WO2010053012A1 - 磁気共鳴イメージング装置及び方法 - Google Patents
磁気共鳴イメージング装置及び方法 Download PDFInfo
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- WO2010053012A1 WO2010053012A1 PCT/JP2009/068222 JP2009068222W WO2010053012A1 WO 2010053012 A1 WO2010053012 A1 WO 2010053012A1 JP 2009068222 W JP2009068222 W JP 2009068222W WO 2010053012 A1 WO2010053012 A1 WO 2010053012A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/4828—Resolving the MR signals of different chemical species, e.g. water-fat imaging
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/483—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
- G01R33/4835—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 of multiple slices
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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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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
- G01R33/48—NMR imaging systems
- G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
- G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
- G01R33/5607—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution by reducing the NMR signal of a particular spin species, e.g. of a chemical species for fat suppression, or of a moving spin species for black-blood imaging
Definitions
- the present invention detects nuclear magnetic resonance (hereinafter referred to as ⁇ NMR '') signals from hydrogen, phosphorus, etc. in a specimen, and images nuclear density distribution, relaxation time distribution, etc. (MRI) technology.
- ⁇ NMR '' nuclear magnetic resonance
- MRI relaxation time distribution, etc.
- the present invention relates to a technique for performing imaging by applying a prepulse that gives a predetermined effect to an NMR signal prior to execution of an NMR signal detection procedure for imaging.
- the B0 direction is defined as the z direction in this specification
- the magnetic moment of the nuclear spin of the tissue composition molecule is Precesses around the B0 direction at the resonance frequency unique to each spin.
- a magnetic field irradiation high frequency magnetic field B1 having a frequency close to the resonance frequency from a direction orthogonal to the B0 direction
- the net magnetic moment M rotates (excited) toward the xy plane, and the net The transverse magnetic moment is generated.
- the MRI apparatus detects the emitted NMR signal (echo signal) and performs signal processing to obtain an image of the living tissue (main imaging).
- a pre-pulse having a special high-frequency magnetic field prior to the main imaging in order to give some effect to the emitted echo signal.
- a pre-pulse called CHESS (Chemical Shift Suppression) pulse is applied to suppress echo signals from hydrogen protons in fat molecules (hereinafter abbreviated as “fatty protons”), and only signals from water molecules are imaged.
- CHESS Chemical Shift Suppression
- CHESS method see, for example, Non-Patent Document 1
- a high-frequency magnetic field (hereinafter abbreviated as “RF”) pulse having a constant magnetic field strength (here, 90 ° flip angle) having a resonance frequency of fat protons is used as a CHESS pulse prior to the main imaging.
- RF high-frequency magnetic field
- the transverse magnetization of the fat protons selectively excited by the CHESS pulse is phase-dispersed, the magnetization of the fat protons disappears immediately before the main imaging, and the signal from the fat protons is suppressed.
- the effect of prepulse decreases with time.
- the longitudinal magnetization of fat protons recovers based on the longitudinal relaxation time of fat protons as time passes immediately after the CHESS pulse is applied.
- the suppression effect is high, and the fat suppression effect decreases with time after application of the CHESS pulse.
- FIG. 12A shows a sequence diagram in multi-slice imaging using a CHESS pulse.
- signals of other slices are acquired within a repetition time TR for one slice to shorten the imaging time.
- the CHESS pulse 201 is applied immediately before the application of each excitation pulse 202 of the main imaging.
- the number given to each excitation pulse 202 is a slice number indicating the slice order of the slice to be measured.
- N slices having slice numbers 1 to N are sequentially imaged.
- Fig. 12 (b) is a schematic diagram of a repeated loop for multi-slice imaging, where the inner loop is the lower loop. As shown in the figure, the measurement of the number of slices with the phase encoding amount of 1 is repeated for the necessary number of phase encoding steps by changing the phase encoding amount.
- FIG. 13 (a) shows a sequence diagram on the RF axis of this system and how the longitudinal magnetization Mz (303) of fat excited by the CHESS pulse 301 is relaxed.
- Reference numeral 302 denotes an excitation pulse for each slice, and a given number is a slice number.
- FIG. 13 (b) shows a repeated loop of measurement using this method.
