating an example of an irradiation coil, and illustrates an example of a planar birdcage coil.
In FIG. 4, two ring conductors 1 a and 1 b that are concentric and different in size on the same plane are interconnected by a plurality of linear conductors 2.
[0010]
FIG. 5 is a circuit diagram showing the birdcage coil. In FIG. 5, a loop a indicates the ring-shaped conductor 1a in FIG. 4, and a loop b indicates the ring-shaped conductor 1b.
[0011]
Loop a and loop b are normally tuned to the magnetic resonance frequency, and capacitor c and coil 1 are used for this tuning.
[0012]
FIG. 6 is a diagram showing a voltage distribution and a current distribution in the loop a shown in FIG.
[0013]
In FIG. 6, since the loop a is tuned to the resonance frequency, the current is maximum and the voltage is minimum at the feeding point d.
[0014]
FIG. 7 is a view showing an example in which the irradiation coil shown in FIG. 4 is mounted in the MR apparatus.
[0015]
In FIG. 7, the irradiation coil 18 is usually disposed in the vicinity of the subject 14 in order to efficiently apply the irradiation pulse to the subject 14.
[0016]
In addition, around the subject 14, a receiving coil 17 for receiving a magnetic resonance signal from the subject 14, a preamplifier 22 for amplifying the received signal, and a signal amplified by the preamplifier 22 are A / D converted. Wiring 23 and the like for connecting to a vessel (not shown) are arranged, and they are usually mounted in a bed for placing the subject 14 thereon.
[0017]
Further, the receiving coil 17, the preamplifier 22, and the wiring 23 may be collectively arranged in one place from the viewpoint of ease of manufacture and operability.
[0018]
[Problems to be solved by the invention]
By the way, the bed on which the patient (subject 14) is placed has a movable structure for adjusting the imaging region, and as a result, the positional relationship among the irradiation coil 18, the preamplifier 22, and the wiring 23 is not fixed. Because it changes.
[0019]
In that case, a potential difference is generated between the potential of the receiving coil 17, the preamplifier 22, and the wiring 23 and the potential of the irradiation coil 18, and high frequency coupling is generated. Since the strength of the high-frequency coupling is affected by the potential difference, if the positional relationship changes, the strength of the high-frequency coupling varies as a result.
[0020]
FIG. 8 is a diagram for simply explaining the fluctuation of the high-frequency coupling.
In FIG. 8, the irradiation coil 18 operates as a QD coil. Here, C shows a part of the capacitor c in the circuit diagram of FIG. 5, and among these, the capacitor 8 is located at the highest voltage location in FIG.
[0021]
Usually, in the case of a QD coil, since two feeding points are arranged so as to be orthogonal, one feeding point does not affect the resonance circuit of the other feeding point.
[0022]
However, as shown in FIG. 7, when only one side of the irradiation coil 18 is coupled by high frequency due to the influence of the wiring 23 or the like (in FIG. 8, the preamplifier 22 and the wiring 23 are in the illustrated positions), The preamplifier 22 and the wiring 23 are positioned at a position where the potential is high. In this case, the high-frequency coupling between the preamplifier 22, the wiring 23, and the irradiation coil 18 varies due to the position variation of the preamplifier 22 and the wiring 23.
[0023]
9 is a diagram for explaining the fluctuation of the potential, and shows the voltage generated in the resonance circuit d (FIG. 9A and d ′ (FIG. 9B)). In such a state, each voltage distribution becomes v1 and v1 ′ indicated by solid lines, and the potential of v1 ′ becomes a base potential at a place where the potential of v1 is the highest (or low), thereby preventing high-frequency coupling.
[0024]
However, when the preamplifier 22 and the wiring 23 in FIG. 8 move relative to the irradiation coil 18, the potential of the resonance circuit d becomes the highest at the point g, and the potential changes as shown by the point g in FIG. 9. Appears (shown in broken line).
[0025]
As a result, as shown by point e in FIG. 9, the position where the base potential of d is moved moves, and the potential difference from d ′ varies.
[0026]
Therefore, the orthogonality of the d and d ′ QD coils is lost, and high frequency coupling is generated between the two coils of the QD coil.
[0027]
Due to this high-frequency coupling, the phase of the high-frequency magnetic field generated from both coils of the QD coil causes an error from 90 °, and as a result, the uniformity of the irradiation pulse is deteriorated and the irradiation efficiency is reduced.
[0028]
However, in the prior art, high frequency coupling between the irradiation coil 18 and the wiring 23 in the vicinity thereof has not been considered.
[0029]
An object of the present invention is to reduce the rate of high-frequency coupling between an irradiation coil and a wiring, improve the irradiation uniformity and irradiation efficiency, and take a high-quality image, and magnetic resonance imaging using the same Is to realize the device.
[0030]
[Means for Solving the Problems]
In order to achieve the above object, the high-frequency coil of the present invention is configured as follows. That is, a first closed curve conductor, a second closed curve conductor disposed inside the first closed curve conductor, and a plurality of first closed loop conductors connecting the first closed curve conductor and the second closed curve conductor. A conductor, a power supply unit to which a high-frequency signal for generating a high-frequency magnetic field pulse is supplied, and a voltage distribution unit for distributing the operating voltage are provided.
