WO2013027539A1 - 磁気共鳴撮影装置、位相値補正方法およびプログラム - Google Patents
磁気共鳴撮影装置、位相値補正方法およびプログラム Download PDFInfo
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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]
- G01R33/46—NMR spectroscopy
- G01R33/4625—Processing of acquired signals, e.g. elimination of phase errors, baseline fitting, chemometric analysis
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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
- G01R33/483—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy
- G01R33/485—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy based on chemical shift information [CSI] or spectroscopic imaging, e.g. to acquire the spatial distributions of metabolites
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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
Definitions
- the present invention relates to spectroscopic imaging, and more particularly to an eddy current correction technique for correcting spectral distortion caused by eddy currents.
- MRS Magnetic Resonance Spectroscopy
- CSI Chemical Shift
- Major metabolites of the human body that can be detected by MRS and MRSI include choline (Cho), creatine (Cr), N-acetylaspartic acid (NAA), lactic acid (Lac), and the like. From the amount of these metabolites, it is possible to determine the degree of progression and early diagnosis of metabolic disorders such as cancer. It is also considered possible to perform non-invasive diagnosis of tumor malignancy.
- an eddy current is generated due to a gradient magnetic field applied during measurement. Eddy currents cause static magnetic field inhomogeneity spatially and temporally and distort the shape of the spectrum obtained by measurement.
- This spectral distortion is usually corrected using the phase value of the signal of the substance having a signal intensity greater than that of the metabolite.
- water is used as a substance having a signal intensity greater than that of a metabolite (see, for example, Non-Patent Document 1).
- a spatial and temporal phase value is calculated from a water FID (free induction decay) signal, phase correction is performed on metabolite image data, and spectral distortion due to eddy current is reduced. to correct.
- CSI or MRSI has a very small number of matrices (voxels) to be measured, about 8 ⁇ 8 to 32 ⁇ 32, from the viewpoint of measurement time and SNR (signal to noise ratio). For this reason, truncation occurs due to the Fourier transform performed in the image reconstruction, and a signal of a distant voxel is mixed. As a result, when the static magnetic field inhomogeneity exists, a water signal having a frequency different from that of the water signal in the target voxel is mixed.
- phase skip region is a region in which the variation in the amount of change in the phase value per unit time is prominent and larger than other portions.
- the magnitude of the phase change amount in this phase jump region is proportional to the concentration of the mixed water signal.
- FIG. 17 (a) shows the spectrum of the water signal by computer simulation.
- Water 1 and Water 2 in FIG. 17A are water signal spectra having frequencies of 2 Hz and 5 Hz, respectively, and a concentration ratio of 1.0 to 0.9.
- FIG. 17B shows the time change of the phase value in the time domain of the water FID signal when these two signals coexist.
- a phase jump proportional to the concentration ratio occurs at a time interval of 1 / ⁇ f with respect to the frequency difference ⁇ f.
- static magnetic field fluctuations due to eddy currents are added to cause a gradual phase change.
- FIG. 17C shows a spectrum before eddy current correction
- FIG. 17D shows a spectrum after eddy current correction.
- FIG. 17D when correction is performed using phase values having phase jumps, ringing artifacts are generated due to phase jumps, and the spectrum is deteriorated conversely by eddy current correction processing.
- Non-Patent Document 3 there is a method of reducing ringing artifacts by applying a low-pass filter to the spectrum of the water signal (for example, see Non-Patent Document 2).
- a method of correcting a phase jump appearing in the phase value of the water FID signal and reducing ringing artifacts see, for example, Non-Patent Document 3.
- the timing at which the absolute value intensity of the FID signal of water in the time domain takes an extreme value is defined as the occurrence of a phase jump.
- the range to be removed as a phase skip is determined using the first derivative of the phase value of the water FID signal with respect to time t.
- the vicinity of the above-described phase jump occurrence location of the first derivative is fitted with a model function, and the full width at half maximum (full width at half maximum, FWHM) of the fitted model function is determined. Then, the phase value in the determined range is corrected on the phase value.
- Non-Patent Document 2 cannot completely eliminate the ringing artifact because the phase skipping low frequency component remains. Further, since the high-frequency component of the eddy current is cut, a sufficient eddy current correction effect cannot be obtained.
- Non-Patent Document 3 when a phase jump occurs even if there is no extreme value in the absolute value intensity of the FID signal of water in the time domain, the phase jump is not corrected. In addition, when there are a plurality of locations where the phase change is steep, it is difficult to identify and extract the location where the phase jumps. In addition, when there are a plurality of phase jumps having different phase change amounts, it is necessary to perform fitting for each phase jump occurrence location in order to increase the fitting accuracy, and the processing becomes complicated. Further, the range for correcting the phase jump is determined on the primary differential value, but the correction itself is performed on the phase value. Therefore, it is necessary to determine the phase change amount of the correction area in accordance with the time change of the phase value, and to correct the correction area and the other areas so as to be smoothly connected, so that the processing becomes complicated.
- the present invention has been made in view of the above circumstances, and it is an object of the present invention to suppress correction artifacts with a simple method and improve correction accuracy when correcting spectral distortion due to eddy currents in an MRI apparatus.
- the phase jump of the phase value used for correction is determined in advance. To correct.
- the phase jump correction first, a portion with a small amount of phase change is specified using the first-order differential value of the phase value, and the other portions are specified as phase jump occurrence locations. Then, the identified phase jump occurrence location is removed on the primary time differential value.
- the phase jump occurrence location is specified as a location that changes by a predetermined threshold value or more within a predetermined range using a first-order time differential value.
- a static magnetic field application unit that applies a static magnetic field to the subject
- a gradient magnetic field application unit that applies a gradient magnetic field to the subject
- a high-frequency magnetic field pulse irradiation unit that irradiates the subject with a high-frequency magnetic field pulse
- a magnetic resonance imaging apparatus comprising: a receiving unit that receives a nuclear magnetic resonance signal from the subject; and a control unit, wherein the control unit includes the gradient magnetic field applying unit, the high-frequency magnetic field pulse irradiating unit, and the receiving unit.
