JP2003310572A - Magnetic resonance imaging apparatus and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging (MRI) apparatus in which continuous images of an excellent picture quality are obtained by suppressing the occurrence of an artifact due to an eddy current or residual magnetic field caused by a gradient magnetic field in imaging utilizing an SSFP (steady state free precision) status. <P>SOLUTION: In the MRI apparatus, when continuously carrying out a pulse sequence of repeatedly applying a high frequency magnetic field and performing phase encoding and nuclear magnetic resonant signal measurement within a repeat time, the gradient magnetic field is controlled to make the order to phase encoding to be changed at the interval of the repeat time alternate in the order (ascending order) of changing a polarity from negative to positive and the order (descending order) of changing the polarity from positive to negative. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to nuclear magnetic resonance (N
Magnetic Resonance Imaging (MRI) utilizing MR phenomenon
Regarding the equipment, especially steady state (SSFP: Steady State Free P
In a pulse sequence that maintains the recession, the generation of eddy currents and the residual magnetic field due to changes in the gradient magnetic field are suppressed,
The present invention relates to a technique for suppressing the generation of false images due to the incompleteness of the SSFP state.

[0002]

2. Description of the Related Art MRI is an NMR method for generating a high frequency magnetic field in a subject placed in a static magnetic field.
This is a method of detecting a signal, processing the signal, and imaging. The nuclear spin system can be brought to a steady state by continuously applying a high-frequency magnetic field, and a continuous imaging method for measuring an NMR signal in such a steady state has been widely put into practical use. A gradient magnetic field is applied between the continuously applied high-frequency magnetic field and the high-frequency magnetic field, that is, during the repetition time TR to encode the position information in the NMR signal.

Normally, the number of phase encodes required to obtain one tomographic image is set to 64, 128, 256, etc., and accordingly, the strength of the gradient magnetic field is centered around 0 and the negative maximum value to the positive maximum value. The value is set stepwise and is changed from positive to negative or from negative to positive for each TR. In the case of capturing a plurality of images, if the phase encoding of one image is controlled from positive to negative, the subsequent images are similarly controlled stepwise from positive to negative.

Here, in order to maintain a perfect SSFP state,
The gradient magnetic field must be applied so that the phases of the spins dispersed by the application of the gradient magnetic field are aligned again. If the phases of spins are not aligned when the next high-frequency magnetic field is applied,
False images (shading artifacts) occur due to imperfections in the SSFP state. SSFP to align the spin phases
In the pulse sequence utilizing the state, after the NMR signal is measured, a gradient magnetic field (rewind pulse) having the same strength as the gradient magnetic field for encoding the NMR signal and having the opposite polarity is applied in TR.

[0005]

As described above, in the pulse sequence using the SSFP state, the spin phases must be aligned when the next high frequency magnetic field is applied, but when the gradient magnetic field is applied. The spin phases may not be aligned due to the generated eddy current and residual magnetic field. The influence of such an eddy current and the residual magnetic field can be reduced as the difference in the intensity of the gradient magnetic field used in the adjacent TR is smaller, but in the conventional continuous imaging method, the same gradient magnetic field control method is adopted for each image. Therefore, the phase encoding amount between adjacent TRs may change from a negative maximum value to a positive maximum value, which causes a large eddy current or a residual magnetic field, resulting in a false image. Was.

Therefore, the present invention provides an MRI apparatus which can suppress the occurrence of artifacts due to eddy currents and residual magnetic fields due to gradient magnetic fields in imaging using the SSFP state, and can obtain continuous images with good image quality. The purpose is to

[0007]

The MRI apparatus of the present invention which achieves the above-mentioned object, is a means for generating a high frequency magnetic field for causing nuclear magnetic resonance in a subject placed in a static magnetic field, A means for generating a gradient magnetic field for phase-encoding an NMR signal generated by the subject, an image forming means for forming a tomographic image of the subject using the NMR signal, a high-frequency magnetic field generating means and a gradient magnetic field generating means. And a control means for controlling, wherein the control means continuously applies a high-frequency magnetic field and continuously executes a pulse sequence for performing phase encoding and measurement of an NMR signal within a repetition time, at which time it changes at each repetition time. The order of phase encoding to be changed is the order in which the polarity changes from negative to positive.
The gradient magnetic field is controlled so that (ascending order) and the order of changing from positive to negative (descending order) alternate.

