WO2010035569A1 - 磁気共鳴イメージング装置 - Google Patents
磁気共鳴イメージング装置 Download PDFInfo
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- WO2010035569A1 WO2010035569A1 PCT/JP2009/063108 JP2009063108W WO2010035569A1 WO 2010035569 A1 WO2010035569 A1 WO 2010035569A1 JP 2009063108 W JP2009063108 W JP 2009063108W WO 2010035569 A1 WO2010035569 A1 WO 2010035569A1
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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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- 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
- G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
- G01R33/563—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution of moving material, e.g. flow contrast angiography
- G01R33/56341—Diffusion imaging
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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
- A61B5/72—Signal processing specially adapted for physiological signals or for diagnostic purposes
- A61B5/7235—Details of waveform analysis
- A61B5/7253—Details of waveform analysis characterised by using transforms
- A61B5/7257—Details of waveform analysis characterised by using transforms using Fourier transforms
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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
- G01R33/5615—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE]
- G01R33/5616—Echo train techniques involving acquiring plural, differently encoded, echo signals after one RF excitation, e.g. using gradient refocusing in echo planar imaging [EPI], RF refocusing in rapid acquisition with relaxation enhancement [RARE] or using both RF and gradient refocusing in gradient and spin echo imaging [GRASE] using gradient refocusing, e.g. EPI
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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
- G01R33/56563—Correction of image distortions, e.g. due to magnetic field inhomogeneities caused by a distortion of the main magnetic field B0, e.g. temporal variation of the magnitude or spatial inhomogeneity of B0
Definitions
- the present invention relates to magnetic resonance imaging technology.
- the present invention relates to a technique for correcting the influence of magnetic field fluctuations caused by eddy currents and vibration in diffusion weighted imaging.
- a magnetic resonance imaging (MRI) apparatus is a medical image diagnostic apparatus that applies a high-frequency magnetic field and a gradient magnetic field to a subject placed in a static magnetic field, measures a signal generated from the subject by nuclear magnetic resonance, and forms an image. .
- Diffusion-weighted imaging in which diffusion of water molecules is emphasized, is taken as a method capable of capturing an image in which an acute cerebral infarction or tumor is emphasized with a high signal. Since acute cerebral infarction is in a state of cell edema, diffusion is suppressed. In addition, the spread of cells is suppressed because the cells are dense. For this reason, in diffusion weighted imaging, the diffusion coefficient of these parts is smaller than that of other tissues, and the signal is measured high.
- a typical method for photographing diffusion-weighted images is the diffusion-weighted echo planar method.
- This method is an ultra high-speed imaging method in which an MPG (motion probing gradient: diffusion-weighted gradient magnetic field) pulse, which is a gradient magnetic field pulse for emphasizing diffusion, is added to the echo planar method that can be photographed with one shot. Since the MPG pulse generally has a high intensity and a relatively long application time, magnetic field fluctuations caused by eddy currents and vibrations generated thereby degrade the image quality.
- MPG motion probing gradient: diffusion-weighted gradient magnetic field
- the intensity of the gradient magnetic field pulse for readout and the gradient magnetic field pulse for phase encoding is made zero, and the signal is measured.
- This measurement is performed at the positions of a plurality of slices in a predetermined slice direction, and the time variation of the magnetic field due to the MPG at each slice position is measured.
- this magnetic field variation is fitted with a linear function in the slice direction, and the time variation of the static magnetic field and the time variation of the gradient magnetic field in the slice direction are obtained.
- the constant term of the obtained linear function is a static magnetic field component generated by MPG, and the primary term is a gradient magnetic field component in the slice direction.
- phase shift (phase offset) and distortion of k-space data are calculated from the static magnetic field component and the gradient magnetic field component measured as described above, the image is corrected by phase offset correction and gridding, and MPG is used. Eliminate the effects of eddy currents.
- Papadakis NG Gradient Premphasis Calibration in Diffusion-Weighted Echo-Planar Imaging, Magn. Reson. In Med. 2000; 44: 616-624.
- Smponias T k-space Correction of Eddy Current-Induced Distributions in DW EPI, Proc. Intl. Soc. Mag. Reson. Med. 2004; 11: 2187.
- the intensity of the readout gradient magnetic field pulse and the phase encoding gradient magnetic field pulse is set to zero when measuring the eddy current, the magnetic field variation distributed in the slice plane is integrated and can be measured. Can not.
- the slice direction at the time of eddy current measurement is the x direction
- the magnetic field fluctuation in the y direction or the z direction cannot be measured.
- the correction value of the gradient magnetic field component in the x direction obtained is a constant independent of y or z. Therefore, for example, when taking a diffusion weighted image in which the slice direction is the z direction, the same correction value is used for correcting the eddy current in the x direction in the slice plane regardless of the slice position z.
- the eddy current is generally not spatially uniform, the optimum correction value varies depending on the slice position z. Therefore, in the above method, an appropriate correction value cannot be obtained, and accurate correction cannot be performed.
