JP2833488B2 - Magnetic resonance tomography equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance tomography apparatus, and more particularly to a magnetic resonance tomography apparatus. A first gradient magnetic field coil that generates a gradient magnetic field pulse based on a waveform signal of a predetermined shape from a generator; and a leakage magnetic field that is attached to the first gradient magnetic field coil and that is generated outside the first gradient magnetic field coil. An active shield type gradient magnetic field coil comprising a second gradient magnetic field coil for generating a magnetic field in a direction opposite to the leakage magnetic field for canceling, and attenuating an attenuated waveform signal having a time constant different from this based on the waveform signal A correction waveform signal generated by the waveform signal generation means, and a correction waveform signal based on the waveform signal and the attenuated waveform signal is generated by the correction waveform signal generation means; By providing the serial first gradient coil, a magnetic resonance imaging apparatus having a correction circuit for correcting the waveform distortion of the gradient magnetic field pulses due to leakage magnetic field of the first gradient coil.

[0002]

2. Description of the Related Art A conventional apparatus of this type will be described with reference to FIG. This figure is a longitudinal sectional view of a gantry cryocontainer used in this apparatus. The superconducting magnet 7 for generating a static magnetic field in the body axis direction of the subject M is built in the inner container 10 filled with a cryogenic cryogen. Further, around the superconducting magnet 7, a shielding coil 9 for preventing leakage of a static magnetic field is provided. Inner container 10
Is covered with a first heat shield plate 11 and a second heat shield plate 12 made of two layers made of a material having high thermal conductivity such as aluminum. These are accommodated in a vacuum vessel 13 having a through hole, and a gradient magnetic field coil 14 for irradiating a subject M with a gradient magnetic field pulse is provided in the through hole. A subject M is inserted into the inside of the device, and a waveform signal of a predetermined shape from a waveform generator (for example, FIG. 7A)
Is input to the gradient magnetic field coil 14 via the gradient magnetic field power supply, and a gradient magnetic field pulse is applied to the subject M. Almost simultaneously with the application of the gradient magnetic field pulse, a high-frequency pulse is irradiated from an RF coil (not shown) to
Is selectively excited, and a resonance signal from this region of interest is detected by the RF coil.

However, when a gradient magnetic field pulse is applied, the inner container 1 is caused by a leakage magnetic field from the gradient magnetic field coil 14.
An eddy current is generated in the zero and first and second heat shield plates 11 and 12, and the eddy current generates a magnetic field in a direction that obstructs the gradient magnetic field pulse. In particular, the inner container 10 is cooled by the cryogenic cryogen and has a very low electric resistance, so that eddy current is likely to be generated for a long time (for example, on the order of seconds), and is further disposed around the inner container 10. The first and second heat shield plates 11 and 12 are cooled by this, and the same is true. Therefore, the gradient magnetic field coil 1
The gradient magnetic field pulse generated from No. 4 is rounded at the rising / falling portion (FIG. 8).
(B)). The characteristic degradation caused by the leakage magnetic field of the gradient magnetic field pulse (hereinafter, referred to as forward characteristic degradation) affects the selective excitation, and as a result, there is a problem that the reconstructed image obtained from the resonance signal is deteriorated. .

[0004] Therefore, a first method for solving this problem is as follows.
As a method of (1), a correction circuit is provided between the waveform generator and the gradient magnetic field power supply to correct the above-described characteristic deterioration of the gradient magnetic field pulse in the positive direction. A correction circuit for performing this correction will be described with reference to FIG.

