JPH10201730A - Magnetic resonance imaging system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the uniformity of magnetic fields to make the image resolution better in measuring any cross-section by calculating the current values to be run into each channel of the magnetostatic correctors to impress them thereunto, based both on the distribution of magnetostatic fields obtained via the magnetostatic field distribution measuring instrument and the characteristics obtained via the characteristics measuring instrument. SOLUTION: The computer 318 controls the sequencer 307 to execute the pulse sequence to calculate the distribution of magnetostatic fields. Impressing a current into a specified channel of the system coil 318 obtains a distribution of magnetostatic fields. Then, repeating this operation by changing the channels into where a current is impressed at a time, obtains a distribution of magnetostatic fields for each channel when a current is impressed thereunto. The system characteristics required for correcting the magnetostatic fields are obtained from a difference between the distribution of magnetostatic fields, when a current is impressed into a specified channel and that in case of impressing no current into the system coil 318. Controlling the system's power supply to impress a current into the system coil 318 executes correcting the magnetostatic fields.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance diagnostic apparatus (hereinafter, referred to as "MRI apparatus"), and more particularly to an MRI apparatus characterized in correcting static magnetic field inhomogeneity.

[0002]

2. Description of the Related Art In recent years, an MRI apparatus has become an important diagnostic means for a disease as well as an X-ray CT as an image diagnostic apparatus having an excellent ability to depict a tissue. In this MRI apparatus, since the non-uniformity of the static magnetic field distribution is one of the causes of distortion of an image and a spectrum, the uniformity of the static magnetic field distribution is required. In particular, when performing high-speed imaging such as the echo planar imaging (EPI) method, a high static magnetic field uniformity is required. In the case of high-precision calculations such as spectroscopic images, in order to reduce the non-uniformity of the static magnetic field caused by inserting the subject into the magnetic field, the non-uniformity of the static magnetic field for each subject is reduced. Correction is needed.

[0003] In order to correct the non-uniformity of the static magnetic field, a passive shim in which a magnetic material is arranged or an active shim for adjusting a current flowing through a magnetic field correction coil (shim coil) is used. Quality of the static magnetic field adjustment directly affects the image quality.

[0004] The shim coil is usually composed of three or more channels of static magnetic field generating coils for generating magnetic fields orthogonal to each other, for example, magnetic fields corresponding to each term of the spherical harmonic function. Such adjustment of the static magnetic field by the shim coil is performed by superimposing a static magnetic field generated by the main coil and an additional static magnetic field generated by applying a current to each shim coil. That is, in order to correct the non-uniformity of the static magnetic field, it is necessary to find a combination in which the current value (the shim current value) flowing through the shim coil gives the optimum static magnetic field distribution. Done in steps.

First, the characteristics (shim characteristics) of the shim coil are represented by M
As a value unique to the RI apparatus, it is measured in advance using a homogeneous sample such as water. This shim characteristic is a change in the static magnetic field distribution per unit current flowing in the shim coil. When the current flowing in the shim coil k is changed by a small amount ΔIk, the static magnetic field at each position (denoted as “pixel j”) is changed. Change amount ΔB
By calculating j, the characteristic of the shim coil k for each pixel j can be obtained.
Can be represented by a matrix.

(Equation 1) Such shim characteristics are stored as a matrix for each slice thickness, slice interval, and slice position, or in the form of an expansion coefficient developed by a spherical harmonic function.

Then, the subject is inserted into the magnetic field to measure the static magnetic field distribution. As shown in an example in FIG. 3, the distribution of the static magnetic field is often obtained from the phase information of the echo signal obtained by reversing the gradient magnetic field.

