JP2002191575A - Magnetic resonance imaging instrument - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging instrument using an irradia tion coil with which the rate of the high-frequency connection of an irradiation coil to wiring, or the like, is reduced, an irradiation uniformity degree and irradiation efficiency are improved and thus an image with excellent image quality is photographed. SOLUTION: In a planar birdcage coil being the irradiation coil, resonance capacities 30, 31, 32 and 33 are serially arranged at positions with high potential in the birdcage coil. Thus, the change in operation potential is restricted and potential change is reduced concerning an exterior main cause. That is, the potential deviation of the birdcage coil owing to a change in the position of a pre-amplifier, or the like, with respect to the birdcage coil is reduced so as to restrict the change of the high-frequency connection. Thereby, the rate of the high-frequency connection of the irradiation coil to wiring, or the like, is reduced, the irradiation uniformity degree and irradiation efficiency are improved and the excellent image is photographed in the magnetic resonance imaging instrument using the irradiation coil.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a magnetic resonance imaging apparatus for imaging a desired portion of a subject using magnetic resonance.

[0002]

2. Description of the Related Art Magnetic resonance imaging (hereinafter referred to as MRI)
Is a device that measures the density distribution and relaxation time distribution of nuclear spins at a desired inspection site in a subject using the nuclear magnetic resonance phenomenon, and displays an image of the cross section of the subject from the measurement data. It is.

A nuclear spin of a subject placed in a uniform and powerful magnetic field generator performs precession about a direction of the magnetic field at a frequency (Larmor frequency) determined by the strength of the magnetic field.

When a subject is irradiated with a high-frequency pulse having a frequency equal to the Larmor frequency from the outside, spins are excited and transit to a high energy state (nuclear magnetic resonance phenomenon).

When the irradiation is stopped, the spin returns to the original low energy state with a time constant corresponding to each state, and at this time, the subject emits an electromagnetic wave (NMR signal) to the outside. This is detected by a high-frequency receiving coil tuned to that frequency.

At this time, a gradient magnetic field of three axes of X-axis, Y-axis and Z-axis is applied to the magnetic field space in order to add positional information to the space.

[0007] As a result, it is possible to capture position information in space as frequency information.

For irradiation of the high-frequency pulse, an irradiation coil for generating 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 to improve irradiation uniformity over a wide range of the magnetic field space, and various coils have been used.

FIG. 4 is a view showing an example of an irradiation coil, and shows an example of a planar birdcage coil. FIG.
In FIG. 2, two ring-shaped conductors 1 a and 1 b which are concentric and different in size on the same plane are interconnected by a plurality of linear conductors 2.

FIG. 5 is a diagram showing a circuit diagram of the birdcage coil. In FIG. 5, the loop a
Shows the ring-shaped conductor 1a in FIG. 4, and the loop b shows the ring-shaped conductor 1b.

The loops a and b are usually tuned to the magnetic resonance frequency, and this tuning uses a capacitor c and a coil 1.

FIG. 6 is a diagram showing a voltage distribution and a current distribution in the loop a shown in FIG.

In FIG. 6, since the loop a is tuned to the resonance frequency, the current becomes maximum and the voltage becomes minimum at the feeding point d.

FIG. 7 shows that the irradiation coil shown in FIG.
FIG. 3 is a diagram illustrating an example implemented in the device.

In FIG. 7, the irradiation coil 18 is usually
In order to apply an irradiation pulse to the subject 14 efficiently, it is arranged near the subject 14.

The object 14 is surrounded by the object 14.
And a preamplifier 22 for amplifying the received signal, and a wiring 23 for connecting the signal amplified by the preamplifier 22 to an A / D converter (not shown). Etc. are arranged,
They are usually mounted in a bed on which the subject 14 is placed.

The receiving coil 17, the preamplifier 22, and the wiring 23 may be collectively arranged at one place from the viewpoint of ease of manufacture and operability.

[0018]

The bed on which the patient (subject 14) is placed has a movable structure for adjusting the imaging region. As a result, the irradiation coil 18 and
Since the positional relationship between the preamplifier 22 and the wiring 23 is not fixed, it changes.

In that case, the receiving coil 17, the preamplifier 2
2 and the potential of the wiring 23 and the potential of the irradiation coil 18 cause a potential difference to generate high-frequency coupling. Since the strength of the high-frequency coupling is affected by the potential difference, a change in the positional relationship results in a change in the strength of the high-frequency coupling.

FIG. 8 is a diagram for simply explaining the fluctuation of the high frequency coupling. In FIG. 8, the irradiation coil 18 is Q
It operates as a D coil. Here, C shows a part of the capacitor c in the circuit diagram of FIG.
The capacitor 8 is located at the highest voltage in FIG.

