Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/48—NMR imaging systems
  • G01R33/54—Signal processing systems, e.g. using pulse sequences ; Generation or control of pulse sequences; Operator console
  • G01R33/56—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution
  • G01R33/563—Image enhancement or correction, e.g. subtraction or averaging techniques, e.g. improvement of signal-to-noise ratio and resolution of moving material, e.g. flow contrast angiography
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/44—Arrangements or instruments for measuring magnetic variables involving magnetic resonance using nuclear magnetic resonance [NMR]
  • G01R33/46—NMR spectroscopy
  • G01R33/4616—NMR spectroscopy using specific RF pulses or specific modulation schemes, e.g. stochastic excitation, adiabatic RF pulses, composite pulses, binomial pulses, Shinnar-le-Roux pulses, spectrally selective pulses not being used for spatial selection
• • 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
  • G01R33/543—Control of the operation of the MR system, e.g. setting of acquisition parameters prior to or during MR data acquisition, dynamic shimming, use of one or more scout images for scan plane prescription
• • 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 relates to a magnetic resonance apparatus comprising scanning section for performing a sequence for acquiring MR (magnetic resonance) signals from a region to be imaged in a subject.
• NPL 1 Wang, et al, "Improved Suppression of Plaque-Mimicking
• the MSDE technique is a technique involving using a refocusing pulse to decrease longitudinal magnetization of a moving object lower than longitudinal magnetization Mz of stationary tissue, and is an excellent method for suppressing signals from a moving object such as blood. Therefore, the MSDE technique is applied to black-blood imaging for rendering a blood vessel wall by suppressing blood signals.
• the MSDE technique is also used for reducing artifacts caused by signals from a moving object (for example, blood).
• the MSDE technique is easily affected by inhomogeneity in a static magnetic field (B0) and a transmission magnetic field (B l), so that in case that the B0 or B l value has large variability in a region to be imaged, there appears a region in which the longitudinal magnetization Mz of stationary tissue does not reach one (or a value close to one) in the region to be imaged. Therefore, it is impossible to make a difference between the longitudinal magnetization of stationary tissue and that of moving tissue, problematically causing degradation of image quality.
• the present invention in one aspect is a magnetic resonance apparatus comprising scanning section for performing a sequence for acquiring MR signals from a region to be imaged in a subject, wherein said scanning section performs a sequence including:
• a first composite pulse applied for exciting said region to be imaged said composite pulse comprising an ⁇ ⁇ ⁇ pulse having a flip angle set to a and a phase set to (po, and a ⁇ pulse being applied after said ⁇ ⁇ ⁇ pulse and having a flip angle set to ⁇ and a phase set to ⁇ ;
• n (> 1) second composite pulses applied after said first composite pulse for refocusing magnetization vectors dispersed within said region to be imaged, a k (1 ⁇ k ⁇ n)-th one of said n second composite pulses being constructed to comprise a ek pulse having a flip angle set to ⁇ and a phase set to 9k, a 2a ⁇ pk pulse being applied after said ek pulse and having a flip angle set to 2a and a phase set to q3 ⁇ 4, and a ek pulse being applied after said 2 ⁇ ⁇ ⁇ pulse and having a flip angle set to ⁇ and a phase set to 9k; and
• a third composite pulse applied after said n composite pulses for bringing transverse magnetization of the magnetization vectors in said region to be imaged back to longitudinal magnetization
• said third composite pulse comprising a ⁇ ⁇ + ⁇ pulse having a flip angle set to ⁇ and a phase set to ⁇ ⁇ + ⁇ , and an ⁇ ⁇ + ⁇ pulse being applied after said ⁇ ⁇ + ⁇ pulse and having a flip angle set to a and a phase set to ⁇ ⁇ + ⁇ , where 9k, (pk, ⁇ ⁇ + ⁇ , and ⁇ ⁇ + ⁇ are expressed by the following equations:
• 5k an angle determined according to the value of k, in a range
• mk an integer determined according to the value of k
• ⁇ + ⁇ an angle determined according to the value of n+1, in a range of 80° ⁇ ⁇ ⁇ + ⁇ ⁇ 100°
• the technique may be made less affected by B0 inhomogeneity and/or B l inhomogeneity. Therefore, a high-quality image may be obtained.
• FIG. 1 is a schematic diagram of a magnetic resonance apparatus in one embodiment of the present invention
• FIG. 2 is a diagram showing processing a processor 9 executes
• FIG. 3 is a diagram schematically showing a region to be imaged in the present embodiment
• FIG. 4 is a diagram showing an example of a conventional scan
• FIG. 5 is a diagram explaining a result of a simulation of behavior of magnetization vectors in stationary tissue while a preparation sequence DP is performed;
• FIG. 6 is a diagram explaining a result of a simulation of behavior of magnetization vectors for an intensity of a transmission magnetic field lower than an ideal value
• FIG. 7 is a diagram explaining a result of a simulation of behavior of magnetization vectors for an intensity of the transmission magnetic field higher than an ideal value
• FIG. 8 is a diagram showing a preparation sequence DP 0 used in the present embodiment.