In this method, a 90 ° radio frequency pulse 201 (hereinafter, referred to as a “90 ° pulse”) is applied along with the application of a slice gradient magnetic field Gs 203 for slice selection, and nuclear spins (hereinafter, referred to as “90 ° pulses”) constituting a subject tissue. (Referred to simply as spin), and a phase encoding gradient magnetic field Gp205 is applied to change the phase of the spin. After τ hours from the 90 ° pulse, a 180 ° high-frequency pulse 202 (hereinafter referred to as “180 ° pulse”) is applied together with the slice gradient magnetic field Gs204. Thereafter, a readout gradient magnetic field Gr206 is applied, and subsequently, an inversion pulse 207 of the readout gradient magnetic field Gr206 is applied to generate a spin echo (not shown). In this case, the reversal timing of the read-out gradient magnetic field Gr is such that the interval 212 between the 180 ° pulse and the point at which the echo signal is maximized is the interval 211 (= τ) between the 90 ° pulse and the 180 ° pulse, and a very short time. Adjust so that it differs by ε. When a time τ elapses after the application of the 180 ° pulse, the phase change of the spin caused by the non-uniformity of the static magnetic field immediately after the application of the 90 ° pulse is completely canceled, and thus the image S (x, y,
z) contains only the phase information generated by the non-uniformity of the static magnetic field during the minute time ε. This image S (x, y, z)
The real and imaginary parts of Sr (x, y, z) and Si
Expressed as (x, y, z), the static magnetic field inhomogeneous distribution E (x,
y, z) can be represented by equation (2).

(Equation 2) In the equation (2), γ represents a gyromagnetic ratio.

After obtaining the static magnetic field distribution in this manner, a shim current value is calculated using shim characteristics measured in advance. The shim current value needs to generate a magnetic field having the same magnitude in the opposite direction to cancel the static magnetic field inhomogeneity, and can be obtained by solving equation (1).
Then, by passing the calculated current to each shim coil,
Correct static magnetic field inhomogeneity.

[0009]

However, the above M
When the shim characteristics are prepared as a matrix in the RI apparatus, it is actually impossible to prepare a characteristic matrix corresponding to every slice thickness, slice interval, and slice position.
Therefore, in a section where shim characteristics are not prepared, it is not possible to calculate a current value flowing through each shim coil. Therefore, the cross section in which the non-uniformity of the static magnetic field can be corrected is limited to the section in which the characteristic matrix of the shim coil is obtained.

When the characteristics of the shim coil are prepared as the expansion coefficient, a calculation error generated at the time of expansion is included. For this reason, even if the magnetic field uniformity is adjusted, the magnetic field is not completely uniform due to a calculation error. In particular, when a sequence or the like that reflects the magnetic field inhomogeneity is used, the image is deteriorated. There is.

According to the present invention, the magnetic field uniformity is high in the measurement of any cross section by improving such a conventional apparatus.
It is another object of the present invention to provide an MRI apparatus having good image resolution.

[0012]

In order to achieve the above object, the MRI apparatus of the present invention performs static magnetic field correction for each slice to be imaged, and performs shim characteristics necessary for the correction during the static magnetic field correction. Is to be measured.

That is, the MRI apparatus of the present invention comprises: a static magnetic field generating means for applying a static magnetic field to the subject; a static magnetic field correcting means for correcting the non-uniformity of the static magnetic field;
A gradient magnetic field generating means for applying a frequency encoding gradient magnetic field and a phase encoding gradient magnetic field, a transmission system for irradiating the subject with a high-frequency pulse for causing nuclear magnetic resonance in nuclei of atoms constituting a living tissue of the subject, and a gradient magnetic field generation A sequencer that controls the means and the transmission system and repeatedly applies each gradient magnetic field and a high-frequency pulse in a predetermined pulse sequence, a detection system that detects a nuclear magnetic resonance signal as a measurement signal, and a signal that is detected by the detection system as data. A central control system for controlling the entire apparatus as well as image reconstruction, input means for inputting conditions to the central control system, and output means for displaying and storing data processed by the central control system And a static magnetic field distribution measuring means for measuring the inhomogeneity of the static magnetic field distribution, and a characteristic measuring means for measuring characteristics of the static magnetic field correcting means. The static magnetic field distribution measuring means obtains a static magnetic field distribution from a pair of echo signals generated before and after a predetermined minute time with reference to the time when the echo time has elapsed, and the characteristic measuring means is provided for a specific channel of the static magnetic field correcting means. A current is sequentially applied, and a correction characteristic is obtained from a difference between a static magnetic field distribution when a current is applied to a specific channel and a static magnetic field distribution when no current is applied to the static magnetic field correction means. Based on the static magnetic field distribution obtained by the distribution measuring means and the characteristics obtained by the characteristic measuring means, the value of the current flowing through the static magnetic field correcting means is calculated, and the current is applied to the static magnetic field correcting means.

