JPH08107888A - Fast magnetic resonance spectroscopic imaging method and apparatus therefor - Google Patents

Abstract

PURPOSE: To implement an MR spectroscopic imaging of a living body in a short time. CONSTITUTION: In a first process 11, inverted two phases, i.e., a narrow band π/2RF pulse 13 and a narrow band -π/2RF pulse 14 are irradiated continuously to flip back the spinning of a single spectrum in a measuring slice so that all spectra in the slice keep vertical magnetization in a thermal equilibrium state while suppressing an interfering peak outside the slice by a powerful crusher inclined magnetic field 17. Subsequently, in a second process 12, a desired spectrum alone is excited by a narrow band RF pulse 15 to form an image by an echo planer method. The measurement of other desired spectra is implemented likewise continuously without providing a waiting time for vertical relaxation. Accordingly the need for repeated operation for encoding space information or spectral information is eliminated, thereby enabling the formation of a spectroscopic imaging in a very short time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an imaging method for a nuclear magnetic resonance spectroscopic imaging method using a medical magnetic resonance diagnostic apparatus.

[0002]

2. Description of the Related Art A spectroscopic imaging method (also referred to as chemical shift imaging, hereinafter referred to as MRSI method) using a medical magnetic resonance diagnostic (MRI) apparatus is a method for imaging the distribution of a specific chemical species in a living body. And M
Unlike RI, not only morphological information but also chemical information on glucose metabolism, energy metabolism, etc. can be obtained, so that it has a great feature that diagnosis at an early stage of disease is possible. This M
The RSI method takes advantage of the fact that the chemical shift δ differs depending on the chemical species even in the same atomic nucleus.
Information is also obtained on coordinate axes (spectral axes). Therefore, a multidimensional Fourier transform method for performing a multidimensional Fourier transform on the dimension including the spatial axis in the spatial axis,
Various MRSI methods such as a phase encoding method of phase encoding about a spectrum axis have been tried. These M
Since the RSI method decomposes nuclear magnetization for each spectrum and measures only a specific component, the absolute amount of nuclear spins to be measured is M
Since it is essentially less than RI, and therefore has low sensitivity, it takes a long time to realize a sufficient SN ratio,
Until now, clinical application has been difficult.

For example, the 3D-CSI (Chemical Shift Imaging) method (Maudsley, AA et al., J. Magn. Reson.
51 , 147 (1983)), a multidimensional Fourier transform method
It encodes all information related to three-dimensional or three-dimensional spatial coordinates in a phase, takes in data without a magnetic field gradient, and obtains a two-dimensional or three-dimensional chemical shift image by three-dimensional or four-dimensional Fourier transform. Even in the case of a plane (two-dimensional) measurement, since the spatial axes in two directions are phase-encoded, there is a drawback that the measurement requires a long time. As an example, when the repetition time (TR) is 1 s and the image matrix is 32 × 32, it takes about 17 minutes. From the viewpoint of clinical application, this degree of matrix is the upper limit.
As described above, in the multidimensional CSI method, if the number of matrices is increased, the measurement time becomes enormous. Therefore, the spatial resolution must be significantly limited as compared with MRI.

In addition, the phase encoding method changes the time from the π / 2 excitation pulse to the π pulse (Se
(pponen method) or by changing the value of the gradient magnetic field between the π / 2 excitation pulse and the π pulse, the spectral information is phase-encoded. However, also in this case, it is necessary to repeat the phase encoding in each of the one direction of the spatial axis and the spectrum axis, and the measurement time must be long.

On the other hand, another MRS aiming at high speed
As a flow of the I method, a measurement sequence (echo planar shift mapping, hereinafter abbreviated as EPSM) to which the echo planar method is applied has been devised (Mansfiel).
d, PJPhys. D, 16 , L235 (1983) and Matsui, S. et
al., J. Am. Chem. Soc., 107 , 2817 (1985)). This is a speed-up method in the case of plane measurement, in which only one of the spatial axes is phase-encoded and one of the remaining spatial axis and the spectral axis is frequency-encoded at the same time to reduce the number of excitation iterations by one dimension. is there. The disadvantage of this method is that the spatial coordinate and the spectral coordinate are encoded at the same time, so that the sampling interval of the spectral coordinate is widened in the phase space (k space), and thus the measured spectral band is narrowed. This means that if the spectrum of the subject exists outside the set measurement spectrum band, it is folded back into the band and becomes an artifact. In order to avoid this problem, measures have been devised to suppress unnecessary spectrum outside the measurement spectrum band prior to measurement.