- Measurement control means for controlling the operation of the means to obtain a nuclear magnetic resonance signal of a desired metabolite at each measurement point, eddy current correction means for correcting eddy current of the nuclear magnetic resonance signal, and eddy current correction Display information generating means for generating display information from each magnetic resonance signal for each measurement point corrected by the means, wherein the eddy current correcting means has a FID of a correction substance having a signal intensity higher than that of a metabolite to be measured.
- Trust Phase value calculating means for calculating the phase value for each measurement point, and phase value correcting means for correcting the phase skip of the phase value to obtain a corrected phase value, the phase value correcting means,
- a primary differential value calculating means for calculating a primary time differential value of the phase value for each measurement point;
- a threshold calculating means for calculating a threshold value for identifying a phase jump generation region in which a phase jump occurs;
- the phase jump generation region specifying means for specifying the phase jump generation region of the phase value, and correcting the primary time differential value of the phase jump generation region by the phase
- a phase jump correction unit for correcting a phase jump of the value, wherein the eddy current correction unit performs the eddy current correction using the phase value after the phase jump correction.
- a static magnetic field applying means for applying a static magnetic field to the subject a gradient magnetic field applying means for applying a gradient magnetic field to the subject; a high frequency magnetic field pulse irradiating means for irradiating the subject with a high frequency magnetic field pulse;
- a nuclear magnetic resonance signal of a desired metabolite is measured at each measurement point.
- an eddy current correction means for performing eddy current correction of the nuclear magnetic resonance signal using a phase value for each measurement point of the FID signal of the correction substance having a signal intensity larger than that of the metabolite to be measured,
- a magnetic resonance imaging apparatus comprising: display information generating means for generating display information from each magnetic resonance signal for each measurement point corrected by the eddy current correcting means, a phase value for correcting a phase jump of the phase value
- a phase jump generation region specifying step for specifying the phase jump generation region of the phase value, and the first time differential value of the phase jump generation region.
- a phase jump correcting step for correcting a phase jump of the phase value by correcting, and a corrected phase value calculating step for obtaining a corrected phase value from the corrected first time differential value.
- a static magnetic field applying means for applying a static magnetic field to the subject a gradient magnetic field applying means for applying a gradient magnetic field to the subject; a high frequency magnetic field pulse irradiating means for irradiating the subject with a high frequency magnetic field pulse;
- a nuclear magnetic resonance signal of a desired metabolite is measured at each measurement point.
- an eddy current correction means for performing eddy current correction of the nuclear magnetic resonance signal using a phase value for each measurement point of the FID signal of the correction substance having a signal intensity larger than that of the metabolite to be measured
- Display information generating means for generating display information from each magnetic resonance signal for each measurement point after correction by the eddy current correction means, and a computer of the magnetic resonance imaging apparatus for the primary time of the phase value for each measurement point
- the threshold value calculating means for calculating the threshold value for specifying the phase jump occurrence region in which the phase jump occurs, the phase and the first time differential value
- the phase A phase jump generation area specifying means for specifying the phase jump generation area of the value, and correcting the first time differential value of the phase jump generation area to correct the phase jump of the phase value
- Provided is a program for functioning as a phase jump correcting means for obtaining a corrected phase value from the first time differential value.
- (A)-(c) is an external view of the nuclear magnetic resonance imaging apparatus of embodiment of this invention.
- 1 is a functional configuration diagram of a nuclear magnetic resonance imaging apparatus according to an embodiment of the present invention. It is a functional block diagram of the computer with which the nuclear magnetic resonance apparatus of embodiment of this invention is provided. It is a flowchart for demonstrating the flow of the whole measurement of embodiment of this invention. It is a figure which shows an example of the MRSI pulse sequence of embodiment of this invention.
- (A)-(c) is a figure for demonstrating the area
- FIG. 5 is a graph of a metabolic spectrum by a computer simulation result when eddy current correction is performed using a phase value that is not subjected to phase jump correction according to the embodiment of the present invention
- (c) is a phase of the embodiment according to the present invention. It is a graph of the metabolism spectrum by the computer simulation result at the time of implementing eddy current correction
- FIG. 1 is an external view of the MRI apparatus of this embodiment.
- FIG. 1A shows a horizontal magnetic field type MRI apparatus 100 using a tunnel magnet that generates a static magnetic field with a solenoid coil.
- FIG. 1B shows a hamburger type (open type) vertical magnetic field type MRI apparatus 120 in which magnets are separated into upper and lower sides in order to enhance the feeling of opening.
- FIG. 1C shows an MRI apparatus 130 that uses the same tunnel-type magnet as in FIG. 1A and has a feeling of openness by shortening the depth of the magnet and tilting it obliquely. In the present embodiment, any of these MRI apparatuses having these appearances can be used.
- the MRI apparatus of the present embodiment is not limited to these forms.
- various known MRI apparatuses can be used regardless of the form and type of the apparatus.
- the MRI apparatus 100 is representative.
- FIG. 2 is a functional configuration diagram of the MRI apparatus 100 of the present embodiment.
- the MRI apparatus 100 of the present embodiment includes a static magnetic field generating magnet 102 that is a static magnetic field applying means for applying a static magnetic field to a space in which a subject 101 is placed, an x direction, a y direction, and a z direction.
- a gradient magnetic field coil 103 which is a gradient magnetic field application means for generating a gradient magnetic field in each direction and applying a gradient magnetic field to the subject, a shim coil 104 for adjusting a static magnetic field distribution, and a high-frequency magnetic field for the measurement region of the subject 101
- a measuring high-frequency coil 105 (hereinafter simply referred to as a transmitting coil) that is a high-frequency magnetic field pulse irradiating means for irradiating a pulse, and a receiving high-frequency coil 106 (hereinafter referred to as a receiving means that receives a nuclear magnetic resonance signal generated from the subject 101).
