JP2004313276A - Mr imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To acquire an image which represents suppressed signals from a tissue wherein both of T1 and T2 are long such as CSF, and an image for which such a signal suppression is not applied within a short time. <P>SOLUTION: A 180°pulse 53 of a pulse sequence B for forcible excitation is applied at a point when ESP has passed from the last 180° pulse 52 of a pulse sequence A for signal generation. Then at a point when a lateral magnetization has started focusing after the ESP/2, a positive 90° pulse 54 is applied in an odd-numbered sequence of (a), and a vertical magnetization in the opposite direction from the direction of a static magnetic field is presented. Thus, the softening of a proton with a longer T1 is delayed, and a signal generated in an even-numbered sequence of (b) is suppressed, and a negative 90° pulse 54 is applied in the even-numbered sequence of (b) to present the vertical magnetization in the same direction as the direction of the static magnetic field. Thus, the softening of the proton with the longer T1 is quickened, and a signal which is generated in the odd-numbered sequence of (a) is prevented from being suppressed. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an MR imaging apparatus that performs imaging using an NMR (nuclear magnetic resonance) phenomenon, and more particularly to an MR imaging apparatus that obtains an image at a high speed by an imaging scan method called a fast spin echo method.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, a high-speed spin echo method (hereinafter, referred to as FSE (Fast Spin Echo) method) has been known as an imaging scan method of an MR imaging apparatus (see Non-Patent Document 1 below). In the FSE method, first, after applying a 90 ° pulse (excitation pulse), a plurality of 180 ° pulses (refocus pulse) are applied, and a slice selection gradient magnetic field pulse is applied simultaneously with each of these RF pulses. . Then, a readout (and frequency encoding) gradient magnetic field pulse is applied to generate a plurality of spin echo signals after each 180 ° pulse. Immediately before the generation of these signals, a phase encoding gradient magnetic field pulse is added, and phase encoding is performed with respect to predetermined uniaxial position information. The amount of application of each phase encoding gradient magnetic field pulse corresponds to the amount of phase encoding such that data obtained from these signals is arranged at different places in the phase direction on the K space (raw data space). Let it. In these three gradient magnetic fields, the gradient directions of the magnetic field strength are in any three orthogonal axes.
[0003]
[Non-patent document 1]
"RARE Imaging: A Fast Imaging Method for Clinical MR", Magnetic Resonance in Medicine, 3, pp 823-833, 1986.
[0004]
According to this FSE method, data to be arranged on many different lines in the K space can be obtained in one TR (one repetition time of a pulse sequence), so that the number of TRs can be reduced and high-speed imaging can be performed. Here, an echo having a desired contrast is arranged near the center (low-frequency region) of the K space, and other echoes that are temporally close to the echo arranged near the center are sequentially adjacent to the K space. The amount of phase encoding, which is arranged in such a manner, is given to each echo.
[0005]
In the GRASE (GRadian And Spin Echo) method disclosed in Patent Document 1 and Non-patent Document 2, the following pulse sequence is employed. After applying a 90 ° pulse (excitation pulse), a plurality of 180 ° pulses (refocusing pulse) are applied, and simultaneously with each of these RF pulses, a pulse of a gradient magnetic field Gs for slice selection is applied. A pulse of the readout (and frequency encoding) gradient magnetic field Gr is applied within the interval of, and this Gr pulse is switched a plurality of times after each 180 ° pulse, and added to the signal of the spin echo before and after the pulse. Gradient echo signals are generated, and pulses of the gradient magnetic field Gp for phase encoding are respectively added immediately before the generation of these signals, and the application amount of each Gp pulse is determined by data obtained from those signals. Corresponds to the amount of phase encoding that is arranged sequentially in the phase direction on the K space (raw data space) Is that.
[0006]
[Patent Document 1]
U.S. Pat. No. 5,270,654
[0007]
[Non-patent document 2]
K. Oshio and D.S. A. Feinberg “GRASE (Gradient and Spin-Echo) Imaging: A Novel Fast MRI Technology” Magnetic Resonance in Medicine 20, 344-349, 1991
[0008]
In such an FSE method or a GRASE method, an image having an arbitrary contrast can be obtained by controlling the relationship between the TR and the arrangement of the echoes in the K space. For example, a proton-density-weighted image or a T2-weighted image can be obtained. Can be.
