JPH11309130A - Mri device and mr image pickup method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide high-quality MR images by exhibiting the function of prepulses such as fat restraint of higher accuracy and stability while allowing for the phenomenon called 'B0 shift' resulting from the nonuniformity of a static magnetic field and manufacturing variations in specification of gradient magnetic field coil. SOLUTION: An image pickup sequence including a procedure whereby a gradient magnetic field is applied to a subject via physical X, Y, and Z channels is executed to collect MR signals from the subject. A means is provided for controlling the RF frequency of the prepulse of the image pickup sequence in order to correct the influence of a B0 shift resulting from the application pattern of the gradient magnetic field applied via the X, Y, and Z channels.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to MRI (Magnetic Resonance Imaging) utilizing the magnetic resonance phenomenon of nuclear spins in a subject, and more particularly, to improving the uniformity of a static magnetic field in which the subject is placed by reducing fat. The present invention relates to an MRI apparatus and an MR imaging method for performing suppression (suppression of collecting MR signals from fat).

[0002]

2. Description of the Related Art A medical MR apparatus magnetically excites a nuclear spin of a subject placed in a static magnetic field with a high frequency signal of a Larmor frequency, and generates an image based on an MR signal generated by the excitation. Is a device for reconstructing the spectrum and obtaining spectrum data.

When an MR image of a subject is obtained, fat present in a cross section may cause an artifact due to a chemical shift, and MR from fat may be used for diagnostic reasons.
So-called fat suppression, in which signals are not collected as much as possible, is an essential matter. As one method of this fat suppression, a technique of magnetically saturating nuclear spins of fat with a frequency-selective fat suppression pulse before executing a normal image data acquisition sequence is known.

In order to make the fat suppressing effect effective, it is necessary to increase the uniformity of the imaging region of the static magnetic field by shimming. Conventionally, when the imaging region is a three-dimensional region as in multi-slice imaging, shimming is performed using a slice at the spatial center position of the imaging region or a slice at the center of the magnetic field.

For this reason, at the position where the shimming is performed (for example, the position of the center slice in the imaging region), the excitation frequency range of the fat suppression pulse exactly matches the resonance curve of fat, and a reliable fat suppression effect can be obtained. . However, as the scan position moves away from the shimming execution position in the slice direction (for example, a slice position at the end of the imaging region), the water and fat spectra shift on the frequency axis due to differences in the composition of the sites and the like. As a result, the excitation frequency range of the fat suppression pulse deviates from the fat resonance curve, and the fat suppression effect has been reduced by half.

As one of the measures, Japanese Patent Laid-Open Publication No. 9-164
A method proposed in Japanese Patent Application No. 124 (Japanese Patent Application No. 7-324660) is known. According to the method described in this publication, two or more slices forming a three-dimensional imaging region are used.
The optimal shimming value and the frequency shift Δf for each of the remaining slices are estimated from the results of the primary shimming performed on an arbitrary slice equal to or larger than the plane, and fat suppression is performed in the optimal shimming state for all slices based on these estimated values. It is to do.

[0007]

However, the method described in the above-mentioned publication does not consider a magnetic field change phenomenon called a so-called " _B0 shift", and thus cannot be said to be sufficient as a measure for suppressing fat.

The “B ₀ shift” refers to a phenomenon that generates a magnetic field change having no spatial distribution, which occurs with a time constant (about 200 msec to about 1 sec) significantly longer than the pulse sequence length (for example, “Hofman”). M., J. of Compt. Assi
st. Tomogr. 19 (1), 56-62, 1995 ”). It is known that the amount of the “B ₀ shift” depends on the difference in the gradient magnetic field channel and the difference in the application pattern of the gradient magnetic field pulse waveform. That is, even when the supply the same current to each channel, more like a manufacturing error of a gradient magnetic field coil, with different B ₀ shift amount for each channel. Further, in order to obtain an MR image, it is necessary to apply gradient magnetic fields having pulse waveforms different from each other in each of the slice direction, the phase encode direction, and the readout direction. However, the B ₀ shift amount is different.

[0009] Therefore, optimal to shimming state, even considering the deviation Δf of the frequency, B ₀ shift amount different pulse sequences and for each for each imaging slice approach publication mentioned above. For example, even if the shimming value and the fat suppression sequence are set to be optimal fat suppression in, for example, an axial section, when the imaging section is changed to a coronal plane or a sagittal plane, an FOV (field of view) is used.
The fat suppression state changes when the slice thickness or slice thickness is changed. Therefore, the lack of consideration the B ₀ shift, by performing and stable fat suppression with high accuracy was in a difficult situation it now.

The present invention has been made in view of such inconveniences in the prior art, and has been described in connection with "B", which is caused by non-uniformity of a static magnetic field, manufacturing variation of a gradient coil specification, and the like.
It is an object of the present invention to provide a high-quality MR image by exhibiting a more accurate and stable prepulse function such as fat suppression in consideration of a phenomenon called “ ₀ shift”.

[0011]

In order to achieve the above object, according to the MRI apparatus of the present invention, an imaging sequence including a procedure for applying a gradient magnetic field to a subject through a plurality of physical channels is executed. an MRI apparatus so as to acquire MR signals from the subject Te, the imaging sequence in order to correct the influence of the resulting from the application pattern of the gradient magnetic field B ₀ shift is applied via the plurality of channels Is provided.

