WO2012005137A1 - 磁気共鳴イメージング装置及びrfパルス制御方法 - Google Patents
磁気共鳴イメージング装置及びrfパルス制御方法 Download PDFInfo
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- WO2012005137A1 WO2012005137A1 PCT/JP2011/064737 JP2011064737W WO2012005137A1 WO 2012005137 A1 WO2012005137 A1 WO 2012005137A1 JP 2011064737 W JP2011064737 W JP 2011064737W WO 2012005137 A1 WO2012005137 A1 WO 2012005137A1
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- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
- G01R33/543—Control of the operation of the MR system, e.g. setting of acquisition parameters prior to or during MR data acquisition, dynamic shimming, use of one or more scout images for scan plane prescription
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
- G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
- G01R33/561—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reduction of the scanning time, i.e. fast acquiring systems, e.g. using echo-planar pulse sequences
- G01R33/5613—Generating steady state signals, e.g. low flip angle sequences [FLASH]
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
- G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
- G01R33/565—Correction of image distortions, e.g. due to magnetic field inhomogeneities
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- G—PHYSICS
- G06—COMPUTING; CALCULATING OR COUNTING
- G06F—ELECTRIC DIGITAL DATA PROCESSING
- G06F17/00—Digital computing or data processing equipment or methods, specially adapted for specific functions
- G06F17/10—Complex mathematical operations
- G06F17/11—Complex mathematical operations for solving equations, e.g. nonlinear equations, general mathematical optimization problems
Definitions
- the present invention relates to a magnetic resonance imaging (hereinafter referred to as “MRI”) apparatus, and more particularly to application control of a high frequency magnetic field pulse that leads to a steady state free precession motion state in a short time.
- MRI magnetic resonance imaging
- MRI equipment measures nuclear magnetic resonance (NMR) signals generated by the nuclear spins that make up the body of a subject, especially the human body, and the shape and function of the head, abdomen, limbs, etc. in two or three dimensions It is a device that automatically images.
- NMR nuclear magnetic resonance
- the NMR signal is given different phase encoding depending on the gradient magnetic field, frequency-encoded, and measured as time series data.
- the measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.
- a pulse sequence (hereinafter referred to as an SSFP sequence) that measures an echo signal by making the magnetization a steady state free precession motion (SSFP; Steady State Free Free Precession, hereinafter referred to as a steady state). )It has been known.
- SSFP sequence high-frequency magnetic field pulses (hereinafter referred to as RF pulses) are irradiated with a very short repetition time TR of about several ms to excite nuclear spins (hereinafter abbreviated as spins).
- spins nuclear spins
- the spin converges to a steady state in which excitation and relaxation are balanced after a time of about the longitudinal relaxation time (T1) has elapsed. While the echo signal measured in the steady state has a high signal-to-noise ratio (SNR; Signal to Noize Ratio), it is necessary to irradiate the RF pulse continuously until the steady state is reached.
- SNR signal-to-noise ratio
- a typical application is cine imaging.
- transient states the state in the middle of the spin reaching the steady state from the thermal equilibrium state is called a transient state.
- excitation and relaxation are not balanced, and the spin is unstable, so the intensity of the echo signal often oscillates. This vibration causes artifacts such as ghosting and blurring in the reconstructed image. Therefore, when measuring the echo signal in the transient state, a start-up sequence is executed as a preparation sequence before the data acquisition sequence for measuring the echo signal for image reconstruction, and the echo signal at the time of echo data collection is Suppresses vibration.
- ⁇ / 2 method for example, Patent Document 1 as one method of the start-up sequence.
- an RF pulse having a flip angle that is half the flip angle ( ⁇ ) of the RF pulse for echo data collection is irradiated before TR / 2 before the irradiation time of the first RF pulse.
- on-resonance spin for spins having the same resonance frequency as the RF pulse irradiation frequency
- vibration can be suppressed, but the spin having a resonance frequency different from the RF pulse irradiation frequency.
- off-resonance spin vibration cannot be sufficiently suppressed.
- Linear-Flip-Angle method for example, Non-Patent Document 1
- the Linear Flip Angle method is a method of irradiating an RF pulse at a certain time interval TR while linearly increasing the flip angle as shown in the equation (1).
