WO2005000116A1 - 磁気共鳴撮影装置 - Google Patents
磁気共鳴撮影装置 Download PDFInfo
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- WO2005000116A1 WO2005000116A1 PCT/JP2004/007992 JP2004007992W WO2005000116A1 WO 2005000116 A1 WO2005000116 A1 WO 2005000116A1 JP 2004007992 W JP2004007992 W JP 2004007992W WO 2005000116 A1 WO2005000116 A1 WO 2005000116A1
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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
- A61B5/41—Detecting, measuring or recording for evaluating the immune or lymphatic systems
- A61B5/411—Detecting or monitoring allergy or intolerance reactions to an allergenic agent or substance
-
- 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
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/41—Detecting, measuring or recording for evaluating the immune or lymphatic systems
- A61B5/414—Evaluating particular organs or parts of the immune or lymphatic systems
- A61B5/415—Evaluating particular organs or parts of the immune or lymphatic systems the glands, e.g. tonsils, adenoids or thymus
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/41—Detecting, measuring or recording for evaluating the immune or lymphatic systems
- A61B5/414—Evaluating particular organs or parts of the immune or lymphatic systems
- A61B5/416—Evaluating particular organs or parts of the immune or lymphatic systems the spleen
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/41—Detecting, measuring or recording for evaluating the immune or lymphatic systems
- A61B5/414—Evaluating particular organs or parts of the immune or lymphatic systems
- A61B5/418—Evaluating particular organs or parts of the immune or lymphatic systems lymph vessels, ducts or nodes
-
- 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
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/389—Field stabilisation, e.g. by field measurements and control means or indirectly by current stabilisation
-
- 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
-
- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/0033—Features or image-related aspects of imaging apparatus classified in A61B5/00, e.g. for MRI, optical tomography or impedance tomography apparatus; arrangements of imaging apparatus in a room
- A61B5/0037—Performing a preliminary scan, e.g. a prescan for identifying a region of interest
Definitions
- the present invention relates to a magnetic resonance imaging apparatus, and particularly to an apparatus suitable for measuring a magnetic resonance signal including information on a chemical shift.
- a magnetic resonance imaging apparatus irradiates a subject placed in a static magnetic field with a high-frequency magnetic field of a specific frequency, thereby exciting nuclear magnetization of hydrogen nuclei contained in the subject (magnetic resonance phenomenon), It can detect physical and chemical information by detecting magnetic resonance signals generated from specimens.
- magnetic resonance imaging which is widely used, acquires images reflecting the density distribution of hydrogen nuclei mainly contained in water molecules in a subject. ing.
- a magnetic resonance spectrum that separates magnetic resonance signals for each molecule based on the difference in resonance frequency (hereinafter referred to as chemical shift) due to the difference in chemical bonds between various molecules including hydrogen nuclei.
- MRSI Magnetic Resonance Spectroscopic Imaging
- the transmission frequency when irradiating a high-frequency magnetic field and the reception frequency when detecting a magnetic resonance signal were set on the assumption that the static magnetic field strength was constant over time.
- spectrum measurement pre-measurement for resonance frequency detection
- the static magnetic field strength is temporally constant (that is, the resonance frequency is assumed to be constant).
- the static magnetic field strength may change during MRS measurement, depending on the structure and characteristics of the magnet that generates the static magnetic field and the measurement environment.
- the water suppression rate gradually decreases with the shift of the resonance frequency, the excitation slice position gradually shifts, There is a problem that the SNR improvement effect cannot be obtained by integration.
- An object of the present invention is to provide a magnetic resonance imaging apparatus that enables highly accurate spectrum measurement even when the resonance frequency changes during measurement.
- the time change characteristic of the water resonance frequency is measured in advance before MRS or MR SI measurement, and the Predicts the amount of change in the water resonance frequency of RS or MRSI measurement, and based on the predicted value, the transmission frequency of the high-frequency magnetic field irradiated with the water signal suppression pulse sequence, MRS or; for excitation and inversion in the sequence of MRSI measurement
- the setting values of the transmission frequency of the high-frequency magnetic field for use and the reception frequency at the time of detecting the magnetic resonance signal are changed every moment during the measurement.