[0006]

As described above, the conventional 3D-CSI method has a problem that it requires a long measurement time, and is essentially unsuitable for high-speed measurement. Although the EPSM method has a problem that the measurement spectrum band is narrow, it has room for improvement and has high potential. An example of such a device is the method of Hirata et al. Using a narrow band excitation radio frequency (RF) pulse (Hirata, S. et al., Proce.
edings of the 12th SMRM 3 , 1266 (1993)). This method obtains a spectral scopic image for a limited spectral region. As shown in FIG.
Prior to the spectroscopic imaging 42 by SM, a step 41 for exciting only a narrow band spectrum of about 2.5 ppm is provided. That is, after applying the narrow-band selective excitation RF pulse 43, an RF pulse 44 having a phase different from that of the RF pulse 43 by π is applied together with the inverted slice selective positive gradient magnetic field Gz to obtain a desired spectrum excited by the RF pulse 43. After the magnetization is saved to the longitudinal magnetization by flipback, the crusher gradient magnetic field 47
Is applied to pseudo-saturate the transverse magnetization of the unnecessary spectrum.
Then, a slice-selective π / 2 RF pulse is applied to excite only a narrow band spectrum, a phase-encoding gradient magnetic field 48 is applied, and a reading gradient magnetic field 401 to be inverted is continuously applied to generate a plurality of echoes. Signal 402
Is what you get.

That is, even in the band-limited EPSM method, in the spectroscopic imaging step 42, one axis of the spatial coordinate is encoded by the phase encode gradient magnetic field 48, and the other one axis of the spatial coordinate and the spectral axis. A frequency-encoded echo signal is obtained, but at this time, the problem of aliasing due to the narrow measurement band in EPSM is avoided by the narrow band selective excitation RF pulses 43, 44, 45. Further, even with this RF pulse, unnecessary spectral peaks in adjacent slices may be excited, but this problem is solved by pseudo saturation by the crusher gradient magnetic field 47 after flipback. Furthermore, in this method, when the measurement is repeated to measure a plurality of narrow bands, longitudinal magnetization is preserved in other spectral bands in the measurement slice,
Since continuous excitation of other bands is possible without providing a waiting time of the repetition time (TR), the extension of the measurement time due to the repetition of the measurement is greatly relaxed. Therefore, the limitation of the matrix that comes from the measurement time is 3D-
Compared with the CSI method, it becomes gentler.

However, in this case as well, there is no relation to one axis of spatial coordinates.
Since it is phase-encoded, one spectrum
Repetition of excitation is still necessary in the band measurement. When
By the time, it is the main nuclide for MR observation in vivo.¹H and³¹
Considering P, it can be a target for in vivo measurement.
The compound is quantitative¹In the case of H, N-acetyl asparagi
Acid (NAA), choline, creatine and creatine
It is limited to a few peaks such as phosphoric acid and lactic acid. these
Other than GABA, glutamine, glutamic acid, inosine
There are also peaks for acid, taurine, etc., but these are for a long time.
Although it can be observed in the spectrum measurement that integrates
Not suitable for imaging in a short time. ³¹Similarly for P
α, β, γ-ATP, creatine phosphate, PDE, PM
Only a few peaks, such as E and inorganic phosphoric acid, are the imaging pairs.
Become an elephant.

