JP5758230B2 - Magnetic resonance imaging apparatus and inversion RF pulse phase control method - Google Patents

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as “NMR”) signals from hydrogen, phosphorus, etc. in a subject, and images nuclear density distribution, relaxation time distribution, etc. It relates to diffusion-weighted imaging in an apparatus called “MRI”.

  An MRI apparatus is an apparatus that detects an echo signal generated in a subject by applying a high-frequency magnetic field (RF) pulse to the subject placed in a static magnetic field, and processes the signal to image it.

  There are many echo signal measurement methods such as spin echo (SE) method and gradient echo (GE) method in MRI apparatuses. As one of the echo signal measurement methods, there is a diffusion weighted imaging (DWI) method in which an image is acquired by reflecting the intensity of Brownian motion of water molecules in the tissue of a subject in the echo signal intensity.

  In the DWI method, generally, an MPG pulse (Motion Probing Gradient) pulse, which is a high-intensity gradient magnetic field pulse, is applied after applying an excitation RF pulse and after applying an inverted RF pulse. This MPG pulse is applied, and an echo signal for reconstructing one diffusion weighted image by applying different phase encoding amounts is measured.

  This measurement unit that measures echo signals mainly uses the Echo Planar Imaging (EPI) method, which enables high-speed imaging, but Fast Spin Echo (FSE), which is considered to be resistant to image distortion due to static magnetic field inhomogeneity. It is also feasible in law.

  In the DWI method using the FSE method, it is known to use the PROPELLER method for imaging while turning in k-space as in [Non-patent Document 1]. In the PROPELLER method, the phase disturbance of the echo signal due to body movement does not form an image in a specific direction, so that the artifact can be made inconspicuous.

  Furthermore, the FSE method enables imaging using Carr Purcell Meiboom Gill (CPMG) conditions. FSE performs nuclear spin refocusing several times with an inverted RF pulse with a flip angle of 180 ° after excitation of the nuclear spin with an excitation RF pulse with a flip angle of 90 °. At that time, inversion by the inversion RF pulse may cause a component whose flip angle is not exactly 180 °. The CPMG method is applied so that the phase of the excitation RF pulse and the inversion RF pulse are orthogonal (that is, the phase of the excitation RF pulse and the inversion RF pulse is 90 degrees different) so that the inversion error of such components is not cumulatively added. It is.

  However, in the DWI method, it is necessary to apply a high-intensity MPG pulse in order to impart phase rotation to the magnetization of water molecules with an extremely slow flow rate. For this reason, the nuclear spin changes locally due to irregular magnetic fields such as eddy currents and remanent magnetization caused by the application of MPG pulses and other gradient magnetic fields, and the relationship between the phase of the excitation RF pulse and the inversion RF pulse is the CPMG method. It is difficult to maintain the conditions specified by the CPMG law.

  On the other hand, as in [Patent Document 1] and [Non-Patent Document 2], in the rotating coordinate system of nuclear spins, the phase of the second and subsequent inverted RF pulses is determined based on the first inverted RF pulse. The second has an orthogonal phase, the third has the same phase as the first, the fourth has the same phase as the second, and the even and odd phases have orthogonal phases. As described above, a method of applying an inversion RF pulse and corresponding to a local spin phase change is used.

  However, the phase difference between the even-numbered echo signal and the odd-numbered echo signal needs to be corrected due to the phase difference between the even-numbered and odd-numbered inverted RF pulses. Therefore, two even-numbered echo signals and two odd-numbered echo signals are measured as phase reference signals without applying phase encoding. Based on the phase between even-numbered echo signals and odd-numbered echo signals, the even-numbered echo signals and odd-numbered echo signals in this measurement are phase-corrected to reduce image quality degradation due to local spin phase changes caused by MPG pulse application. It is possible to reduce.

US Pat. No. 6,882,148

Pipe JG, `` Motion Correction With PROPELLER MRI, Application to Head Motion and Free Breathing Cardiac Imaging '', Magenetic Resonance in Medichine, p.963-969, Vol.42, 1999 Pipe JG, Farhing VG, Forbes KP, `` Multiple Diffusion-Weighted FSE Using PROPELLER MRI '', Magenetic Resonance in Medichine, p. 45-52, Vol. 47, 2002

  However, as the simulation described in [Patent Document 1] [Non-Patent Document 2], if the flip angle of the inverted RF pulse is not tilted exactly 180 degrees, it depends on the initial phase of the first inverted RF pulse. Thus, the detected echo signal value changes. In particular, the change in the echo signal value detected third is large. Therefore, the S / N varies depending on the initial phase. In addition, when an MPG pulse is applied, the optimum initial phase cannot be determined due to the influence of eddy currents.