[0005] The correction circuit 20 comprises a primary correction unit 21 to a fourth correction unit 24 and a correction waveform signal generation unit 25. Each of the correction units 21 to 24 receives the waveform signal from the waveform generator, adjusts the signal strength in gain adjustment units 21 a to 24 a including a variable resistor and an inverting amplifier, and obtains a capacitor, a variable resistor, and a non-inverting amplifier. The rising / falling portion of the waveform signal is adjusted to an attenuated waveform signal having a time constant different from that by the attenuated waveform signal generators 21b to 24b. These attenuated waveform signal generators 21b to 24b are configured such that capacitors and variable resistors having different values of capacitance, that is, time constants are different from each other. This is inner container 1
Since the electric resistances of the heat shield plates 11 and 12 are different from each other (because the respective temperatures are different), the decay times of the eddy currents generated differ from each other, and these effects are individually corrected. In addition, each of the correction units 21 to 24
Are configured so that the time constant decreases in order from the primary correction unit 21, and the correction units 21 to 23 correct the effects of the inner container 10 and the heat shield plates 11 and 12. The quaternary correction unit 24 includes primary correction units 21 to 21.
It is preliminarily added in consideration of the case where the correction cannot be performed by the tertiary correction unit 23. Each of these correction units 21 to 24
Is superimposed on the waveform signal from the waveform generator and output (corrected waveform signal) by the corrected waveform signal generator 25 composed of a non-inverting amplifier.

By interposing the correction circuit 20 between the waveform generator and the gradient magnetic field power supply, the waveform signal (FIG. 9A) from the waveform generator is converted into a primary correction circuit 21 through a fourth correction circuit. By superimposing the 24 different attenuated waveform signals (FIG. 9B), it is possible to compensate for the deterioration in the forward characteristic of the gradient magnetic field pulse caused by the leakage magnetic field of the gradient magnetic field coil. That is, an area S ₁ ′ (FIG. 9 (b)) corresponding to an area S ₁ (FIG. 7 (b)) missing from the waveform signal (FIG. 7 (a)) due to deterioration is added to the waveform signal in advance, and A corrected waveform signal is obtained by subtracting an area S ₂ ′ ((FIG. 9 (b)) corresponding to an area S ₂ (FIG. 7 (b)) increased by deterioration from the signal (FIG. 7 (a)) from the waveform signal in advance. By applying the gradient magnetic field power to the gradient magnetic field power supply, the characteristics of the gradient magnetic field pulse in the forward direction are compensated.

As a second method, a coil for preventing leakage of a magnetic field from the gradient magnetic field coil 14 is provided to prevent the forward characteristic deterioration of the gradient magnetic field pulse. This gradient coils 14, the inner coil 14 ₁ for irradiating a gradient magnetic field pulses to the subject M, a magnetic field opposite to the gradient magnetic field pulse generated by the inner coil 14 _1, to the outside generating a magnetic field of the same strength and the leakage magnetic field (cancellation for magnetic field pulse) is performed by configuring outer coil 14 _2. The gradient magnetic field coil having such a configuration is called an active shield type gradient magnetic field coil. This outer coil 14 ₂
Is attached to the outside of the inner coil 14 ₁ (see FIG. 6), the inner coil so as to cancel only the leakage magnetic field of the outer without interfering with the gradient magnetic field pulses emitted from the inner coil 14 ₁ to the subject M It is attached from 14 ₁ at a predetermined distance.

As described above, there are the first method and the second method as a method for preventing the forward characteristic deterioration of the gradient magnetic field pulse, but only the second method (active shield type gradient coil). Since the leakage magnetic field of the gradient magnetic field coil cannot be completely prevented, usually, the first method (correction circuit) is used in combination to prevent the forward characteristic deterioration of the gradient magnetic field pulse.

[0009]

By using the above-described correction method of the first method and the active shield type gradient coil of the second method together, it is possible to prevent the forward characteristic deterioration of the gradient magnetic field pulse. When the correction is made, there are the following problems.

[0010] That is, the outer coil 14 ₂ of the active shield gradient coil 14, the gradient magnetic field pulse generated by the inner coil 14 _1, is configured to cancel only the leakage magnetic field generated outside of the inner coil 14 ₁ Yes, but actually the inner coil 14 ₁
Or cause an assembly error of the outer coil 14 ₂ and, in some cases each of the magnetic field intensity distribution is different. So that the direction of magnetic field pulse for canceling than leakage magnetic field occurring outside the inner coil 14 ₁ is stronger in such a case. Then the magnetic field is the difference, since the opposite direction to the magnetic field pulse and turned by inner coil 14 ₁ of the direction of the leakage magnetic field cancellation, gradient pulses and the magnetic field due to eddy currents rising portion thereof is due to the leakage magnetic field Is enhanced by a magnetic field in the opposite direction (due to the eddy current caused by the canceling magnetic field pulse) (that is, the magnetic field in the same direction as the gradient magnetic field pulse) and overshoots, and the falling portion undershoots (see FIG. 10). That is, characteristic deterioration of the gradient magnetic field pulse (hereinafter referred to as characteristic deterioration in the opposite direction) occurs in a state where an extra area S ₃ is added in the rising part and an extra area S ₄ is subtracted in the falling part. become.