In the case where the static magnetic field correction means has a plurality of channels, the characteristic measuring means for measuring the characteristics of the static magnetic field correction means sequentially supplies current to a specific channel of the static magnetic field correction means when measuring the static magnetic field distribution. Is applied, and a correction characteristic is obtained for each channel from a difference between a static magnetic field distribution when a current is applied to a specific channel and a static magnetic field distribution when no current is applied to the static magnetic field correction unit. Based on the static magnetic field distribution obtained by the static magnetic field distribution measuring means and the correction characteristics obtained by the characteristic measuring means, a current value flowing through each channel of the static magnetic field correcting means is calculated, and a current is applied to the static magnetic field correcting means. Things.

Preferably, the sequencer applies a 180 ° high-frequency pulse before applying a current to the static magnetic field correction means. Also, the characteristic measuring means may apply a current applied to each channel of the static magnetic field correcting means to a first current having an opposite polarity.
And a second pulse. At this time, the time integral of the current of each polarity is set to zero. Further, the characteristic measuring means continuously applies the pulse of the first polarity to be applied to the static magnetic field correcting means at least while acquiring a pair of echoes used by the static magnetic field distribution measuring means to obtain the static magnetic field distribution. Preferably, it is

In the above-mentioned MRI apparatus of the present invention, the static magnetic field distribution measuring means firstly generates a pair of static magnetic field distributions generated before and after a predetermined minute time with reference to the lapse of the echo time in a state where no current is applied to the static magnetic field correcting means. The static magnetic field distribution is obtained from the echo signal of. Next, a current is applied to a specific channel of the static magnetic field correction means, and a static magnetic field distribution is obtained from a pair of echo signals in the same manner as described above. The channel to which the current is applied is sequentially changed, and the same is repeated. The static magnetic field distribution is obtained for the case where the current is applied to each channel.

Then, a correction characteristic required for performing the correction is obtained from a difference between the static magnetic field distribution when the current is applied to the specific channel and the static magnetic field distribution when the current is not applied to the static magnetic field correction means. After obtaining the correction characteristics, a predetermined calculation is performed, and the static magnetic field is corrected by the static magnetic field correction means. Since the correction characteristic is measured each time the static magnetic field correction is performed, nonuniformity of the static magnetic field distribution can be corrected with high accuracy.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An MRI apparatus according to the present invention will be described below with reference to embodiments. FIG. 2 shows a schematic configuration of an embodiment of an MRI apparatus to which the present invention is applied.
The RI apparatus includes a static magnetic field generating system 302 that is a static magnetic field generating unit that applies a static magnetic field to a subject, and a static magnetic field correcting unit (a shim coil 318 and a shim power supply 31) that corrects a static magnetic field inhomogeneity.
9), a gradient magnetic field generation system 303 which is a gradient magnetic field generating means for applying a slice-direction gradient magnetic field, a frequency encoding gradient magnetic field, and a phase encoding gradient magnetic field to the subject, and nuclear magnetic nuclei of atoms constituting a living tissue of the subject. A transmission system 304 for irradiating a subject with a high-frequency pulse causing resonance,
A sequencer 307 that controls the gradient magnetic field generation system 303 and the transmission system 304 to repeatedly apply each gradient magnetic field and high frequency pulse in a predetermined pulse sequence, and a detection system that detects a nuclear magnetic resonance signal generated from the subject as a measurement signal. 305, a computer 308 which is a central control system for reconstructing an image by using a signal detected by the detection system 305 as data, and controlling the entire apparatus, and a computer 30
8 has an input system 321 as input means for inputting conditions, and an output system 306 as output means for displaying and storing data processed by the computer 308.

