JP2002085369A - Eddy compensation device and eddy compensation method for magnetic resonance system - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To stably and accurately perform eddy magnetic field compensation and suppress an increase in circuit scale even when so-called cross-term compensation of eddy magnetic fields is performed. An eddy compensator for a magnetic resonance imaging apparatus having means for generating a gradient magnetic field to be superimposed on a static magnetic field. This device is provided with an eddy compensator 21 for compensating for an eddy magnetic field generated with the generation of a gradient magnetic field, and the compensator 21 is configured to process digital signals. Compensator 2
1 is a sampling circuit 3 for sampling and digitizing the control waveform data of the gradient magnetic field at regular intervals.
1 and an A / D converter 32; a compensation circuit 33 for calculating eddy compensation data based on the digital data based on a time constant and a gain; and an adder 34 for adding the eddy compensation data to control waveform data. .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a magnetic field fluctuation (distortion of a gradient magnetic field waveform) caused by an eddy current induced by a gradient magnetic field pulse in a magnetic resonance system for performing imaging and spectrum analysis based on a magnetic resonance phenomenon. The present invention relates to a vortex compensation device and a vortex compensation method for compensating (correcting) a vortex.

[0002]

2. Description of the Related Art Magnetic resonance imaging (MRI) apparatuses are now widely used as essential modalities in medical practice.

[0003] Magnetic resonance imaging performed by this apparatus magnetically excites a nuclear spin of an object placed in a static magnetic field with a high frequency signal of the Larmor frequency, and generates an echo signal generated by the excitation. This is an imaging method for reconstructing an image from the MR signal of FIG. This magnetic resonance imaging is extremely effective as a method for noninvasively obtaining an anatomical sectional view of a human body. In particular, it is widely used as a diagnostic method for the central nervous system such as the brain covered by bone.

On the other hand, magnetic resonance imaging is accompanied by a problem due to eddy current. Since a gradient magnetic field required for performing magnetic resonance imaging is generated as a pulse-like magnetic field by a gradient magnetic field coil and a gradient magnetic field power supply (amplifier), an electric conductor (for example, a heat shield of a static magnetic field magnet) is provided around the gradient magnetic field coil. Eddy currents occur in the conductor when the gradient magnetic field pulse rises and falls, and the eddy currents generate a fluctuating magnetic field called an eddy magnetic field. When this eddy magnetic field is generated,
The gradient magnetic field is distorted instead of having a waveform according to the control value commanded by the sequencer. When the gradient magnetic field waveform is distorted, the image quality deteriorates, for example, an image flows in the phase encoding direction, an artifact occurs in the image, and the resolution decreases.

[0005] Due to its physical properties, this eddy magnetic field (ie, eddy current) includes spatially zero-order and first-order components, as well as second-order and higher-order components. The zero-order component refers to an eddy magnetic field component that has no spatial distribution and behaves as if the center frequency fluctuated with time. The strength and time constant of the eddy magnetic field depend on the order, but are usually composed of a plurality of components. The time constant is typically a few milliseconds to a few seconds
ec.

Since it is one of the important issues in magnetic resonance imaging to improve the image quality by minimizing the influence of the eddy magnetic field, improvement based on a pulse sequence, an eddy current compensation circuit, a shielded gradient magnetic field coil Various measures are taken according to the order and properties of the eddy magnetic field, such as coil design.

It is known that the primary eddy magnetic field component can be suppressed by controlling the gradient magnetic field waveform.
For this reason, a eddy compensation circuit provided with a plurality of analog circuits each having an integrator having a variable time constant and an amplifier having a variable gain is provided in the gradient magnetic field amplifier. A gradient magnetic field control waveform corresponding to the control value is input to the eddy compensation circuit, and an exponential decay waveform based on each time constant and each gain is formed using this waveform.
The plurality of exponential decay waveforms are superimposed on the gradient magnetic field control waveform. As a result, waveform components for compensating for the eddy magnetic field formed of the exponential decay waveform are superimposed on the rising and falling portions of the gradient magnetic field control waveform.