By the way, the correction circuit which is the first method is as follows.
In the rising portion of the gradient magnetic field pulse, an extra area S ₁ ′ is added, and in the falling portion, the area S ₂ ′ is subtracted to correct the forward characteristic deterioration of the gradient magnetic field pulse. Accordingly, when the above-described characteristic deterioration in the opposite direction occurs using the active shield type gradient coil 14 which is the second method, the gradient magnetic field pulse is corrected by the correction circuit which is the first method. There is a problem that it is not possible.

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and even if an active shield type gradient coil causes characteristic degradation in a gradient magnetic field pulse in the opposite direction, the characteristic degradation is corrected. It is an object of the present invention to provide a magnetic resonance tomography apparatus capable of performing the above.

[0013]

The present invention has the following configuration to achieve the above object. That is, the magnetic resonance tomography apparatus according to the present invention provides a container having a predetermined shape from a waveform generator in a static magnetic field generated by a superconducting magnet immersed in a cryogenic cryogen of an inner container covered with a heat shield plate. A first gradient magnetic field coil for generating a gradient magnetic field pulse based on a waveform signal; and the leakage magnetic field provided to the first gradient magnetic field coil for canceling a leakage magnetic field generated outside the first gradient magnetic field coil. Comprises an active shield type gradient magnetic field coil comprising a second gradient magnetic field coil for generating a canceling magnetic field pulse in the opposite direction, and based on the waveform signal, generates an attenuated waveform signal having a different time constant from the attenuated waveform signal generation means. And a correction waveform signal based on the waveform signal and the attenuated waveform signal is generated by a correction waveform signal generation means. In the magnetic resonance tomography apparatus having a correction circuit for correcting the waveform distortion of the gradient magnetic field pulse caused by the leakage magnetic field of the first gradient magnetic field coil by applying the correction signal to the first gradient magnetic field coil, the correction circuit includes: Signal inverting means for inverting the polarity of the waveform signal from the waveform generator; and interposing between the waveform generator and the attenuated waveform signal generating means, the waveform signal from the waveform generator and the inverted waveform Switching means for switching between signals and inputting the signals to the attenuated waveform signal generating means.

[0014]

The operation of the present invention is as follows. That is, when the intensity of the canceling magnetic field pulse from the second gradient magnetic field coil is lower than the intensity of the leakage magnetic field from the first gradient magnetic field coil, an eddy current is generated in the heat shield plate or the inner container by the leakage magnetic field, The eddy current generates a magnetic field in a direction that obstructs the gradient magnetic field pulse of the first gradient magnetic field coil, and waveform distortion (positive forward characteristic deterioration) occurs in the gradient magnetic field pulse. In this case, the switching means is switched so that the waveform signal from the waveform generator is input to the attenuated waveform signal generating means. Thereby, the attenuation waveform signal generation means generates an attenuation waveform signal having a time constant different from the waveform signal. The correction waveform signal generating means generates a correction waveform signal based on the waveform signal from the waveform generator and the attenuation waveform signal, and applies the correction waveform signal to the first gradient magnetic field coil to thereby generate a correction signal for the first gradient magnetic field coil. Waveform distortion of a gradient magnetic field pulse (positive characteristic deterioration) caused by a leakage magnetic field can be corrected.

