WO2013002231A1 - 磁気共鳴イメージング装置および高周波磁場決定方法 - Google Patents
磁気共鳴イメージング装置および高周波磁場決定方法 Download PDFInfo
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- WO2013002231A1 WO2013002231A1 PCT/JP2012/066314 JP2012066314W WO2013002231A1 WO 2013002231 A1 WO2013002231 A1 WO 2013002231A1 JP 2012066314 W JP2012066314 W JP 2012066314W WO 2013002231 A1 WO2013002231 A1 WO 2013002231A1
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves
- A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
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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
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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
- G01R33/56518—Correction of image distortions, e.g. due to magnetic field inhomogeneities due to eddy currents, e.g. caused by switching of the gradient magnetic field
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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
- G01R33/56572—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by a distortion of a gradient magnetic field, e.g. non-linearity of a gradient magnetic field
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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
- G01R33/58—Calibration of imaging systems, e.g. using test probes, Phantoms; Calibration objects or fiducial markers such as active or passive RF coils surrounding an MR active material
- G01R33/583—Calibration of signal excitation or detection systems, e.g. for optimal RF excitation power or frequency
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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
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/288—Provisions within MR facilities for enhancing safety during MR, e.g. reduction of the specific absorption rate [SAR], detection of ferromagnetic objects in the scanner room
Definitions
- the present invention relates to a nuclear magnetic resonance imaging (MRI) technique that measures nuclear magnetic resonance (NMR) signals from protons in a subject and visualizes proton density distribution, relaxation time distribution, and the like.
- MRI nuclear magnetic resonance imaging
- NMR nuclear magnetic resonance
- the present invention relates to a technique for setting an imaging position and an imaging area on a subject.
- the MRI apparatus that executes the MRI examination visualizes (images) a slice (imaging section) at an arbitrary position of the subject set in the static magnetic field space.
- the slice position and slice thickness of the imaging target are determined by the slice selection gradient magnetic field and the high-frequency radio wave pulse that excites proton magnetization. Therefore, by adjusting the waveform (intensity) of the slice selective gradient magnetic field and the irradiation frequency and waveform of the radio wave pulse, it is possible to excite only the proton magnetization in the slice of the desired thickness at the desired position.
- a radio wave pulse that excites proton magnetization is referred to as an excitation RF pulse.
- the center in the slice thickness direction of the slice to be imaged is referred to as a slice position or an imaging position.
- the slice selection gradient magnetic field is generated and applied by passing a current through a gradient coil incorporated in the MRI apparatus.
- a linear gradient magnetic field that changes linearly with respect to the application time is used as the slice selective gradient magnetic field.
- a sinc function-like excitation RF pulse is used to make the excitation profile rectangular.
- a VERSE (variable selective excitation) method that requires lower excitation power than the Sinc function-like excitation RF pulse is used in the high magnetic field apparatus (for example, see Non-Patent Document 1).
- an excitation RF pulse is applied while changing the application intensity of the slice selective gradient magnetic field.
- a non-linear gradient magnetic field that changes non-linearly with respect to the application time is used as the slice selective gradient magnetic field.
- an excitation RF pulse having an amplitude lower than that of the Sinc function is used. Since the excitation power is proportional to the square of the amplitude of the excitation RF pulse, the VERSE method requires a lower excitation power than when a linear gradient magnetic field is used.
- the irradiation frequency of the excitation RF pulse is determined based on the theoretical gradient magnetic field waveform.
- the slice selective gradient magnetic field (applied gradient magnetic field) waveform actually applied is distorted as described above, the obtained magnetic field gradient is also distorted. As a result, an error occurs in the excited position, and the image quality deteriorates.
- the distortion of the gradient magnetic field waveform causes the excitation range and intensity (excitation profile) to collapse, further degrading the image quality.
- the non-linear gradient magnetic field has a larger distortion of the gradient magnetic field waveform than the linear gradient magnetic field, the profile is likely to collapse, and it is difficult to improve the image quality.
- the present invention has been made in view of the above circumstances, and an object thereof is to provide a technique for obtaining a high-quality image even when a slice selection gradient magnetic field waveform is distorted by eddy current or vibration.
- the present invention calculates high-frequency magnetic field information based on the output gradient magnetic field waveform applied in accordance with the input gradient magnetic field waveform set in the pulse sequence, and sets the calculated high-frequency magnetic field information in the pulse sequence. And imaging is performed using the pulse sequence by which the high frequency magnetic field information was set in this way.
- Flowchart of excitation RF pulse determination processing according to the embodiment of the present invention (A) is an explanatory diagram for explaining the input gradient magnetic field waveform and the output gradient magnetic field waveform of the embodiment of the present invention, (B) is a phase change and output gradient by the input gradient magnetic field waveform of the embodiment of the present invention Explanatory diagram for explaining phase change due to magnetic field waveform Explanatory drawing for demonstrating the pulse sequence created by the sequence creation part of embodiment of this invention (A) shows a simulation result when using an excitation RF pulse set by an imaging parameter using a linear gradient magnetic field, and (B) shows an excitation obtained using the method of the embodiment of the present invention using a linear gradient magnetic field.
- Explanatory diagram for explaining simulation results when using RF pulses (A) shows a simulation result when using an excitation RF pulse set by an imaging parameter using a non-linear gradient magnetic field, and (B) shows an excitation obtained using the method of the embodiment of the present invention using a non-linear gradient magnetic field.
- Explanatory diagram for explaining simulation results when using RF pulses (A) is an explanatory diagram for explaining the comparison results of excitation RF pulses when a linear gradient magnetic field is used and when a nonlinear gradient magnetic field is used, and (B) is a case where a linear gradient magnetic field is used and a nonlinear gradient magnetic field.