- ⁇ (n) is the absolute value of the flip angle of the nth RF pulse
- N is the total number of RF pulses in the start-up sequence
- a ⁇ / N. That is, the flip angle ⁇ (n) specified by the equation (1) is a value on a straight line with an offset (intercept) of 0 (zero) and an inclination of a.
- the flip angle is ⁇ (n) ⁇ ( ⁇ 1) n .
- vibrations can be suppressed for off-resonance spins compared to the ⁇ / 2 method, but vibrations occur for on-resonance spins. This vibration can be suppressed by setting the total number of RF pulses to about several tens of times.
- the echo signal oscillates not only for off-resonance spins but also for on-resonance spins.
- the echo signal is transmitted with the vibration remaining. Must be measured.
- artifacts such as ghosting and blurring may occur in the image.
- the present invention has been made in view of the above problems, and even if the total number of RF pulses of the start-up sequence leading to a steady state is reduced, the vibration of the echo signal is reduced, and a ghost or blur is added to the image.
- the purpose is to prevent artifacts such as rings from occurring.
- the RF pulse sequence of the start-up sequence has a flip angle that increases monotonously with an offset.
- the sum between two adjacent terms in a monotonically increasing number sequence is the flip angle.
- the MRI apparatus of the present invention includes a start-up sequence that suppresses spin oscillation in a transient state by an RF pulse sequence in which the flip angle monotonously increases, and an SSFP sequence that measures an echo signal in a steady state.
- a calculation processing unit that generates an imaging sequence and a measurement control unit that controls measurement of an echo signal from the subject based on the imaging sequence, and the RF pulse sequence has a flip angle that monotonously increases with an offset It is characterized by being.
- the RF pulse control method of the present invention includes the steps of setting the number of RF pulses in the RF pulse sequence and the flip angle of the RF pulse in the SSFP sequence, and the number of RF pulses in the set RF pulse sequence and the RF in the SSFP sequence. And a flip angle calculation step for obtaining an RF pulse sequence by obtaining a flip angle that monotonously increases with an offset based on the flip angle of the pulse.
- the vibration of the echo signal is reduced, and ghost, blurring, etc. Artifacts do not occur, and high-quality images can be acquired.
- FIG. 6 is a diagram illustrating a monotonically increasing number sequence (a) of the first embodiment and a flip angle (b) of an RF pulse sequence based on the monotonically increasing number sequence.
- the figure which shows the flip angle of the RF pulse sequence of Start-up sequence when N is 5 times.
- the figure which shows the behavior of the spin of on resonance. (a) shows the case where the Start-up sequence of FIG. 3 is used, and (b) shows the case where the conventional Linear Flip Angle method is used.
- FIG. 3 is a diagram illustrating functional blocks of an arithmetic processing unit according to the first embodiment.
- 3 is a flowchart illustrating an operation flow of functional blocks of an arithmetic processing unit according to the first embodiment. The figure which shows an example of the input screen of imaging conditions.
- FIG. 10 is a diagram showing a monotonically increasing number sequence (a) of Example 2 and a flip angle (b) of an RF pulse sequence based on the monotonically increasing number sequence.
- FIG. 11 is a diagram showing a result of numerical calculation of vibrations in a transient state for off-resonance spins when the start-up sequence of FIG. 10 is used.
- FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention.
- This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject 101.
- a static magnetic field generating magnet 102, a gradient magnetic field coil 103, a gradient magnetic field power supply 109, and an RF transmission coil 104, an RF transmitter 110, an RF receiver coil 105, a signal detector 106, a signal processor 107, a measurement controller 111, an overall controller 108, a display / operation unit 113, and a subject 101 are mounted.
- a bed 112 for taking the top plate into and out of the static magnetic field generating magnet 102.
- the static magnetic field generating magnet 102 generates a uniform static magnetic field in the direction perpendicular to the body axis of the subject 101 in the vertical magnetic field method and in the body axis direction in the horizontal magnetic field method.
- a permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the.
- the gradient magnetic field coil 103 is a coil wound in the three-axis directions of X, Y, and Z that are the real space coordinate system (stationary coordinate system) of the MRI apparatus, and each gradient magnetic field coil is a gradient magnetic field that drives it.
- a current is supplied to the power source 109.
- the gradient magnetic field power supply 109 of each gradient coil is driven according to a command from the measurement control unit 111 described later, and supplies a current to each gradient coil.
