JP4679968B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI) apparatus, and more particularly to an MRI apparatus suitable for photographing water-fat separated images.

  An MRI apparatus detects an NMR (nuclear magnetic resonance) signal of protons in a subject by applying a high-frequency magnetic field pulse to the subject placed in a static magnetic field, and performs signal processing on the signal to image it. An imaging device. As one of the high-speed imaging methods performed by the MRI apparatus, the magnetization is made to be in a steady state (SSFP: Steady State Free Precession) by continuously applying a high-frequency magnetic field pulse, and the phase dispersion of the magnetization by the gradient magnetic field is repeated (TR) There is a high-speed imaging sequence using a coherent SSFP state that makes it zero between. Non-Patent Document 1 describes an example of the imaging sequence.

  On the other hand, protons in the specimen include protons such as water, protein, fat, etc. Among them, the NMR signal due to fat protons is characterized by a relatively higher signal than other protons. Have. However, in clinical practice, it is often desired to obtain an image in which water and fat are separated.

A CHESS (Chemical Shift Selective) method or a Dixon method can be used as a method for acquiring an image in which a fat component is suppressed. The CHESS method uses a difference in the resonance frequency of water / fat magnetization due to chemical shift, irradiates a frequency-selective high-frequency magnetic field pulse, and suppresses the fat signal by saturating only the fat signal in advance. The Dixon method uses the Larmor frequency difference between water and fat magnetization to acquire nuclear magnetic resonance signals when the water and fat magnetization phases are the same and opposite phases, and adds and subtracts each. This is a method for obtaining an image in which water and fat are separated by processing. Non-Patent Document 2 describes a method of separating and acquiring a water signal and a fat signal by addition / subtraction processing in a coherent SSFP sequence.
FISP-a new fast MRI sequence. Oppelt A., et al. Electromedica 54, 15-18 (1986) Linear Combination Steady-State Free Precession MRI S. Vasanawala, et al. Magnetic Resonance in Medicine 43: 82-90 (2000)

  However, when acquiring a water / fat separation image in a coherent SSFP system sequence, using the CHESS method gradually restores fat magnetization during imaging, so a selective high-frequency magnetic field pulse for fat signal saturation is added. Irradiation must be performed, making it difficult to maintain the SSFP state. As a result, problems such as generation of artifacts and change in contrast occur. Further, in the method of Non-Patent Document 2, when a position shift occurs in the subject during imaging, an error occurs in the addition / subtraction process, and an artifact is generated in the water / fat separation image.

  An object of the present invention is to provide an MRI apparatus capable of separating a water / fat signal without calculating between acquired measurement data and acquiring a water / fat separated image at high speed.

  In order to achieve the above object, the present invention provides the following MRI apparatus. That is, by repeatedly irradiating a high-frequency magnetic field pulse at a predetermined time interval (TR) to bring the magnetization into a steady state and applying the gradient magnetic field so that the phase dispersion of the magnetization becomes zero for each TR, the coherent The MRI apparatus executes a pulse sequence for measuring a nuclear magnetic resonance signal in a steady state. At this time, TR is set to a time interval such that water magnetization and fat magnetization are in opposite phases, and the phase of the high frequency magnetic field pulse irradiated for each TR is relative to the phase of precession of water magnetization. The phase difference is set to change by a predetermined magnitude at a predetermined irradiation interval. Thereby, it becomes possible to acquire a water magnetization signal and a fat magnetization signal alternately for every TR.

  For example, the phase difference of a high-frequency magnetic field pulse changes by 180 ° for every 2TR, such as 0 °, 0 °, 180 °, 180 °, 0 °, 0 °, etc., with respect to the phase of water magnetization. Can be set.

  By changing the gradient magnetic field pulse that gives the phase encoding every 2TR, the signal received by the receiving means is divided into two types: the signal acquired by the odd-numbered TR and the signal acquired by the even-numbered TR. It is possible to reconstruct a water image and a fat image from the other.

