JP2004201756A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus which emits RF magnetic fields showing a good power efficiency and RF magnetic fields of a good distribution of magnetic field uniformity by switching them. <P>SOLUTION: An RF emission coil (6) comprises a plurality of RF emission coils (6a, 6b), and is equipped with a switching means (15) to switch the connection between the plurality of RF magnetic field emission coils (6a, 6b) and an RF magnetic field power source (7). <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus (hereinafter, referred to as an MRI apparatus), and more particularly to an MRI apparatus that switches between a high-frequency magnetic field having good power efficiency and a high-frequency magnetic field having good magnetic field uniformity distribution.
[0002]
[Prior art]
An MRI apparatus uses a nuclear magnetic resonance phenomenon that occurs in nuclei of atoms constituting a subject when a subject placed in a uniform static magnetic field is irradiated with a high-frequency magnetic field, and generates a nuclear magnetic resonance signal from the subject ( Hereafter, an NMR signal is detected, and an image is reconstructed using the NMR signal, thereby obtaining a magnetic resonance image (hereinafter, referred to as an MRI image) representing the physical properties of the subject.
[0003]
The performance of a high-frequency magnetic field irradiation coil (hereinafter referred to as an RF irradiation coil) for irradiating a high-frequency magnetic field is determined by the power efficiency of the irradiating high-frequency magnetic field and the uniformity of the magnetic field. , Power efficiency and magnetic field uniformity are designed to satisfy required performance. A power-efficient RF irradiation coil can reduce the cost of a high-frequency magnetic field power supply. On the other hand, an RF irradiation coil with good magnetic field uniformity can improve image quality.
[0004]
However, the power efficiency and the magnetic field uniformity have conflicting performances. That is, in one RF irradiation coil, when the power efficiency is improved, the magnetic field uniformity is reduced, and when the magnetic field uniformity is improved, the power efficiency is reduced. Here, the power efficiency indicates how strong a high-frequency magnetic field can be generated per unit power, and the magnetic field uniformity indicates how spatially the generated high-frequency magnetic field is spatially uniform.
[0005]
Therefore, conventionally, a single RF irradiation coil has been optimized so as to appropriately satisfy the respective performances. In other words, conventionally, both performances have not been equally optimized.
[0006]
On the other hand, depending on the pulse sequence used for imaging, there is a difference between a case where priority is given to the power efficiency of irradiation of the high-frequency magnetic field and a case where priority is given to magnetic field uniformity. For example, in the spin echo method, a high-energy pulse of 180 ° pulse is used as a high-frequency magnetic field, so that an RF irradiation coil giving priority to power efficiency is required.In the gradient echo method, a high-frequency magnetic field having a small flip angle is required. Since a pulse of less than or equal to ° is used, the power efficiency may be small. Rather, a high-frequency magnetic field giving priority to magnetic field uniformity has been desired.
[0007]
The problem regarding the conflicting performance of the coil characteristics was the same for the gradient coil. For example, in the case of a gradient magnetic field coil, high linearity of the generated gradient magnetic field and driving at a high speed are not compatible.
[0008]
As a well-known technique for solving this problem, there is an MRI apparatus provided with a gradient coil and a correction coil for correcting the linearity of the gradient coil. Here, the use of the correction coil is turned on and off when a gradient magnetic field with high linearity is required and when driving at a high speed is required (see Patent Document 1).
[0009]
[Patent Document 1]
Japanese Unexamined Patent Publication No. 2002-528148
[Problems to be solved by the invention]
However, there has not been proposed any technique for solving the above-described problem regarding the conflicting performance in the RF irradiation coil.
An object of the present invention is to provide a magnetic resonance imaging apparatus capable of switching and irradiating a high-frequency magnetic field having good power efficiency and a high-frequency magnetic field having good magnetic field uniformity distribution in a high-frequency magnetic field.
[0011]
[Means for Solving the Problems]
The above object is a static magnetic field generating means for generating a static magnetic field in the imaging region of the subject, a gradient magnetic field coil for generating a gradient magnetic field in the imaging region, and a high frequency magnetic field irradiation coil for irradiating the imaging region with a high frequency magnetic field, A high-frequency magnetic field power supply for supplying a drive current to the high-frequency magnetic field irradiation coil, a high-frequency magnetic field reception coil for detecting a nuclear magnetic resonance signal generated from the imaging region, and information obtained from a reception signal received by the high-frequency magnetic field reception coil A magnetic resonance imaging apparatus comprising: a display unit for displaying an image based on the high frequency magnetic field irradiation coil, wherein the high frequency magnetic field irradiation coil comprises a plurality of high frequency magnetic field irradiation coils; This is achieved by a magnetic resonance imaging apparatus characterized by comprising means for switching the selection of connection with the magnetic resonance imaging apparatus.