[0009]
In addition to these methods, it is also known to obtain an image in which a signal of a specific tissue is attenuated by adding an IR (Inversion Recovery) pulse before a pulse sequence. An example thereof is the FLAIR (FLuid Attenuated IR) method. This is because the time interval between the IR pulse and the 90 ° pulse (excitation pulse) is adjusted, and the excitation pulse is applied when the longitudinal magnetization of the specific tissue has recovered to near zero after the IR pulse. The idea is to suppress the signal. Further, a method of selectively attenuating a signal of a specific tissue using a Driven Inversion pulse as a preparation pulse has been proposed in Non-Patent Document 3 below.
[0010]
[Non-Patent Document 3]
"Cooperative T1 and T2 effects on contrast using a new drive inversion spin echo (DISE) MRI pulse sequence", Magnetic Resonance, Canada, Canada
[0011]
[Problems to be solved by the invention]
However, according to the conventional FSE method or GRASE method, a problem occurs in an image containing a specific tissue, for example, cerebrospinal fluid (hereinafter, CSF). In the FSE method and the GRASE method, since a plurality of echoes are arranged in one K space, refocusing is repeated and echoes that occur later in the TR are also collected. Relatively higher than the signal from the tissue. Therefore, when a proton-enhanced density image is collected by these methods, the CSF has a higher signal than in the normal SE method, and the boundary between the brain parenchyma and the CSF becomes unclear.
[0012]
In this regard, according to the FLAIR method, a signal from the CSF can be suppressed, and a focal lesion of the brain near the cortex or the ventricle can be observed. Therefore, conventionally, in diagnosis, both images obtained by the FSE method or the GRASE method and images obtained by the FLAIR method are taken, and these images are used in combination. However, in the FLAIR method and the method using the driven inversion pulse, a sufficient recovery time is required, so that the imaging time is greatly extended, and the S / N of a substantial portion other than the CSF is reduced. Obtaining an image is impossible in principle.
[0013]
In view of the above, the present invention provides an image in which a signal from an arbitrary tissue such as CSF, which has a long T1 and T2, is suppressed when a proton density weighted image or a T2 weighted image is obtained at high speed using the FSE method or the GRASE method. It is an object of the present invention to provide an MR imaging apparatus improved so that two types of images including an image without signal suppression can be obtained in a shorter time as compared with a case where the FLAIR method or the like is used.
[0014]
[Means for Solving the Problems]
In order to achieve the above object, in the MR imaging apparatus according to the first aspect of the present invention,
Static magnetic field generating means for generating a static magnetic field in the space where the subject is placed,
When the first axis, the second axis, and the third axis are three arbitrary orthogonal axes, a slice selection gradient magnetic field pulse in the first axis direction and a phase encoding gradient magnetic field in the second axis direction are provided in the space. Gradient magnetic field pulse applying means for applying a pulse and a readout gradient magnetic field pulse in the third axis direction;
RF transmitting means for applying an excitation RF pulse and a refocus RF pulse in the space,
Receiving means for receiving the echo signal, performing phase detection, sampling and A / D converting the data to obtain data;
The RF transmission means, the gradient magnetic field pulse applying means, and the receiving means are controlled to perform a signal generation pulse sequence for generating a signal by applying one excitation RF pulse and then sequentially applying a plurality of refocus RF pulses. , The amount of phase encoding for each signal is determined so that data from each signal is arranged on each line in the K space, and a refocus RF pulse is applied after the pulse sequence, and then the polarity is positive for each repetition time. Control means for repeating a pulse sequence, to which a pulse train for forced excitation for applying an excitation pulse inverted to negative is added;
Each of the two K spaces in which data collected at the repetition time when the polarity of the excitation pulse added at the end of the pulse train for forced excitation is positive and data collected at the repetition time when the polarity is negative is arranged from each of the two K spaces. Image reconstruction means for reconstructing an image;
It is characterized by having.