Therefore, when the MRSI measurement is viewed from the viewpoint of imaging rather than acquisition of spectrum, the above-mentioned 3
In the D-CSI method and the EPSM method (including the band-limited EPSM method), the number of spectral data points is 3D-C, respectively.
256-1024 by SI method, 16-64 by EPSM method
And there are too many wastes. Moreover, the spatial resolution must be sacrificed in order to bring the overall measurement time into a practical range. In addition, the phase encoding method Sepponen
There is a method of Guimaraes et al. In the fast MRSI method that combines the echo method and the echo planar method (Guimaraes, AR et al.,
Proceedings of the 12th SMRM 1 , 43 (1993)), but even with this method it is necessary to iterate to phase-encode the spectral information, and it can be said that the amount of spectral data is excessive.

That is, in each of the above-mentioned methods, the spectrum of the multi-voxel can be obtained simultaneously with the spectroscopic image, but it can be said that there is a waste of measurement when only the former is required. The present invention has been made from such a viewpoint, and an object thereof is to provide a method for acquiring a desired distribution image of a relatively small number of spectra in a short time. Another object of the present invention is to provide an MRSI method capable of obtaining a desired spectroscopic shift image with high temporal resolution and spatial resolution.

[0011]

To achieve the above object, the fast MRSI method of the present invention comprises a first step for selecting a specific single spectrum and a fast step for imaging measurement on that spectrum. And a second step including an imaging method, and the first and second steps are repeated at least one or more times while changing the target spectrum. It should be noted that the single spectrum herein refers to individual spectrum peaks (resonance lines) split by a chemical shift.

The first step is preferably the narrow band selective excitation R used in the band-limited EPSM method of Hirata et al.
A method similar to the spectrum selection including F pulse irradiation, flipback, and pseudo saturation gradient magnetic field is adopted,
At this time, as the spectrum-selective RF pulse (narrow-band RF pulse), an RF pulse whose bandwidth is narrower than the difference in frequency between the target single spectrum and the spectrum adjacent thereto is used, and in the second step Causes only specific spectra to be measured. That is, an RF pulse whose bandwidth is narrower than the frequency difference between the spectrum of interest and the spectrum adjacent thereto is irradiated at the same time as the slice selective gradient magnetic field, and then only this RF pulse and the phase are π.
The same RF pulse changed by (180 °) is applied at the same time as the slice selective gradient magnetic field whose sign is reversed. The first and second RF pulse irradiations are continuously performed. This makes the first
The magnetization excited by one RF pulse is flipped back. After the second irradiation of the high frequency pulse, a high-intensity gradient magnetic field is applied to at least one of the three orthogonal spatial axes to pseudo-saturate the magnetization of other spectra in the adjacent slices. In this state, the imaging measurement of the second step is performed.

The second step is a high-speed imaging method such as an echo planar method or a gradient echo method. Particularly, an echo planar method combining a blip-shaped gradient magnetic field and a gradient magnetic field which is reversed at a short interval in a direction orthogonal to the blip-shaped gradient magnetic field is used. It is suitable. The first step and the second step are repeatedly executed by sequentially changing the band of the spectrum-selective RF pulse in the first step, without providing a waiting time for longitudinal relaxation of the magnetization, to obtain all desired plural spectra. About the image acquisition.

The magnetic resonance imaging apparatus of the present invention realizes the MRSI method of the present invention, and includes a sequencer for irradiating the gradient magnetic field and the RF pulse in a predetermined sequence. A first step including narrowband RF pulse irradiation to selectively excite only a single spectrum within a selected slice of
The second step of acquiring image data at high speed with respect to the selectively excited spectrum is repeated at least one or more times by changing the spectrum of interest.

[0015]

In the present invention, since the highly accurate spectrum selective excitation and the high speed image method are combined, it is not necessary to encode the spectral information, and a high resolution image can be obtained at high speed only for the necessary spectral components. it can.
In particular, an image without chemical shift artifacts can be obtained by performing spectrum selection including narrow-band selective excitation RF pulse irradiation, flipback, and pseudo-saturation gradient magnetic field. In addition, by using the echo planar method as the high-speed imaging method in the second step, a two-dimensional image is formed by one excitation, and the excitation is repeated for encoding spatial coordinates like the 3D-CSI method and EPSM method. Therefore, an image can be formed at an extremely high speed. still,
Since it is only possible to obtain an image of one spectral peak in a single measurement, it is necessary to iterate to image different peaks, but this iteration is necessary for at most several peaks for which image information is available. I only have to do it a few times. On the other hand, since the matrix can be set in the same manner as the echo planar method, 64 ² to 128 ² are possible, and 3D-
It is possible to obtain image data that greatly exceeds the CSI method and the EPSM method.