  In addition, the gradient magnetic field pulse to which the phase encoding is applied is changed every n times 2TR, and the signal received by the receiving means is classified into two types, that is, the signal acquired by the odd-numbered TR and the signal acquired by the even-numbered TR. It is also possible to reconstruct a water image from one of the accumulated signals and a fat image from the other by integrating each n.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First, the configuration of the MRI apparatus of the present embodiment will be specifically described with reference to FIG. The MRI apparatus obtains a tomographic image of a subject using a magnetic resonance phenomenon, and includes a static magnetic field generation apparatus 1, a gradient magnetic field generation system 2, a transmission system 3, a reception system 4, and a signal processing system. 5, a sequencer 6, a central processing unit (CPU) 7, an operation unit 8, and a bed 27 on which the subject 9 is mounted.

  The static magnetic field generator 1 is a device that generates a uniform static magnetic field around the subject 9 mounted on the bed 27 in the body axis direction or in a direction perpendicular to the body axis. As the static magnetic field generator 1, a device including a permanent magnet, a normal conducting magnet, or a superconducting magnet as a magnetic field generating source can be used. The gradient magnetic field generation system 2 includes a gradient magnetic field coil 10 and a gradient magnetic field power supply 11 and applies gradient magnetic fields Gs, Gp, Gr in three orthogonal directions to the subject 9. Depending on how the gradient magnetic field is applied, the slice plane for the subject 9 is set and the position information is added to the NMR signal.

  The transmission system 3 receives a signal transmitted from a sequencer 6 to be described later, generates a high-frequency magnetic field (RF) pulse for causing nuclear magnetic resonance in the atomic nucleus constituting the biological tissue of the subject 9, The object 9 is irradiated with a high frequency oscillator 12, a modulator 13, a high frequency amplifier 14, and a high frequency irradiation coil 15. The high-frequency signal output from the high-frequency oscillator 12 is modulated by the modulator 13 in accordance with the control signal from the sequencer 6, further amplified by the high-frequency amplifier 14, and then applied to the high-frequency irradiation coil 15 disposed close to the subject 9. Supplied. As a result, the subject 9 is irradiated with an RF pulse from the high-frequency irradiation coil 15.

  The receiving system 4 detects a nuclear magnetic resonance signal (echo signal) emitted by nuclear magnetic resonance of the nucleus of the biological tissue of the subject 9, and includes a high-frequency receiving coil 16, an amplifier 17, a quadrature detector 18, and the like. And an A / D converter 19. The echo signal is received by the high-frequency receiving coil 16 arranged close to the subject 9, a desired NMR signal is detected by the amplifier 17 and the quadrature detector 18, and converted into a digital quantity by the A / D converter 19. Further, two series of collected data sampled by the quadrature detector 18 at a timing according to a command from the sequencer 6. This signal is sent to the signal processing system 5.

  The signal processing system 5 performs an image reconstruction calculation using the echo signal detected by the reception system 4 and displays the reconstructed image. The signal processing system 5 includes a CPU 7, a ROM 20, a RAM 21, a data storage unit such as a magneto-optical disk 22 and a magnetic disk 24, and a display 23. The ROM 20 stores in advance various programs executed by the CPU 7, invariant parameters used in the execution, and the like. The CPU 7 reads and executes a program stored in the ROM 20 to perform processing such as Fourier transform, correction coefficient calculation, image reconstruction, etc. on the echo signal obtained by the receiving system 4, and obtains the obtained tomographic image and the like. It is displayed on the display 23. Also, temporal image analysis processing and the like are performed according to the program. The RAM 21 temporarily stores data such as measurement parameters obtained by measurement and echo signals detected by the reception system 4. A tomographic image reconstructed by the CPU 7 is recorded on the magneto-optical disk 22 and the magnetic disk 24.

  The operation unit 8 is used by a user to input control information of processing performed in the signal processing system 5 and imaging conditions, and includes a mouse 25 and a keyboard 26.

