JP3980396B2 - Magnetic resonance imaging system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic resonance imaging apparatus, and more particularly to a technique for obtaining a magnetic resonance imaging image (hereinafter referred to as “MRI image”) by applying a flip-back method to an IR sequence.
[0002]
[Prior art]
The flip-back method adds an RF refocusing pulse at the end of the SE sequence, etc., and an RF recovery pulse controlled in the opposite phase to the RF excitation pulse (with a phase difference of 180 °) to promote longitudinal relaxation and shorten the repetition time. It is a method to do. The flip back method is described in JP-A-60-177251.
[0003]
The IR sequence is a sequence in which an RF inversion pulse is added before the SE sequence or the like, and is known as one method for giving contrast of a living tissue during imaging. The IR sequence is described in “Introduction to MRI“ Re ”” p.184 (by Riki Araki, Nanedo).
[0004]
[Problems to be solved by the invention]
By applying the flip back method to the IR sequence, the repetition time can be shortened, so it is possible to significantly shorten the scan time and obtain an MRI image with high resolution and excellent image quality. In addition, when the phase of the RF recovery pulse is opposite to that of the RF excitation pulse as in the normal flip-back method, there are problems that artifacts occur in the MRI image and the S / N of the image is reduced. For this reason, the imaging method which applied the flip back method to IR sequence was not performed conventionally.
[0005]
An object of the present invention is to obtain a good MRI image by applying a flip-back method to an IR sequence.
[0006]
[Means for Solving the Problems]
The object is arranged in the static magnetic field including a static magnetic field generating means for generating a static magnetic field, an RF pulse generating means for generating an RF pulse in the static magnetic field, a first component and a second component. Measurement control means for measuring the nuclear magnetic resonance signal by applying the RF pulse to the subject based on a predetermined pulse sequence, the pulse sequence reversing the longitudinal magnetization of the subject. A pulse, an RF excitation pulse for generating transverse magnetization from the reversed longitudinal magnetization, an even number of RF refocusing pulses for refocusing the phase of the transverse magnetization, and recovering the refocused transverse magnetization to longitudinal magnetization An RF recovery pulse, and the measurement control means adjusts the phase of the RF recovery pulse in accordance with the positive and negative longitudinal magnetization at the application time point of the RF excitation pulse of the first component and the second component. System The magnetic resonance imaging apparatus, the measurement control unit, said have you to the application time point of the RF excitation pulse, the longitudinal magnetization of the first component is negative, when the longitudinal magnetization of the second component is positive The magnetic resonance imaging apparatus is characterized in that the phase of the RF recovery pulse is the same as the phase of the RF excitation pulse .
[0007]
The purpose of the measurement control unit is when the longitudinal magnetization of the first component is negative and the longitudinal magnetization of the second component is negative or zero at the time of application of the RF excitation pulse. Is achieved by a magnetic resonance imaging apparatus characterized in that the phase of the RF recovery pulse is the same as the phase of the RF excitation pulse .
[0008]
The purpose of the measurement control unit is when the longitudinal magnetization of the first component is positive or zero and the longitudinal magnetization of the second component is positive at the time of application of the RF excitation pulse. Is achieved by a magnetic resonance imaging apparatus characterized in that the phase of the RF recovery pulse is reversed from the phase of the RF excitation pulse .
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a first embodiment of the present invention will be described in detail with reference to the drawings. First, the configuration of the magnetic resonance imaging apparatus will be described with reference to FIG. In FIG. 1, the magnetic resonance imaging apparatus is roughly classified into a central processing unit (hereinafter abbreviated as CPU) 1, a sequencer 2, a transmission system 3, a static magnetic field generating magnet 4, a reception system 5, and signal processing. It consists of the system 6.
[0010]
The CPU 1 controls the sequencer 2, the transmission system 3, the reception system 5, and the signal processing system 6 according to a predetermined program. The sequencer 2 operates based on a control command from the CPU 1, and sends various commands necessary for collecting image data of the tomographic plane of the subject 7 to the transmission system 3, the gradient magnetic field generation system 21, and the reception system 5. ing.