- the present invention has been made in view of the above circumstances.
- diffusion-weighted imaging distortion of k-space data due to a time-varying defective magnetic field generated by eddy current and vibration accompanying application of a diffusion-weighted gradient magnetic field pulse is obtained.
- characteristic data for correcting distortion of k-space data is calculated for each position in the slice direction from the peak shift of the echo depending on whether or not the MPG pulse is applied.
- characteristic data a distortion amount and a phase offset in the readout direction and the phase encoding direction of the slice plane are calculated. Then, the data in the k space is corrected using the characteristic data calculated at the time of image reconstruction.
- an imaging means for detecting a magnetic resonance signal generated from the subject by applying a high-frequency magnetic field and a gradient magnetic field to the subject placed in a static magnetic field, and the magnetic resonance detected by the imaging means
- a magnetic resonance imaging apparatus comprising a computing means for processing a signal, the imaging means, and a control means for controlling the computing means, wherein the imaging means is a magnetic resonance signal according to a pulse sequence including application of a diffusion-weighted gradient magnetic field pulse.
- a reference data acquisition means for acquiring reference data for detecting a distortion amount of k-space data due to the diffusion-weighted gradient magnetic field pulse at an arbitrary position in the slice direction,
- the calculation means is configured to read out and phase out an arbitrary position in the slice direction from the reference data.
- Characteristic data calculating means for calculating the distortion amount and phase offset amount in the encode direction as characteristic data of the distortion amount of the k-space data, and magnetic resonance obtained by the diffusion weighted imaging execution means using the characteristic data
- a magnetic resonance imaging apparatus comprising correction means for correcting k-space data constituted by signals, and image reconstruction means for reconstructing an image from data corrected by the correction means.
- FIG. 1 It is a block diagram which shows schematic structure of the MRI apparatus of embodiment of this invention. It is the sequence diagram of DWEPI in embodiment of this invention, (a) uses a standard MPG pulse, (b) uses a bipolar type MPG pulse. It is a figure for demonstrating k space in embodiment of this invention, (a) is based on the general echo planar method, (b) shows based on DWEPI. It is a figure which shows an example of the reference data in embodiment of this invention, (a) is a thing when not applying an MPG pulse, (b) is a thing at the time of MPG pulse application. It is a figure for demonstrating the characteristic data calculation process in embodiment of this invention. It is a processing flow at the time of imaging
- FIG. 1 is a block diagram showing a schematic configuration of the MRI apparatus 100 of the present embodiment.
- the MRI apparatus 100 irradiates a high-frequency magnetic field and a nuclear magnetism with a magnet 101 that generates a static magnetic field, a gradient coil 102 that generates a gradient magnetic field, a sequencer 104, a gradient magnetic field power source 105, a high-frequency magnetic field generator 106, and the like.
- a probe 107 for detecting a resonance signal, a receiver 108, a calculator 109, a display 110, and a storage medium 111 are provided.
- a subject (for example, a living body) 103 is placed on a bed (table) in a static magnetic field space generated by a magnet 101.
- the sequencer 104 sends commands to the gradient magnetic field power source 105 and the high frequency magnetic field generator 106 to generate a gradient magnetic field and a high frequency magnetic field, respectively.
- the high frequency magnetic field is applied to the subject 103 through the probe 107.
- the nuclear magnetic resonance signal generated from the subject 103 is received by the probe 107 and detected by the receiver 108.
- the sequencer 104 sets a nuclear magnetic resonance frequency (detection reference frequency f0) as a reference for detection.
- the detected signal is sent to the computer 109, where signal processing such as image reconstruction is performed.
- the result is displayed on the display 110.
- the detected signal and measurement conditions can be stored in the storage medium 111 as necessary.
- a gradient magnetic field coil 102 in which a gradient magnetic field power source 105 is wound in three axes of x, y, and z in accordance with a command from a sequencer 104 described later as a gradient magnetic field including a diffusion-weighted gradient magnetic field pulse.
- Gradient magnetic fields Gx, Gy, and Gz in the three-axis directions of x, y, and z are generated and applied.
- a slice gradient magnetic field pulse (Gs) is applied in one of x, y, and z to set a slice plane (position) with respect to the subject 103, and the phase encode gradient in the remaining two directions.
- a magnetic field pulse (Gp) and a readout gradient magnetic field pulse (Gr) are applied to encode position information in each direction in the nuclear magnetic resonance signal.
- the sequencer 104 normally performs control so that each device operates at a timing and intensity programmed in advance.
- a program that particularly describes a high-frequency magnetic field, a gradient magnetic field, and timing and intensity of signal reception is called a pulse sequence.
- the MRI apparatus 100 of the present embodiment captures a diffusion weighted image. For this reason, DWEPI (Diffusion Weighted Echo Planar Imaging) including an MPG pulse is provided as a pulse sequence.