- multiple magnetic resonance signals measured in the MRS or MRSI measurement sequence are shifted according to the frequency change (this frequency change is predicted based on the previously measured time change characteristics of the water resonance frequency). And add them.
- measurement for detecting the water resonance frequency is performed every predetermined number of times, and based on this detection value, Set the transmission frequency of the high-frequency magnetic field applied in the water signal suppression pulse sequence, the transmission frequency of the excitation and inversion high-frequency magnetic field in the MRS or MRSI measurement sequence, and the reception frequency when detecting the magnetic resonance signal.
- the time variation characteristic of the water resonance frequency is measured in advance before the MRS or MRSI measurement, and the amount of change in the water resonance frequency during the MRS or MRSI measurement is predicted from the measured time variation characteristic, and the prediction is performed.
- the magnetic resonance imaging apparatus of the present invention includes a means for generating a static magnetic field, a gradient magnetic field generating means for generating a gradient magnetic field, and a high-frequency magnetic field generation for generating a high-frequency magnetic field.
- Generating means measuring means for measuring a magnetic resonance signal generated from the subject, calculating means for performing an operation on the measured magnetic resonance signal, and storage for storing the measured magnetic resonance signal and a calculation result by the calculating means Means, a gradient magnetic field generating means, a high frequency magnetic field generating means, a measuring means, a calculating means, and a sequence controlling means for controlling the operation of each part by setting operating conditions in each part of the storing means.
- the sequence control means may: (1) perform a preliminary operation for measuring the resonance frequency of water for each predetermined number of measurements of the magnetic resonance signal; Performing measurement, (2) detecting the resonance frequency of water from a magnetic resonance spectrum obtained by Fourier transforming the magnetic resonance signal obtained in the preliminary measurement, and (3) detecting the water frequency in (2). Setting the transmission frequency or Z of the high-frequency magnetic field applied to the subject and the reception frequency when measuring the magnetic resonance signal in the pulse sequence executed after the preliminary measurement based on the resonance frequency of Perform control.
- the sequence control means includes: (1) applying a high-frequency magnetic field and a gradient magnetic field to the subject to execute a water suppression sequence for suppressing a water signal; and (2) applying a high-frequency magnetic field to the subject. And applying a gradient magnetic field to selectively excite a predetermined poxel and execute a spectrum measurement sequence for measuring a magnetic resonance signal generated from the predetermined poxel; (3) (1) and (2) are performed a plurality of times. If the repetition is performed, a pre-measurement sequence for measuring the resonance frequency of water is performed prior to the execution of the predetermined number of times (1) and (2), and the detection is performed in (4) and (3).
- the transmission frequency of the high-frequency magnetic field applied in the water suppression sequence is set based on the resonance frequency of the water, and the transmission frequency of the high-frequency magnetic field applied to selectively excite the predetermined poxels in the spectrum measurement sequence is set. Number or / and, And setting a reception frequency when detecting a magnetic resonance signal generated from the predetermined poxel.
- the sequence control means includes: (1) applying a high-frequency magnetic field and a gradient magnetic field to the subject to execute a water suppression sequence for suppressing a water signal; Applying a magnetic field and a gradient magnetic field to selectively excite a predetermined poxel and execute a spectrum measurement sequence for measuring a magnetic resonance signal generated from the predetermined poxel; (3)
- the magnetic resonance signal obtained by performing (1) and (2) every time a predetermined number of times (1) and (2) are performed Detecting a water signal peak from a magnetic resonance spectrum obtained by performing a Fourier transform on the water signal peak, and calculating the signal strength of the water signal peak.
- the calculated signal strength of the water signal peak is equal to or more than a predetermined value. If the water resonance frequency has shifted, it is determined that the water resonance frequency has shifted.If it is determined in (5) and (4) that the water resonance frequency has shifted, a preliminary measurement for measuring the water resonance frequency is performed.
- the pulse sequence executed after the preliminary measurement Setting the transmission frequency of the high-frequency magnetic field applied in the pressure sequence, and / or setting the transmission frequency of the high-frequency magnetic field applied to selectively excite the predetermined poxel in the spectrum measurement sequence, and / or And setting a reception frequency when detecting a magnetic resonance signal generated from the predetermined poxel.