  Therefore, an object of the present invention has been made in view of the above problems, and in the DWI method for applying an MPG pulse, an MRI apparatus capable of acquiring an image with a stable S / N irrespective of eddy currents and residual magnetic fields, and An inversion RF pulse phase control method is provided.

  In order to achieve the above-described object, the present invention is configured to rotate each blade of a plurality of parallel trajectories around the origin in the k space and measure each echo data of the hybrid radial type FSE sequence that measures echo data of each blade. Controls the initial phase of the pulse.

  Specifically, an optimal initial phase is obtained in advance by preliminary measurement or the like, and the initial phase of the inverted RF pulse or the excitation RF pulse is set. At that time, the initial phase of the inversion RF pulse or the excitation RF pulse is set differently for each blade. Alternatively, the initial phase of the inversion RF pulse or excitation RF pulse in all blades is made the same.

  According to the MRI apparatus and the inversion RF pulse phase control method of the present invention, an image can be acquired with a stable S / N irrespective of eddy current or residual magnetic field in the DWI method in which an MPG pulse is applied.

The block diagram of the whole basic composition in one Example of the MRI apparatus which concerns on this invention Graph showing the relationship between the initial phase value (δ) of the inverted RF pulse and S / N when the MPG pulse is applied to the same slice in the same direction with the same applied intensity Graph showing the relationship between the initial phase value (δ) of the inverted RF pulse and S / N when the MPG pulse application direction is changed The image 401 at the initial phase 201 and the image 402 at the initial phase 202 shown in FIG. Sequence chart of an example of a hybrid radial type DW-FSE sequence Sequence chart of an example of the hybrid radial type DW-FSE sequence of Example 1 Functional block diagram (a) of the arithmetic processing unit 8 is a functional block diagram of the first and third embodiments, and (b) is a functional block diagram of the second embodiment. Flowchart showing the processing flow of the first embodiment Flowchart showing the processing flow of the second embodiment Graph showing the relationship between the initial phase δ0 of the inverted RF pulse of each blade and the S / N of the measured echo signal in Example 2 The image 1101 at the initial phase 1001 and the image 1102 at the initial phase 1002 shown in FIG. Flowchart showing the processing flow of the third embodiment

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  First, an overall outline of an example of an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention. This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject.As shown in FIG. 1, the MRI apparatus includes a static magnetic field generation system 2, a gradient magnetic field generation system 3, a transmission system 5, The receiving system 6, the signal processing system 7, the measurement control unit 4, and the arithmetic processing unit 8 are configured.

  The static magnetic field generation system 2 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 1 if the vertical magnetic field method is used, and in the direction of the body axis if the horizontal magnetic field method is used. Thus, a permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the subject 1.

  The gradient magnetic field generating system 3 includes a gradient magnetic field coil 9 wound in the three-axis directions of X, Y, and Z, which is a coordinate system (stationary coordinate system) of the MRI apparatus, and a gradient magnetic field power source 10 that drives each gradient magnetic field coil. The gradient magnetic field Gx, Gy, Gz is applied in the three axis directions of X, Y, and Z by driving the gradient magnetic field power supply 10 of each coil in accordance with a command from the measurement control unit 4 described later. At the time of imaging, a slice direction gradient magnetic field pulse (Gs) 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 measurement control unit 4 is a control unit that repeatedly applies a high-frequency magnetic field pulse (hereinafter referred to as an “RF pulse”) and a gradient magnetic field pulse in a certain predetermined pulse sequence, operates under the control of the arithmetic processing unit 8, and Various commands necessary for tomographic image data collection are sent to the transmission system 5, the gradient magnetic field generation system 3, and the reception system 6.