  Further, the CPU 7 executes an imaging program stored in the ROM 20 to create an imaging pulse sequence for realizing an imaging method designated by the user, and transfers it to the sequencer 6. The sequencer 6 outputs a control signal to the modulator 13 of the transmission system 3, the gradient magnetic field power supply 11 and the A / D converter 19 of the reception system in accordance with the imaging pulse sequence, so that the RF pulse and the gradient magnetic field pulse are predetermined. It is applied to the subject 9 at the timing, and the generated echo signal is detected at a predetermined timing.

  Next, the imaging pulse sequence of this embodiment will be described with reference to FIGS.

  This imaging pulse sequence is an imaging sequence based on the gradient echo method of obtaining one echo signal 106 by irradiating the RF pulse 101 once as shown in FIG. 2, with a short time interval (TR) interval as shown in FIG. The imaging is performed in a steady state (SSFP) by repeating the process continuously. In each imaging sequence, a compensation gradient magnetic field 109 is applied in pair with a gradient magnetic field pulse 103 that gives a phase encoding offset, so that the magnetization phases are aligned again (coherence) when the next RF pulse is applied. In the present embodiment, TR is a time interval such that the magnetization becomes SSFP, and the phase of fat magnetization becomes the phase of water magnetization from the precession frequency difference (chemical shift difference) of water magnetization and fat magnetization. On the other hand, the time interval is set so that the phase is opposite (phase difference 180). In addition, the phase of the sequentially applied RF pulse 101 is 180.degree. For every two, such as 180.degree., 180.degree., 0.degree., 0.degree. The phase difference is changed. Thereby, the echo signal of the water magnetization which suppressed fat magnetization, and the echo signal of the fat magnetization which suppressed water magnetization can be acquired alternately.

  The imaging pulse sequence will be described more specifically with reference to FIGS. In the 1TR imaging sequence, as shown in FIG. 2, first, the RF pulse 101 is irradiated from the high-frequency irradiation coil 15. The RF pulse 101 is an RF pulse 101 that tilts water and fat magnetization by a predetermined flip angle α °, and has a predetermined phase difference (0 ° or 180 °) with respect to the phase of precession of water magnetization at the time of irradiation. Have Simultaneously with the irradiation of the RF pulse 101, a gradient magnetic field pulse (Gs) 102 in the slice direction is applied to select a slice. Subsequently, a rephasing gradient magnetic field pulse (Gs) 111 in the slice direction for collecting the magnetization dispersed by the slice selective gradient magnetic field pulse 102, an offset gradient magnetic field pulse (Gp) 103 in the phase encoding direction, and an offset gradient magnetic field in the reading direction. A pulse (Gr) 104 is applied, and then a readout gradient magnetic field pulse 105 is applied. The reception system 4 receives the data 107 of the echo signal 106 generated during the application of the read gradient magnetic field pulse 105. In FIG. 2, the rephasing gradient magnetic field pulse 111, the offset gradient magnetic field pulse 103, and the offset gradient magnetic field pulse 104 are applied at different timings, but can be applied simultaneously.

  Subsequently, a spoiling gradient magnetic field 110 in the readout direction and a rewind gradient magnetic field pulse 109 in the phase direction are applied. Further, an offset gradient magnetic field pulse 108 for the slice direction gradient magnetic field pulse 102 of the next RF pulse 101 is applied. In this way, by applying the compensation gradient magnetic field 109 in combination with the gradient magnetic field pulse 103 that gives the phase encoding offset, the phase of magnetization is again aligned (coherence) when the next RF pulse is applied.

  The sequence shown in FIG. 2 is repeated while changing the phase encoding gradient magnetic field pulse 103 and the rewind gradient magnetic field pulse 109 every 2TR at intervals of time TR as shown in FIG. This is continued until the echo signal 106 having the number of data necessary for image reconstruction is acquired.