- the sequence control means irradiates the subject with the high-frequency magnetic field at least once, and applies the magnetic resonance signal generated after the irradiation of the high-frequency magnetic field in a state where the applied intensity of the gradient magnetic field is almost zero. It includes a control for measuring, calculating magnetic resonance spectrum information from the measured magnetic resonance signal, and performing magnetic resonance spectrum measurement.
- the sequence control means includes: (1) measuring a first magnetic resonance signal generated from a poxel to be measured in magnetic resonance spectrum measurement in a first time interval; ) Detecting the resonance frequency F1 of water from the first magnetic resonance spectrum obtained by Fourier transforming the first magnetic resonance signal; (3) detecting a predetermined frequency after the measurement of the first magnetic resonance signal; Measuring a second magnetic resonance signal generated from the poxel in a second time interval after the time; (4) a second magnetic resonance signal obtained by performing a Fourier transform on the second magnetic resonance signal; Detecting the resonance frequency F2 of water from the vector, and (5) calculating the time variation of the resonance frequency of water based on F1 and F2.
- the sequence control means includes: (1) measuring a first magnetic resonance signal generated from a poxel to be measured in magnetic resonance spectrum measurement in a first time interval; ) Detecting the resonance frequency F1 of water from the first magnetic resonance spectrum obtained by Fourier transforming the first magnetic resonance signal; (3) detecting a predetermined frequency after the measurement of the first magnetic resonance signal Measuring a second magnetic resonance signal generated from the poxel in a second time interval after the time; (4) a second magnetic resonance signal obtained by performing a Fourier transform on the second magnetic resonance signal; Detecting the resonance frequency F2 of the water from the vector, (5) based on F1 and F2, in the measurement time for measuring the magnetic resonance signal after the end of the measurement of the second magnetic resonance signal, Water resonance Estimating the time variation of the frequency; (6) using the time variation of the estimated resonance frequency to determine the transmission frequency or Z of the high-frequency magnetic field and the reception frequency for receiving the magnetic resonance signal generated from the above-mentioned pixels.
- the magnetic resonance signal generated from the above-described poxel is measured. (7) Measurement of the second magnetic resonance signal After the end, control of (6) is repeated several times.
- the sequence control means may: (1) measure the resonance frequency of water every time the magnetic resonance signal is measured a predetermined number of times; Preliminary measurement— (2) Detecting the resonance frequency of water from the magnetic resonance spectrum obtained by Fourier transforming the magnetic resonance signal obtained in the preliminary measurement; (3) Detecting the resonance frequency in (2) Based on the resonance frequency of the water, the transmission frequency of the high-frequency magnetic field applied to the subject and / or the reception frequency when the magnetic resonance signal is measured in the spectrum measurement sequence executed after the preliminary measurement Setting and control of.
- FIG. 1 is an external view of a magnetic resonance imaging apparatus to which the present invention is applied
- FIG. 2 is a diagram showing a configuration example of a magnetic resonance imaging apparatus to which the present invention is applied
- FIG. 3 is used in an embodiment of the present invention.
- FIG. 4 is a diagram showing an example of an MRS pulse sequence to be used
- FIG. 4 is a diagram showing an example of a pulse sequence for suppressing a water signal used in the embodiment of the present invention
- FIG. 5 is a diagram showing an embodiment of the present invention.
- MRS measurement procedure assuming that the static magnetic field strength is constant over time
- FIG. 6 shows (a) the positioning of the imaging poxels in the flowchart of FIG. 5, and (b) the flow chart of FIG. 5 when the static magnetic field intensity changes over time.
- FIG. 7 is a flowchart showing an MRS measurement procedure according to the first embodiment of the present invention
- FIG. 8 is a diagram showing (a) a position of an imaging pixel
- FIG. b) A diagram showing an example of MRS measurement results measured according to the flowchart of FIG. 7 when the static magnetic field intensity changes over time.
- FIG. 9 is a flowchart showing a procedure of MRS measurement in the second embodiment of the present invention.
- Figures 10 and 10 show (a) a diagram showing the positions of the imaging poxels, and (b) a diagram of the MRS measured in accordance with the flow chart of Figure 9 when there is no temporal continuity in the change characteristics of the static magnetic field strength.