• FIG. 9 is a diagram explaining composite pulses A, B, and C.
• FIG. 10 is a diagram showing a preparation sequence DPi having ( ⁇ , ⁇ , ⁇ 3 ⁇ 4 ⁇ , ⁇ , mi, m 2 ) set to values listed in Table 1 ;
• FIG. 13 is a diagram explaining a difference between planes SI and S4;
• FIG. 15 is a diagram explaining a difference between planes S2 and S5;
• FIG. 16 is a diagram showing results of simulations
• FIG. 17 is a diagram showing a preparation sequence DP 2 having ( ⁇ , ⁇ , ⁇ 3 ⁇ 4 ⁇ , ⁇ , mi, m 2 ) set to values listed in Table 2;
• FIG. 18 is a diagram showing a preparation sequence DP 3 having ( ⁇ , ⁇ , ⁇ , ⁇ , mi, m 2 ) set to values listed in Table 3;
• FIG. 19 is a diagram showing a result of a simulation for the preparation sequence DP 2 ;
• FIG. 20 is a diagram showing a result of a simulation for the preparation sequence DP 3 ;
• FIG. 21 is a diagram showing a preparation sequence DP 4 having ( ⁇ , ⁇ , ⁇ 3 ⁇ 4 ⁇ , ⁇ , mi, m 2 ) set to values listed in Table 4;
• FIG. 22 is a diagram showing a preparation sequence DP5 having ( ⁇ , ⁇ , ⁇ , ⁇ , mi, m 2 ) set to values listed in Table 5;
• FIG. 23 is a diagram showing results of simulations
• FIG. 24 is a diagram showing a preparation sequence DP 6 in which the composite pulse B is applied a plurality of times;
• FIG. 26 is a diagram showing a preparation sequence DP 7 having ( ⁇ , ⁇ , ⁇ 3 ⁇ 4 ⁇ , ⁇ ) set to values listed in Table 6;
• FIG. 27 is a diagram showing a preparation sequence DP 8 having phases set to values in EQS. (30) - (33);
• FIG. 28 is a diagram showing a result of a simulation.
• FIG. 1 is a schematic diagram of a magnetic resonance apparatus in one embodiment of the present invention.
• a magnetic resonance apparatus (referred to hereinbelow as “MR apparatus") 100 comprises a magnet 2, a table 3, an RF receive coil (referred to hereinbelow as “receive coil”) 4, and the like.
• the magnet 2 has a bore 21 into which a subject 13 is inserted.
• the magnet 2 comprises a superconductive coil for generating a static magnetic field, a gradient coil for applying gradient pulses, an RF coil for applying RF pulses, and the like.
• the table 3 has a cradle 3a for supporting the subject 13.
• the cradle 3a is configured to be movable into the bore 21.
• the subject 13 is carried into the bore 21 by the cradle 3 a.
• the receive coil 4 receives magnetic resonance signals from the subject 13.
• the MR apparatus 100 further comprises a transmitter 5, a gradient power supply 6, a receiver 7, a computer 8, an operating section 11, a display section 12, and the like.
• the transmitter 5 supplies electric current to the RF coil.
• the gradient power supply 6 supplies electric current to the gradient coil.
• the receiver 7 applies signal processing such as detection/demodulation to signals received by the receive coil 4.
• the magnet 2, receive coil 4, transmitter 5, gradient power supply 6, and receiver 7 together represent the scanning section.
• the computer 8 controls operations of several sections in the MR apparatus 100 to implement several kinds of operation of the MR apparatus 100, including an operation of transmitting required information to the display section 12, an operation of reconstructing an image, etc.
• the computer 8 comprises a processor 9, a memory 10, and the like.
• the processor 9 loads thereon a program stored in the memory 10, and executes processing written in the program.
• FIG. 2 shows processing the processor 9 executes.
• the processor 9 constitutes image producing unit 91, etc. by loading thereon a program stored in the memory 10.
• the image producing unit 91 produces an image of a region to be imaged based on data received from the receiver 7.
• the processor 9 is an example for constructing the image producing unit 91, and it functions as the unit by executing a program stored in the memory 10.
• the operating section 11 is operated by an operator to input several kinds of information to the computer 8.
• the display section 12 displays several kinds of information.
• the MR apparatus 100 is constructed as described above.
• FIG. 3 is a diagram schematically showing a region to be imaged in the present embodiment.
• an SI direction designates a superior-inferior direction, an RL direction a right-left direction, and an AP direction an anterior-posterior direction, of a subject.
• the present embodiment will address a case in which a scan is conducted for imaging a blood vessel wall of a blood vessel A while weakening signals from blood a.
• a scan is conducted for imaging a blood vessel wall of a blood vessel A while weakening signals from blood a.
• an conventional scan used for imaging the blood vessel wall will be explained before explaining a scan in accordance with the present embodiment in order to clarify advantageous effects of the scan in the present embodiment.
• FIG. 4 is a diagram showing an example of a conventional scan.
• FIG. 4 a scan SC separated into a plurality of periods of time Wl— Wz is shown.
