JP2004024902A - Magnetic resonance imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an MR image in which a repeating time TR is not extended and an MR signal from fat is suppressed in the case of performing multi-slice photographing for solving various kinds of problems using a sequence containing a prepulse. <P>SOLUTION: This magnetic resonance imaging apparatus has a function of exciting a spin of proton contained in water on a slice cross section of a patient placed in a static magnetic field and making the MR signal about this spin excitation into an image. The apparatus is provided with a means for applying a 90° RF (radio frequency) pulse capable of selectively exciting only the proton contained in the water and an applying means of a tilted magnetic field for slicing the cross section with application of this 90° RF pulse. <P>COPYRIGHT: (C)2004,JPO

Description

The present invention relates to a magnetic resonance imaging apparatus, and in particular, an MR signal from fat using a sequence including a procedure of applying a pre-pulse before a scan for imaging or a sequence of selectively exciting only water in a subject. The present invention relates to a magnetic resonance imaging apparatus with an improved suppression effect.

In MR imaging, obtaining a so-called MTC (Magnetization Transfer Contrast) effect may have clinical significance.

FIG. 17 shows the spectra of protons contained in water, fat and polymer. As can be seen from the figure, protons contained in water have a resonance frequency of about 64 MHz under a magnetic field of, for example, 1.5 Tesla, while protons contained in fat are, as is well known, due to a chemical shift. It shifts to the lower frequency side by 0.5 PPM (the right side in the figure), and shifts by about 224 Hz under the above-mentioned magnetic field. On the other hand, the protons contained in the polymer have a very wide frequency range.

Here, if a frequency shifted from the resonance frequency of the protons of water, for example, 500 Hz is selectively excited before executing the normal imaging sequence, the signal level from the protons of the polymer decreases as shown by the broken line. The level of the MR signal from the protons contained in the water also decreases as shown by the broken line. This is considered to be because the protons of water cross-relax or exchange with the protons of the polymer, and are already known as the MTC (Magnetization Transfer Contrast) effect.

利用 By utilizing such an MTC effect, an image having a contrast different from the conventional one can be obtained according to the proportion of the polymer present. In addition, since it has the effect of greatly reducing the signal level of the substantial part as compared with the signal level of the blood vessel part, it is also applied to angiography for rendering thin blood vessels.

FIG. 18 shows an example of a pulse for selectively exciting polymer protons. The pulse shown in the figure is a so-called binomial pulse composed of a plurality of pulse groups whose pulse length is a ratio represented by a binomial distribution and whose polarity is alternately reversed. : 2: 1 (such by specified as 1 2 1 pulse following a binomial pulse), so the total time of about 1msec in consideration of the frequency of the above, shorten the time τ between pulses It has been set.

Figure 19 is a curve showing the magnetization characteristics of the 1 2 1 pulse, the magnetization Mz is the longitudinal axis on the horizontal axis are taken the shift amount from the center frequency f _0. This curve shows that protons having a spectral value near the center frequency are not consequently excited and have no effect on the imaging sequence following the binomial pulse, but have a spectral value at 500 Hz or more from the center frequency. It is shown that the proton having is excited by the binomial pulse and no magnetization remains, and the level of the MR signal obtained in the subsequent imaging sequence decreases. In other words, a frequency range in which a signal can be extracted in a later photographing sequence corresponds to a peak of the curve in the figure, and a frequency range in which no signal is extracted corresponds to a valley of the curve in the figure.

Therefore, when the center frequency f ₀ of such a binomial pulse is applied in conformity with the resonance frequency of water protons, water protons are not excited, but polymer protons are excited at valleys of 500 Hz or more. This means that the level of the MR signal from the polymer can be reduced in the subsequent imaging sequence. As described above, the above-mentioned MTC effect can be obtained by previously selectively exciting polymer protons using a binomial pulse.

On the other hand, in MR imaging, a so-called fat-suppressing imaging method is often employed so that fat, which is said to have little clinical significance, does not appear in the image. There are many imaging methods for suppressing fat, and one of them is a method using a binomial pulse that can excite protons in a frequency-selective manner.

As described above, in the valley portion of the magnetization Mz of the binomial pulse, since the signal level appearing in the subsequent imaging sequence decreases, the binomial pulse is set so that the resonance frequency of protons contained in fat comes to the valley portion. Set the pulse.

Figure 20 is a diagram showing a 1 3 3 1 pulse which is set in this way. Here, the time axis is set so that the resonance frequency of the protons contained in the water is at the center frequency f ₀ and the resonance frequency of the protons contained in the fat is at a valley portion shifted from f ₀ by about 220 Hz to the low frequency side. On t, the time τ between pulses is set to be as long as about 2.3 msec.

印 加 If such a binomial pulse is applied and then a normal imaging sequence is executed, only protons in free water can be imaged, and the signal level from protons contained in fat can be suppressed.

By the way, in a magnetic resonance imaging (MRI) apparatus, the uniformity of a static magnetic field is an important factor in determining the quality of a spectrum (SN ratio and resolution). The adjustment means by the shim coil is provided. In the case of medical MRI, a primary shim (gradient shim) of x, y, z is used as a means for adjusting the MRI from the viewpoint of avoiding long-term restraint of the patient. The above separation has been performed.

