WO2011037064A1 - 磁気共鳴イメージング装置および照射周波数調整方法 - Google Patents
磁気共鳴イメージング装置および照射周波数調整方法 Download PDFInfo
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/05—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves
- A61B5/055—Detecting, measuring or recording for diagnosis by means of electric currents or magnetic fields; Measuring using microwaves or radio waves involving electronic [EMR] or nuclear [NMR] magnetic resonance, e.g. magnetic resonance imaging
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/48—Other medical applications
- A61B5/4887—Locating particular structures in or on the body
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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]
- G01R33/446—Multifrequency selective RF pulses, e.g. multinuclear acquisition mode
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- 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
- G01R33/4833—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy using spatially selective excitation of the volume of interest, e.g. selecting non-orthogonal or inclined slices
- G01R33/4836—NMR imaging systems with selection of signals or spectra from particular regions of the volume, e.g. in vivo spectroscopy using spatially selective excitation of the volume of interest, e.g. selecting non-orthogonal or inclined slices using an RF pulse being spatially selective in more than one spatial dimension, e.g. a 2D pencil-beam excitation pulse
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- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
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Definitions
- the present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI) technique, and more particularly to an imaging technique based on two-dimensional selective excitation that selectively excites a region restricted in an arbitrary two-dimensional direction.
- MRI magnetic resonance imaging
- MRI uses a radio wave (hereinafter referred to as RF) and a gradient magnetic field to selectively excite an arbitrary plane having a predetermined thickness only in a one-dimensional direction.
- RF radio wave
- SS two-dimensional spatial-spatial
- the SS method can obtain a signal by exciting only the inside of a selected region constrained in the two-dimensional direction, signals from outside the region can be effectively suppressed.
- This SS method is used, for example, in a navigator echo sequence (hereinafter referred to as “navigation echo”) for tracking the movement of the diaphragm (see, for example, Non-Patent Document 2).
- Navigation echo a navigator echo sequence
- NaviEcho the vicinity of the diaphragm is excited in a cylinder shape in the body axis direction by the SS method, and the respiratory movement is monitored by detecting the temporal change of the diaphragm position in the axial direction of the region excited in the cylinder shape from the signal generated from this area.
- the RF irradiation frequency used for main imaging is usually determined based on the resonance frequency of nuclear magnetization obtained from signals collected in a scan (hereinafter referred to as pre-scan) performed prior to main imaging.
- pre-scan signals are collected from the entire photographing area of the main photographing.
- the effect of the difference ⁇ F of about 100 to several tens [Hz] on the excitation profile is greatly different between the one-dimensional slice selective excitation and the SS method.
- the slice selective gradient magnetic field strength Gs is a small Gs of 1 [mT / m] and there is a difference ⁇ F of 50 [Hz ⁇
- the slice position is shifted by about 1 [mm]. This shift amount is about one pixel of the image and has almost no effect on the image.
- the obtained excitation profile is deformed from a desired shape.
- the diameter of the excitation profile (hereinafter referred to as ⁇ ) increases, and the flip angle (hereinafter referred to as FA) decreases accordingly.
- the FA of the desired excitation profile is 90 [deg] and ⁇ is 30 [mm]
- the RF Duration (irradiation time) is 8 [ms]
- the slice selective gradient magnetic field strength Gs is 1 [mT / m].
- ⁇ F of 50 [Hz ⁇ is present, ⁇ of the obtained excitation profile is about 37 [mm], and FA is about 80 [deg] (89%).
- the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a technique capable of obtaining a high-quality image using a two-dimensional selective excitation method even when there is a static magnetic field inhomogeneity.
- the present invention measures the static magnetic field inhomogeneity in the region of particular interest in the selective excitation region excited by 2DRF, and reflects the measurement result in the imaging sequence using 2DRF.
- the resonance frequency of magnetization obtained from the measurement result is set to the irradiation frequency of 2DRF.
- a shim gradient magnetic field is applied so as to correct the magnetic field inhomogeneity obtained from the measurement result.
- a selective excitation irradiation frequency which is an irradiation frequency of a high-frequency magnetic field when executing a two-dimensional selective excitation type pulse sequence, and a region of interest in an area excited by the two-dimensional selective excitation type pulse sequence
- An irradiation frequency adjusting means for reducing a deviation of the magnetization frequency from the resonance frequency, and a control means for executing the two-dimensional selective excitation type pulse sequence using the result obtained by the irradiation frequency adjusting means.
- an irradiation frequency adjusting method for adjusting an irradiation frequency when executing a two-dimensional selective excitation type pulse sequence wherein the region of interest is an area excited by the two-dimensional selective excitation type pulse sequence
- a region of interest setting step a signal collection step of collecting an echo signal from the region of interest using a preset initial irradiation frequency, an irradiation frequency is calculated from the echo signal collected from the region of interest, and the 2
- a frequency setting step of setting the selective excitation irradiation frequency which is an irradiation frequency of a dimension selective excitation type pulse sequence, to provide an irradiation frequency adjusting method.
- a high-quality image can be obtained using the two-dimensional selective excitation method even if there is a static magnetic field inhomogeneity.
- Functional block diagram of the MRI apparatus of the first embodiment (a) is a pulse sequence diagram by the conventional excitation method, (b) is a pulse sequence diagram of the SS method of the first embodiment.
- Functional block diagram of the control unit of the first embodiment Flowchart of irradiation frequency adjustment processing of the first embodiment Illustration of UI screen of the first embodiment (a) is an explanatory diagram of a region excited by the orthogonal three-section excitation method of the first embodiment, and (b) is a pulse sequence diagram of the orthogonal sectional excitation method.
- (a) is an explanatory diagram of a region excited by the 2D orthogonal 1D method of the first embodiment
- (b) is a pulse sequence diagram of the 2D orthogonal 1D method.
- a) is an explanatory diagram of a region excited by the 2D pre-saturation method of the first embodiment
- (b) is a pulse sequence diagram of the 2D pre-saturation method.