[0008]

However, since the above-mentioned eddy compensation circuit is composed of an analog circuit, there is a problem that the temperature of circuit elements forming an integrator greatly varies and varies. Further, at the time of installation of the magnetic resonance imaging apparatus, it is necessary to adjust a time constant and a gain for eddy compensation, but there is a problem that the manual adjustment is troublesome.

Further, when so-called cross-term compensation for compensating for the eddy magnetic field generated between the channels of the gradient magnetic field, there is a problem that the circuit configuration becomes particularly complicated.

Furthermore, since the conventional eddy compensation circuit focuses on the eddy magnetic field having a short time constant, the rising and falling portions of the gradient magnetic field control waveform can be compensated for. However, in order to obtain a higher quality MR image, this is insufficient and it is necessary to compensate for a vortex component having a long time constant. In the case of eddy compensation using a conventional analog circuit,
When the time constant becomes longer, the accuracy of the eddy compensation decreases due to a change over time.

SUMMARY OF THE INVENTION The present invention has been made to solve such problems of the prior art, and it is a first object of the present invention to stably and accurately perform eddy magnetic field compensation.

It is a second object of the present invention to stably and accurately perform eddy magnetic field compensation and to suppress an increase in circuit scale even when so-called cross-term compensation of eddy magnetic fields is performed. .

A third object of the present invention is to automatically adjust the time constant and gain of the eddy compensation when the time constant and gain are adjusted, thereby reducing the trouble of the adjustment.

[0014]

In order to achieve the above object, a magnetic resonance system eddy compensator according to the present invention comprises a gradient magnetic field generating means for generating a gradient magnetic field superimposed on a static magnetic field. And a eddy magnetic field compensating means for compensating for an eddy magnetic field generated in accordance with the generation of the eddy magnetic field.

According to a preferred embodiment, the eddy magnetic field compensating means includes a digitizing means for sampling the control waveform data of the gradient magnetic field at regular intervals and digitizing the data, and a time constant and a gain based on the digital data. And an adding means for adding the eddy compensation data to the control waveform data.

According to another preferred example, the eddy magnetic field compensating means samples the control waveform data of the gradient magnetic field at a fixed interval and digitizes the control waveform data based on digital data to obtain a eddy magnetic field by a time constant and a gain. Computing means for computing compensation data and adding means for adding the eddy compensation data to the control waveform data are provided.

For example, the arithmetic means has a digital type integrator for generating an exponential decay waveform of the time constant.
Further, the calculation means may include a plurality of digital-type integrators for generating exponential decay waveforms for each of the plurality of different time constants as the time constant.

Further, the gradient magnetic field generating means comprises a plurality of channels of gradient magnetic field generating means, and the calculating means comprises:
The apparatus may include a self-channel operation unit for calculating the eddy compensation data for the own channel and another channel operation unit for calculating the eddy compensation data for another channel. The self-channel calculating means and the other-channel calculating means may each include a plurality of digital-type integrators each generating an exponential decay waveform for each of the plurality of different time constants as the time constant.

Preferably, the integrator is configured to continuously perform an integration operation for at least 10 seconds at a clock cycle of 4 μs. It is preferable that the integrator is an arithmetic unit that replaces the generation of the exponential decay waveform with a multiplication of the integrated data of one clock before by the time constant for each clock. Further, as another example, it is preferable that the integrator is an integrator having redundancy of 8 bits or more with respect to the control waveform data.

According to a further preferred example, measurement command means for commanding measurement of the time constant and intensity of the eddy magnetic field under a predetermined time constant and gain for compensating the eddy magnetic field, and Determining means for determining whether or not the eddy magnetic field has converged to an allowable value; and a re-measurement command for correcting the time constant and / or gain and performing the measurement again when the result of the determination is not converged. Means, the eddy magnetic field compensating means is digitized by sampling control waveform data of the gradient magnetic field used for the measurement at regular intervals and digitized, and the measurement is designated based on the digital data. It is provided with a calculating means for calculating eddy compensation data based on a time constant and a gain, and an adding means for adding the eddy compensation data to the control waveform data.

In each of the above-described configurations, for example, the eddy magnetic field is a primary eddy magnetic field component.