- the MRI apparatus 100 of this embodiment includes a bed 112 on which a subject 101 is placed, a magnet 102 that generates a static magnetic field, a gradient magnetic field coil 103 that applies a gradient magnetic field to a static magnetic field space, and a high-frequency magnetic field that is applied to the subject 101
- the bed 112 inserts the subject 101 into the static magnetic field space formed by the magnet 102 and retracts the subject from the static magnetic field space.
- the bed 112 is driven by a bed driving unit 113.
- the bed driving unit 113 controls at least the movement of the bed 112 in the body axis direction in accordance with a control signal given from the sequencer 111.
- the body axis direction is the z direction
- the two directions perpendicular to the z direction are the direction perpendicular to the bed surface and the other (the direction perpendicular to the z direction and the y direction). Is the x direction.
- the gradient magnetic field coil 103 together with the gradient magnetic field power source 109, constitutes a gradient magnetic field generation system that applies a gradient magnetic field to the static magnetic field space.
- the gradient magnetic field coil 103 includes coils that generate gradient magnetic fields in three directions of x, y, and z, respectively, and applies gradient magnetic fields that are orthogonal to each other to the imaging region.
- the gradient magnetic field is applied according to a current supplied from the gradient magnetic field power source 109 to the gradient magnetic field coil 103 in accordance with a signal from the sequencer 111.
- Each gradient magnetic field is used as a slice selection gradient magnetic field that determines an imaging position (imaging slice) for offset imaging, a phase encode gradient magnetic field that imparts phase encoding, and a read gradient magnetic field that imparts read encoding.
- Each role can be set in any direction.
- the slice selection gradient magnetic field for determining the imaging position is referred to as a gradient magnetic field.
- the RF transmission coil 104 constitutes an RF application system that applies a high-frequency magnetic field (excitation RF pulse) to the subject 101 together with the RF transmission unit 110.
- the excitation RF pulse is applied according to a signal sent from the RF transmission unit 110 to the RF transmission coil 104 in accordance with an instruction from the sequencer 111.
- the irradiation frequency, phase, and amplitude waveform of the applied excitation RF pulse are determined in advance and set in a pulse sequence.
- an excitation RF pulse having an irradiation frequency and a bandwidth capable of exciting a desired slice thickness at a desired slice position is selected and applied.
- a FID free induction decay
- the RF receiving coil 105 and the signal detection unit 106 constitute a signal detection system that detects an echo signal generated from the subject 101.
- the echo signal is received by the RF receiving coil 105 and detected by the signal detection unit 106.
- the detected signal is subjected to processing such as FFT (Fast Fourier Transform) in the arithmetic unit 107 and converted into an image signal.
- the obtained image is displayed on the display unit 108.
- the sequencer 111 is responsible for controlling the gradient magnetic field power source 109, the RF transmission unit 110, the signal detection unit 106, the bed driving unit 113, and the display unit 108 in accordance with commands and signals from the input unit 114 and the calculation unit 107.
- the control time chart is generally called a pulse sequence.
- the computing unit 107 receives setting or changing of imaging parameters via the input unit 114. Using the received imaging parameter and information such as the gradient magnetic field intensity stored in the storage unit 115, the calculation unit 107 determines an excitation RF pulse corresponding to the gradient magnetic field actually applied during imaging. The operation unit 107 further creates a pulse sequence using the determined gradient magnetic field and excitation RF pulse, and sends a command to the sequencer 111 according to the created pulse sequence.
- the calculation unit 107 of the present embodiment calculates an excitation RF pulse according to a gradient magnetic field waveform (output gradient magnetic field waveform) that is actually applied at the time of imaging.
- a determination unit 210, a sequence creation unit 240 that reflects the calculated excitation RF pulse in the pulse sequence, and an imaging unit 250 that executes imaging according to the pulse sequence are provided.
- the excitation RF pulse determination unit 210 of the present embodiment determines the output gradient magnetic field waveform by actual measurement using a gradient magnetic field waveform (input gradient magnetic field waveform) determined according to the imaging parameter. For this reason, the excitation RF pulse determination unit 210 of the present embodiment includes an input gradient magnetic field determination unit 220 that determines an input gradient magnetic field waveform from imaging parameters, and a gradient magnetic field waveform ( An output gradient magnetic field determining unit 230 for determining an output gradient magnetic field waveform).
- the operator sets imaging parameters via the input unit 114.
- the calculation unit 107 receives an imaging parameter (step S1101).
- the input gradient magnetic field determination unit 220 determines the input gradient magnetic field waveform Gs_in (t) based on the imaging parameters (input gradient magnetic field determination processing; step S1102).
- the output gradient magnetic field determination unit 230 determines the output gradient magnetic field waveform Gs_out (t) actually applied at the time of imaging based on the imaging parameter and the calculated input gradient magnetic field waveform (output gradient magnetic field determination process; step S1103) ).
- the excitation RF pulse determination unit 210 determines an excitation RF pulse based on the imaging parameter and the output gradient magnetic field waveform (excitation RF pulse determination processing; step S1104).
- the sequence creation unit 240 creates a pulse sequence reflecting the input imaging parameters and the determined excitation RF pulse (step S1105).
- the imaging unit 250 operates each unit according to the created pulse sequence and executes imaging (step S1106). That is, at the time of imaging, the imaging unit 250 irradiates the excitation RF pulse determined from the RF transmission coil 104.
- step S1102 Details of the processing of each step from step S1102 to step S1104 will be described.