- Flowchart of another example of irradiation frequency adjustment processing of the first embodiment (a) is a graph showing the relationship between ⁇ F and FA in the first embodiment
- (b) is a graph showing the relationship between ⁇ F and ⁇ in the first embodiment.
- Explanatory drawing of the static magnetic field map of the second embodiment Functional block diagram of the control unit of the third embodiment
- Explanatory drawing of the imaging result of the local selection area of a third embodiment (a)-(c) is explanatory drawing of the projection result to each axial direction of the resonant frequency of 3rd embodiment.
- FIG. 1 is a functional block diagram of the MRI apparatus 100 of the present embodiment.
- the MRI apparatus 100 of the present embodiment includes a magnet 102, a gradient magnetic field coil 103, a radio frequency magnetic field (RF) coil 104, an RF probe 105, a gradient magnetic field power source 106, an RF transmission unit 107, and a signal detection unit 108.
- RF radio frequency magnetic field
- the magnet 102 generates a static magnetic field in an area around the subject 101 (examination space).
- the gradient magnetic field coil 103 is composed of coils in three directions of X, Y, and Z, and each generates a gradient magnetic field in the examination space in accordance with a signal from the gradient magnetic field power supply 106.
- the RF coil 104 applies (irradiates) RF to the examination space in accordance with a signal from the RF transmission unit 107.
- the RF probe 105 detects an MR signal generated by the subject 101.
- a signal received by the RF probe 105 is detected by the signal detection unit 108, subjected to signal processing by the signal processing unit 109, and input to the control unit 110.
- the control unit 110 reconstructs an image from the input signal and displays it on the display unit 111.
- control unit 110 performs operations of the gradient magnetic field power source 106, the RF transmission unit 107, and the signal detection unit 108 in accordance with imaging parameters input from the operator via the control time chart and the operation unit 112 held in advance.
- the control time chart is generally called a pulse sequence.
- the bed 113 is for the subject to lie down.
- the MRI apparatus 100 may further include a shim coil that corrects the static magnetic field inhomogeneity in the examination space and a shim power source that supplies current to the shim coil.
- the subject of MRI imaging is the main constituent substance of the subject 102, proton.
- proton the main constituent substance of the subject 102, proton.
- FIG. 2 is a diagram for explaining the pulse sequence by the SS method of the present embodiment in comparison with the pulse sequence by the conventional excitation method.
- FIG. 2 (a) shows a pulse sequence by the conventional excitation method
- FIG. 2 (b) shows a pulse sequence by the SS method used in this embodiment.
- the conventional method an example of selectively exciting an arbitrary slice in which only the position in the z-axis direction is specified will be described.
- the SS method shows an example of selectively exciting an arbitrary columnar region in which only the shape on the xy plane is specified.
- the shape specified on the xy plane is a circle.
- RF, Gx, Gy, and Gz are the application of a high-frequency magnetic field (RF) pulse, a gradient magnetic field in the x-axis direction, a gradient magnetic field in the y-axis direction, and a gradient magnetic field in the z direction, respectively. It is a timing chart.
- a constant slice selective gradient magnetic field (Gz) 202 is applied in the z-axis direction when RF 201 is applied.
- Gz constant slice selective gradient magnetic field
- RF (2DRF) 211 is applied together with an oscillating gradient magnetic field (Gx) 212 in the x-axis direction and an oscillating gradient magnetic field (Gy) 213 in the y-axis direction.
- Gx oscillating gradient magnetic field
- Gy oscillating gradient magnetic field
- ⁇ ⁇ ⁇ ⁇ Perform Fourier transform on the echo signal (data) placed in k-space to obtain an image.
- values such as 128, 256, and 512 are usually selected for one image.
- values such as 128, 256, 512, and 1024 are selected.
- irradiation frequency adjustment processing the RF irradiation frequency applied when collecting the echo signal in the irradiation frequency adjustment processing is determined by a conventional method, that is, the irradiation frequency determined based on the echo signal obtained from the entire imaging region.
- FIG. 3 is a functional block diagram of the control unit 110 of the MRI apparatus 100 of the present embodiment.
- the irradiation frequency adjustment process is realized by the control unit 110.
- the control unit 110 includes an excitation region setting unit 320, a signal collection unit 330, an irradiation frequency setting unit 340, and a UI control unit 350, as shown in FIG.
- the control unit 110 of the present embodiment includes a CPU, a memory, and a storage device, and the above functions are realized by loading a program stored in the storage device into the memory and executing the program.
- FIG. 4 is a processing flow of 2DRF irradiation frequency adjustment processing of the present embodiment. Prior to this irradiation frequency adjustment processing, the entire irradiation frequency F0 is determined from the signal of the entire imaging region by a conventional method.
- the UI control unit 350 displays a UI screen on the display unit 111, and receives input of a two-dimensional selection region excited by 2DRF and a local selection region that is a region of particular interest in the two-dimensional selection region (step S1101). ).
- the UI control unit 350 receives input of the two-dimensional excitation selection region and the local selection region via the UI screen, the UI control unit 350 notifies the excitation region setting unit 320 of the region.
- the excitation area setting unit 320 sets imaging parameters so as to excite the received local selection area by a predetermined imaging sequence (step S1102). At this time, the overall irradiation frequency F0 is used as the RF irradiation frequency.
- the signal collection unit 330 executes the above imaging sequence with the imaging parameters set in step S1102, and obtains an echo signal from the local selection region (step S1103). Then, the irradiation frequency setting unit 340 calculates the irradiation frequency F1 based on the echo signal from the local selection region (step S1104), and uses the calculated irradiation frequency F1 in the RF (2DRF) irradiation frequency (SS). (Irradiation frequency) Fss is set (step S1105). Details of the processes in steps S1103 and S1104 will be described later.
- the SS irradiation frequency Fss is determined according to the above procedure, and imaging by the SS method is performed. For example, when the SS method is used for main imaging, the control unit 110 sets the SS irradiation frequency Fss as the RF irradiation frequency in the main imaging, and performs imaging. In addition, when the SS method is used for pre-scanning and then main imaging is performed, the control unit 110 performs imaging using the SS irradiation frequency Fss during pre-scanning and the whole imaging frequency F0 during main imaging.