- DWEPI Delivery Weighted Echo Planar Imaging
- the computer 109 instructs the sequencer 104 to measure the nuclear magnetic resonance signal (echo) according to the DWEPI sequence, and arranges the measured echo in the k space,
- An image reconstruction unit that reconstructs an image from the arranged echoes and an influence due to magnetic field fluctuations (hereinafter referred to as defective magnetic fields) caused by eddy currents and vibrations caused by MPG pulses from the echoes arranged in the k space are removed.
- a correction processing unit that performs correction is provided.
- a reference data acquisition unit that performs measurement (reference measurement) for acquiring reference data serving as a basis for calculating characteristic data used for correction through the sequencer 104 prior to actual imaging, and reference data acquired by the reference data acquisition unit
- a characteristic data calculation unit for calculating characteristic data used for correction by the correction processing unit.
- a pulse sequence (hereinafter referred to as a DWEPI sequence) that realizes the DWEPI method used when the diffusion enhancement measurement unit performs diffusion enhancement imaging (main imaging) in the MRI apparatus 100 of the present embodiment will be described.
- 2A and 2B are sequence diagrams of the DWEPI sequence.
- RF, Gs, Gp, and Gr represent axes of a high-frequency magnetic field, a slice gradient magnetic field, a phase encoding gradient magnetic field, and a readout gradient magnetic field, respectively.
- the slice direction in which the slice gradient magnetic field pulse Gs is applied to determine the slice position is defined as the z direction
- the readout direction in which the readout gradient magnetic field pulse Gr is applied is defined in the x direction
- the phase encode gradient is defined in the y direction.
- a high frequency magnetic field (RF) pulse 202 having a proton resonance frequency fh is applied along with application of a slice direction gradient magnetic field pulse 201 in the z direction, and within the target object.
- the proton of a predetermined slice is excited.
- the 180-degree pulse 208 is irradiated and read.
- a plurality of magnetic resonance signals (echoes) 207 are measured while applying a dephasing readout gradient magnetic field 205 and a positive and negative alternating readout gradient magnetic field pulse 206.
- a blip-shaped gradient magnetic field 210 is applied every time the echo 207 is measured.
- MPG pulses 211 and 212 are applied before and after the 180-degree pulse 208.
- the MPG pulse may be applied in any of the slice direction, the readout direction, and the phase encoding direction.
- an MPG pulse having a large intensity and a long application time is required. For this reason, when performing diffusion weighted imaging, eddy currents generated by MPG pulses and vibrations associated with application of MPG pulses cannot be ignored.
- the shape of the MPG pulse includes a standard MPG pulse 211 applied in only one of positive and negative directions shown in FIG. 2A, and a bipolar type MPG pulse 212 shown in FIG. 2B.
- the bipolar MPG pulse 212 is a combination of positive and negative pulses, it is possible to suppress the generation of eddy currents compared to the standard MPG pulse 211. However, it cannot be completely suppressed. In the present embodiment, the influence of eddy currents is corrected using actual measurement results, so that it does not depend on the shape of the MPG pulse. Therefore, the MPG pulse used for the DWEPI sequence may have any shape of the MPG pulse 211 and the MPG pulse 212.
- the MPG pulse 211 is used will be described as an example.
- echoes measured by the echo planar method are arranged along the lead-out direction (kr) in the k space as shown in FIG.
- the case where the phase encoding amount of each echo is ⁇ 8 to 63 (the number of echoes is 72) and the number of sampling points in the readout direction is 128 is shown. Since the echo planar method is used, the number of echoes in the phase encoding direction (kp) is suppressed.
- the image reconstruction unit reconstructs an image of 128 ⁇ 128 pixels using the half Fourier method and filling the data in the phase encoding direction by calculation.
- the DWEPI sequence when executed, a defective magnetic field that fluctuates in time due to eddy current and vibration accompanying MPG pulse application is generated. Since the spatial main component of the defective magnetic field is a linear component, the defective magnetic field causes distortion in the k-space data as shown in FIG. Therefore, if normal image reconstruction processing is performed as it is, artifacts (distortion and blurring) occur in the obtained image.
- the reference data acquisition unit performs reference measurement prior to the main imaging to acquire reference data
- the characteristic data calculation unit references the defective magnetic field characteristic data that causes distortion in the k-space data. Calculate from the data. Thereafter, actual photographing is performed, and the distortion of k-space data obtained by the main photographing is removed by the correction processing unit using the characteristic data.
- the characteristic data means the distortion amount of k-space data per unit gradient magnetic field strength of the MPG pulse (distortion amount in the readout direction: dkr, distortion amount in the phase encoding direction: dkp) and the phase offset amount (p 0 ). It is.
- reference data acquisition processing by the reference data acquisition unit will be described.
- the same DWEPI sequence as in the main shooting is executed as the reference measurement using the same shooting parameters as in the main shooting, and reference data for calculating the characteristic data is acquired.