[0012]
The plurality of high-frequency magnetic field irradiation coils are achieved by a magnetic resonance imaging apparatus including a high-frequency magnetic field irradiation coil for irradiating a high-frequency magnetic field with high power efficiency and a high-frequency magnetic field irradiation coil for irradiating a high-frequency magnetic field with good magnetic field uniformity. You.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be specifically described with reference to the accompanying drawings.
FIG. 1 shows an overall configuration of an MRI apparatus employing a vertical magnetic field system according to the present invention. The MRI apparatus includes a static magnetic field generating means (including a superconducting coil, a normal conducting coil, a permanent magnet, and the like) 1 for generating a static magnetic field in an imaging region of the subject 12, and an XYZ orthogonal coordinate system for the imaging region. An X direction gradient magnetic field generating coil 2 for generating a gradient magnetic field, a Y direction gradient magnetic field generating coil 3 for generating a gradient magnetic field in the Y direction, a Z direction gradient magnetic field generating coil 4 for generating a gradient magnetic field in the Z direction, A gradient magnetic field power supply 5 for supplying power to the magnetic field generating coils 2 to 4; an RF irradiation coil 6 for irradiating the imaging region with a high frequency magnetic field; and a high frequency magnetic field power supply for supplying power to the RF irradiation coil 6 for high frequency magnetic field irradiation 7, an RF receiving coil 8 for receiving a high-frequency magnetic field emitted by nuclear magnetic resonance of the living tissue of the imaging region, a high-frequency transmitting and receiving unit 9 for transmitting and receiving the high-frequency magnetic field, and generating the gradient magnetic field and generating a high-frequency magnetic field. Control of transmission and reception Performs in accordance with the pulse sequence, and the computer 10 to perform the original to the image reconstruction calculation reception signal obtained by receiving the radio frequency magnetic field comprises a display unit 11 for displaying an image generated by the computer 10.
[0014]
Next, an enlarged view of a cross-sectional structure of the RF irradiation coil in the first embodiment of the MRI apparatus according to the present invention is shown in FIG. 6a is a first RF irradiation coil for irradiating a radio frequency magnetic field to the imaging site of the subject 12, 6b is a second RF irradiation coil for irradiating the radio frequency magnetic field to the imaging site, RF irradiation coil 6 has a two-stage structure. Further, 15 is a switch for switching which of the first RF irradiation coil and the second RF irradiation coil is connected to the high-frequency magnetic field power supply 7 by an instruction from the computer 10, and 16 is a switch for changing the high-frequency magnetic field from the gradient coil 2 to An RF shield for preventing unnecessary coupling with the gradient magnetic field coils 2 to 4 due to leakage to the 4 side.
[0015]
FIG. 3 is a plan view of the first RF irradiation coil 6a and the second RF irradiation coil 6b viewed from the Z direction. FIG. 3A is a plan view of the first RF irradiation coil 6a, and FIG. 3B is a plan view of the second RF irradiation coil 6b. However, each RF irradiation coil is a double ring multi-element type and has the same shape, but the outer shape d1 of the first RF irradiation coil 6a is a similar shape smaller than the outer shape d2 of the second RF irradiation coil 6b. (D1 <d2).
[0016]
In this case, a schematic diagram showing the magnetic field distribution per unit power of each coil along the X axis is shown in FIG. FIG. 4A is a schematic diagram of a magnetic field distribution of the first RF irradiation coil 6a, and FIG. 4B is a schematic diagram of a magnetic field distribution of the second RF irradiation coil 6b. According to this, comparing the magnetic field intensity peak values, it can be seen that the first irradiation coil 6a has a higher peak value. This is because the first RF irradiation coil 6a is a smaller coil, and therefore has higher power efficiency. On the other hand, comparing the distribution of the magnetic field homogeneity, the second RF irradiation coil 6b is larger than the first RF irradiation coil 6a, so a uniform high-frequency magnetic field can be generated in a large space, and the magnetic field uniformity is good. It turns out that it is a coil. That is, when d1 <d2, the first RF irradiation coil 6a is a coil having good power efficiency, and the second RF irradiation coil 6b is a coil having good magnetic field uniformity.
[0017]
With the above configuration, by switching the switch 15 between the first RF irradiation coil 6a having good power efficiency and the second RF irradiation coil 6b having good magnetic field uniformity, a high frequency magnetic field having different characteristics depending on the sequence to be used is irradiated. can do.