- FIG. 5 is an example of a UI screen 400 that is generated by the UI control unit 350 and displayed on the display unit 111 when these areas are selected.
- the UI screen 400 displays a positioning image 410 acquired in advance.
- the operator sets a two-dimensional excitation selection area 401 and a local selection area 402.
- the two-dimensional excitation selection region 401 is a cylinder type region excited by the SS method
- the local selection region 402 is a region of particular interest in the two-dimensional excitation selection region.
- the cylinder shape is the same as that of the two-dimensional excitation selection region 401 and has the same cross-sectional radius.
- the two-dimensional excitation selection region 401 and the local selection region 402 may be configured to accept any input first.
- the two-dimensional excitation selection region 401 can be set at an arbitrary position and angle as indicated by an arrow in the figure, and then the local selection region 402 is indicated by an arrow in the figure.
- the cylinder is slidable in the direction of the cylinder axis of the cylinder within an area along the cylinder set as the two-dimensional excitation selection area 401.
- the two-dimensional excitation selection region 401 is received as a cylinder coaxial with the set local selection region 402.
- the two-dimensional excitation selection region 401 and the local selection region 402 are both cylinders (cylindrical shapes) and their cross-sectional shapes are circular, but this is not restrictive. These cross-sectional shapes can be arbitrarily set.
- FIG. 6 (a) is a diagram for explaining a region excited by orthogonal three-section excitation.
- FIG. 6 (b) is a pulse sequence diagram of the orthogonal three-section excitation method.
- each cross section is shown as transparent for the sake of explanation.
- a rectangular parallelepiped region (intersection region) 524 that intersects three orthogonal sections is excited.
- the intersecting region 524 is excited so that the cylindrical local selection region 402 is inscribed.
- the first gradient magnetic field 512 is applied in the x-axis direction (Gx) together with the 90-degree pulse 511, and a predetermined cross section (first One cross-section) 521 is excited.
- the magnetization of the region where the cross section (second cross section) 522 and the first cross section 521 intersect is excited.
- TE / 2 hours after the application of the first 180-degree pulse 513 a third gradient magnetic field 516 is applied in the z-axis direction (Gz) together with the second 180-degree pulse 515, and the z-axis direction specified thereby is applied.
- the nuclear magnetization of the region 524 where the cross section (third cross section) 523, the first cross section 521, and the second cross section 522 intersect is excited. Then, the generated echo signal 517 is collected at a timing of TE / 4 hours after the application of the second 180-degree pulse 515.
- the order of application axes to which the gradient magnetic field is applied does not matter.
- the signal collecting unit 330 executes the orthogonal cross section excitation method and collects echo signals without encoding.
- the collected signal is Fourier transformed in the time direction.
- a histogram of resonance frequencies in the local selection region 402 is obtained.
- the irradiation frequency setting unit 340 scans the obtained histogram, and specifies a frequency that is the center of gravity of a band of ⁇ several tens [Hz ⁇ around the maximum peak. Then, the frequency is calculated as the irradiation frequency and set to the SS irradiation frequency Fss.
- the frequency for obtaining the maximum peak may be set as the SS irradiation frequency Fss.
- the center of gravity of the entire band of the histogram may be set as the SS irradiation frequency Fss.
- the irradiation frequency used for 2DRF is determined using an echo signal obtained from a region that substantially matches the region actually excited by 2DRF. Therefore, since the irradiation frequency used for 2DRF substantially matches the resonance frequency of nuclear magnetization in the region, deformation of the excitation profile of 2DRF due to the difference ⁇ F between them can be suppressed.
- selective excitation can be performed by RF having a desired excitation profile, so that a desired region can be excited with high accuracy,
- the purpose of selective excitation can be achieved with high accuracy. Therefore, a high quality image can be obtained.
- the orthogonal three-section excitation method is used as an imaging sequence for collecting signals from the local selection region 402 by the signal acquisition unit 320 in step S1103 . It is not limited to this.
- two-dimensional selective excitation may be used. An example using two-dimensional selective excitation will be described below.
- FIG. 7 is a diagram for explaining the excitation region and imaging sequence in the case of combining two-dimensional selective excitation and excitation of one cross section orthogonal to the axis of the cylindrical region excited by this two-dimensional selective excitation (2D orthogonal 1D method).
- FIG. FIG. 7 (a) is a diagram for explaining a region excited by the 2D orthogonal 1D method
- FIG. 7 (b) is a pulse sequence diagram of the 2D orthogonal 1D method.
- the cross section and the cylinder region are shown as transparent for explanation.
- a 90-degree pulse (2DRF) 531 and a first oscillating gradient magnetic field 532 in the x-axis direction (Gx) and a second oscillating gradient magnetic field 533 in the y-axis direction (Gy) are applied,
- the cylindrical region 541 is excited.
- a gradient magnetic field 535 is applied along with the 180 degree pulse 534 in the z-axis direction (Gz), and the nuclear magnetization of the crossing region 543 between the cross section 542 and the cylindrical region 541 Returns the phase of.
- the generated echo signal 536 is collected at a timing of TE / 2 hours after the application of the 180-degree pulse 534.
- the processing for the obtained echo signal and the calculation method of the SS irradiation frequency Fss are the same as in the above embodiment.
- the imaging parameters are set so that the intersection area 543 matches the local selection area 402.
- the irradiation frequency used for the 90-degree pulse 531 in the imaging sequence is the entire irradiation frequency F0 determined in advance by a conventional method.
- FIG. 8A is a diagram for explaining a region excited by the 2D pre-saturation method, and is a diagram seen from the x-axis direction.
- FIG. 8 (b) is a pulse sequence diagram of the 2D pre-saturation method.
- the first gradient magnetic field 552 is applied in the z-axis direction (Gz) together with the first pre-saturation pulse 551, and the magnetization in the first region 562 is lost.