[0012] Preferably, said control means includes execution means for executing the measurement sequence for determining the quantity representing the B ₀ shift, to correct the influence of the B ₀ shift from measurement data obtained by the execution of this measurement sequence Calculating means for calculating a correction amount for calculating the correction amount, and correcting means for reflecting the correction amount calculated by the calculating means on the imaging sequence.

More specifically, the imaging sequence includes a pre-sequence including a pre-pulse applied to the subject, and a data acquisition sequence for acquiring an MR signal for imaging from the subject. The pre-sequence is a sequence including an RF pulse for fat suppression as the pre-pulse and a gradient spoiler pulse applied to at least one of the plurality of channels after the application of the RF pulse. For example, the gradient magnetic field spoiler pulse includes three pulses applied to physical channels of the X axis, the Y axis, and the Z axis as the plurality of channels.

[0014] Preferably, the execution means is configured to execute the measurement sequence at an appropriate timing before the execution of the imaging sequence. This timing is, for example, when the MRI apparatus is installed on site. In this case, the measurement sequence is based on the X axis, Y
The gradient magnetic field spoiler and the sequence portion including the gradient magnetic field are provided for each combination of the gradient magnetic field application patterns for the slice, readout, and phase encoding used for the channels of the axis and the Z axis and the data acquisition sequence. It consists of a first half sequence that is repeated a plurality of times, and a second half sequence including application of an RF pulse and acquisition of an FID signal executed after the first half sequence. For example, the calculation unit is configured to calculate a coefficient representing the B ₀ shift for each combination based on the measurement data, and the correction unit is configured to calculate the B ₀ shift based on a desired combination of the coefficients. First means for obtaining the amount, second means for obtaining the correction amount from the B ₀ shift amount, and third means for changing the frequency of the RF pulse for fat suppression of the pre-sequence by the correction amount. And

Further, for example, the timing at which the execution means executes the measurement sequence is at the time of actual imaging of the subject. As an example of this case, the measurement sequence may include channels for the X-axis, Y-axis, and Z-axis and slices used in the data acquisition sequence.
For the first half sequence to repeat the sequence portion including the gradient magnetic field spoiler and its gradient magnetic field depending on the combination actually used for the imaging of the application pattern of the gradient magnetic field for readout and phase encoding, respectively,
The second half sequence includes application of an RF pulse and acquisition of an FID signal executed after the first half sequence. For example, the calculation unit is configured to calculate a frequency shift amount corresponding to the B ₀ shift amount based on the measurement data, and the correction unit obtains the correction amount from the frequency shift amount. Means, and second means for changing the frequency of the RF pulse for fat suppression in the pre-sequence by the correction amount.

The MRI apparatus further includes means for performing primary shimming on a three-dimensional area including the imaging region of the subject to obtain an optimum shimming value and a shift amount of a spectrum on a frequency axis. Means may be provided for correcting the imaging sequence based on the optimum shimming value and the amount of deviation on the frequency axis.

On the other hand, the MR imaging method according to the present invention
During the acquisition of an MR signal from the subject by executing an imaging sequence including a procedure for applying a gradient magnetic field to the subject via the plurality of physical channels of the I device, the gradient magnetic field is applied through the plurality of channels. wherein the amount representing the effects of resulting from the B ₀ shift to application pattern of the gradient magnetic field determined, and corrects the imaging sequence in accordance with this amount.

[0018]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

First Embodiment A first embodiment of the present invention will be described with reference to FIGS.

FIG. 1 shows a schematic configuration of an MRI (magnetic resonance imaging) apparatus according to this embodiment.

This MRI apparatus comprises a bed on which a subject P is placed, a static magnetic field generator for generating a static magnetic field, a gradient magnetic field generator for adding positional information to the static magnetic field, and a transceiver for transmitting and receiving high-frequency signals. And a control / arithmetic unit for controlling the whole system and reconstructing an image, and an electrocardiographic measuring unit for measuring an ECG signal as a signal indicating a cardiac phase of the subject P.

The static magnetic field generating section includes, for example, a gantry having a superconducting type 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) formed in the gantry. static magnetic field H ₀ in the axial direction (Z axis direction) of the)
Generate. The subject P is loosely inserted into this opening.
The couch portion includes a couch body and a top plate on which the subject P is placed,
This top plate can be inserted into the opening of the gantry (that is, the magnet 1) so as to be able to retract.

The magnet 1 is provided with a shim coil 14 for performing the second or higher order shimming. When performing the second or higher order shimming, a current for homogenizing the static magnetic field is supplied to the shim coil 14 from the shim coil power supply 15 under the control of the host computer described later.

The gradient magnetic field generator has a gradient magnetic field coil unit 3 incorporated in the magnet 1. 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 the X, Y, and Z-axis directions as physical axes of the gantry, which are orthogonal to each other. .
The gradient magnetic field unit further includes a gradient magnetic field power supply 4 for supplying a current to the x, y, z coils 3x to 3z. The gradient power supply 4 controls x, y, and x under the control of a sequencer 5 described later.
A pulse current for generating a gradient magnetic field is supplied to the z coils 3x to 3z.