  The transmission system 5 irradiates the subject 1 with RF pulses 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 includes a high frequency oscillator 11, a modulator 12, and a high frequency amplifier. 13 and a high frequency coil (transmission coil) 14a on the transmission side. The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the measurement control unit 4, and the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13 and then close to the subject 1. The RF pulse is irradiated to the subject 1 by supplying it to the arranged high frequency coil 14a.

The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 1, and receives a high-frequency coil (receiving coil) 14b on the receiving side and a signal amplifier 15 And a quadrature phase detector 16 and an A / D converter 17. After the NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15, The signals are divided into two orthogonal signals by the quadrature phase detector 16 at the timing according to the command from the measurement control unit 4, converted into digital quantities by the A / D converter 17, and sent to the signal processing system 7.
The signal processing system 7 performs various data processing and display and storage of processing results, and has an external storage device such as an optical disk 19 and a magnetic disk 18 and a display 20 composed of a CRT, etc. Is input to the arithmetic processing unit 8, the arithmetic processing unit 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 20, Recording is performed on the magnetic disk 18 or the like of the storage device.

  The operation unit 25 inputs various control information of the MRI apparatus and 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 20, and the operator controls various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.

  At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used clinically. By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

  First, the relationship between the initial phase value (δ) of the inverted RF pulse and the S / N of the measured echo signal will be described. FIG. 2 is a graph showing the relationship between the initial phase value (Δ) of the inverted RF pulse and S / N when the MPG pulse is applied to the same slice in the same direction with the same applied intensity. FIG. 3 is a graph showing the relationship between the initial phase value (Δ) of the inverted RF pulse and S / N when the application direction of the MPG pulse is changed. □ indicates the case where an MPG pulse is applied in the AP (Anterior-Posterior) direction, Δ indicates the RL (Right-Left) direction, and ○ indicates the HF (Head-Feet) direction. In both cases, the horizontal axis represents the initial phase value (δ), and the vertical axis represents the S / N of the echo signal.

  From FIG. 2, it is understood that the S / N is the highest at the initial phase 201 and the S / N is the lowest at the initial phase 202. FIG. 4 shows an image 401 at the initial phase 201 and an image 402 at the initial phase 202. 2 and 3, it is understood that the relationship between the initial phase and S / N changes due to the influence of the MPG pulse, and the manner of the change also depends on the application direction of the MPG pulse.

  According to the present invention, when the phase of the inverted RF pulse is orthogonal at an odd number and an even number, the S / N does not change due to the difference in the initial phase as shown in FIG. By controlling the initial phase of the inverted RF pulse, a stable image quality is provided regardless of the application intensity, application direction, and slice position of the MPG pulse.

  Therefore, the present invention sets the initial phase of the inverted RF pulse or the excitation RF pulse for each blade so that the SN does not change due to the difference in the initial phase. Note that changing the initial phase of the inverted RF pulse has the same meaning as changing the initial phase of the excitation RF pulse.

  Hereinafter, each example of the present invention is described in detail.

(DWI sequence)
First, a hybrid radial type DW-FSE sequence (hereinafter referred to as a Hybrid DWFSE sequence) according to the present invention will be described with reference to a sequence chart shown in FIG. The sequence chart of FIG. 5 shows a pulse sequence for measuring an echo signal of a blade whose rotation angle is zero (that is, parallel to the kx axis of k space). When measuring the echo signals of blades at other rotation angles, the gradient magnetic field pulse waveforms shown in this sequence chart may be distributed to three axes according to the rotation angles.

  In the sequence chart of FIG. 5, RF, MPG, Gs, Gp, Gf, and Echo represent an RF pulse, an MPG pulse, a slice gradient magnetic field, a phase encode gradient magnetic field, a frequency encode gradient magnetic field, and an echo signal, respectively. A 90-degree excitation RF pulse 71 and a slice selective gradient magnetic field 78 are applied together to excite a desired slice. Next, the first MPG pulse 76 is applied to one or more of the slice axis, the phase encode axis, and the frequency encode axis. The intensity of Brownian motion of water molecules in the axial direction to which the MPG pulse is applied is reflected in the echo signal intensity.