In the first step, the first and second RFs are used.
Since pulse irradiation is continuously performed and flipback is performed within a short time, the addition of this step does not significantly extend the entire image measurement time.
Furthermore, by performing spectrum selection including flip-back and quasi-saturation gradient magnetic fields, different spectral peaks in the slices can be sequentially measured without waiting for TR and the spectroscopic images of multiple spectra can be shortened. You can get in time.

[0017]

Embodiments of the MRSI method of the present invention will be described in detail below with reference to the drawings. FIG. 4 is a diagram showing a schematic configuration of a magnetic resonance imaging apparatus to which the present invention is applied. This apparatus has a static magnetic field generating magnet 30 for generating a static magnetic field in a space in which a subject 41 is placed, and a static magnetic field generating magnet 30 in this space. Gradient magnetic field generator 35 and gradient magnetic field coil 36 for generating a gradient magnetic field
A high-frequency transmitter 32 and an RF coil 34 for irradiating the subject with RF pulses; a receiving RF coil 37 and a signal receiver 33 as detecting means for detecting a magnetic resonance signal generated from the subject; An image creating unit 38 that performs signal processing on the signal received by the signal receiving unit 33 to form an image, a display unit 40 that displays an image, and a control unit 3 that controls these.
1, and the control unit 31 includes a sequencer for irradiating the gradient magnetic field and the RF pulse in a predetermined sequence.

The static magnetic field generating magnet 30 generates a strong and uniform static magnetic field in the horizontal or vertical direction around the subject 41, and typically generates a magnetic field having a magnetic field strength of 0.1T to 4.7T. . A superconducting magnet, a normal conducting magnet, or a permanent magnet is used as the magnet. The RF coil 34 has a frequency of 4 MHz to 200 MHz depending on the output of the high frequency transmitter 32.
Generate a high frequency magnetic field. The gradient magnetic field coil 36 receives the output of the gradient magnetic field generator 35, and intersects the three orthogonal axes X, Y, and Z.
The directional gradient magnetic fields Gx, Gy, and Gz are generated. Receiver 3
3 receives the signal of the RF coil 37, and the output is subjected to processing such as Fourier transform and image reconstruction in the image creating unit 28,
After that, it is displayed on the display unit 40. The sequencer is equipped with a pulse sequence according to the MRSI method described below.

FIG. 1 is a pulse sequence showing an embodiment of the MRSI method of the present invention, which comprises a first step 11 for selectively exciting only a target spectrum and an echo planar method for acquiring image data. And a second step 12 consisting of. In the first step 11, a very narrow band π / 2 RF pulse 13 and −π / 2 RF pulse 1 for exciting the magnetization of a target spectral peak in a thin slice plane.
4 and a slice selective gradient magnetic field Gz in the Z direction, which is applied simultaneously with the RF pulses 13 and 14, and crusher gradient magnetic fields Gz, Gy, and Gx17 having high intensity.

The first and second RF pulses 13, 14 are
As shown in FIG. 2, the bandwidth Δν _RF is narrower than the frequency difference Δνδ between the target single spectrum A and the adjacent spectrum B, and the first RF pulse 1
3 is applied to the subject together with the gradient magnetic field Gz in the positive direction. Only the phase of the second RF pulse is opposite to that of the first RF pulse, and together with the negative gradient magnetic field Gz, the RF pulse 13
Is applied immediately after. These two RF pulses cause the longitudinal magnetization of the target spectral peak in the observation slice to flip back in the z-axis direction, while the magnetizations of other spectra excited in the slice adjacent to the observation slice are Exists, it remains in the xy plane as transverse magnetization.
The crusher gradient magnetic field diffuses the residual transverse magnetization phase so that the transverse magnetization component does not contribute to image formation in the second step.