Further, the eddy compensation method for a magnetic resonance system according to the present invention is applied to a magnetic resonance system for generating a gradient magnetic field superimposed on a static magnetic field. The eddy compensation data is obtained by a time constant and a gain based on the digital data, and the eddy compensation data is added to the control waveform data to form a command signal for generating the gradient magnetic field.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment according to the present invention will be described below with reference to FIGS. In this embodiment, a magnetic resonance imaging (MRI) apparatus implemented as a magnetic resonance system according to the present invention will be described.
The present invention can be applied to a magnetic resonance spectroscopy (MRS) device or a magnetic resonance spectroscopic imaging (MRSI) device.

FIG. 1 shows a schematic configuration of a magnetic resonance imaging apparatus according to this embodiment. This magnetic resonance imaging apparatus functionally includes a bed portion on which a patient P as a subject is placed, a static magnetic field generating portion for generating a static magnetic field, a gradient magnetic field generating portion for adding positional information to the static magnetic field, A transmitting and receiving unit for transmitting and receiving a high-frequency signal; a control and arithmetic unit for controlling the entire system and image reconstruction; an electrocardiographic measuring unit for measuring an ECG signal as a signal representing a cardiac phase of the subject P; And a breath-hold command unit for commanding the user to hold his / her breath.

The static magnetic field generation unit includes, for example, a superconducting magnet 1 and a static magnetic field power supply 2 for supplying a current to the magnet 1, and has a cylindrical opening (diagnostic space) into which the subject P is loosely inserted. the axial direction (Z axis direction) to generate a static magnetic field H _0.
The magnet part is provided with a shim coil 14. The shim coil 14 is supplied with a current for homogenizing a static magnetic field from a shim coil power supply 15 under the control of a host computer described later. The couch part can retreatably insert the top plate on which the subject P is placed into the opening of the magnet 1.

The gradient magnetic field generator includes a gradient magnetic field coil unit 3 incorporated in the magnet 1 and a gradient magnetic field power supply 4 connected to the unit 3.

The gradient magnetic field coil unit 3 includes three sets (types) of x, y, and z coils 3x to 3z for generating gradient magnetic fields in X, Y, and Z axis directions orthogonal to each other. The gradient magnetic field power supply 4 supplies a pulse current for generating a gradient magnetic field to the x, y, and z coils 3x to 3z under the control of a sequencer 5 described later.

The waveform of this pulse current is, as described later,
The eddy compensation component according to the present invention is formed by receiving eddy compensation according to the present invention, and is superimposed on a control waveform having a waveform area corresponding to a desired control value.

The x, y, z coils 3x from the gradient magnetic field power supply 4
By controlling the pulse current supplied to ~ 3z,
The gradient magnetic fields in the three axes X, Y, and Z directions, which are physical axes, are combined, and the slice gradient magnetic field G _S , the phase encode direction gradient magnetic field G _E , and the readout direction (frequency encode direction) gradient magnetic field G _R are orthogonal to each other. Can be arbitrarily set and changed in each logical axis direction. Slice direction, phase encoding direction, and gradient magnetic fields in the readout direction are superimposed on the static magnetic field H _0.

The transmitting / receiving section includes an RF coil 7 disposed near the subject P in the imaging space in the magnet 1,
And a transmitter 8T and a receiver 8R, which are connected to each other. The transmitter 8T and the receiver 8R are connected to a sequencer 5 described later.
It operates under the control of. The transmitter 8T supplies the RF coil 7 with an RF current pulse having a Larmor frequency for causing nuclear magnetic resonance (NMR). The receiver 8R has R
The F-coil 7 receives an MR signal (high-frequency signal) such as an echo signal, and pre-amplifies the signal, intermediate frequency conversion,
After performing various kinds of signal processing such as phase detection, low frequency amplification, and filtering, A / D conversion is performed to generate digital amount of echo data (raw data) according to the MR signal.

The control / arithmetic unit includes a sequencer (also called a sequence controller) 5, a host computer 6, an arithmetic unit 10, a storage unit 11, and a display unit 1.
2 and an input device 13.