Further, when the intensity of the canceling magnetic field pulse from the second gradient magnetic field coil is stronger than the strength of the leakage magnetic field of the first gradient magnetic field coil, the eddy current is applied to the heat shield plate or the inner container by the canceling magnetic field pulse. Is generated, and a magnetic field is generated by the eddy current in a direction in which the gradient magnetic field pulse of the first gradient magnetic field coil is strengthened, thereby causing waveform distortion (reverse characteristic deterioration) in the gradient magnetic field pulse. In this case, the switching means is switched so that the waveform signal from the waveform generator is input to the signal inversion means, and the waveform signal is inverted. Based on the inverted waveform signal, the attenuated waveform signal generation means generates an attenuated waveform signal having a time constant different from the waveform signal in a direction opposite to the case where the intensity of the canceling magnetic field pulse is smaller than the intensity of the leakage magnetic field. Generate. Then, the correction waveform signal generation means generates a correction waveform signal based on the waveform signal and the attenuation waveform signal, and supplies the correction waveform signal to the first gradient magnetic field coil, thereby generating a correction waveform signal having the same direction as the gradient magnetic field pulse of the first gradient magnetic field coil. The waveform distortion of the gradient magnetic field pulse due to the magnetic field (due to eddy current generated by the canceling magnetic field pulse of the second gradient magnetic field coil) (reverse characteristic deterioration) can be corrected.

[0016]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a perspective view illustrating a schematic configuration of the magnetic resonance tomography apparatus according to the embodiment. In the figure, reference numeral 1 denotes a gantry into which the subject M is inserted when capturing a tomographic image.
Inside the gantry 1, a cryogenic container 2 for a superconducting magnet (hereinafter simply referred to as a cryogenic container 2) is inserted.
The low-temperature container 2 is formed as a cylindrical body as shown by a dotted line in the figure, and a hollow portion thereof is a through hole 3 for inserting the subject M.

On the front side of the gantry 1, a bed 4 that moves forward and backward with respect to the through hole 3 is installed on the floor via a base 6. The base 6 is provided with a moving mechanism (not shown) for moving the bed 4 forward and backward. The subject M lying on the bed 4 is guided into the through-hole 3 by the moving movement of the bed 4, and is subjected to magnetic resonance. Tomography is performed.

Next, a description will be given with reference to FIG. 2 showing a longitudinal sectional view of the cryogenic container 2 and a block diagram of a control circuit. In the through hole 3 into which the subject M is inserted, a plurality of solenoid-type superconducting magnets 7 that generate a uniform static magnetic field in the body axis direction of the subject M are provided along the cylinder axis of the low-temperature container 2. I have. A cylindrical magnetic shield plate 8 is provided so as to cover the outer peripheral surface of each superconducting magnet 7, and two shields that generate a magnetic field opposite to the static magnetic field are provided on the outer peripheral portion of the magnetic shield plate 8. Coils 9 are arranged in parallel along the cylinder axis of the magnetic shield plate 8. The superconducting magnet 7, the magnetic shield plate 8, and the shielding coil 9
Inner vessel 10 filled with a cryogenic refrigerant (for example, liquid helium) for maintaining superconducting state of superconducting magnet 7
It is stored inside.

A first heat shield plate 11 made of a material having high thermal conductivity such as aluminum is provided around the inner container 10.
The periphery of the first heat shield plate 11 is covered with a second heat shield plate 12 formed of a similar material. The first heat shield plate 11 is maintained at approximately 20K by the cooling action of a refrigerator (not shown), and the second heat shield plate 12 is maintained at approximately 80K. The inner container 10 and the first and second heat shield plates 11 and 12 are housed in a vacuum container 13 at room temperature.