- step S1102 the input gradient magnetic field determination process by the input gradient magnetic field determination unit 220 in step S1102 will be described.
- the input gradient magnetic field determining unit 220 calculates a theoretical gradient magnetic field waveform of a preset gradient magnetic field.
- a linear gradient magnetic field either a linear gradient magnetic field or a non-linear gradient magnetic field is used.
- a linear gradient magnetic field is determined as an example will be described.
- FIG. 4 (A) is a graph showing the time change of gradient magnetic field strength (Gradient strength).
- Gs_flat (t) of the flat portion 504f is obtained by the following equation (1).
- Gs_flat (t) TBW / ( ⁇ ⁇ zw ⁇ D) (1)
- TBW time-bandwidth
- ⁇ the gyromagnetic ratio
- zw a slice thickness and is given as an imaging parameter.
- D is the application time and is equal to the excitation time of the excitation RF pulse.
- the gradient magnetic field strength Gs_flat (t) of the flat portion 504f is changed to the input gradient magnetic field waveform Gs_in (t) for the sake of simplicity.
- a gradient magnetic field intensity changes according to the application time of a gradient magnetic field.
- the ideal waveform of the nonlinear gradient magnetic field is as shown at 604 in FIG.
- the detailed calculation method of the ideal waveform 604 is as disclosed in Non-Patent Document 1. That is, the gradient magnetic field waveform Gs_in (t) in the case of the nonlinear gradient magnetic field is expressed by the following equation (2).
- Gs_in (t) Gs_linear (t) ⁇ vRF (t) / oRF (t) (2)
- Gs_linear (t) is the gradient magnetic field waveform of the linear gradient magnetic field
- vRF (t) is the excitation RF pulse waveform corresponding to the nonlinear gradient magnetic field
- oRF (t) is the excitation RF pulse corresponding to the linear gradient magnetic field. It is a waveform.
- vRF (t) is created by increasing the amplitudes on both sides so that the maximum amplitude is ⁇ times the maximum amplitude of oRF (t) so as not to change the area of oRF (t) and the irradiation time.
- ⁇ is 1 or less, and the determination of ⁇ does not exceed the slew rate of the slice selective gradient magnetic field.
- reference numeral 605 denotes an output gradient magnetic field waveform.
- the output gradient magnetic field waveform Gs_out (t) is distorted by eddy current or vibration and becomes a waveform indicated by 505 in FIG.
- the output gradient magnetic field waveform is calculated using the technique disclosed in Non-Patent Document 2.
- the gradient magnetic field is applied or not applied, and the amount of gradient magnetic field applied per unit time is calculated from the amount of change in the phase difference of each signal intensity per unit time. Get. By comparing this with the application amount of the theoretical gradient magnetic field, the amount of distortion is grasped.
- the output gradient magnetic field determination unit 230 of the present embodiment calculates and determines the output gradient magnetic field waveform 505Gs_out (t) by executing a gradient magnetic field waveform calculation sequence. This gradient magnetic field waveform calculation sequence is executed prior to the main imaging.
- the gradient magnetic field waveform calculation sequence 300 is shown in FIGS. 5 (A) and 5 (B).
- RF indicates the timing of RF pulse application
- AD indicates data acquisition
- Gs indicates the timing of slice gradient magnetic field application.
- the gradient magnetic field waveform calculation sequence 300 includes a first pulse sequence 310 shown in FIG. 5 (A) and a second pulse sequence 320 shown in FIG. 5 (B).
- the first pulse sequence 310 and the second pulse sequence 320 include an RF pulse 301 and a slice selective gradient magnetic field 303 that is applied simultaneously with the RF pulse 301.
- the first pulse sequence 310 performs data acquisition 302 without applying the slice gradient magnetic field.
- the second pulse sequence data acquisition 302 is performed while applying the second slice selection gradient magnetic field 304.
- the second slice selection gradient magnetic field 304 has the same gradient magnetic field waveform 504Gs_in (t) as the slice selection gradient magnetic field (input gradient magnetic field) used for the main imaging. That is, the gradient magnetic field is specified by the same imaging parameter as the slice selection gradient magnetic field used for the main imaging.
- the output gradient magnetic field determination unit 230 of the present embodiment executes the first pulse sequence 310 and the second pulse sequence 320. Then, using the first echo signal obtained in the first pulse sequence 310 and the second echo signal obtained in the second pulse sequence 320, the echo signal based on the presence or absence of the second slice selective gradient magnetic field 304 Get the phase change.
- the output gradient magnetic field determination unit 230 obtains the time change of the application amount of the second slice selection gradient magnetic field 304, that is, the output gradient magnetic field waveform 505Gs_out (t) of the slice selection gradient magnetic field used for the main imaging. .
- step S1104 the excitation RF pulse determination process by the excitation RF pulse determination unit 210 in step S1104 will be described.
- the excitation RF pulse is expressed by the following formula (3).
- RF RFa (t) ⁇ exp (j2 ⁇ ⁇ RFf ⁇ t + RFp (t)) (3)
- RFa (t) is the pulse waveform (amplitude waveform) of the excitation RF pulse
- RFp (t) is the phase of the excitation RF pulse
- RFf is the irradiation frequency of the excitation RF pulse
- j is the imaginary unit, It is a symbol.
- the excitation RF pulse determination unit 210 calculates the excitation RF pulse irradiation frequency RFf, phase RFp (t), and amplitude waveform RFa (t), and determines the excitation RF pulse. As shown in FIG. 6, the excitation RF pulse determination unit 210 first calculates the irradiation frequency RFf using the output gradient magnetic field waveform Gs_out (t) (irradiation frequency RFf calculation processing; step S1201). Then, the amount of change in phase due to the change in the applied gradient magnetic field from the input gradient magnetic field waveform to the output gradient magnetic field waveform is calculated as the phase RFp (t) (phase RFp calculation process; step S1202).