Preferably, the voltage distribution means includes a plurality of resonant capacitive elements arranged in series in at least one of the first closed-curved conductor portions between two adjacent first conductors.
Preferably, the voltage dispersion means is disposed in the first closed curve conductor portion corresponding to a position shifted by approximately 90 ° with respect to the power feeding unit.
In order to achieve the above object, a magnetic resonance imaging apparatus of the present invention is configured as follows. That is, a static magnetic field generator for generating a static magnetic field, a gradient magnetic field generating coil for generating a gradient magnetic field in the static magnetic field, and a high-frequency magnetic field pulse for inducing nuclear magnetic resonance in a subject placed in the static magnetic field are generated. A high-frequency coil; a reception coil that receives an echo signal generated by nuclear magnetic resonance; and a signal processing unit that reconstructs an image of the subject using the echo signal. It has a coil.
[0031]
If the irradiation coil is configured to disperse the operating voltage of the irradiation coil for applying the irradiation pulse to the subject, the potential change of the irradiation coil becomes gentle, and the periphery of the irradiation coil Even if the position of the preamplifier or the like arranged in the position fluctuates, the change in high-frequency coupling between the preamplifier and the irradiation coil can be reduced.
[0032]
In other words, in the present invention, it is possible to prevent the strength of the high-frequency coupling from fluctuating depending on the position of the peripheral object by locally reducing the potential change of the irradiation coil.
[0033]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.
FIG. 3 is a block diagram showing a schematic overall configuration of the MRI apparatus according to the present invention.
[0034]
In FIG. 3, the MRI apparatus obtains a tomographic image of the subject 14 using a nuclear magnetic resonance (NMR) phenomenon, and includes a magnetic field generator 11, an MRI unit 12, a gradient magnetic field coil 21, and an irradiation coil. 18, a receiving coil 17, a bed 16, and a display device 15.
[0035]
The magnetic field generator 11 generates a strong and uniform static magnetic field on the subject 14, and a magnetic field generating means such as a permanent magnet system or a superconducting system is arranged in a space having a certain extent around the subject 14. .
[0036]
The MRI unit 12 includes a control device 10 that controls various pulse sequences in imaging, a high-speed image data calculation device 13, a gradient magnetic field power supply 20, and a high-frequency device 19.
[0037]
The gradient magnetic field coils 21 are arranged one by one on the three axes of the X axis, the Y axis, and the Z axis, and necessary gradients around the subject 14 by the output current of the gradient magnetic field power source 20 controlled by the control device 10. A magnetic field space is formed, and position information is given to the NMR signal.
[0038]
The high-frequency device 19 irradiates the subject 14 with a high-frequency pulse for spin excitation by the irradiation coil 18 under the control of the control device 10.
[0039]
The resulting NMR signal is detected by the receiving coil 17, the image reconstruction operation is performed on the signal data collected by the high frequency device 19, and the obtained MRI image is output to the display device 15. Yes.
[0040]
Here, the structure of the irradiation coil 18 which is one Embodiment of this invention is demonstrated using FIG.
[0041]
In FIG. 1, a plurality of linear conductors 4 are connected at equal intervals on the circumference of a ring-shaped conductor 3 and a ring-shaped conductor 5 having a smaller diameter than the ring-shaped conductor 3. Thereby, the ring-shaped conductor 3 and the ring-shaped conductor 5 are mutually connected.
[0042]
The number of straight conductors 4 is eight in one embodiment of the present invention. However, when the orthogonal transmission (QD transmission) method is used, the number of straight conductors 4 needs to be 4n (n is a natural number). For this reason, the number of straight conductors 4 is suitably 4-16.
[0043]
Resonance capacitive elements 8 are arranged in series with each other on the circumference of the ring-shaped conductor 3 and between the connection portions of the linear conductor 4, and between the circumference of the ring-shaped conductor 5 and the connection portion of the linear conductor 7. Resonant capacitors (not shown) are connected in series to form the irradiation coil 18.
[0044]
Here, the irradiation coil 18 is supplied with a high-frequency signal from the high-frequency device 19 from a feeding point 30 and a feeding point 31. A plurality of resonance capacitive elements are provided in series at a position where the voltage is highest when viewed from the feeding points 30 and 31 (a position rotated by 90 degrees, that is, a position where the resonance capacitors 32 and 33 are disposed).
[0045]
In FIG. 1, the resonance capacitor 32 (33) is composed of three resonance capacitors. However, the resonance capacitor 32 (33) only needs to have a plurality of resonance capacitors. The plurality of resonance capacitors 32 (33) are set so that the series combined capacitance is equivalent to the resonance capacitor 8 in FIG. 1, and thus the plurality of capacitors constituting the resonance capacitor 32 (33) constitute a resonance capacitor. The number n to be multiplied by the resonance capacitance 8 is obtained.