- the second gradient magnetic field 554 is applied in the z-axis direction (Gz) together with the second pre-saturation pulse 553, and the magnetization in the second region 563 is lost.
- the magnetization in either the first region 562 or the second region 563 may be lost first.
- a first oscillating gradient magnetic field 556 in the x-axis direction (Gx) and a second oscillating gradient magnetic field 557 in the y-axis direction (Gy) are applied together with the 90-degree pulse (2DRF) 555, and the inside of the cylindrical region 561 A region (non-intersecting region) 567 outside the first region 562 and the second region 563 is excited.
- echo signals generated at the timing after TE time from the application of the 90-degree pulse (2DRF) 555 are collected.
- the processing for the obtained echo signal and the calculation method of the SS irradiation frequency Fss are the same as in the above embodiment.
- the imaging parameters are set so that the non-intersecting region 567 matches the local selection region 402. Further, the irradiation frequency used for the 90-degree pulse 554 in the imaging sequence is the entire irradiation frequency F0 determined in advance by a conventional method.
- an irradiation frequency registered in advance may be used instead of the overall irradiation frequency F0 obtained by measurement in advance. This is because the static magnetic field changes somewhat when the subject enters, but the amount of change is slight, so that the value obtained by multiplying the magnetic rotation ratio ⁇ and the static magnetic field strength B0 is ⁇ F Is the same level. Further, the frequency used in the imaging sequence of the 2D orthogonal 1D method or the 2D pre-saturation method executed for calculating the SS irradiation frequency Fss is shifted from the resonance frequency in the local selection region 402 by ⁇ F. Therefore, the excitation profile of 2DRF here is different from the desired one.
- the obtained irradiation frequency F1 is closer to the actual resonance frequency in the local selection region 402 than the entire irradiation frequency F0. Become.
- step S1103 when the 2D orthogonal 1D method and the 2D presaturation method are used for signal collection from the local selection region 402 in step S1103, feedback control may be performed.
- the processing flow at this time is shown in FIG.
- the processing up to step S1104 is the same as in FIG.
- the irradiation frequency can be set with higher accuracy than in the above embodiment.
- the threshold value dF is determined by the 2D RF excitation profile diameter ⁇ required for imaging and the excitation profile flip angle FA.
- the control unit 110 may be configured to be held in advance in the storage device of the MRI apparatus 100, or may be configured such that the operator determines and inputs. Further, the irradiation frequency setting unit 340 may be configured to calculate from the accuracy of the 2DRF diameter ⁇ and the flip angle FA set in advance by the operator.
- FIG. 10 (a) is a graph showing the relationship between the difference ⁇ F and the flip angle FA
- FIG. 10 (b) is a graph showing the relationship between the difference ⁇ F and the diameter ⁇ .
- FIGS. 10 (a) and 10 (b) are graphs when Duration is 8 [ms], the diameter ⁇ is 30 [mm], and the flip angle FA is 90 [deg]. Note that the shapes of these graphs vary depending on the 2DRF waveform, Duration (irradiation time), and k-space trajectory (shape of the oscillating gradient magnetic field).
- the accuracy (allowable range) of the set flip angle FA is defined as dFA, and a band dF FA corresponding to dFA is calculated from the maximum flip angle FA.
- the accuracy (allowable range) of the set diameter ⁇ is d ⁇ , and the band dF ⁇ corresponding to d ⁇ is calculated from the minimum diameter ⁇ .
- the smaller of dF FA and dF ⁇ is set as the threshold value dF.
- the feedback control may be defined by the number of times. That is, the repetition number N (N is a natural number) is registered in advance. Then, the processing of step S1103, step S1104, and step S1107 in FIG. 9 by the signal collection unit 330 and the irradiation frequency setting unit 340 is executed N times that is a predetermined number of times.
- the irradiation frequency setting unit 340 calculates ⁇ F each time the irradiation frequency F1 is calculated, and whether it is within the threshold or the number of times F1 is calculated reaches a predetermined number N times. It is determined whether or not, and when either is achieved, the process is terminated.
- a warning indicating that the accuracy has not been reached may be displayed on the display unit 111.
- the warning is generated from warning screen generation data held in advance by the irradiation frequency setting unit 340 and displayed on the UI control unit 350. Then, in this state, it may be configured to accept selection of whether to start shooting.
- the irradiation frequency setting unit 340 When receiving an instruction to start via the UI control unit 350 from the operator, the irradiation frequency setting unit 340 adds the minimum value ⁇ Fmin of ⁇ F to F0 (F0 + ⁇ Fmin) as the irradiation frequency Fss of 2DRF. On the other hand, when an instruction not to start is received, the feedback control is repeated.
- the SS irradiation frequency Fss is determined from the histogram obtained by collecting the signals from the region substantially matching the local selection region 402 without encoding and performing the Fourier transform in the time direction.
- volume data of a region including the local selection region 402 is acquired, the static magnetic field distribution is obtained, and the SS irradiation frequency Fss is determined from the average value of the static magnetic field strength.
- an imaging sequence capable of reconstructing a two-dimensional image is used as an imaging sequence for collecting signals from the local selection region 402.
- the overall irradiation frequency F0 is used as the irradiation frequency.
- the excitation area setting unit 320 sets imaging parameters so as to excite a predetermined slice including the local selection area.
- the signal collection unit 330 collects echo signals from the set slice including the local selection region 402. Then, the collected echo signal is subjected to a two-dimensional Fourier transform to obtain volume data of a region including the local selection region 402.
- This volume data is a complex image.
- the irradiation frequency setting unit 340 uses the phase component of the complex image to generate a static magnetic field map 600 as shown in FIG. Then, in this static magnetic field map 600, the average value of the static magnetic field strength of the region 602 corresponding to the local selection unit 402 is obtained, and the corresponding frequency F2 is defined as Fss. It should be noted that it is desirable that the area for acquiring the volume data includes the local selection unit 402 and selects an area perpendicular to the axis of the two-dimensional excitation selection area 401.