- Figure 11 shows an example of the measurement results.
- FIG. 3 is a diagram showing an example of a pulse sequence of MRS measurement applicable to the embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION
- FIG. 1 is an external view of a magnetic resonance imaging apparatus to which the present invention is applied.
- Fig. 1 (a) shows a magnetic resonance imaging system using a tunnel-type magnet that generates a static magnetic field with a solenoid coil.
- Fig. 1 (b) shows a hamburger type with a magnet separated vertically to increase the sense of openness. Is a magnetic resonance imaging apparatus.
- Fig. 1 (c) is the same tunnel type magnetic resonance imaging apparatus as Fig. 1 (a), but the depth of the magnet is shortened and the magnet is tilted obliquely to enhance the open feeling.
- FIG. 2 is a diagram showing a configuration example of a magnetic resonance imaging apparatus to which the present invention is applied.
- the subject 1 is placed in a space where a static magnetic field generated by a static magnetic field generating magnet 2 and a gradient magnetic field in three orthogonal directions generated by a gradient magnetic field generating coil 3 are applied. Change the current flowing through each coil In some cases, a shim coil 11 that can adjust the uniformity of the static magnetic field is provided.
- the subject 1 is irradiated with a high-frequency magnetic field generated by the probe 4 to cause a magnetic resonance phenomenon, and a magnetic resonance signal generated from the subject 1 is detected by the probe 4.
- the high-frequency magnetic field to be irradiated is generated by the transmitter 8, and the detected magnetic resonance signal is sent to the computer 5 through the receiver 9.
- the computer 5 performs various arithmetic processing on the magnetic resonance signal to generate spectrum information and image information, and displays the information on the display 6 and records the information on the storage device 13 ( If necessary, measurement conditions and the like are also recorded in the storage device 13).
- the power supply 12 for driving the shim coil 11, the power supply 7 for driving the gradient magnetic field generating coil 3, the transmitter 8 and the receiver 9 are controlled by the sequence controller 10.
- FIG. 2 shows an example in which the probe 4 is used for both transmission and reception, a transmission probe and a reception probe may be provided separately.
- the pulse sequence used in the embodiment of the present invention will be described.
- FIG. 3 is a diagram showing an example of an MRS measurement pulse sequence (MRS pulse sequence) used in the embodiment of the present invention.
- MRS pulse sequence MRS pulse sequence
- the first gradient field (X-axis gradient field) for selecting the first slice (plane perpendicular to the X axis) is called Gs1 and 90 ° pulse.
- Gs1 and 90 ° pulse By simultaneously applying the first high-frequency magnetic field RF1, the nuclear magnetization in the first slice can be excited.
- TE is the echo time
- TR is the repetition time.
- a second gradient magnetic field (gradient magnetic field in the Y-axis direction) G s 2 for selecting the second slice (plane perpendicular to the Y-axis) is called Gs 2 and 180 ° pulse.
- Gs 2 and 180 ° pulse a second gradient magnetic field (gradient magnetic field in the Y-axis direction) G s 2 for selecting the second slice (plane perpendicular to the Y-axis)
- the nuclear magnetization in the first slice excited by RF1 is reduced.
- the nuclear magnetization included in the second slice can be inverted by 180 °.
- a third gradient magnetic field (a gradient magnetic field in the Z-axis direction) Gs3 for selecting a third slice (a plane perpendicular to the Z-axis) and a second pulse called a 180 ° pulse
- the nuclear magnetization included in the third slice among the nuclear magnetization in the intersection region of the first slice and the second slice inverted by RF2 is again reproduced.
- ° Can be inverted.
- G s 1 ′ applied immediately after the application of G s 1 is a gradient magnetic field for rephase (phase return) to G s 1.
- Gd1 and Gd1, applied before and after the application of RF2, and Gd2 and Gs2, do not disturb the phase of the nuclear magnetization excited by the irradiation of RF1 (G The phase change is canceled by d 1 and G d 1, and the phase change is canceled by G d 2 and G s 2.), the nuclear magnetization excited by the irradiation of RF 2 is diffused (phase disorder). ).