- the amplitude waveform RFa (t) of the excitation RF pulse is calculated using the output gradient magnetic field waveform Gs_out (t) and the input gradient magnetic field waveform Gs_in (t) (amplitude waveform RFa calculation processing; step S1203).
- the irradiation frequency RFf of the excitation RF pulse is generally calculated by the following equation (4) based on the intensity Gs of the slice selection gradient magnetic field and the imaging position Z.
- RFf ⁇ ⁇ (Gs ⁇ Z) + ⁇ ⁇ B0 (4)
- ⁇ is a magnetic rotation ratio
- B0 is the static magnetic field strength.
- the excitation RF pulse determination unit 210 of the present embodiment uses the average value Ave (Gs_out (t)) in the time direction of Gs_out (t) for the slice selection gradient magnetic field strength Gs in the above equation (4), and uses the following equation: In (5), the irradiation frequency RFf of the excitation RF pulse is calculated.
- OD is an imaging position represented by a distance from the magnetic field center (offset distance OD).
- offset imaging imaging in which the imaging position is not the magnetic field center, that is, imaging in which the offset distance is not 0 is referred to as offset imaging.
- the excitation RF pulse determination unit 210 calculates the phase change due to the gradient magnetic field distortion as the phase RFp (t) of the excitation RF pulse.
- the phase RFp of the excitation RF pulse is 0.
- FIG. 7A is a graph showing the time change of gradient magnetic field strength (Gradient strength)
- FIG. 7B is a graph showing the time change of phase change (RF Phase diff).
- phase RFp (t) is expressed by the following formula (6).
- RFp (t) (gk (t) -gl (t)) ⁇ RFf ⁇ D (6)
- D is the application time of the gradient magnetic field and the excitation RF pulse.
- the phase RFp of the excitation RF pulse is set to 0. However, if the excitation RF pulse has a phase other than the phase change due to the gradient magnetic field, it is necessary. In response, the phase value is added to RFp.
- the pulse waveform RFa (t) of the excitation RF pulse is designed so that the excited range (cross section of slice thickness (imaging region thickness); hereinafter referred to as excitation profile) is rectangular.
- the excitation profile is a Fourier transform of the pulse waveform of the excitation RF pulse.
- a sinc function waveform is used as the pulse waveform RFa (t) of the excitation RF pulse.
- the amplitude waveform RFa (t) of the excitation RF pulse to be used is It is expressed by (7).
- RFa (t) RFa_in (t) ⁇ Gs_out (t) / Gs_in (t) (7)
- the excitation RF pulse waveform RFa_in (t) is calculated from the slice thickness and the bandwidth given as imaging parameters.
- the excitation RF pulse determination unit 210 calculates the irradiation frequency RFf, phase RFp (t), and amplitude waveform RFa of the excitation RF pulse, and determines the excitation RF pulse.
- the sequence creation unit 240 creates a pulse sequence so that an excitation RF pulse having the calculated irradiation frequency RFf, phase RFp (t), and amplitude waveform RFa (t) is applied during imaging.
- an excitation RF pulse 401 that is phase-modulated with the irradiation frequency RFf as a modulation frequency is set in RFch, and the calculated phase RFp (t) 403 is set in RFpch.
- the waveform of the excitation RF pulse 401 is RFa (t).
- the input gradient magnetic field waveform Gs_in (t) 402 calculated from the imaging parameters is set in the gradient magnetic field GSch.
- the imaging unit 250 executes imaging according to the obtained pulse sequence.
- FIG. 9 shows an example in which a linear gradient magnetic field is used as the slice selection gradient magnetic field.
- FIG. 9A shows an example in which the excitation RF pulse set by the imaging parameter is used as it is.
- FIG. 9B shows an example in which the excitation RF pulse determined according to the above embodiment is used.
- FIG. 9 (A) and FIG. 9 (B) respectively, the state of change of signal intensity (Signal magnitude) according to position (position), time (time) change of gradient magnetic field strength (Gradient strength), The time change of the amplitude waveform (RF amplitude) of the excitation RF pulse and the time change of the phase (RF phase diff) of the excitation RF pulse are shown.
- the waveform RFa_in (t) of the set excitation RF pulse is set to 503, and the phase is set to 506.
- the excitation profile is 501 when the slice selection gradient magnetic field is the input gradient magnetic field waveform Gs_in (t) 504.
- the excitation RF pulse 503 is designed so that its center is the imaging position 510.
- the slice selective gradient magnetic field actually applied has an output gradient magnetic field waveform Gs_out (t) 505. Therefore, when the designed excitation RF pulse (amplitude waveform is 503, phase is 506) is used as it is, the obtained slice profile is 502, and the imaging position is shifted to 511.
- the excitation RF pulse (amplitude waveform RFa (t) is 508 and phase RFp (t) is 509) determined by the method of the present embodiment, the obtained slice profile is 507 and almost overlapped with the excitation profile 501. Accordingly, the imaging position is also substantially the same as 510.
- the excitation RF pulse is determined by the method of the present embodiment, a desired imaging slice is excited with high accuracy without any deviation of the excitation profile itself or imaging position.
- FIG. 10 (A) shows an example of using the excitation RF pulse set by the imaging parameter as it is.
- FIG. 10B is an example in the case of using the excitation RF pulse determined according to the above embodiment.