Among them, the host computer 6 controls the operation of the entire apparatus by the stored software procedure,
The sequencer 5 is instructed to execute pulse scanning by instructing pulse sequence information, and the arithmetic unit 10 is instructed with necessary information such as timing for image reconstruction. As the pulse sequence, F based on two-dimensional or three-dimensional scanning
E (gradient echo) method, FFE (fast FE) method,
SE (spin echo) method, FSE (fast SE) method, FA
A pulse train based on the SE (high-speed Asymmetric SE: imaging method in which the half-Fourier method is combined with the high-speed SE method), the EPI (echo planar imaging) method, or the like is used.

The sequencer 5 has a CPU and a memory, stores pulse sequence information sent from the host computer 6, and controls the operations of the gradient magnetic field power supply 4, the transmitter 8T, and the receiver 8R according to the information. At the same time, the echo data (digital amount) of the MR signal output from the receiver 8R is input once and transferred to the arithmetic unit 10. Here, the pulse sequence information is all information necessary for operating the gradient magnetic field power supply 4, the transmitter 8T, and the receiver 8R in accordance with a series of pulse sequences, for example, x, y, and z coils 3x to 3z. The intensity of the pulse current applied to the
It contains information about the application timing and the like.

The arithmetic unit 10 receives the echo data (raw data) output from the receiver 8R through the sequencer 5, and inputs a two-dimensional or three-dimensional k-space (both a Fourier space and a frequency space) on its internal memory. ), And the two-dimensional or three-dimensional Fourier transform is applied to each set of the echo data to reconstruct the image data in the real space. The arithmetic unit 10 can also perform a process of synthesizing data relating to the image, a difference operation process, and the like as necessary.

The storage unit 11 can store not only reconstructed image data but also image data that has been subjected to the above-described synthesizing processing and differential processing. The display 12 displays the reconstructed image. In addition, information regarding imaging conditions (imaging parameters), pulse sequences, image synthesis, and difference calculation desired by the operator can be input to the host computer 6 via the input device 13. For this reason, the input device 13 and the display device 12 form a user interface.

On the other hand, the voice generator 16 is provided as an element of the breath-hold command unit when a scan by the breath-hold method is required.
The voice generator 16 can emit a breath-hold start and a breath-hold end message as voice when instructed by the host computer 6.

Further, the electrocardiogram measuring section is provided with an ECG sensor 17 for detecting an ECG signal as an electric signal by attaching the ECG signal to the body surface of the subject, and performing various processes including digitizing process on the sensor signal to obtain a host computer. 6 and an ECG unit 18 for outputting to the sequencer 5. The measurement signal from the electrocardiogram measurement unit is used by the sequencer 5 or the host computer 6 when performing a scan by the electrocardiogram synchronization method. Thereby, the synchronization timing based on the ECG synchronization method can be set appropriately.

Next, the gradient magnetic field power supply 4 having the function of compensating for the eddy magnetic field due to the gradient magnetic field according to the present invention will be described in further detail together with its operation. Here, the eddy magnetic field to be compensated is a spatially first-order eddy magnetic field.

As shown in FIG. 1, this gradient magnetic field power supply 4
A vortex compensator 21 that receives a control magnetic field gradient waveform (waveform of a gradient magnetic field pulse based on pulse sequence information) output from the sequencer 5 and performs eddy compensation on the control waveform, D / A converters 22X to 22Z for each channel provided on the output side of the compensator 21 and gradient magnetic field amplifiers 23X to 23Z for each channel provided on the output side of the converter.

The eddy compensator 21 and the D / A converter 22
X to 22Z may be separated and independent from the gradient magnetic field power supply 4 and may be interposed between the gradient magnetic field power supply 4 and the sequencer 5 as a single unit, or may be incorporated in the sequencer 5.

As shown in FIG. 2, the eddy compensator 21 is a digital circuit including a processor equipped with a CPU for eddy compensation.
Moreover, the so-called cross-term compensation can be performed.

More specifically, sampling circuits 31X, 31Y, 31 for inputting and sampling the X-channel control waveform G _Xin , the Y-channel control waveform G _Yin , and the Z-channel control waveform G _Zin of the gradient magnetic field, respectively.
Z, and A / D converters 32X, 32Y, and 32Z disposed on the output side of each of the sampling circuits.