- the first half slice measured immediately after applying the CHESS pulse has a high fat suppression effect and a good fat suppression image is obtained, but it is measured over time after applying the CHESS pulse.
- the latter half of the slice has a low fat suppression effect, and a desired fat suppression image cannot be obtained.
- the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a technique for obtaining a good prepulse effect for all slices while suppressing the number of prepulse applications in multi-slice imaging using a prepulse together.
- the k space is divided into a number of segments different from the number of slices in the phase encoding direction, and the slice and the segment to be measured for each pre-pulse application are changed in a predetermined order, Measure until the segment to be measured is completed.
- the segment measurement order is set so that the segment in the low spatial frequency region of the k space is measured at the timing when the effect of the prepulse is the highest.
- a magnetic resonance imaging apparatus for imaging a plurality of different slice planes
- prepulse application means for applying a prepulse that affects the magnetization in all slice planes to be measured
- one slice plane Measuring means for measuring the echo signal obtained by providing the phase encoding amount in k-space
- control means for controlling the operation of the pre-pulse applying means and the measuring means, the control means comprising: After the first pre-pulse application, the measurement is performed once for each slice surface to be measured according to a predetermined order, and the phase encoding amount is changed in a predetermined order to change all the slices in all slices.
- First control means for controlling to repeat until the k space is filled, and the number of slices to be measured and the number of times different from the multiple are measured.
- Second control means for controlling the prepulse application means so as to apply the prepulse every time the prepulse is applied, and the first control means has a low spatial frequency region of k-space at a timing when the effect of the prepulse is large.
- a magnetic resonance imaging apparatus characterized by setting an initial phase encoding amount of each slice so as to perform measurement.
- a magnetic resonance imaging method for imaging a plurality of different slice planes a prepulse application step for applying a prepulse that affects the magnetization in all slice planes to be measured, and one phase encoding for one slice plane
- a measurement step for performing measurement to arrange the echo signal obtained by giving a quantity in the k-space and a control step for controlling operations of the prepulse application step and the measurement step, the control step comprising: After applying the pre-pulse, for each slice surface to be measured, the measurement is performed once according to a predetermined order, and the phase encoding amount is changed in a predetermined order to change all k spaces of all slices.
- a first control step for controlling to repeat until filling, and the number of slices to be measured and its multiples A second control step for controlling the prepulse application step so as to apply the prepulse each time a different number of measurements are performed, wherein the first control step is a k-space at a timing when the effect of the prepulse is large.
- the magnetic resonance imaging method is characterized in that the initial phase encoding amount of each slice is set so as to measure the low spatial frequency region.
- FIG. 1 is a block diagram showing an overall configuration of an MRI apparatus according to an embodiment of the present invention. It is a sequence diagram of the imaging sequence using a CHESS pulse.
- (a) is a sequence diagram of an imaging sequence according to the embodiment of the present invention, and (b) is a diagram for explaining a measurement region in k-space. It is a sequence diagram of the imaging sequence of the embodiment of the present invention. It is a schematic diagram of the repetition loop of measurement control of the embodiment of the present invention.
- (a) is a sequence diagram of an imaging sequence in the case of three slices according to the embodiment of the present invention, and (b) is a schematic diagram of the repetition loop. It is a figure for demonstrating the measurement area
- FIG. 1 It is a sequence diagram of an imaging sequence using another pre-pulse of an embodiment of the present invention,
- (a) is a T2 preparation pulse,
- (b) is a pre-saturation pulse,
- (c) is a sequence diagram of an IR pulse .
- (a) is a sequence diagram when measurement is performed in sequential order in the embodiment of the present invention, and
- (b) is a diagram for explaining a measurement region of the k space.
- (a) is another sequence diagram in the case of performing measurement in sequential order in the embodiment of the present invention, and (b) is a diagram for explaining the measurement region of the k space.
- (a) is another sequence diagram of the imaging sequence of the embodiment of the present invention, and (b) is a diagram for explaining the measurement region of the k space.
- (a) is a sequence diagram of multi-slice imaging using a conventional CHESS pulse
- (b) is a schematic diagram of the repetitive loop.
- (a) is a sequence diagram of multi-slice imaging using a conventional CHESS pulse
- (b) is a schematic diagram of the repetitive loop.