[0015]
After one excitation RF pulse is applied, a plurality of refocusing RF pulses are sequentially applied to generate a signal to generate a signal pulse sequence (for example, by the FSE method or the GRASE method), and data from each signal is generated. Is arranged in each line on the K space, and the amount of phase encoding for each signal is determined. This makes it possible to obtain a plurality of pieces of data having different phase encoding amounts within one repetition period, similarly to a pulse sequence based on a normal FSE method, a GRASE method, or the like. That is, the magnetic moment of the protons is defeated by the excitation pulse to produce transverse magnetization, and when the phases are separated, the phase is realigned by the refocusing pulse to generate a spin echo signal. The generation of signals with the same phase by adding the focus pulse is repeated a plurality of times. Since refocusing is repeated in this manner, a signal from a proton having a long T2 becomes relatively large. Thereafter, a pulse train for forced excitation is added to form 1TR. This pulse train for forced excitation applies a refocus pulse and then applies an excitation pulse, and inverts the polarity of the last excitation pulse between positive and negative for each TR.
[0016]
The refocusing pulse in the pulse train for forced excitation aligns phases that have been separated before then. Then, when the phases are aligned again, an excitation pulse is given. When the polarity of the last excitation pulse is positive, the transverse magnetization is further defeated by the positive excitation pulse, and the longitudinal magnetization is in a direction opposite to the first longitudinal magnetization. For this reason, protons having a long T1 and not yet recovered at this point recover from the longitudinal magnetization in the opposite direction, and therefore do not return to the saturated state by the start of the next TR, and the signal intensity at the next TR becomes lower. Become smaller. On the other hand, the short T1 proton is sufficiently recovered when the excitation pulse of the pulse train for forced excitation is applied and is excited by the excitation pulse, but is recovered by the start of the next TR. Absent. As a result, in the next TR after the TR to which the positive excitation pulse is given, the signal from the tissue that is long in both T1 and T2 is attenuated, so that this TR (TR in which the excitation pulse is positive in the immediately preceding TR) When the image is reconstructed by the two-dimensional Fourier transform method from the K space in which the data collected in the above is arranged, it is possible to obtain an image in which a signal of a tissue such as CSF which is long at both T1 and T2 is suppressed.
[0017]
On the other hand, when the polarity of the last excitation pulse is negative, the transverse magnetization is inverted in the opposite direction by this negative excitation pulse, that is, the longitudinal magnetization is in the same direction as the first longitudinal magnetization. Therefore, protons that have a long T1 and have not yet been recovered at this time are forcibly recovered to a saturated state. At this point, the spin remaining as the transverse magnetization depends on the time from the first excitation pulse (of the pulse sequence for signal generation), but when this time is long (the number of generated signals in the pulse sequence for signal generation is large) In, the spin of a substance having a long T2 value becomes dominant. Therefore, the magnetization of the tissue such as CSF that is long at both T1 and T2 is returned to the initial longitudinal magnetization. In this state, the next TR starts, and in the next TR, the intensity of signals from these tissues increases. Therefore, when the polarity of the excitation pulse of the pulse train for forced excitation of the immediately preceding TR is negative, the image is reconstructed by the two-dimensional Fourier transform method from the K space where the data collected by the subsequent TR is arranged, and the CSF Thus, it is possible to obtain an image having a strong intensity without suppressing a signal of a long tissue in both T1 and T2.
[0018]
Therefore, such two types of images can be obtained in a shorter time than in the case where the FLAIR method or the Driven Inversion pulse is used, and an accurate diagnosis can be made by using these two types of images together.
[0019]
As in claim 2, when the number of refocusing pulses in the pulse sequence for signal generation of the pulse sequence is an odd number, when the pulse train for forced excitation is added after adding an odd number of refocusing pulses for dummy, Since the total number of refocusing pulses is an odd number, complete reverse and forward longitudinal magnetization can be realized with positive and negative excitation pulses in the pulse train for forced excitation. If the excitation pulse of the signal generation pulse sequence is tilted by, for example, 80 ° instead of 90 °, one refocusing pulse becomes 100 ° and returns to 80 ° by the next refocusing pulse. In this way, if the total number of refocusing pulses is even, the flip angle α ° that was first defeated is maintained, but if it is odd, {90+ (90−α)} ° = (180−α) °. Therefore, when the last positive excitation pulse in the pulse train for forced excitation is further tilted by α °, longitudinal magnetization in the opposite direction by 180 ° is obtained.
[0020]
If the gradient magnetic field pulse in the first axis direction and the gradient magnetic field pulse in the third axis direction are added as rephasing pulses in the pulse sequence for forced excitation of the pulse sequence as in claim 3, the excitation pulse of the pulse sequence for forced excitation is added. , The phases dispersed in the first axis direction and the third axis direction at the time of applying are more uniform.