Specifically, the time TR is set to the following time τ. That is, in order to make the phase of fat magnetization opposite to the phase of water magnetization (the phase difference is 180 °), the time is set in advance from the following equation (1). Note that δ is a chemical shift difference between water magnetization and fat magnetization.
TR = 1 / (2 × δ) (1)

The phase difference θ between the water magnetization and the fat magnetization due to the time TR is affected by the nonuniformity of the static magnetic field strength B ₀ , but the change in the phase difference between water and fat due to the static magnetic field nonuniformity is slight. Can be ignored. Further, the phase of the RF pulse 101 is shifted with respect to the phase of water magnetization and fat magnetization due to non-uniformity of the static magnetic field, but this phase shift is different from that of water magnetization and fat magnetization unless the static magnetic field inhomogeneity changes. And fixed amount. Therefore, as long as extreme static magnetic field inhomogeneity does not occur, this phase shift is permissible and does not affect the function of water / fat separation according to the present embodiment.

  Here, the movement of water magnetization and fat magnetization in the imaging pulse sequence of FIG. 3 will be described with reference to FIGS. 3 and 4 show the movement of magnetization as seen from the rotational coordinate system of water precession. Therefore, in FIGS. 3 and 4, the water magnetization is shown not to rotate (precession), and the fat magnetization seems to rotate (precession) at a resonance frequency difference (chemical shift) from the water magnetization. It is expressed in Here, assuming that the RF pulse 101 is applied in the order of 180 °, 180 °, 0 °, and 0 ° from the initial state, the water magnetization and fat that are both upright in the initial state as shown in FIG. Magnetization is tilted (flipped) in the direction of 180 ° of the rotational coordinate system of water magnetization, that is, −α ° from the rotation axis, together with the RF pulse 101 having a first phase difference of 180 ° and a flip angle of α ° (FIG. 4). (B)). (Hereinafter, the inclination of the rotation axis of water magnetization in the direction of 180 ° is referred to as “−α ° degree flip”, and the inclination of the rotation coordinate system in the direction of 0 ° is referred to as “+ α ° degree flip”). Since TR is set so that the phase of fat magnetization is opposite to the phase of water magnetization as described above, rotation of fat magnetization is performed while time TR elapses until the second RF pulse 101. The coordinate system rotates to an angle that is opposite in phase to the rotational coordinate system of water magnetization, and fat magnetization is tilted in the direction opposite to water magnetization (+ α °) around the rotation axis (see FIG. 4 (c)). By receiving the second RF pulse 101 having a phase difference of 180 ° in this state, the fat magnetization and water magnetization are further flipped by −α °, so that the fat magnetization is almost upright and the water magnetization is further inclined ( FIG. 4 (d)).

  Thereby, water magnetization and fat magnetization are separated, the echo signal 106 due to fat magnetization is suppressed, and the signal 106 due to water magnetization can be acquired. In FIG. 4 (d), the difference in tilt angle between water magnetization and fat magnetization is 2α. However, if irradiation with the TR interval of the RF pulse 101 is continued thereafter, as in the normal TrueFISP sequence, Under the influence of T1 relaxation, the flip angle gradually converges, and finally the water and fat magnetization are in a steady state (the state shown in FIG. 3A) with a tilt angle difference α. Note that the signal 106 acquired until the steady state in FIG. 3 is not used for image reconstruction.

  An RF pulse 101 having a phase difference of 0 ° and a flip angle α ° with respect to the rotating coordinate system of water magnetization is generated with respect to the water magnetization and fat magnetization ((a) in FIG. 3) that are in a steady state in the separated state. When irradiated, both magnetizations are in the opposite direction to that when receiving the RF pulse 101 having a phase difference of 180 °, that is, + α ° flipping ((b) in FIG. 3). Thereby, the water magnetization is almost upright, and the fat magnetization is inclined + α ° from the rotation axis. Therefore, the echo signal 106 acquired by this TR is a fat magnetization echo signal, and the water magnetization signal is suppressed.

  While the time TR until the next RF pulse 101 is irradiated, the rotational coordinate system of fat magnetization rotates 180 degrees, so that the fat magnetization is inclined by -α ° from the rotation axis ( (C) of FIG. In this state, the RF pulse 101 having a phase difference of 0 ° with respect to the water magnetization is irradiated, so both magnetizations are flipped by + α °, the water magnetization is inclined + α ° from the rotation axis, and the fat magnetization is upright ( (D) of FIG. The signal 107 acquired by the TR immediately after is an echo signal of water magnetization, and the fat magnetization signal is suppressed.