The second step 12 consists of an image measurement sequence using the spin-echo type echo planar method in the embodiment of FIG. 1, and the π / 2 RF pulse 15 for image acquisition and the spin due to inhomogeneity of the static magnetic field etc. The πRF pulse 16 for correcting the phase dispersion is emitted. The RF pulse 15 is
Narrow band π / similar to the RF pulses 13 and 14 in the first step
Two pulses are applied together with a positive or negative gradient magnetic field Gz for slice selection. This pulse 15 is preferably carried out immediately after a short waiting time for the decay of the eddy currents associated with the crusher gradient field 17.

Since the πRF pulse 16 is a pulse for correcting the phase dispersion of spins due to inhomogeneity of the static magnetic field, even if it is a wideband pulse, the RF pulse 15
Similarly, frequency selective pulses may be used. When the frequency selective pulse is used, the length of the π pulse becomes slightly longer, but since the spectrum selectivity can be increased, it can be used to reduce an interfering water signal in the case of ¹ H, for example.

In the illustrated echo planar method, the signal 102
In order to obtain, a blip-shaped gradient magnetic field Gy19 and a gradient magnetic field Gx101 reversing at short intervals are applied.
The gradient magnetic field Gx18 applied prior to the inverting gradient magnetic field Gx101 is a warp gradient magnetic field or offset gradient magnetic field for determining the data acquisition start position in the k space.

MR using the pulse sequence described above
The SI method will be described in more detail. FIG. 2 schematically shows a part of the spectrum of the subject, and in order to simplify the explanation, two spectra A and B having different resonance frequencies are shown.
(Spectral peaks 21, 22) are shown. still,
The horizontal axis in the figure represents frequency. When one of these spectrum peaks is the spectrum peak 21 to be measured and the other is the adjacent peak 22, the bandwidth Δν _RF of the RF pulses 13 and 14 is set smaller than the interval Δνδ between the adjacent peaks, as already described. (Equation (1)).

[0025]

[Equation 1]

The bandwidth Δν _RF of the _RF pulse is Δν _RF = n / T (where n is the number of zero-cross points of the sinc function and T is the pulse length, when the sinc function is used as the pulse shape. ) Is given. Therefore, a desired bandwidth can be obtained by appropriately selecting the zero-cross point and the pulse length. By irradiating such a narrow band RF pulse 13 together with the positive gradient magnetic field Gz, only the spin of the desired peak 21 is excited in the measurement slice S ₀ as shown in FIG. π / 2 falls. On the other hand,
The spin of the peak 22 is not excited in this slice, but outside the slice ZB, when the gradient magnetic field strength Gz satisfies the following equation (2), ν _B = γ / 2π (H ₀ + Gz · Z B) , Ν _B is the resonance frequency of the spectrum peak 22, γ
Is the gyromagnetic ratio, and H ₀ is the static magnetic field strength.) The longitudinal magnetization falls by π / 2. In the figure, it is assumed that the peak 22 is excited in the adjacent slice S ₊₁ .

Immediately after this, when a −π / 2 RF pulse 14 whose phase has been changed by π is applied together with the negative gradient magnetic field Gz, the magnetization of the spectrum A becomes longitudinal magnetization by flipback as shown in FIG. 3B. Although returning, the magnetization of the spectrum B in the slice S ₊₁ does not change and remains in the xy plane as transverse magnetization. On the other hand, the RF pulse 14 causes the magnetization of the spectrum B to fall by -π / 2 in the slice S _-1 on the opposite side (at a symmetrical position with respect to the slice S ₀ ). Subsequently, when the crusher gradient magnetic field 17 is applied in the three-axis directions, the spectrum B in the slices S ₋₁ and S ₊₁ has a spin phase dispersed and is pseudo saturated as shown in FIG. 3C. As a result of the above first step, the disturbing peak 22 in the adjacent slice is removed, and the longitudinal magnetization of the target peak 21 returns to the initial thermal equilibrium state.