[0011]
The transmission system 3 includes a high-frequency oscillator 8, a modulator 9, and an irradiation coil 11. The modulator 9 amplitude-modulates a reference high-frequency pulse output from the high-frequency oscillator 8 according to a command from the sequencer 2, and the amplitude modulation is performed. The high-frequency pulse is amplified via the high-frequency amplifier 10 and supplied to the irradiation coil 11, so that the subject is irradiated with a predetermined pulse-shaped electromagnetic wave.
[0012]
The static magnetic field generating magnet 4 is for generating a uniform static magnetic field in a predetermined direction around the subject 7. In the static magnetic field generating magnet 4, an irradiation coil 11, a gradient magnetic field coil 13, and a receiving coil 14 are arranged. The gradient magnetic field coil 13 is included in the gradient magnetic field generation system 21, receives a current supplied from the gradient magnetic field power supply 12, and generates a gradient magnetic field under the control of the sequencer 2. The reception system includes a reception coil 14, an amplifier 15 connected to the reception coil 14, a detection circuit 16, and an analog / digital converter (hereinafter referred to as ADC) 17, and receives an NMR signal from the subject 7. When the coil 14 detects, the NMR signal is converted into a digital quantity via the amplifier 15, the detection circuit 16, and the ADC 17, and is also converted into image reproduction data by the detection circuit 16 and the ADC 17 at a timing according to a command from the sequencer 2. , Is sent to CPU1.
[0013]
The signal processing system 6 includes an external storage device such as a magnetic disk 20 and an optical disk 19 and a display 18 such as a CRT. When data from the reception system 5 is input to the CPU 1, the CPU 1 performs signal processing and image re-processing. Processing such as configuration is executed, and the resulting image of the desired tomographic plane of the subject 7 is displayed on the display 18 and stored in the magnetic disk 20 of the external storage device or the like.
[0014]
Next, an IR sequence to which the flip-back method is applied will be described with reference to FIG. In FIG. 2, the RF pulse, time time, slice gradient magnetic field Gs, phase encoding gradient magnetic field Gp, and readout gradient magnetic field Gf are shown as time elapses from the top.
[0015]
In the RF pulse in FIG. 2, 101 is an RF inversion pulse, 102 is an RF excitation pulse, 103 and 104 are RF refocusing pulses, and 105 is an RF recovery pulse. Here, in the RF inversion pulse (180 °) y, RF excitation pulse (90 °) x, RF refocusing pulse (180 °) -y103, (180 °) and (90 °) are the excitation angles of nuclear spins (flip The subscript x, y, or -y indicates the irradiation phase of the RF pulse on the axis of the coordinate system in which the nuclear spin to be imaged rotates at the resonance frequency (Larmor frequency). A slice gradient magnetic field 116 is a gradient magnetic field for slice direction dephasing, and the phase of the nuclear spin is rephased by the first half of the slice selective gradient magnetic field pulse 117 that follows. Of the readout gradient magnetic field, 133 is a readout direction rephase pulse. The above is an example in which the flip-back method is applied to the IR sequence to shorten the repetition time.
[0016]
In this embodiment, when the RF recovery pulse is irradiated in the opposite phase to the RF excitation pulse as in the normal flip-back method, it takes time to recover reversely, and thus it takes a long time to repeat, and the MRI image There have been problems such as the occurrence of artifacts and the reduction in image S / N.
[0017]
In the first embodiment, the setting of the reversal time TI is classified into the following three types as the difference in the sign of the longitudinal magnetization of the object at the time TI, and the sign of the RF recovery pulse to be irradiated is To change. Details will be described below. However, this example is for imaging an object consisting of two tissues with different T ₁ values. Of the two tissues, the longer T ₁ value is assumed to be tissue 1, and the shorter one is assumed to be tissue 2. In addition, T ₂ value of tissue 1 is not short.
[0018]
Specifically, when CSF (cerebrospinal fluid) is assumed as the tissue 1, typically, T ₁ = 2000 ms and T ₂ = 300 ms.