- FIG. 1 is a block diagram showing the overall configuration of the MRI apparatus 100 of the present embodiment.
- the MRI apparatus 100 of the present embodiment obtains a tomographic image of a subject using an NMR phenomenon, and includes a static magnetic field generation system 2, a gradient magnetic field generation system 3, a transmission system 5, a reception system 6, and a signal.
- a processing system 7, a sequencer 4, and a central processing unit (CPU) 8 are provided.
- the static magnetic field generation system 2 is a vertical magnetic field system, the static magnetic field is uniform in the direction perpendicular to the body axis in the space around the subject 1, and if it is a horizontal magnetic field system, the static magnetic field is uniform in the body axis direction of the subject 1. This is realized by a permanent magnet type, normal conducting type or superconducting type static magnetic field generating source arranged around the subject 1.
- the gradient magnetic field generation system 3 includes a gradient magnetic field coil 9 wound in the three axes of X, Y, and Z, which is the coordinate system (stationary coordinate system) of the MRI apparatus 100, and a gradient magnetic field power source that drives each gradient magnetic field coil 10 and.
- Gradient magnetic fields Gx, Gy, Gz are applied in the three axial directions of X, Y, and Z by driving the gradient magnetic field power supply 10 of each gradient coil in accordance with a command from the sequencer 4 described later.
- a slice direction gradient magnetic field pulse is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining two orthogonal to the slice plane and orthogonal to each other
- a phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in one direction, and position information in each direction is encoded into an echo signal.
- the transmission system 5 irradiates the subject 1 with a high-frequency magnetic field (RF) pulse in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 1, and the high-frequency oscillator 11 and the modulator 12, a high frequency amplifier 13, and a high frequency coil (transmission coil) 14 a on the transmission side.
- the RF pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the sequencer 4 to be described later, amplified by the high-frequency amplifier 13, and transmitted from the transmission coil 14a disposed close to the subject 1.
- the specimen 1 is irradiated.
- the receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nuclear spins of the atoms constituting the living tissue of the subject 1, and receives signals from the high-frequency coil (receiving coil) 14b on the receiving side.
- An amplifier 15, a quadrature detector 16, and an A / D converter 17 are provided.
- the response NMR signal induced by the RF pulse irradiated from the transmission coil 14a is detected by the reception coil 14b arranged close to the subject 1, amplified by the signal amplifier 15, and sent from the sequencer 4 described later.
- the sequencer 4 is controlled to repeatedly apply RF pulses and gradient magnetic field pulses according to a predetermined pulse sequence. It operates under the control of the CPU 8 and is necessary for data collection to reconstruct a tomographic image of the subject 1.
- Various commands are sent to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6.
- the transmission system 5 and the gradient magnetic field generation system 3 respectively control the application timing, period and intensity of the RF pulse, and the application timing, period and intensity of the gradient magnetic field pulse according to instructions from the sequencer 4.
- the receiving system 6 detects an echo signal in accordance with an instruction from the sequencer 4.
- the pulse sequence is created in advance according to the purpose of measurement, and stored as a program and data in a storage device 18 or the like which will be described later.
- the signal processing system 7 performs various data processing and display and storage of processing results, and includes a CPU 8, a storage device 18 such as a ROM and a RAM, an external storage device 19 such as an optical disk and a magnetic disk, and a display device 20. It consists of.
- the CPU 8 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 1 as a result on the display device 20 and also stores it. Record in device 18 or external storage device 19.
- the operation unit 25 receives input of various control information of the MRI apparatus 100 itself and various control information of processing performed in the signal processing system 7, and includes a trackball or mouse 23 and a keyboard 24.
- the operation unit 25 is disposed close to the display device 20, and the operator interactively inputs information necessary for various processes of the MRI apparatus 100 via the operation unit 25 while viewing the display device 20.
- the transmission coil 14a and the gradient magnetic field coil 9 are opposed to the subject 1 in the static magnetic field space of the static magnetic field generation system 2 into which the subject 1 is inserted, in the case of the vertical magnetic field method, If it is a horizontal magnetic field system, it is installed so as to surround the subject 1. Further, the receiving coil 14b is disposed so as to face the subject 1 or surround the subject 1.
- the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is the main constituent material of the subject, as is widely used in clinical practice.
