JPS6382638A - Magnetic resonance imaging apparatus - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention provides a magnetic field generating coil that applies a rectangular gradient magnetic field to a subject placed in a static magnetic field. The present invention relates to a magnetic resonance imaging device.

(Prior art) A magnetic resonance (MR) phenomenon is generated by placing a subject in a static magnetic field and applying a high-frequency magnetic field in a direction perpendicular to the static magnetic field. A magnetic resonance imaging (MRI) apparatus is known that applies gradient magnetic fields in three axial directions to image MR radiation obtained from a subject.

In order to obtain high-quality images with this MRI device, the above-mentioned X@It, 'l! It is desirable that the waveforms of the gradient magnetic fields applied in the three directions of the I and Z axes be rectangular as shown by the solid lines in FIG. 9(a). This is obtained by supplying a rectangular pulsed current to the gradient magnetic field generating coil.

However, in reality, due to the influence of eddy currents flowing through conductors such as superconducting magnets, shielding materials, frames, various coils, etc. placed around the gradient magnetic field generating coils, the gradient magnetic field waveform changes as shown in Figure 9. The image will be distorted as shown by the dotted line in a).

For this reason, the strength of the applied gradient magnetic field is not constant and is constantly changing, making it impossible to obtain high-quality images. In order to eliminate this drawback, if you take into account the amount of the waveform in advance and supply a current that overshoots as shown by the solid line in Figure 9(b), the result will be as shown in the dotted line in Figure 9(b). It is possible to modify the image into a rectangle.

However, the conductor and the gradient magnetic field generating coil, for example, the Y-axis direction coil (hereinafter referred to as the Y-axis direction coil,
The same is true for the axial coil.
If it is asymmetric along the Y-axis direction as shown in FIG. 10, that is, the distance L1 between the conductor 1 and the upper coil 2Δ and lower coil 2B of the Ym-direction coil 2. L2
are arranged in a relationship such as 11>12, the following inconvenience occurs.

Regarding the influence of eddy current flowing through the conductor 1, the magnetic field formed by the eddy current caused by the upper coil 2A and the magnetic field formed by the eddy current caused by the lower coil 2B become unbalanced. FIG. 12 shows characteristics showing this situation, where the vertical axis shows the magnetic field strength, the horizontal axis shows the distance in the Y-axis direction, and each straight line (a) to (C) represents the characteristics as shown. As shown in the figure, based on the asymmetric arrangement described above, the center of the eddy current magnetic field represented by the straight line (C) is 0.
It does not pass through the point, but passes through a position shifted by ye from the point. For this reason, the influence of eddy currents differs from place to place, so even if an overshoot current is supplied to the Y-axis coil as shown by the solid line in Fig. 9(b),
) will no longer be able to obtain a rectangle like the dotted line.

As shown in FIG. 11(a), the YIFl 11-direction coil is composed of four saddle-shaped coil segments 2, two of which are provided at the upper and lower positions across the space 3 in which the subject is to be placed.
A, 2B, 2C, and 2D, as shown in FIG. 11(b), a current for generating a gradient magnetic field supplied from a signal source 4 via an amplifier 5 is applied in series to four coil segments 2A to 2D. It is now possible to

(Problems to be Solved by the Invention) As described above, in the conventional MRI apparatus, when the arrangement relationship between the gradient magnetic field generating coil and the conductor arranged around the coil is asymmetric, the gradient magnetic field There is a problem in that even if an overshooting current is supplied to the generating coil, the gradient magnetic field waveform does not become rectangular.

The present invention has been made in response to the above-mentioned problems, and is an MRI system in which a rectangular gradient magnetic field waveform can be generated even when the arrangement relationship between the gradient magnetic field generating coil and the conductor is asymmetric.
The purpose is to provide an I location.

[Structure of the Invention] (Means for Solving the Problems) In order to achieve the above object, the present invention provides first and second coils adjacent to at least a pair of gradient magnetic field generating coils.
eddy current correction coils are disposed, and current supply means is provided for independently supplying gradient magnetic field generation current to both correction coils.

