JPH08154914A - Irradiation coil for magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE: To achieves a securing of a space of a subject sufficiently with the side of the subject being released and moreover, an improving of irradiation efficiency and irradiation uniformity. CONSTITUTION: Plane coils 14a-14d are coils each made planar in substance as obtained by the deformation of a saddle type coil. The two plane coils 14a and 14b and the plane coils 14c and 14d are arranged on two axes orthogonal to each other to be placed above and below a subject 1 as a set of irradiation coils 24 and 25. Frequency signals by the respective plane coils 14a and 14b (14c and 14d) are corrected in phase difference using a phase shifter. The orthogonal arrangement of the two coils can improve irradiation efficiency. Moreover, the plane coils each have a plurality of elements in parallel and currents flowing through the individual elements are weighted to irradiate the high frequency signals thereby achieving an improved quality of picture with higher uniformity of the irradiation.

Description

Detailed Description of the Invention

[0001]

[Field of Industrial Use and Problems to be Solved by the Invention]
The present invention relates to an irradiation coil for a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus), and in particular, a side of the object is opened to secure a sufficient object space, and further improve irradiation uniformity and irradiation efficiency. And an irradiation coil for an MRI apparatus.

[0002]

2. Description of the Related Art An MRI apparatus irradiates an atomic nucleus constituting a living tissue with a high frequency wave to cause magnetic resonance, receives a magnetic resonance signal generated by the resonance by a receiving coil, and performs a Fourier transform on the received magnetic resonance signal. It is performed on-the-fly and reconstructed into an image, which is widely used to obtain a tomographic image at an arbitrary portion of the subject.

An irradiation coil for generating a high-frequency magnetic field in a direction orthogonal to the static magnetic field is used for irradiation of the high-frequency signal. The irradiation coil has been researched and improved to improve the uniformity because the amplitude of the applied high frequency wave directly affects the SN ratio of the reconstructed image, and various coils such as saddle type and solenoid type coils have been developed. It is considered. Such an irradiation coil is usually incorporated in a gantry that generates a magnetic field, and is configured so that the center of the gantry is along the opening through which the subject enters and leaves, and the side of the subject is open. is not. FIG. 9 is a diagram showing an example of the saddle type irradiation coil configured as described above. In addition, in FIG.
The irradiation coil 33 is tuned to the magnetic resonance frequency by a capacitor.

By the way, the irradiation coil 33 should be made as small as possible in order to improve the irradiation efficiency. However, when the diagnostic region of interest is offset to the side of the body, it is necessary to bring that side to the center of the static magnetic field. The reason for bringing the side to the center of the static magnetic field is to obtain a good image without distortion in the diagnostic region of interest. As a result, the irradiation coil 33 and the subject interfere with each other to prevent lateral imaging of the body, or the subject is in a cramped posture, and the posture of the subject to be photographed becomes unstable.

For this reason, a laterally widened saddle type, that is, an enlarged irradiation coil on the side of the subject has been developed. This can improve the stability of the posture of the subject without sacrificing the irradiation uniformity as described above, but the irradiation efficiency decreases due to the larger shape of the irradiation coil. Therefore, the amplitude of the high-frequency signal required to cause magnetic resonance increases, and a high-output high-frequency amplifier is required, which makes the device expensive.

Therefore, the irradiation coil according to the prior art has been compromised so as to meet the requirements such as irradiation uniformity, irradiation efficiency, and securing of the space of the subject. On the other hand, recently, an intervening MRI apparatus has also been developed that removes the feeling of pressure on the subject and allows the subject placed in an open measurement space to freely approach. In the intervening device, the irradiation coil is also opened on the side of the subject, which makes it easier to approach the subject being imaged and perform medical treatment. Therefore, an irradiation coil whose side is open is required.

Therefore, the present invention provides an irradiation coil for an MRI apparatus which is capable of ensuring a sufficient measurement space in which the side of the subject is opened and in which the subject is placed, and which can improve the irradiation uniformity and the irradiation efficiency. With the goal.