- FIG. 10 (A) and FIG. 10 (B) Each graph of FIG. 10 (A) and FIG. 10 (B), the state of change of signal intensity (Signal magnitude) according to position (position), time (time) change of gradient magnetic field strength (Gradient strength), The time change of the amplitude waveform (RF amplitude) of the excitation RF pulse and the time change of the phase (RF phase diff) of the excitation RF pulse are shown.
- the input gradient magnetic field waveform Gs_in (t) calculated from the imaging parameters is 604, and the actually applied output gradient magnetic field waveform Gs_out (t) is 605.
- the waveform of the excitation RF pulse RFa_in (t) to be set is 603 and the phase is 606.
- the excitation profile (slice profile) is 601 when the slice selection gradient magnetic field is the input gradient magnetic field waveform Gs_in (t) 604.
- the excitation RF pulse 603 is designed so that its center 610 is an imaging position.
- the slice selective gradient magnetic field that is actually applied has an output gradient magnetic field waveform Gs_out (t) 605. Therefore, when the designed excitation RF pulse (amplitude waveform is 603 and phase is 606) is used as it is, the obtained slice profile is 602, and the imaging position is shifted to 611.
- the imaging position is also substantially the same as 610.
- the excitation RF pulse is determined by the method of the present embodiment, the desired imaging slice is excited with high accuracy without the deviation of the profile itself and the imaging position.
- the excitation RF pulse is determined according to the output gradient magnetic field waveform actually applied during imaging. That is, the irradiation frequency RFf and the phase RFp (t) are determined based on the actually applied gradient magnetic field waveform Gs_out (t). Therefore, even when the gradient magnetic field changes from the input gradient magnetic field waveform Gs_in (t) to the output gradient magnetic field waveform Gs_out (t), a desired slice position can be excited. The same applies to the amplitude waveform RFa (t).
- the present embodiment even if distortion occurs in the slice selection gradient magnetic field waveform that is actually applied due to eddy current or vibration, the frequency, phase, An excitation RF pulse having an amplitude waveform is irradiated. Therefore, a desired slice can be excited with high accuracy. Thereby, the image quality is improved.
- the other imaging conditions are the same.
- the graph of FIG. 11 (A) shows how the signal intensity (Signal magnitude) changes according to the position (position), the gradient magnetic field strength (Gradient ⁇ strength) changes in time (time), and the amplitude waveform of the excitation RF pulse ( This shows the time change of RF (amplitude) and the time change of the phase of the RF pulse (RF phase diff).
- the amplitude waveform RFa (t) of the excitation RF pulse determined by the method of this embodiment is 703
- the phase change RFp (t) is 707
- the excitation profile ( 701 is the slice profile. Further, the obtained image is set to 709 shown in FIG.
- the amplitude waveform RFa (t) of the excitation RF pulse determined by the method of this embodiment is 704
- the phase change RFp (t) is 708, and the excitation
- the profile (slice profile) is set to 702. Further, the obtained image is assumed to be 710 in FIG.
- the slice profile 701 and the slice profile 702 substantially match the waveform and the imaging position.
- the image 709 and the image 710 have almost the same image quality.
- the value of the amplitude waveform 704 is smaller than that of the amplitude waveform 703, and the required RF excitation power is low. Therefore, it was shown that, under the same imaging conditions, an equivalent image can be obtained with a low excitation power by using a non-linear gradient magnetic field for the slice selection gradient magnetic field.
- the excitation RF pulse having the frequency, phase, and amplitude waveform determined based on the actually applied gradient magnetic field waveform can be obtained. Irradiated. Therefore, a desired slice can be excited as in the case of using a linear gradient magnetic field for the slice selective gradient magnetic field.
- the output gradient magnetic field determination unit 230 executes a gradient magnetic field waveform calculation sequence using the slice selection gradient magnetic field waveform (input gradient magnetic field waveform; Gs_in (t)) used for the main imaging for each main imaging. By doing so, the output gradient magnetic field waveform Gs_out (t) is determined.
- the method for determining the output gradient magnetic field waveform Gs_out (t) is not limited to this.
- the gradient magnetic field strength and slice thickness are inversely proportional. Therefore, for example, the output gradient magnetic field waveform Gs_out (t) of the main imaging, which is different from the imaging in which the output gradient magnetic field waveform Gs_out (t) is obtained by the above-described method, only in the slice thickness, It can be obtained by equation (8).
- Gs_out (t) c ⁇ Tb ⁇ Gsb (t) / Tg (8)
- Gsb (t) is the base gradient magnetic field waveform calculated by executing the gradient magnetic field waveform calculation sequence 300
- Tb is the slice thickness at that time
- Tg is the output gradient magnetic field waveform Gs_out (t) This is the slice thickness of the actual imaging to be calculated.
- C is a correlation coefficient. The correlation coefficient c is obtained in advance by calculating the output gradient magnetic field waveform by executing the gradient magnetic field waveform calculation sequence 300 with two or more different slice thicknesses.
- the output gradient magnetic field waveform Gs_out (t) may be calculated and determined using a system transfer function ts (t) unique to the MRI apparatus 100. That is, the output gradient magnetic field waveform Gs_out (t) can be calculated by the following equation (9) using the input gradient magnetic field waveform Gs_in (t) and the system transfer function ts (t).
- t is the application time of the slice selective gradient magnetic field
- ⁇ is a variable that satisfies 0 ⁇ ⁇ ⁇ t.
- the system transfer function ts (t) is a time-damping response function having a plurality of time constants and gains from characteristics such as eddy currents.
- a response function that can be used as the system transfer function ts (t) for example, an exponential function represented by the following equation (10) can be used.