[0043] Furthermore A / D converter 32 _X of X channel side, eddy compensation circuit for the Y channel and the Z channel eddy compensation circuit 33 _X and another channel of the own X channel _{33 X-Y} and _{33 X-Z} Connected to each other. Similarly, A / D converter 32 _Y of the Y channel side further eddy compensation circuit for X-channel and Z-channel eddy compensation circuit 33 _Y and other channels of its own Y channel _{33 Y-X} and _{33 Y-Z} Connected to each other. Similarly, the A / D converter 32 _Z on the Z channel side
It is further respectively connected to the eddy compensation circuit 33 _Z-X and 33 _Z-Y against X channel and Y channel eddy compensation circuit 33 _Z and other channels of its own Z channels.

The nine vortex compensation circuits 33 _{X to} 3 described above.
_{3 Y, 33 X-Y,} 33 X-Z, 33 Y -X, 33
_{Y-Z, 33 Z-X} , of _{33 Z-Y,} respectively, as shown in FIG. 3, the five integrators 331A~331E for vortex compensation is configured using a processor and a register, and, in the digital circuit And one adder 332 configured to mutually add the output signals of these integrators.

The five integrators 331A to 331E have a time constant τ (τ1 to τ5) and a gain g (g1 to g5), respectively.
Are set independently. Time constants τ1 to τ5 and gain g
1 to g5 are set in accordance with the decay state of the eddy magnetic field. In order to accurately perform eddy compensation, a plurality of types of independently settable constants and gains are set rather than a single set of time constant and gain. It is desirable to be able to set pairs. The gain g is an amount recognized as a contribution ratio of the attenuation component having a certain time constant to the control waveform height.

Therefore, each of the integrators 331 (331A to 331A)
31E) is obtained by using the given time constant τ (τ1 to τ5) and gain g (g1 to g5), “control input × τ ^{n · s}
× g ”to obtain an exponential decay waveform component as an eddy compensation component. Here, n is the number of calculations per second, and s is the continuation time for calculating the eddy compensation. As an example, when the operation clock cycle = 4 μs, n = 250,000
And the duration s at that time is set to, for example, s = 3 seconds.

However, each integrator 331 performs a multiplication equivalent to this operation instead of performing the above-described operation for each clock. That is, at the first clock 1, “control input × τ × g
(= M1) ”is changed to“ M1 × τ (= M
2) is changed to “M2 × τ (= M3)” at the next clock 3.
At the next clock 4, “M3 × τ (= M4)”
In such a manner, while the value calculated by the previous clock is stored in the register, the multiplication of multiplying this value by the time constant τ is repeated “ns” times in the next clock.
Thus, when the control input is a rectangular wave, the correction amount basically expressed by the exponential decay function can be obtained more easily, quickly, and continuously.

The purpose of this embodiment is to compensate for the spatially primary eddy magnetic field, including the eddy magnetic field component having a long time constant. It is desirable to carry out continuously for 3 seconds, preferably about 10 seconds. Therefore, when each integrator 331 is configured by a fixed-point processor, the time constant τ is
To calculate continuously for 3 seconds while retaining the data of the previous clock, the clock cycle = 4 μs (250,000 times /
), Each integrator 331 has a total of 26 bits (1
A bit length of 8 bits + 8 bits of error (redundancy) is set. To calculate this continuously for 10 seconds, a bit length of 28 bits is set in each integrator 331 as a whole. As a result, a sufficient number of bits for compensating the eddy magnetic field component having a long time constant, that is, the calculation accuracy is secured.

Although each integrator 331 requires software, it can be constituted by a floating point type processor. In this case, it is more advantageous than the fixed-point method in terms of suppressing the circuit scale.

On the other hand, the gain g in each integrator 331
Is an amount that is closely entangled with the hard surface, and varies depending on how the coil is wound. Generally, since it is within about 3% of the control waveform height, about 14 bits are allocated.