• a preparation sequence DP and a data acquisition sequence DAQ are performed.
• the preparation sequence DP comprises three RF pulses (90 x pulse Po, 180_ x pulse Pi, 90 x pulse P 2 ).
• the 90 x pulse Po is an excitation pulse for exciting a region to be imaged.
• the 180-x pulse Pi is a refocusing pulse for refocusing dispersed magnetization vectors.
• the 90 x pulse P2 is a flip-back pulse for bringing transverse magnetization of the magnetization vectors back to longitudinal magnetization.
• the preparation sequence DP also comprises MPG (motion probing gradient) for dispersing the phase. While in the present embodiment, MPG is applied in a single axis, it may be applied in a plurality of axes.
• MPG motion probing gradient
• Coordinate axes for representing the phase of an RF pulse are illustrated in the lower portion of FIG. 4.
• a z-axis indicates a direction of the static magnetic field.
• the symbol "90 x " for the 90 x pulses Po and P2 indicates that the RF pulses have a flip angle of 90° and a phase set in an x-axis. Therefore, the 90 x pulses Po and P2 are RF pulses for rotating magnetization by 90° around the x-axis.
• the symbol " 180_ x " for the 180_ x pulse Pi indicates that the RF pulse has a flip angle of 180° and a phase set in the -x-axis (an axis offset from the x-axis by 180° in the xy -plane). Therefore, the 180 x pulse Pi is an RF pulse for rotating magnetization by 180° around the -x-axis.
• longitudinal magnetization of moving tissue may be sufficiently decreased as compared with that of stationary tissue (for example, blood vessel wall).
• the data acquisition sequence DAQ is performed for acquiring imaging data from the region to be imaged.
• RF pulses and gradient magnetic fields used in the data acquisition sequence DAQ are not shown in the drawing. Since longitudinal magnetization of moving tissue is decreased by the preparation sequence DP, MR signals having signals from the moving tissue sufficiently weakened as compared with those from stationary tissue may be acquired by performing the data acquisition sequence DAQ.
• the preparation sequence DP and data acquisition sequence DAQ are performed in the other periods of time W2 — Wz similarly to the period of time Wl .
• data acquisition in the periods of time Wl - Wz data in k-space required for image reconstruction may be acquired.
• FIG. 5 is a diagram explaining a result of the simulation of behavior of magnetization vectors in stationary tissue while the preparation sequence DP is performed.
• FIG. 5 will be explained.
• FIG. 5(a) shows a magnetization vector V at time point tO immediately before the 90 x pulse Po is applied
• FIG. 5(b) shows the same at time point tl immediately after the 90 x pulse Po is applied.
• the magnetization vector V rotates by 90° around the x-axis by the 90 x pulse Po.
• MPG is applied.
• FIG. 5(c) shows phase dispersion occurring from time point tl to time point t2. While the phase dispersion gives rise to vectors having an arbitrary phase between 0° and 360°, the dispersed magnetization vectors are represented by two magnetization vectors VI and V2 in FIG. 5(c) for convenience of explanation.
• the magnetization vector VI has a phase of 0°
• the magnetization vector V2 has a phase of 180°.
• FIG. 5(c) also shows a plane S surrounded by vertices of the magnetization vectors generated by the phase dispersion. Immediately after time point t2, a 180_ x pulse Pi is applied.
• FIG. 5(d) shows the magnetization vectors at time point t3 immediately after the 180_ x pulse Pi is applied.
• the magnetization vectors rotate by 180° around the -x-axis by the 180_ x pulse Pi. Therefore, the positions of the magnetization vectors VI and V2 are exchanged.
• MPG is applied.
• the dispersed magnetization vectors are refocused between time points t3 and t4.
• FIG. 5(e) shows the magnetization vectors V refocused between time points t3 and t4.
• the magnetization vectors V are refocused in the y-axis.
• the period of time between time points t3 and t4 is the same as that between time points tl and t2.
• a 90 x pulse P2 is applied.
• FIG. 5(f) shows the magnetization vector V at time point t5 immediately after the 90 x pulse P2 is applied.
• the magnetization vector V rotates by 90° around the x-axis by the 90 x pulse P 2 .
• moving tissue is affected by MPG, so that causing longitudinal magnetization is smaller than one. Therefore, the value of longitudinal magnetization may be adjusted according to the velocity of movement of tissue.
• FIG. 6 is a diagram explaining a result of a simulation of behavior of magnetization vectors for an intensity of the transmission magnetic field lower than an ideal value.
• FIG. 6(a) shows a magnetization vector V at time point tO immediately before the 90 x pulse Po is applied
• FIG. 6(c) shows phase dispersion occurring from time point tl to time point t2. While the phase dispersion gives rise to vectors having an arbitrary phase between 0° and 360°, the dispersed magnetization vectors are represented by two magnetization vectors VI and V2 in FIG. 6(c) for convenience of explanation.
• the magnetization vector VI has a phase of 0°
• the magnetization vector V2 has a phase of 180°.
• FIG. 6(c) also shows a plane S surrounded by vertices of the magnetization vectors generated by the phase dispersion. Immediately after time point t2, a 180_ x pulse Pi is applied.