- gradient magnetic fields Gx, Gy, and Gz are generated in the three-axis directions of X, Y, and Z.
- a slice gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 101, orthogonal to the slice plane and orthogonal to each other.
- Phase encoding gradient magnetic field pulse (Gp) and frequency encoding (leadout) gradient magnetic field pulse (Gf) are applied in the remaining two directions, and position information in each direction is encoded in the NMR signal (echo signal). .
- the RF transmission coil 104 is a coil that irradiates the subject 101 with an RF pulse, and is connected to the RF transmission unit 110 and supplied with a high-frequency pulse current. As a result, an NMR phenomenon is induced in the spins of atoms constituting the living tissue of the subject 101.
- the RF transmission unit 110 is driven in accordance with a command from the measurement control unit 111 described later, and the RF transmission coil 104 is arranged in proximity to the subject 101 after the high frequency pulse is amplitude-modulated and amplified. , The subject 101 is irradiated with an RF pulse.
- the RF receiving coil 105 is a coil that receives an echo signal emitted by the NMR phenomenon of spin that constitutes the living tissue of the subject 101.
- the received echo signal is connected to the signal detecting unit 106 and is received by the signal detecting unit 106. Sent.
- the signal detection unit 106 performs detection processing of the echo signal received by the RF receiving coil 105. Specifically, the echo signal of the response of the subject 101 induced by the RF pulse irradiated from the RF transmission coil 104 is received by the RF receiving coil 105 disposed in the vicinity of the subject 101, and measurement control described later is performed. In accordance with a command from the unit 111, the signal detection unit 106 amplifies the received echo signal, divides the signal into two orthogonal signals by quadrature detection, and samples each by a predetermined number (for example, 128, 256, 512, etc.) Each sampling signal is A / D converted into a digital quantity and sent to a signal processing unit 107 described later. Therefore, the echo signal is obtained as time-series digital data (hereinafter referred to as echo data) composed of a predetermined number of sampling data.
- echo data time-series digital data
- the signal processing unit 107 performs various processes on the echo data, and sends the processed echo data to the measurement control unit 111.
- the measurement control unit 111 mainly transmits various commands for collecting echo data necessary for reconstruction of the tomographic image of the subject 101 to the gradient magnetic field power source 109, the RF transmission unit 110, and the signal detection unit 106. And a control unit for controlling them. Specifically, the measurement control unit 111 operates under the control of the overall control unit 108 described later, and controls the gradient magnetic field power source 109, the RF transmission unit 110, and the signal detection unit 106 based on a predetermined pulse sequence. The echo necessary for reconstructing the image of the imaging region of the subject 101 is repeatedly executed by irradiating the subject 101 with the RF pulse and applying the gradient magnetic field pulse and detecting the echo signal from the subject 101. Control data collection.
- the application amount of the phase encoding gradient magnetic field is changed in the case of two-dimensional imaging, and the application amount of the slice encoding gradient magnetic field is further changed in the case of three-dimensional imaging.
- Values such as 128, 256, and 512 are normally selected as the number of phase encodings, and values such as 16, 32, and 64 are normally selected as the number of slice encodings.
- the overall control unit 108 controls the measurement control unit 111 and controls various data processing and processing result display and storage, and includes an arithmetic processing unit 114 having a CPU and a memory, an optical disc, And a storage unit 115 such as a magnetic disk.
- the measurement control unit 111 is controlled to execute the collection of echo data, and when the echo data is input from the measurement control unit 111, the arithmetic processing unit 114 converts the encoded information applied to the echo data. Based on this, it is stored in an area corresponding to the K space in the memory.
- a group of echo data stored in an area corresponding to the K space in the memory is also referred to as K space data.
- the arithmetic processing unit 114 performs processing such as signal processing and image reconstruction by Fourier transform on the K space data, and displays the resulting image of the subject 101 on the display / operation unit 113 described later. And is recorded in the storage unit 115.
- the display / operation unit 113 includes a display unit for displaying the reconstructed image of the subject 101, a trackball or a mouse and a keyboard for inputting various control information of the MRI apparatus and control information for processing performed by the overall control unit 108. Etc., and an operation unit.
- the operation unit is disposed in the vicinity of the display unit, and an operator interactively controls various processes of the MRI apparatus through the operation unit while looking at the display unit.
- the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is the main constituent material of the subject, as is widely used in clinical practice.