[0008]

In order to achieve the above object, the irradiation coil for an MRI apparatus of the present invention comprises a plane coil formed on substantially the same plane, preferably a pair of plane coils. The specimen is arranged so as to face each other with the top and bottom of the specimen sandwiched therebetween. The irradiation coil generates a high frequency magnetic field in a direction orthogonal to the static magnetic field direction, and when the static magnetic field direction is the vertical direction, the planar coil of the present invention has a shape obtained by deforming a saddle type coil. A flat coil obtained by modifying the saddle type coil suitable for the present invention has at least a pair of element parts and a return part through which a return current from the element parts flows, and two flat coil shapes in which currents of opposite directions flow. This is equivalent to placing the loops next to each other.

[0009] More preferably, the plane coil is one in which the element portion has a plurality of parallel pattern structures,
In particular, it is desirable to have a means for weighting the current flowing in each of the plurality of parallel patterns of the element portion. Furthermore, the plane coil is preferably arranged on two orthogonal axes, provided with means for supplying a phase-corrected high frequency pulse.

[0010]

According to the irradiation coil for an MRI apparatus configured as described above, first, all sides of the subject are opened, and the subject is unstable even if the body is displaced from the center of the static magnetic field. You don't have to feel. In particular, by using an improved substantially flat irradiation coil of the saddle type, it is possible to improve the degree of freedom of space in the MRI apparatus which is opened laterally.

Further, by making the irradiation coil a plurality of element parts, it is possible to improve the irradiation uniformity of the high-frequency signal. Irradiation with higher uniformity can be performed. Furthermore, the irradiation efficiency is improved by arranging the irradiation coils on two orthogonal axes to form one set, and correcting the phase difference of the high-frequency pulse supplied to each irradiation coil to irradiate the high-frequency signal. You can

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a block diagram showing an embodiment of the MRI apparatus according to the present invention. This MRI apparatus includes a static magnetic field generating magnetic circuit 2, a gradient magnetic field generating system 3, a transmitting system 4, and a receiving system 5.
A signal processing system 6, a sequencer 7, and a central processing unit (CPU) 8.

The static magnetic field generating magnetic circuit 2 is for generating a uniform static magnetic field around the subject 1 in an arbitrary direction. In this embodiment, the direction of the static magnetic field is vertical. Inside the static magnetic field generating magnetic circuit 2, a gradient magnetic field coil 9 for generating a gradient magnetic field, a high frequency coil (reception coil) 15 of the reception system 5, and high frequency coils (irradiation coils) 24 and 25 of the transmission system 4 are set. ing.

The gradient magnetic field generating system 3 includes a gradient magnetic field coil 9 having a structure capable of independently applying a gradient magnetic field in mutually orthogonal Cartesian coordinate axis directions, that is, in the X axis direction, the Y axis direction and the Z axis direction, and a gradient magnetic field. It is composed of a gradient magnetic field power supply 10 for supplying a current to the coil 9 and a sequencer 7 for controlling the gradient magnetic field power supply 10. The transmission system 4 includes a high frequency generator 11, a modulator 12, a phase shifter 23, and high frequency coils (irradiation coils) 24 and 25. The irradiation coil 24 has a pair of plane coils 14a and 14b, and as will be described later, 14a and 14b are arranged on two orthogonal axes. Similarly, the irradiation coil 25 includes a pair of flat coils 14c,
14d and are arranged on two orthogonal axes. The irradiation coils 24 and 25 are arranged to face each other with the upper and lower sides of the subject being sandwiched therebetween. In response to a command from the sequencer 7, the high frequency pulse from the high frequency generator 11 is supplied to the high frequency amplifiers 13a, 13b, 13c,
The subject 1 is irradiated with a predetermined pulsed electromagnetic wave by being amplified via 13d and supplied to the irradiation coils 24 and 25. At this time, the phase shifter 23 shifts the phase of the high frequency pulse supplied to the two planar coils 14a and 14b by 90 degrees. Thereby, the phase-corrected high-frequency pulse is emitted.