[0021]
As described in claim 4, the pulse sequence is sequentially performed a plurality of times at different times within one TR, and the frequencies of the excitation RF pulse and the refocus pulse are made different for each pulse sequence, so that the pulse sequence is different for each pulse sequence. Since a signal for a slice can be obtained, a plurality of different slices can be imaged in one TR.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
Next, embodiments of the present invention will be described in detail with reference to the drawings. The MR imaging apparatus according to the present invention is configured as shown in FIG. In FIG. 1, a main magnet 11 generates a strong static magnetic field, and a subject (not shown) is arranged in the static magnetic field space. Further, the gradient magnetic field coils 12 are provided in three sets so as to generate three gradient magnetic fields Gx, Gy, Gz whose magnetic field strengths incline in three orthogonal axes of X, Y, Z so as to be superimposed on the static magnetic field. Have been. An RF coil 13 for transmission and an RF coil 14 for receiving NMR signals are attached to the subject.
[0023]
The host computer 21 controls the entire system, and the sequencer 22 under the control of the host computer 21 collects data for reconstructing an image of a desired cross section of the subject (see FIG. 2 later). Are transmitted to the transmission system, the reception system, and the gradient magnetic field generation system. As for the gradient magnetic field generation, a predetermined pulse waveform relating to Gx, Gy, Gz is generated at a predetermined timing from the waveform generator 15 and sent to the gradient magnetic field power supply 16, and the gradient magnetic field coil 12 generates Gx, Gy and Gz are generated. The gradient magnetic field Gs for slice selection, the gradient magnetic field Gr for reading (for frequency encoding), and the gradient magnetic field Gp for phase encoding shown in the pulse sequence of FIG. 2 have their magnetic field inclines in one direction of each of three arbitrary orthogonal axes. This is a gradient magnetic field that can be made in any direction by using any one of Gx, Gy, and Gz, or by combining some of them.
[0024]
Further, the waveform generator 15 generates an RF pulse waveform at a predetermined timing under the control of the sequencer 22 and sends the RF pulse waveform to the amplitude modulator 24. The RF signal from the RF signal generator 23 is sent to the amplitude modulator 24 as a carrier, and the carrier is amplitude-modulated according to the waveform signal from the waveform generator 15. The RF signal generator 23 is set by the host computer 21 so as to generate an RF signal having a frequency corresponding to the resonance frequency of the subject. The output of the amplitude modulator 24 is sent to the RF coil 13 via the RF power amplifier 25. In this manner, the waveform and timing of the RF signal transmitted from the RF coil 13 are determined by the sequencer 22, so that the subject is irradiated with the 90 ° pulse and the 180 ° pulse shown in FIG.
[0025]
The NMR signal generated from the subject is received by the receiving RF coil 14 and sent to the phase detector 27 via the preamplifier 26. An RF signal, which is a carrier of a transmission RF pulse, is sent from the RF signal generator 23 to the phase detector 27, and the signal is used as a reference signal to perform phase detection. The A / D converter 28 samples the detection signal from the phase detector 27 and converts it into digital data according to the sampling pulse from the sampling pulse generator 29 whose timing, frequency, and the like are controlled by the sequencer 22. This digital data is taken into the host computer 21 and subjected to Fourier transform processing by the image reconstruction device 33. The image thus reconstructed is displayed on the display device 32. The indicator 31 is a keyboard, a mouse, and the like for an operator or the like to give necessary instructions to the host computer 21.
[0026]
In such an MR imaging apparatus, a pulse sequence as shown in FIGS. 2A and 2B is performed under the control of the host computer 21 and the sequencer 22. In FIG. 2, 1TR includes a pulse sequence A for signal generation and a subsequent pulse train B for forced excitation. The pulse sequence A for signal generation is based on the FSE method. The pulse sequence of FIG. 2A and the pulse sequence of FIG. 2B are alternately repeated, for example, the former is an odd number of times, and the latter is an even number of times.