(Function) Since the current supply means supplies a correction current to the first and second eddy current correction coils to cancel the deviation of the eddy current magnetic field, the center of the eddy current magnetic field cannot pass through the zero point. can. Therefore, since a rectangular gradient magnetic field waveform is obtained, a high quality image can be obtained.

(Example) Fig. 1 is a perspective view showing the arrangement of gradient magnetic field generating coils, for example, Y@ direction coils, used in an MRI apparatus according to an embodiment of the present invention. 4 saddle-shaped coil segments 1, 2 each
2A, 12B.

First Y-axis direction coil 12 composed of 12C and 12D
and two saddle-shaped coil segments 14A disposed above the space 13 and close to the segments 12A and 12B of the first Y-axis coil 12.

1=IB, and two saddle-shaped coil segments 15A disposed below the space 13 and close to the segments 12C and 120;
The second eddy current correction coil 15 composed of '1.58
A Y-axis direction coil is constructed from these.

2 and 3 show the first Y-axis direction coil 12 and the second Y-axis direction coil 12.
Axial coil (first and second eddy current correction coil 14.
15) is a block diagram showing a circuit for supplying current for generating slope m scratches to the Y-axis direction coil configured by 15). In FIGS. 2 and 3, reference numeral 20 denotes a signal source that generates an inclined fa pumping field generation current.

16 is an overcurrent compensation circuit, 18A is a first active eddy current drive circuit, and 18B is a second active eddy current drive circuit.

17A is a first gradient magnetic field amplifier, 17B is a second gradient magnetic field amplifier, and 17C is a third gradient magnetic field amplifier.

4 and 5 show the eddy current compensation circuit 16 and the first eddy current compensation circuit 16.
This circuit shows a specific configuration of the second active eddy current drive circuits 18A and 18B. In FIG. 4, A1 to A4 are operational amplifiers, R1 to R3 and Rs to R8 are resistors, R4 and Rs are variable resistors, and C1 is a capacitor.

In addition, in Fig. 5, A1 to 8th are operational amplifiers, R1 to R3゜R6 to Rs, Rn to R+s are resistors, R4,
Rs.

R9 and Rho are variable resistors, and C1 and C2 are capacitors.

31 and 32 are changeover switches.

The operation of the eddy current correction circuit 16 shown in FIG. 4 will be explained below. The input signal level of 1 to the operational amplifier is Vl, and the output signal level is 2. Set the output signal level of operational amplifier A3 to V
3. When the output signal level of the operational amplifier A4 is 4, the output signal levels V2, V3, and V4 are expressed by the following equations.

When a stepped input signal as shown in Fig. 6 (a) is added as v1, the output signal level v4 is expressed as shown in Fig. 6 (b), and this output signal level V4 is used for eddy current compensation. can do.

Note that the signal ■p is expressed as shown in the following equation.

Next, 1. of Fig. 5. second active eddy current drive circuit 18A;
The operation of 18B will be explained. The input signal level of operational amplifier A1 is Vl, the output signal level is V2, the output signal level of operational amplifier A3 is V3, and the output signal level of operational amplifier A6 is V4.

When the output signal level of operational amplifier A5 is V5, the output signal level of operational amplifier A7 is V6, and the output signal level of operational amplifier 8 is ■7, selector switch 31.32
is switched to the lower position as shown in FIG. 5, each output signal level V2.

V3, V4, Vs, V6, and V7 are expressed by the following equations.

When a stepped input signal as shown in FIG. 7(a) is added as v1, the output signal level V7 can be expressed as shown in FIG. 7(b). Here, the signal 8Vp is expressed by the following equation.

(Left below) When the changeover switches SL and S2 are switched to the upper position, the respective output signal levels V3 and Ve.