- a defective magnetic field due to vibration or eddy current is not spatially uniform, and the characteristic data depends on the direction and position of the slice. For this reason, characteristic data of a plurality of slice positions is required for each slice direction.
- at least characteristic data in the slice direction that is the same as the slice direction in the main photographing is necessary.
- the slice direction of the reference measurement is set to the same direction as the slice direction of the main shooting, shooting is performed at a plurality of slice positions, and the slice data in the same direction as the slice direction of the main shooting is obtained from the obtained reference data.
- characteristic data is calculated will be described as an example.
- the following description will be made assuming that the slice direction of the main photographing is the z direction.
- the distortion amount dkr in the lead-out direction is an echo (first reference) measured by setting all the gradient magnetic field pulses (204 and 210) in the phase encoding direction to zero in the DWEPI sequence shown in FIG. Data) can be observed as a change in peak position (peak shift) depending on the presence or absence of an MPG pulse.
- FIG. 4 shows an example of first reference data obtained by setting all gradient magnetic field pulses in the phase encoding direction to zero in the DWEPI sequence.
- FIG. 4A shows an example when no MPG pulse is applied
- FIG. 4B shows an example when an MPG pulse is applied in any one of x, y, and z directions.
- the echo peak position 281 (the portion with high luminance) is located at the center (origin) in the readout direction (kr direction) regardless of the echo number.
- the MPG pulse 211 when the MPG pulse 211 is applied, the echo peak position 281 'shifts in the readout direction (kr direction) as the latter half echoes.
- the distortion of the k-space data due to the eddy current generated by the MPG pulse 211 appears as a shift of the echo peak position 281. Therefore, the distortion of the k-space data can be measured by detecting the amount of deviation of the echo peak position 281 'when the MPG pulse 211 is applied from the echo peak position 281 when the MPG pulse is not applied.
- the k-space data distortion amount dkr in the lead-out direction is calculated from the peak shift amount depending on the presence or absence of the MPG pulse of the first reference data acquired as described above.
- the peak shift amount of the echo is equal to the primary component of the phase of the projection data obtained by inverse Fourier transform of the echo from the principle of Fourier transform. That is, when the peak position is shifted by one point in the kr direction, the phase difference between the ends of the projection data changes by 2 ⁇ . Therefore, the k-space data distortion amount dkr in the lead-out direction is obtained by applying projection data obtained by performing inverse Fourier transform on the first reference data obtained by applying the MPG pulse in the lead-out direction and without applying the MPG pulse.
- the phase offset amount p 0 is a difference in peak phase of each echo of the reference data depending on the presence or absence of the first MPG pulse.
- the phase of the echo peak is equal to the zero order component of the phase of the projection data obtained by inverse Fourier transforming the echo from the principle of Fourier transform. Therefore, the phase offset amount p 0 is obtained as the zeroth-order component of the phase difference between the projections with or without MPG of the first reference data.
- the reference data acquisition unit applies MPG pulses to the x-, y-, and z-axes respectively for the first set of the readout direction and the phase encoding direction as reference measurements.
- a total of four types of sequences, which are not performed, are executed at a plurality of slice positions with the pulse intensity of the phase encoding gradient magnetic field pulse all set to 0, and first reference data is acquired.
- the readout direction and the phase encoding direction are switched, and the same four types of sequences are executed at the same plurality of slice positions to obtain second reference data.
- the pulse sequence to be executed basically follows the DWEPI sequence shown in FIG. 2A except for the above conditions.
- the measurement parameters of the reference measurement for performing the above-described eight types (eight sets) of multi-slice photographing can be set as follows, for example.
- the values of measurement parameters other than the measurement parameters to be changed are the same in each of the eight sets of reference measurements.
- it is desirable that values of measurement parameters other than the change parameter and the phase encoding gradient magnetic field pulse intensity are matched with the imaging conditions of the main imaging.
- the characteristic data calculated from the reference data depends on the slice direction and the on / off time of the MPG pulse. Therefore, the slice direction and the application time and application interval of the MPG pulse are matched to the imaging parameters of the main imaging as much as possible.
- the intensity of the MPG pulse in the reference measurement is adjusted so that the peak position can be detected up to the last echo.
- the bad magnetic field is proportional to the MPG pulse intensity.
- the intensity of the MPG pulse equivalent to that at the time of actual imaging is used, the intensity is too large and the echo attenuates in a short time due to the defective magnetic field, and the peak position may not be detected until the last echo. Therefore, the MPG pulse intensity in the reference measurement is often set smaller than the same pulse intensity in the main imaging.
- Characteristic data calculation unit using the reference data s 0 obtained in Reference shooting 8 sets the reference data acquisition unit has acquired, k-space data distortion amount (DKR, dkp) and the phase offset p 0, respectively, the reference data It is calculated as a function of the center time and slice position of each echo at the time of acquisition.