The through hole 3 is provided with a solenoid type gradient magnetic field coil 14 along the inner peripheral surface thereof. The gradient magnetic field coil 14 is usually moved in a three-dimensional direction (X, Y, Z directions (subject M) in a static magnetic field generated by the superconducting magnet 7.
In order to generate gradient magnetic fields orthogonal to each other in the direction of the body axis), three gradient magnetic field coils are provided, but for the sake of simplicity, one gradient magnetic field coil 14 is used for simplicity.
The gradient coil 14 has a double structure of the outer coil 14 ₂ corresponding to the inner coil 14 ₁ and the second gradient coil corresponding to the first gradient coil.
To illuminate the gradient magnetic field pulses to the subject M is a inner coil 14 _1, the outer coil 14 ₂ that are parallel in the outer peripheral surface of the inner coil 14 ₁ is leaked from the inner coil 14 ₁ to its outward leakage This is for irradiating a magnetic field (a canceling magnetic field pulse) in a direction opposite to the gradient magnetic field pulse to cancel the magnetic field. A gradient magnetic field coil having a double structure that generates such a canceling magnetic field pulse is called an active shield type gradient magnetic field coil.

The waveform generator 15 includes a correction circuit 20 described later.
, A waveform signal of a predetermined shape (for example, a square wave), and a correction circuit 20 previously corrects the waveform distortion of the gradient magnetic field pulse. The correction circuit 20 supplies the corrected waveform signal to the gradient power supply 30. The gradient magnetic field power supply 30 supplies the gradient magnetic field power supply 30 to the gradient magnetic field coil 14 so that the gradient magnetic field pulse of a predetermined shape is irradiated onto the subject M from the gradient magnetic field coil 14.

Next, reference will be made to the circuit diagram of the correction circuit 20 shown in FIG. Correction circuit 20 includes a signal inverter 19, the changeover switch S ₁ to S _4, a primary correction unit 21 to the fourth order correction unit 24, and a correction waveform signal generator 25. Since the components of the primary correction unit 21 to the fourth correction unit 24 are common, the primary correction unit 21 will be described here as an example. The primary correction unit 21 includes a gain adjustment unit 21a and an attenuated waveform signal generation unit 21b. The gain adjustment unit 21a has the polarity inverted by the waveform signal from the waveform generator 15 or the signal inversion unit 19. Adjust the voltage level of the waveform signal. This gain adjustment unit 21a includes a variable resistor R ₁₁
It is constituted by an inverting amplifier OP ₁₁ and a variable resistor R ₁₁
To adjust the voltage level of the signal.

The signal whose voltage level has been adjusted by the gain adjuster 21a is supplied to an attenuated waveform signal generator 21b.
Attenuation waveform signal generating unit 21b, and a capacitor C ₁ and a differentiator circuit comprising a variable resistor R ₂₁ Prefecture, Tokyo noninverting amplifier OP ₂₁ for amplifying the differentiated signal. In the decay waveform signal generating unit 21b, so that a variable resistor R ₂₁ to adjust the time constant of the rising / falling edge of the differential waveform by varying and adjusting the attenuation waveform signal of the time constant different from the input signal Has become.

The primary correction section 21 is configured as described above,
The secondary corrector 22 to the fourth corrector 24 are similarly configured. However, the time constants of the respective correction units are set to be different from each other. For example, the time constants are set so as to become smaller sequentially from the primary correction unit 21. That is, C ₁ R ₂₁ > C
₂ R _22> C ₃ values such that R _23> C ₄ R ₂₄ are determined. The reason why the plurality of correction units are constituted is that the electric resistance of the inner container 10 and the heat shield plates 11 and 12 (because the cooling temperature is different) is different, and thus the decay time of the eddy current generated respectively. Are different, so that these effects are individually corrected. The primary compensator 21 generates the magnetic field having the longest time constant (attributable to the eddy current) at the lowest temperature.
The heat shield plate 11 and the tertiary correction unit 23 correct the effects of the second heat shield plate 12 having the highest temperature. The quaternary correction unit 24 includes the primary correction unit 2
It is preliminarily added in consideration of a case where correction cannot be performed even by the first to third order correction units 23. Each of the attenuated waveform signal generators 21b to 24b corresponds to an attenuated waveform signal generator in the present invention.

The output signals of the first to fourth correction units 21 to 24, that is, the attenuated waveform signals having different time constants, are supplied to the corrected waveform signal generation unit 25 together with the waveform signal from the waveform generator 15. Correction waveform signal generator 2
Reference numeral 5 denotes a non-inverting amplifier, which combines and amplifies different attenuation waveform signals and waveform signals from the waveform generator 15 and outputs the combined signals to the gradient magnetic field power supply 30. Note that the correction waveform signal generator 25 corresponds to a correction waveform signal generator in the present invention.