- proton the main constituent material of the subject
- the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.
- each flip angle of the RF pulse sequence constituting the Start-up sequence is controlled so that the offset (intercept) becomes a value on a straight line other than 0 (zero).
- this embodiment will be described in detail with reference to FIGS.
- ⁇ represents the flip angle of the SSFP sequence at the time of echo data collection
- N represents the number of RF pulses in the start-up sequence.
- ⁇ is the limit that the longitudinal relaxation time T1 and the transverse relaxation time T2 are sufficiently longer than the repetition time TR, and the relaxation process can be ignored.
- the on resonance This means the absolute value of the angle that the spin makes with the Mz axis.
- the absolute value ⁇ (n) of the flip angle of the nth RF pulse is calculated from ⁇ (n) according to the following equation (3).
- the flip angle of the RF pulse sequence of the start-up sequence is ⁇ / 10, ⁇ 3 ⁇ / 10, 5 ⁇ / 10, ⁇ 7 ⁇ / 10, 9 ⁇ / 10 as shown in FIG. It becomes.
- ⁇ 90 deg., 9 deg., ⁇ 27 deg., 45 deg., ⁇ 63 deg., 81 deg.
- the flip angle of the conventional Linear Flip Angle method expressed by Equation (1) is 18 deg., ⁇ 36 deg., 54 deg., ⁇ 72 geg., 90 deg.
- the gradient magnetic field pulse waveform is omitted.
- the pulse sequence described in Patent Document 2 can be used, and thus detailed description thereof is omitted here.
- Fig. 4 shows the behavior of on-resonance spin when the Start-up sequence is used.
- (a) shows the case where the start-up sequence of this embodiment is used, and for comparison,
- (b) shows the case where the conventional Linear ⁇ Flip Angle method is used.
- the spin behavior on the Mx-Mz plane when executing the Start-up sequence is shown, and the numerical value in the figure represents the number of times of RF pulse irradiation.
- the flip angle of the RF pulse in this embodiment shown in the figure is 9 deg., -27 deg., 45 deg., -63 deg., 81 deg., and in the case of the conventional Linear ⁇ Flip Angle method shown in (b)
- the flip angle of the RF pulse is 18 deg., ⁇ 36 deg., 54 deg., ⁇ 72 geg., 90 deg.
- the on-resonance spin applied with the start-up sequence of this example moves alternately with the Mz axis every time an RF pulse is applied, but at the end of the start-up sequence, the Mz axis Is exactly ⁇ / 2, so that the vibration of the echo signal can be suppressed.
- the on-resonance spin to which the conventional Linear-Flip-Angle start-up sequence is applied moves alternately across the Mz axis each time an RF pulse is applied, and at the end of the start-up sequence, 401 and Since it vibrates between 402, the echo signal will also vibrate.
- Fig. 5 shows the numerical calculation results of the vibration in the transient state for the off-resonance spin.
- the spin whose vertical axis is zero is defined as “on resonance”, and the other spins are defined as “off resonance” (the same applies to FIG. 11 described later).
- (a) shows the case where the start-up sequence of this embodiment is used, and for comparison,
- (b) shows the case where the conventional Linear ⁇ Flip Angle method is used.
- the horizontal axis represents the number of times of RF pulse irradiation
- the vertical axis represents the offset frequency between repetition times TR
- the absolute value of the transverse magnetization is displayed in a grace case.
- Fig. 6 shows the result of numerical calculation of the flip angle dependence of the vibration in the transient state for the on-resonance spin using the Start-up sequence.
- (a) shows the case where the start-up sequence of this embodiment is used, and for comparison,
- (c) shows the case where the conventional LinearLineFlip Angle method is used.
- the horizontal axis indicates the number of RF pulses
- the vertical axis indicates the flip angle
- the absolute value of the transverse magnetization is displayed in gray scale as in FIG.
- the parameters used for the numerical calculation are the same as in FIG.
- Figures (b) and (d) show the absolute values of transverse magnetization at the number of RF pulses for spins with flip angles FA of 45, 90, and 135 degrees, respectively, from figures (a) and (c).
- FIG. 5B shows that no vibration occurs in the start-up sequence of this embodiment. This tendency is the same for an arbitrary flip angle.