- each MPG pulse application shaft m (m is one of x, y, z) strain amount DKR m for each of the lead-out direction, the phase encoding direction strain amount dkp m, and the phase offset p 0m is calculated according to the following equation group (1).
- Each value in the formula group (1) is normalized by the MPG pulse intensity and the visual field.
- the reference data (echo) s 0 * was measured under the conditions of the subscripts * (readout direction and phase encoding direction, MPG pulse application axis) in the above eight sets of reference measurements. It means reference data (echo).
- AP and RL indicate a set of the readout direction and the phase encoding direction
- m and b 1 indicate that the MPG pulse is applied in the m-axis direction
- b 0 indicates that the MPG pulse is not applied.
- Show. B 1 also represents the pulse intensity of the MPG pulse.
- T ′ is the center time of each echo in the reference measurement
- n e0 is the number of echoes in the reference measurement
- T e0 is the echo interval in the reference measurement
- T 00 is the first echo from the off time of the last MPG pulse in the reference measurement.
- n s0 is the number of slices in the reference measurement
- FT ⁇ 1 [*] is the one-dimensional inverse Fourier transform
- Arg [*] is the variable *
- the functions that return the phase of each point, F 1,1 [*] and F 1,0 [*], respectively, use only data of “region with signal”, and a linear coefficient and a constant term as a result of linear function fitting.
- n r, n p the lead-out direction of the field of view of each AP and RL
- n r, n p is the AP and RL
- Doauto is the direction of the number of sampling points. Note that the domain of definition (sampling point coordinate range) for fitting is [ ⁇ n r / 2, n r / 2-1], [ ⁇ n p / 2, n p / 2-1] for AP and RL, respectively. ].
- the “region with a signal” is, for example, a region of a pixel having a luminance value greater than or equal to a predetermined value in the projection of the first measured echo (first echo).
- a maximum value of 0.7 of the luminance value of the projection of the first echo ⁇ 0.7 is set as a threshold value, and an area that is equal to or greater than the threshold value among the pixels of the projection of the first echo is expressed as “signal Is an area.
- the same area is referred to as an “area with a signal” in all echoes.
- the coefficient (0.7) for obtaining the threshold varies depending on conditions such as the S / N of the data.
- FIG. 5 is a diagram for explaining the flow of the characteristic data calculation process of the present embodiment.
- m a description will be given by using m as a representative.
- the characteristic data calculation unit for each of AP and RL, the reference data (501, 503) obtained by the sequence of applying the MPG pulse to the m-axis and the reference data (502) obtained by the sequence of not applying it. , 504) are respectively subjected to one-dimensional inverse Fourier transform (steps 505, 506, 507, 508) to obtain projection data.
- the projection data calculated from the reference data obtained by applying the MPG pulse is divided by the projection data calculated from the reference data obtained without applying the MPG pulse (steps 509 and 510). Since this processing is division of complex data, it is equivalent to obtaining a phase difference between projections.
- a phase value is calculated from each result (complex data) (steps 511 and 512), and fitting is performed using a linear function.
- the first-order coefficient and constant term of the linear function are calculated, and for RL, the first-order coefficient is calculated (steps 513, 514, and 515).
- the results are normalized using the MPG pulse intensity and the field of view (516, 517, 518) in the readout direction (steps 519, 520, 521), and the characteristic data (522, 523, 524) are obtained. obtain.
- the correction processing unit applies to the k-space data obtained by the actual photographing.
- the correction process to be performed will be described.
- the main photographing is performed according to the DWEPI sequence by the diffusion weighting measurement unit.
- the case where the slice direction is the z direction as described above, and the readout direction is the x direction and the phase encoding direction is the y direction will be described as an example.
- the obtained imaging data s ′ * is expressed as the following (2) as a function of the center time t of each echo and the slice position z at the time of actual imaging.
- S ′ * indicates that it is photographing data obtained under the condition of the subscript * at the time of actual photographing.
- g x, g y, g z is the intensity of MPG pulses for each axis at the time of the shooting
- n e is the number of echoes during the imaging
- T e is the echo spacing during the main photographing
- T 0 is the last time of the shooting
- n s is the number of slices at the time of actual imaging.
- Correction processing unit first, using the phase offset p 0, performs phase offset correction.
- the phase offset correction amount of the imaging data s ′ * is obtained as a proportional addition of the MPG pulse intensity g m of the characteristic data p 0m for each MPG pulse application axis. Therefore, the photographing data s ′′ * after the phase offset correction is obtained by the following equation (3). I is an imaginary unit.
- k-space data distortion correction for correcting k-space data distortion is performed using the lead-out direction distortion amount dkr and the phase encoding direction distortion amount dkp.
- the distortion correction of k-space data is performed by shooting data s ''.
- the signal value of the latticed sampling points (kr, kp) is obtained by performing the coding.
- w r and w p are the fields of view in the readout direction and the phase encoding direction of the main imaging, respectively.