Therefore, in the second step, the RF pulse 13
If a narrow band RF pulse 15 similar to the above is applied together with the slice selection gradient magnetic field Gz, only the spectrum A in the slice S ₀ can be excited this time, and thereafter the spectrum A is formed by forming an image by the echo planar method. Only the image can be obtained. That is, by applying the blip-shaped gradient magnetic field Gy19 and the inversion gradient magnetic field Gx101 at extremely short intervals, the echo signal 1
For example, 32 to 128 are generated.

The image creating unit processes the signals thus obtained to reconstruct an image, but the data of each voxel collected here is only the image data of the spectrum A. Therefore, for different spectra B, the first step and the second step are similarly performed by changing the frequency of the carrier wave of the RF pulse. In this case, in the first step for the spectrum A, the peaks other than the spectrum A in the slice S ₀ have not been excited yet, and the longitudinal magnetization is preserved. Therefore, it is possible to immediately measure another spectrum of S ₀ without providing a repetition time TR for recovery of longitudinal magnetization. Although only two peaks are shown in the figure, similarly, measurement can be repeated for a plurality of spectra to be measured, and finally image data of a plurality of spectra can be acquired.

To give concrete values, in the case of ¹ H, the main measurement objects are NAA (2.0 ppm), choline (3.2 ppm), creatine (3.0 ppm), lactic acid (1.3 ppm). And so on (spectral coordinates are shown in parentheses). Therefore, the interval to the adjacent spectrum is 0.6p
Assuming pm, the static magnetic field strength is 4.0 T, the Larmor frequency is 170 MHz, and the frequency difference between adjacent spectra is Δν.
δ = 102 Hz. If the RF pulse bandwidth is Δν
Assuming _RF = Δνδ, and assuming that it is a sinc type with four zero-cross points, the slice thickness of 1 mm
, The gradient magnetic field strength is Gz = 2.4 mT / m, and R
The length of the F pulse is 39 ms.

Δν _RF = n / T (3) Δν _RF = γ / 2π (Gz · ΔZ) (4) (where γ is the gyromagnetic ratio of ¹ H and ΔZ is the slice thickness). Since the RF pulse is longer than the RF pulse of a normal sequence (several ms), signal attenuation due to the transverse relaxation time T ₂ poses a problem, but it can be measured in a spectrum with a long T ₂ . For example, NAA is an important substance in the brain, but this T ₂ is sufficiently long, about 300 ms. Therefore, the MRSI according to the present invention enables imaging. In a typical example, the time required for the first step is about 90 ms, and the time required for the second step is about 100 ms, for a total of about 190 m.
The spectral image for one spectrum is obtained at s. When sequentially imaging different n types of peaks, since continuous excitation can be performed as described above, the total measurement time is 190
It is only extended to about n times ms.

In the MRSI method according to the present invention, necessary signal addition processing is further performed according to the pixel size. This can improve the S / N ratio. In this case, since the MRSI method of the present invention is an ultra-high-speed method conforming to the echo planar method as described above, signal addition can be performed in synchronization with the heartbeat time phase and the respiratory phase, so that the body movement A spectral image free of artifacts can be obtained.

For simplicity in the above description, the slice S ₀
The case where only spectrum B exists outside of
Even if there are other spectra, these are the first
It is pseudo-saturated in exactly the same way by the process and is removed from the echo signal measurement in the second process. Furthermore, the MRSI of the present invention
In the method, in order to further improve the selectivity of the spectrum, for example, in the case of ¹ H, a step of suppressing the signal from the spin of water or fat may be added before the first step. CHESS can be used to suppress water or fat.
A publicly known technique such as a method, a binary pulse, etc. can be adopted.

The embodiment of FIG. 1 shows a typical sequence for the purpose of explanation, and the details can be changed. For example, the crusher gradient magnetic field 17 is not necessarily required for all three axes, and in some cases, the most effective z-axis can be used. Although the spin echo type echo planar sequence is shown as the second step, a gradient echo type sequence may be used. In this case, in the second step of FIG.
The π pulse 16 applied next to is omitted. This π
The pulse is for preventing image deterioration due to non-uniformity of the static magnetic field of the apparatus as described above, and is not essential for image formation. Therefore, in this gradient-eco-type sequence, although there is image deterioration due to magnetic field inhomogeneity of the apparatus, measurement time can be shortened and it is effective in an apparatus with good magnetic field uniformity.