- g 1 , g 2 , and g 3 are gains
- ⁇ 1 , ⁇ 2 , and ⁇ 3 are time constants. Since these gains and time constants are values depending on the apparatus, each MRI apparatus may be measured once, for example, at the time of installation.
- the gain and time constant of the system transfer function ts (t) are not limited to those obtained by measurement.
- the optimum value may be determined while changing the gain and the time constant.
- the candidate Gs_app (t) of the output gradient magnetic field waveform Gs_app (t) is calculated using the above equations (9) and (10), and the gradient magnetic field waveform calculation sequence 300 is executed.
- the gain and time constant when the similarity is the highest are employed.
- the similarity between the output gradient magnetic field waveform candidate Gs_app (t) and the base gradient magnetic field waveform Gsb (t) is evaluated using the least square sum shown in the following equation (11).
- the gain and time constant of Equation (10) may be calculated after performing Laplace transform and z transform on Equation (9).
- the output gradient magnetic field determination unit 230 calculates the output gradient magnetic field waveform Gs_out (t) with respect to the input gradient magnetic field waveform Gs_in (t) for each main imaging or every time the imaging parameter changes. Although it comprises, it is not restricted to this.
- the output gradient magnetic field waveform Gs_out (t) with respect to the input gradient magnetic field waveform Gs_in (t) determined by the imaging parameter may be obtained by some method.
- the output gradient magnetic field waveform Gs_out (t) with respect to the input gradient magnetic field waveform Gs_in (t) is held in the storage unit 115 as a database for each representative imaging parameter.
- the output gradient magnetic field determination unit 230 calculates the output gradient magnetic field waveform Gs_out (t) held in the database in association with the input gradient magnetic field waveform Gs_in (t) corresponding to the imaging parameter instead of calculating for each imaging. You may comprise so that it may extract.
- the cross-sectional imaging in which the imaging cross section is parallel to any one of the xy plane, the yz plane, and the zx plane has been described as an example.
- oblique imaging that is cross-sectional imaging at an arbitrary angle may be used.
- the output gradient magnetic field waveform Gs_out (t) in the case of oblique imaging is expressed by the following equation (12).
- G_x (t), G_y (t), and G_z (t) are output gradient magnetic field waveforms in the x-axis, y-axis, and z-axis directions, respectively, and wx, wy, and wz depend on the oblique angle Weight.
- the weight is calculated from the rotation coordinates and the oblique angle set by the user. It is preferable that the data is stored in the storage unit 115 in advance.
- the output gradient magnetic field waveforms G_x (t), G_y (t), and G_z (t) in the x-axis, y-axis, and z-axis directions are calculated by any of the methods described above.
- the excitation RF pulse determination unit 210 calculates the irradiation frequency RFf of the excitation RF pulse and the phase change amount RFp (t) from the obtained output gradient magnetic field waveform Gs_out (t). Not limited to this. The calculation may be performed by correcting the irradiation frequency RFf_in and the phase RFp_in (t) of the excitation RF pulse determined in advance for each sequence. The irradiation frequency RFf_in and the phase RFp_in (t) are calculated from an ideal gradient magnetic field, that is, an input gradient magnetic field waveform Gs_in (t).
- the correction amount ⁇ RFf of the irradiation frequency RFf and the correction amount ⁇ RFp (t) of the phase RFp (t) are expressed by the following equations (13) and (14), respectively.
- ⁇ RFf ⁇ ⁇ (Ave (Gs_in (t))-Ave (Gs_out (t))) ⁇ OD (13)
- Ave (Gs_in (t)) is the average value of the input gradient magnetic field waveform Gs_in (t) in the time direction
- Ave (Gs_out (t)) is the average value of the output gradient magnetic field waveform Gs_out (t) in the time direction. It is.
- ⁇ RFp (t) (gcal (t) -gk (t)) ⁇ RFf ⁇ D (14)
- gcal (t) is the amount of change in phase in one cycle due to the input gradient magnetic field waveform Gs_in (t), and is normalized so that the maximum position is 2 ⁇ .
- the process of determining the excitation RF pulse according to the input gradient magnetic field Gs_in (t) is performed in the arithmetic unit 107 provided in the MRI apparatus 100, but is not limited thereto.
- the excitation RF pulse may be determined in accordance with the input gradient magnetic field Gs_in (t) in an information processing apparatus that can transmit and receive data to and from the MRI apparatus 100 and is independent of the MRI apparatus 100.