• the dispersed magnetization vectors are refocused between time points t3 and t4. However, since the magnetization vectors have rotated by only 144° as shown in FIG. 6(d), the dispersed magnetization vectors cannot completely be refocused.
• FIG. 6(e) shows the magnetization vectors refocused between time points t3 and t4. Comparing the magnetization vector VI with V2 in FIG. 6(e), the phase of the magnetization vector VI is shifted from that of the magnetization vector V2. Therefore, by connecting the vertices of the magnetization vectors, a plane SI having a certain spread appears, proving that the magnetization vectors are not completely refocused.
• a 90 x pulse P2 is applied.
• FIG. 7 is a diagram explaining a result of a simulation of behavior of magnetization vectors for an intensity of the transmission magnetic field higher than an ideal value.
• FIG. 7(a) shows a magnetization vector V at time point tO immediately before the 90 x pulse Po is applied
• FIG. 7(c) shows phase dispersion occurring from time point tl to time point t2. While the phase dispersion gives rise to vectors having an arbitrary phase between 0° and 360°, the dispersed magnetization vectors are represented by two magnetization vectors VI and V2 in FIG. 7(c) for convenience of explanation.
• the magnetization vector VI has a phase of 0°
• the magnetization vector V2 has a phase of 180°.
• FIG. 7(c) also shows a plane S surrounded by vertices of the magnetization vectors generated by the phase dispersion. Immediately after time point t2, a 180_ x pulse Pi is applied.
• the dispersed magnetization vectors are refocused between time points t3 and t4. However, since the magnetization vectors have rotated by 216° as shown in FIG. 7(d), the dispersed magnetization vectors cannot completely be refocused.
• FIG. 7(e) shows the magnetization vectors refocused between time points t3 and t4.
• the preparation sequence DP poses a problem that longitudinal magnetization in stationary tissue cannot sufficiently be brought back to one because it is affected by Bl inhomogeneity. Accordingly, in the present embodiment, the preparation sequence is constructed to be capable of bringing longitudinal magnetization in stationary tissue back to one (or a value close to one). Now preparation sequences used in accordance with the present embodiment will be described hereinbelow.
• FIG. 8 is a diagram showing a preparation sequence DP 0 used in the present embodiment.
• the preparation sequence DPo in the present embodiment comprises a composite pulse A, a composite pulse B, and a composite pulse C.
• the three composite pulses A, B, and C will now be described one by one.
• FIG. 9 is a diagram explaining the composite pulses A, B, and C.
• the composite pulse A is an excitation pulse for exciting a region to be imaged, and comprises an ⁇ ⁇ 0 pulse P 0 i and a ⁇ ⁇ ⁇ pulse P 0 2- [0058]
• FIG. 9(a) is a diagram explaining the composite pulse A.
• the ⁇ ⁇ ⁇ pulse Poi represents an RF pulse having a flip angle set to a and a phase set to (po (an axis rotated by an angle (po with respect to the x-axis within the xy-plane: (po-axis).
• the ⁇ pulse P02 represents an RF pulse having a flip angle set to ⁇ and a phase set to ⁇ (an axis rotated by an angle ⁇ with respect to the x-axis within the xy- plane: ⁇ -axis).
• the composite pulse B is a refocusing pulse for refocusing magnetization vectors, and comprises a ⁇ ⁇ pulse Pn, a 2 ⁇ ⁇ ⁇ pulse P12, and a ⁇ ⁇ pulse P1 3 .
• FIG. 9(b) is a diagram explaining the composite pulse B.
• the 2 ⁇ ⁇ ⁇ pulse P12 represents an RF pulse having a flip angle set to 2a and a phase set to (pi (an axis rotated by an angle (pi with respect to the x-axis within the xy-plane: (pi-axis), (pi is set to satisfy the following equation:
• ⁇ an angle in a range of 80° ⁇ ⁇ ⁇ 100°
• mi an integer.
• the ⁇ ⁇ pulse (Pn and P1 3 ) represents an RF pulse having a flip angle set to ⁇ and a phase set to ⁇ (an axis rotated by an angle ⁇ with respect to the x-axis within the xy-plane: ⁇ -axis).
• ⁇ is set to satisfy the following equation:
• ⁇ a phase of the ⁇ pulse P02 in the composite pulse A
• ⁇ a phase difference
• the composite pulse C is a flip-back pulse for bringing transverse magnetization of a magnetization vector back to longitudinal magnetization, and comprises a ⁇ 2 pulse P21 and an ⁇ ⁇ 2 pulse P22.
• FIG. 9(c) is a diagram explaining the composite pulse C.
• the ⁇ 2 pulse P21 represents an RF pulse having a flip angle set to ⁇ and a phase set to ⁇ 2 (an axis rotated by an angle ⁇ 2 with respect to the x-axis within the xy- plane: 02-axis).
• ⁇ 2 is set to satisfy the following equation:
• ⁇ a phase of the ⁇ pulse P02 in the composite pulse A, and %2 - a phase difference.