In the embodiment of FIG. 1, the blip 19 and the rectangular wave 101 are used as the gradient magnetic fields Gy and Gz in the second step, but DC may be used instead of the blip 19, and the rectangular wave 101 may be used. Alternatively, a sine wave can be used, and they can be appropriately combined and used. When the blip 19 and the rectangular wave 101 are used, a rectangular scan can be performed in the k space as shown in FIG. Due to the characteristics of the magnetic field coil, it is difficult to form a perfect rectangular wave, and it is difficult to reverse the magnetic field in a short time. Therefore, for example, as shown in FIGS. And a sine wave may be combined. In this case, since the data is acquired by scanning the non-lattice points in the k space, the data is appropriately interpolated to reconstruct the image.

The echo planar method in the second step is not limited to the one shown in the figure, and a k-space division type echo planar method can also be used. The k-space is divided into, for example, a method of dividing the k-space into a central portion and both end portions, or
It is possible to adopt an interlace that shifts one scan and divides it. In this case, the first step is required for each divided measurement, and since the measurement is repeated for the same spectrum, the waiting time TR must be inserted for each division. Therefore, although the whole measurement time is significantly longer than the case where the measurement is performed without division, a special high-speed high-intensity gradient magnetic field generating means which is generally required in the echo planar method becomes unnecessary.

Further, the MRSI method of the present invention is not the echo planar method described above as the second step or various modifications thereof, but is a high-speed method such as a FLASH method using a pulse having a small flip angle as an RF pulse for image measurement. It is also possible to adopt the gradient method. Also in this case,
Compared with the echo planar method that can form an image with one shot, the total measurement time is significantly longer, but since image measurement for a single spectrum is repeated only for the required spectrum, it is useless compared to conventional MRSI. The number of measurements can be reduced.

From the above description, the advantages of the MRSI method of the present invention over the conventional MRSI method are clear.
It will be appreciated that the present invention also improves upon conventional echo-planar imaging methods. That is, the echo planar method is often carried out in a high magnetic field apparatus, and chemical shift artifacts become a problem in a high magnetic field, but since the MRSI method of the present invention can realize imaging for a single spectrum, Chemical shift artifacts can be removed. For example, when water is selected as a target peak (spectrum) and the MRSI method of the present invention is carried out, an echo planar image from which fat is removed can be obtained.

[0039]

As described above, according to the present invention, the first step of selecting a single spectrum and the second step of the high speed image forming method are combined, and these steps are repeated for a desired kind of spectrum. As a result, it is possible to obtain a spectroscopic image of a required spectrum without requiring an iterative operation for encoding the spectral information and reducing the spatial resolution of the image in a short time.

In particular, as a first step, a sequence for suppressing an interference peak outside a slice by flipback and a high-intensity gradient magnetic field is adopted, and as an second step, an echo planar method is adopted. Since continuous measurement can be performed, all spectroscopic images can be acquired in a short time. For example, a distribution image of a specific spectrum can be obtained in a short time of about 200 ms, and n kinds of spectra can be imaged in a short time of about n × 200 ms. This enables time-resolved measurement of brain function using spectroscopic images.

Further, according to the MRSI method of the present invention, since one spectrum can be imaged in a short time in this way, a spectroscopic image free from body motion artifacts can be easily obtained by using heartbeat and respiratory synchronization together. Obtainable. Also, an echo planar image without chemical shift artifacts such as fat artifacts can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a sequence of one embodiment of an MRSI method of the present invention.

FIG. 2 is a diagram showing a relationship between a peak interval of a spectrum and a bandwidth of an RF pulse.

3 is a diagram showing a behavior of magnetization of two spectral peaks in three adjacent slices in the first step of the sequence of FIG. 1. FIG.

FIG. 4 is a diagram showing a schematic configuration of an MRI apparatus to which the present invention is applied.