[0027]
First, referring to FIG. 2A, after one positive 90 ° pulse (excitation RF pulse) 51 is applied, a plurality (two in this case) of 180 ° pulses (refocus RF pulse) 52, At the same time as adding the RF pulses 51, 52, 52, pulses 61, 63, 63 of the gradient magnetic field Gs for slice selection are applied simultaneously. The Gs pulse 62 of the opposite polarity is a rephase pulse for aligning the phase disturbed by the Gs pulse 61. Since the Gs pulse 63 is applied before and after the 180 ° pulse 52, the influence on the phase is canceled out before and after the 180 ° pulse 52. A pulse 81 of a gradient magnetic field Gr for reading (and for frequency encoding) is applied after the 90 ° pulse 51, and a Gr pulse 82 is given after the 180 ° pulse 52, and after each of the 180 ° pulses 52, 52. Generates a spin echo signal. A phase encoding gradient magnetic field Gp pulse 71 is applied before each signal is generated, and after a signal is generated, a rephasing Gp pulse 72 having the same integral value and opposite polarity is added so as to cancel the effect.
[0028]
Assuming that the time from the 90 ° pulse 51 to the focus of the echo is ESP (Echo Space), a 180 ° pulse 52 is added at the point of ESP / 2 from the 90 ° pulse 51, and the time interval between the 180 ° pulse 52 and ESP is It repeats several times, such as applying the next 180 ° pulse 52 (here, it repeats twice).
[0029]
When ESP has elapsed from the last 180 ° pulse 52 of the signal generation pulse sequence A, the 180 ° pulse 53 of the forced excitation pulse train B is applied simultaneously with the Gs pulse 64. Further, a positive 90 ° pulse 54 is applied together with the Gs pulse 66 at the time point after the ESP / 2. The Gs pulse 65 immediately before that is a re-phase pulse given in advance in order to cancel the phase dispersion and align the phases because the phase in the slice direction (slice thickness direction) is dispersed by the Gs pulse 66. The Gr pulse 83 is also a rephase pulse, and is used to recover the disorder of the phase due to the Gr pulse 82 (the latter half of the Gr pulse 82) applied after the echo focus time. Finally, the Gs pulse 67, the Gp pulse 73, and the Gr pulse 84 are applied, and the residual magnetization is spoiled.
[0030]
In the pulse sequence of FIG. 2B, the same pulse sequence A for signal generation as described above is performed, and the pulse train B for forced excitation is almost the same. However, the polarity of the 90 ° pulse 54 added at the end becomes negative. Only the differences.
[0031]
The Gp pulse 71 is different for each signal in 1TR in both FIGS. 2 (a) and 2 (b), and is changed little by little with each repetition as shown by an arrow. Thus, each of the two K spaces is completely filled with the data obtained by the respective pulse sequences of FIGS. 2A and 2B.
[0032]
Here, the behavior of the magnetic moment of the proton spin will be described with reference to FIG. 3. Since the relaxation is reduced before the 90 ° pulse 51, the direction of the static magnetic field is indicated by the thick arrow in FIG. The longitudinal magnetization is oriented to Z. RF pulses such as 90 ° pulses 51, 54, 180 ° pulses 52, 53 are based on CPMG conditions or CP conditions, and the positive 90 ° pulse 51 irradiated from the X direction changes the longitudinal magnetization in the Z direction to X. Rotation by 90 ° about the axis results in transverse magnetization oriented in the Y direction as shown in FIG. Next, at the time when the time has elapsed and the phase of the transverse magnetization has been separated as shown in FIG. 3C (at the time of ESP / 2), the 180 ° pulse 52 is irradiated from the Y direction. As shown in (d), the transverse magnetization is rotated by 180 ° around the Y axis, so that the transverse magnetization that has been heading in the dispersion direction starts to converge in a direction in which it is aligned again. As shown in (2), the light is focused (focused) on the Y axis. At this point where the phases converge, a large signal is generated. If the time further elapses, the phase is dispersed again as shown in FIG. 3 (c), and as shown in FIG. 3 (d), the phase is rotated 180 ° around the Y direction by the 180 ° pulse 52 from the Y direction. Focusing is performed on the Y axis as shown in FIG. Here, FIGS. 3C to 3E are repeated twice.