The receiving system 5 comprises a receiving coil 15, an operational amplifier 16, a quadrature detector 17, and an A / D converter 18. When the receiving coil 15 detects the magnetic field resonance signal from the subject 1, the signal is converted into a digital amount by the A / D converter 18 via the operational amplifier 16 and the quadrature phase detector 17, and the command of the sequencer 7 is sent. The data is converted into two series of various data sampled by the quadrature detector 17 at a timing and sent to the CPU 8.
Although the receiving coil 15 is shown at a position apart from the subject in the figure, it is actually located near the subject.

The signal processing system 6 has an external storage device 20 such as a magnetic disk 20a and a magnetic tape 20b, a display 21 including a CRT and a keyboard 22. The sequencer 7 operates based on a control command from the CPU 8 and sends various commands necessary for collecting tomographic image data of the subject 1 to the transmission system 4, the gradient magnetic field generation system 3 of the static magnetic field generation magnetic circuit 2, and the reception system 5. I am sending it to.

The CPU 8 controls each of the sequencer 7, the transmission system 4, the reception system 5, and the signal processing system 6 according to a predetermined program. When the data from the receiving system 5 is input to the CPU 8, the CPU 8 executes signal processing, image reconstruction processing, etc.
The desired cross-sectional image is displayed on the display 21 and is also stored in the external storage device 20, for example, on the magnetic disk 20a.

Next, a concrete configuration example of the irradiation coils 24 and 25 will be described with reference to FIG. In FIG. 1A, reference numerals 1 and 24 and 25 denote the subject and the irradiation coil as in FIG. The irradiation coils 24 and 25 are arranged above and below the subject 1, respectively. FIG. 2B is a diagram for explaining an example of the irradiation coil 24 in FIG.
5 is also configured similarly to this.

The irradiation coil 24 (25) consists of two identical coils 14a, 14b (14c, 14d),
These are arranged on two axes orthogonal to each other. FIG. 3 shows the shape of the irradiation coil 14a. For the sake of explanation, FIG. 3 shows the simplest flat coil obtained by modifying the saddle type coil. That is, this planar coil has at least a pair of element parts 26 and a return part 27 through which a feedback current from the element parts 26 flows, and two loops through which currents of opposite directions flow are arranged adjacent to each other. The element portions 26 and the return portions 27 are formed on substantially the same plane. The irradiation coil is tuned to the magnetic resonance frequency by a capacitor 28. The “substantially the same plane” as used herein may be a structure in which the side of the subject is open, and may be a curved surface to some extent according to the curved surface of the body of the subject. Irradiation coil 1
Reference numeral 4a is connected to the modulator 12 and the high frequency oscillator 11 via the phase shifter 23, and is configured to emit a high frequency signal.

The coil 14b also has the same structure as the coil 14a, and the modulator 1 is connected via the phase shifter 23.
2. Connected to the high frequency generator 11. The high-frequency pulse supplied to these irradiation coils passes through the phase shifter 23, so that the phase-corrected high-frequency signal is emitted from each of the irradiation coils 14a and 14b.

FIG. 4 is a diagram for explaining the magnetic flux direction generated by the irradiation coils 14a and 14b orthogonally arranged as described above when a high frequency signal is irradiated, and the intensity. The magnetic flux vector generated by the irradiation coil 14a is 29, and the irradiation coil 1 is
When the magnetic flux vector produced by 4b is 30, the phases of the high frequency pulses supplied to both irradiation coils are shifted by 90 degrees, and the magnetic flux vector 29 and the magnetic flux vector 30 are combined into a magnetic flux vector 31. The magnitude of the magnetic flux vector 31 formed by this synthesis is √2 when the magnitudes of the magnetic flux vector 29 and the magnetic flux vector 30 are both 1. That is, by arranging the two irradiation coils orthogonally and driving them with phase correction, the irradiation efficiency can be improved to about 1.4 times that in the case of one irradiation coil.