- p 0m (t, z), dkr m (t, z) and dkp m (t, z) are p 0m (t ′, z ′), dkr m (t ′, z ′) and dkp m (t ′, z ′), respectively. Is obtained by linear interpolation in the directions of t ′ and z ′.
- FIG. 6 is a processing flow at the time of photographing according to the present embodiment.
- the reference data acquisition unit sets the slice direction to the same direction as the main shooting and performs reference measurement (step 602) to obtain reference data (603).
- the characteristic data calculation unit performs characteristic data calculation processing using the reference data (step 604), and calculates characteristic data (605).
- the diffusion weighted measurement unit performs diffusion weighted photographing using the photographing parameters determined in step 601 (step 606), and obtains photographing data (607).
- the correction processing unit performs phase offset correction on the photographic data using the phase offset amount p 0 in the characteristic data (step 608). Thereafter, the correction processing unit performs k-space data distortion amount correction using the k-space data distortion amount (dkr, dkp) by gridding on the data after the phase offset correction processing (step 609). Data (610) is obtained.
- the image reconstruction unit performs a two-dimensional inverse half Fourier transform (2D half FT ⁇ 1 ) on the k-space data after the k-space data distortion amount correction (step 611) to obtain an image (612).
- Each graph is a value of three of the 20 slices described above. As shown in the figure, the value of the characteristic data varies greatly depending on the slice. In the present embodiment, since the characteristic data is calculated by measuring reference data for each slice, correction with an optimal correction value is possible for each slice, that is, according to the position in the slice direction.
- FIG. 8 shows an example in which correction is performed according to the above procedure.
- FIG. 8 shows one image when 20 multi-slice imaging is performed.
- 8A is an image before correction
- FIG. 8B is an image after correction
- FIG. 8C is an image when the pulse intensity of the MPG pulse is zero.
- the image contrast is emphasized so as to be displayed.
- the image shown in FIG. 8A shows that distortion and blurring are caused by the MPG pulse.
- FIG. 8B by performing the correction processing of the present embodiment, distortion and blurring can be suppressed, and an image equivalent to the image shown in FIG. 8C can be obtained. I can confirm. Note that the noise in FIG. 8B is large because of the effect of diffusion emphasis.
- the k-space data distortion amount and phase offset amount in the readout direction and the phase encoding direction by the diffusion-weighted gradient magnetic field pulse in the slice plane Calculated as characteristic data, and the k-space data obtained in the actual photographing is corrected using the characteristic data. Therefore, a highly accurate correction value can be obtained for each slice. For this reason, the accuracy of correction is also improved, and the image quality of an image obtained by diffusion weighted imaging is improved. That is, according to the present embodiment, a magnetic resonance imaging apparatus capable of accurately correcting the influence of a defective magnetic field caused by eddy current and vibration caused by a diffusion-weighted gradient magnetic field pulse and suppressing blur and distortion of a diffusion-weighted image is provided. it can.
- FIG. 9 shows the result of correcting the data photographed under the same conditions by the conventional method and the result of correcting by the method of this embodiment.
- FIG. 9A shows the result of correcting the conventional method, that is, all slices with the same correction value
- FIG. 9B shows the method of the present embodiment, that is, the correction value for each slice. It is the result corrected using.
- a circle is displayed at the original contour position of the subject, and the lower figure in each figure is displayed with the image contrast emphasized.
- FIG. 9A the blur seen near the top of the circular phantom is suppressed in FIG. 9B, and it can be confirmed that a uniform image is obtained as a whole.
- the characteristic data must be calculated for each slice direction. For this reason, in the above embodiment, the slice direction in which the reference measurement that is the basis for calculating the characteristic data is matched with the slice direction (z direction as an example) at the time of actual imaging, and the minimum necessary characteristic data is calculated. The case where correction is performed has been described as an example. However, the calculation of the characteristic data is not limited to this.
- Defective magnetic field depends on the device (magnet and gradient coil) and not on the subject.
- the device characteristic data is unique to each device, and once measured, the device characteristic data can be commonly used in subsequent photographing. Therefore, in order to shorten the imaging time, data in all slice directions necessary for subsequent main imaging may be acquired in advance and held in the storage medium 111. In this case, in the processing flow shown in FIG. 6, step 602 and step 604 are repeated for each slice direction to acquire characteristic data in each direction.
- the slice direction, the lead-out direction, and the phase encoding direction in the reference measurement are matched with each direction of the main imaging is described as an example, but the present invention is not limited thereto.
- the readout direction and the phase encoding direction may be reversed.
- the direction of each axis may be deviated within a predetermined range (about 20 to 30 degrees).
- the slice position z ′ and the echo peak time t ′ at the time of reference measurement are not necessarily the same as the slice position z and the echo peak time t at the time of actual imaging is described as an example.
- the shooting parameters for reference shooting may be set so as to match.
- linear interpolation becomes unnecessary, and the accuracy of correction increases.