• EQ. (8) is expressed by the following equation:
• the ⁇ ⁇ 2 pulse P22 represents an RF pulse having a flip angle set to a and a phase set to ⁇ 2 (an axis rotated by an angle ⁇ 2 with respect to the x-axis within the xy-plane: q3 ⁇ 4-axis).
• ⁇ 2 is set to satisfy the following equation:
• ⁇ 2 in EQ. (11) is expressed by EQ. (9). Therefore, substituting EQ. (9) into EQ. (11), ⁇ 2 may be expressed by the following equation:
• the preparation sequence DP 0 is constructed as shown in FIG. 9.
• a data acquisition sequence DAQ (see FIG. 8) for acquiring data from the region to be imaged is performed.
• the preparation sequence DPo and data acquisition sequence DAQ are alternately performed.
• Data acquired by the data acquisition sequence DAQ are sent to the computer 8.
• the image producing unit 91 (see FIG. 2) produces an image based on the data acquired by the data acquisition sequence DAQ.
• the aforementioned parameters ( ⁇ , ⁇ , (po, ⁇ , ⁇ , mi, m 2 ) are used to define the flip angles and phases in the composite pulses. Values of the parameters ( ⁇ , ⁇ , (po, ⁇ , ⁇ , mi, m 2 ) may be set according to the imaging conditions. To enable better understandings of the flip angles and phases in the composite pulses used in the preparation sequence DPo, let us consider a case in which the parameters ( ⁇ , ⁇ , (po, ⁇ , ⁇ , mi, m 2 ) are set to values listed in Table 1.
• the flip angles and phases in the composite pulses may be represented as in FIG. 10.
• the preparation sequence is designated by symbol "DPi.” Now the preparation sequence DPi shown in FIG. 10 will be described separately for the composite pulses A, B, and C.
• FIG. 10(a) is a diagram explaining the composite pulse A.
• FIG. 10(b) is a diagram explaining the composite pulse B.
• phase set to (pi 180° (i.e., the -x-axis).
• FIG. 10(c) is a diagram explaining the composite pulse C.
• ⁇ 2 ⁇ 0 + 2 ⁇ + 90° m 2
• FIGS. 11 — 15 show diagrams explaining results of the simulations of behavior of magnetization vectors in stationary tissue while the preparation sequence DPi is performed.
• FIGS. 11 - 15 will be explained.
• FIG. 11(a) shows a magnetization vector V at time point tO immediately before the 90 x pulse Poi is applied
• a 90 y pulse P02 is applied.
• FIG. 11(d) shows phase dispersion occurring from time point t02 to time point ti l. While the phase dispersion gives rise to vectors having an arbitrary phase between 0° and 360°, the dispersed magnetization vectors are represented by two magnetization vectors VI and V2 in FIG. 11(d) for convenience of explanation.
• the magnetization vector VI has a phase of 0°
• the magnetization vector V2 has a phase of 180°.
• FIG. 11(d) also shows a plane S surrounded by vertices of the magnetization vectors generated by the phase dispersion. Immediately after time point ti l, the composite pulse B is applied.
• the dispersed magnetization vectors are refocused between time points tl4 and t21.
• FIG. 11(h) shows the magnetization vectors V refocused between time points tl4 and t21.
• the magnetization vectors V are refocused in the y-axis.
• the composite pulse C is applied.
• the preparation sequence DPi can bring longitudinal magnetization in stationary tissue back to one, similarly to the preparation sequence DP (see FIG. 5).
• FIG. 12 will be explained.
• FIG. 12(a) shows a magnetization vector V at time point tO immediately before the 90 x pulse P01 is applied
• a 90 y pulse P02 is applied.
• FIG. 12(d) shows phase dispersion occurring from time point t02 to time point ti l. While the phase dispersion gives rise to vectors having an arbitrary phase between 0° and 360°, the dispersed magnetization vectors are represented by two magnetization vectors VI and V2 in FIG. 12(d) for convenience of explanation.
• the magnetization vector VI has a phase of 0°
• the magnetization vector V2 has a phase of 180°.
• FIG. 12(d) also shows a plane S surrounded by vertices of the magnetization vectors generated by the phase dispersion. Immediately after time point ti l, the composite pulse B is applied.
• the magnetization vectors may be rotated by an angle close to 180° around the -x-axis by applying the composite pulse B. After the composite pulse B is applied, MPG is applied.
• the dispersed magnetization vectors are refocused between time points tl4 and t21.
• FIG. 12(h) shows the magnetization vectors refocused between time points tl4 and t21.
• the magnetization vectors are not completely refocused because they are affected by B l inhomogeneity.
• the magnetization vector V2 is refocused to a position sufficiently close to the magnetization vector VI, the area of a plane S4 surrounded by vertices of the magnetization vectors is sufficiently reduced.
• the positions of the magnetization vectors VI and V2 are offset from the y-axis, the magnetization vectors VI and V2 contain an Mx component.
• the composite pulse C is applied.
• FIG. 13 is a diagram explaining a difference between the planes S 1 and S4.