In the above MRI apparatus, the static magnetic field generation system 3
02 is composed of an electromagnet or a permanent magnet. The static magnetic field correction means for correcting the nonuniformity of the static magnetic field generated by the static magnetic field generation system includes a shim coil 318 having a plurality of channels.
And a shim power supply 319 for applying a current to each channel of the shim coil 318. This shim coil 3
Reference numeral 18 generally has about 18 channels.

The gradient magnetic field generation system 303 has orthogonal x, y
Gradient coil 30 that generates gradient magnetic fields Gx, Gy, and Gz whose magnetic field strength changes linearly in three directions of z and z.
9a and 9b, and a gradient magnetic field power supply 310 for supplying a current thereto.

The transmitting system 304 includes a transmitting coil 314a for irradiating the subject with a high-frequency magnetic field, a synthesizer 311 for generating a high-frequency signal to be applied to the transmitting coil 314a, a modulator 812 for modulating the high-frequency signal under the control of the sequencer 307, and Amplifier 313 for amplifying the modulated signal
It has. Further, the detection system 305 includes a detection coil 314b for detecting a magnetic resonance signal generated from the subject,
An amplifier 315 that amplifies the detection signal, a quadrature phase detector 316 that performs quadrature phase detection on the amplified signal, and an A / D converter 3 that converts the analog output to a digital signal.
17 is provided. Note that the transmission coil 314a and the detection coil 314b may be provided separately as in this embodiment, but may be used for both transmission and reception.

A keyboard 322 and a mouse 323 are provided as an input system 321, and a read-only memory (ROM) 324 for storing data during calculation or final data, and a random access memory (RA) are provided as an output system 306.
M) 325, a magnetic disk 326, and a magneto-optical disk 327, and a display 328 for displaying image processing data corresponding to the spin density distribution, relaxation time distribution, spectrum distribution, and the like.

In the MRI apparatus having such a configuration,
After various measurement conditions are input from the keyboard 322 to the computer 308, the computer 308 controls and operates the entire apparatus according to these conditions.

In accordance with a command from the computer 308, the sequencer 307 controls the operation timing of the three gradient magnetic field power supplies 310 and superimposes the static magnetic field applied by the static magnetic field generation system 302 on the gradient magnetic field coils 309a and 309.
A gradient magnetic field is applied by b. On the other hand, in the transmission system 304, the signal output from the synthesizer 311 is modulated by the modulator 812 whose operation timing is controlled by the sequencer 307, and then amplified by the amplifier 313. When this signal flows through the transmission coil 314a, the subject is irradiated with a predetermined pulsed high-frequency magnetic field.

By the operation of the sequencer 307 controlled by the computer 308, the high-frequency pulse RF,
A predetermined pulse sequence of the magnetic field by the gradient magnetic fields Gs, Gp, and Gr is executed.

A resonance signal is generated from the subject irradiated with these magnetic fields, and detected by the detection coil 314 b of the detection system 305. The detected signal is amplified by an amplifier 315 and then divided into two streams by a quadrature detector 316.
Each signal is converted into digital measurement data by the A / D converter 317. At this time, the sequencer 307 controls the data collection timing of the A / D converter 317. This measurement data is input to the computer 308, and the image is reconstructed by two-dimensional Fourier transform or the like. The reconstructed image is displayed on the display 328 of the output system 306, and is stored as processing data in an external storage medium such as a magnetic disk 326 or a magneto-optical disk 327.

Further, the MRI apparatus has a static magnetic field generation system 3
02, a static magnetic field distribution measuring means (not shown) for measuring the distribution of the static magnetic field generated by the sensor 02, a characteristic measuring means (not shown) for measuring the shim characteristic of the shim coil 318, and the static magnetic field distribution and the shim characteristic. Calculating means (not shown) for calculating a shim current value to be applied to each channel of the shim coil 318 based on the following formulas. These means are executed by the computer 308 in this embodiment.