According to this MRI apparatus, for example, when one set of NMR signals is measured by changing the phase encode in ascending order at each repetition time and then the next one set of NMR signals is measured from the next repetition time. In addition, since the phase encoding is changed in descending order, the difference between the phase encoding at one repetition time and the phase encoding at the next repetition time can be minimized,
As a result, the effects of the eddy current and the residual magnetic field caused by the gradient magnetic field can be suppressed, and an image in which light and shade stripe artifacts are suppressed can be obtained.

As one form of the MRI apparatus of the present invention,
The control means sequentially changes the order of phase encoding to be changed at each repetition time from the maximum negative value to the maximum positive value, and then sequentially changes the maximum positive value to the maximum negative value.

Further, the MRI apparatus of the present invention continuously applies a high frequency magnetic field and continuously executes a pulse sequence for performing phase encoding in two axial directions and measurement of an NMR signal within a repeating time. The step of repeatedly acquiring the nuclear magnetic resonance signal while fixing the first phase encoding and sequentially changing the second phase encoding is repeated while sequentially changing the first phase encoding, and the first phase encoding at each step is performed. The gradient magnetic field is controlled so that the ascending order and the descending order are alternated.

In this MRI apparatus, when one of the biaxial phase encodes is fixed and a series of phase encodes is performed on the other, the other phase encode order is changed from ascending order to descending order every time one phase encode is changed. By changing from the descending order to the ascending order, it is possible to maintain the integrity of the SSFP state by reducing the change of the other phase encoding when changing the one phase encoding which becomes the outer loop.

In the MRI apparatus of the present invention, the control means is
The pulse sequence is controlled in synchronism with the biological signal of the subject, and a plurality of time phases obtained by dividing one biological signal and the next biological signal are separated. The gradient magnetic field is controlled so that the order changes from negative to positive (ascending order), and in the next time phase, the order changes from positive to negative (descending order).

In this MRI apparatus, the change in phase encoding when the time phase changes can be reduced, and the integrity of the SSFP state can be maintained. As a result, it is possible to eliminate the artifacts caused by the incompleteness of SSFP in the images obtained for each time phase. Furthermore, the MRI apparatus of the present invention is
In the repetition time, a rewind pulse having the same intensity as the gradient magnetic field pulse for imparting phase encoding but the polarity is reversed is included.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the MRI apparatus of the present invention will be described below. FIG. 1 is a diagram showing an overall outline of an MRI apparatus to which the present invention is applied. This MRI apparatus includes a static magnetic field magnet 102 that forms a uniform static magnetic field in a space in which a subject 101 is placed, a gradient magnetic field coil 103 that is arranged in this space to give a gradient to the static magnetic field, and a tissue of the subject 101. An RF coil 104 that generates a high-frequency magnetic field that causes nuclear magnetic resonance in the nuclear spins of the constituent atoms, an RF coil 105 that detects an NMR signal generated from the subject 101, and a magnetic field space formed by the static magnetic field magnet 102. A bed 112 for inserting the sample 101 is provided.

The gradient magnetic field coils 103 are three orthogonal to each other.
It is composed of three gradient magnetic field coils wound in the axial direction, and each is driven by a current supplied from a gradient magnetic field power source 109 to generate a gradient magnetic field in the three axial directions. By applying these gradient magnetic fields, the slice plane of the subject can be set and position information can be added to the NMR signal (echo signal).

The RF coil 104 is driven by a transmission system 110 having a high-frequency oscillator and a modulator, and the high-frequency pulse output from the high-frequency oscillator is supplied to the RF coil 104 to irradiate the subject 101 with electromagnetic waves. To be done.

The echo signal detected by the RF coil 105 is
It is sent to a signal processing system 107 via a receiving system 106 equipped with a quadrature detector and an A / D converter. The signal processing system 107 is a receiving system.
Using the echo signal received at 106, processing such as Fourier transform, correction coefficient calculation, and image reconstruction is performed. Signal processing system 10
The image reconstructed by the process of 7 is displayed on the display unit 108.

The operations of the gradient magnetic field power source 109, the transmission system 110 and the signal processing system 107 are controlled by the control system 111 according to a pulse sequence determined by the imaging method. In the present invention, continuous imaging using a short TR gradient echo pulse sequence is executed as an imaging method, and the encoding order in the pulse sequence is controlled so that the amount of change in phase encoding for each TR is reduced. Such control is specifically control of the order in which the strength and polarity of the gradient magnetic field in the three-axis directions are changed in the pulse sequence, and when continuous imaging as specifically described below is selected as the imaging method. , Automatically or by user specification.