[0033]
When the time elapses from the last 180 ° pulse 52 of the pulse sequence A for signal generation, the transverse magnetization is dispersed as shown in FIG. 3 (f), so the 180 ° pulse from the Y direction at the time of ESP / 2 53, the transverse magnetization is rotated around the Y axis as shown in FIG. 3 (g), and when the time ESP / 2 has elapsed, the Y axis is focused as shown in FIG. 3 (h). At this time, in the pulse sequence of FIG. 2A, a positive 90 ° pulse 54 is applied again from the X-axis direction, and in the pulse sequence of FIG. 2B, a negative 90 ° pulse 54 is applied (X-axis direction). From the opposite direction). Therefore, when the 90 ° pulse 54 is applied from the plus direction of the X-axis, the coherent transverse magnetization is further rotated by 90 ° around the X-axis, and as shown in FIG. -Z direction). On the other hand, when the 90 ° pulse 54 is applied from the minus direction of the X axis, the coherent transverse magnetization rotates 90 ° in a direction to return around the X axis, and as shown in FIG. It becomes magnetization.
[0034]
Spins having negative longitudinal magnetization as shown in FIG. 3 (i) are spins whose transverse magnetization has not attenuated at this time, and depend on ETL (Echo Train Length). The ETL is the time from the first 90 ° pulse 51 to the last 180 ° pulse 53. When the ETL is long, the spin of a substance having a long T2 value becomes dominant in the longitudinal magnetization of −Z. In the human body, CSF can be cited as a substance having a long T2 value, but CSF also has a long T1 value. For this reason, the spin of a substance long in both T1 and T2 such as CSF is negatively excited and cannot be sufficiently recovered by the start time of the next TR, so that it is excited by the 90 ° pulse 51 given at the start time of the next TR. Thus, the magnitude of the magnetization is suppressed. On the other hand, since the magnetization of the substance having a shorter T1 and T2 has been recovered at the time of application of the 90 ° pulse 54, the material is excited again by the application of the 90 ° pulse 54, is tilted 90 °, and faces the Y-axis direction. However, since both T1 and T2 are short, they are recovered before the next TR starts, and the magnitude of the magnetization excited by the 90 ° pulse 51 at the head of the next TR does not decrease and the next TR does not occur. There is no signal suppression in TR. The same applies to a spin of a material having a long T2 and a short T1, and recovers before the next TR starts, and the signal is not suppressed. Therefore, by increasing the ETL, it is possible to selectively reduce only a signal from a substance having a long T1 and T2 in the next TR (even number of TRs) as compared with a signal from another tissue. .
[0035]
On the other hand, in the odd number of TRs, the negative 90 ° pulse 54 added to the end of the pulse train B for the immediately preceding TR (the even number of TRs) forcibly excites the spin of a substance long in both T1 and T2 such as CSF. Since the magnetization is returned to the longitudinal magnetization in the Z direction, the spin of the tissue such as CSF is sufficiently excited by the first 90 ° pulse 51 of the odd number of TRs, and the spin from the tissue generated in the odd number of TRs is generated. The signal is not suppressed.
[0036]
Therefore, by performing a two-dimensional Fourier transform on the K space filled with the data collected by the pulse sequence (odd number of TRs) of FIG. 2A, an image in which the tissue such as CSF is long, both T1 and T2, is suppressed. In addition, by performing a two-dimensional Fourier transform on the K space filled with the data collected by the pulse sequence (even number of TRs) in FIG. 2B, even a tissue having a long T1 and T2 such as CSF can be suppressed. An image that is not performed can be obtained.
[0037]
4A and 4B show an example in which a dummy pulse train C is added before the forced excitation pulse train B when the number of 180 ° pulses 52 of the signal generation pulse sequence A is odd. Also in this case, as in FIGS. 2A and 2B, the polarity of the 90 ° pulse 54 of the forced excitation pulse train B is positive in the pulse sequence of FIG. 4A and is positive in the pulse sequence of FIG. Negative. The pulse sequence shown in FIG. 4A is odd-numbered, and the pulse sequence shown in FIG. 4B is even-numbered.
[0038]
The dummy pulse train C in FIGS. 4A and 4B includes one 180 ° pulse (refocus pulse) 55 irradiated from the Y direction, a slice selection gradient magnetic field Gs pulse 68 applied at the same time, And a gradient magnetic field Gr pulse 85 for rephasing the readout axis. The 180 ° pulse 55 is given at an ESP interval from the last 180 ° pulse 52 of the signal generation pulse sequence A. In the dummy pulse train C, signal sampling is not performed when a Gr pulse is applied.