Normally, in the case of a QD coil, two feed points are arranged so as to be orthogonal to each other.
It does not affect the resonance circuit at the other feeding point.

However, as shown in FIG. 7, when only one side of the irradiation coil 18 is coupled at a high frequency due to the influence of the wiring 23 and the like (in FIG. 8, the preamplifier 22 and the wiring 23 are connected).
Means the position shown in the drawing), and the preamplifier 22 and the wiring 23 are located at a position where the potential of the irradiation coil 18 is high. In this case, the high-frequency coupling between the preamplifier 22, the wiring 23, and the irradiation coil 18 is performed by the preamplifier 22, the wiring 23,
Fluctuates due to the position fluctuation.

FIG. 9 is a diagram for explaining the fluctuation of the potential, and shows the voltages generated in the resonance circuits d (FIG. 9A and d '(FIG. 9B)). In an ideal state, each voltage distribution becomes v1 and v1 'shown by a solid line, and at a place where the potential of v1 is the highest (or lowest), the potential of v1' becomes a base potential to prevent high-frequency coupling. I have.

However, the preamplifier 22 shown in FIG.
When the wiring 23 moves relatively to the irradiation coil 18, the position of the resonance circuit d at the point g has the highest potential, and the potential changes as shown at the point g in FIG. 9 (shown by a broken line).

As a result, as shown at point e in FIG. 9, the position at which the base potential of d moves moves, and the potential difference from d 'fluctuates.

Therefore, the orthogonality of the dD and d 'QD coils is lost, and high-frequency coupling occurs between the two QD coils.

This high-frequency coupling causes an error in the phase of the high-frequency magnetic field generated from both coils of the QD coil from 90 °, resulting in deterioration of irradiation pulse uniformity and deterioration of irradiation efficiency.

However, in the prior art, no consideration has been given to the high-frequency coupling between the irradiation coil 18 and the wiring 23 and the like in the vicinity thereof.

An object of the present invention is to provide an irradiation coil capable of reducing the ratio of high-frequency coupling between an irradiation coil and a wiring, improving irradiation uniformity and irradiation efficiency, and photographing a high-quality image, and using the same. The object is to realize a magnetic resonance imaging apparatus.

[0030]

In order to achieve the above object, the present invention is configured as follows. (1) A magnetic circuit that applies a static magnetic field to the subject, a gradient coil that applies a slice gradient magnetic field, a readout gradient magnetic field, and an encoding gradient magnetic field to the subject, and an irradiation pulse that causes magnetic resonance to be applied to the subject. An irradiation coil, a reception coil that detects a magnetic resonance signal, and a magnetic resonance imaging apparatus including an image reconstruction unit that obtains an image of a target object using the detection signal detected by the reception coil, wherein the irradiation coil is Means for dispersing an operating voltage for applying an irradiation pulse to the subject.

If the means for dispersing the operating voltage of the irradiation coil for applying the irradiation pulse to the subject is provided in the irradiation coil, the change in the potential of the irradiation coil becomes gentle, and the irradiation coil becomes gentle. Even if the position of a preamplifier or the like arranged around the coil fluctuates, a change in high-frequency coupling between the preamplifier or the like and the irradiation coil can be reduced.

That is, in the present invention, the change in the potential of the irradiation coil is locally reduced, so that the strength of the high-frequency coupling can be prevented from fluctuating depending on the position of the peripheral object.

[0033]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. FIG.
FIG. 2 is a block diagram illustrating an overall configuration outline of the RI apparatus.

In FIG. 3, the MRI apparatus obtains a tomographic image of the subject 14 by utilizing a nuclear magnetic resonance (NMR) phenomenon, and includes a magnetic field generator 11 and an MRI unit 12.
, A gradient coil 21, an irradiation coil 18, a receiving coil 17, a bed 16, and a display device 15.

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 type or a superconducting type is arranged in a certain space around the subject 14. Have been.

The MRI unit 12 includes a control device 10 for controlling various pulse sequences in imaging.
, High-speed image data calculation device 13, and gradient magnetic field power supply 2
0 and a high-frequency device 19.

The gradient magnetic field coils 21 are arranged in a set on each of the three axes of the X axis, the Y axis, and the Z axis, and are arranged around the subject 14 by an output current of the gradient magnetic field power supply 20 controlled by the controller 10. A necessary gradient magnetic field space is formed, and positional information is given to the NMR signal.

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.

The resulting NMR signal is transmitted to the receiving coil 1
7, the image data is calculated by the arithmetic unit 13 on the signal data collected by the high-frequency device 19, and the obtained MR is obtained.
The I image is output to the display device 15.

Here, the configuration of the irradiation coil 18 according to one embodiment of the present invention will be described with reference to FIG.