- the gradient magnetic field coil 103 and the shim coil 104 are driven by a gradient magnetic field power supply unit 112 and a shim power supply unit 113, respectively.
- a case where separate transmission coils 105 and reception coils 106 are used will be described as an example.
- the transmission coil 105 and the reception coil 106 are configured as a single coil. May be.
- the high-frequency magnetic field irradiated by the transmission coil 105 is generated by the transmitter 107.
- the nuclear magnetic resonance signal detected by the receiving coil 106 is sent to the computer 109 through the receiver 108.
- the sequence controller 114 controls the operations of the gradient magnetic field power supply unit 112 that is a drive power supply for the gradient coil 103, the shim power supply unit 113 that is the drive power supply for the shim coil 104, the transmitter 107, and the receiver 108. Controls the application of a gradient magnetic field, a high-frequency magnetic field, and the reception of a nuclear magnetic resonance signal.
- the control time chart is called a pulse sequence, is preset according to measurement, and is stored in a storage device or the like included in the computer 109 described later.
- the computer 109 is a control unit that performs various arithmetic processes on the received nuclear magnetic resonance signal to generate image information and spectrum information, and controls the overall operation of the MRI apparatus 100.
- the computer 109 is an information processing apparatus including a CPU, a memory, a storage device, and the like, and a display 110, an external storage device 111, an input device 115, and the like are connected to the computer 109.
- the display 110 is an interface for displaying results obtained by the arithmetic processing to the operator.
- the input device 115 is an interface for an operator to input conditions, parameters, and the like necessary for the arithmetic processing performed in the present embodiment.
- the external storage device 111 holds, together with the storage device, data used for various arithmetic processes executed by the computer 109, data obtained by the arithmetic processes, input conditions, parameters, and the like.
- FIG. 3 is a functional block diagram of the computer 109 of this embodiment.
- the computer 109 of this embodiment includes a measurement control unit 210, a display information generation unit 220, and an eddy current correction unit 230.
- the measurement control unit 210 operates the sequence control device 14 in accordance with the pulse sequence and controls each unit to perform measurement to obtain a nuclear magnetic resonance signal.
- a nuclear magnetic resonance signal of a desired metabolite is obtained for each measurement point.
- the eddy current correction unit 230 performs eddy current correction for correcting the spectral distortion due to the eddy current of the nuclear magnetic resonance signal obtained by the measurement.
- the display information generation unit 220 performs various arithmetic processes on the nuclear magnetic resonance signal after correcting the spectral distortion due to the eddy current to generate display information such as image information and spectrum information.
- the eddy current correction unit 230 applies spectral distortion caused by eddy currents to a FID signal (free induction decay signal; hereinafter referred to as eddy current correction) of a substance (correction substance; water in this embodiment) having a signal intensity higher than that of a metabolite. This is corrected using the phase value of the signal. At this time, the correction is performed after phase jump correction. Therefore, the eddy current correction unit 230 according to the present embodiment corrects the phase value calculation unit 240 that calculates the phase value from the eddy current correction signal, the phase jump in the phase value, and obtains the corrected phase value. A value correction unit 250.
- the various functions realized by the computer 9 are realized by the CPU loading a program stored in the storage device into the memory and executing it.
- at least one function is realized by an information processing apparatus that is independent of the MRI apparatus 100 and capable of transmitting and receiving data to and from the MRI apparatus 100. It may be.
- FIG. 4 is a flowchart for explaining the overall measurement flow of this embodiment.
- N the number of sampling points
- t n is the discrete value representing the time in the n-th measurement point.
- T 0 represents the measurement start time.
- the phase value calculation unit 240 calculates the phase value ⁇ (t n ) of the water FID signal for each measurement point from the obtained water FID signal F (t n ) (step S1102).
- the phase value ⁇ (t n ) of the water FID signal is calculated from the measured water FID signal F (t n ) according to the following equation (1).
- ⁇ (t n ) tan ⁇ 1 (Im (F (t n ))) / (Re (F (t n ))) (1)
- tan -1 is the arctangent function
- Im (F (t n )) is the imaginary part of the complex number F (t n )
- Re (F (t n )) is the real part of the complex number F (t n ).
- phase value correcting unit 250 corrects the phase jump of the phase value ⁇ (t n), obtaining a phase value after correction corrected phase value ⁇ c (t n) (step S1103).
- the measurement control unit 210 controls the sequence control device 14 according to a predetermined pulse sequence, performs water suppression measurement (step S1104), and obtains a metabolite signal S (t n ).
- the eddy current correction unit 230 performs eddy current correction for correcting the obtained metabolite signal S (t n ) with the corrected phase value ⁇ c (t n ) (step S1105), and metabolism after the eddy current correction.
- the substance signal S ecc (t n ) is obtained.
- Eddy current corrected metabolite signal S ecc (t n) the metabolite of a material signal S (t n), using the phase value of the FID signal of water after the phase jump correction ⁇ c (t n), the following formula Calculated according to (2).
- S ecc (t n ) S (t n ) ⁇ exp ( ⁇ i ⁇ ⁇ c (t n )) (2)
- i is an imaginary unit.
- the display information generation unit 220 performs a Fourier transform on the metabolite signal Secc (t n ) after eddy current correction to obtain a spectrum or distribution image of the metabolite (step S1106).
- an example of a pulse sequence used by the measurement control unit 210 for the above measurement (the non-water suppression measurement in step S1101 and the water suppression measurement in step S1104) will be described.
- a description will be given by taking as an example a region selective MRSI pulse sequence (hereinafter referred to as an MRSI pulse sequence) for imaging a metabolite.
- FIG. 5 is an example of the MRSI pulse sequence 300.
- RF indicates the application timing of the high-frequency magnetic field pulse.