The x, y, z coils 3x from the gradient magnetic field power supply 4
By controlling the ratio and magnitude of the pulse current supplied to .about.3z, the gradient magnetic fields in the three axes X, Y, and Z are combined, and the gradient magnetic field Gss in the slice direction, the gradient magnetic field Gpe in the phase encoding direction, and the readout are read out. The direction (frequency encoding direction) gradient magnetic field Gro can be arbitrarily set or changed. Slice direction, phase encoding direction and readout direction perpendicular forms a logical axis together, the gradient magnetic field in each direction are superimposed on the static magnetic field H _0.

The transmission / reception unit includes a high-frequency coil 7 disposed near the subject P in the diagnostic space in the magnet 1, and a transmitter 8T and a receiver 8R connected to the coil 7. The transmitter 8T and the receiver 8R supply an RF current pulse having a Larmor frequency for exciting magnetic resonance (MR) to the high-frequency coil 7 under the control of a sequencer 5 described later, and the high-frequency coil 7 MR received
A signal (high-frequency signal) is received and subjected to various kinds of signal processing to form a corresponding digital 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, a display 12, an input unit 13, and a sound generator 16 are provided.

The host computer 6 has a function of instructing the sequencer 5 on pulse sequence information in accordance with the stored software procedure and controlling the operation of the entire apparatus including the sequencer 5.

The MRI apparatus of the present embodiment employs a scan method in which a pre-sequence including a fat suppression pulse is executed, so-called fat suppression is performed, and then an image data acquisition sequence is executed. The pre-sequence and the data acquisition sequence form the imaging sequence of the present invention. By this scanning method, M
MR data in which the R signal is suppressed is collected, and fat at the diagnosis site can be suppressed. To realize this, the host computer 6 measures the B ₀ shift amount when the MRI apparatus is installed in a hospital or the like, and adjusts the offset of the gradient magnetic field and the B ₀ shift amount based on primary shimming during actual imaging. The adjustment of the RF frequency of the fat suppression pulse in accordance with the measured value of is performed. These processes will be described later.

The sequencer 5 has a CPU and a memory. As a result, the sequencer 5 is connected to the host computer 6
Is stored in the memory, and a series of operations of the gradient magnetic field power supply 4, the transmitter 8R, and the receiver 8T are controlled according to this information. Here, the pulse sequence information is all information necessary to operate the gradient magnetic field power supply 4, the transmitter 8R, the receiver 8T, and the like in accordance with a series of pulse sequences, for example, the x, y, and z coils 3x to 3x. 3z includes information on the intensity of the pulse current applied to 3z, the application time, the application timing, and the like.

The pulse sequence may be a two-dimensional scan or a three-dimensional scan,
The form of the pulse train includes an SE (spin echo) method, an FE (field gradient echo) method,
FSE (Fast Asymmetric) method, FASE (Fast Asymmetric)
Any pulse train such as the SE) method and the EPI method may be used.

The arithmetic unit 10 inputs the digital data of the MR signal from the receiver 8R via the sequencer 5 and converts the digital data into original data (also called raw data) to Fourier space (also called k-space or frequency space). And a two-dimensional or three-dimensional Fourier transform process for reconstructing the original data into a real space image is performed. The function of the arithmetic unit 10 may be configured to be provided to the host computer 6.

The storage unit 11 can store image data to which original data and reconstructed image data have been applied. The display 12 displays an image. Information such as scan conditions and pulse sequences can be input to the host computer 6 via the input device 13. The function of the arithmetic unit 10 may be configured to be provided to the host computer 6.

The voice generator 14 can emit a breath-hold start and a breath-hold end message as voice when instructed by the sequencer 5 or the host computer 6.

Further, the electrocardiogram measuring section is provided with an ECG sensor 17 which is attached to the body surface of the subject to detect an ECG signal as an electric signal, and performs various processes including digitizing process on the sensor signal to perform sequencer 5 processing. And an ECG unit 18 for outputting to the The measurement signal by this electrocardiograph is
It is used by the sequencer 5 as a timing signal when executing a scan sequence. As a result, the synchronization timing for ECG synchronization can be appropriately set, and an ECG-gated scan based on the set synchronization timing can be performed to collect MR original (raw) data.

Next, the operation of the MRI apparatus of this embodiment will be described with reference to FIGS.

[0037] First, the amount of operation of the measurements on previously conducted in advance B ₀ shift, for example, when installing the MRI apparatus in the medical field such as hospitals. Note that the measurement of the amount related to the _B0 shift may be performed before capturing the fat suppression image, and an example of such measurement timing is “at the time of device installation”.
It is not necessarily limited to “at the time of installation of the device”. For example, only the measurement of the _B0 shift may be performed periodically, or may be performed at the time of periodic maintenance.

[0038] For example, in time of installation, in a state that caused the static magnetic field by driving the static magnetic field magnet 1, put a phantom simulating a characteristic of a living body for diagnostic space, preliminary measurement of the B ₀ shift is performed.