As a result, in each eddy compensation circuit 33, five
Time constants given by the integrators 331A to 331E
τ (τ1 to τ5) and gain g (g1 to g5)
The eddy compensation components CP1 to CP5 represented by exponential decay waveforms perform
Is calculated. The eddy compensation components CP1 to CP5 are added to an adder 33.
2 and one eddy compensation component G_ed
dy(G_{eddy (x)}, G_{eddy (Y)}, G
_{eddy (Z)}, G_{eddy ( xy)}, G
_{eddy (XZ)}, G_{eddy (YX)}, G
_{eddy (YZ )}, G_{eddy (ZX)}, G
_{eddy (ZY)}). As a result,
Channel eddy compensation circuit 33_X, 33_Y, 33_ZFrom the tilt
Eddy magnetic field generated in own channel by gradient magnetic field control waveform
Of eddy compensation component for compensating (cancelling) the effect of vortex
A value is generated. Meanwhile, cross-term channel compensation
Circuit 33G _xy, 33_X-Z, 33_Y-X, 33
_YZ, 33_Z-X, 33_ZYFrom your own channel
Vortices generated in other channels by the gradient magnetic field control waveform of
Generates eddy compensation component command values to compensate for magnetic field effects
Is done.

Returning to the eddy compensator 21 shown in FIG. 2, as shown in the figure, three digital type adders 34X to 34Z are provided on the output side.

Of these, the first adder 34X has an A / D
Converters 32X and three eddy compensation circuit ₃₃ X, 3
3 _Y-X, the output signal of the _{33 Z-X} are input, the signal _{G Xout} is generated this four inputs are added together. This signal G
_Xout is returned as an analog signal by a D / A converter 22X at the subsequent stage as a control waveform signal in which the eddy of the gradient magnetic field of the X channel is compensated, and then output to the gradient magnetic field amplifier 23X. As a result, a gradient magnetic field pulse current is applied to the x coils 3x, 3x from the gradient magnetic field amplifier 23X based on a predetermined pulse sequence and eddy compensated for the eddy magnetic field of its own channel and the eddy magnetic field from another channel.

Similarly, A / D is added to the second adder 34Y.
A converter 32Y and three vortex compensation circuits 33 _XY , 3
Output signals of 3 _Y and 33 _Z−Y are input, and the four inputs are added to generate a signal G _Yout . This signal G
_Yout is returned as an analog signal by the subsequent D / A converter 22Y as a control waveform signal in which the eddy of the gradient magnetic field of the Y channel is eddy, and then output to the gradient magnetic field amplifier 23Y. As a result, a gradient magnetic field pulse current based on a predetermined pulse sequence and eddy-compensated for the eddy magnetic field of the own channel and the eddy magnetic field from another channel is applied to the x coils 3y and 3y from the gradient magnetic field amplifier 23Y.

Similarly, the third adder 34Z includes an A / D converter 32Z and three eddy compensation circuits 33 _XZ ,
Output signals of 33 _YZ and 33 _Z are input, and these four inputs are added to generate a signal G _Zout . This signal G
_Zout is returned to the analog signal by the D / A converter 22Z at the subsequent stage as a control waveform signal in which the eddy of the gradient magnetic field of the Z channel is compensated, and then output to the gradient magnetic field amplifier 23Z. As a result, the gradient magnetic field pulse current based on the predetermined pulse sequence and eddy-compensated for the eddy magnetic field of the own channel and the eddy magnetic field from another channel is applied to the z coils 3z, 3z from the gradient magnetic field amplifier 23Z.

As a result, the spatial primary eddy magnetic field due to the gradient magnetic field pulses generated from the gradient magnetic field coils 3x to 3z of the X, Y and Z channels can be extremely effectively changed by the application of the eddy magnetic field compensation component described above. Is suppressed. Therefore, the influence of the first-order eddy magnetic field included in the acquired MR signal is extremely reduced, and the image quality is significantly improved.

According to this embodiment, since the eddy magnetic field is spatially compensated for the primary eddy magnetic field as described above, in addition to improving the image quality by suppressing the eddy magnetic field, a conventional analog type eddy compensator is used. Various remarkable advantages can be enjoyed as compared with the case where. That is, since the eddy compensator is constituted by a digital processing circuit, changes and variations due to the temperature of the circuit element are smaller than those of the analog element, and stable operation can be performed. In addition, since it is configured with a digital processing circuit, even when performing so-called cross-term compensation, which compensates for eddy magnetic fields generated between channels of the gradient magnetic field, the circuit configuration and circuit size are larger than when an analog circuit is used. Increase and complexity can be suppressed.