- Gx, Gy, and Gz indicate application timings of gradient magnetic field pulses in the x, y, and z directions, respectively.
- a / D indicates a signal measurement period.
- the MRSI pulse sequence 300 shown in FIG. 5 is the same as a known MRSI pulse sequence, and a predetermined region of interest is selectively excited using one excitation pulse RF1 and two inversion pulses RF2 and RF3.
- An FID signal (free induction decay) FID1 is obtained from the region of interest.
- FIGS. 6 (a) to 6 (c) The regions excited according to the MRSI pulse sequence 300 are shown in FIGS. 6 (a) to 6 (c).
- 6 (a) to 6 (c) are positioning scout images obtained by the measurement performed prior to the main measurement.
- FIG. 6 (a) is a transformer image 410 and
- FIG. 6 (b) is a sagittal image.
- Image 420, FIG. 6C is a coronal image 430.
- a high-frequency magnetic field RF1 and gradient magnetic field pulses Gs1 and Gs1 ′ in the z direction are applied to excite the cross section 401 in the z direction.
- a high frequency magnetic field RF2 and a gradient magnetic field pulse Gs2 in the y direction are applied after TE / 4 (where TE is an echo time).
- TE is an echo time
- the high frequency magnetic field RF3 and the gradient magnetic field pulse Gs3 in the x direction are applied after TE / 2 from the application of the high frequency magnetic field RF2.
- the gradient magnetic field pulses Gd1 to Gd3 and Gd1 ′ to Gd3 ′ in each direction rephase the phase of nuclear magnetization excited by the high-frequency magnetic field RF1, and dephase the phase of nuclear magnetization excited by RF2 and RF3. It is a gradient magnetic field. Further, the phase encode gradient magnetic fields Gp1 and Gp2 are applied after the high-frequency magnetic field RF3. Thus, the nuclear magnetic resonance signal of the region of interest 404 is obtained.
- the correction of the phase value of the present embodiment is correction of phase jump of the phase value used for eddy current correction.
- the phase skip correction is performed by specifying the phase jump occurrence location on the phase value and removing the phase jump at the location.
- a phase value that changes with time has an inflection point at a point where a phase jump occurs, and has a maximum value and a minimum value in the vicinity thereof. That is, the primary time differential value of the phase value (hereinafter referred to as the primary differential value) has a peak shape that is convex upward or downward in the vicinity of the location where the phase jump occurs. In the present embodiment, this is used to identify a phase jump region in the phase value. That is, in the primary differential value of the phase value, a range and position other than this peak shape are extracted, and the other range and position are specified as a phase jump occurrence location.
- the phase value correction unit 250 determines whether the portion where the phase change exhibits the above characteristics is due to the influence of the eddy current or due to the phase jump, and specifies only the one due to the phase jump.
- the amount of phase change due to eddy current is smaller than the amount of phase change due to phase jump. Therefore, among the phase changes, those whose amount of change is smaller than a predetermined value are phase changes due to eddy currents, and those whose amount is greater than a predetermined value are due to phase skipping.
- the phase change amount in a predetermined time domain of the first derivative value of the phase value at each measurement point is used.
- the predetermined time region is a region from the measurement start time to a predetermined time that is greatly influenced by the phase change due to the eddy current.
- the phase change amount is calculated as an evaluation value at each point of the primary differential value.
- a portion where the evaluation value is smaller than the threshold is set as a phase change region due to the influence of eddy current or a phase change region due to non-uniform static magnetic field, and the other portion is set as a phase jump generation region.
- a phase change region caused by the influence of eddy currents or a phase change region caused by non-uniform static magnetic fields is referred to herein as a phase skip non-occurrence region.
- Correction is performed by connecting the primary differential values of the measurement points in the phase skip non-occurrence area by interpolation.
- the corrected phase value is obtained from the first-order differential value after connection.
- the phase value correction unit 250 differentiates the phase value with respect to time and obtains a primary differential value, and a calculated primary differential value.
- the threshold value calculated by the threshold value calculation unit 252 of the present embodiment first specifies a region where no phase jump has occurred (phase jump non-occurrence region) based on the threshold value, and sets the other regions as phase jump occurrence regions. By doing so, it is a threshold value for specifying the phase jump generation region. Therefore, the phase skip generation area specifying unit 253 specifies a region having a primary differential value equal to or less than the threshold on the primary differential value as a phase skip non-occurrence region, and specifies the other regions as phase jump generation regions. .
- FIG. 7 is a flowchart for explaining the flow of the phase value correction processing of the present embodiment.
- the primary differential value calculation unit 251 calculates the primary differential value ⁇ z ′ (t n ) from the phase value ⁇ (t n ) of the FID signal of water calculated by the phase value calculation unit 240 (step S1201). .
- the threshold value calculation unit 252 calculates a threshold value P th used when specifying a phase skip non-occurrence region from the primary differential value ⁇ z ′ (t n ) (step S1202).
- phase jump generation area specifying unit 253 specifies a phase jump non-occurrence area from the primary differential value ⁇ z ′ (t n ) using the threshold value P th , and other phase jump generation areas PJ. Is specified (step S1203).
- phase jump correction unit 254 corrects the phase jump of the phase value by removing the phase jump generation region PJ from the primary differential value ⁇ z ′ (t n ), interpolating between them and returning to the phase value. (Step S1204). Thereby, the phase value correcting unit 250 obtains a corrected phase value ⁇ c (t n ).
- the first-order differential value calculation unit 251 first performs a phase return connection process on the phase value ⁇ (t n ) calculated by the phase value calculation unit 240, and obtains the phase value ⁇ z (t n ) after the phase return connection process. obtain. Then, the primary differential value calculation unit 251 obtains a primary differential function ⁇ z ′ (t n ) by differentiating the phase value ⁇ z (t n ) after the phase return connection processing with time.
- the horizontal axis represents time (ms) from the start of measurement of the water FID signal
- the vertical axis represents the phase value (rad).