- MRI apparatus 101 subject, 102 magnet, 103 gradient coil, 104 RF transmission coil, 105 RF reception coil, 106 signal detection unit, 107 calculation unit, 108 display unit, 109 gradient magnetic field power supply, 110 RF transmission unit, 111 Sequencer, 112 bed, 113 bed drive unit, 114 input unit, 115 storage unit, 210 excitation RF pulse determination unit, 220 input gradient magnetic field determination unit, 230 output gradient magnetic field determination unit, 240 sequence creation unit, 250 imaging unit, 300 tilt Magnetic field waveform calculation sequence, 301 RF pulse, 302 data acquisition, 303 slice selection gradient magnetic field, 304 slice selection gradient magnetic field, 310 pulse sequence, 320 pulse sequence, 401 excitation RF pulse, 402 input gradient magnetic field waveform, 403 phase, 501 excitation profile , 502 excitation profile, 503 amplitude waveform, 504 input gradient magnetic field waveform, 504d falling part, 504f flat part, 504u standing Edge, 505 Output gradient magnetic field waveform, 506 phase, 50
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Abstract
Description
演算部107は、さらに、決定した傾斜磁場および励起RFパルスを用いてパルスシーケンスを作成し、作成したパルスシーケンスに従って、シーケンサ111に命令を送る。
まず、ステップS1102の、入力傾斜磁場決定部220による入力傾斜磁場決定処理について説明する。
ここで、TBW(time-bandwidth)は、励起RFパルス波形において、励起RFパルスの強度が0となる回数である。これは、励起RFパルスに使用するRFパルス波形毎に決まった値であり、既知である。γは磁気回転比である。zwはスライス厚であり、撮像パラメータとして与えられる。Dは、印加時間であり、励起RFパルスの励起時間と等しい。なお、立ち上がり部504uと立ち下がり部504dとは、励起RFパルスには関係しないため、説明を簡単にするため、フラット部分504fの傾斜磁場強度Gs_flat(t)を、入力傾斜磁場波形Gs_in(t)とする。
すなわち、非線形勾配磁場の場合の傾斜磁場波形Gs_in(t)は、以下の式(2)で表される。
ここで、Gs_linear(t)は線形勾配磁場の傾斜磁場波形であり、vRF(t)は非線形勾配磁場に対応する励起RFパルス波形であり、oRF(t)は線形勾配磁場に対応する励起RFパルス波形である。また、vRF(t)はoRF(t)の面積と照射時間を変えないよう、最大振幅がoRF(t)の最大振幅のα倍になるように、両サイドの振幅を上げて作成する。但し、αは1以下であり、αの決定はスライス選択傾斜磁場のスリューレートを超えないようする。なお、図4(B)において、605は、出力傾斜磁場波形である。
RF=RFa(t)・exp(j2π・RFf・t+RFp(t)) (3)
ここで、RFa(t)は、励起RFパルスのパルス波形(振幅波形)、RFp(t)は、励起RFパルスの位相、RFfは、励起RFパルスの照射周波数、jは虚数単位、・は乗算記号である。
ここで、γは磁気回転比である。B0は、静磁場強度である。
ここで、ODは、磁場中心からの距離(オフセット距離OD)で表される撮像位置である。オフセット距離ODは、ユーザが入力する。なお、撮像位置が磁場中心でない撮像、すなわち、オフセット距離が0でない撮像を、オフセット撮像と呼ぶ。
なお、ここでは、gk(t)は、位相の累積和の最大値が2πとなるよう規格化してある。また、図7(A)は、傾斜磁場強度(Gradient strength)の時間変化を、図7(B)は、位相変化(RF Phase diff)の時間変化を示すグラフである。
RFp(t)=(gk(t)-gl(t))・RFf・D (6)
ここで、Dは、傾斜磁場および励起RFパルスの印加時間である。
なお、励起RFパルス波形RFa_in(t)は、撮像パラメータとして与えられたスライス厚とバンド幅とにより算出される。
ここで、Gsb(t)は、上記傾斜磁場波形算出シーケンス300を実行して算出したベースとなる傾斜磁場波形、Tbは、その時のスライス厚、一方、Tgは、出力傾斜磁場波形Gs_out(t)算出対象の本撮像のスライス厚である。また、cは、相関係数である。相関係数cは、予め、2以上の異なるスライス厚で、上記傾斜磁場波形算出シーケンス300を実行して出力傾斜磁場波形をそれぞれ算出し、求めておく。
ここで、tは、スライス選択傾斜磁場の印加時間であり、τは、0≦τ≦tを満たす変数である。また、システム伝達関数ts(t)は、渦電流などの特性から、複数の時定数とゲインとを持つ、時間的に減衰する応答関数である。システム伝達関数ts(t)として用いることができる応答関数には、例えば、以下の式(10)で表される指数関数を用いることができる。
ここで、g1、g2、g3はゲイン、τ1、τ2、τ3は時定数である。これらのゲインおよび時定数は装置に依存する値であるため、各MRI装置について、たとえば、据付時などに、1回測定すればよい。
さらに、式(10)のゲインと時定数とは、式(9)に対して、ラプラス変換やz変換を行った後に算出してもよい。
ここで、G_x(t)、G_y(t)、G_z(t)は、それぞれ、x軸、y軸、z軸方向の出力傾斜磁場波形であり、wx、wy、wzは、オブリークの角度に応じた重みである。重みは、回転座標とユーザが設定するオブリークの角度により計算される。事前に記憶部115に保持されていることが好ましい。
ここで、Ave(Gs_in(t))は、入力傾斜磁場波形Gs_in(t)の時間方向の平均値、Ave(Gs_out(t))は、出力傾斜磁場波形Gs_out(t)の時間方向の平均値である。