[0039]
Assuming that there is no dummy pulse train C, if the number of 180 ° pulses 52 of the signal generation pulse sequence A is an odd number, the number of 180 ° pulses 53 of the forced excitation pulse train B is added, and an even number of 180 ° pulses 53 is added. The ° pulse will be applied before the 90 ° pulse 54. At this time, if the tilt angle of the 90 ° pulses 51 and 54 does not accurately become 90 ° due to an error in the waveforms of the 90 ° pulses 51 and 54, the last pulse sequence in FIG. The 90 ° pulse 54 cannot realize the longitudinal magnetization of −Z. For example, if the 90 ° pulses 51 and 54 actually cause only a 85 ° fall, the first 90 ° pulse 51 falls by 85 °, so that the first 180 ° pulse 52 becomes 95 ° and the second 180 ° pulse 52 gives 85 °, the third 180 ° pulse 52 becomes 95 °. Thus, if the total number of the 180 ° pulses 52 and 53 is even, the angle is 85 °, and it can be further defeated by 85 ° by the 90 ° pulse 54 added at the end. Does not. On the other hand, if the total number of the 180 ° pulses 52 and 53 is an odd number, the angle can be lowered from 95 ° to 85 °, so that −Z can be realized.
[0040]
Therefore, when the number of 180 ° pulses 52 of the signal generation pulse sequence A is odd, an odd number (at least one) of 180 ° pulses 55 (and the Gs pulse 68 and the Gr pulse 85) are applied. By adding one 180 ° pulse 53 of the pulse train B for forced excitation, the total number can be made odd, and -Z magnetization can be ensured. When an odd number (three or more) of the 180 ° pulses 55 (and the Gs pulse 68 and the Gr pulse 85) are applied, the pulses are applied at ESP intervals. A 180 ° pulse 53 of pulse train B is applied.
[0041]
By adding the dummy pulse train C in this manner, -Z magnetization can be reliably realized in an odd number of TRs in which the pulse sequence shown in FIG. 4A is performed. However, it also means that it is possible to change the relaxed state of longitudinal magnetization by selecting an arbitrary flip angle, for example, setting the flip angle to 60 °. Further, by adding the dummy pulse train C, the length of the ETL can be arbitrarily increased.
[0042]
However, the pulse sequence shown in FIG. In this case, assuming that the angle to be tilted by the 90 ° pulses 51 and 54 is 85 ° as in the previous example, the total number of the 180 ° pulses 52, 53 and 55 is an odd number. Therefore, even if it is tilted by -85 °, it does not become 0 ° (+ Z). If the total number of the 180 ° pulses 52, 53, and 55 is even, the last 180 ° pulse 55 is 85 °, so that it can be tilted by −85 ° to 0 ° (+ Z). However, in FIG. 4B, it is not necessary to accurately generate the magnetization in the + Z direction. In other words, even if the CSF or the like is not perfect, even if it is forcibly returned to a certain saturated state, it is sufficient in that the signal from these is not suppressed.
[0043]
Multi-slice imaging can also be performed with such a pulse sequence as shown in FIGS. When imaging three slices SL1, SL2, and SL3 as shown in FIG. 5, as shown in FIG. 6A, the pulse sequence of FIG. 2A or FIG. After that, the pulse sequence shown in FIG. 2B or FIG. 4B is repeated three times in even number 1TR. If the pulse sequence in 1TR is assumed to be PS1, PS2, and PS3 in order, the data of slice SL1 is collected by PS1, the data of slice SL2 is collected by PS2, and the data of slice SL3 is collected by PS3. The frequency of the carrier of the RF pulse (90 ° pulse 51, 54, 180 ° pulse 52, 53, etc.) depends on the position of SL1 in the slice direction (slice thickness direction), and the frequency of the carrier of the RF pulse in PS2. According to the position of SL2, and the frequency of the carrier of the RF pulse in PS3 corresponds to the position of SL3. Then, from the relationship between the frequency of the carrier of the RF pulse and the gradient magnetic field Gs, only SL1 is selectively excited in PS1, only SL2 is selectively excited in PS2, and only SL3 is selectively excited in PS3. In each of SL1, SL2, and SL3, TR is excited only once as shown in FIG. 6B, and does not affect other slices.