The host computer 6 and the sequencer 5 execute the processing shown in FIG. First, the operation information including the preliminary measurement instruction B ₀ shift is transmitted to the host computer 6 through the input device 13 from the operator (step 49). In response to this command, the host computer 6 instructs the sequencer 5 to start pre-measurement, and sends to the sequencer 5 idle data such as data on various gradient magnetic field waveforms used for this measurement (step 50).

[0040] Thus, the sequencer 5, come out of B ₀ shift start state which command is waited while determining whether a pre-measured (step 51), in FIG. 3 representing a measurement sequence of the present invention In the sequence, a blanking sequence shown in the first half of the left side is instructed (step 52).

This idle beating sequence is executed in order to calculate a coefficient relating to the B ₀ shift for each physical channel and for each gradient magnetic field waveform, and to calculate the B ₀ shift amount of the entire apparatus. Thus, in this idle beating sequence, the slice gradient magnetic field waveform Gss, the readout gradient magnetic field Gro, and the phase encoding gradient magnetic field Gpe are applied to each of the physical axes X, Y, and Z of the apparatus, and a total of 9 It is continuously executed a plurality of times (for example, about 3 to 20 times) for each of (= 3 × 3) combinations (step 53).

For example, the idle shot sequence shown in FIG.
The gradient magnetic field spoiler Gspl-a applied first in the axial direction,
Subsequent gradient magnetic field for slice Gss and end spoiler (gradient magnetic field spoiler) Gsp applied thereafter
lb. Although the spoiler Gspl-a, b is not always essential, in the actual fat suppression sequence, this spoiler is often applied after the application of the fat suppression pulse or at the end of the data acquisition sequence. In such a case, in order to measure the amount relating to B ₀ shift as much as possible the combined state to the actual sequence, as shown in FIG. 3, the spoiler Gspl-
It is desirable to apply a and b. This X axis, Spoiler G
For one combination of the spl-a, the slice gradient magnetic field Gss, and the end spoiler Gspl-b, the gradient magnetic field power supply 4 is driven, and the gradient magnetic fields Gspl-a, Gss are transmitted through the x coils 3x, 3x as described above. , Gspl-b are pulsed multiple times.

Here, the reason why the blank beating is performed a plurality of times is B ₀
This is due to the formation of the shift phenomenon, which is to apply a gradient magnetic field and reflect the magnetic field to the static magnetic field magnet to create a state in which the time constant of the stable (saturated) magnetic field change is long. Therefore, the number of times of idling may be defined as a plurality of times corresponding to such a long time constant of the magnetic field change.

When a plurality of times of blanking of the gradient magnetic field with respect to this one combination are completed, as shown in the latter half of FIG.
A pulse is applied from the RF coil 7 to excite the nuclear spin (step 54). The rising and strength of the gradient magnetic field applied at the time of excitation of the RF pulse is considered so as not to cause eddy current as much as possible. The flip angle of this RF pulse is arbitrary.

By this excitation, FID (free induction decay)
A signal is generated and this signal is received by the RF coil 7. The FID signal received by the RF coil 7 is output from the receiver 8R to the host computer 6 as a digital FID signal.

Then, the host computer 6 inputs a digital amount of the FID signal for a predetermined period (for example, 200 ms) from the state of waiting until then (step 55).

Next, the host computer 6 sends the input FI
Fourier transform is performed on the D signal to obtain its frequency spectrum distribution (step 56). Further, the deviation Δf from the resonance center frequency f ₀ of the spectrum of the spectrum when no pre-pulse is applied to the resonance center frequency f ₀ ′ of the water obtained above as a predetermined reference at a predetermined magnetic field strength is calculated. (Step 57: See FIG. 4).

Here, depending on the time constant of the target B ₀ shift, it is also possible to extend the data collection time by using a spin echo signal by a 90 ° -180 ° pulse to improve the measurement accuracy.

Next, the host computer 6 calculates the deviation amount Δ
The coefficient a11 corresponding to f is calculated by an appropriate method and set at a predetermined address position in the built-in nonvolatile memory.
Is stored in the corresponding element position of the "3.times.3" matrix (step 58).

As described above, the processing of steps 51 to 54 by the sequencer 5 and the processing of steps 55 to 58 by the host computer 6 include the gradient magnetic field waveforms Gs, Gro and Gpe for the X axis, Y axis and Z axis, respectively. Alternately, the process is performed nine times in total (steps 59 and 60).

Since the value of the phase encode amount itself is a component whose value is changed every repetition time, in the above-described blanking sequence, the inclination simulating the phase encode amount in the physical axis direction corresponding to the phase encode direction. It is desirable not to apply a magnetic field and to stop at an end spoiler.

Next, a description will be given of a method of calculating the _B0 shift amount of the entire apparatus, which is executed at the time of actual imaging, using the coefficient data a11,..., A33 obtained as described above. This processing is shown in FIG. This processing is performed by the host computer 6 before the execution of the data collection sequence for imaging.

The host computer 6 inputs operation information from the input device 13 and determines the correspondence of one or more kinds of gradient magnetic fields applied in each physical axis direction from the operation information (step 71, 72). Thereby, as an example, the X-axis direction is applied to the application of the slice gradient magnetic field Gss,
It is determined that the axial direction is used for applying the readout gradient magnetic field Gro and the Z-axis direction is used for applying the phase encoding gradient magnetic field Gpe. This example is a case where the imaging slice is not oblique (when the imaging slice is orthogonal to the physical axis, for example). When obliquely observing an imaging slice, there is a physical axis on which two or more types of gradient magnetic field components are superimposed.