ここで、gcal(t)は、入力傾斜磁場波形Gs_in(t)による、1サイクルの位相の変化量であり、最大位置が2πになるよう規格化したものである。
RFp(t)=RFp_in(t)+ΔRFp(t) (16)
また、本実施形態では、入力傾斜磁場Gs_in(t)に応じて励起RFパルスを決定する処理を、MRI装置100が備える演算部107内で行っているが、これに限られない。例えば、MRI装置100とデータの送受信が可能な、MRI装置100とは独立した情報処理装置内で、入力傾斜磁場Gs_in(t)に応じて励起RFパルスを決定するよう構成してもよい。
Claims (14)
- 静磁場を発生する磁石と、静磁場空間に傾斜磁場を印加する傾斜磁場発生部と、被検体に高周波磁場を印加する高周波磁場印加部と、被検体から発生する核磁気共鳴信号を検出する信号検出部と、傾斜磁場発生部、高周波磁場印加部および信号検出部の動作を制御する計測制御部と、予め定められたパルスシーケンスに従って前記計測制御部に制御の指示を行うとともに、前記信号検出部が検出した核磁気共鳴信号からの画像の再構成を含む演算を行う演算部と、を備える磁気共鳴イメージング装置であって、
前記演算部は、前記パルスシーケンスに従って入力される入力傾斜磁場波形に応じて前記傾斜磁場発生部が印加する出力傾斜磁場波形に基づいて、前記高周波磁場印加部から印加する高周波磁場情報を決定する高周波磁場決定部と、
前記高周波磁場決定部が決定した高周波磁場情報を前記パルスシーケンスに設定するシーケンス作成部と、を備えること
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であって、
前記高周波磁場決定部は、
前記出力傾斜磁場波形を用いて前記高周波磁場の照射周波数を算出する周波数算出部と、
印加される傾斜磁場が前記入力傾斜磁場波形から前記出力傾斜磁場波形に変化したことによる位相変化量を算出する位相算出部と、を備え、
前記シーケンス作成部は、前記算出した位相変化量を前記パルスシーケンスの高周波磁場情報に設定すること
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であって、
前記シーケンス作成部は、前記算出した照射周波数を変調周波数として前記パルスシーケンスの高周波磁場情報に設定すること
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であって、
前記高周波磁場決定部は、前記入力傾斜磁場波形と前記出力傾斜磁場波形とを用いて前記高周波磁場の振幅波形を算出する振幅波形算出部をさらに備え、
前記シーケンス作成部は、前記算出した振幅波形を前記パルスシーケンスの高周波磁場情報に設定すること
を特徴とする磁気共鳴イメージング装置。 - 請求項1記載の磁気共鳴イメージング装置であって、
前記高周波磁場決定部は、
撮像パラメータから前記入力傾斜磁場波形を決定する入力傾斜磁場決定部と、
前記入力傾斜磁場波形から前記出力傾斜磁場波形を決定する出力傾斜磁場決定部と、をさらに備えること
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であって、
前記周波数算出部は、前記出力傾斜磁場波形の強度の平均値を用いて前記高周波磁場の周波数を算出し、
前記位相算出部は、前記出力傾斜磁場波形による1サイクルの位相変化量を用いて前記位相変化量を算出すること
を特徴とする磁気共鳴イメージング装置。 - 請求項1記載の磁気共鳴イメージング装置であって、
前記高周波磁場決定部は、
前記高周波磁場の周波数の補正量と、前記高周波磁場の位相の補正量とを算出する補正量算出部を備え、
前記補正量を用い、予め定められた高周波磁場の周波数と位相とを補正し、前記パルスシーケンスに設定する高周波磁場情報を決定すること
を特徴とする磁気共鳴イメージング装置。 - 請求項5記載の磁気共鳴イメージング装置であって、
前記演算部は、前記入力傾斜磁場波形および撮像パラメータ毎に、予め算出した出力傾斜磁場波形を、前記入力傾斜磁場波形に対応づけて記憶する傾斜磁場波形データベースをさらに備え、
前記出力傾斜磁場決定部は、前記入力傾斜磁場波形に対応する出力傾斜磁場波形を、前記傾斜磁場波形データベースから抽出し、前記出力傾斜磁場波形を決定すること
を特徴とする磁気共鳴イメージング装置。 - 請求項5記載の磁気共鳴イメージング装置であって、
前記出力傾斜磁場決定部は、所定のパルスシーケンスを、本撮像で用いる傾斜磁場を印加して実行することにより得た第一の結果と、当該傾斜磁場を印加せずに実行することにより得た第二の結果とを比較することにより、前記出力傾斜磁場波形を決定すること
を特徴とする磁気共鳴イメージング装置。 - 請求項5記載の磁気共鳴イメージング装置であって、
前記演算部は、複数の出力傾斜磁場波形から相関係数を算出する相関係数算出部をさらに備え、
前記出力傾斜磁場決定部は、前記撮像パラメータと前記相関係数とを用いて、前記出力傾斜磁場波形を決定すること
を特徴とする磁気共鳴イメージング装置。 - 請求項5記載の磁気共鳴イメージング装置であって、
前記出力傾斜磁場決定部は、前記入力傾斜磁場波形と予め定めたシステム伝達関数とを用い、畳み込み積分により前記出力傾斜磁場波形を決定すること
を特徴とする磁気共鳴イメージング装置。 - 請求項5記載の磁気共鳴イメージング装置であって、
前記出力傾斜磁場決定部は、ユーザが入力するオブリークの角度を用い、前記出力傾斜磁場波形を決定すること
を特徴とする磁気共鳴イメージング装置。 - 磁気共鳴イメージング装置において撮像に用いるパルスシーケンスに設定する高周波磁場情報を決定する高周波磁場決定方法であって、
前記パルスシーケンスに設定された入力傾斜磁場波形に応じて印加される出力傾斜磁場波形に基づいて、前記高周波磁場情報を算出する高周波磁場算出ステップを備えること
を特徴とする高周波磁場決定方法。 - 請求項13記載の高周波磁場決定方法であって、
前記高周波磁場算出ステップは、
前記出力傾斜磁場波形の平均値を用いて前記高周波磁場の周波数を算出する周波数算出ステップと、
前記入力傾斜磁場波形から前記出力傾斜磁場波形に変化したことによる、印加される高周波磁場の位相変化量を算出する位相算出ステップと、を備えること、
を特徴とする高周波磁場決定方法。
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JPWO2013002231A1 (ja) | 2015-02-23 |
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JP6078465B2 (ja) | 2017-02-08 |
CN103635135A (zh) | 2014-03-12 |
CN103635135B (zh) | 2016-01-13 |
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