- proton the main constituent material of the subject
- the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.
- a pulse sequence in pre-pulse combined multi-slice imaging, a pulse sequence is constructed so that the data of the low spatial frequency space region of k space of each slice can be acquired at the timing when the effect of the pre-pulse is the highest, and imaging is controlled.
- the pulse sequence of the present embodiment for realizing this will be described.
- a case where a CHESS pulse is used as a pre-pulse will be described as an example.
- FIG. 2 is a sequence chart of a fat suppression imaging sequence using a general CHESS pulse.
- the fat suppression imaging sequence acquires a fat suppression pulse unit 412 for suppressing echo signals from fat protons using CHESS pulses and a subject image in which signals from fat tissue are suppressed.
- a main imaging pulse unit 413 that measures an echo signal for the purpose.
- RF, Gr, Gs, and Gp indicate timings of application of RF pulse, readout gradient magnetic field, slice gradient magnetic field, and phase encode gradient magnetic field, respectively. Echo indicates the timing of echo signal detection.
- RF pulse RF
- slice gradient magnetic field Gs
- phase encoding gradient magnetic field Gp
- readout gradient magnetic field Gr
- the fat suppression pulse unit 412 is a pulse sequence unit for eliminating the magnetization of fat protons in the imaging region.
- a CHESS pulse 401 for selectively exciting fat protons is applied with non-slice selection, ie without application of a slice gradient magnetic field.
- the flip angle of the CHESS pulse 401 is set to a predetermined angle ( ⁇ °). Since the CHESS pulse 401 is applied with non-slice selection, in the case of multi-slice imaging, the CHESS pulse 401 is applied to all slices to be imaged.
- a spoiler gradient magnetic field pulse 404 for phase dispersion of transverse magnetization of fat protons excited by the CHESS pulse 401 follows.
- the spoiler gradient magnetic field pulse 404 of the fat suppression pulse unit 412 is applied to the three axes Gr, Gs, and Gp, but it is sufficient that it is applied to at least one axis.
- the main imaging pulse unit 413 is a pulse sequence unit that measures an echo signal for reconstructing the image of the subject 1, and an arbitrary pulse sequence can be applied.
- an example using a known spin echo sequence is shown as an example.
- 90 ° excitation pulse 402, 180 ° refocus pulse 403, slice selective gradient magnetic fields 405 and 406, readout gradient magnetic fields 408 and 409, and phase encode gradient magnetic field 410 are applied to measure echo signal 411.
- a spoiler gradient magnetic field 407 is applied to the three axes.
- the main imaging pulse unit 413 measures the echo signal 411 for the image of the subject 1 in a state where the magnetization of the fat protons is lost by the immediately preceding fat saturation pulse unit 412. As a result, a fat-suppressed image in which signals from adipose tissue are suppressed can be acquired.
- the main imaging pulse unit 413 (excitation pulse 302) has the same phase encoding amount after the fat suppression pulse unit 412 (CHESS pulse 301).
- the combination is repeated by the number of phase encoding steps to obtain a fat-suppressed image of each slice cross section.
- the phase encoding amount and the CHESS pulse application timing are set so that the measurement of the low spatial frequency region in the k space of each slice is performed at the timing when the CHESS pulse is most effective immediately after the CHESS pulse is applied. Control.
- a specific measurement control method will be described.
- FIG. 3 is a diagram for explaining the measurement control of the present embodiment.
- FIG. 3 (a) is a sequence diagram of an imaging sequence in the measurement control of the present embodiment.
- FIG. 3B is a diagram for explaining a temporal change of the k space region 610 that is a measurement target of the main imaging in the measurement control of the present embodiment.
- detecting an echo signal with one phase encoding amount for one slice and arranging the data of the echo signal in the k space is referred to as measurement.
- the sequence diagram for explaining the subsequent imaging sequence only the timing of the RF pulse sequence is shown. For each measurement of the imaging pulse unit 413, the RF pulse is represented by one excitation pulse.
- the excitation pulse 602 includes a 90 ° pulse and a 180 ° pulse as shown in FIG. In FIG. 3 (a), this is represented by a single excitation pulse 602.
- the recovery state of the longitudinal magnetization Mz of the fat protons after application of the CHESS pulse 601 is also shown together with the sequence diagram in order to explain the temporal change of the slice to be measured for the main imaging. Show.