  Further, a phase pulse 81 is applied to the frequency encode axis, and the phase of the excited spin is dispersed in advance. The application amount of the phase pulse 81 is ½ of the application amount of a readout gradient magnetic field 83 and the like which will be described later. Next, an inversion RF pulse 72 and a slice selective gradient magnetic field 79 are applied to invert the spin phase of the same slice by 180 degrees. Next, the second MPG pulse 77 having the same application amount as that of the first MPG pulse 76 is applied to the same axis as the application axis of the first MPG pulse 76. As a result, the phase of the stationary spin dispersed by the first MPG pulse 76 returns to its original state, but the phase dispersion of the water molecules moving in Brownian motion does not return to zero, and the state is reflected in the echo signal. Is done. Next, the echo signal 83 is measured by applying a phase encode pulse 80 having a predetermined phase encode amount and a read gradient magnetic field 82.

  After measuring the echo signal 83, a phase encode pulse having the opposite polarity to the applied phase encode pulse 80 is applied to cancel the application amount of the phase encode pulse 80 (return to zero). Then, an inversion RF pulse 73 and a slice selection gradient magnetic field 79 are applied to invert the spin phase of the same slice by 180 degrees. Similarly, the phase encode pulse 80 and the read gradient magnetic field 82 are applied, and the echo signals 84, 85, 86,... Are measured for each inverted RF pulse 74, 75,.

  A first embodiment of the MRI apparatus and the inversion RF pulse phase control method of the present invention will be described. The first embodiment uses the initial phase of the inverted RF pulse for each blade and the phase of the reference echo signal measured by the previous blade. Hereinafter, the present embodiment will be described with reference to the drawings.

(Inverted RF pulse phase setting)
First, the phase calculation of the inversion RF pulse of the jth (j = 1, 2,..., Total blade number) blade in the FSE sequence will be described. The following phases are set for each inverted RF pulse.

  (Y + α + δj) is set as the phase of the first inversion RF pulse 72. Here, Y is a phase value orthogonal to the phase of the excitation RF pulse 71. Α is a phase value that changes between the application center of the excitation RF pulse 71 and the application center of the first inversion RF pulse 72. Further, δj is an initial phase of the inversion RF pulse set for the jth blade.

  (X + 2α + β / 2 + δj) is set as the phase of the second inversion RF pulse 73. Here, X is the phase value of the excitation RF pulse 71. Β is a phase that changes at the time of inter-echo time, which is the application interval of the inverted RF pulse.

  (Y + 2α + 3β / 2 + δj) is set as the phase of the third inversion RF pulse 74.

(X + 2α + 5β / 2 + δj) is set as the phase of the fourth inversion RF pulse 75.
Thereafter, the same setting is made as follows.

Phase of (2n + 1) th inverted RF pulse = (Y + 2α + β / 2 + β · 2n + δj),
Phase of (2n) th inverted RF pulse = (X + 2α + β / 2 + β · 2 (n-1) + δj) (1)
Set. Here, n = 1, 2, 3.

  Alternatively, instead of setting the initial phase of the inverted RF pulse, -δj may be set as the initial phase of the excitation RF pulse.

Next, Δj set for the jth blade will be described. Set an appropriate value for δ1 of the first blade. However, Δ1 = π / 4 is preferable as the optimum theoretical value of the initial phase. Furthermore, if the phase of the even echo signal and the odd echo signal, which are reference echo signals detected after the measurement of the jth blade, is R1j and R2j, δj + 1 = (R1j-R2j) / 2 + π / 4 (2)
Is the optimal initial phase.

  That is, the initial phase of the inversion RF pulse is set for each blade. As the initial phase, the phase of the reference echo signal measured by the immediately preceding blade is used.

  Thus, by setting the phase, it is possible to provide stable image quality regardless of the application intensity, application time, and slice of the MPG pulse.

(Description of pulse sequence)
An example of the HybridDWFSE sequence according to the present embodiment is shown in FIG. The sequence chart shown in FIG. 6 does not apply the phase encode pulse 80 in order to measure the first two echo signals as reference echo signals described later in the sequence chart of the HybridDWFSE sequence shown in FIG. Others are the same as those in the sequence chart of FIG.

(Function block diagram)
Next, each function of the arithmetic processing unit 8 for performing measurement of the HybridDWFSE sequence according to the present embodiment will be described with reference to a functional block diagram shown in FIG. The function of the arithmetic processing unit 8 of this embodiment includes an initial phase setting unit 701 and an inverted pulse phase setting unit 702.