5 (a), (b) and (c) are diagrams showing different examples of echo signal acquisition methods according to the second step of the MRSI method of the present invention.

FIG. 6 is a diagram showing a sequence of a conventional MRSI method.

[Explanation of symbols]

 11 ... 1st step 12 ... 2nd step 13, 14, 15, 16 ... RF pulse 17 ... Crusher gradient magnetic field pulse 102 ...・ ・ ・ Echo signal 21 ・ ・ ・ ・ Target peak 22 ・ ・ ・ ・ ・ Interference peak 30 ・ ・ ・ ・ ・ ・ Static magnetic field generating magnet (static magnetic field generating means) 31 ・ ・ ・ ・ ・ Control unit ( Sequencer) 32 ・ ・ ・ ・ ・ ・ High frequency magnetic field generating means 34 ・ ・ ・ ・ ・ ・ RF coil (high frequency magnetic field generating means) 35 ・ ・ ・ ・ ・ ・ (Gradient magnetic field generating means) 36 ・ ・ ・ ・ ・Coil (gradient magnetic field generating means) 37 .... RF probe (detecting means) 38 .... Detecting means 41 ..

Claims (5)

[Claims] 1. A plurality of high-frequency magnetic fields and a gradient magnetic field are applied to a subject placed in a static magnetic field, magnetic resonance signals generated from the subject are detected, and the magnetic resonance signals are processed to obtain respective signals. In a magnetic resonance spectroscopic imaging method for forming an image of a spectrum, a first step comprising irradiation with a narrow band radio frequency pulse that selectively excites only a single spectral peak in a slice,
A second step consisting of high-speed imaging, in which a gradient magnetic field is applied to image a distribution of the spectral peaks and a plurality of magnetic resonance signals are collected, and the target spectral peaks are changed, and at least one or more times are repeated. A high-speed magnetic resonance spectroscopic imaging method characterized by: 2. The first step comprises, as the high-frequency pulse in the narrow band, first and second high-frequency pulses whose bandwidth is narrower than a difference in frequency between a target spectrum peak and a spectrum peak adjacent thereto. Pulse is used to simultaneously irradiate the first high-frequency pulse and the slice-selective gradient magnetic field,
In succession to this, only the first high-frequency pulse and the phase are 2
The second high-frequency pulse changed by π is irradiated simultaneously with the gradient magnetic field whose sign is inverted from that of the slice-selective gradient magnetic field, and after the irradiation of the second high-frequency pulse, a high-intensity gradient magnetic field is generated in three orthogonal spaces. The high-speed magnetic resonance spectroscopic imaging method according to claim 1, wherein the magnetic field is applied to at least one of the axes. 3. The high-speed magnetic resonance spectroscopic imaging method according to claim 1, wherein an echo planar method is carried out in the second step. 4. A desired combination of the first step and the second step is repeatedly executed while sequentially changing the frequency of the carrier wave of the narrow-band high-frequency pulse and without providing a waiting time for longitudinal relaxation of magnetization. The high-speed magnetic resonance spectroscopic imaging method according to claim 2, wherein images are acquired for all of the plurality of spectral peaks. 5. A static magnetic field generating means for generating a uniform static magnetic field in a space in which the subject is placed, a gradient magnetic field generating means for generating a gradient magnetic field superimposed on the static magnetic field, and a desired band for the subject. High frequency magnetic field generating means for irradiating the high frequency magnetic field pulse, detecting means for detecting a magnetic resonance signal generated from the subject, and the gradient magnetic field generating means for irradiating the gradient magnetic field and the high frequency pulse according to a predetermined sequence. And a magnetic resonance imaging apparatus including a sequencer that controls driving of the high-frequency magnetic field generation unit, and an image creating unit that processes the magnetic resonance signal to form an image, wherein the sequencer selects the subject. A first step including narrow band radio frequency pulse irradiation to selectively excite only a single spectral peak in the sliced slice, and selectively excite A magnetic resonance imaging apparatus comprising a sequence for repeating a second step of acquiring image data at high speed for a spectrum peak and repeating at least one or more times while changing a target spectrum peak.