- control unit 110 performs imaging for irradiating 2DRF using the irradiation frequency Fss calculated in the above procedure.
- the present embodiment it is possible to obtain the irradiation frequency Fss corresponding to the static magnetic field strength of the region excited by 2DRF. Therefore, as in the first embodiment, since the irradiation frequency used for 2DRF substantially matches the resonance frequency of nuclear magnetization in the region, deformation of the 2DRF excitation profile due to the difference ⁇ F between them can be suppressed.
- the volume data to be acquired may be one that can reconstruct an image with a low spatial resolution.
- the result of shim imaging may be used instead of the static magnetic field map.
- the shim imaging is an imaging for determining a current value to be passed through a shim gradient magnetic field coil for eliminating the static magnetic field inhomogeneity, and is performed before the main imaging. Similar to the volume data, the shim image obtained by shim imaging reflects the static magnetic field distribution. Using this shim imaging result eliminates the need for pre-scanning to obtain a static magnetic field distribution, shortens the entire imaging time, and improves efficiency.
- the MRI apparatus 100 includes a shim coil and a shim power source.
- the MRI apparatus of this embodiment basically has the same configuration as that of the second embodiment. However, a shim coil and a shim power source are provided.
- the irradiation frequency Fss that matches the resonance frequency of nuclear magnetization in the local selection region 402 is calculated based on the obtained static magnetic field distribution (volume data, shim image). However, in this embodiment, based on these, the static magnetic field is adjusted so that the resonance frequency of the nuclear magnetization in the region matches the total irradiation frequency F0 obtained in advance.
- FIG. 12 is a functional block diagram of the control unit 110 of the present embodiment.
- the present embodiment includes a magnetic field adjustment unit 360 that adjusts the magnetic field in the region instead of the irradiation frequency setting unit 340.
- the magnetic field adjustment processing by the magnetic field adjustment unit 360 which is different from the second embodiment, will be described in the present embodiment.
- Other configurations and other processes are the same as those in the second embodiment.
- the magnetic field adjustment unit 360 of the present embodiment improves the non-uniformity of the static magnetic field in the region to match the resonance frequency of nuclear magnetization in the local selection region 402 with the overall resonance frequency F0.
- the shim current value Is for correcting the static magnetic field inhomogeneity in the local selection region 402 is calculated by the conventional method from the volume data or shim image obtained by the signal collecting unit 330 by the same method as in the second embodiment. To do. Note that the shim current is calculated only for the axis whose current value can be switched during measurement (scanning). Then, the magnetic field adjustment unit 360 controls the shim power source so that the applied current value to the shim coil in the axial direction becomes the calculated Is only during the application of 2DRF.
- a shim current value Is is calculated with the static magnetic field strength B1 in the local selection region 402 as the static magnetic field strength B0 that realizes the overall frequency F0.
- the local selection region 402 has a cylinder shape and the axial direction thereof coincides with the z-axis direction of the examination space will be described as an example.
- the shim coil can correct the static magnetic field strength component to the primary component in each axial direction. That is, a current value that matches the overall irradiation frequency F0 and the resonance frequency F calculated from the zeroth-order component of the static magnetic field strength B1 is calculated.
- FIG. 13 shows an imaging result of the local selection region 402.
- a plurality of measurement points 801 are set on the circumferences of the upper surface and the lower surface of the local selection region 402 having a cylinder shape with a diameter of ⁇ and a thickness of D. It should be noted that a plurality of measurement points 801 are desirably arranged on the axis of this cylinder.
- the resonance frequency F calculated from the static magnetic field strength B1 at each measurement point 801 is projected (Fx, Fy, Fz) in the x-axis, y-axis, and z-axis directions, respectively, and the plotted results are shown in FIG. 14A shows the result of projection in the x-axis direction, FIG. 14B shows the result of projection in the y-axis direction, and FIG. 14C shows the result of projection in the z-axis direction.
- the horizontal axis represents the position in each axial direction, and the vertical axis represents the resonance frequency F calculated from the static magnetic field strength B1.
- Each axial projection result is approximated by a linear expression.
- the shim current value is determined so that these all pass through F0 and have a slope of 0.
- the shim current value Is calculated by the magnetic field adjustment unit 360 is applied only during the application of 2DRF by the above-described method, and imaging is performed. In other cases, the shim current value Is is set to 0 or a shim current value that improves the static magnetic field inhomogeneity of the entire imaging region is applied to perform imaging.
- a desired excitation profile can be obtained for 2DRF even when the static magnetic field is not uniform in the local selection region 402.
- the correction order of the static magnetic field inhomogeneity by the shim coil is not limited to the above.
- the projection results of the static magnetic field strength B1 at each measurement point 801 in each axial direction can be approximated within a range of orders in which the shim coil in the axial direction can correct the static magnetic field strength.
- the method of calculating the shim current value that achieves the static magnetic field uniformity is not limited to the above method. Various general techniques can be used.
- the static magnetic field inhomogeneity may be corrected using the gradient magnetic field by the gradient magnetic field coil 103. That is, control is performed so that the same amount of current as the shim current value Is calculated by the above method is supplied to each gradient coil 103 as an offset only while 2DRF is being applied from the gradient magnetic field power source 106.
- the static magnetic field nonuniformity of the local selection region 402 can be corrected during 2DRF application even when the MRI apparatus 100 does not include a shim coil. Therefore, 2DRF can be executed at an irradiation frequency with no difference ⁇ F, and deformation of the excitation profile of 2DRF can be suppressed.
- the RF irradiation gain and the gradient magnetic field strength may be adjusted by using the difference ⁇ F1 still remaining after correcting the static magnetic field inhomogeneity by the shim magnetic field by the shim coil or the gradient magnetic field by the gradient magnetic field coil.
- the magnetic field adjustment unit 360 generates the static magnetic field map 600 from the volume data by the same method as in the second embodiment. Then, the magnetic field generated by the obtained shim current value Is or the gradient magnetic field correction current (referred to as a gradient magnetic field for correction) is added to the static magnetic field map 600.