  The initial phase setting unit 701 sets the initial phase (Δj) of the inverted RF pulse of the jth blade based on the equation (2). For example, as described above, π / 4 is preferable as a theoretical value as the initial phase (Δ1) of the inversion RF pulse of the first blade. Then, using the reference echo signal measured by the jth blade, the initial phase (Δj + 1) of the inverted RF pulse of the (j + 1) th blade is set based on the equation (2).

  The inverted pulse phase setting unit 702 is based on the initial phase (δj) of the inverted RF pulse of the jth blade set by the initial phase setting unit 701 and the equation (1), and the phase of each inverted RF pulse of the jth blade. Set. Sets the phase of each inversion RF pulse of the jth blade.

(flowchart)
Next, the processing flow of the present embodiment performed in cooperation with the above functions will be described based on the flowchart shown in FIG.

  In step 801, the inverted RF pulse phase setting unit 702 sets 1 to the blade counter (j).

  In step 802, the inversion RF pulse phase setting unit 702 compares the blade counter (j) with the total number of blades, and when the blade counter (j) is less than the total number of blades, the process proceeds to step 803, and the above is completed. In this case, measurement of the HybridDWFSE sequence is terminated.

  In step 803, the initial phase setting unit 701 sets the initial phase (Δj) of the inverted RF pulse of the jth blade. The setting method is as described above. In particular, the initial phase setting unit 701 sets the initial phase (Δj + 1) of the inverted RF pulse of the (j + 1) th blade using the reference echo signal measured by the jth blade in step 805 described later.

  In step 804, the inverted RF pulse phase setting unit 702 sets the phase of each inverted RF pulse of the jth blade based on the initial phase (Δj) of the inverted RF pulse of the jth blade and Equation (1). Then, the inverted RF pulse phase setting unit 702 notifies the measurement control unit 4 of the phase of each inverted RF pulse of the set jth blade.

  In step 805, the measurement control unit 4 measures the echo signal of the jth blade while controlling the phase of each inverted RF pulse to be the phase of each inverted RF pulse notified in step 804. Subsequently, the reference echo signals of the even echo signal and the odd echo signal are measured. Then, the measured reference echo signal data is notified to the inverted RF pulse phase setting unit 702.

In step 806, the inverted RF pulse phase setting unit 702 increments the blade counter (j) and proceeds to step 802 and thereafter.
The above is the outline of the processing flow of the first embodiment.

  According to the processing flow of the first embodiment, the image 401 in FIG. 4 can be stably obtained at the SN 201 in FIG.

  As described above, the MRI apparatus and the inverted RF pulse phase control method of the first embodiment use the initial phase of the inverted RF pulse for each blade and the phase of the reference echo signal measured by the previous blade. As a result, in the DWI method in which an MPG pulse is applied, an image can be acquired with a stable S / N regardless of eddy current or residual magnetic field.

  A second embodiment of the MRI apparatus and the inversion RF pulse phase control method of the present invention will be described. The second embodiment basically has the same configuration as that of the first embodiment. However, a preliminary measurement without applying phase encoding is performed, and the initial phase set for the inverted RF pulse is calculated using the phase of the reference echo signal measured in the preliminary measurement, and the calculated initial phase is applied to all blades regardless of the blade. Applies in common. Hereinafter, the second embodiment will be described in detail.

(Inverted RF pulse phase setting)
First, a method of calculating the phase of the inverted RF pulse that is commonly applied to all blades of the HybridDWFSE sequence will be described.

Preliminary measurement is performed to measure the reference echo signal without applying phase encoding, and the phase of the detected even echo signal and odd echo signal is R1p and R2p, respectively. Δ = (R1p-R2p) / 2 + π / 4 ( 3)
Is the initial phase of the inverted RF pulse. Using this phase, the phase of each inverted RF pulse for all blades is calculated as follows:

  Set (Y + α + δ) as the phase of the first inversion RF pulse. Here, Y is a phase value orthogonal to the phase of the excitation RF pulse. Α is a phase changing between the application center of the excitation RF pulse and the application center of the inversion RF pulse.