- the vibration of the echo signal can be suppressed at an arbitrary flip angle, but the conventional Linear Flip Angle In the method, it can be understood that the echo signal vibrates at any flip angle. Therefore, by using the start-up sequence of the present embodiment, it is possible to suppress vibration even when a flip angle shifts due to uneven irradiation.
- FIG. 6 shows the numerical calculation results for the on-resonance spin, but the off-resonance spin is similar to that in FIG. It is possible to sufficiently suppress the vibration with respect to the spin at.
- the first term of ⁇ (n) is 0, but vibration can be suppressed even with an arbitrary value.
- FIG. 7 shows each function of the arithmetic processing unit 114 that obtains the flip angle of each RF pulse constituting the start-up sequence of the above-described embodiment and starts the SSFP sequence including the start-up sequence.
- the arithmetic processing unit 114 of the present embodiment includes an imaging condition setting unit 701, an RF pulse sequence setting unit 702, a pulse sequence setting unit 703, and an imaging control unit 704.
- the imaging condition setting unit 701 is connected to the display / operation unit 113 and the storage unit 115, and causes the display unit to display an input screen for accepting input of imaging condition setting (ie, imaging parameter value).
- imaging condition setting ie, imaging parameter value
- FIG. 9 shows an example of the input screen.
- This input screen includes at least the flip angle ⁇ (901) in the SSFP sequence that is the data acquisition sequence, and the RF pulse number N (902) in the RF pulse sequence in the start-up sequence that is the preparation sequence before collecting the echo data.
- a region for receiving input of the value is displayed for each imaging condition including.
- the operator inputs the value of each imaging condition via the operation unit.
- the imaging condition setting unit 701 stores various imaging condition values input by the operator in the memory as the various imaging condition setting values.
- predetermined values stored in the storage unit 115 may be used.
- the RF pulse sequence setting unit 702 is connected to the imaging condition setting unit 701 and calculates and sets the flip angle of the RF pulse sequence of the start-up sequence based on the input imaging conditions.
- the RF pulse sequence setting unit 702 uses the values of the flip angle ⁇ and the number of RF pulses N input by the operator and stored in the memory or read from the storage unit 115, based on the formula (2).
- An equality number sequence ⁇ (n) is created, and the absolute value ⁇ (n) of the flip angle of the nth RF pulse is calculated based on the equation (3) using the generated difference number sequence ⁇ (n).
- the absolute value ⁇ (n) of the flip angle of the nth RF pulse may be directly calculated based on the equation (4).
- the pulse sequence generation unit 703 is connected to the RF pulse sequence setting unit 702 and the storage unit 115, and sets application timings of RF pulses and gradient magnetic field pulses based on imaging conditions input by the operator and stored in the memory.
- a pal sequence is generated for specific data. Since the pulse sequence of the present embodiment includes a start-up sequence as a preparation sequence and an SSFP sequence as a data collection sequence, the pulse sequence setting unit 703 continuously executes these two pulse sequences. Specific data for each is generated. Specifically, the start-up sequence data specifically defines the RF pulse sequence of the flip angle represented by the equation (5) in which the irradiation phase is added to the flip angle set by the RF pulse sequence setting unit 702. This is data on RF pulses and gradient magnetic field pulses.
- the SSFP sequence data is data on an RF pulse and a gradient magnetic field pulse that define the measurement of an echo signal for image reconstruction by irradiating an RF pulse with a flip angle ⁇ .
- the imaging control unit 704 is connected to the pulse sequence generation unit 703, the storage unit 115, and the measurement control unit 111, notifies the measurement control unit 111 of specific data of the pulse sequence generated by the pulse sequence generation unit 703, and performs measurement control.
- the unit 111 is caused to execute the pulse sequence to start imaging.
- the imaging condition setting unit 701 displays an input screen for accepting an input of imaging condition setting on the display unit.
- the operator inputs and sets imaging conditions via this input screen.
- the imaging condition setting unit 701 reads the imaging condition value stored in advance in the storage unit 115.
- the imaging condition setting unit 701 stores the values of various imaging conditions that are input or read in the memory of the arithmetic processing unit 114.
- step 802 the RF pulse sequence setting unit 702 calculates the flip angle of the RF pulse sequence of the start-up sequence based on the value of the imaging condition stored in the memory in step 801, and stores it in the memory of the arithmetic processing unit 114.