- slice i the slice ms [i] in which the slice order is determined so as to measure i-th is referred to as slice i.
- the measurement order of each slice is fixed, and in order to measure the low spatial frequency region of the k space of each slice immediately after application of the CHESS pulse 601, the number of measurements to make a round of all the slices (N), It sets so that the frequency
- the CHESS pulse 601 is applied every measurement of the number of times of adding 1 to the number N of slices will be described as an example.
- the CHESS pulse 601 is inserted at intervals of performing N (number of slices) +1 measurement.
- N + 1 measurement is performed at the application interval of the CHESS pulse 601.
- the phase encoding amount is adjusted so that the low spatial frequency region of k-space is measured immediately after the CHESS pulse 601 is applied. Specifically, first, the k-space is divided into segments of the number of times of measurement (N + 1) performed at the application interval of adjacent CHESS pulses 601 in the phase encoding direction, and each time the CHESS pulse 601 is applied, the CHESS pulse For each measurement, the measurement target segment is changed so that the spatial frequency region becomes higher in order from the low spatial frequency region in each segment in order of the time from application of 601 in the earliest time.
- the k space is divided into segments of the number of slices N plus 1 in the phase encoding direction. Then, as shown in FIG. 3 (b), measurement is performed by changing the phase encoding amount in a centric order for each slice for each slice.
- ⁇ Numbers are assigned to each segment obtained in the centric order in order from the central area of k-space. Specifically, as shown in FIG. 3 (b), the positive and negative regions are alternately arranged around the kx axis, respectively, Seg # 1, Seg # 2, Seg # i, Seg # N, Seg # N + 1 is assigned.
- N is an even number
- (N + 1) is an odd number
- Seg # 1 is a region including the center of the k space.
- N + 1) is an even number
- Seg # 1 and Seg # 2 are the regions closest to the center of k-space
- Seg # 2 is Seg # 1 with respect to the center of k-space.
- the target area is assigned to each segment obtained in the centric order in order from the central area of k-space.
- the positive and negative regions are alternately arranged around the kx axis, respectively, Seg # 1, Seg # 2, Seg # i, Seg # N, Seg # N + 1 is assigned.
- N is an even number
- phase encoding steps in each segment are as follows. Assuming that the number of phase encoding steps (total number of phase encoding steps) necessary to reconstruct an image of one slice is Ps, in the present embodiment, this is divided into (N + 1) segments. Therefore, the number of phase encoding steps Psn in one segment can be obtained by dividing Ps by (N + 1). Basically, Psn is the integer part of the result of dividing Ps by (N + 1). The remainder obtained by dividing Ps by (N + 1) is assigned in order from the segment with the earlier measurement order, here the segment on the low spatial frequency region side. Therefore, the phase encoding step number Psn of each segment is an integer part int (Ps / (N + 1)) or int (Ps / (N + 1)) + 1 of Ps / (N + 1).
- the Seg # 1 region which is the low spatial frequency region of space, is measured.
- measurement is performed by applying one of the phase encode steps int (Ps / (N + 1) (or int (Ps / (N + 1) +1)) included in Seg # 1.
- FIG. 5 shows a schematic diagram of an iterative loop in the measurement control of this embodiment.
- a CHESS pulse 601 is applied as the fat suppression pulse unit 412
- the main imaging pulse unit 413 one phase encoding amount is given to each slice and the excitation pulse 602 is added.
- Multi-slice measurement by applying.
- the segment starts measurement from the same area from the one shifted from the previous application of the CHESS pulse 601, and then the slice is changed in a predetermined measurement order.
- the segment is changed in a centric order, and the measurement is performed N + 1 times when the excitation pulse 602 is applied.
- Measurements by the fat suppression pulse unit 412 and the main imaging pulse unit 413 are performed PN times (PN is a minimum natural number of Ps ⁇ N / (N + 1) or more until Ps is filled). The number of phase encoding steps per slice) is repeated, and the image is reconstructed using the obtained results. When measuring the same segment of the same slice, a phase encoding amount that has not been measured so far is given.
- FIG. 6 (a) is a sequence diagram showing only the RF pulses (701, 702) of the present embodiment.