For this purpose, the computer 308 controls the sequencer 307, executes a predetermined pulse sequence described later, and measures the static magnetic field distribution (static magnetic field distribution measuring means).
In addition, a current is applied to a predetermined channel of the shim coil 318 to obtain a static magnetic field distribution. At this time, the current application channel is sequentially changed and repeated, and the static magnetic field distribution when the current is applied to each channel is obtained. Then, from the difference between the static magnetic field distribution when a current is applied to the specific channel and the static magnetic field distribution when no current is applied to the shim coil 318, a shim characteristic necessary for performing the static magnetic field correction is obtained (characteristic measuring means). .

Each time the static magnetic field correction is performed, the shim characteristic is measured, and based on the obtained static magnetic field distribution and the shim characteristic, the correction calculating means calculates a current value flowing through each channel of the shim coil 318 ( Calculation means). Computer 30
8, the sequencer 307 controls the shim power supply 319 to apply a current to the shim coil 318, whereby the static magnetic field correction is performed.

Next, a pulse sequence for static magnetic field correction and shim characteristic measurement in the MRI apparatus of the present invention will be described with reference to FIG. In this example, a case where the number of channels of the shim coil 318 is n and an application to two-dimensional imaging will be described for simplicity.

First, a 90 ° pulse 101 is applied to excite spins in a slice together with a slice selection gradient magnetic field Gs105 for slice selection. The slice selection gradient magnetic field Gs105 is a slice selection gradient magnetic field Gs1.
06 and the slice selection gradient magnetic field Gs107 inverted for the rephase, and the areas of the slice selection gradient magnetic field Gs106 and the slice selection gradient magnetic field Gs107 applied after the center of the 90 ° pulse are equalized. . Subsequent to the application of the slice selection gradient magnetic field Gs106, a phase encoding gradient magnetic field Gp111 is applied to change the phase of the spin in the phase encoding direction.

Thereafter, the slice selection gradient magnetic field Gs108
With 180 to reverse the spins in the slice
゜ Apply the pulse 102. At this time, 90 ° pulse 10
The time from the center of 1 to the center of the 180 ° pulse 102 is set to の of the echo time (TE).

Next, the readout gradient magnetic field Gr114
And a readout gradient magnetic field Gr115 obtained by inverting the readout gradient magnetic field Gr115
And the readout gradient magnetic field Gr11 reversed once more
6 and a readout gradient magnetic field Gr113 composed of the readout gradient magnetic field Gr114 and the readout gradient magnetic field Gr116 at the respective centers thereof.
23 and the echo 124 are acquired. At this time, the center of the readout gradient magnetic field Gr115 is 180 ° pulse 10
The center of the readout gradient magnetic field Gr114 is Δt time before the center of the readout gradient magnetic field Gr115, and the center of the readout gradient magnetic field Gr116 is the readout gradient magnetic field Gr11.
It corresponds to Δt time after the center of No. 5. Also, the area of the readout gradient magnetic field Gr114 applied at the time Δt before and the area of the readout gradient magnetic field Gr115 are equal with respect to the center of the readout gradient magnetic field Gr115, and the readout gradient magnetic field Gr115 applied at the time Δt is equal to the readout gradient magnetic field Gr115. The area of the out gradient magnetic field Gr116 is made equal. As a result, a pair of echo signals is generated before the time Δt and after the time Δt with reference to the center of the readout gradient magnetic field Gr115, that is, the time when the TE time when the 90 ° pulse 101 is applied.