Set (X + 2α + β / 2 + δ) as the phase of the second inverted RF pulse. X is the phase of the excitation RF pulse. Β is a phase that changes at the time of inter-echo time, which is the application interval of the inverted RF pulse.
Set (Y + 2α + 3β / 2 + δ) as the phase of the third inverted RF pulse.
Set (X + 2α + 5β / 2 + δ) as the phase of the fourth inversion RF pulse.
Thereafter, the same setting is made as follows.

Phase of (2n + 1) th inverted RF pulse = (Y + 2α + β / 2 + β · 2n + δ),
Phase of (2n) th inverted RF pulse = (X + 2α + β / 2 + β · 2 (n-1) + δ) (4)
Set. Here, n = 1, 2, 3. That is, the calculated initial phase is applied to all blades regardless of the blades. The initial phase of the present embodiment is obtained using the phase of the reference echo signal measured in the preliminary measurement without applying phase encoding.

  In addition, instead of setting the initial phase of the inverted RF pulse, -δ may be set as the initial phase of the excitation RF pulse.

  By setting the above phases for each plate RF pulse, it is possible to provide stable image quality regardless of the application intensity, application time, and slice of the MPG pulse.

(Explanation of pulse sequence)
The HybridDWFSE sequence of this embodiment is the same as the pulse sequence shown in FIG. Further, the preliminary measurement sequence of the present embodiment is one in which the phase encode 80 is not applied in the pulse sequence shown in FIG. The rest is the same as the pulse sequence shown in FIG.

(Function block diagram)
Next, each function of the arithmetic processing unit 8 for performing measurement of the HybridDWFSE sequence according to the present embodiment will be described with reference to a functional block diagram shown in FIG. 7B. The function of the arithmetic processing unit 8 of the present embodiment includes a preliminary measurement setting unit 703 in addition to the functions shown in FIG. 7A described in the first embodiment.

  The initial phase setting unit 701 sets the initial phase (δ) of the inverted RF pulse that is commonly applied to all blades based on the equation (3).

  The inversion pulse phase setting unit 702 sets the phase of each inversion RF pulse of all blades based on the initial phase (δ) of the inversion RF pulse of all blades set by the initial phase setting unit 701 and Equation (4). To do.

  The preliminary measurement setting unit 703 generates control data of the preliminary measurement sequence in which the phase encoding is not applied with the same MPG pulse application intensity, application time, and slice slice as in the main measurement, and notifies the measurement control unit 4 of the control data.

(flowchart)
Next, the processing flow of the present embodiment performed in cooperation with the above functions will be described based on the flowchart shown in FIG.

  In step 901, the preliminary measurement setting unit 703 generates control data for the preliminary measurement sequence and notifies the measurement control unit 4 of the control data. The measurement control unit 4 executes a preliminary measurement sequence using the notified control data, and notifies the initial phase setting unit 701 of the measured echo data of the reference echo signal.

  In step 902, the initial phase setting unit 701 uses the echo data of the preliminary measurement sequence notified in step 901 to calculate the initial phase of the inverted RF pulse that is commonly applied to all blades based on Equation (3). To do.

  In step 903, the inverted RF pulse phase setting unit 702 calculates and sets the phase of each inverted RF pulse based on the initial phase (δ) of the inverted RF pulse calculated in step 902 and Equation (4). .

  In step 904, the inverted RF pulse phase setting unit 702 applies the phase of each inverted RF pulse obtained in step 903 to the phase of the inverted RF pulse in the Hybrid DWFSE sequence of all blades. Then, the inverted RF pal phase setting unit 702 notifies the measurement control unit 4 of the phase of the inverted RF pulse of each blade in the HybridDWFSE sequence.

In step 905, the measurement control unit 4 measures echo signals of all blades.
The above is the outline of the processing flow of the first embodiment.

  Also in the above-described processing flow of the second embodiment, the image 401 in FIG. 4 can be stably obtained at the SN 201 in FIG.

  As described above, the MRI apparatus and the inverted RF pulse phase control method of the second embodiment perform preliminary measurement without applying phase encoding, and set the inverted RF pulse using the phase of the reference echo signal measured in the preliminary measurement. The initial phase to be calculated is calculated, and the calculated initial phase is commonly applied to all blades regardless of the blade. As a result, in the DWI method in which MPG pulses are applied, the calculation and setting of the initial phase of the inverted RF pulse during imaging can be simplified compared to Example 1, and the S is stable regardless of eddy currents or residual magnetic fields. Images can be acquired with / N.