Next, a continuous imaging method using the MRI apparatus having the above configuration will be described. FIG. 2 is a diagram showing an example of a pulse sequence of continuous imaging using the SSFP state, in which the high frequency magnetic field RF, the slice selection gradient magnetic field Gs, the phase encoding gradient magnetic field Gp, and the application timing of the read gradient magnetic field Gr and the echo signal are sequentially shown from the top. Signal generation timing and sampling timing A / D are shown (the same applies hereinafter).
In this pulse sequence, after applying a high-frequency pulse 201 that excites spins in the slice together with a slice selection gradient magnetic field pulse 202 that selects a desired slice,
A pulse 203 that gives an offset for phase encoding and a pulse 204 that gives an offset for a read gradient magnetic field pulse are applied. Next, the readout gradient magnetic field pulse with reversed polarity
205 is applied, and the gradient echo 206 generated thereby is sampled 207 for a predetermined time. After that, before applying the next high-frequency magnetic field, gradient magnetic fields 208, 209, and 210 for aligning spins in the slice are applied. Such a sequence is continuously repeated while changing the magnitude of the phase encode pulse 203. As the magnitude of the phase encode pulse 203 is changed, the rewind pulse 209 in the phase encode direction is also changed.

When the phase encoding is started from a negative maximum value, for example, the echo required for reconstruction of one image is increased by sequentially increasing the phase encoding amount by 1 for each TR and repeating it up to the positive maximum value. Signal (1 set of echo signals)
Is obtained. The next one set of echo signals is also measured by repeating the same sequence, but at this time, the phase encoding is started from the positive maximum value, sequentially decreased by one phase encoding for each TR, and repeated until the negative maximum value. Hereinafter, the descending order and the ascending order are similarly repeated. Rewind pulse
209 is a pulse whose polarity is opposite to that of the phase encode pulse 203 and which has the same intensity, and therefore has the reverse order of the phase encode pulse 203.

FIG. 3 shows how the strength of the phase encode gradient magnetic field changes in this case. In the figure, the phase encode pulse 203 is a white pulse, and the rewind pulse 209 is shown with diagonal lines. Further, in the figure, for simplification, only the pulses that are smaller than the normal phase encode number are shown. As shown in the figure, the gradient magnetic field strength changes so that the difference is always minimized. Therefore, the influence of the eddy current or the residual magnetic field due to the gradient magnetic field can be minimized between one high frequency magnetic field pulse and the next high frequency magnetic field pulse. by this
The integrity of the SSFP state is maintained, and the occurrence of artifacts due to the incompleteness of SSFP can be suppressed in images that are continuously reconstructed.

In fluoroscopy using continuous imaging, some of the data may be shared by adjacent ones of the time-series images obtained continuously. FIG. 3 (a)
In case of not sharing, (b) is in case of sharing. 1
In the conventional method in which the phase encoding of the data of the set is in the same order, when a part of the data is shared, the phase encoding greatly changes in one set of data, and the SSFP state is disturbed. In the method of the present invention, the phase encoding is controlled so that the change in the phase encoding is always minimized.
The condition is maintained. Therefore, by sharing the data, it is possible to prevent the occurrence of artifacts due to the incompleteness of SSFP in continuous images even when the time resolution of image update is increased.

Next, a case of three-dimensional measurement will be described as a second embodiment of the present invention. In the three-dimensional measurement, as shown in FIG. 4, the slice direction encode 411n is used in addition to the phase encode 403. Either of the phase encode and the slice encode may be an inner loop, but in the example shown in the figure, the slice encode is performed for each TR while keeping the phase encode amount constant. In the figure, TR1 and TR2 respectively indicate steps (inner loop) for performing a series of slice encoding by repeating TR.

That is, in the figure, from the high frequency pulse 4011 to the next high frequency pulse 4012, TR1 is the phase encode 403
Applying high-frequency pulse 4011 and echo signal 4 while keeping 1 constant
Slice encode 4111 for the sequence to measure 061
Repeat with TR while changing (and its rewind pulse 4121). Next high frequency pulse 4012 to high frequency pulse 4013
Until then, TR2 slice-encodes the sequence in which the phase encode amount is increased (or decreased) by one step, and the high frequency pulse 4012 is applied and the echo signal 4062 is measured.
TR while changing 12 (and its rewind pulse 4122)
Repeat with. In the series of slice encoding, if the slice encoding is changed in the ascending order in the first repetition TR, the slice encoding is changed in the descending order in the next repetition. Hereinafter, the ascending order and the descending order are alternately repeated to obtain a series of slice-encoded signals for all the phase encode amounts.