Next, the host computer 6 reads out the coefficient a corresponding to the above judgment result from the matrix corresponding to FIG. 5 stored in the internal memory (step 73).
For example, in the case of the above-described example in which the oblique is not caused, the coefficient a11 in the first row and the first column, the coefficient a22 in the second row and the second column, and the coefficient a33 in the third row and the third column are read from the matrix corresponding to FIG. When obliquely imaging a slice, two or more coefficients are read so as to divide one gradient magnetic field according to the superposition ratio of the gradient magnetic field components. For example, the coefficients a11 and a13 are read out for the slice gradient magnetic field Gss in the logical axis direction.

Next, the scanning conditions instructed in the operation information already given, for example, the imaging slice position (including the difference in the imaging cross section such as axial, coronal, sagittal), FOV size, slice thickness, image The read coefficient is adjusted according to the matrix size or the like (step 74). For example, if the measured coefficient a indicates that the slice thickness is 1
When the slice thickness to be imaged is 5 mm, the coefficient is doubled.

[0056] Then, the B ₀ shift amount of the entire basis of the adjusted coefficient device determined by vector composition (or added) (step 75). For example, if the imaging slice is not oblique, as an example,

(Equation 1) Total amount B ₀ by the vector synthesis is required. Case of oblique imaging slice, for example, the calculation of the B ₀ shift component corresponding to one of the gradient, coming into play two or more coefficients which are divided according to the ratio to be oblique. For example,

(Equation 2) Such a degree, thereby B ₀ shift component due to the superimposed component of the gradient magnetic field in one direction is calculated.

Next, the host computer 6 sends the entire B
_{According to the 0} shift amount, R of the fat suppression pulse executed later
The change Δf _SUP of the F frequency is calculated by an appropriate method (step 76). For example, when the B ₀ shift amount is large, R
The change Δf _SUP of the F frequency is increased, and conversely, B ₀
When the shift amount is small, the change amount of the RF frequency Δf _{SU P}
Is calculated to reduce. Then, the RF frequency change Δf _SUP is temporarily stored (step 77).

Subsequently, the host computer 6 shown in FIG.
Perform multi-slice scan with primary shimming,
A fat suppression image will be obtained.

The host computer 6 inputs designation information of a desired three-dimensional imaging area (for example, the head as shown in FIG. 8) in the first step 81 of FIG. Two or more arbitrary slices among a plurality of slices forming an imaging region are designated. For example, as shown in FIG. 8A, the slice Scen at the center position of the three-dimensional region in the body axis Z direction and the slices Sedg1 and Sedg2 at both end positions of the three-dimensional region in the body axis Z direction as shown in FIG. Is adopted automatically or based on a command from an operator.

Next, at step 83, the host computer 6 sends the sequencer 5
And instructs the arithmetic unit 10 to perform primary shimming by single slice. The “shimming” here is a process for correcting the uniformity of the imaging region of the static magnetic field which is disturbed by placing the patient P in a static magnetic field having a uniformity equal to or more than a predetermined value. This primary shimming is performed on a designated slice using, for example, a sequence of the SE method without applying a gradient magnetic field for phase encoding and reading. The resulting echo signal is subjected to Fourier transform to obtain a spectrum (see FIG. 9A). Water resonance curve C of this spectrum
wat and the half width Wwat and W of the fat resonance curve Cfat
fat is calculated, and x coils 3x... 3 are supplied from the gradient magnetic field power supply 4 so that the half widths Wwat and Wfat are minimized.
The offset values (DC current values) flowing through the x, y coils 3y... 3y and the z coils 3z, 3z are adjusted. This series of processing is performed until the minimum values of the half widths Wwat and Wfat are found and the resonance curves Cwat and Cfat of water and fat are surely separated on the spectrum, as shown in FIG. 9B.

At the same time, in step 84, the x coils 3x...
The offset values to be supplied to the x, y coils 3y... 3y and z coils 3z, 3z are determined and stored as primary optimal shimming values Gxn, Gyn, Gzn in the X, Y, Z axis directions.

Further, in step 85, the center frequency f ₀ of the resonance curve Cwat of water is determined as the center frequency of the RF pulse, and the center frequency of fat which is 3.5 ppm lower than the center frequency f ₀ is also estimated. At the same time, the shift amount (frequency offset amount) of the center frequency of fat is calculated.

Next, in step 86, it is determined whether or not primary shimming has been performed for all of the specified plurality of slices. For example, as shown in FIG.
If cen, Sedg1, and Sedg2 are specified, the processing of steps 82 to 85 described above is executed for all three slices.

In the above-described processing, shimming is performed several times by the single slice method.
The processing may be performed such that a plurality of arbitrary slices are shimmed at a time by the multi-slice method.