  A third embodiment of the MRI apparatus and the inversion RF pulse phase control method of the present invention will be described. The third embodiment basically has the same configuration as that of the first embodiment. However, the initial phase from 0 to π is set equally for all blades. Hereinafter, the third embodiment will be described in detail.

(Inverted RF pulse phase setting)
First, the phase calculation of the inverted RF pulse of the jth (j = 1, 2,..., Total blade number) blade in the HybridDWFSE sequence will be described.

  The calculation method of the phase of the inverted RF pulse of the jth blade is the same as the calculation method described in the first embodiment. Specifically, it is calculated based on the equation (1). Therefore, detailed description is omitted. The difference from the first embodiment is the following calculation method of the initial phase Δj of the j-th blade.

The initial phase δj of the inverted RF pulse of the jth (j = 1,2, ..., total blades) blade is
δj = (π / total number of blades) × n + δ0 (6)
And That is, the initial phase from 0 to π is set uniformly for all blades. However, δ0 is an initial value common to all blades.

  Alternatively, instead of setting the initial phase of the inverted RF pulse, -δj may be set as the initial phase of the excitation RF pulse.

  FIG. 10 shows the relationship between Δ0 and the S / N of the measured echo signal. The horizontal axis represents the initial phase value (δ), and the vertical axis represents the S / N of the echo signal. It is understood that the S / N is stable regardless of δ0. FIG. 11 shows an image 1101 at the initial phase 1001 and an image 1102 at the initial phase 1002.

  By uniformly setting the initial phase of 0 to π on all the blades, it is possible to provide stable image quality regardless of the application intensity, application time, and slice position of the MPG pulse.

(Function block diagram)
Next, each function of the arithmetic processing unit 8 for measuring the HybridDWFSE sequence according to the present embodiment will be described. The function of the arithmetic processing unit 8 of the present embodiment is the same as the function block diagram of the first embodiment described with reference to FIG. 7A, but the processing content of each function is different. Hereinafter, the processing content of each functional unit will be described through the description of the processing flow of the third embodiment.

  The initial phase setting unit 701 sets the initial phase (Δj) of the inverted RF pulse for each blade based on the equation (6).

  The inversion pulse phase setting unit 702 sets the phase of each inversion RF pulse of each blade based on the initial phase (δj) of the inversion RF pulse of each blade set by the initial phase setting unit 701 and Equation (1). To do.

(flowchart)
Next, the processing flow of the present embodiment performed in cooperation with the above functions will be described based on the flowchart shown in FIG.

  In step 1201, the initial phase setting unit 701 sets the initial phase of the inverted RF pulse for each blade based on equation (6).

  In step 1202, the inverted RF pal phase setting unit 702 uses the initial phase of the inverted RF pulse for each blade set in step 1101, and calculates the phase of each inverted RF pulse for each blade based on Equation (1). Set. Then, the inverted RF pal phase setting unit 702 notifies the measurement control unit 4 of the phase of each inverted RF pulse of the set jth blade.

  In step 1203, the measurement control unit 4 measures the echo signal of each blade while controlling the phase of each inversion RF pulse of each blade to be the phase of each inversion RF pulse notified in step 1202.

  The above is the outline of the processing flow of the first embodiment.

  By performing the above processing flow of the third embodiment, the image 1101 or 1102 of FIG. 11 can be stably obtained at the SN of FIG.

  As described above, the MRI apparatus and the inversion RF pulse phase control method according to the second embodiment set the initial phases of 0 to π uniformly for all the blades. As a result, in the DWI method to apply the MPG pulse, compared to Examples 1 and 2, the S / N is stable regardless of eddy current and residual magnetic field, without calculating and setting the initial phase of the inverted RF pulse. It becomes possible to acquire an image.