By the way, FIG. 3 shows the simplest form of the saddle type planar coil according to the present invention. , Called magnetic field strength). In this coil, the element portion 26 is composed of two pieces, and the magnetic field generated by the two element portions 26 is used. If the magnetic field strength by these element parts 26 is uniform in the imaging range R, the irradiation uniformity of the high frequency signal will be good, but as shown in FIG. i
And the current −i of the return unit 27 cancel each other out, resulting in non-uniformity in the photographing range R.
Further, since the elements are separated from each other, the magnetic field strength is weakened in the central portion C of the static magnetic field.

Another embodiment for improving such non-uniformity of magnetic field strength is shown in FIGS. FIG. 6 (a),
The irradiation coil shown in (b) has a total of 4 elements 26a, 26b, 26 with the element parts 26 arranged in two parallel.
a ', 26b', and the two parallel division points (feed points) are the two parallel center points. At this time, a current i / 2, which is half the current i flowing through the element portion of the coil in FIG. 3, flows evenly through each element. In addition, since the distances between the elements are close to each other, the homogeneity of the magnetic field strength in the central portion C of the static magnetic field is improved.
However, since the current −i of the return portion 27 is larger than the current i / 2 flowing through the outer elements 26b and 26b ′,
The return section 27 is affected at both ends of the photographing range R, and the uniformity of the magnetic field strength at both ends of the photographing range R is not sufficiently improved.

The irradiation coil shown in FIGS. 7A and 7B is
In the same pattern as that shown in FIG. 1 (b), the element parts 26 are respectively arranged in two parallels, similarly to the irradiation coil shown in FIG. It is a thing. In this way, by changing the length of each element up to the feeding point, the current flowing through the element is weighted, and the outer element 26b,
26b 'is configured to be sufficiently larger than the currents of the inner elements 26a and 26a'.

At this time, the outer element portions 26b, 2
By weighting the elements, the current i ″ flowing through 6b ′ becomes larger than the current i / 2 flowing through the outer element portion in the case of FIG. The influence of the current −i of the portion 27 is reduced, and the distance between the elements is as shown in FIGS.
Therefore, the homogeneity of the magnetic field strength at the center portion C of the static magnetic field can be improved, and the homogeneity of the magnetic field strength at both ends of the imaging range R can be improved.

As a method of weighting the currents flowing through the elements arranged in parallel, a method of adjusting the distance to the feeding point as described above, and a capacitor 32 is inserted in each element portion 26 as shown in FIG. There is a method of adjusting by changing the capacitance of the element, a method of adjusting according to the thickness of the pattern forming the element, and the like. still,
In the above embodiment, the element portions 26 are arranged in parallel in two and are composed of four elements in total. However, even if the number of elements is different, the same effect can be obtained by weighting the current flowing in each element.

Furthermore, in order to improve the homogeneity of the magnetic field strength, it is possible to adjust the distance between the respective elements. For example, in FIG. 6A, when the distance between two parallel elements (26a-26b, 26a'-26b ') is L1 and the distance between inner elements (26a-26a') is L2, L2 should be too narrow. Not, L1 rather than L2
The uniformity of the magnetic field strength is improved by widening.

The planar coils described above can be used alone to form the irradiation coils 24 and 25 shown in FIG. 1A, but it is preferable to use a pair of planar coils as shown in FIG. 1B. One irradiation coil is formed by arranging the coils on two axes orthogonal to each other. The advantages of arranging the two coils orthogonally have already been described. In the embodiments of FIGS. 1 and 2, the case where the two irradiation coils are arranged opposite to each other with the subject in between has been described. However, when it is sufficient to obtain a local magnetic field homogeneity, the irradiation coil is not covered. It may be either the upper side or the lower side of the sample. Also in this case, as the irradiation coil, it is possible to configure a single plane coil having a pattern as illustrated in FIGS. 3 and 6 to 8, or two plane coils may be arranged orthogonally.