[0018]
For example, here, a case is considered in which the subject is first photographed by the spin echo method, and then photographed by the gradient echo method. Since the 180 ° pulse is required in the spin echo method as described above, the power efficiency is prioritized. Therefore, the switch 15 is switched by the computer 10, and the first RF irradiation coil 6a and the high frequency magnetic field power supply 8 are connected. Next, the gradient echo method does not require a 180 ° pulse with a large flip angle, but rather gives priority to magnetic field uniformity, so the computer 10 switches the switch 15, and the second RF irradiation coil 6b and the high-frequency magnetic field power supply 8 Connected.
[0019]
Next, an enlarged view of the cross-sectional structure of the RF irradiation coil in the second embodiment of the MRI apparatus according to the present invention is shown in FIG. This embodiment is an example in which the same small-sized coils as the first RF irradiation coil 6a in the first embodiment are vertically arranged in two pairs (6c and 6d). In the present embodiment, two power-efficient coils (6c and 6d) are arranged at different positions, so that when photographing different photographed parts (near areas A and B), power-efficient high-frequency The magnetic field can be switched by the switch 15 for irradiation. Further, if the switch 15 is switched so that power can be supplied simultaneously to both coils 6c and 6d, a high-frequency magnetic field having good power efficiency can be irradiated to a wide area.
[0020]
Next, an enlarged view of the cross-sectional structure of the RF irradiation coil in the third embodiment of the MRI apparatus according to the present invention is shown in FIG. This embodiment is an example in which a pair of small-sized coils, which are the same as the first RF irradiation coil 6a in the first embodiment, are moved in the horizontal direction by a motor. According to the present embodiment, when an image of the region A is to be imaged, and when an image of the region B is to be imaged, a high-frequency magnetic field with high power efficiency is moved by the motor 17 for an imaging region at an arbitrary position in the middle, so that irradiation can be performed. it can.
[0021]
The present invention is not limited to the above embodiment, and can be implemented in various modifications without departing from the gist of the present invention. For example, in the first embodiment, the first and second RF irradiation coils having a two-stage structure in which the inner ring diameter is changed have been described, but the first RF irradiation coil and the second RF irradiation coil are in the same plane. However, the present invention is also applicable to a case where power efficiency and magnetic field uniformity are changed. Further, in the above embodiment, the case where the upper and lower RF irradiation coils are two pairs has been described. However, the present invention is also applicable to the case where three or more pairs are used. Further, the shape of the RF irradiation coil can be variously changed.
For example, in the above embodiment, the MRI apparatus using the vertical magnetic field method has been described. However, the present invention can be applied to an MRI apparatus using the horizontal magnetic field method, in which case the shape of the RF irradiation coil is a tunnel shape.
[0022]
【The invention's effect】
As described above, according to the MRI apparatus of the present invention, it is possible to switch between a high-frequency magnetic field having good power efficiency and a high-frequency magnetic field having good magnetic field uniformity for irradiation.
[Brief description of the drawings]
FIG. 1 is an overall configuration of an MRI apparatus employing a vertical magnetic field method according to the present invention.
FIG. 2 is an enlarged view of a cross-sectional structure of an RF irradiation coil in the first embodiment of the MRI apparatus according to the present invention.
FIG. 3 is a plan view of a first RF irradiation coil and a second RF irradiation coil as viewed from a Z direction.
FIG. 4 is a schematic diagram showing a magnetic field distribution per unit power of each coil along an X axis.
FIG. 5 is an enlarged view of a cross-sectional structure of an RF irradiation coil in a second embodiment of the MRI apparatus according to the present invention.
FIG. 6 is an enlarged view of a cross-sectional structure of an RF irradiation coil in a third embodiment of the MRI apparatus according to the present invention.
[Explanation of symbols]
6a: first RF irradiation coil 6b: second RF irradiation coil 10: computer 15: switch d1: outer diameter of first RF irradiation coil d2: outer diameter of second RF irradiation coil

Claims (1)

Static magnetic field generating means for generating a static magnetic field in an imaging region of a subject, a gradient coil for generating a gradient magnetic field in the imaging region, a high frequency magnetic field irradiation coil for irradiating the imaging region with a high frequency magnetic field, and applying the high frequency magnetic field A high-frequency magnetic field power supply for supplying a driving current to the coil, a high-frequency magnetic field receiving coil for detecting a nuclear magnetic resonance signal generated from the imaging region, and information obtained from the received signal received by the high-frequency magnetic field receiving coil. In a magnetic resonance imaging apparatus comprising display means for displaying an image, the high-frequency magnetic field irradiation coil is composed of a plurality of high-frequency magnetic field irradiation coils, and the plurality of high-frequency magnetic field irradiation coils are connected to the high-frequency magnetic field power supply. A magnetic resonance imaging apparatus comprising means for switching selection.

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