V7 is expressed as shown in FIGS. 7(c), (d), and (e). Here, V31) and Ve p are expressed as in the following equation.

Therefore, variable resistors R4, R5, R9, RIO and selector switch S1. By combining S2, it is possible to obtain a correction current that cancels the deviation of the eddy current magnetic field when the arrangement relationship between the gradient magnetic field generating coil and the conductor is asymmetrical.

The difference between FIG. 2 and FIG. 3 is that in FIG.
4.15 is supplied by active eddy current drive circuits 18A and 18B provided individually, whereas in FIG.
In this point, correction currents are supplied to coils 14 and 15 including coils 2 and 2.

In the latter case, the correction current is distributed and the coil 12゜14.1
Since the amplifier 17A can be fully utilized, the load on the amplifiers 17B and 17G can be reduced. On the other hand, the former case has the advantage that the load on the amplifier 17A can be reduced.

FIG. 8 shows the gradient magnetic field characteristics obtained by this example, in which the first and second eddy current correction coils 14 and 15 are equipped with an eddy current compensation circuit 16 or an active eddy current drive circuit 18A as described above. By supplying a correction current to 18B to cancel the deviation of the gradient magnetic field, the center of the overcurrent magnetic field can pass through the zero point like a straight line (e). Therefore, high quality images can be obtained.

1st. Second eddy current correction coil 14. '15 can be operated independently, and four coils 14A, 14B,
15A and 15B can also be operated independently. Further, by adjusting the number of turns of each coil, it is possible to use only one active eddy current drive circuit.

[Effects of the Invention] As described above, according to the present invention, the first and second eddy current correction coils are disposed close to the gradient magnetic field generation coil, and correction current is independently supplied to both correction coils. As a result, a rectangular gradient magnetic field waveform can be obtained even when the gradient magnetic field generating coil and the conductor are arranged asymmetrically.

[Brief explanation of the drawing]

FIG. 1 is a layout diagram showing a gradient magnetic field generating coil used in an MRI apparatus according to an embodiment of the present invention, FIGS. 2 and 3 are block diagrams showing a current supply circuit for the gradient magnetic field generating coil, and FIGS. Figure 5 is a circuit diagram showing the specific configuration of the main parts of Figures 2 and 3, and Figure 6 (a> and (b) is a circuit diagram of the
Waveform diagrams showing the operation of the circuit shown in Fig. 7 (a) to (e)
is a waveform diagram showing the operation of the circuit in FIG. 5, FIG. 8 is a gradient magnetic field characteristic diagram obtained by the embodiment of the present invention, and FIGS. 9(a) and (b) are diagrams for explaining the present invention in detail. Waveform diagram, 1st
Figure 0 is a schematic diagram showing a conventional example, Figure 11 (a> and (b))
1 is a layout diagram and a block diagram showing a conventional example, and FIG. 12 is a gradient magnetic field characteristic diagram of the conventional example. 11... Conductor, 12, 12A, 12B, 12C, 12D... Y-axis direction coil, 14.14A, 14B, 15.15A, 15B... Eddy current correction coil, 16... Eddy current Compensation circuit, ``18A, '18B... Active eddy current 'a drive circuit, A1
...Operation amplifier, R4, R5, R9, R+o=variable resistor, SL, 32.
...Choice switch. Agent Patent Attorney Nori Chika Ken Yudai Hu
Norio Figure 12

Claims (1)

[Claims] A high-frequency magnetic field generating coil that applies a high-frequency magnetic field to a subject placed in a static magnetic field and at least a pair of gradient magnetic field generating coils that apply a rectangular gradient magnetic field are provided, and the magnetic resonance signal obtained from the subject is imaged. In the magnetic resonance imaging apparatus that is used in the present invention, first and second eddy current correction coils are arranged in close proximity to the pair of gradient magnetic field generation coils, and the gradient magnetic field generation coils are arranged independently for both correction coils. A magnetic resonance imaging apparatus comprising a current supply means for supplying current.