- FIG. 10 A graph showing the relationship between ⁇ F1 and the flip angle FA and ⁇ F1 and the diameter ⁇ is shown in FIG. 10 (a) when the duty is 8 [ms], the diameter ⁇ is 30 [mm], and the flip angle FA is 90 [deg]. ) And (b) ( ⁇ F is ⁇ F1).
- the magnetic field adjustment unit 360 calculates the RF irradiation gain value so that the product of the RF irradiation gain and the flip angle becomes constant. That is, assuming that the RF gain before correction is RFGain0 and the flip angle before correction is FA0, the corrected RFGain1 is obtained by the following equation (1).
- the magnetic field adjustment unit 360 calculates the gradient magnetic field strength value so that the ratio between the gradient magnetic field strength and the diameter is constant. That is, assuming that the gradient magnetic field strength before correction is GC0 and the diameter before correction is ⁇ 0, the gradient magnetic field strength GC1 after correction is obtained by the following equation (2).
- the control unit 110 controls to execute the imaging sequence using the RF irradiation gain value RFGain1 and the gradient magnetic field strength GC1 obtained by the magnetic field adjustment unit 360.
- the flip angle FA and the diameter ⁇ of the 2DRF can be more accurately controlled, and can be brought close to a desired excitation profile.
- FIG. 15 shows a functional block diagram of the control unit 110 of the present embodiment.
- a functional block diagram based on the control unit 110 of the first embodiment is shown.
- the other structure of the MRI apparatus of this embodiment is the same as that of any of said each embodiment fundamentally.
- different configurations will be mainly described based on the first embodiment.
- FIG. 16 is an example of an imaging sequence executed by the automatic setting unit 370 to identify the local selection region.
- FIG. 17A shows a region 703 in which echo signals are collected in the imaging sequence shown in FIG.
- 701 is a two-dimensional selection region.
- the axial direction of the cylinder-shaped two-dimensional selection region 701 is the z-axis direction.
- 2DRF711 is applied together with the oscillating gradient magnetic fields 712 and 713 as a frequency for exciting only the water signal, and only the water band in the two-dimensional selection region 701 is excited.
- the echo signal 715 is collected while applying the reading gradient magnetic field 714 in the z-axis direction (Gz). Then, the collected echo signal 715 is Fourier transformed.
- the result 704 of the Fourier transform is shown in FIG.
- the result 704 becomes a projection image 704 in the z-axis direction of the two-dimensional selection region 701.
- a location 702 that becomes a high signal on the projection image 704 is a local selection region 402 that is an intersection region between the two-dimensional selection region 701 and the blood vessel.
- the automatic setting unit 370 of the present embodiment uses this, scans the Fourier transform result 704, and discriminates an area that is equal to or greater than a predetermined threshold as the local selection area 402. That is, when the two-dimensional selection area 401 is received via the UI screen 400, the automatic setting unit 370 executes the above-described process and identifies the local selection area 402. Note that the irradiation frequency adjustment processing after the local selection region 402 is specified is the same as in any of the above embodiments.
- the setting of the local selection region is automated in the irradiation frequency adjustment process. Therefore, in addition to the effect obtained by any of the above embodiments, there is an effect that it is possible to further reduce the labor of the operator. Further, since the local selection area is determined from the measurement result, the local selection area can be set with stable accuracy without depending on the skill of the operator.
- the automatic setting unit 370 is added to the configuration of the first embodiment has been described as an example, but the automatic setting unit 370 may be added to the configuration of any of the above embodiments.
- the blood vessel position that is, the local selection region 402 is specified as a position indicating a signal equal to or higher than the threshold value, but the specifying method is not limited to this.
- it may be determined on the profile 721 of the projection image 704 shown in FIG.
- the position 722 showing the maximum signal intensity on the profile 721 is specified as the center position of the local selection region 402.
- the width of the local selection region 402 in the z-axis direction is previously stored in the storage device and used.
- the regions 724 and 725 indicating the signal intensity exceeding the predetermined threshold 726 may be extracted, and the region having the maximum width may be used as the local selection region 402.
- the center position of the region having the maximum width may be specified as the center position of the local selection region 402, and the width in the z-axis direction may be stored in advance in the storage device.
- the local selection region 402 determined from the projection image 704 by any of the above methods may be displayed on the display unit 111 so that adjustment from the operator can be accepted via the UI control unit 350. .
- region used as the object which adjusts irradiation frequency with higher precision can be determined.
- the frequency of 2DRF is set to a band including both water and fat
- TE is set to “Out of Phase” in which the fat signal is suppressed. May be configured to collect.
- the RF Duration can be shortened compared to the case where the band includes only water. Accordingly, with this configuration, the imaging time for specifying the local selection region 402 can be shortened.
- the displayed fat may be rendered with a high signal depending on the shooting conditions.
- a projection image is generated, the above processing is performed, and the local selection region 402 is specified.
- the profile 731 of the projection image 704 obtained by the above method is scanned from both ends 737 toward the center, and a region 733 having a signal value equal to or smaller than a predetermined threshold 732 is determined as a noise region.
- a region 735 having a predetermined width in the direction opposite to the end portion 737 from the edge portion 734 on the side opposite to the end portion 737 of the region 733 determined to be a noise region is determined as the fat of the epidermis.
- the signal of the determined area 735 is removed.
- the width to be used for determining the region 735 may be stored in advance or may be determined on the profile 731.
- the length from the edge portion 734 to the end portion 737 is scanned in the opposite direction and the signal value becomes equal to or less than a predetermined threshold value 736.
- the local selection region 402 may be specified from the phase profile of the projection image 704. That is, the phase of the part with the flow changes greatly as compared with other parts due to this flow.
- the blood vessel position that is, the local selection region 402 can be specified.
- a differential or difference of the phase profile result is taken to detect the position where the extreme value is obtained. In this case, since the blood vessel position is depicted with a high signal, it is not affected by fat.