- the longitudinal magnetization recovery state of fat protons after application of the CHESS pulse 701 is also shown.
- FIG. 6 (b) is a schematic diagram of an iterative loop in this measurement control.
- FIG. 7 shows a measurement region in the k space according to the application time T from each CHESS pulse.
- FIG. 7 is a diagram for explaining a change in the measurement region of the k space.
- the measurement area of the k space at three application intervals is shown in order from the top, with the horizontal axis representing the time T from the CHESS pulse 701 and the vertical axis representing the application interval of the adjacent CHESS pulse 701.
- T 2 , t 3 , t 4 the areas of Seg # 1, Seg # 2, Seg # 3, and Seg # 4 are measured, respectively.
- one encoded data is collected for each segment in each k space in all slices. This is repeated by changing the phase encoding amount several times included in each segment, and the measurement ends when the k space of all slices is filled. Then, an image for three slices is reconstructed from the obtained k-space data.
- the repetition time TR is used, and for each slice, the time when the fat suppression effect is high after application of the CHESS pulse.
- the band measures the low spatial frequency region of the k space, and controls the measurement so that the high spatial frequency region of the k space is measured in the time zone where the fat suppression effect is low. Therefore, according to the present embodiment, for all slices, the acquired data is arranged in the low spatial frequency region of the k space, which has a large influence on the contrast and the like, at the timing when the effect of the prepulse is great.
- a good fat suppression image can be obtained for all slices without applying a CHESS pulse every time an excitation pulse for main imaging is applied (measured).
- an image with the same quality can be obtained with a smaller number of application times of the CHESS pulse, and the overall imaging time can be shortened.
- SAR can be reduced.
- the case where the CHESS pulse is used as the prepulse and fat is suppressed is described as an example, but the present invention is not limited to this.
- the various suppression targets may be water during chemical shift imaging.
- the pre-pulse is not limited thereto.
- a known T2 preparation pulse for imparting T2 contrast a presaturation pulse for suppressing signals in or out of the slice plane, a known IR (Inversion recovery) pulse for imparting T1 contrast, and the like may be used.
- An example of an imaging sequence when these are used as prepulses is shown in FIG.
- FIG. 8 (a) is an example of a T2 preparation pulse
- FIG. 8 (b) is an example of the pre-saturation pulse
- FIG. 8 (c) is an example of an IR pulse.
- the pre-pulse is a CHESS pulse
- a method of measuring each segment in the centric order as described above is suitable.
- various measurement orders such as sequential order and anticentric order can be considered. In any case, it is only necessary to control so that the low frequency region of the k space of each slice is measured at the timing when the effect of the prepulse is the highest.
- FIG. 9 shows an example of moving the measurement target segment in sequential order.
- a case where an IR pulse 901 is used as a pre-pulse will be described as an example.
- FIG. 9 (a) is a sequence diagram showing temporal changes in slices to be measured when measurement control is performed so that measurement is performed in sequential order in units of segments. The recovery state of magnetization is also shown.
- FIG. 9B is a diagram for explaining a temporal change in the k-space region 910 that is a measurement target of the main imaging in the main measurement control example.
- N the number of slices
- the phase encoding step is divided into N + 1 segments.
- the application intensity of the IR pulse 901 is adjusted so that the fat suppression effect of the IR pulse 901 becomes the highest at the N / 2th measurement after the application. Further, it is assumed that N + 1 excitation pulses 902 are applied at an IR pulse 901 application interval, and N + 1 slices are measured.
- the slice to be measured is changed one by one between the application of one IR pulse 901 and the application of the next IR pulse 901. Change the segment one by one in sequential order and measure the number of slices N + 1. This is repeated to fill the k space of all slices.
- the intensity of the IR pulse 901 is adjusted so that the effect of the IR pulse 901 becomes the highest in the middle of the application interval of the IR pulse 901 (at the time of the N / 2th measurement). Control is performed to measure the lowest spatial frequency region in k-space.
- the timing at which the effect of the IR pulse 901 becomes highest is the timing at which the longitudinal magnetization of the fat protons becomes zero.
- the start segment may be adjusted so as to measure the segment including the lowest spatial frequency region at the M-th time.
- Fig. 10 (a) The imaging sequence in this case is shown in Fig. 10 (a).