(Category 1) Mz ₁ <0, Mz ₂ ≦ 0
(Category 2) Mz ₁ <0, Mz ₂ > 0
(Category 3) Mz ₁ ≧ 0, Mz ₂ > 0
In this classification, in the first embodiment, the phase direction of the RF recovery pulse 105 is the same as that of the RF irradiation pulse 102 (Category 1) In-phase (Category 2) In-phase
(Category 3) If the phase is reversed, the longitudinal magnetization is recovered quickly, so that the repetition time is shortened, and there is no problem that an artifact is generated in the MRI image or the S / N of the image is lowered.
[0019]
This will be explained individually for each classification.
In (Category 1), the magnetization vectors of tissue 1 and tissue 2 are in the -z direction at time Ta (or tissue 2 includes 0), so apply an RF excitation pulse (90 °) + x102 As a result, the magnetization vector points in the -y direction. The phase of tissue 1 and tissue 2 applied at this time is maintained even when the RF recovery pulse 105 is applied, so the RF recovery pulse (90 °) + x105 controlled to the same phase as the RF excitation pulse 102 is applied. Then, the magnetization vectors of the tissue 1 and the tissue 2 are both oriented in the + z direction. Note that the longitudinal relaxation components of the tissue 1 and the tissue 2 generated after the irradiation with the RF excitation pulse are irradiated with the RF refocusing pulses (180 °) 103 and 104, and thus become small and can be ignored.
[0020]
In (Category 2), as in (Category 1), the RF recovery pulse in phase with the RF excitation pulse 102 is used to direct the magnetization vector of tissue 1 that faces the -z direction at the Ta point in the + z direction. Irradiate (90 °) + x105. Here, since the magnetization vector of the tissue 2 is already in the + z direction at the time of TI (or 0), when the RF recovery pulse 105 is further irradiated to an object that is in the + y direction by the RF excitation pulse, −z In this case, since the longitudinal relaxation is fast, it will fully recover within the TR time.
FIG. 3 shows a schematic diagram showing the behavior of magnetization showing the state. In FIG. 3, the time indicated by t corresponds to FIG. However, phase dispersion is ignored in FIG.
[0021]
(step 1)
First, when an RF inversion pulse (180 °) y101 is irradiated, the nuclear spins of the two tissues are inverted at t = Ta, and longitudinal Mz ₁ 201 and Mz ₂ 202 are generated. (Fig. 3 (a))
[0022]
(Step 2)
Although the reversed magnetization is longitudinally relaxed, in the case of (Category 2), at the time t = Tb after the reversal, the longitudinal magnetization distribution 203 of the tissue 1 is Mz ₁ <0, and the longitudinal magnetization distribution 204 of the tissue 2 is Mz ₁ > 0. It becomes. (Fig. 3 (b))
[0023]
(Step 3)
Next, when an RF pulse (90 °) × 102 is irradiated, the magnetization vectors of the two tissues at time t = T _{c become} transverse magnetization components Mxy ₁ (<0) 205 and Mxy ₂ (> 0) 206. (Fig. 3 (c))
[0024]
(Step 4)
Next, when the refocusing pulse (180 °) -y103 is irradiated, the two magnetizations refocus at time t = _Td to generate an echo signal, but when the RF recovery pulse (180 °) -y104 is further irradiated, to re-converge at t = T _e. In this case, when irradiated with RF recovery pulse (90 °) x105 was controlled in the same phase as the RF excitation pulse (90 °) x102, longitudinal magnetization Mz ₁ tissue 1 towards T ₁ is long, the time t = T _f Positive and longitudinal relaxation is promoted. On the other hand, the longitudinal relaxation of the structure 2 having a shorter T ₁ is Mz ₂ <0 at t = T _f. However, since the T ₁ value is originally short, there is no problem because the longitudinal relaxation is easy.
[0025]
In (Category 3), since the magnetization vectors are both oriented in the + z direction at Ta time (or tissue 1 is 0), as in the normal flip-back method, the RF recovery pulse (90 °) Irradiate -x.