- the resonance frequency of nuclear magnetization and the irradiation frequency of 2DRF can be matched in the region of interest of the two-dimensional selective excitation region. Therefore, the deviation from the desired one of the diameter ⁇ of the excitation profile of the 2DRF and the flip angle FA due to the non-uniformity of the static magnetic field is reduced. That is, the excitation profile of 2DRF is stabilized.
- the irradiation frequency or the shim current value is switched between the prepulse sequence and the main imaging. Therefore, the optimization of the excitation profile of 2DRF can be realized without affecting the main imaging.
- the excitation size (diameter ⁇ ) of 2DRF and the flip angle FA are stable, so the excitation profile is stable and a high-quality image is obtained. be able to.
- MRI device 101 subject, 102 magnet, 103 gradient coil, 104 RF coil, 105 RF probe, 106 gradient magnetic field power supply, 107 RF transmitter, 108 signal detector, 109 signal processor, 110 controller, 111 display Unit, 112 operation unit, 113 bed, 201 RF, 202 gradient magnetic field, 211 RF, 212 vibration gradient magnetic field, 213 vibration gradient magnetic field, 320 excitation region setting unit, 330 signal collection unit, 340 irradiation frequency setting unit, 350 UI control unit , 360 magnetic field adjustment part, 370 automatic setting part, 400 UI screen, 401 two-dimensional selection area, 402 local selection area, 511 90 degree pulse, 512 first gradient magnetic field, 513 first 180 degree pulse, 514 second Gradient field, 515 Second 180 degree pulse, 516 Third gradient field, 517 Echo signal, 521 First section, 522 Second section, 523 Third section, 524 Crossing area, 531 90 degree pulse, 532 First oscillating gradient magnetic field, 533 Second oscillating gradient magnetic field Field Field
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Abstract
Description
以下、本発明を適用する第一の実施形態について説明する。以下、本発明の実施形態を説明するための全図において、同一機能を有するものは同一符号を付し、その繰り返しの説明は省略する。
次に、本発明を適用する第二の実施形態について説明する。本実施形態のMRI装置100の構成は、基本的に第一の実施形態と同様である。ただし、制御部110が実現する照射周波数調整処理が異なる。
次に、本発明を適用する第三の実施形態を説明する。本実施形態のMRI装置は、基本的に第二の実施形態と同様の構成を有する。ただし、シムコイルおよびシム電源を備える。また、第二の実施形態では、得られた静磁場分布(ボリュームデータ、シム画像)に基づき、局所選択領域402における核磁化の共振周波数に合致する照射周波数Fssを算出する。しかし、本実施形態では、これらに基づき、当該領域の核磁化の共振周波数が予め求めた全体照射周波数F0に合致するよう静磁場を調整する。
また、磁場調整部360は、傾斜磁場強度と直径との比が一定となるよう、傾斜磁場強度値を算出する。すなわち、補正前の傾斜磁場強度をGC0、補正前の直径をΦ0とすると、補正後の傾斜磁場強度GC1は、以下の式(2)で得られる。
制御部110は、2DRF印加時は、磁場調整部360で得たRF照射ゲイン値RFGain1、傾斜磁場強度GC1を用いて、撮影シーケンスを実行するよう制御する。これにより、2DRFのフリップアングルFAおよび直径Φをさらに正確に制御でき、所望の励起プロファイルに近づけることができる。
次に、本発明を適用する第四の実施形態を説明する。本実施形態では、局所選択領域の設定をさらに自動化する。このため、本実施形態の制御部110は、上記各実施形態のいずれかの制御部110が備える機能に、局所選択領域を自動設定する自動設定部370をさらに備える。図15に本実施形態の制御部110の機能ブロック図を示す。ここでは、第一の実施形態の制御部110を基礎とした機能ブロック図を示す。なお、本実施形態のMRI装置の他の構成は、基本的に上記各実施形態のいずれかと同様である。以下、第一の実施形態を基礎として、異なる構成に主眼をおいて説明する。
Claims (15)
- 静磁場中に載置される被検体に所定のパルスシーケンスに従って高周波磁場および傾斜磁場を印加することにより発生するエコー信号を収集し、当該エコー信号から画像を再構成する磁気共鳴イメージング装置であって、
2次元選択励起型のパルスシーケンスを実行する際の高周波磁場の照射周波数である選択励起照射周波数と、前記2次元選択励起型のパルスシーケンスで励起する領域内の着目領域内の磁化の共振周波数とのずれを低減する照射周波数調整手段と、
前記照射周波数調製手段で得た結果を用い、前記2次元選択励起型のパルスシーケンスを実行する制御手段と、を備えること
を特徴とする磁気共鳴イメージング装置。 - 請求項1記載の磁気共鳴イメージング装置であって、
前記照射周波数調整手段は、
前記着目領域を設定する領域設定手段と、
予め設定される初期照射周波数を用い、前記着目領域からエコー信号を収集する信号収集手段と、
前記信号収集手段が前記エコー信号を収集すると、当該エコー信号から照射周波数を算出し、算出した当該照射周波数を前記選択励起照射周波数に設定する周波数設定手段と、を備えること
を特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置であって、
前記初期照射周波数は、本撮影の撮影領域のプリスキャン結果から算出すること
を特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置であって、
前記周波数設定手段は、前記エコー信号を時間方向にフーリエ変換することにより得たヒストグラム上の最大ピークを示す周波数を前記照射周波数として算出すること
を特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置であって、
前記信号収集手段は、前記着目領域の少なくとも一部を含む直方体領域を交差領域とする直交3断面励起型のパルスシーケンスに従って、当該交差領域のエコー信号をエンコード無しで収集すること