- a broken line indicates a plot result of the phase value ⁇ (t n ) calculated from the measurement result
- a solid line indicates a plot result of the phase value ⁇ z (t n ) after the phase return connection processing.
- the phase value ⁇ (t n ) is calculated as a value between ⁇ and + ⁇ .
- the phase of the water FID signal acquired by the MRSI pulse sequence 300 has a value exceeding the range of ⁇ to + ⁇ .
- a phase exceeding the range of ⁇ to + ⁇ is folded back to a value between ⁇ and + ⁇ .
- the values are discontinuous in the folded portion. Therefore, phase wrapping connection processing is performed to remove such temporally discontinuous changes in the phase value, and a phase value ⁇ z (t n ) indicating the original phase change state is obtained.
- phase return connection process can use various existing phase return connection processes. At this time, smoothing may be appropriately performed in order to prevent phase variation due to noise.
- phase value ⁇ z (t n ) after the phase loop-back connection process is simply referred to as a phase value ⁇ z (t n ).
- the threshold value P th is used to specify the measurement point corresponding to the phase skip non-occurrence region in the primary differential value ⁇ z ′ (t n ).
- the threshold calculation unit 252 calculates a threshold P th used for this determination.
- the threshold calculation unit 252 sets a time range in which the influence of the phase change due to the eddy current is large from the primary differential value ⁇ z ′ (t n ) of the phase value ⁇ z (t n ) as the threshold calculation region R. Then, the amount of phase change in the threshold calculation region R is calculated as the threshold value P th as the phase change due to the eddy current.
- the threshold value calculation unit 252 sets a time range from the measurement start time to a predetermined time as the threshold value calculation region R in the primary differential value ⁇ z ′ (t n ) of the phase value ⁇ z (t n ). Then, the absolute value
- the threshold calculation region R is determined using, for example, the absolute value
- the threshold value calculation region R is a time range in which the absolute value
- FIG. 9 is a process flow for explaining the flow of the threshold value calculation process.
- FIG. 10A is a diagram for explaining processing for determining the threshold calculation region R from the plot result of the absolute value
- FIG. 10B is a diagram for explaining a process of determining the threshold value P th using the plot result of the primary differential value ⁇ z ′ (t n ).
- the threshold value calculation unit 252 calculates the absolute value
- the threshold value calculation unit 252 determines that the absolute value
- p is an integer satisfying 1 ⁇ p ⁇ N), and the time range from the measurement start time t 0 to time t P is set as the threshold value calculation region R (step S1302).
- at the apparent transverse magnetization relaxation time T2 * is used as the threshold Sth .
- the threshold value calculation unit 252 selects the primary differential value ⁇ z ′ (t n ) of each measurement point t n within the threshold value calculation region R set in step S1302.
- the maximum value M and the minimum value m are extracted, and the absolute value
- is set as a threshold value P th (step S1303).
- which is the change amount of the primary differential value ⁇ z ′ (t n ) obtained in the threshold calculation region R, is considered to be the change amount due to the eddy current.
- the threshold calculation region (time range) R may be determined not by using the signal intensity F (t n ) of the water FID signal but by directly setting the predetermined time t P.
- the predetermined time t P to be set is determined by determining a period during which the eddy current component is held based on, for example, an empirically known time constant of eddy current. For example, t P is determined to be several tens of milliseconds to several hundreds of milliseconds.
- the method for determining the threshold calculation area (time range) R is not limited to this. It suffices if an area that contains sufficient eddy current information and does not change due to a phase jump can be set as the threshold calculation area (time range) R.
- the threshold value P th may be calculated from the primary differential value ⁇ z ′ (t n ) without using the threshold value calculation region R.
- the primary differential value ⁇ z ′ (t n ) shown in FIG. 10B is divided into a plurality of small regions in the time direction. In each small area, the standard deviation of the primary differential value ⁇ z ′ (t n ) included in the small area is calculated. The minimum standard deviation is determined from the calculated standard deviations for each small area. Then, the minimum standard deviation is multiplied by a predetermined coefficient to obtain a threshold value.
- the threshold value calculated by the above procedure uses a region where the phase change in the first-order differential value ⁇ z ′ (t n ) is gradual as a reference for the non-jumping portion.
- an evaluation value calculation area RE k described later is determined by the determination method of the threshold calculation area (time range) R described above.
- the phase jump generation area specifying unit 253 determines whether or not a phase jump has occurred at each measurement point t n . Determination is performed by comparing the evaluation value E k for each measurement point t k to be determined, and a threshold value P th for the threshold value calculation unit 252 has calculated. When the evaluation value E k is smaller than the threshold value P th , the determination target measurement point t k is determined as a phase jump non-occurrence location.
- Evaluation value E k is the primary differential value ⁇ z included in the evaluation value calculation region RE k having a predetermined time width around the measurement point t k 'of (t n), the maximum value M k and the minimum value m k
- FIG. 11 is a flowchart showing the flow of processing by the phase jump generation area specifying unit 253.
- the phase jump generation area specifying unit 253 first sets the first measurement point t k (k is an integer equal to or greater than 1) (step S1401).
- a measurement point for determining whether or not a phase skip occurs is referred to as an evaluation point.
- the next measurement time (measurement point) t P + 1 of the time t P calculated by the threshold value calculation unit 252 is set as the first evaluation point t k .
- the phase jump generation area specifying unit 253 sets an evaluation value calculation area RE k for the evaluation point t k (step S1402).
- the same range as the threshold value calculation region R around the evaluation point t k is set as the evaluation value calculation region RE k .
- the evaluation value calculation region RE k for the evaluation points t k is set to a range of t k -R / 2 of t k + R / 2.
- the phase jump generation area specifying unit 253 calculates an evaluation value E k of the evaluation point t k (step S1403).
- the maximum value M k and the minimum value m k are calculated from the primary differential values ⁇ z ′ (t n ) of each measurement point t n in the evaluation value calculation region RE k .