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 interconnected.

Although the number of the linear conductors 4 is eight in one embodiment of the present invention, it is necessary to be 4n (n is a natural number) when the quadrature transmission (QD transmission) system is used. Therefore, it is appropriate that the number of the linear conductors 4 is 4 to 16.

Resonant capacitive elements 8 are arranged in series with each other between the connection between the circumference of the ring-shaped conductor 3 and the straight conductor 4, and between the connection between the circumference of the ring-shaped conductor 5 and the connection with the straight conductor 7. In addition, a resonance capacitance element (not shown) is connected in series with each other to form an irradiation coil 18.

Here, a high-frequency signal of the high-frequency device 19 is supplied to the irradiation coil 18 from a feeding point 30 and a feeding point 31. A position where the voltage is highest when viewed from the feeding points 30 and 31 (a position rotated by 90 degrees, that is, the resonance capacitance 3
A plurality of resonance capacitance elements are provided in series at the positions where 2, 33 are arranged.

In FIG. 1, the resonance capacitance 32 (33) is 3
Although it is composed of two resonance capacitance elements, the resonance capacitance 32 (33) may have a plurality of resonance capacitance elements. The plurality of resonance capacitors 32 (33) are set such that the series combined capacitance is equivalent to the resonance capacitor 8 in FIG. 1. Therefore, the plurality of capacitors constituting the resonance capacitor 32 (33) constitute the resonance capacitor. The number n is multiplied by the resonance capacitance 8.

However, in FIG. 1, at the feeding points 30 and 31,
In order to match the impedance with the high-frequency device 19 and perform the irradiation efficiently, the nx resonance capacitance 8
It is not necessary to make the capacity.

Similarly, the resonance capacitors 32 and 33 do not need to have n × resonance capacitance 8 for adjusting the resonance frequency of the irradiation coil 18.

FIG. 2 shows the potential in FIG.

In FIG. 2, the waveform of the potential 41 (dashed line) when the resonance capacitor 32 is constituted by a plurality of capacitors is different from the waveform of the potential 40 (solid line) when the resonance capacitor 32 is constituted by one capacitor. 7, the change in the potential becomes gentler than the potential 40, and the change in the high-frequency coupling to the preamplifier 22, the wiring 23, and the like in FIG.

That is, since the potential 41 has a gentle waveform, even if the relative position between the preamplifier 22, the wiring 23, etc. and the irradiation coil 18 changes, and the potential 41 fluctuates, the slope of the waveform is gentle. Therefore, for example, the potential fluctuation at the point e shown in FIG.

[0051]

As described above, according to the present invention, in a planar birdcage coil, a plurality of resonance capacitors are arranged in series at positions where the potential of the birdcage coil is high, and the operating potential is reduced. Change can be suppressed,
As a result, the configuration is such that the potential change due to an external factor is reduced.

As a result, a potential shift of the planar birdcage coil due to a change in the position of the preamplifier, the wiring, and the like with respect to the planar birdcage coil is reduced, and a change in high-frequency coupling can be suppressed.

Therefore, the present invention uses an irradiation coil capable of reducing the ratio of high-frequency coupling between an irradiation coil and wiring, improving irradiation uniformity and irradiation efficiency, and capturing a high-quality image. A magnetic resonance imaging 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 sectional view of a birdcage coil and its surroundings.

FIG. 8 is a diagram illustrating interference between a birdcage coil and a peripheral portion.

FIG. 9 is a diagram illustrating a potential change when a relative position between a preamplifier or the like and a birdcage coil changes.

[Explanation of symbols]

 REFERENCE SIGNS LIST 3 ring conductor 4 linear conductor 5 ring conductor 6 linear conductor 8 resonance capacity 11 magnetic field generator 12 MRI unit 13 arithmetic unit 14 subject 15 display 16 bed 17 receiving coil 18 irradiation coil 19 high frequency device 20 gradient magnetic field power supply 21 gradient Magnetic field coil 22 Prep 23 Wiring 30 Resonance capacitance 31 Resonance capacitance 32 Resonance capacitance 33 Resonance capacitance

Claims (1)

[Claims] 1. A magnetic circuit for applying a static magnetic field to a subject, a gradient coil for applying a slice gradient magnetic field, a readout gradient magnetic field, and an encoding gradient magnetic field to the subject, and an irradiation pulse for generating magnetic resonance in the subject. A magnetic resonance imaging apparatus comprising: an irradiation coil to be applied; a reception coil for detecting a magnetic resonance signal; and image reconstruction means for obtaining an image of a target object using the detection signal detected by the reception coil. A magnetic resonance imaging apparatus, wherein the coil has means for dispersing an operating voltage for applying an irradiation pulse to the subject.