Further, by using a digital processing circuit, the gradient magnetic field control waveform is sampled at regular intervals and digitized, and based on this digital data, an eddy compensation component represented by an exponentially decreasing waveform is accurately calculated. Because it can perform continuous calculations well, it does not suffer from deterioration of compensation accuracy due to aging as seen in conventional analog circuits, and compensates for eddy components with long time constants with high accuracy can do. Thereby, the power of improving the image quality of the MR image based on eddy compensation can be doubled.

Although there is no particular swing in the above-described embodiment, automation of eddy compensation can be achieved by causing the sequencer 5 to execute the processing outlined in FIG.

Specifically, the sequencer 5 activates a predetermined adjustment pulse sequence (step S1 in FIG. 4) at the time of eddy adjustment when the magnetic resonance imaging apparatus is installed or maintenance and inspection is performed. 7 and the MR signals obtained via the receiver 8R are collected (step S
2). Next, the sequencer 5 calculates an eddy magnetic field (intensity and time constant) from the MR signal by using a conventionally known method (step S3). Next, by determining whether or not the calculated eddy magnetic field falls within a predetermined threshold range, it is determined whether or not the eddy magnetic field has converged to an allowable value (step S4). If the result of this determination is that they have not converged, another time constant τ and gain g are corrected (step S
5). The adjustment sequence is started again, and the above processing is repeated. During this repetition, the eddy magnetic field can be automatically converged to an allowable value.

As a result, there is no need to manually adjust the time constant and the gain for eddy compensation as in the prior art during installation and maintenance and inspection, and the labor involved can be greatly reduced.

If the sequencer 5 is configured to output pulse sequence information as a digital signal having a clock cycle required by the eddy compensator, the sampling circuit and the A / D converter for each channel shown in FIG. The D converter can be omitted.

Further, in the above-described embodiment, an example has been described in which the first-order eddy magnetic field is spatially compensated as the eddy magnetic field. However, this compensation method can be applied to the zero-order eddy magnetic field. . In the case of the zero-order eddy magnetic field, such digital processing may be performed in the same manner, and the control amount may be set to the center frequency f ₀ and the phase of the carrier frequency handled by the transceiver.

It should be noted that the present invention is not limited to the configuration of the above-described embodiment, and it is needless to say that various modifications can be made based on the gist of the claims.

[0065]

As described above, according to the present invention,
Since the eddy compensation component for compensating the eddy magnetic field is generated by digital processing, the eddy magnetic field compensation can be performed stably and accurately. Further, even when so-called cross-term compensation of the eddy magnetic field is performed, an increase in circuit scale and complexity can be suppressed. Furthermore, by using the eddy measurement and eddy compensation techniques, the time constant and gain of eddy compensation can be automatically adjusted during installation and maintenance, greatly reducing the time and effort required for adjustment. it can.

[Brief description of the drawings]

FIG. 1 is a schematic block diagram illustrating an example of a configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention.

FIG. 2 is a functional block diagram showing an outline of eddy compensation centering on an eddy compensator.

FIG. 3 is a functional block diagram showing each eddy compensation circuit more specifically.

FIG. 4 is a flowchart schematically showing an automatic vortex compensation routine;

[Explanation of symbols]

 Reference Signs List 1 magnet 3 gradient magnetic field coil unit 4 gradient magnetic field power supply 5 sequencer 6 host computer 7 RF coil 8T transmitter 8R receiver 10 arithmetic unit 11 storage unit 13 input device 21 vortex compensator 22X-22Z D / A converter 31X-31Z sampling Circuit 32X-32Z A / D converter 33 Eddy compensation circuit 34X-34Z Adder 331A-331E Integrator 332 Adder

Claims (13)