- of the difference between the maximum value M k and the minimum value m k is calculated as the evaluation value E k .
- the phase jump generation area specifying unit 253 compares the calculated evaluation value E k and the threshold value P th (step S1404). If the evaluation value E k is equal to or smaller than the threshold value P th , the phase jump occurs. It is set as a non-occurrence point (step S1406). That is, at this evaluation point t k , it is determined that only a linear phase change due to non-uniform static magnetic field or a phase change due to eddy current has occurred. On the other hand, if the evaluation value E k is larger than the threshold value P th , the evaluation point t k is set as a phase jump generation point (step S1405). That is, it is determined that a phase jump has occurred at this evaluation point t k .
- phase jump occurrence area specifying unit 253 determines whether each measurement point t k is a phase jump occurrence point or a non-occurrence point.
- a continuous phase jump generation point is called a phase jump generation area, and a continuous non-occurrence point is called a non-occurrence area.
- FIG. 12 shows the determination result by the phase jump generation area specifying unit 253 of the present embodiment.
- the horizontal axis represents time (sec) from the start of measurement of the water FID signal
- the vertical axis represents the first-order differential value ⁇ z ′ (t n ) (rad / sec) of the phase of the water FID signal.
- the solid line 501, 'a (t n) group, the broken line 502 the primary differential value ⁇ z of the phase jump generation region' primary differential value ⁇ z not generating region (t n) groups, respectively.
- Incidentally hollow circles in FIG. 12 corresponds to the t P.
- the phase jump generation region specifying unit 253 of the present embodiment specifies the phase jump generation region using the first-order differential value ⁇ z ′ (t n ) of the phase of the water FID signal.
- FIG. 13 is a flowchart for explaining the flow of phase jump correction by the phase jump correction unit 254 of this embodiment.
- the phase skip correction unit 254 connects adjacent non-occurrence areas 501 by interpolation on the plot result of the primary differential value ⁇ z ′ (t n ) (step S1501). Specifically, as shown in FIG. 14, interpolation is performed on the plot result of the primary differential value ⁇ z ′ (t n ) so that the non-occurrence areas 501 are smoothly connected, and the primary differential value ⁇ z ′ ( The value of the phase jump generation region 502 is removed from t n ).
- a known method such as linear interpolation, spline interpolation, cubic interpolation, or the like that connects the ends of adjacent non-occurrence areas 501 with straight lines is used.
- the phase skip correction unit 254 obtains a primary differential function ⁇ c ′ (t n ) after phase jump correction by fitting the plot result 511 after interpolation, and the value after each phase jump correction is obtained as the value of each measurement point.
- a primary differential value ⁇ c ′ (t n ) (511) is obtained (step S1502).
- the fitting is performed using a polynomial, an exponential function, or the like.
- the phase skip correcting unit 254 obtains a corrected phase value ⁇ c (t n ) from the corrected primary differential value ⁇ c ′ (t n ) (step S1503).
- a corrected phase value ⁇ c (t n ) is obtained from the corrected primary differential value ⁇ c ′ (t n ) (step S1503).
- the phase function ⁇ c (t) after phase jump correction is obtained, and each measurement point of the phase function ⁇ c (t) after correction is obtained.
- a phase value ⁇ c (t n ) after phase jump correction is calculated.
- the corrected phase value ⁇ c (t n ) may be calculated as a power sum of the corrected primary differential value ⁇ c ′ (t n ).
- FIG. 1 an example of a graph of the phase value before and after the phase jump correction by the phase jump correction unit 254 is shown in FIG.
- the horizontal axis indicates time (sec)
- the vertical axis indicates phase (rad).
- the broken line is the plot result of the phase value ⁇ z (t n ) of the water FID signal before phase jump correction (graph of the phase function ⁇ z (t))
- the solid line is the phase value of the water FID signal after phase jump correction.
- a plot result of ⁇ c (t n ) (a graph of the phase function ⁇ c (t)) is represented. As shown in the figure, it is understood that the phase jump of the FID signal of water has been eliminated by the phase jump correction process.
- the magnetic resonance apparatus of the present embodiment includes a static magnetic field application unit that applies a static magnetic field to a subject, a gradient magnetic field application unit that applies a gradient magnetic field to the subject, and a high-frequency magnetic field to the subject.
- a magnetic resonance imaging apparatus 100 comprising: a high-frequency magnetic field pulse irradiation means for irradiating a pulse; a reception means for receiving a nuclear magnetic resonance signal from the subject; and a control means, wherein the control means applies the gradient magnetic field application Control unit 210 for controlling the operation of the means, the high-frequency magnetic field pulse irradiating means and the receiving means to obtain a nuclear magnetic resonance signal of a desired metabolite for each measurement point, and eddy current correction of the nuclear magnetic resonance signal An eddy current correction unit 230, and a display information generation unit 220 that generates display information from each magnetic resonance signal for each measurement point corrected by the eddy current correction unit 230.
- phase jump correction unit 254 that corrects the phase jump of the phase value by correcting the first-order differential value of the phase jump generation region
- the eddy current correction unit 230 includes the phase jump correction unit 230. Phase value after correction Used, and performs the eddy current correction.
- the threshold calculation unit 252 sets a predetermined area as a threshold calculation area, and calculates an absolute value of a difference between the maximum value and the minimum value of the primary time differential value in the threshold calculation area as the threshold. Also good.
- the threshold value calculation area may be an area from a measurement start time to a predetermined time.
- the predetermined time may be a time at which the absolute value of the signal intensity of the FID signal of the correction substance becomes the predetermined value earliest.
- the threshold value calculation unit 252 divides the primary time differential value sequence into a plurality of small regions in the time direction, and calculates a standard deviation of the primary time differential value included in the small region for each of the divided small regions.
- the minimum standard deviation may be specified from all the calculated standard deviations, and a value obtained by multiplying the specified minimum standard deviation by a predetermined coefficient may be calculated as a threshold value.