- Gd3 and Gd3, and Gd4 and Gd4 which are applied before and after the application of RF3, do not disturb the phase of nuclear magnetization excited by irradiation of RF1 (G The phase change is canceled by d3 and Gd3, 0 (14 and 0 (the phase change is canceled by 14 ').
- the nuclear magnetization excited by the irradiation of RF3 is dephased (phase disorder). ).
- SINC waveform (sin (t) / t) having a rectangular excitation frequency characteristic is often used for the first high-frequency magnetic field R F1 and the second high-frequency magnetic field R F2 in many cases.
- FIG. 4 is a diagram showing an example of a pulse sequence (water signal suppression pulse sequence) for suppressing a water signal used in the embodiment of the present invention.
- the water signal suppression described in Non-Patent Document 2 Is the way.
- the transmission frequency Ft is set to the resonance frequency Fw of water
- the excitation frequency band ⁇ F High-frequency magnetic field with t set to about the water peak width ⁇ F w (high-frequency magnetic field for water excitation) Irradiation with RF w 1 (selective excitation of water core magnetization).
- a gradient magnetic field for phase G dw1 is applied (water Pseudo-saturation of nuclear magnetization).
- the high frequency magnetic field RF w 1 has a narrow band excitation frequency characteristic. Gaussian waveforms are often used.
- GX as gradient magnetic field diffusion AIDS, G y is an example of applying a gradient magnetic field of any one axis of the G z, G x, G y , 3 axes G z All gradient magnetic fields may be applied simultaneously, or any two axes may be applied simultaneously.
- the signal of the weak metabolite can be measured by performing the excitation and detection according to the sequence of FIG.
- the flip angle of the high frequency magnetic field RFw for water excitation is often set to around 90 °, but for the gradient magnetic field Gdw for phase, various combinations and numerical values are used as the number of applied axes and applied strength.
- the metabolite signal that can be detected in a living body is often very weak, so a large number of integrations are performed with the aim of improving the signal-to-noise ratio (SNR) of the obtained spectrum. Often done.
- SNR signal-to-noise ratio
- FIG. 5 shows the transmission frequency when irradiating a high-frequency magnetic field and the reception frequency when detecting a magnetic resonance signal, assuming that the static magnetic field strength is constant over time (the resonance frequency is constant) in the embodiment of the present invention.
- FIG. 7 is a flowchart showing a procedure of MRS measurement when is set only once. The outline of the shooting procedure is described below.
- STEP05-01 Determine the imaging poxel V1 of the subject.
- STEP05-02 If necessary, perform shimming to improve the uniformity of the static magnetic field.
- STEP05-03 Using the MRS sequence, a magnetic resonance signal Sig generated from a predetermined region including the imaging poxel V1 is acquired.
- STEP05-04 Calculate the magnetic resonance spectrum by applying the Fourier transform to the obtained magnetic resonance signal.
- STEP05-05 Detect the water resonance frequency Fw from the magnetic resonance spectrum.
- STEP05-06 Based on the detected Fw value, the transmission frequency of the high-frequency magnetic field irradiated in the process of suppressing the water signal, the transmission frequency of the high-frequency magnetic field irradiated to selectively excite the imaging poxel V1, and the imaging Set each value of the reception frequency when detecting the magnetic resonance signal generated from the poxel V1.
- STEP05-07 Measure the spectrum of metabolites by continuously performing the water signal suppression pulse sequence shown in Fig. 4 and the MRS sequence shown in Fig. 3.
- STEP05-08 Repeat STEP05-07 as necessary to perform signal integration.
- the transmission frequency when irradiating a high-frequency magnetic field and the reception frequency when detecting a magnetic resonance signal are set only once, assuming that the static magnetic field strength is constant over time. If the static magnetic field strength changes over time for some reason, as the number of integration increases, the position of the measurement poxel shifts, the position of the measurement peak shifts, and a sufficient integration effect cannot be obtained. Insufficient suppression of water signal.
- Fig. 6 shows an example of the measurement results when the measurement was performed according to the procedure shown in Fig. 5 when the static magnetic field strength changed over time.
- Fig. 6 (a) shows the position of three orthogonal slices (first slice, second slice, and third slice) on the magnetic resonance image in the procedure for determining the imaging poxel position shown in Fig. 5.