[0026]
If the phase of the RF recovery pulse is controlled as in this method, the longitudinal magnetization is recovered quickly, so the repetition time is shortened, and there are no problems such as artifacts in the MRI image or a decrease in the S / N of the image. .
[0027]
Next, a second embodiment of the present invention will be described with reference to the drawings. In this example, the flip-back method is applied to a high-speed spin echo type IR sequence, and the sequence is shown in FIG. FIG. 4 shows a time diagram showing a part of this sequence. In FIG. 4, 301 is an RF inversion pulse, 302 is an RF excitation pulse, 303, 304, 305 and 306 are RF refocusing pulses, and 307 is an RF recovery pulse. Of the slice gradient magnetic field, 318 is a slice direction phase pulse, and the phase of the nuclear spin is rephased by the first half of the slice selective gradient magnetic field pulse 319 that follows. Of the readout gradient magnetic field, 334 is a readout direction rephase pulse.
[0028]
In this sequence, if the number of RF refocusing pulses applied is odd, the RF refocusing is incomplete in the z direction, and after the last RF refocusing pulse is irradiated, the magnetization vector of the tissue 1 is the xy plane. It will deviate from. In that case, when the RF recovery pulse is irradiated for the first time, the magnetization vector of the tissue 1 is not accurately in the + z direction.
[0029]
Therefore, in this embodiment, the number of times of applying the RF refocusing pulse by applying the dummy pulse is set to an even number. As a result, the imperfection of the RF refocusing in the z direction can be canceled, so that the magnetization vector of the tissue 1 is in the xy plane after the last RF refocusing pulse is irradiated. Therefore, after the RF recovery pulse irradiation, the magnetization vector of the tissue 1 is accurately oriented in the + z direction, so that the longitudinal magnetization is recovered quickly, the repetition time is shortened, and thus an artifact is generated in the MRI image or the image The problem of lowering the S / N is eliminated.
[0030]
More specifically, a case where it is assumed that an object composed of two tissues having different T ₁ using this sequence is assumed will be described below. Within the two organizations, the longer T ₁ is organization 1 and the other is organization 2. The time in FIG. 4 corresponds to FIG. The nuclear spins of the two tissues that have received the RF inversion pulse (180 °) y301 are both reversed at t = Ta to generate longitudinal magnetization Mz ₁ 201 and M _z 2 202 (same as in FIG. 3A). Although inverted magnetization is longitudinal relaxation, the longitudinal magnetization components of the tissue 1 at time t = T _b after inversion time TI is Mz ₁ <0 203, the longitudinal magnetization components of the tissue 2 Mz _2> becomes 0 204. (The case of Fig. 3 (b) and (Category 2) will be explained). When the RF excitation pulse (90 °) × 302 is irradiated, the magnetization vectors of the two tissues become Mxy ₁ <0 205, Mxy ₂ > 0 206 at the time t = Tc (same as FIG. 3C). When the refocusing pulse (180 °) + y 303 is irradiated, the transverse magnetizations of the two tissues refocus at the time t = Td to generate an echo signal. Further, the process of generating an echo signal each time the refocusing pulse (180 °) + y 304, 305,... Is repeated is repeated. Finally, after the echo signal acquisition is completed, the transverse magnetizations Mxy ₁ and Mxy ₂ of the _two tissues that have received the RF refocusing pulse 306 are refocused at t = Te. At this time, when the RF recovery pulse (90 °) x307 controlled in the same phase as the excitation pulse is irradiated, the longitudinal magnetization of the tissue 1 with the longer T ₁ becomes Mxy ₂ > 0 207 at t = T _f , and the longitudinal relaxation is Promoted.