- the phase jump generation area specifying unit 253 includes a maximum value and a minimum value of the primary time differential value included in an evaluation value calculation area having a predetermined time width centered on a predetermined evaluation point. An absolute value of the difference is calculated as an evaluation value, and for each measurement point, the evaluation value calculated using the measurement point as the evaluation point is compared with the threshold value, and the evaluation value is greater than the threshold value. May be specified as the phase jump generation region, and other measurement points may be specified as the non-occurrence region.
- the phase skip correction unit 254 may correct the phase skip by connecting the first time differential values of the non-occurrence region by interpolation.
- the interpolation may be linear interpolation that connects the first-order differential values at the ends of the adjacent non-occurrence regions with a straight line.
- the phase skip correction unit 254 may use a sum of powers of the first-order differential value after the interpolation as the phase value after the phase skip correction.
- the phase skip correction unit 254 calculates a value corresponding to each measurement point of a function obtained by integrating the primary time differential value obtained by fitting the primary time differential value after the interpolation. It is good also as a phase value after amendment.
- the phase jump is corrected in the eddy current correction processing for correcting the spectral distortion due to the eddy current using the phase value of the FID signal of the substance whose signal intensity is larger than the metabolite to be measured.
- the eddy current correction is performed using the phase value.
- phase value after phase jump correction is obtained by removing the phase jump occurrence location specified by using the primary differential value of the phase value on the primary differential value. That is, the corrected phase value is obtained as the sum of powers of the post-interpolation primary differential value obtained by interpolating between the primary differential values of the measurement points other than the phase jump occurrence location and the specified measurement point.
- the phase jump occurrence location is specified as a location that changes by a predetermined threshold value or more within a predetermined range using a primary differential value.
- the signal strength of the eddy current correction signal is used to identify the time range affected by the eddy current, and the amount of change in the phase value of the eddy current correction signal during that time is used as the threshold value.
- the evaluation unit R is determined. And using these, the primary differential value of a phase value is evaluated and a phase jump generation
- the present embodiment it is possible to identify the phase jump occurrence location regardless of the presence or absence of the extreme value of the absolute value intensity of the FID signal whose phase value is to be corrected. Moreover, the identification can be performed by a simple calculation. Since the location to be corrected is specified with high accuracy, the accuracy of correction is also increased.
- the phase jump of the phase value used for eddy current correction can be removed efficiently and accurately. And, since the eddy current correction processing of the nuclear magnetic resonance signal to be measured is performed using the phase value corrected accurately, ringing artifacts can be effectively prevented and the spectral distortion due to eddy current is corrected well. can do. Therefore, high quality display information can be obtained.
- the baseline is searched and the protruding portion is excluded.
- a protruding part is searched. Therefore, according to the present embodiment, compared to the method disclosed in Non-Patent Document 3, there is no fitting process for a region determined to be out of phase. Further, since the phase skip correction process is performed on the first-order differential value, the phase change amount of the correction region is determined in accordance with the time change of the phase value used in the method disclosed in Non-Patent Document 3, and the correction is performed. The process of correcting so that the area and the other areas are smoothly connected can be omitted. For this reason, the amount of processing can be greatly reduced, the load is small, and the result can be obtained at high speed.