- FIG. 6 (b) shows a spectrum obtained from this imaging poxel.
- the peaks of choline and creatine which should be observable separately from each other, are superimposed and buried in the water signal undersuppression.
- the half width of the NAA peak is also wide.
- a preliminary measurement for measuring the time change characteristic of the water resonance frequency is performed in advance before performing the MRS measurement, and the time change of the water resonance frequency is performed. From the characteristics, the amount of change in the water resonance frequency of MRS measurement is predicted, and based on the predicted value, the transmission frequency of the high-frequency magnetic field irradiated in the water signal suppression pulse sequence, the high-frequency magnetic field for excitation and inversion in the MRS sequence.
- FIG. 7 is a flowchart showing a procedure of MRS measurement in the first embodiment of the present invention. Hereinafter, a specific photographing procedure will be described.
- STEP07-01 First, the imaging poxel V1 of the subject is determined.
- STEP07-02 If necessary, perform shimming to improve the uniformity of the static magnetic field. O
- STEP07-01 and STEP07-02 may be performed in any order.
- STEP07-03 The first magnetic resonance signal Sig1 generated from the imaging poxel V1 at the first time t1 is acquired using the MRS pulse sequence shown in FIG.
- STEP07-04 Calculate the first magnetic resonance spectrum by applying Fourier transform to Sig1.
- STEP07-05 Detect the water resonance frequency F w 1 from the first magnetic resonance spectrum.
- STEP07-06 Associate t 1 with F w 1 and save.
- STEP07-07 At a second time t2 after a predetermined time from the time t1, a second magnetic resonance signal Sig2 generated from the imaging bottom cell V1 is acquired.
- STEP07-09 Detect the water resonance frequency F w 2 from the second magnetic resonance spectrum.
- STEP07-10 It is possible to calculate the saved time t1, Fw1, t2 and Fw2, force, and the time change characteristic of water resonance frequency (Fw2-Fwl) (t2-tl). come.
- STEP07-12 Based on the estimated value (change amount) of the water resonance frequency, the transmission frequency F wt (i) of the high-frequency magnetic field applied to suppress the water signal set in each measurement M i, the imaging poxel V Set the transmission frequency F t (i) of the high-frequency magnetic field applied to selectively excite and invert 1 and the reception frequency F r (i) when detecting the magnetic resonance signal generated from the imaging poxel V1.
- Equation 2 the transmission frequency F wt (i) of the high-frequency magnetic field applied to suppress the water signal set in each measurement M i
- the imaging poxel V Set the transmission frequency F t (i) of the high-frequency magnetic field applied to selectively excite and invert 1 and the reception frequency F r (i) when detecting the magnetic resonance signal generated from the imaging poxel V1.
- STEP07-13 Using the calculated set values (Fwt (i), Ft (i), and Fr (i)), the sequence shown in Figs. Measure the spectrum.
- STEP07-14 Repeat steps 07-13 while integrating each set value (Fwt (i), Ft (i), Fr (i)) to the value calculated above to perform signal integration.
- FIG. 8 shows an example of a measurement result when the measurement is performed according to the procedure of FIG. 7 when the static magnetic field strength changes with time.
- FIG. 8 (a) shows the positions of the imaging voxels determined on the magnetic resonance image by the first slice, the second slice, and the third slice
- FIG. 8 (b) shows the positions obtained from these imaging voxels. Shows the spectrum. Compared to Fig. 6 (b), in Fig. 8 (b), the water signal is sufficiently suppressed, and the half-width of the NAA peak is narrower. In addition, choline and creatine peaks were also separated.
- the water resonance frequency at two given times is measured to calculate the time change characteristic of the water resonance frequency
- the water resonance frequency at three or more times is measured and the time change characteristic is calculated.
- the least-squares fitting method or the like can be used to calculate the water resonance frequency time change characteristics with higher accuracy.
- Embodiment 2 in which a sufficient accuracy improvement effect can be expected even when there is no temporal continuity in the change characteristic of the water resonance frequency is described below.