The gain adjusters 21a to 24a on the input side of the correctors 21 to 24 are connected to changeover switches S _{1 to} S ₄ having two input and one output terminals, respectively.
The signal line of the waveform generator 15 is connected to one input terminal a of each of the changeover switches S _{1 to} S ₄ , and the other input terminal b is connected to the signal inverting section 19 of an inverting amplifier. Output terminal is connected. Therefore, by changing each of the changeover switches S _{1 to} S ₄ to the input terminal a side or the input terminal b side, each of the correction units 21 to 24 is changed.
Is input with a waveform signal or its inverted waveform signal. Each of the changeover switches S _{1 to} S
Reference numerals ₄ correspond to switching means in the present invention, and the signal inverting section 19 corresponds to signal inverting means.

Next, the attenuated waveform signal generated by each correction unit and the corrected waveform signal obtained by combining the waveform signal and the attenuated waveform signal will be described with reference to the waveform diagram of FIG. In addition,
FIG. 4 shows a case where all the changeover switches are set to the input terminal b side (the side to which the inverted waveform signal is input).

FIG. 4A shows a waveform signal (square wave) of the waveform generator 15. The polarity of this waveform signal is inverted by the signal inverting section 19. And this inverted waveform signal is input to the primary correction unit 21 via the changeover switch S _1. FIG. 4B shows an example of an attenuated waveform signal appearing at the output of the primary correction unit 21 at this time. In the figure, reference symbol L indicates a signal voltage level, and t indicates a time (related to a time constant) at which the voltage level becomes, for example, 1 / e (about 36%). Voltage level L is adjusted by varying the variable resistance R ₁₁ of the gain adjustment section 21a,
Time t is adjusted by varying the variable resistance R ₂₁ of the attenuation waveform signal generating unit 21b. FIG. 4C shows an example of an attenuated waveform signal appearing at the output of the secondary correction unit 22.
(D) and FIG. 4 (e) show an example of an attenuated waveform signal appearing in the outputs of the tertiary corrector 23 and the fourth corrector 24, respectively.
Then, these attenuated waveform signals are output to the corrected waveform signal generator 2.
FIG. 4 (f) shows an example of the corrected waveform signal synthesized and amplified in FIG.
Shown in

In the magnetic resonance tomography apparatus configured as described above, the following adjustments are made when the apparatus is assembled. First, a measuring instrument for measuring the waveform signal of the gradient magnetic field pulse is disposed in the through hole 3 almost at the position where the subject M is located. And from the waveform generator 15, a predetermined shape, for example,
A waveform signal of a square wave is output, and at this time, a signal waveform of a gradient magnetic field pulse output from the gradient magnetic field coil is observed. FIG. 7 shows the signal waveform of the gradient magnetic field pulse observed at this time.
The rising portion of the square wave is rounded pulling falling portion with (by the area S ₁ lack) of the tail (increased by the area S ₂₎ distortion (positive direction of the characteristic degradation) occurs as shown in (b). This magnetic field strength of the magnetic field pulse for canceling emitted from the outer coil 14 ₂ in order to cancel the leakage magnetic field of the inner coil 14 ₁ can not be canceled completely the leakage magnetic field to less than the leakage magnetic field, therefore leakage Magnetic field is inner container 10, first heat shield plate 11, second
This is because an eddy current is generated across the heat shield plate 12, and the eddy current generates a magnetic field in a direction that prevents the gradient magnetic field pulse. In this case, the changeover switches S _{1 to} S ₄ of the correction circuit 20 are set to the input terminal a side, and the respective variable resistances of the primary correction unit 21 to the quaternary correction unit 24 are adjusted to obtain a correction waveform signal as shown in FIG. The adjustment is performed so that the corrected waveform signal of (b) is obtained. Then, as a gradient magnetic field pulse, FIG.
As shown in (c), the same gradient magnetic field pulse waveform as the signal waveform can be obtained. That is, it is possible to correct the forward characteristic deterioration of the gradient magnetic field pulse due to the leakage magnetic field.