  1 subject, 2 static magnetic field generation system, 3 gradient magnetic field generation system, 4 measurement control unit, 5 transmission system, 6 reception system, 7 signal processing system, 8 operation processing unit, 9 gradient magnetic field coil, 10 gradient magnetic field power supply, 11 High-frequency transmitter, 12 modulator, 13 high-frequency amplifier, 14a high-frequency coil (transmitting coil), 14b high-frequency coil (receiving coil), 15 signal amplifier, 16 quadrature detector, 17 A / D converter, 18 magnetic disk, 19 Optical disc, 20 Display, 21 ROM, 22 RAM, 23 Trackball or mouse, 24 Keyboard, 71 Excitation RF pulse, 72 1st inversion RF pulse, 73 2nd inversion RF pulse, 74 3rd inversion RF pulse, 75 4th inversion RF pulse, 76 forward MPG pulse, 77 backward MPG pulse, 78 excitation RF slice selection magnetic field pulse, 79 inversion RF slice selection magnetic field pulse, 80 phase encoding pulse, 81 phase pulse, 82 frequency encoding pulse, 83 first echo, 84 2nd echo, 85 3rd echo, 86 4th echo

Claims (7)

an arithmetic processing unit for setting a phase of each inversion RF pulse of a hybrid radial type high-speed spin echo sequence for measuring echo data of each blade by rotating a blade having a plurality of parallel trajectories around the origin in k-space; ,
A measurement control unit for measuring the echo data of each blade by setting the phase of each inversion RF pulse of the high-speed spin echo sequence to the set value;
A magnetic resonance imaging apparatus comprising:
The arithmetic processing unit includes:
An initial phase setting unit for setting an initial phase of the inversion RF pulse or excitation RF pulse;
An inverted RF pulse phase setting unit that sets the phase of the second and subsequent inverted RF pulses based on the initial phase, and
The magnetic resonance imaging apparatus, wherein the initial phase setting unit sets the initial phase of the inversion RF pulse differently for each blade. The magnetic resonance imaging apparatus according to claim 1.
The magnetic resonance imaging apparatus, wherein the initial phase setting unit sets an initial phase of the inverted RF pulse in one blade using a phase of a reference echo signal measured by the immediately preceding blade. an arithmetic processing unit for setting a phase of each inversion RF pulse of a hybrid radial type high-speed spin echo sequence for measuring echo data of each blade by rotating a blade having a plurality of parallel trajectories around the origin in k-space; ,
A measurement control unit for measuring the echo data of each blade by setting the phase of each inversion RF pulse of the high-speed spin echo sequence to the set value;
A magnetic resonance imaging apparatus comprising:
The arithmetic processing unit includes:
An initial phase setting unit for setting an initial phase of the inverted RF pulse;
An inverted RF pulse phase setting unit that sets the phase of the second and subsequent inverted RF pulses based on the initial phase, and
The magnetic resonance imaging apparatus, wherein the initial phase setting unit makes the initial phase of the inverted RF pulse in all blades the same. The magnetic resonance imaging apparatus according to claim 3.
The arithmetic processing unit further includes a preliminary measurement setting unit that generates control data of a preliminary measurement sequence that does not apply phase encoding,
The measurement control unit measures the reference echo signal by executing the preliminary measurement sequence,
The magnetic resonance imaging apparatus, wherein the initial phase setting unit sets an initial phase of the inverted RF pulse using a phase of the reference echo signal. The magnetic resonance imaging apparatus according to claim 1.
The initial phase setting unit sets an initial phase of 0 to π uniformly for all blades. Inverted RF pulse phase for controlling the phase of each inverted RF pulse in a hybrid radial type high-speed spin echo sequence in which the blade data consisting of a plurality of parallel trajectories is rotated around the origin in k-space to measure echo data of each blade A control method,
An initial phase setting step for setting an initial phase of the inversion RF pulse or excitation RF pulse;
An inverted RF pulse phase setting step for setting the phase of the second and subsequent inverted RF pulses based on the initial phase;
With
The inversion RF pulse phase control step is characterized in that the inversion RF pulse phase setting step sets the initial phase of the inversion RF pulse different for each blade. Inverted RF pulse phase for controlling the phase of each inverted RF pulse in a hybrid radial type high-speed spin echo sequence in which the blade data consisting of a plurality of parallel trajectories is rotated around the origin in k-space to measure echo data of each blade A control method,
An initial phase setting step for setting an initial phase of the inverted RF pulse;
An inverted RF pulse phase setting step for setting the phase of the second and subsequent inverted RF pulses based on the initial phase;
With
In the inversion RF pulse phase setting step, the initial phase of the inversion RF pulse in all blades is made the same.

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