• FIG. 13(a) is a diagram showing the plane SI, a projection profile Sly of the plane SI in the y-axis direction, and a projection profile Six of the plane SI in the x-axis direction, in sequence from the left.
• FIG. 13(b) is a diagram showing the plane S4, a projection profile S4y of the plane S4 in the y-axis direction, and a projection profile S4x of the plane S4 in the x-axis direction, in sequence from the left.
• the plane S4 has an area sufficiently smaller than that of the plane SI. Therefore, the preparation sequence DPi using the composite pulses is more capable of bringing longitudinal magnetization Mz back to a value close to one than the preparation sequence DP using single RF pulses is, so that a high-quality image may be obtained.
• FIG. 14 will be explained.
• FIG. 14(a) shows a magnetization vector V at time point tO immediately before the 90 x pulse P 0 1 is applied
• FIG. 14(d) shows phase dispersion occurring from time point t02 to time point ti l. While the phase dispersion gives rise to vectors having an arbitrary phase between 0° and 360°, the dispersed magnetization vectors are represented by two magnetization vectors VI and V2 in FIG. 14(d) for convenience of explanation.
• the magnetization vector VI has a phase of 0°
• the magnetization vector V2 has a phase of 180°.
• FIG. 14(d) also shows a plane S surrounded by vertices of the magnetization vectors generated by the phase dispersion. Immediately after time point ti l, the composite pulse B is applied.
• the magnetization vectors may be rotated by an angle close to 180° around the -x-axis by applying the composite pulse B. After the composite pulse B is applied, MPG is applied.
• the dispersed magnetization vectors are refocused between time points tl4 and t21 (MPG is applied).
• FIG. 14(h) shows the magnetization vectors refocused between time points tl4 and t21.
• the magnetization vectors are not completely refocused because they are affected by Bl inhomogeneity.
• the magnetization vector V2 is refocused to a position sufficiently close to the magnetization vector VI, the area of a plane S5 surrounded by vertices of the magnetization vectors is sufficiently reduced.
• the positions of the magnetization vectors VI and V2 are offset from the y-axis, the magnetization vectors VI and V2 contain an Mx component.
• the composite pulse C is applied.
• FIG. 15 is a diagram explaining a difference between the planes S2 and S5.
• FIG. 15(a) is a diagram showing the plane S2, a projection profile S2y of the plane S2 in the y-axis direction, and a projection profile S2x of the plane S2 in the x-axis direction, in sequence from the left.
• FIG. 15(b) is a diagram showing the plane S5, a projection profile S5y of the plane S5 in the y-axis direction, and a projection profile S5x of the plane S5 in the x-axis direction, in sequence from the left.
• FIG. 16 is a diagram showing results of the simulations.
• FIG. 16(a) is a diagram showing a result of the simulation for the preparation sequence DPi using the composite pulses (see FIG. 10).
• FIG. 16(a) shows an Mz map HI representing the value of longitudinal magnetization immediately after performing the preparation sequence DPi.
• a horizontal axis of the Mz map HI represents ⁇ 1, and a vertical axis represents ⁇ 0.
• the Mz map HI indicates the value of longitudinal magnetization Mz by gray scale. A color in the Mz map HI closer to white implies higher longitudinal magnetization, while that closer to black implies lower longitudinal magnetization.
• FIG. 16(b) shows a comparative example, which is a result of the simulation for the preparation sequence DP using single RF pulses (see FIG. 4).
• FIG. 16(b) shows an Mz map H2 representing the value of longitudinal magnetization immediately after performing the preparation sequence DP.
• On the right of the Mz map H2 are shown two profiles F21 and F22.
• ⁇ 180° is set (see Table 1) in the preparation sequence DPi (see FIG. 10).
• ⁇ is not limited to 180° and may be set to a value other than 180°.
• Tables 2 and 3 have different values of ⁇ and the same values of ( ⁇ , ⁇ , (po, ⁇ 3 ⁇ 4 mi, m 2 ).
• FIG. 17 is a diagram showing the preparation sequence DP 2 with ( ⁇ , ⁇ , (po, ⁇ , ⁇ , mi, m 2 ) set to values listed in Table 2.
• FIG. 17(a) is a diagram explaining the composite pulse A.
• the ⁇ ⁇ ⁇ pulse P 01 is a 90 x pulse as in FIG. 10.
• the ⁇ pulse P 0 2 is a 90 y pulse as in FIG. 10.
• FIG. 17(b) is a diagram explaining the composite pulse B.
• FIG. 17(c) is a diagram explaining the composite pulse C.
• ⁇ 2 ⁇ 0 + 2 ⁇ + 90° x m 2
• FIG. 18 is a diagram showing the preparation sequence DP 3 with ( ⁇ , ⁇ , (po, ⁇ , ⁇ , mi, ⁇ 2) set to values listed in Table 3.
• FIG. 18(a) is a diagram explaining the composite pulse A.
• the ⁇ ⁇ ⁇ pulse Poi is a 90 x pulse
• the ⁇ pulse P02 is a 90 y pulse.
• FIG. 18(b) is a diagram explaining the composite pulse B.