- a case where an IR pulse 1001 is used as a pre-pulse will be described as an example. Further, similarly to the above, it is assumed that the number of slices N + 1 is applied at the application interval of the IR pulse 1001, and the measurement is performed N + 1 times.
- the start segment is adjusted to measure Seg # N / 2).
- TI is larger than M ⁇ TR / (N + 1), that is, when only M times (M is an integer equal to or smaller than N / 2) can be measured at TI time, as shown in FIG.
- the number of slices N + 1 is measured at the application interval of a pre-pulse such as a CHESS pulse.
- the number of slices may not be the same as the number of slices N, and may be the number of slices N + L (L is an integer other than 0 and satisfies ⁇ N ⁇ L ⁇ N).
- the phase encoding step is divided by the number of times of measurement.
- FIG. 11 (a) shows the imaging sequence at this time
- FIG. 11 (b) shows changes in the measurement segment of the k space 1110.
- N + L excitation pulses 1102 are applied at the pre-pulse 1101 application interval, slices are changed in a predetermined order, and N + L measurements are performed. Further, the phase encoding step is divided into segments corresponding to the number of times of measurement performed at the application interval of the pre-pulse 1101, and the phase encoding amount is changed on a segment basis for each measurement performed at the application interval.
- FIG. 11 shows a case where measurement is performed in sequential order in segment units.
- the measurement is performed in the same centric order.
- the number of application times of the CHESS pulse can be further reduced as compared to the above embodiment, and the SAR can be further reduced. Also, the entire imaging time can be shortened.
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Abstract
Description
以下、本発明を適用する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。
Claims (14)
- 複数の異なるスライス面の撮像を行う磁気共鳴イメージング装置であって、
計測対象の全スライス面内の磁化に影響を与えるプリパルスを印加するプリパルス印加手段と、1のスライス面について1の位相エンコード量を付与して取得したエコー信号をk空間に配置する計測を行う計測手段と、前記プリパルス印加手段と前記計測手段との動作を制御する制御手段と、を備え、
前記制御手段は、
最初のプリパルス印加後に、前記計測対象の全スライス面について、予め定められた順序に従って前記計測をそれぞれ1回実行することを、位相エンコード量を予め定められた順序で変化させて全スライスの全k空間を充填するまで繰返すよう制御する第一の制御手段と、
前記計測対象のスライス数およびその倍数と異なる回数の計測が行われる毎に前記プリパルスを印加するよう前記プリパルス印加手段を制御する第二の制御手段と、を備え、
前記第一の制御手段は、前記プリパルスの効果が大きいタイミングでk空間の低空間周波数領域の計測を行うよう各スライスの初期位相エンコード量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に記載の磁気共鳴イメージング装置であって、
前記プリパルスは、効果が単調減少するパルスであって、
前記第一の制御手段は、k空間を前記プリパルス印加手段が前記プリパルスを印加する間に行われる計測の回数分のセグメントに分割し、セントリックオーダで前記セグメント単位で前記計測毎に前記位相エンコード量を変化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1に記載の磁気共鳴イメージング装置であって、