Next, in the signal waveform in which the gradient magnetic field pulse is observed, the rising portion of the square wave has an overshoot (increased by an area S ₃ ) and the falling portion has an undershoot (an area S ₄ ) as shown in FIG. ) And distortion (reverse characteristic degradation) occurs. It such as by assembling errors of the coil, the magnetic field intensity of the irradiated cancellation for magnetic field pulse from the outer coil 14 ₂ in order to cancel the leakage magnetic field of the inner coil 14 _1, the magnetic field pulse for canceling the order is larger than the leakage magnetic field contents Vessel 1
This is because an eddy current is generated across the first heat shield plate 11 and the second heat shield plate 12, and the eddy current generates a magnetic field in a direction to strengthen the gradient magnetic field pulse. Alternatively, the outer coil 14 ₂ and the inner coil 14 ₁ of the magnetic field intensity distribution is different, because the same eddy current is generated. In this case, the changeover switches S _{1 to} S ₄ of the correction circuit 20 are set to the input terminal b side, and the primary correction circuit 21
In addition, each variable resistor of the quaternary correction circuit 24 is adjusted so that the waveform of FIG. 4F is obtained as a correction waveform signal. In other words, 'with subtracting previously, the area S ₄ corresponding to the area S ₄ which lacks the falling portion' area S ₃ corresponding to the area S ₃ that is increased at the rising portion of the gradient magnetic field pulses to advance additional correction waveform signal Gradient magnetic field power supply 3
0. As a result, it is possible to correct the reverse characteristic deterioration such as overshoot or undershoot that occurs in the gradient magnetic field pulse due to the magnetic field intensity of the canceling magnetic field pulse being too strong.

Next, a practical procedure for correcting characteristic deterioration in the opposite direction will be described with reference to FIG. This figure shows the observation waveform of the gradient magnetic field pulse.

The correction is performed by paying attention to the falling part of the pulse, and the adjustment is performed in order from a region having a long time constant, that is, a region apart from the falling part. First, assuming that regions t ₃ , t ₂ , and t ₁ are close to the falling portion (that is, corresponding to a correction unit having a short time constant), the region t ₁ is the primary correction unit 21 and the region t ₂ is the secondary correction unit. Part 22,
The region t ₃ corresponds to an adjustment region in the tertiary correction unit 23. Therefore, by varying the variable resistance R _11, R ₂₁ of first primary correction unit 21, a waveform signal which is pulling the tail diagonally in the region t ₁ is adjusted to be horizontal at the ground level. Next, the variable resistances R ₁₂ and R ₂₂ of the secondary correction unit 22 are varied to adjust the waveform signal that is obliquely trailing in the region t ₂ so as to be horizontal at the ground level. Further, the variable resistor R of the third-order correcting unit 23
_13, and the R ₂₃ variable, and likewise adjusted to target a region t _3. Finally, the variable resistors R ₁₄ and R ₂₄ of the quaternary correction unit 24 are varied to adjust the rising / falling portions and the waveforms that could not be adjusted by the primary correction unit 21 to the tertiary correction unit 23. Do.

In the above description, the case where the characteristic deterioration in the forward direction and the reverse direction has occurred separately has been described as an example. However, due to the assembly error of the active shield type gradient coil 14, both the forward and reverse directions are deteriorated. Due to the distributed existence of the leakage magnetic field, both characteristics may deteriorate at the same time as shown by the two-dot chain line in FIG. This figure shows a waveform signal of a gradient magnetic field pulse when strong reverse characteristic deterioration and weak positive characteristic deterioration occur simultaneously. In such a case, be to perform appropriately switched adjusted to the changeover switch S ₁ to S ₄ of the input terminal a side or the input terminal b side (positive direction characteristic degradation) (reverse characteristic degradation) I just need.

In this embodiment, the correction circuit 20 is provided with four correction units (primary correction units 21 to 24). However, depending on the degree of characteristic deterioration and the degree of desired correction, one correction unit may be used. One correction unit may be provided, or five or more correction units may be provided.