- reference data is acquired by switching m in three ways of x, y, and z, and characteristic data is calculated so that correction can be made regardless of the direction in which the MPG pulse is applied in actual imaging.
- the acquisition of the reference data and the calculation of the characteristic data are not necessarily performed for the three axes x, y, and z. You may comprise so that only the reference measurement which applies an MPG pulse to the same direction as the application direction of the MPG pulse at the time of this imaging
- photography may be performed.
- the number of reference measurements can be reduced to shorten the measurement time.
- the number of reference measurements can be reduced, so that the measurement time can be shortened.
- linear interpolation is performed when the imaging parameters for reference measurement are set so that the slice position z ′ and echo peak time t ′ at the time of reference measurement coincide with the slice position z and echo peak time t at the time of actual imaging. Becomes unnecessary and the correction accuracy is improved.
- the number of slices for reference measurement may be the same as that for actual photographing. For example, when the number of slices for actual photographing is one, the number of slices for reference measurement may be one.
- the k-space data distortion amount (dkr, dkp) and the phase offset amount p 0 obtained by the procedure described in the above embodiment have a systematic slight difference between the odd value and the even value. There is. In that case, the correction accuracy may be improved by averaging the values adjacent in time according to the following equation group (5).
- correction of the influence of the defective magnetic field by the diffusion weighted gradient magnetic field pulse is targeted, but the present invention is not limited to this. It can also be used to correct the influence of a bad magnetic field that cannot be ignored by gradient magnetic field pulses that generate eddy currents and vibrations.
- 101 Magnet for generating a static magnetic field
- 102 Gradient magnetic field coil
- 103 Subject
- 104 Sequencer
- 105 Gradient magnetic field power source
- 106 High-frequency magnetic field generator
- 107 Probe
- 108 Receiver
- 109 Computer
- 110 Display
- 111 Storage medium
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Abstract
Description
以下、本発明を適用する第一の実施形態について説明する。以下、本発明の実施形態を説明するための全図において、同一機能を有するものは同一符号を付し、その繰り返しの説明は省略する。
スライス方向:z方向、
リードアウト方向および位相エンコード方向:x方向およびy方向とするもの(以下、APと呼ぶ。)とy方向およびx方向とするもの(以下、RLと呼ぶ。)の2種類、
MPGパルス:x軸方向に印加するもの、y軸方向に印加するもの、z軸方向に印加するもの、いずれにも印加しないものの4種類、
位相エンコード傾斜磁場パルス強度:0、
スライス数:20枚、
スライス間隔:1cm、
エコー間隔(IET):1ms、
マルチエコー数:100
*の各エコーのk空間上の実際の位置(kr’、kp’)と、格子状のサンプリング点(kr、kp)とが、以下の式群(4)の関係にあることを用いてグリッディングすることにより、格子状のサンプリング点(kr、kp)の信号値を求める。
Claims (12)
- 静磁場の中に置かれた被検体に高周波磁場および傾斜磁場を印加して、前記被検体から発生する磁気共鳴信号を検出する撮影手段と、前記撮影手段で検出した磁気共鳴信号を処理する演算手段と、前記撮影手段および前記演算手段を制御する制御手段とを備える磁気共鳴イメージング装置であって、
前記撮影手段は
拡散強調傾斜磁場パルスの印加を含むパルスシーケンスに従って磁気共鳴信号を検出する拡散強調撮影実行手段と、
前記拡散強調傾斜磁場パルスによるk空間データのひずみ量をスライス方向の任意の位置において検出するためのリファレンスデータを取得するリファレンスデータ取得手段と、を備え、
前記演算手段は、
前記リファレンスデータから前記スライス方向の任意の位置のリードアウト方向および位相エンコード方向それぞれのひずみ量と位相オフセット量とを前記k空間データのひずみ量の特性データとして算出する特性データ算出手段と、