  During the time TR until the next RF pulse 101 is irradiated, the rotational coordinate system of fat magnetization rotates 180 degrees, but since the fat magnetization at this time is in an upright state, the direction of the inclination does not change (see FIG. 3 (e)). When an RF pulse 101 having a phase difference of 180 ° is irradiated in this state, both magnetizations are flipped by −α °, the water magnetization is in an upright state, and the fat magnetization is inclined by −α ° from the rotation axis. (FIG. 3 (f)). The signal 107 acquired by the immediately following TR is an echo signal of fat magnetization, and the water magnetization signal is suppressed.

  During the time TR until the next RF pulse 101 is irradiated, the rotational coordinate system of fat magnetization is rotated by 180 degrees, so that the fat magnetization is inclined by + α ° from the rotation axis ((g in FIG. 3 )). When an RF pulse 101 having a phase difference of 180 ° is irradiated in this state, both magnetizations are flipped by −α °, the water magnetization is inclined by −α ° from the rotation axis, and the fat magnetization becomes upright (FIG. 3). (h)). The signal 107 acquired by the TR immediately after is an echo signal of water magnetization, and the fat magnetization signal is suppressed.

  During the time TR until the next RF pulse 101 is irradiated, the rotational coordinate system of fat magnetization rotates 180 degrees with respect to the water magnetization, but since the fat magnetization is in an upright state, the inclination direction does not change. . Accordingly, the state returns to the state of FIG. 3A, and the magnetization states of FIGS. 3A to 3D are repeated thereafter.

  At this time, as shown in FIG. 5, the phase encoding gradient magnetic field pulse 103 and the rewinding gradient magnetic field pulse 109 are changed every 2TR, and repeatedly until data 107 of all the phase encoding numbers necessary for image reconstruction are acquired. Then, the imaging pulse sequence of FIG. 3 is performed. The acquired data 107 is separated from the odd-numbered data 107 and the even-numbered data 107 as shown in FIG. 6 and is subjected to processing such as two-dimensional Fourier transform, respectively, so that image reconstruction is performed. I do. Thereby, a water image and a fat image can be obtained. Since the phase of the echo signal changes depending on whether the magnetization is inclined in the positive direction or in the negative direction, adjustments such as reversing the detection phase of the receiving system 4 at the time of reception are performed.

  As described above, the imaging pulse sequence of the present embodiment applies an RF pulse having a predetermined phase difference with respect to the phase of the precession of water magnetization to the coherent magnetization in a steady state. The water / fat signal can be separated without calculating between the acquired measurement data, and the water / fat separated image can be obtained at high speed.

  In order to stabilize the transition in the transition period from the state in which the flip angle difference between the water magnetization and the fat magnetization in FIG. 4 is 2α to the steady state (state (a) in FIG. 3) of the flip angle difference α, Although it is possible to set the flip angle of the RF pulse 101 irradiated in the period to α / 2, the difference in tilt angle of water fat magnetization converges to α without using such a method. Even when the flip angle is set to α / 2 in the transition period, the phase of the RF pulse 101 changes every two of 180 °, 180 °, 0 °, 0 °... As described above. And TR is set to the value determined by equation (1).

  When signal integration is performed to improve the SN ratio, the phase encoding gradient magnetic field pulse 103 and the rewind gradient magnetic field pulse 109 are changed by n times 2TR (for example, twice in FIG. 7) as shown in FIG. Then, it is only necessary to accumulate n pieces of odd-numbered data 107 and n pieces of even-numbered data 107, and the SN ratio can be improved without performing complicated calculations.

  In the present embodiment, the case of an imaging pulse sequence that acquires an echo signal by the 2D gradient echo method has been described, but various physiological signal synchronous imaging such as 3D imaging method, dynamic imaging method, fluoroscopy, and electrocardiographic synchronization are described. The same applies to the law.