[0044]
In the above-described pulse sequence (FIGS. 2 and 4), the pulse sequence A for signal generation is based on the SFE method, but it is needless to say that the pulse sequence A based on the GRASE method can be used. Further, it goes without saying that the ETL is not limited to the pulse sequence (FIGS. 2 and 4). In addition, various changes can be made without departing from the spirit of the present invention.
[0045]
【The invention's effect】
As described above, according to the MR imaging apparatus of the present invention, the pulse train for forced excitation including the refocusing pulse and the excitation pulse is added after the pulse sequence for signal generation, and the polarity of the excitation pulse of the pulse train for forced excitation is added. Alternately inverts the signal from the tissue with long T1 and T2 in the TR immediately after the excitation pulse of the positive polarity is given, and suppresses the signal of the tissue such as CSF with long T1 and T2. A proton density-weighted image or a T2-weighted image can be obtained, and a proton density-weighted image or a T2-weighted image without such signal suppression can be obtained in TR immediately after a negative-polarity excitation pulse is applied. Thus, two types of images can be obtained in a short imaging time.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an MR imaging apparatus according to an embodiment of the present invention.
FIG. 2 is a time chart showing a pulse sequence performed in the embodiment.
FIG. 3 is a conceptual diagram showing each of the behaviors of magnetization in the embodiment.
FIG. 4 is a time chart showing a pulse sequence performed in another embodiment.
FIG. 5 is a schematic diagram showing a positional relationship between slices during multi-slice.
FIG. 6 is a time chart showing a sequence at the time of multi-slice.
[Explanation of symbols]
11 Main magnet for generating static magnetic field
12 gradient coil
13 RF coil for transmission
14 RF coil for reception
15 Waveform generator
16 Power supply for gradient magnetic field
21 Host computer
22 PLC
23 RF signal generator
24 Amplitude modulator
25 RF power amplifier
26 Preamplifier
27 Phase detector
28 A / D converter
29 Sampling pulse generator
31 Indicator
32 Display device
33 Image Reconstruction Device
A pulse sequence for signal generation
B Pulse train for forced excitation
C Dummy pulse train
51, 54 90 ° pulse
52, 53, 55 180 ° pulse

Claims (4)

Static magnetic field generating means for generating a static magnetic field in the space where the subject is placed,
When the first axis, the second axis, and the third axis are three arbitrary orthogonal axes, a slice selection gradient magnetic field pulse in the first axis direction and a phase encoding gradient magnetic field in the second axis direction are provided in the space. Gradient magnetic field pulse applying means for applying a pulse and a readout gradient magnetic field pulse in the third axis direction;
RF transmitting means for applying an excitation RF pulse and a refocus RF pulse in the space,
Receiving means for receiving the echo signal, performing phase detection, sampling and A / D converting the data to obtain data;
The RF transmission means, the gradient magnetic field pulse applying means, and the receiving means are controlled to perform a signal generation pulse sequence for generating a signal by applying one excitation RF pulse and then sequentially applying a plurality of refocus RF pulses. , The amount of phase encoding for each signal is determined so that data from each signal is arranged on each line in the K space, and a refocus RF pulse is applied after the pulse sequence, and then the polarity is positive for each repetition time. Control means for repeating a pulse sequence, to which a pulse train for forced excitation for applying an excitation pulse inverted to negative is added;
Each of the two K spaces in which data collected at the repetition time when the polarity of the excitation pulse added at the end of the pulse train for forced excitation is positive and data collected at the repetition time when the polarity is negative is arranged from each of the two K spaces. An MR imaging apparatus comprising: image reconstruction means for reconstructing an image. 2. The pulse sequence for forced excitation is added after adding an odd number of dummy refocusing pulses when the number of refocusing pulses in the pulse sequence for signal generation of the pulse sequence is odd. MR imaging apparatus. 3. The MR imaging according to claim 1, wherein a gradient magnetic field pulse in a first axis direction and a gradient magnetic field pulse in a third axis direction are added as rephase pulses in the pulse train for forced excitation of the pulse sequence. apparatus. In one repetition period, the above pulse sequence is sequentially performed a plurality of times at different times, and the frequencies of the excitation RF pulse and the refocus pulse are made different for each pulse sequence, so that signals for different slices in each pulse sequence are generated. 4. The MR imaging apparatus according to claim 1, wherein the MR imaging apparatus is obtained.

Images