Further, in the above embodiments, the case where the direction of the static magnetic field is the vertical direction has been shown, but the static magnetic field may be in any direction. For example, when a static magnetic field magnet of a superconducting system or a normal conducting system is used as a static magnetic field generating magnetic circuit, the static magnetic field direction is generally horizontal, but in this case, the irradiation coil may be a solenoid type planar coil. Good. However, since the static magnetic field magnet of the superconducting type or the normal conducting type is not opened laterally by itself, the present invention uses the above-mentioned irradiation coil in an MRI apparatus in which the static magnetic field is vertical and laterally opened. It is more suitable to use.

[0030]

As described above, according to the present invention, a substantially planar coil, particularly an irradiation coil obtained by deforming a saddle type coil is used, and the irradiation coil is arranged above and below the object to be examined. The sides can be opened, and the posture of the subject can be stabilized. As a result, in an intervening device that is open to the side and can freely approach the subject placed in the measurement space, it is further possible to approach the subject being imaged and perform medical treatment. It will be easy.

Further, the irradiation coil is provided with a plurality of element parts to improve the irradiation uniformity of the high-frequency signal, and the current flowing through each element part is weighted for irradiation so that irradiation with a higher uniformity is performed in the photographing range. be able to. Further, the irradiation coils are orthogonal to each other.
Efficient by irradiating a high-frequency signal after correcting the phase difference in the high-frequency pulse supplied to each irradiation coil by arranging one set on one axis and one set above and below the subject, respectively. An irradiation coil can be obtained.

The MRI apparatus equipped with such an irradiation coil can stabilize the posture of the subject and can efficiently irradiate a high-frequency signal with high uniformity, so that the image quality can be improved. effective.

[Brief description of drawings]

FIG. 1A is a diagram showing a positional relationship of irradiation coils according to an embodiment of the present invention, and FIG. 1B is a diagram showing a pattern of one irradiation coil.

FIG. 2 is a block diagram showing an embodiment of an MRI apparatus to which the present invention is applied.

FIG. 3 is a diagram showing a shape of an irradiation coil according to an embodiment.

FIG. 4 is a diagram for explaining a magnetic flux direction and a magnetic field strength produced by the irradiation coil in FIG. 1 (b).

5 is a graph showing the magnetic field strength of the coil of FIG.

FIG. 6 is a view showing another embodiment of the irradiation coil, (a)
Is a diagram showing its shape, and (b) is a graph showing magnetic field strength.

FIG. 7 is a diagram showing another embodiment of the irradiation coil, (a)
Is a diagram showing its shape, and (b) is a graph showing magnetic field strength.

FIG. 8 is a view showing another embodiment of the irradiation coil.

FIG. 9 is a view showing a conventional irradiation coil.

[Explanation of symbols]

1 --- Subject 4--Transmission system 14a, 14b, 14c, 14d, 24, 25-High-frequency coil (irradiation coil) 23 --- Phase shifter 26a, 26a ', 26b, 26b' ... Element part 27 ... Return part

Claims (2)

[Claims] 1. An irradiation coil for a magnetic resonance imaging apparatus for generating a high-frequency magnetic field in a direction orthogonal to a static magnetic field direction, the irradiation coil comprising planar coils formed on substantially the same plane, and the planar coils are at least one pair. For a magnetic resonance imaging apparatus having two element parts and a return part through which a return current from the element part flows Irradiation coil. 2. The irradiation coil for a magnetic resonance imaging apparatus according to claim 1, wherein the planar coils are arranged so as to face each other with the upper and lower sides of the subject being sandwiched therebetween.