• FIG. 18(c) is a diagram explaining the composite pulse C.
• FIG. 19 is a diagram showing a result of the simulation for the preparation sequence DP 2
• is not limited to
• 90°, and it may be
• ⁇ 90° is shown in Table 5. [Table 5]
• Table 5 shows an example of
• FIG. 21 is a diagram showing a preparation sequence DP 4 having ( ⁇ , ⁇ , (po, ⁇ , ⁇ , mi, m 2 ) set to values listed in Table 4.
• the preparation sequence DP 4 in FIG. 21 has a different flip angle and the same phase. Therefore, only the flip angle of the preparation sequence DP 4 in FIG. 21 will be described.
• FIG. 22 is a diagram showing a preparation sequence DP 5 having ( ⁇ , ⁇ , ⁇ , ⁇ , ⁇ , mi, m 2 ) set to values listed in Table 5.
• FIG. 22(a) is a diagram explaining the composite pulse A.
• FIG. 22(b) is a diagram explaining the composite pulse B.
• FIG. 22(c) is a diagram explaining the composite pulse C.
• ⁇ 2 ⁇ 0 + 2 ⁇ + 90° x m 2
• the preparation sequences DP 4 and DP 5 in FIGS. 21 and 22 are capable of bringing longitudinal magnetization Mz of magnetization vectors back to a value close to one even in the presence of B0 and/or Bl inhomogeneity in the region to be imaged, similarly to the sequence DPi shown in FIG. 10.
• the values of ⁇ 1 and ⁇ 0 were changed, and simulations were conducted for studying the value of longitudinal magnetization of magnetization vectors immediately after performing the preparation sequences DP 4 and DP 5 in FIGS. 21 and 22 (immediately after applying the composite pulse C). Results of the simulations will be explained hereinbelow (see FIG. 23).
• FIG. 23 is a diagram showing results of the simulations.
• FIG. 23(a) is a diagram showing a result of the simulation for the preparation sequence DP 4 in FIG. 21.
• FIG. 23(a) shows an Mz map H3 representing the value of longitudinal magnetization immediately after performing the preparation sequence DP 4 .
• On the right of the Mz map M3 are shown two profiles F31 and F32.
• FIG. 23(b) shows a result of the simulation for the preparation sequence DP 5 in FIG. 22.
• FIG. 23(b) shows a Mz map H4 representing the value of longitudinal magnetization immediately after performing the preparation sequence DP5.
• On the right of the Mz map H4 are shown two profiles F41 and F42.
• the profile F42 represents Mz at
• the present invention may be configured to apply the composite pulse B n (> 1) times.
• An example in which the composite pulse B is applied n times will be explained hereinbelow.
• FIG. 24 is a diagram showing a preparation sequence DP 6 in which the composite pulse B is applied n times.
• the preparation sequence DP6 comprises a composite pulse A, n (> 1) composite pulses B, and a composite pulse C.
• n composite pulses B
• C composite pulse C
• FIG. 24(a) is a diagram explaining the composite pulse A.
• the composite pulse A is similar to that (see FIG. 9(a)) described earlier.
• the ⁇ ⁇ ⁇ pulse Poi is an RF pulse having a flip angle set to a and a phase set to (po (an axis rotated by an angle (po with respect to the x-axis within the xy-plane: (po-axis).
• the ⁇ pulse P02 is an RF pulse having a flip angle set to ⁇ and a phase set to ⁇ (an axis rotated by an angle ⁇ with respect to the x-axis within the xy-plane: ⁇ -axis).
• the k-th composite pulse Bk comprises a ek pulse Pki, a 2a ⁇ pk pulse Pk2, and a ek pulse Pk3.
• FIG. 24(b) is a diagram explaining the k-th composite pulse Bk.
• the 2a ⁇ pk pulse P k2 represents an RF pulse having a flip angle set to 2a and a phase set to 3 ⁇ 4 (an axis rotated by an angle 3 ⁇ 4 with respect to the x-axis within the xy-plane: (p k -axis).
• 3 ⁇ 4 is set to satisfy the following equation:
• 5k an angle in a range of 80° ⁇ 5k ⁇ 100°
• m k an integer.
• EQ. (14) is expressed by the following equation:
• the pek pulse (Pki and P ⁇ ) will be explained.
• the e k pulse (P k i and Pk 3 ) represents an RF pulse having a flip angle set to ⁇ and a phase set to 9 k (an axis rotated by an angle 9 k with respect to the x-axis within the xy-plane: 9 k -axis).
• 9 k is set to satisfy the following equation:
• ⁇ k in EQ. (17) is expressed by EQ. (15), similarly to ⁇ k in EQ. (13). Therefore, substituting EQ. (15) into EQ. (17), 9 k may be expressed by the following equation:
• FIG. 24(c) is a diagram explaining the composite pulse C.
• the composite pulse C is a flip-back pulse for bringing transverse magnetization of magnetization vectors back to longitudinal magnetization, and comprises a ⁇ ⁇ + ⁇ pulse P n +i,i and an ⁇ ⁇ + ⁇ pulse P n +i, 2 .