The host computer 6 then moves the processing to step 87. In step 87, the primary optimal shimming value of the remaining slice of the desired three-dimensional imaging region BR (see FIG. 8B) is estimated and calculated. Specifically, based on the primary optimal shimming value obtained as described above (in the example of FIG. 8, the primary optimal shimming value of each of the three slices Scen, Sedg1, and Sedg2 at the center and the end), Estimation calculation is performed by higher-order curve fitting.

When this is completed, in step 88, the center frequency of the fat resonance curve Cfat of the remaining slices is estimated and calculated by curve fitting as well, and the deviation amount (frequency offset amount) of this center frequency of fat is calculated. .

Further, the host computer 6 advances the processing to step 89, reads the primary optimum shimming value (offset value of the gradient magnetic field) of each slice obtained in steps 84 and 87 from the built-in memory, and reads out the information in step 85
And 88, the deviation amount (frequency offset amount) of the center frequency of the fat resonance curve is read, and these are given to the sequencer 5 together with information such as imaging conditions.

Further, the host computer 6 executes step 90
Then, the frequency change Δf _SUP of the fat suppression pulse corresponding to the B ₀ shift amount, which has already been determined by executing the above-described processing of FIG. 6, is read from the built-in memory, and this change is provided to the sequencer 5. .

In response to the processing of steps 89 and 90, the sequencer 5 changes the frequency and gradient magnetic field of the pulse sequence of the fat suppression SE method related to the multi-slice scan to be executed from now on. That is,
A magnetic field offset value Gzn (Gxn, Gyn) that forms a first-order optimum shimming value is added to each of the slice, readout, and phase encoding gradient magnetic fields for each slice. Further, for each slice, the RF frequency of the fat suppression pulse Pfat is changed by adding the above-described frequency offset value and the change Δf _SUP according to the B ₀ shift amount, or the like.

When the preparation is completed, the host computer 6
Shifts to the process of step 91, and instructs the sequencer 5 and the arithmetic unit 10 to perform a multi-slice scan. As a result, the designated three-dimensional imaging region BR is, for example, as shown in FIG.
Is scanned by a multi-slice method (SE method) based on a CHESS (chemical shift selective) method using a fat suppression pulse Pfat as a pre-pulse.

As can be seen from FIG. 10, the sequencer 5
Are given primary optimum shimming values Gxn, Gx for each encode of each slice in the multi-slice scan.
yn and Gzn are applied as offset amounts. By applying the offset amount, the magnetic field uniformity of each slice plane to be imaged maintains the static magnetic field uniformity at a level at which the resonance curves Cwat and Cfat of water and fat are well separated in all slices as shown in FIG. And fat suppression pulse Pfa
Since the excitation frequency range of t is adjusted for each slice based on the center frequency information, it exactly matches the fat resonance curve Cfat.

At the same time, the RF of the fat suppression pulse Pfat
Frequency is automatically set to a value obtained by taking into account the B ₀ shift amount of the device. This fine adjustment of the RF frequency allows the excitation range of the fat suppression pulse Pfat to exactly match the fat resonance curve for each slice. That is, it is possible to almost completely eliminate a slice in which the excitation range partially deviates from the fat resonance curve due to the B ₀ shift phenomenon. Therefore, unlike the conventional method described above,
Even if the B ₀ shift phenomenon occurs, its influence is reliably eliminated, and a highly accurate fat suppression effect can be obtained.

As a result, the actual primary shimming is performed only on the first plurality of slices. However, the actual primary shimming is reduced by 3 by appropriately estimating the shimming values of the remaining slices from the actually measured values. A magnetic field uniform state almost equivalent to that performed over the entire three-dimensional imaging region can be achieved, and as a result,
Water and fat can be reliably separated over a wide three-dimensional area.

[0074] Thus, fat saturation pulse Whatever slice is favorably worked, hardly influenced even B ₀ shift, the accuracy is high and is stable fat suppression effect exerted, no artifacts due to fat Alternatively, a plurality of high-quality multi-slice images with almost no image quality can be obtained. Since the actual shimming is of the first order, the shimming is also simple. In particular, it is not necessary to provide a special shimming coil, and the shimming can be easily performed by also using the gradient coil.

Particularly, in this embodiment, in the process of calculating the B ₀ shift amount, scanning conditions such as the slice position, the FOV, and the slice thickness are taken into consideration. even if changes in sagittal cross-section, also even when the slice thickness is varied, always can set the frequency of the fat saturation pulse which can avoid the influence of B ₀ shift, thereby, possible to realize a fat suppression precision.

The B ₀ shift amount is calculated and calculated each time each image is picked up by using a coefficient value unique to the apparatus which is previously obtained and stored at the time of installation or the like. Not only the scan conditions, but also the scan in which the correspondence between the physical axis of the device and the tilt application direction (logical axis) changes, and the scan in which oblique / non-oblique is performed
It is possible to cope with a case where imaging conditions vary greatly individually, and it is possible to provide a fat suppression method which is very flexible and versatile.

Second Embodiment An MRI apparatus according to a second embodiment of the present invention will be described with reference to FIGS. Note that the hardware configuration of the MRI apparatus according to this embodiment is the same as or similar to that of the first embodiment, and thus the description thereof will be omitted or simplified.