- the pulse sequence for suppressing the water signal shown in Fig. 4 and the MRS pulse sequence shown in Fig. 3 are continuously performed and the MRS measurement including the repeated measurement is performed, during the repeated measurement,
- a preliminary measurement of the magnetic resonance signal for detecting the water resonance frequency is performed at a predetermined number of repetitions, a water resonance frequency of the repetition measurement performed after the preliminary measurement is calculated, and the repetitive measurement is performed based on the calculated value.
- FIG. 9 is a flowchart illustrating a procedure of MRS measurement in the second embodiment of the present invention. Hereinafter, a specific photographing procedure will be described.
- STEP09-01 Determine the imaging poxel V1 of the subject.
- STEP09-02 If necessary, perform shimming to improve the uniformity of the static magnetic field.
- STEP09-08 Detect the first water resonance frequency Fw1 from the first magnetic resonance spectrum.
- the set value of the reception frequency F r (i) for detecting the magnetic resonance signal generated from the imaging poxel V1 is calculated according to (Equation 5), (Equation 6), and (Equation 7), respectively.
- F t (i) Fwl ⁇ : ⁇ ( number 5)
- Ft (i) Fwl ⁇ ( number 6)
- FIG. 10 is an example of a measurement result when the measurement is performed in accordance with the procedure of FIG. 9 when there is no temporal continuity in the change characteristic of the static magnetic field intensity.
- FIG. 10 (a) shows the position of the radiographic poxel determined on the magnetic resonance image
- FIG. 10 (b) shows the spectrum insulted from this radiographic poxel.
- the water signal is sufficiently suppressed, and the half-width of the NAA peak is narrower.
- the peaks for choline and creatine could be separated.
- the second embodiment in the L-times spectrum measurement after the detection of the water resonance frequency, the case where various frequencies are set based on the detected water resonance frequency itself has been described. After estimating the water resonance frequency at each measurement time during L times by performing the estimation process of the above, various frequency settings may be performed based on the estimated values.
- the preliminary measurement of the magnetic resonance signal for detecting the water resonance frequency is separately performed. Therefore, the total measurement time becomes longer according to “the ratio of the number of preliminary measurements to the number of measurements in FIGS. 4 and 3”.
- the ratio of the number of preliminary measurements to the number of measurements in Figs. 4 and 3 can be reduced, thus increasing the measurement time. Is small, but the frequency of instantaneous changes in the resonance frequency is high or completely unknown, it is necessary to increase the ratio of the number of preliminary measurements to the number of measurements in Figs. 4 and 3. The measurement time greatly increases.
- the MRS pulse shown in Fig. 3 must be used.
- the excitation bands of the first high-frequency magnetic field RF1 and the second high-frequency magnetic field RF2 in Fig. 3 are narrowed, and they are included in water in the preliminary measurement. It is sufficient to excite nuclear magnetization and not excite nuclear magnetization contained in metabolites. If the nuclear magnetization contained in the metabolite is not excited during the preliminary measurement, the longitudinal relaxation of the nuclear magnetization contained in the metabolite proceeds without interruption during the preliminary measurement. If the preliminary measurement is performed during the idle time, the pulse sequence for suppressing the water signal shown in Fig. 4 and the MRS pulse sequence shown in Fig.
- a SINC waveform or a Gaussian waveform having a narrow band excitation frequency characteristic about the peak width of a water signal may be used.
- the flip angle of the first high-frequency magnetic field RF1 irradiated by the MRS pulse sequence should be set to 9 times. It may be set smaller than 0 degrees. If the nuclear magnetization contained in the metabolite is excited during pre-measurement so as not to be overturned, it takes a long time for the longitudinal magnetization of nuclear spins contained in the metabolite to fully recover after the preliminary measurement. If the pre-measurement is performed during the idle time of the repetitive measurement time of the MRS measurement, the pulse sequence for suppressing the water signal shown in Fig. 4 and the MRS pulse shown in Fig.
- the imaging poxel V to be measured in the MRS measurement is used.
- Poxel V2 different from 1 may be used as the poxel to be measured in the preliminary measurement performed to detect the water resonance frequency. (If V2 is selected in the vicinity of imaging poxel V1, the time variation of the resonance frequency in both poxels The characteristics are equivalent). If the imaging poxel V1 to be measured in the MRS measurement is not excited during the preliminary measurement, the longitudinal relaxation of the nuclear magnetization contained in the imaging Votacell V1 proceeds smoothly during the preliminary measurement.