- Step 803 the pulse sequence generation unit 703, based on the imaging condition value stored in the memory in Step 801 and the flip angle of the RF pulse sequence of the Start-up sequence stored in the memory in Step 802, Specific data for defining the flip angle of the RF pulse, the gradient magnetic field pulse shape, the application timing thereof, and the like are obtained, and specific data for continuously executing the Start-up sequence and the SSFP sequence is generated.
- step 804 the imaging control unit 704 notifies the measurement control unit 111 of specific data of the pulse sequence generated by the pulse sequence generation unit 703 in step 803, and causes the measurement control unit 111 to execute the generated pulse sequence. To start imaging.
- the MRI apparatus and the RF pulse control method of the present embodiment linearly increase the flip angle of each RF pulse constituting the RF pulse sequence of the start-up sequence with an appropriate offset. Thereby, it is possible to suppress vibration of the on-resonance spin. Therefore, even if the total number of start-up sequence RF pulses that lead to a steady state is reduced, the vibration of the echo signal is reduced, and artifacts such as ghosting and blurring do not occur in the image, and a high-quality image is acquired. be able to. That is, echo data collection can be started with a small number of idle shots.
- a start-up sequence is performed using a number sequence that smoothly changes at the rising edge and the contact point with the ⁇ / 2 line, that is, a number sequence in which the difference between adjacent two terms monotonously increases and then decreases monotonically. Determine each flip angle of the RF pulse sequence.
- this embodiment will be described in detail with reference to FIGS.
- the number sequence ⁇ is created based on the straight line connecting the two points 0 and ⁇ / 2.
- it may be a monotonically increasing number sequence. Therefore, as a result of numerical calculation by changing the monotonically increasing number sequence variously, the number sequence that smoothly changes at the rise and the contact point with the ⁇ / 2 line, that is, the difference between adjacent binomials increases monotonously after the monotonic increase.
- the inventor has discovered that by using the decreasing number sequence to determine each flip angle of the RF pulse sequence of the Start-up sequence, the oscillation of the echo signal is suppressed for a wider offset frequency.
- ⁇ (n) ⁇ / 4 ⁇ (1-cos (n ⁇ / N)) (6)
- This number sequence is shown in FIG.
- ⁇ (n) ⁇ / 2 ⁇ (1- ⁇ ⁇ cos (n ⁇ / N ⁇ ))
- n 1, 2,. , N (7)
- ⁇ 2 (1 + cos ( ⁇ / N)) / 2
- sin ( ⁇ ) sin ( ⁇ / N) / 2 ⁇
- cos ( ⁇ ) (1 + cos ( ⁇ / N)) / 2 ⁇ .
- FIG. 11 shows the result of numerical calculation of the vibration of the echo signal in the transient state for the off-resonance spin (in FIG. 11, the vertical axis is a spin other than zero).
- the coordinate axes and the method of numerical calculation are the same as in FIG. Comparing FIG. 5 (a) with FIG. 11, it can be seen that FIG. 11 can suppress the vibration with respect to a wider frequency offset. Note that FIG. 11 shows that vibration is also suppressed for spins of on-resonance (spins whose vertical axis is zero in FIG. 11).
- each function of the arithmetic processing unit 114 that obtains the flip angle of each RF pulse in the start-up sequence of the above-described embodiment and starts the SSFP sequence including the start-up sequence will be described.
- Each function of the arithmetic processing unit 114 in the present embodiment is the same as that of the first embodiment shown in FIG. 7, but the processing contents of the RF pulse sequence setting unit 702 are different.
- the processing contents of the RF pulse sequence setting unit 702 are different.
- only different portions will be described, and description of the same portions will be omitted.
- the RF pulse sequence setting unit 702 of the arithmetic processing unit 114 of the present embodiment creates an arithmetic sequence ⁇ (n) based on the equation (6), and uses the generated arithmetic sequence ⁇ (n) (3 ) To calculate the absolute value ⁇ (n) of the flip angle of the nth RF pulse.
- the absolute value ⁇ (n) of the flip angle of the nth RF pulse may be directly calculated based on the equation (7).
- processing flow of this embodiment which is performed in cooperation with the functional units of the arithmetic processing unit 114 of this embodiment, is the same as the processing flow of the first embodiment shown in FIG.
- the MRI apparatus and the RF pulse control method of the present embodiment use the rising angle and the flip angle calculated from the equation (3) based on the numerical sequence that smoothly changes at the point of contact with the ⁇ / 2 line.