The MRI apparatus according to this embodiment is characterized in that an amount relating to the B ₀ shift is estimated and calculated each time imaging is performed. In the case of the first embodiment, a coefficient unique to the apparatus according to the application pattern of the gradient magnetic field that is the basis of the B ₀ shift amount is obtained and stored in advance, which is different.

At the time of imaging, the host computer 6 of the MRI apparatus performs, for example, prior to the execution of the scan shown in FIG.
The processing shown in a series of steps shown in FIG. 12 is executed. In FIG. 12, the processing of steps 49 to 57 is the same as that of the step of the same reference numeral in FIG.

Step 5 in the process shown in FIG.
An example of the idle driving sequence instructed in 2 is shown in the first half of FIG. In other words, based on the blanking information (step 50), the spoiler Gspl-a, the gradient magnetic field waveform, and the logical axes using the end spoiler Gspl-b (slice gradient magnetic field Gss, readout gradient magnetic field Gro, phase encoding gradient) are used. According to the magnetic field Gpe), the physical axes X, Y,
Since a combination of magnetic field waveforms applied in the Z direction is uniquely determined, a plurality of blank shots are first performed only for one uniquely determined combination (step 52,
53). Also in this case, it is desirable not to apply the gradient magnetic field for phase encoding itself. An excitation pulse is applied after the blank hit, and the FID signal is collected (step 5).
4).

Then, the input of the FID signal and the Fourier transform are performed (steps 55 and 56), and the deviation Δf of the spectrum from the resonance center frequency f ₀ of the water, which is a predetermined reference at a predetermined magnetic field strength (see FIG. 4) is obtained (step 57). This frequency deviation Δf is B ₀
This is the amount corresponding to the shift.

Therefore, the host computer 6 calculates the frequency change Δf _SUP of the fat suppression pulse by an appropriate method according to the frequency shift amount Δf, and stores this (step 10).
1, 102). Thereafter, similarly to the first embodiment, for example, a multi-slice scan shown in FIG. 7 is performed, and a plurality of slice images are obtained.

[0083] can be carried out in this way precisely avoiding highly accurate and stable fat suppress the influence of even B ₀ shift by the present embodiment. On the other hand, the frequency adjustment of the fat suppression pulse caused by the B ₀ shift need only be performed once in a spot manner at the time of imaging based on desired imaging conditions (including scanning conditions). For this reason, there is also an advantage that data processing for frequency adjustment is simplified and simple.

In each of the above embodiments, three slices for measuring the primary optimum shimming value are exemplified first, but the present invention is not necessarily limited to this. The primary optimum shimming values of the remaining slices are not limited to three. As long as the slice position can be estimated and calculated, measurement of at least two slices, such as a combination of a center slice and a slice at one end, may be performed.

Further, in each of the above embodiments, the CHES
The case where the S method is implemented under the SE method has been described.
The same can be applied to the E method or the FE method. The fat suppression pulse used in the CHESS method as the pre-sequence may be a binomial pulse, a sink function, a Gaussian function, or the like.

Further, in each of the above embodiments, the gradient magnetic field waveform of a spoiler or the like applied in the idle driving sequence is as follows:
In measuring the B ₀ shift or an amount reflecting the B ₀ shift, it is sufficient that the application is performed under the same conditions as the fat suppression sequence and the data collection sequence (that is, the present sequence) as much as possible. Appropriate deformation is possible, such as limiting to one direction or one direction, or changing the gradient magnetic field or the applied polarity of the spoiler.

[0087] Furthermore, in the above embodiments, the pre-sequence has been representatively described as applying a pre-pulse of fat suppression (fat suppression pulse), prepulse of the present invention is affected by the B ₀ shift prepulse Any pulse can be applied, for example, a pre-pulse for pre-saturating a blood flow spin flowing into an imaging slice, or MTC (magnetization transfer contrast).
A pre-pulse (MTC pulse) that produces an effect may be used.

In addition, it will be understood by those skilled in the art that various modifications and developments can be made within the scope of each invention described in the claims. It will be clear from the description.

[0089]

As described above, the MR according to the present invention is used.
The I device and MR imaging method to measure or estimate the amount or B ₀ shift amount itself represents the effect of B ₀ shift,
By controlling the RF frequency of, for example, pre-pulse imaging sequence in response to the measurement result or estimation result, which is adapted to correct the influence of the resulting the B ₀ shift to application pattern of the gradient magnetic field applied via a plurality of channels, For example, when performing a multi-slice scan, a pre-pulse for fat suppression can be applied to each of a plurality of slices with high fat suppression accuracy, and fat suppression can be performed more accurately than the conventional imaging method that does not consider the _B0 shift phenomenon. For example, it is possible to provide a stable and high quality MR image with a remarkably low S / N.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an example of an MRI apparatus according to an embodiment of the present invention.

2 is a flowchart showing an example of processing for measuring the coefficient for B ₀ shift according to the first embodiment.

FIG. 3 is a sequence including a blanking sequence for measuring a coefficient related to a B ₀ shift.

FIG. 4 is a diagram illustrating a frequency shift amount.

FIG. 5 is a schematic diagram of a matrix for storing coefficients.

FIG. 6 is a flowchart illustrating a processing example for obtaining a frequency change of a fat suppression pulse corresponding to a B ₀ shift amount during imaging.