FIG. 5 shows how the intensity of the phase encode gradient magnetic field changes in this case. As shown in the figure, even when the phase encoding, which is the outer loop, is changed in three-dimensional measurement, the difference between the strength and polarity of the slice gradient magnetic field in the adjacent TR is minimized, so the integrity of SSFP is maintained. It is possible to suppress the occurrence of artifacts due to the incompleteness of SSFP in a three-dimensional image.

In FIG. 4, the case where the phase encode is constant and the slice encode is changed in TR1, TR2, ... Has been described, but conversely, the slice encode may be fixed and the phase encode may be changed. .

Next, as still another embodiment of the present invention,
A case of performing an electrocardiographic synchronized measurement will be described. FIG. 6 is a diagram showing a time chart of electrocardiographic synchronized measurement. In the electrocardiographic synchronized measurement, for example, biological signals P1, P2, ...
.. is divided into a plurality (for example, 7 to 10) of time phases T1 to T7, and an image for each time phase is formed based on the echo signal obtained in each of these time phases. Therefore, in one cycle from one biological signal to the next biological signal, an echo signal of a part of the phase encode amount is measured for each time phase, and an echo signal of all phase encodes is obtained by repeating a plurality of cycles.

In the example of FIG. 6, one cycle R1, R2, ... Is divided into seven time phases, and in one time phase, the sequence TR shown in FIG. 2 is repeated four times and four echo signals are measured. There is. In the figure, Gp indicates the amount of phase encoding added to each of these four signals. In this case, assuming that the number of phase encodes is N, it is possible to measure an echo signal required for image reconstruction of one temporal phase in N / 4 cycles. In each cycle, the phase encoding applied to the four signals is
The order of phase encoding in adjacent time phases should be different from each other. That is, for example, when the phase encoding is performed in the descending order in the first time phase T1, the phase encoding is performed in the ascending order in the next time phase T2. Hereinafter, the descending order and the ascending order are repeated. In the first time phase T1 of the second cycle R2, the phase encoding offset amount is changed, and phase encoding is performed in the reverse order of the last time phase T7 of the first cycle R1. In the illustrated example, since the number of time phases is odd and the phase encoding in the last time phase is in descending order, the second cycle starts with ascending phase encoding. If the number of time phases in the cycle is even, the first time phase is always in the same order.

In FIG. 6, the rewind pulse is omitted and only the phase encode gradient magnetic field is shown. However, in reality, as shown in FIG. 7, the phase encode pulse 703 has the same intensity and the opposite polarity is the rewind pulse. 709 is inserted.

FIG. 8 shows an example of k-space scanning when phase encoding is performed in this order. In the figure, the numbers circled indicate the order of phase encoding. The solid line shows the echo signal measured in the first cycle R1, and the dotted line shows the echo signal measured in the second cycle R2.

By changing the order of phase encoding each time the phase changes, it is possible to keep the change in the intensity and polarity of the phase encode gradient magnetic field to a minimum between the time phases. Therefore, the influence of the eddy current and the residual magnetic field due to the gradient magnetic field can be minimized. This makes it possible to suppress artifacts due to incompleteness of SSFP in images obtained for each time phase.

In FIG. 6, in order to simplify the explanation,
The case where one cycle of the biomedical signal is divisible by the number of time phases is shown, but the actual biomedical signal varies and cannot be divided by the number of time phases, and it is between the last time phase and the first time phase of the next cycle. You need to take intervals. An example of such spacing is shown in FIG. Gp indicates the polarity and strength of the phase encode applied in each TR. In the interval between the cycles, the sequence is repeated while maintaining the last encoded state in the last time phase, and the biological signal of the next cycle is used as a trigger to start the phase encoding of the first time phase of the cycle. As a result, continuous imaging can be performed while maintaining the SSFP state.

The embodiment of continuous imaging using the MRI apparatus of the present invention has been described above. However, the MRI apparatus of the present invention uses SSFP.
Any imaging method that repeats a series of encoding while maintaining the state can be applied to imaging methods other than three-dimensional measurement and electrocardiographic synchronized imaging. Further, in the above embodiment, the case where one echo signal is measured in TR has been described, but the number of signals measured in TR is not limited to one.