• the ⁇ + ⁇ pulse P n+ i,i represents an RF pulse having a flip angle set to ⁇ and a phase set to 9 n+ i (an axis rotated by an angle 9 n+ i with respect to the x-axis within the xy-plane: 9 n+ i-axis).
• 9 n+ i is set to satisfy the following equation:
• ⁇ ⁇ + ⁇ an angle in a range of 80° ⁇ ⁇ ⁇ + ⁇ ⁇ 100°
• the ⁇ ⁇ ⁇ + ⁇ pulse P n+ i,2 represents an RF pulse having a flip angle set to a and a phase set to ⁇ ⁇ + ⁇ (an axis rotated by an angle ⁇ ⁇ + ⁇ with respect to the x-axis within the xy-plane: (p n+ i-axis).
• ⁇ ⁇ + ⁇ is set to satisfy the following equation:
• ⁇ + ⁇ in EQ. (23) is expressed by EQ. (21), similarly to ⁇ dress + ⁇ in EQ. (19). Therefore, substituting EQ. (21) into EQ. (23), ⁇ ⁇ + ⁇ may be expressed by the following equation:
• the preparation sequence DP 6 shown in FIG. 24 is constructed as above.
• the value of n may be set to an arbitrary integer value.
• the preparation sequence DP 6 comprises a composite pulse A, composite pulses Bl, B2, and B3, and a composite pulse C.
• the flip angles and phases in the composite pulses are given as in FIG. 26.
• the preparation sequence is designated by symbol “DP 7 .”
• the preparation sequence DP7 shown in FIG. 26 will be explained hereinbelow.
• FIG. 26(a) is a diagram explaining the composite pulse A.
• FIG. 26(bl) is a diagram explaining the composite pulse Bi.
• FIG. 26(b2) is a diagram explaining the composite pulse B2.
• ⁇ 2 ⁇ 0 + 2 ⁇ + 90° x m 2
• ⁇ 2 ⁇ 0 + 2 ⁇ + 90° m 2
• FIG. 26(b3) is a diagram explaining the composite pulse B 3 .
• ⁇ 3 ⁇ 0 + 3 ⁇ + 90° x m 3
• ⁇ 3 ⁇ 0 + 3 ⁇ + 90° x m 3
• FIG. 26(c) is a diagram explaining the composite pulse C.
• ⁇ 4 ⁇ 0 + 4 ⁇ + 90° x m 4
• the ⁇ 4 pulse P 41 is a pulse having a flip angle of ⁇
• FIG. 27 shows a preparation sequence DP 8 having phases set to the values in EQS. (34)— (37). It can be seen from FIG. 27 that (po— ⁇ 4 are set so that 0° (the x-axis) and 90° (the y-axis) alternately appear, while ⁇ — ⁇ 4 are set so that 100° and 190° alternately appear.
• the preparation sequence DP 8 in FIG. 27 is capable of bringing longitudinal magnetization Mz of magnetization vectors back to a value close to one even in the presence of B0 and/or Bl inhomogeneity in the region to be imaged. To verify this, the values of ⁇ 0 and ⁇ 1 were changed, and simulations were conducted for studying the value of longitudinal magnetization of magnetization vectors immediately after performing the preparation sequence DPs in FIG. 27 (immediately after applying the composite pulse C). A result of the simulation will be explained hereinbelow (see FIG. 28).
• FIG. 28 is a diagram showing a result of the simulation.
• FIG. 28 shows an Mz map H5 representing the value of longitudinal magnetization immediately after performing the preparation sequence DPs in FIG. 27.
• On the right of the Mz map H5 are shown two profiles F51 and F52.
• may be set to
• 90°,
• 5 k and ⁇ ⁇ + ⁇ are not limited to 90° insofar as they have values close to 90°.
• 5 k and ⁇ + i included in a range of 80° ⁇ 5 k , ⁇ ⁇ + ⁇ ⁇ 100° may provide a high-quality image.
• the values of 5 k and ⁇ ⁇ + ⁇ may be set differently according to the values of k and n+1.
• the preparation sequences DP 0 — DPs each have one gradient magnetic field (MPG) between a composite pulse and a next composite pulse.
• MPG gradient magnetic field
• a plurality of gradient magnetic fields may be provided between a composite pulse and a next composite pulse.
• the plurality of gradient magnetic fields may have the same polarity or different polarities.
• the plurality of gradient magnetic fields may be provided as bipolar gradient magnetic fields comprised of a gradient magnetic field with positive polarity and that with negative polarity.
• no gradient magnetic field may be provided between a composite pulse and a next composite pulse.
• a killer gradient magnetic field for eliminating transverse magnetization may be applied immediately after applying the composite pulse C.
• an RF pulse for achieving fat suppression and/or T2 weighting may be applied before applying the composite pulse A.
• preparation sequences DP 0 — DPs may be performed in synchronization with physiological signals such as heart beat signals and/or respiration signals.

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• 2014
  • 2014-08-29 JP JP2014176550A patent/JP6258162B2/ja not_active Expired - Fee Related
• 2015
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