前記特性データを用いて、前記拡散強調撮影実行手段で得られた磁気共鳴信号により構成されたk空間データを補正する補正手段と、
前記補正手段による補正後のデータから画像を再構成する画像再構成手段と、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、
前記リファレンスデータ取得手段は、
前記拡散強調撮影実行手段が実行するパルスシーケンスにおいて位相エンコード傾斜磁場パルスのパルス強度を0とし、前記拡散強調傾斜磁場パルスを印加するシーケンスと、
当該パルスシーケンスにおいて位相エンコード傾斜磁場パルスのパルス強度を0とし、前記拡散強調傾斜磁場パルスをいずれの軸にも印加しないシーケンスと、
当該パルスシーケンスのリードアウト方向および位相エンコード方向を互いに入れ替えて、位相エンコード傾斜磁場パルスのパルス強度を0とし、前記拡散強調傾斜磁場パルスを印加するシーケンスと、
当該パルスシーケンスのリードアウト方向および位相エンコード方向を互いに入れ替えて、位相エンコード傾斜磁場パルスのパルス強度を0とし、前記拡散強調傾斜磁場パルスをいずれの軸にも印加しないシーケンスと、をそれぞれ実行し、前記リファレンスデータを取得すること
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、
前記特性データ算出手段は、
前記k空間データのリードアウト方向のひずみ量および前記位相オフセット量を、前記リードアウト方向および位相エンコード方向を当該パルスシーケンスと同方向に設定して前記拡散強調傾斜磁場パルスを印加するシーケンスから得たリファレンスデータの、前記リードアウト方向および位相エンコード方向を当該方向に設定して前記拡散強調傾斜磁場パルスを印加しないシーケンスから得たリファレンスデータからの、リードアウト方向のシフト量を用いて算出し、
前記k空間データの位相エンコード方向のひずみ量を、前記リードアウト方向および位相エンコード方向を当該パルスシーケンスと互いに入れ替えて前記拡散強調傾斜磁場パルスを印加するシーケンスから得たリファレンスデータの、前記リードアウト方向および位相エンコード方向を互いに入れ替えて前記拡散強調傾斜磁場パルスを印加しないシーケンスから得たリファレンスデータからの、リードアウト方向のシフト量を用いて算出すること
を特徴とする磁気共鳴イメージング装置。 - 請求項3に記載の磁気共鳴イメージング装置において、
前記特性データ算出手段は、
前記前記k空間データのリードアウト方向のひずみ量および位相エンコード方向のひずみ量の前記リードアウト方向のシフト量を、前記拡散強調傾斜磁場パルスを印加して得たリファレンスデータをリードアウト方向に逆フーリエ変換して得られたプロジェクションデータと、当該拡散強調傾斜磁場パルスを印加せずに得たリファレンスデータをリードアウト方向に逆フーリエ変換して得たプロジェクションデータとの、位相差の一次成分として算出し、
前記位相オフセット量の前記シフト量を、前記拡散強調傾斜磁場パルスを印加して得たリファレンスデータをリードアウト方向に逆フーリエ変換して得られたプロジェクションデータと、当該拡散強調傾斜磁場パルスを印加せずに得たリファレンスデータをリードアウト方向に逆フーリエ変換して得たプロジェクションデータとの、位相差の0次成分として算出すること
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、
スライス位置は複数あり、
前記補正手段は、前記補正の対象となるk空間データのスライス位置の前記位相オフセット量と前記リードアウト方向および位相エンコード方向それぞれのひずみ量とを、前記リファレンスデータを取得した複数のスライス位置毎の特性データから算出し前記補正を行うこと
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、
前記補正の対象となるk空間データのスライス位置の前記位相オフセット量と前記リードアウト方向および位相エンコード方向それぞれのひずみ量とは、前記リファレンスデータを取得した複数のスライス位置毎の特性データを、前記スライス方向に線形補間することにより算出されること
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、
前記補正手段は、前記位相オフセット量を用い、前記k空間データに対し位相オフセット補正を行い、当該位相オフセット補正後の前記k空間データに対し、前記特性データ算出手段で算出したリードアウト方向および位相エンコード方向のひずみ量を用いてグリッディングにより補正を行うこと
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、
前記リファレンスデータを取得時のリードアウト方向は、前記拡散強調撮影実行手段が実行する前記パルスシーケンスのリードアウト方向および位相エンコード方向のいずれか一方に略一致すること
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において、
前記リファレンスデータ取得時に印加する拡散強調傾斜磁場パルスの強度は、前記拡散強調撮影実行手段が前記パルスシーケンスにおいて印加する拡散強調傾斜磁場パルスの強度より小さいこと
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、
前記リファレンスデータ取得手段が前記リファレンスデータを取得するために実行するパルスシーケンスの撮影パラメータは、前記位相エンコード傾斜磁場パルスの強度と、前記拡散強調傾斜磁場パルスの強度以外は、前記拡散強調撮影実行手段が実行するパルスシーケンスの撮影パラメータと同一であること
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であって、
前記特性データ算出手段は、前記特性データを、前記リファレンスデータ取得手段が前記リファレンスデータを取得するために実行するパルスシーケンスにおけるリードアウト方向および位相エンコード方向の視野と、前記拡散強調傾斜磁場パルスの印加強度とにより規格化すること
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、
前記拡散強調傾斜磁場パルスを印加するシーケンスでは、当該拡散強調傾斜磁場パルスを、x、y、zの各軸にそれぞれ印加すること
を特徴とする磁気共鳴イメージング装置。
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KR101821487B1 (ko) | 2015-03-30 | 2018-01-23 | 지멘스 악티엔게젤샤프트 | 보상 확산 기반 확산 영상 |
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US8587310B2 (en) | 2013-11-19 |
JPWO2010035569A1 (ja) | 2012-02-23 |
US20110112393A1 (en) | 2011-05-12 |
JP5138043B2 (ja) | 2013-02-06 |
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