- the pulse sequence for suppressing the water signal shown in Fig. 4 and the MRS sensor shown in Fig. 3 without extending the entire measurement time It is possible to repeat the measurement and the preliminary measurement in which the pulse sequence is continuously performed.
- MRS measurement often sets a long repetition time of about 2 seconds because the longitudinal relaxation time of metabolites is long. There is often an idle time of about 1 second when no high-frequency magnetic field or gradient magnetic field is applied or magnetic resonance signals are detected.
- the poxel V2 to be measured is selectively excited in the preliminary measurement, it is necessary to selectively excite three orthogonal slices different from the three orthogonal slices including the imaging Votacell V1 to be measured in the MRS measurement .
- Specific changes in the MRS sequence include the gradient magnetic fields for slice selection & 3 1, 0 3 2 and 0 3 3 in FIG.
- the transmission frequencies of the second high-frequency magnetic field RF2 and the third high-frequency magnetic field RF3 may be changed (selective excitation of three orthogonal slices separated by at least the width of each slice).
- a preliminary measurement for detecting the water resonance frequency must not be performed every predetermined number of times.
- the preliminary measurement may be performed only when the resonance frequency is shifted.
- the water sequence in each spectrum obtained by performing the pulse sequence for suppressing the water signal shown in Fig. 4 and the MRS pulse sequence shown in Fig. 3 continuously is measured. Monitor the change in signal peak intensity (peak area), and if the resonance frequency shifts and the water signal peak intensity (peak area) increases beyond a predetermined value, determine that the resonance frequency has shifted and perform preliminary measurement. What should I do?
- the predetermined value the absolute value of the water signal peak intensity (peak area) may be specified, or the water signal peak intensity (peak area) in the spectrum obtained in the first or previous measurement. Alternatively, a relative value may be used.
- the example in which the reception frequency when detecting the magnetic resonance signal is corrected has been described.
- the measurement can be performed.
- the same effect as in the case of correcting the received frequency can be obtained by post-processing. That is, the peak position of the residual water signal or the peak position of the metabolite signal is detected for each spectrum data before integration, and the residual water signal is detected for all the spectrum data. If the post-processing is performed after the peak position or the peak position of the metabolite signal becomes the same, and the integration processing is performed, a sufficient addition effect can be obtained. Note that the metabolite signal intensity in each spectrum is extremely small, The peak position of the metabolite signal may be detected after summing up the spectrum data for several times before and after.
- the pulse sequence shown in FIG. 3 has been described as an example of the MRS sequence. However, similar effects can be obtained with MRS sequences other than FIG.
- FIG. 11 is a diagram showing another example of the MRS pulse sequence applicable to the embodiment of the present invention.
- TR is the repetition time
- TE is the echo time
- TM is the time indicating the irradiation interval between the second high-frequency magnetic field pulse RF2 and the third high-frequency magnetic field pulse RF3.
- the first gradient magnetic field (the gradient magnetic field in the X-axis direction) for selecting the first slice (plane perpendicular to the X-axis) is called G sl and 90 ° pulse.
- the second slice (plane perpendicular to the Y-axis)
- the second gradient magnetic field for selection (the gradient magnetic field in the Y-axis direction) G s 2 and the second gradient called 90 ° pulse
- the nuclear magnetization also included in the second slice can be rotated by 90 °.
- Third gradient magnetic field for selecting the third slice (plane perpendicular to the Z-axis) after TM from RF2 irradiation Gs3 and third high-frequency magnetic field called 90 ° pulse
- RF3 Third gradient magnetic field
- the nuclear magnetization in the intersection of the first slice and the second slice rotated by RF2 which is also included in the third slice, is rotated by 90 ° again. it can.
- the magnetic resonance signal S ig whose echo time is the time after TEZ 2 from the irradiation of RF 3 Generate 1
- G s 1 ′ immediately after application of G s 1, G s 2 ′ immediately after application of G s 2, and G s 3 immediately after application of G s 3 are G sl, G s 2, and G s 3, respectively.
- This is a gradient magnetic field for rephase (phase reversal) of the phase.
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US7518362B2 (en) | 2009-04-14 |
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