[0031]
The above embodiment is an example, and as another form, for example, the RF reinversion pulse does not need to be a 180 ° pulse, and may be an RF pulse having an effect of inverting the phase of the nuclear spin. In addition, the magnitude and number of RF inversion pulses are not limited to one at 180 °, and even if a plurality of RF pulses having an appropriate flip angle are applied, if the longitudinal magnetization is inverted, the above example and Similar effects can be obtained. Further, the above embodiment also satisfied when of three or more organizations with different T _1, by controlling the phase direction of the RF recovery pulse as best value of T ₁ to promote recovery of the longitudinal magnetization of tissue long In the same manner, an MRI image free from artifacts can be obtained. In the above embodiment, the number or phase of the RF refocusing pulse and the RF recovery pulse is controlled by the positive / negative of the longitudinal magnetization in the TI time of the angular nucleus spins of a plurality of tissues in the subject. Variable control is also possible.
[0032]
【The invention's effect】
As described above, according to the magnetic resonance imaging apparatus of the present invention, the longitudinal magnetization is recovered quickly, so that the repetition time is shortened, artifacts occur in the MRI image, and the S / N of the image decreases. Disappear.
[Brief description of the drawings]
FIG. 1 shows a configuration example of a magnetic resonance imaging apparatus.
FIG. 2 is a diagram showing an IR sequence to which a flip-back method is applied.
FIG. 3 is a schematic diagram showing the behavior of magnetization when imaging an object composed of two tissues (tissue 1 and tissue 2) having different T ₁ using the sequence of FIG. 2 (the time indicated by t is a diagram) Corresponding to 2).
FIG. 4 is a diagram when the flip-back method is applied to a high-speed spin echo type IR sequence.
[Explanation of symbols]
101,301… RF inversion pulse
102,302… RF excitation pulse
103,104,303,304,305,306… RF refocusing pulse
105,307… RF recovery pulse
111,112,114,115,117,311,312,314,315,316,317,319… slice selective gradient magnetic field pulse
113,116,313,318… Slice direction rephase gradient magnetic field pulse
121,122,321,324… Phase encoding gradient magnetic field
131,133,331,334… Reading direction rephase gradient magnetic field pulse
132,332,333… Reading gradient magnetic field pulse
201… longitudinal magnetization of tissue 1
202 ... Longitudinal magnetization of tissue 2
205 ... Transverse magnetization of tissue 1
206 ... Transverse magnetization of tissue 2

Claims (3)

An object disposed in the static magnetic field includes a static magnetic field generating means for generating a static magnetic field, an RF pulse generating means for generating an RF pulse in the static magnetic field, and a first component and a second component. And a measurement control means for measuring the nuclear magnetic resonance signal by applying the RF pulse based on a predetermined pulse sequence,
The pulse sequence includes an RF inversion pulse that reverses the longitudinal magnetization of the subject, an RF excitation pulse that generates transverse magnetization from the inverted longitudinal magnetization, and an even number of RF refocusing that refocuses the phase of the transverse magnetization. a pulse, the RF recovery pulse to recover the re-focussed transverse magnetization in longitudinal magnetization possess,
Wherein the measurement control means, said in response to positive and negative longitudinal magnetization in the application time point of the RF excitation pulse of the first component second component, in the magnetic resonance imaging apparatus that controls the phase of the RF recovery pulse ,
Wherein the measurement control unit, said have you to the application time point of the RF excitation pulse, a negative longitudinal magnetization of the first component, when the longitudinal magnetization of the second component is positive, the RF recovery pulse The magnetic resonance imaging apparatus is characterized in that the phase of the same is the same as the phase of the RF excitation pulse . 2. The magnetic resonance imaging apparatus according to claim 1, wherein at the time of application of the RF excitation pulse, the measurement control unit has negative longitudinal magnetization of the first component and negative longitudinal magnetization of the second component. If zero, the magnetic resonance imaging apparatus is characterized in that the phase of the RF recovery pulse is the same as the phase of the RF excitation pulse . 3. The magnetic resonance imaging apparatus according to claim 1 , wherein the measurement control unit has a longitudinal magnetization of the first component that is positive or zero at the application time of the RF excitation pulse, and a longitudinal direction of the second component. A magnetic resonance imaging apparatus characterized in that when the magnetization is positive, the phase of the RF recovery pulse is reversed from the phase of the RF excitation pulse .

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