[0035]

As is apparent from the above description, according to the present invention, the switching means is switched so that the waveform signal from the waveform generator is input to the signal inversion means, and the polarity of the waveform signal is inverted. The attenuated waveform signal generating means is in the opposite direction to the case where the intensity of the canceling magnetic field pulse is smaller than the intensity of the leakage magnetic field (when the forward characteristic degradation occurs) based on the inverted waveform signal. , And generates an attenuated waveform signal having a time constant different from that of the waveform signal. Then, the correction waveform signal generation means generates a correction waveform signal based on the waveform signal and the attenuation waveform signal, and supplies the correction waveform signal to the first gradient magnetic field coil, thereby generating a correction waveform signal having the same direction as the gradient magnetic field pulse of the first gradient magnetic field coil. The waveform distortion of the gradient magnetic field pulse due to the magnetic field can be corrected by the eddy current generated by the canceling magnetic field pulse of the second gradient magnetic field coil. Therefore, even if the characteristics of the gradient magnetic field pulse are reversed in the opposite direction due to the active shield type gradient coil, the characteristic degradation can be corrected.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a schematic configuration of a magnetic resonance tomography apparatus according to an embodiment.

FIG. 2 is a diagram showing a vertical cross-sectional view of a cryogenic container and a block diagram of a control circuit according to the embodiment.

FIG. 3 is a circuit diagram of a correction circuit according to the embodiment.

FIG. 4 is a waveform diagram of a correction circuit.

FIG. 5 is a diagram provided for describing a procedure for correcting characteristic deterioration.

FIG. 6 is a longitudinal sectional view of a low-temperature container according to a conventional example.

FIG. 7 is a diagram provided to explain forward characteristic deterioration.

FIG. 8 is a circuit diagram of a correction circuit according to a conventional example.

FIG. 9 is a diagram for explaining correction of characteristic deterioration in a forward direction;

FIG. 10 is a diagram provided for explaining characteristic deterioration in the opposite direction.

[Explanation of symbols]

14 ... gradient magnetic field coil 14 ₁ ... inner coil 14 ₂ ... outer coil 15 ... waveform generator 19 ... signal inversion unit 20 ... correction circuit 21 ... primary correction unit 22 ... secondary correction unit 23 ... tertiary correction unit 24 ... 4 Next correction unit 25 ... Correction waveform signal generation unit 21a, 22a, 23a, 24a ... Gain adjustment unit 21b, 22b, 23b, 24b ... Attenuation waveform signal generation unit 30 ... Gradient magnetic field power supply

Claims (1)

(57) [Claims] 1. A gradient magnetic field pulse based on a waveform signal of a predetermined shape from a waveform generator is generated in a static magnetic field generated by a superconducting magnet immersed in a cryogenic refrigerant in an inner container covered with a heat shield plate. A first gradient magnetic field coil that is generated, and a canceling magnetic field pulse that is attached to the first gradient magnetic field coil and that is opposite to the leakage magnetic field to cancel a leakage magnetic field that is generated outside the first gradient magnetic field coil. And an active shield type gradient magnetic field coil comprising a second gradient magnetic field coil for generating an attenuated waveform signal, wherein an attenuated waveform signal having a different time constant is generated by an attenuated waveform signal generating means based on the waveform signal, and Generating a correction waveform signal based on the attenuation waveform signal by a correction waveform signal generating means, and applying the generated correction waveform signal to the first gradient coil. Therefore, in a magnetic resonance tomography apparatus having a correction circuit for correcting waveform distortion of the gradient magnetic field pulse caused by a leakage magnetic field of the first gradient magnetic field coil, the correction circuit includes a correction circuit for converting a waveform signal from the waveform generator. Signal inverting means for inverting the polarity, and interposing between the waveform generator and the attenuated waveform signal generating means, switching between the waveform signal from the waveform generator and the inverted waveform signal to produce the attenuated waveform signal A magnetic resonance tomography apparatus comprising: a switching unit for inputting to the generation unit.