- a start-up sequence comprising a plurality of RF pulse sequences is used.
- 1 subject 2 static magnetic field generation system, 3 gradient magnetic field generation system, 4 sequencer, 5 transmission system, 6 reception system, 7 signal processing system, 8 central processing unit (CPU), 9 gradient magnetic field coil, 10 gradient magnetic field power supply, 11 High frequency transmitter, 12 modulator, 13 high frequency amplifier, 14a high frequency coil (transmitting coil), 14b high frequency coil (receiving coil), 15 signal amplifier, 16 quadrature phase detector, 17 A / D converter, 18 magnetic disk, 19 optical disc, 20 display, 21 ROM, 22 RAM, 23 trackball or mouse, 24 keyboard
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Abstract
Description
ここで、φ(n)はn番目のRFパルスのフリップ角の絶対値、NはStart-upシーケンスのRFパルスの総数であり、a=α/Nである。即ち、(1)式で指定されるフリップ角φ(n)は、オフセット(切片)が0(ゼロ)で傾きがaの直線上の値となる。実際には、RFパルスは照射位相を180deg.ずらしながら照射するため、フリップ角はφ(n)×(-1)nとなる。Linear Flip Angle法では、オフレゾナンスのスピンについては、α/2法よりも振動を抑制可能であるが、オンレゾナンスのスピンについては振動が生じる。この振動は、RFパルスの総数を数10回程度とすれば抑制可能である。
2次元スライス面の撮像時には、スライス面(撮像断面)に直交する方向にスライス傾斜磁場パルス(Gs)が印加されて被検体101に対するスライス面が設定され、そのスライス面に直交して且つ互いに直交する残りの2つの方向に位相エンコード傾斜磁場パルス(Gp)と周波数エンコード(リードアウト)傾斜磁場パルス(Gf)が印加されて、NMR信号(エコー信号)にそれぞれの方向の位置情報がエンコードされる。
ここで、αはエコーデータ収集時のSSFPシーケンスのフリップ角を、NはStart-upシーケンスのRFパルス数を表す。例えば、Nが5回の場合は、α/10、2α/10、3α/10、4α/10、5α/10(=α/2)となる。このΘ(n)は、縦緩和時間T1と横緩和時間T2が繰り返し時間TRに較べて十分長く、緩和過程が無視できるような極限で、n番目のRFパルスを照射した後で、オンレゾナンスのスピンがMz軸とのなす角の絶対値を意味する。
φ(n)=Θ(n)+Θ(n-1) 2≦n≦Nのとき (3)
(2)式を(3)式に代入して解くと、下記(4)式のようになる。
ここで、傾き:a=α/N、オフセット(切片):b=-α/(2N)である。(4)式で表される直線の例としてN=5の場合を図2(b)に示す。つまり、本実施例のStart-upシーケンスを構成するRFパルス系列の各RFパルスのフリップ角は、オフセット(切片)が0(ゼロ)でない直線上の値となり、この直線に沿ってフリップ角がRFパルスの照射毎に単調に増加する。フリップ角が単調に増加するという点では従来のLinear Flip Angle法と同様であるが、従来のLinear Flip Angle法でのフリップ角の絶対値は、(1)式のようにオフセットがないのに対して、本実施例のフリップ角制御では(4)式のようにオフセットbが存在する点が異なる。
例えば、Nが5回の場合、Start-upシーケンスのRFパルス系列のフリップ角は、図3に示す様に、α/10,-3α/10,5α/10,-7α/10,9α/10となる。更にα=90deg.の場合、9deg.,-27deg.,45deg.,-63deg.,81deg.となる。これに対して、(1)式で示す従来のLinear Flip Angle法のフリップ角は、同一条件で18deg.,-36deg.,54deg.,-72geg.,90deg.である。なお、図3では、傾斜磁場パルス波形を省略してある。傾斜磁場パルス波形の詳細は、例えば特許文献2に記載のパルスシーケンスを用いることができるので、ここでの詳細説明は省略する。
Θ(n)=α/4×(1-cos(nπ/N)) (6)
が考えられる。この数列を図10(a)に示す。(6)式の数列について、(3)式に従い漸化式を解くと