[0046]
However, in FIG. 1, the feeding points 30 and 31 do not necessarily have a capacity of n × resonance capacity 8 in order to match the impedance matching with the high-frequency device 19 and perform irradiation efficiently.
[0047]
Similarly, the resonance capacitors 32 and 33 need not have a capacity of n × resonance capacitor 8 in order to adjust the resonance frequency of the irradiation coil 18.
[0048]
FIG. 2 shows the potential in FIG.
[0049]
In FIG. 2, the waveform of the potential 41 (broken line) when the resonance capacitor 32 is configured with a plurality of capacitors is the waveform of the potential 40 (solid line) when the resonance capacitor 32 is configured with one capacitor. Compared with FIG. 7, the change in potential becomes gentle, and the change in high-frequency coupling with respect to the preamplifier 22, the wiring 23, etc. in FIG. 7 can be reduced.
[0050]
That is, since the potential 41 has a gentle waveform, even if the relative position between the preamplifier 22 and the wiring 23 and the irradiation coil 18 changes and the potential 41 fluctuates, the waveform slope is gentle. For example, the potential fluctuation at the point e shown in FIG.
[0051]
【The invention's effect】
As described above, according to the present invention, in a planar birdcage coil, a plurality of resonance capacitors are arranged in series at a position where the potential of the birdcage coil is high, thereby suppressing a change in operating potential. As a result, the potential change with respect to an external factor is reduced.
[0052]
Thereby, the potential deviation of the planar birdcage coil due to the change in the position of the preamplifier, the wiring, etc. with respect to the planar birdcage coil is reduced, and the fluctuation of the high frequency coupling can be suppressed.
[0053]
Therefore, the present invention reduces the ratio of high-frequency coupling between the irradiation coil and the wiring, improves the irradiation uniformity and the irradiation efficiency, and can take a good image, and magnetic resonance imaging using the same An apparatus can be realized.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a planar birdcage coil according to an embodiment of the present invention.
FIG. 2 is a potential diagram of a birdcage coil according to an embodiment of the present invention.
FIG. 3 is a schematic configuration diagram of an MR apparatus to which the present invention is applied.
FIG. 4 is a schematic configuration diagram of a conventional birdcage coil.
FIG. 5 is a circuit diagram of a conventional birdcage coil.
FIG. 6 is an explanatory diagram of an operating potential of a birdcage coil.
FIG. 7 is a schematic cross-sectional view of a birdcage coil and its surroundings.
FIG. 8 is a diagram for explaining interference between a birdcage coil and a peripheral portion;
FIG. 9 is a diagram for explaining potential change when the relative position between the preamplifier and the like and the birdcage coil varies.
[Explanation of symbols]
3 Ring-shaped conductor 4 Linear conductor 5 Ring-shaped conductor 6 Linear conductor 8 Resonant capacitance 11 Magnetic field generator 12 MRI unit 13 Arithmetic device 14 Subject 15 Display device 16 Bed 17 Reception coil 18 Irradiation coil 19 High-frequency device 20 Gradient magnetic field power supply 21 Inclination Magnetic field coil 22 Print 23 Wiring 30 Resonance capacity 31 Resonance capacity 32 Resonance capacity 33 Resonance capacity

Claims (4)

A substantially annular first closed curve conductor, a substantially annular second closed curve conductor disposed inside the first closed curve conductor, and the first closed curve conductor and the second closed curve conductor are connected to each other. A first conductor disposed so as to divide the first closed curve conductor and the second closed curve conductor at an equal angle, and a power feeding unit to which a high frequency signal for generating a high frequency magnetic field pulse is supplied. In the high-frequency coil of the QD transmission method, which is arranged on the right and left sides of two locations from 90 degrees different directions ,
Voltage distribution means for distributing the operating voltage, the voltage distribution means being on the opposite side to the right side of the two power supply units and on the opposite side to the left side of the two power supply units In the region of the first closed curved conductor sandwiched between the corresponding adjacent first conductors, a first number of the plurality of resonant capacitive elements arranged in series are provided, and the first of the plurality of resonant capacitive elements Is the first closed curve sandwiched between the adjacent first conductors other than the region of the first closed curve conductor sandwiched between the adjacent first conductors corresponding to the two opposite sides. A high-frequency coil, wherein the number of resonant capacitive elements arranged in a conductor region is greater than a second number . The high frequency coil according to claim 1, wherein the number of the first conductors is 4n. The high frequency coil according to claim 1 or 2, wherein the number of the first conductors is eight. A static magnetic field generating device for generating a static magnetic field, a gradient magnetic field generating coil for generating a gradient magnetic field in the static magnetic field, and a high-frequency magnetic field pulse for inducing nuclear magnetic resonance in a subject arranged in the static magnetic field In a magnetic resonance imaging apparatus comprising: a high-frequency coil; a reception coil that receives an echo signal generated by the nuclear magnetic resonance; and a signal processing unit that reconstructs an image of the subject using the echo signal.
A magnetic resonance imaging apparatus comprising the high-frequency coil according to claim 1 as the high-frequency coil.

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