[Claims] 1. An eddy compensator for a magnetic resonance system having a gradient magnetic field generating means for generating a gradient magnetic field superimposed on a static magnetic field, wherein the eddy magnetic field compensation means for compensating for the eddy magnetic field generated with the generation of the gradient magnetic field is provided. A vortex compensator for a magnetic resonance system, wherein the eddy magnetic field compensating means is configured to process the digital signal. 2. The eddy compensator for a magnetic resonance system according to claim 1, wherein the eddy magnetic field compensating means samples the control waveform data of the gradient magnetic field at a constant interval and digitizes the data. An eddy compensator for a magnetic resonance system, comprising: an arithmetic means for calculating eddy compensation data based on a time constant and a gain based on data; and an adding means for adding the eddy compensation data to the control waveform data. 3. The eddy compensator for a magnetic resonance system according to claim 1, wherein said eddy magnetic field compensating means samples control waveform data of said gradient magnetic field at regular intervals and digitizes the data based on digital data. An eddy compensator for a magnetic resonance system, comprising: an arithmetic means for calculating eddy compensation data based on a constant and a gain; and an adding means for adding the eddy compensation data to the control waveform data. 4. An eddy compensator for a magnetic resonance system according to claim 2, wherein said calculating means has a digital type integrator for generating an exponential decay waveform of said time constant. 5. The eddy compensator for a magnetic resonance system according to claim 2, wherein said arithmetic means generates a plurality of digital type integrators for generating exponential decay waveforms for each of a plurality of different time constants as said time constant. Eddy compensator for a magnetic resonance system having a magnetic field. 6. The eddy compensator for a magnetic resonance system according to claim 2, wherein said gradient magnetic field generating means comprises a plurality of channels of gradient magnetic field generating means, and said arithmetic means outputs the eddy compensation data for its own channel. An eddy compensator for a magnetic resonance system, comprising: a self-channel calculating means for calculating; and another-channel calculating means for calculating the eddy compensation data for another channel. 7. The eddy compensator for a magnetic resonance system according to claim 2, wherein the self-channel operation unit and the other-channel operation unit each have an exponential decay for each of the plurality of different time constants as the time constant. An eddy compensator for a magnetic resonance system having a plurality of digital-type integrators for generating a waveform. 8. The eddy compensator for a magnetic resonance system according to claim 4, 5 or 7, wherein the integrator has at least 10 clock cycles of 4 μs.
An eddy compensator for a magnetic resonance system configured to continuously perform integral operations for seconds. 9. The eddy compensator for a magnetic resonance system according to claim 4, 5 or 7, wherein the integrator converts the generation of the exponential attenuation waveform into integrated data one clock before every clock. An eddy compensator for a magnetic resonance system, which is an arithmetic unit that is replaced by multiplication by a time constant. 10. The vortex compensator for a magnetic resonance system according to claim 4, wherein the integrator has at least 8 bits of redundancy for the control waveform data. Eddy compensator for magnetic resonance system, which is an integrator. 11. The eddy compensator for a magnetic resonance system according to claim 1, wherein a measurement commanding a measurement of the time constant and intensity of the eddy magnetic field under a predetermined time constant and gain for compensating the eddy magnetic field. Commanding means; determining means for determining whether or not the eddy magnetic field has converged to an allowable value based on the measurement result; and when the result of the determination has not converged, correcting the time constant and / or the gain. Re-measurement command means for executing the measurement again, the eddy magnetic field compensation means digitizes control waveform data of the gradient magnetic field used for the measurement by sampling at regular intervals, and digitizing the digital data. A magnetic means comprising: arithmetic means for calculating eddy compensation data based on the time constant and gain designated for the measurement based on the measurement; and addition means for adding the eddy compensation data to the control waveform data. Vortex compensator tinnitus systems. 12. The eddy compensator for a magnetic resonance system according to claim 1, wherein the eddy magnetic field is a primary eddy magnetic field component. 13. A vortex compensation method for a magnetic resonance system in which a gradient magnetic field superimposed on a static magnetic field is generated, wherein control waveform data of the gradient magnetic field is sampled at regular intervals and digitized, and based on the digital data. An eddy compensation method for a magnetic resonance system, comprising: performing eddy compensation data based on a time constant and a gain; and adding the eddy compensation data to the control waveform data to form a command signal for generating the gradient magnetic field.