φ(n)=α/2×(1-β×cos(nπ/N-γ))、n=1,2,…,N (7)
となる。ここで、β2=(1+cos(π/N))/2、sin(γ)=sin(π/N)/2β、cos(γ)=(1+cos(π/N))/2βである。(7)式で表される曲線の例としてN=5の場合を図10(b)に示す。実際には、照射位相を180deg.ずらしながら各RFパルスを照射するため、n番目のRFパルスのフリップ角は(5)式のように表される。
Claims (15)
- フリップ角が単調増加するRFパルス系列により過渡状態でのスピンの振動を抑制するStart-upシーケンスと、定常状態でエコー信号を計測するSSFPシーケンスとからなる撮像シーケンスを生成する演算処理部と、
前記撮像シーケンスに基づいて被検体からのエコー信号の計測を制御する計測制御部と、
を備えた磁気共鳴イメージングであって、
前記RFパルス系列は、オフセットを有して単調増加するフリップ角であることを特徴とする磁気共鳴イメージング装置。 - 請求項1記載の磁気共鳴イメージング装置において、
前記オフセットは、前記SSFPシーケンスのRFパルスのフリップ角(α)と、前記RFパルス系列のRFパルス数(N)に応じて変わる値であることを特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置において、
前記SSFPシーケンスのRFパルスのフリップ角(α)と、前記RFパルス系列のRFパルス数(N)の入力を受け付ける入力手段を備えていることを特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置において、
前記RFパルス系列のフリップ角は、単調増加数列の隣り合う二項間の和であることを特徴とする磁気共鳴イメージング装置。 - 請求項4記載の磁気共鳴イメージング装置において、
前記単調増加数列は、ゼロと前記SSFPシーケンスのRFパルスのフリップ角(α)の半分の値との間の値をとることを特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置において、
前記RFパルス系列のフリップ角は、線形に増加することを特徴とする磁気共鳴イメージング装置。 - 請求項6記載の磁気共鳴イメージング装置において、
前記RFパルス系列のフリップ角は、傾きがα/N、オフセットが-α/(2N)である直線に沿って増加することを特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置において、
前記RFパルス系列のフリップ角は、隣り合う二項間の差が単調増加の後単調減少する数列の隣り合う二項間の和であることを特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置において、
前記RFパルス系列のフリップ角は、立ち上がり、及び、α/2の直線との接点で滑らかに変化する数列の隣り合う二項間の和であることを特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置において、
前記RFパルス系列のフリップ角は、曲線に沿って増加することを特徴とする磁気共鳴イメージング装置。 - 請求項10記載の磁気共鳴イメージング装置において、
前記RFパルス系列のn(n=1,2,・・・,N)番目のフリップ角φ(n)は、
φ(n)=α/2×(1-β×cos(nπ/N-γ))
β2=(1+cos(π/N))/2
sin(γ)=sin(π/N)/2β
cos(γ)=(1+cos(π/N))/2β
で表されることを特徴とする磁気共鳴イメージング装置。 - フリップ角が単調増加するRFパルス系列により過渡状態でのスピンの振動を抑制するStart-upシーケンスと、定常状態でエコー信号を計測するSSFPシーケンスとからなる撮像シーケンスにおける前記RFパルス系列のフリップ角を制御するRFパルス制御方法であって、
前記RFパルス系列のRFパルス数と、前記SSFPシーケンスにおけるRFパルスのフリップ角とを設定するステップと、
前記設定されたRFパルス系列のRFパルス数とSSFPシーケンスにおけるRFパルスのフリップ角とに基づいて、オフセットを有して単調増加するフリップ角を求めて前記RFパルス系列とするフリップ角算出ステップと、
前記求めたフリップ角のRFパルス系列を有する撮像シーケンスを実行するステップと、
を有することを特徴とするRFパルス制御方法。 - 請求項12記載のRFパルス制御方法において、
前記フリップ角算出ステップは、単調増加数列の隣り合う二項間の和として、前記RFパルス系列の各RFパルスのフリップ角を求めることを特徴とするRFパルス制御方法。 - 請求項13記載のRFパルス制御方法において、
前記単調増加数列は、初項がゼロでない、等差数列であることを特徴とするRFパルス制御方法。 - 請求項13記載のRFパルス制御方法において、
前記単調増加数列は、隣り合う二項間の差が単調増加の後単調減少する数列であることを特徴とするRFパルス制御方法。
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