Subsequently, after the TE time when the 180 ° pulse 102 is applied, the 180 ° pulse 103 and the slice selection gradient magnetic field Gs109 are simultaneously applied, and after TE / 2 hours, the readout gradient magnetic field Gr117 is changed to the readout gradient magnetic field. Echo 125 is applied in the same manner as Gr113.
And the echo 126. At this time, the magnetic field 119 is also applied by the channel of the shim coil (hereinafter, referred to as “shim channel”) 1 together with the readout gradient magnetic field Gr117. The magnetic field 119 includes a magnetic field 120 generated when a current ΔI1 flows through the shim channel 1,
It comprises a reversed magnetic field 121 generated when a current in the opposite direction is applied, and the magnetic field 120 is applied at least during the application of the readout gradient magnetic field Gr117,
The pulse width of the magnetic field 121 is shorter than that of the magnetic field 120. In addition, by making the areas of the magnetic field 120 and the magnetic field 121 equal and canceling the influence of the magnetic field generated by flowing a current through the shim channel 1, the same measurement can be performed by another shim channel performed later. Thus, an echo 125 and an echo 126 when a constant current ΔI1 flows in the shim channel 1 are obtained.

Similarly, a 180 ° pulse is applied at intervals of TE, and TE /
Applying three readout gradient magnetic fields centered on the two subsequent time points and applying currents to different shim channels at the time of applying the three readout gradient magnetic fields to obtain two echo signals are repeated. , For n shim channels, 2n for a total of 2 (n
+1) echo signals are obtained.

Such a sequence is repeated while changing the number of phase encodes of the phase encoding gradient magnetic field 111, and a plurality of echo signals such as 64, 128, etc. are acquired, so that an echo necessary for imaging is obtained for each echo. Is obtained. 2 (n + 1) images are obtained by grouping the 2 (n + 1) echo signals acquired in the above sequence for each phase encoding and performing a two-dimensional Fourier transform.

Next, a method of obtaining the static magnetic field distribution and the shim characteristics for each channel from the data of these 2 (n + 1) images will be described.

First, a static magnetic field distribution is obtained from a pair of echo signals 123 and 124 obtained when no current is applied to the shim coil. This method is the same as the conventional method for measuring the static magnetic field distribution shown in FIG. 3, which is a method for obtaining the static magnetic field distribution from the phase information of the echo signal.
Images F10 (x, y, z) from 23 and 124, respectively
And F20 (x, y, z). The ratio F20 (x,
When the real and imaginary parts of (y, z) / F10 (x, y, z) are represented by Sr0 (x, y, z) and Si0 (x, y, z), respectively, From equation (3), the static magnetic field distribution E0
(X, y, z) is determined.

(Equation 3) This is the static magnetic field distribution to be corrected.
This is indicated by p0. In FIG. 1, what is expressed as map is substantially the same as the static magnetic field distribution E (x, y, z). Further, when obtaining the static magnetic field distribution by the equation (3), the processing of the equation (3) was performed in order to make the discontinuous phase distribution caused by the phase change exceeding the range of -π to π continuous. Thereafter, a known phase unrubbing process may be added as necessary.

Next, a pair of echo signals 125 and 126 obtained when the current ΔI1 is supplied to the shim channel 1
From images F11 (x, y, z) and F21, respectively.
(X, y, z) and the ratio F21 (x, y, z) /
A static magnetic field distribution E1 (x, y, z) is obtained from the real part Sr1 (x, y, z) and the imaginary part Si1 (x, y, z) of F11 (x, y, z) by Expression (2). . This is map1, which is a static magnetic field distribution when the current ΔI1 flows through the shim coil 1. If the difference between E0 (x, y, z) and E1 (x, y, z) is taken for each pixel, the characteristics of the shim coil when the current flowing through the shim channel 1 is changed by a small amount ΔI1 (the shim characteristics). 1) is obtained. That is, the shim characteristic 1 indicates the amount of change ΔBj of the static magnetic field at the pixel j when the current ΔI1 flows through the shim channel 1.

Similarly, a pair of echo signals 125 and 12 obtained when a current ΔI 2 is supplied to the shim channel 2
6 to F12 (x, y, z) and F22, respectively.
(X, y, z) is obtained and the ratio F22 (x, y, z)
/ F12 (x, y, z) from the real part Sr2 (x, y, z) and the imaginary part Si2 (x, y, z) from the static magnetic field distribution E2 (x,
y, z) are determined. This is map2, which is the static magnetic field distribution when the current ΔI2 flows through the shim channel 2. Shim characteristics can be obtained from the difference between E0 (x, y, z) and E2 (x, y, z) as described above.