前記プリパルスは、印加後所定のタイミングで最大の効果を奏するパルスであって、
前記第一の制御手段は、k空間を前記プリパルス印加手段が前記プリパルスを印加する間に行われる計測の回数分のセグメントに分割し、シーケンシャルオーダで前記セグメント単位で計測毎に前記位相エンコード量を変化させるとともに、前記プリパルスの効果が最も大きいタイミングに、最も低空間周波数領域を含むセグメント内に配置するデータを取得するよう初期位相エンコード量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に記載の磁気共鳴イメージング装置であって、
前記プリパルスは、脂肪分子の水素プロトンからのエコー信号を抑制するパルスであることを特徴とする磁気共鳴イメージング装置。 - 請求項1に記載の磁気共鳴イメージング装置であって、
前記プリパルスは、不要な組織を選択的に励起するパルスであることを特徴とする磁気共鳴イメージング装置。 - 請求項1に記載の磁気共鳴イメージング装置であって、
前記プリパルスは、T2プリパレーションパルスであることを特徴とする磁気共鳴イメージング装置。 - 請求項1に記載の磁気共鳴イメージング装置であって、
前記プリパルスは、IR(Inversion Recovery)パルスであることを特徴とする磁気共鳴イメージング装置。 - 複数の異なるスライス面の撮像を行う磁気共鳴イメージング方法であって、
計測対象の全スライス面内の磁化に影響を与えるプリパルスを印加するプリパルス印加ステップと、1のスライス面について1の位相エンコード量を付与して取得したエコー信号をk空間に配置する計測を行う計測ステップと、前記プリパルス印加ステップと前記計測ステップとの動作を制御する制御ステップと、を備え、
前記制御ステップは、
最初のプリパルス印加後に、前記計測対象の全スライス面について、予め定められた順序に従って前記計測をそれぞれ1回実行することを、位相エンコード量を予め定められた順序で変化させて全スライスの全k空間を充填するまで繰返すよう制御する第一の制御ステップと、
前記計測対象のスライス数およびその倍数と異なる回数の計測が行われる毎に前記プリパルスを印加するよう前記プリパルス印加ステップを制御する第二の制御ステップと、を備え、
前記第一の制御ステップは、前記プリパルスの効果が大きいタイミングでk空間の低空間周波数領域の計測を行うよう各スライスの初期位相エンコード量を設定することを特徴とする磁気共鳴イメージング方法。 - 請求項8に記載の磁気共鳴イメージング方法であって、
前記プリパルスは、効果が単調減少するパルスであって、
前記第一の制御ステップは、k空間を前記プリパルス印加ステップが前記プリパルスを印加する間に行われる計測の回数分のセグメントに分割し、セントリックオーダで前記セグメント単位で前記計測毎に前記位相エンコード量を変化させることを特徴とする磁気共鳴イメージング方法。 - 請求項8に記載の磁気共鳴イメージング方法であって、
前記プリパルスは、印加後所定のタイミングで最大の効果を奏するパルスであって、
前記第一の制御ステップは、k空間を前記プリパルス印加ステップが前記プリパルスを印加する間に行われる計測の回数分のセグメントに分割し、シーケンシャルオーダで前記セグメント単位で計測毎に前記位相エンコード量を変化させるとともに、前記プリパルスの効果が最も大きいタイミングに、最も低空間周波数領域を含むセグメント内に配置するデータを取得するよう初期位相エンコード量を設定することを特徴とする磁気共鳴イメージング方法。 - 請求項8に記載の磁気共鳴イメージング方法であって、
前記プリパルスは、脂肪分子の水素プロトンからのエコー信号を抑制するパルスであることを特徴とする磁気共鳴イメージング装置。 - 請求項8に記載の磁気共鳴イメージング方法であって、
前記プリパルスは、不要な組織を選択的に励起するパルスであることを特徴とする磁気共鳴イメージング方法。 - 請求項8に記載の磁気共鳴イメージング方法であって、
前記プリパルスは、T2プリパレーションパルスであることを特徴とする磁気共鳴イメージング方法。 - 請求項8に記載の磁気共鳴イメージング方法であって、
前記プリパルスは、IR(Inversion Recovery)パルスであることを特徴とする磁気共鳴イメージング方法。
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JP2016190093A (ja) * | 2016-08-15 | 2016-11-10 | 東芝メディカルシステムズ株式会社 | 磁気共鳴撮像装置 |
CN106175765A (zh) * | 2015-04-29 | 2016-12-07 | Ge医疗系统环球技术有限公司 | 磁共振成像系统及方法 |
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US9995811B2 (en) | 2012-01-13 | 2018-06-12 | Toshiba Medical Systems Corporation | Magnetic resonance imaging apparatus |
JP2014046124A (ja) * | 2012-09-04 | 2014-03-17 | Hitachi Medical Corp | 磁気共鳴イメージング装置及びマルチエコー計測方法 |
CN106175765A (zh) * | 2015-04-29 | 2016-12-07 | Ge医疗系统环球技术有限公司 | 磁共振成像系统及方法 |
CN106175765B (zh) * | 2015-04-29 | 2021-02-09 | Ge医疗系统环球技术有限公司 | 磁共振成像系统及方法 |
JP2016190093A (ja) * | 2016-08-15 | 2016-11-10 | 東芝メディカルシステムズ株式会社 | 磁気共鳴撮像装置 |
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