FIG. 7 is a flowchart showing a procedure for imaging including primary shimming processing and frequency adjustment of a fat suppression pulse during imaging.

FIG. 8 is a view for explaining the relationship between designated slices and remaining slices in a multi-slice scan.

FIG. 9 is a spectrum diagram illustrating an outline of shimming processing.

FIG. 10 is a pulse sequence showing an example for imaging according to the embodiment.

FIG. 11 is a view for explaining the fat suppression effect of the embodiment.

FIG. 12 is a flowchart illustrating a processing example for measuring a frequency change of a fat suppression pulse according to a B ₀ shift amount according to the second embodiment.

FIG. 13 is a sequence including a blanking sequence for measuring a frequency change according to the B ₀ shift amount.

[Explanation of symbols]

 Reference Signs List 1 magnet 2 static magnetic field power supply 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 12 display 13 input unit

Claims (14)

[Claims] 1. An M sequencer for acquiring an MR signal from a subject by executing an imaging sequence including a procedure of applying a gradient magnetic field to the subject via a plurality of physical channels.
In RI apparatus, MRI apparatus characterized by comprising control means for controlling the imaging sequence in order to correct the influence of the resulting the B ₀ shift to application pattern of the gradient magnetic field applied via the plurality of channels . 2. A first aspect of the invention, said control means includes execution means for executing the measurement sequence for determining the quantity representing the B ₀ shift, said B ₀ from the measured data by the execution of this measurement sequence An MRI apparatus comprising: a calculating unit that calculates a correction amount for correcting an influence of a shift; and a correcting unit that reflects the correction amount calculated by the calculating unit on the imaging sequence. 3. The MRI apparatus according to claim 2, wherein the imaging sequence includes a pre-sequence including a pre-pulse applied to the subject, and a data acquisition sequence for acquiring an MR signal for imaging from the subject. . 4. The invention according to claim 3, wherein the pre-sequence includes an RF pulse for fat suppression as the pre-pulse and a gradient applied to at least one of the plurality of channels after the application of the RF pulse. An MRI apparatus which is a sequence including a magnetic field spoiler pulse. 5. The MRI apparatus according to claim 4, wherein the gradient magnetic field spoiler pulse includes three pulses applied to physical channels of X axis, Y axis, and Z axis as the plurality of channels. 6. The MRI apparatus according to claim 5, wherein the execution unit is configured to execute the measurement sequence at an appropriate timing before the execution of the imaging sequence. 7. The MRI apparatus according to claim 6, wherein the execution unit executes the measurement sequence at a time when the MRI apparatus is installed at a site. 8. The invention according to claim 7, wherein the measurement sequence includes a channel for each of the X-axis, Y-axis, and Z-axis and a slice, a readout, and a phase encode used in the data acquisition sequence. For each combination of gradient magnetic field application patterns,
The first half of the sequence that repeats the gradient magnetic field spoiler and the sequence portion including the gradient magnetic field a plurality of times, and the second half of the sequence including the application of the RF pulse and the acquisition of the FID signal performed after the first half of the sequence.
RI equipment. 9. The invention of claim 8, wherein the calculation means may be configured by means for calculating a coefficient representative of the B ₀ shift for each of the combinations on the basis of the measurement data, the correction means is the First means for obtaining the B ₀ shift amount based on a desired combination of coefficients; second means for obtaining the correction amount from the B ₀ shift amount; and a frequency of the pre-sequence fat suppression RF pulse. And a third means for changing the value by the correction amount.
RI equipment. 10. The MRI apparatus according to claim 6, wherein the execution unit executes the measurement sequence at the time of actual imaging of the subject. 11. The invention according to claim 10, wherein the measurement sequence includes a channel for each of the X-axis, Y-axis, and Z-axis, and a slice, a readout, and a phase encode used in the data acquisition sequence. The first half of the sequence that repeats the gradient magnetic field spoiler and the sequence part including the gradient magnetic field a plurality of times according to the combination actually used for the imaging of the application pattern of the gradient magnetic field, and the application of the RF pulse executed after this first half sequence and An MRI apparatus comprising a second half sequence including acquisition of an FID signal. 12. The invention according to claim 11, wherein the calculation means is configured to calculate a frequency shift amount corresponding to the B ₀ shift amount based on the measurement data, and the correction means is configured to: First means for obtaining the correction amount from the frequency shift amount; and R for fat suppression in the pre-sequence.
A second means for changing the frequency of the F pulse with the correction amount. 13. A device according to claim 1, wherein primary shimming is performed on a three-dimensional region including the imaging part of the subject to obtain an optimum shimming value and a shift amount of a spectrum on a frequency axis. An MRI apparatus comprising: means for correcting the imaging sequence based on the optimum shimming value and a shift amount on a frequency axis. 14. An MR imaging method in which an imaging sequence including a procedure of applying a gradient magnetic field to a subject via a plurality of physical channels of an MRI apparatus is executed to acquire an MR signal from the subject. An MRI imaging method comprising: obtaining an amount representing an influence of a _B0 shift caused by an application pattern of the gradient magnetic field applied through the plurality of channels; and correcting the imaging sequence according to the amount.