As described above, if the characteristics are obtained for all of the n shim channels and applied to the equation (1), a matrix representing the shim characteristics can be obtained.

(Equation 4) The shim characteristics obtained for the predetermined slice in this way are stored in the storage means (RAM 325) in the computer 8.

Next, based on the static magnetic field distribution E0 (x, y, z) obtained without the shim current and the shim characteristics,
The shim current value required to generate a magnetic field having the same magnitude in the opposite direction to the static magnetic field distribution E0 (x, y, z) is calculated by solving equation (1). By flowing the calculated current through each shim channel, the non-uniformity of the static magnetic field is corrected.

After the correction of the static magnetic field as described above, imaging can be performed by the EPI method, the spectroscopic image method, or the like, as in the usual case. As a result, it is possible to eliminate image distortion or the like due to non-uniformity of the static magnetic field.

As described above, by simultaneously performing the measurement of the shim characteristics and the static magnetic field correction based on the measured shim characteristics for the slice to be imaged, the static magnetic field correction can be performed with extremely high accuracy, and a large amount of shim characteristic data is prepared in advance. You don't have to. In the case of imaging a plurality of slices such as a multi-slice, a similar effect can be obtained by performing the above-described measurement of the shim characteristics and the correction of the static magnetic field for each desired slice.

In the above embodiment, the MRI apparatus using the shim coil 318 as the static magnetic field correction means has been described. However, the present invention is applicable to an MRI apparatus having a gradient magnetic field coil and a static magnetic field correction function without using a shim coil. Is similarly applicable. In this case, the number of channels of the static magnetic field correction means is usually about three channels, but a single channel number can be applied.

In the above embodiment, the 90 ° pulse is set to 1
Although the shim characteristics for all the shim channels are obtained after the application once, the shim characteristics need not be obtained at once for all the shim channels, and the shim characteristics may be obtained by dividing the shim channel into a plurality of times as necessary. For example, when the echo signal may be attenuated, the shim characteristics of several shim channels may be obtained by the first 90 ° pulse, and the remaining shim characteristics may be obtained by the application of the second and subsequent 90 ° pulses.

[0047]

As described above, according to the present invention, the non-uniformity of the static magnetic field distribution and the correction characteristics (shim characteristics) of the static magnetic field correction means (shim coil) can be obtained at one time by one imaging.
From these, the optimum current value to be passed through the shim coil can be determined. Therefore, the shim characteristics can be measured each time static magnetic field correction is required, that is, for each subject. For this reason, there is no need to prepare shim characteristics in advance, and correction of non-uniform magnetic fields can be performed at any slice thickness, slice interval,
This can be performed accurately in accordance with the slice position. Thereby, the non-uniformity of the static magnetic field caused by inserting the subject into the static magnetic field can be reduced, and the distortion of the image and the spectrum caused by the non-uniformity can be eliminated.

[Brief description of the drawings]

FIG. 1 is a diagram showing a pulse sequence and shim characteristic measurement by the MRI apparatus of the present invention.

FIG. 2 is a block diagram of the MRI apparatus of the present invention.

FIG. 3 is a diagram showing a pulse sequence for measuring an inhomogeneous distribution of a static magnetic field in a conventional MRI apparatus.

[Explanation of symbols]

302 Static magnetic field generating system (static magnetic field generating means) 318 Shim coil (static magnetic field correcting means) 319 Shim power supply (static magnetic field correcting means) 303 Gradient magnetic field generating system (gradient magnetic field generating means) 304 Transmission system 305 Detection system 307 Sequencer 308 Computer ( Central control system, static magnetic field distribution measuring means, characteristic measuring means, static magnetic field correcting means) 321 input system (input means) 306 output system (output means)

Claims (5)