[0034]

According to the present invention, in continuous imaging using the SSFP state, by constantly reducing the change in the phase encoding amount for each TR, the influence of the eddy current and the residual magnetic field due to the gradient magnetic field can be suppressed and stable. The steady state can be maintained. This can provide a good image without artifacts due to SSFP imperfections. In particular, in three-dimensional measurement that encodes in the two-axis direction, or in electrocardiographic synchronized imaging that obtains continuous images for each time phase, when the encoding step of the outer loop is changed or when the time phase and the biological signal cycle change, The change can be minimized, and the generation of light and shade stripe artifacts in a three-dimensional image or an image for each time phase can be suppressed.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of an MRI apparatus to which the present invention is applied.

FIG. 2 is a diagram showing an example of a pulse sequence of continuous imaging adopted by the MRI apparatus of the present invention.

3A and 3B are views for explaining phase encoding control in the continuous imaging of FIG.

FIG. 4 is a diagram showing an example of a pulse sequence for continuous three-dimensional imaging adopted by the MRI apparatus of the present invention.

5A and 5B are views for explaining phase encoding control in the continuous imaging of FIG.

FIG. 6 is a diagram showing an example of a time chart of ECG gated imaging adopted by the MRI apparatus of the present invention.

FIG. 7 is a diagram showing an example of a pulse sequence for ECG gated imaging.

FIG. 8 is a diagram showing k-space scanning for ECG gated imaging in FIG. 6;

FIG. 9 is a diagram showing another example of a pulse sequence for ECG gated imaging.

[Explanation of symbols] 101 ... Subject 102: Static magnetic field magnet 103 ... Gradient field coil 104: RF coil for irradiation 105 ・ ・ ・ RF coil for reception 107 ... Signal processing system 108 ... Display 111 ... Control system

Claims (5)

[Claims] 1. A means for generating a high-frequency magnetic field for causing nuclear magnetic resonance in a subject placed in a static magnetic field, and a gradient magnetic field for phase-encoding a nuclear magnetic resonance signal generated by the subject by nuclear magnetic resonance. A magnetic resonance imaging apparatus including a generating unit, an image forming unit that forms a tomographic image of the subject using the nuclear magnetic resonance signal, and a control unit that controls the high-frequency magnetic field generating unit and the gradient magnetic field generating unit. In the above, the control means continuously executes a pulse sequence in which a high-frequency magnetic field is repeatedly applied and phase encoding and measurement of a nuclear magnetic resonance signal are continuously performed within a repetition time, and at that time, a phase encoding of the phase encoding which is changed at each repetition time is performed. The gradient magnetic field is controlled so that the order in which the polarity changes from negative to positive (ascending order) and the order in which the polarity changes from positive to negative (descending order) alternate. A magnetic resonance imaging apparatus characterized by the above. 2. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit sequentially changes the phase encoding order changed at each repetition time from the maximum negative polarity value to the maximum positive polarity value. A magnetic resonance imaging apparatus, characterized in that a positive maximum value and a negative maximum value are sequentially changed. 3. The magnetic resonance imaging apparatus according to claim 1 or 2, wherein the control means repeatedly applies a high frequency magnetic field and performs biaxial phase encoding and nuclear magnetic resonance signal measurement within a repetition time. The step of continuously executing the pulse sequence, at which time the step of acquiring the nuclear magnetic resonance signal repeatedly while fixing the first phase encoding and sequentially changing the second phase encoding is performed by sequentially changing the first phase encoding. The magnetic resonance imaging apparatus characterized in that the gradient magnetic field is controlled so that the order of the first phase encoding in each step is repeated so as to alternate between ascending order and descending order. 4. The magnetic resonance imaging apparatus according to claim 1, wherein the control unit controls a pulse sequence in synchronization with a biological signal of the subject, and controls a pulse sequence between one biological signal and a next biological signal. The phase encoding order is the order in which the polarity changes from negative to positive (ascending order) for a certain time phase of a plurality of time phases obtained by dividing, and the order in which the polarity changes from positive to negative (descending order) for the next time phase. ), The magnetic resonance imaging apparatus is characterized by controlling the gradient magnetic field. 5. The magnetic resonance according to claim 1, further comprising a rewind pulse having the same intensity as the gradient magnetic field pulse for imparting phase encoding but having the opposite polarity within the repetition time. Imaging equipment.