1 is a block diagram showing a configuration of an MRI apparatus according to an embodiment. Explanatory drawing which shows the detailed content of the imaging pulse sequence of this Embodiment. Explanatory drawing which shows the imaging pulse sequence of this Embodiment, and explanatory drawing which shows the state of fat magnetization of water magnetization. In the imaging pulse sequence of this Embodiment, explanatory drawing which shows the state from which water magnetization and fat magnetization become an antiphase from an initial state. Explanatory drawing which shows changing the phase encoding amount for every 2TR in the imaging pulse sequence of this Embodiment. Explanatory drawing which isolate | separates odd-numbered data and even-numbered data in the imaging pulse sequence of this Embodiment, and reconfigure | reconstructs a water image and a fat image, respectively. Explanatory drawing which shows the case where several data are integrated | accumulated in the imaging pulse sequence of this Embodiment for SN ratio improvement.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Static magnetic field generator, 2 ... Gradient magnetic field generation system, 3 ... Transmission system, 4 ... Reception system, 5 ... Signal processing system, 6 ... Sequencer, 7 ... CPU, 8 ... operation unit, 9 ... subject, 10 ... gradient magnetic field coil, 11 ... gradient magnetic field power supply, 12 ... high frequency oscillator, 13 ... modulator, 14 ... High frequency amplifier, 15 ... high frequency irradiation coil, 16 ... high frequency receiving coil, 17 ... high frequency amplifier, 18 ... quadrature phase detector, 19 ... A / D converter, 20 ... ROM , 21 ... RAM, 22 ... magneto-optical disk, 23 ... display, 24 ... magnetic disk, 25 ... trackball or mouse, 26 ... keyboard.

Claims (4)

Irradiating means for irradiating a subject placed in a static magnetic field with a high-frequency magnetic field pulse, receiving means for receiving a nuclear magnetic resonance signal generated from the subject by irradiation of the high-frequency magnetic field pulse, and adding position information to the signal A gradient magnetic field generating means for generating a gradient magnetic field, a control means for operating the irradiation means, the gradient magnetic field generating means, and the receiving means in accordance with a predetermined pulse sequence; and an image of the subject based on the signal. Signal processing means for reconfiguration,
The pulse sequence repeatedly irradiates the high-frequency magnetic field pulse at a predetermined time interval (TR) to bring the magnetization into a steady state, and applies the gradient magnetic field so that the phase dispersion of the magnetization becomes 0 for each TR. This is a sequence for measuring a nuclear magnetic resonance signal in a coherent steady state, and the TR is set to a time interval in which water magnetization and fat magnetization are in opposite phases, and the high frequency magnetic field irradiated for each TR The magnetic resonance imaging apparatus characterized in that the phase of the pulse is set so that the phase difference changes by a predetermined magnitude at a predetermined irradiation interval with respect to the phase of precession of water magnetization.   2. The magnetic resonance imaging apparatus according to claim 1, wherein the phase of the high-frequency magnetic field pulse is set such that a phase difference changes by 180 ° every 2TR with respect to the phase of the water magnetization. Magnetic resonance imaging apparatus.   3. The magnetic resonance imaging apparatus according to claim 1, wherein the pulse sequence changes a gradient magnetic field pulse to which phase encoding is applied every 2 TR, and a signal received by the receiving unit is obtained by an odd-numbered TR. A magnetic resonance imaging apparatus, wherein a signal obtained by an even-numbered TR is classified into two types, and a water image is reconstructed from one and a fat image is reconstructed from the other. 3. The magnetic resonance imaging apparatus according to claim 1, wherein the pulse sequence changes a gradient magnetic field pulse to which phase encoding is applied every n times of 2TR, and obtains a signal received by the receiving means with an odd-numbered TR. Magnetic resonance imaging, wherein the signal is divided into two types, the signal obtained by the even-numbered TR and the signals obtained by the even-numbered TR, each of which is integrated n, and a water image is reconstructed from one of the accumulated signals and a fat image is reconstructed from the other apparatus.

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