[Claims] 1. A static magnetic field generating means for applying a static magnetic field to a subject, a static magnetic field correcting means for correcting static magnetic field inhomogeneity, and a slice direction gradient magnetic field, a frequency encoding gradient magnetic field, and a phase encoding gradient magnetic field are applied to the subject. A gradient magnetic field generating means, a transmitting system for irradiating the subject with a high-frequency pulse for causing nuclear magnetic resonance in nuclei of atoms constituting the living tissue of the subject, and the gradient magnetic field generating means and the transmitting system. A sequencer that controls and repeatedly applies each gradient magnetic field and a high-frequency pulse in a predetermined pulse sequence, a detection system that detects a nuclear magnetic resonance signal as a measurement signal, and image reconstruction using the signal detected by the detection system as data A central control system for controlling the entire apparatus, input means for inputting conditions to the central control system, and data processed by the central control system. A magnetic resonance diagnostic apparatus provided with output means for displaying and storing data, a static magnetic field distribution measuring means for measuring nonuniformity of a static magnetic field distribution, and characteristic measurement for measuring characteristics of the static magnetic field correcting means. The static magnetic field distribution measuring means obtains a static magnetic field distribution from a pair of echo signals generated before and after a predetermined minute time with reference to the time when the echo time has elapsed, and the characteristic measuring means comprises: The static magnetic field distribution when a current is applied to the correction means is obtained, and the characteristic of the static magnetic field correction means is obtained from the difference between the static magnetic field distribution and the static magnetic field distribution when no current is applied to the static magnetic field correction means. The static magnetic field correction means calculates a current value flowing through each channel of the static magnetic field correction means based on the static magnetic field distribution obtained by the static magnetic field distribution measurement means and the characteristics obtained by the characteristic measurement means, A magnetic resonance diagnostic apparatus, wherein 2. A static magnetic field generating means for applying a static magnetic field to a subject, a static magnetic field correcting means having a plurality of channels and correcting static magnetic field inhomogeneity, a slice direction gradient magnetic field, a frequency encoding gradient A gradient magnetic field generating means for applying a magnetic field and a phase encoding gradient magnetic field, a transmission system for irradiating the subject with a high-frequency pulse for causing nuclear magnetic resonance in nuclei of atoms constituting the living tissue of the subject, and the gradient magnetic field generation A sequencer that controls the means and the transmission system, repeatedly applies each gradient magnetic field and a high-frequency pulse in a predetermined pulse sequence, a detection system that detects a nuclear magnetic resonance signal as a measurement signal, and a signal detected by the detection system. And a central control system for controlling the entire apparatus, input means for inputting conditions to the central control system, and A magnetic resonance diagnostic apparatus having output means for displaying and storing data processed by a control system, wherein a static magnetic field distribution measuring means for measuring nonuniformity of a static magnetic field distribution, and characteristics of the static magnetic field correcting means The static magnetic field distribution measuring means obtains a static magnetic field distribution from a pair of echo signals generated before and after a predetermined minute time with reference to the time when the echo time has elapsed, the characteristic measuring means Is a difference between a static magnetic field distribution when the current is applied to the specific channel and a static magnetic field distribution when the current is not applied to the static magnetic field corrector. From the static magnetic field correction means, the static magnetic field correction means, based on the static magnetic field distribution obtained by the static magnetic field distribution measurement means and the characteristics obtained by the characteristic measurement means A magnetic resonance diagnostic apparatus comprising: calculating a current value flowing through each channel of the static magnetic field correction means; and applying a current. 3. The apparatus according to claim 1, wherein the sequencer applies a 180 ° high-frequency pulse before applying a current to the static magnetic field correction unit. 4. The characteristic measuring means gives currents applied to the respective channels of the static magnetic field correcting means as first and second pulses having opposite polarities, and the time integrated value of the current of each polarity is zero. 4. The magnetic resonance diagnostic apparatus according to claim 2, wherein: 5. The characteristic measuring means acquires the pulse of the first polarity applied to the static magnetic field correcting means, at least a pair of echoes used by the static magnetic field distribution measuring means to obtain a static magnetic field distribution. 6. The magnetic resonance diagnostic apparatus according to claim 5, wherein the voltage is continuously applied during the operation.