を特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置であって、
前記信号収集手段は、前記着目領域が内接する柱状領域を励起領域とする2次元選択励起と、当該柱状領域に直交する断面であって前記着目領域が内接する断面を励起領域とする1次元選択励起とを組み合わせたパルスシーケンスに従って、前記着目領域のエコー信号をエンコード無しで収集すること
を特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置であって、
前記信号収集手段は、前記柱状領域に直交する断面であって前記着目領域が内接する断面以外の領域からの信号を抑制する抑制パルスと、当該着目領域が内接する柱状領域を励起領域とする2次元選択励起と、を組み合わせたパルスシーケンスに従って、前記着目領域のエコー信号をエンコード無しで収集すること
を特徴とする磁気共鳴イメージング装置。 - 請求項6記載の磁気共鳴イメージング装置であって、
前記周波数設定手段は、前記照射周波数算出後、前記選択励起照射周波数と前記初期照射周波数との差の絶対値を算出し、当該絶対値が予め定めた閾値との大小を比較する比較手段を備え、
前記絶対値が前記閾値より大きいとの比較結果を得た場合は、当該照射周波数を前記初期照射周波数として前記信号収集手段に前記着目領域からエコー信号を収集させること
を特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置であって、
前記信号収集手段は、前記着目領域を含む領域からのボリュームデータを取得し、
前記周波数設定手段は、前記ボリュームデータの静磁場分布の平均値を前記照射周波数として算出すること
を特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置であって、
撮影領域の静磁場不均一を補正するシム傾斜磁場発生手段をさらに備え、
前記信号収集手段は、前記着目領域を含む領域のシム画像を取得し、
前記周波数設定手段は、前記シム画像により得られる静磁場分布の平均値を前記照射周波数と算出すること
を特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置であって、
前記照射周波数調整手段は、2次元選択励起型以外のパルスシーケンスの高周波磁場に、前記初期照射周波数を設定すること
を特徴とする磁気共鳴イメージング装置。 - 請求項1記載の磁気共鳴イメージング装置であって、
前記着目領域を含む領域のシム画像を取得するシム画像取得手段と、
撮影領域の静磁場不均一を補正するシム傾斜磁場発生手段と、をさらに備え、
前記照射周波数調整手段は、2次元選択励起型のパルスシーケンスの前記高周波磁場に前記初期照射周波数を設定するとともに、当該2次元選択励起型のパルスシーケンス実行時は、前記シム画像により得られる静磁場分布に従って、前記着目領域内の静磁場分布を均一にするよう前記シム傾斜磁場発生手段を制御すること
を特徴とする磁気共鳴イメージング装置。 - 請求項11記載の磁気共鳴イメージング装置であって、
前記照射周波数調整手段は、
前記着目領域内の共振周波数を前記初期照射周波数に合致させた後、前記高周波磁場のゲインと前記傾斜磁場強度とを調整する調整手段を備えること
を特徴とする磁気共鳴イメージング装置。 - 請求項2記載の磁気共鳴イメージング装置であって、
前記領域設定手段は、前記2次元選択励起型のパルスシーケンスに従って操作者から指定される2次元選択領域からエコー信号を収集し、当該エコー信号をフーリエ変換して得たプロジェクション像に基づいて前記着目領域を決定し、設定すること
を特徴とする磁気共鳴イメージング装置。 - 磁気共鳴イメージング装置において、2次元選択励起型パルスシーケンスを実行する際の照射周波数を調整する照射周波数調整方法であって、
前記2次元選択励起型のパルスシーケンスで励起する領域内の着目領域を設定する着目領域設定ステップと、
予め設定される初期照射周波数を用い、前記着目領域からエコー信号を収集する信号収集ステップと、
前記着目領域から収集したエコー信号から照射周波数を算出し、算出した当該照射周波数を、前記2次元選択励起型のパルスシーケンスの照射周波数である選択励起照射周波数に設定する周波数設定ステップと、を備えること
を特徴とする照射周波数調整方法。
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US (1) | US9615768B2 (ja) |
JP (1) | JP5823865B2 (ja) |
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Cited By (2)
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JP2013192957A (ja) * | 2012-03-20 | 2013-09-30 | Siemens Ag | 磁気共鳴システム駆動制御シーケンスを求める方法、磁気共鳴システムを動作させる方法、磁気共鳴システムおよびコンピュータプログラム |
KR101751404B1 (ko) | 2016-03-17 | 2017-06-28 | 삼성전자 주식회사 | 자기장 모니터링 프로브, 이를 포함하는 자기 공명 영상 장치 및 그 제어방법 |
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JP6162131B2 (ja) * | 2012-09-20 | 2017-07-12 | 株式会社日立製作所 | 磁気共鳴イメージング装置および磁気共鳴イメージング方法 |
KR101967240B1 (ko) * | 2012-10-10 | 2019-04-09 | 삼성전자주식회사 | 피검체의 움직임을 보정하여 mri 영상을 획득하는 방법 및 mri 장치 |
DE102016225705A1 (de) * | 2016-12-21 | 2018-06-21 | Siemens Healthcare Gmbh | Erzeugung korrigierter Magnetresonanz-Messdaten |
DE102020202830A1 (de) * | 2020-03-05 | 2021-09-09 | Siemens Healthcare Gmbh | Magnetresonanztomograph und Verfahren zum Betrieb mit dynamischer B0-Kompensation |
CN113848519A (zh) * | 2021-09-23 | 2021-12-28 | 合肥工业大学 | 一种用于固态单自旋磁共振频率实时测量的方法 |
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JP2013192957A (ja) * | 2012-03-20 | 2013-09-30 | Siemens Ag | 磁気共鳴システム駆動制御シーケンスを求める方法、磁気共鳴システムを動作させる方法、磁気共鳴システムおよびコンピュータプログラム |
KR101751404B1 (ko) | 2016-03-17 | 2017-06-28 | 삼성전자 주식회사 | 자기장 모니터링 프로브, 이를 포함하는 자기 공명 영상 장치 및 그 제어방법 |
US10247794B2 (en) | 2016-03-17 | 2019-04-02 | Samsung Electronics Co., Ltd. | Magnetic field monitoring probe, magnetic resonance imaging apparatus including the same, and method for controlling the same |
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US20120176132A1 (en) | 2012-07-12 |
US9615768B2 (en) | 2017-04-11 |
JP5823865B2 (ja) | 2015-11-25 |
JPWO2011037064A1 (ja) | 2013-02-21 |
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