- FIG. 16A shows a metabolite spectrum when eddy current correction is not performed
- FIG. 16B shows a case where eddy current correction is performed using a phase value that is not subjected to phase jump correction according to this embodiment
- FIG. 16C shows the metabolite spectrum when the eddy current correction is performed using the phase value obtained by performing the phase jump correction according to the present embodiment.
- the phase value used for eddy current correction is obtained from the FID signal of water is taken as an example, but the present invention is not limited to this.
- the signal is not limited to the water FID signal as long as it is a signal of a substance having a signal intensity higher than that of the metabolite to be measured.
- DESCRIPTION OF SYMBOLS 100 MRI apparatus, 101: Subject, 102: Static magnetic field production
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Abstract
Description
φ(tn)=tan-1 (Im(F(tn)))/(Re(F(tn))) (1)
ここで、tan-1は、アークタンジェント関数、Im(F(tn))は複素数F(tn)の虚部、Re(F(tn))は複素数F(tn)の実部を表す。
Secc(tn)=S(tn)・exp(-i・φc(tn)) (2)
ここで、iは虚数単位である。
Sth=|F(T2*)|=|F(0)×exp(-1)| (3)
Claims (12)
- 被検体に静磁場を印加する静磁場印加手段と、前記被検体に傾斜磁場を印加する傾斜磁場印加手段と、前記被検体に高周波磁場パルスを照射する高周波磁場パルス照射手段と、前記被検体から核磁気共鳴信号を受信する受信手段と、制御手段と、を備える磁気共鳴撮影装置であって、
前記制御手段は、
前記傾斜磁場印加手段と前記高周波磁場パルス照射手段と前記受信手段との動作を制御して、所望の代謝物質の核磁気共鳴信号を計測点毎に得る計測制御手段と、
前記核磁気共鳴信号の渦電流補正を行う渦電流補正手段と、
前記渦電流補正手段で補正後の計測点毎の各磁気共鳴信号から表示情報を生成する表示情報生成手段と、を備え、
前記渦電流補正手段は、
計測対象の代謝物質より信号強度の大きい補正用物質のFID信号の位相値を前記計測点毎に算出する位相値算出手段と、
前記位相値の位相とびを補正し、補正後の位相値を得る位相値補正手段と、を備え、
前記位相値補正手段は、
前記計測点毎の位相値の1次時間微分値を算出する1次微分値算出手段と、
位相とびが発生する位相とび発生領域を特定するための閾値を算出する閾値算出手段と、
前記閾値と前記1次時間微分値とを用いて、前記位相値の記位相とび発生領域を特定する位相とび発生領域特定手段と、
前記位相とび発生領域の前記1次時間微分値を補正することにより前記位相値の位相とびを補正する位相とび補正手段と、を備え、
前記渦電流補正手段は、前記位相とび補正後の位相値を用い、前記渦電流補正を行うこと
を特徴とする磁気共鳴撮影装置。 - 請求項1記載の磁気共鳴撮影装置であって、
前記閾値算出手段は、
所定の領域を閾値算出領域として設定し、当該閾値算出領域内の、前記1次時間微分値の最大値と最小値との差分の絶対値を前記閾値として算出すること
を特徴とする磁気共鳴撮影装置。 - 請求項2記載の磁気共鳴撮影装置であって、
前記閾値算出領域は、計測開始時刻から所定の時刻までの領域であること
を特徴とする磁気共鳴撮影装置。 - 請求項3記載の磁気共鳴撮影装置であって、
前記所定の時刻は、前記補正用物質のFID信号の信号強度の絶対値が最も早く所定の値となる時刻であること
を特徴とする磁気共鳴撮影装置。 - 請求項1記載の磁気共鳴撮影装置であって、
前記閾値算出手段は、
前記1次時間微分値列を、時間方向に複数の小領域に分割し、分割後の小領域毎に、当該小領域に含まれる1次時間微分値の標準偏差を算出し、算出した全標準偏差の中から最小の標準偏差を特定し、特定された最小の標準偏差に所定の係数をかけた値を閾値として算出すること
を特徴とする磁気共鳴撮影装置。 - 請求項1記載の磁気共鳴撮影装置であって、
前記位相とび発生領域特定手段は、予め定めた評価点を中心とする所定の時間幅である評価値算出領域内に含まれる、前記1次時間微分値の最大値と最小値との差分の絶対値を評価値として算出し、前記計測点毎に、当該計測点を前記評価点として算出された前記評価値と前記閾値とを比較して、当該評価値が前記閾値より大きい計測点を、前記位相とび発生領域として特定し、他の計測点を不発生領域として特定すること
を特徴とする磁気共鳴撮影装置。 - 請求項6記載の磁気共鳴撮影装置であって、
前記位相とび補正手段は、前記不発生領域の前記1次時間微分値間を、補間により接続し、前記位相とびを補正すること
を特徴とする磁気共鳴撮影装置。 - 請求項7記載の磁気共鳴撮影装置であって、
前記補間は、隣接する前記不発生領域の端部の前記1次時間微分値間を直線で接続する直線補間であること
を特徴とする磁気共鳴撮影装置。 - 請求項7記載の磁気共鳴撮影装置であって、
前記位相とび補正手段は、前記補間後の前記1次時間微分値の累乗和を、前記位相とび補正後の位相値とすること
を特徴とする磁気共鳴撮影装置。 - 請求項7記載の磁気共鳴撮影装置であって、
前記位相とび補正手段は、前記補間後の前記1次時間微分値をフィッティングして得た1次時間微分値を積分して得た関数の、各計測点に対応する値を、前記補正後の位相値とすること
を特徴とする磁気共鳴撮影装置。 - 被検体に静磁場を印加する静磁場印加手段と、前記被検体に傾斜磁場を印加する傾斜磁場印加手段と、前記被検体に高周波磁場パルスを照射する高周波磁場パルス照射手段と、前記被検体から核磁気共鳴信号を受信する受信手段と、前記傾斜磁場印加手段と前記高周波磁場パルス照射手段と前記受信手段との動作を制御して、所望の代謝物質の核磁気共鳴信号を計測点毎に得る計測制御手段と、計測対象の代謝物質より信号強度の大きい補正用物質のFID信号の計測点毎の位相値を用いて前記核磁気共鳴信号の渦電流補正を行う渦電流補正手段と、前記渦電流補正手段で補正後の計測点毎の各磁気共鳴信号から表示情報を生成する表示情報生成手段と、を備える磁気共鳴撮影装置において、前記位相値の位相とびを補正する位相値補正方法であって、
計測点毎の前記位相値の1次時間微分値を算出する1次微分値算出ステップと、
位相とびが発生する位相とび発生領域を特定するための閾値を算出する閾値算出ステップと、
前記閾値と前記1次時間微分値とを用いて、前記位相値の前記位相とび発生領域を特定する位相とび発生領域特定ステップと、
前記位相とび発生領域の前記1次時間微分値を補正することにより、前記位相値の位相とびを補正する位相とび補正ステップと、
前記補正後の前記1次時間微分値から補正後の位相値を得る補正後位相値算出ステップと、を備えること
を特徴とする位相値補正方法。 - 被検体に静磁場を印加する静磁場印加手段と、前記被検体に傾斜磁場を印加する傾斜磁場印加手段と、前記被検体に高周波磁場パルスを照射する高周波磁場パルス照射手段と、前記被検体から核磁気共鳴信号を受信する受信手段と、前記傾斜磁場印加手段と前記高周波磁場パルス照射手段と前記受信手段との動作を制御して、所望の代謝物質の核磁気共鳴信号を計測点毎に得る計測制御手段と、計測対象の代謝物質より信号強度の大きい補正用物質のFID信号の計測点毎の位相値を用いて前記核磁気共鳴信号の渦電流補正を行う渦電流補正手段と、前記渦電流補正手段で補正後の計測点毎の各磁気共鳴信号から表示情報を生成する表示情報生成手段と、を備える磁気共鳴撮影装置のコンピュータを、
計測点毎の前記位相値の1次時間微分値を算出する1次微分値算出手段、
位相とびが発生する位相とび発生領域を特定するための閾値を算出する閾値算出手段、
前記閾値と前記1次時間微分値とを用いて、前記位相値の前記位相とび発生領域を特定する位相とび発生領域特定手段、および、
前記位相とび発生領域の前記1次時間微分値を補正することにより、前記位相値の位相とびを補正し、補正後の前記1次時間微分値から補正後の位相値を得る位相とび補正手段として機能させるためのプログラム。
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