However, 1 3 3 1 pulse added to suppress the MR signals from fat is pre-pulse, the length of the entire pulse becomes about 7 msec, since the repetition time TR is longer, the repetition time TR There is a problem that it is difficult to use in ultrafast scan or angiography, which uses a shorter length. On the other hand, if τ is shortened in order to shorten the repetition time TR, the peak portion in the graph of the magnetization Mz characteristic expands, fat is also imaged, and the effect of fat suppression is reduced. A conflicting problem. Further, since the pre-pulse is a pulse to the last, the pre-pulse is easily affected by the non-uniformity of the static magnetic field, and the image quality sometimes deteriorates. Furthermore, since the frequency range outside the resonance point of water is excited (off-resonance), the MTC effect also occurs, and the S / N ratio is reduced accordingly.

Further, the problem of securing the uniformity of the static magnetic field by the above-described conventional primary shim will be considered. Even if shimming is performed by the first-order shim, the disturbance of the higher-order (second-order or higher) magnetic field component of the static magnetic field still remains, and the non-uniformity of the higher-order component causes the resonance curve of water to change the pre-pulse ( For example, a state of deviating from the center frequency f _{0 of (} 1 3 1 binomial pulse) frequently occurs.

FIG. 21 shows an example of a frequency change at each slice position due to non-uniformity of a higher-order magnetic field component when the slice direction is plotted on the vertical axis and the frequency f is plotted on the horizontal axis. According to the position of the slice plane away from the center of the slice direction, the deviation of the same frequency f = f ₁ becomes remarkable.

As a result, especially when multi-slice imaging activates a pre-pulse aiming at fat suppression as described above, the center slice plane in the slice direction aligned with the isocenter position depends on the effect of the primary shim. For example, as shown in FIG. 22A, although the resonance curve of water coincides with the center frequency f ₀ , on the slice plane shifted from the center in the slice direction, due to the disturbance of the higher-order magnetic field component described above, for example resonance curve of water as shown in FIG. 5 (b) is shifted from the center frequency f _0, the resonance curves of water and fat departing from the peaks and valleys of the pre-pulse. Therefore, when multi-slice imaging is performed, the effect of fat suppression can be exerted on the slice plane near the center in the slice direction, but as the position deviates from the vicinity of the center, the effect of fat suppression becomes smaller, and the image is reduced according to the slice position. There is an unresolved problem that the quality of the product deteriorates.

The present invention has been made in view of the various circumstances described above, and in particular, in view of various problems using a sequence including a pre-pulse, when performing multi-slice imaging, without increasing the repetition time TR and reducing fat. It is an object of the present invention to obtain an MR image in which the MR signal is suppressed.

In order to achieve the above object, the magnetic resonance imaging apparatus according to the first, third, fourth, and sixth aspects of the present invention provides a magnetic resonance imaging apparatus for detecting the spin of protons contained in water in a slice section of a subject placed in a static magnetic field. RF pulse applying means for exciting and imaging an MR signal related to the spin excitation, and applying a 90 ° RF pulse capable of selectively exciting only protons contained in the water; A gradient magnetic field applying means for applying a gradient magnetic field for slicing the cross section at the same time as the application of the pulse is provided as a main part.

The magnetic resonance imaging apparatus according to the second to sixth aspects of the present invention excites proton spins contained in water in a plurality of slice cross sections of a subject placed in a static magnetic field, and generates an MR signal related to the spin excitation. And a first RF pulse applying means for applying a 90 ° RF pulse capable of selectively exciting only protons contained in the water, and the 90 ° RF pulse Simultaneously applying a first gradient magnetic field applying means for applying a first gradient magnetic field having a first magnetic field strength for slicing each of the plurality of slice sections, and the 90 ° RF pulse and the first gradient magnetic field. A second RF pulse applying means for applying a 180 ° RF pulse for inverting the spin excited by the 90 ° RF pulse after a lapse of a predetermined time after the application of the gradient magnetic field; And a second gradient magnetic field applying means for applying a second gradient magnetic field having a second magnetic field intensity higher than the first magnetic field intensity simultaneously with the application of the 80 ° RF pulse. .

According to the first, third, fourth and sixth aspects of the present invention, a gradient magnetic field for slicing the cross section is applied simultaneously with the application of a 90 ° RF pulse capable of selectively exciting only protons contained in water. You. Thereby, only protons of water are selectively excited, and fat is not excited.

According to the second to sixth aspects of the present invention, the first magnetic field intensity for slicing each of the plurality of cross sections is simultaneously adjusted with the application of a 90 ° RF pulse capable of selectively exciting only protons contained in water. Is applied. After a lapse of a predetermined time after the application, a 180 ° RF pulse for reversing the spin excited by the 90 ° RF pulse is applied simultaneously with the second gradient magnetic field having the second magnetic field strength larger than the first magnetic field strength. Is done. Since the intensity of the first and second gradient magnetic fields and the frequency bands of the 90 ° RF pulse and the 180 ° RF pulse are different as described above, only water protons are selectively excited by the 90 ° RF pulse, and then 180 ° It is refocused by an RF pulse and collected as an MR signal. However, spins contained in fat are only excited by the 180 ° RF pulse, and are hardly collected as MR signals. As a result, it is not necessary to use a pre-pulse, so that the MTC effect does not occur and the level of the MR signal from fat can be suppressed.

In the magnetic resonance imaging apparatus according to the first to sixth aspects of the present invention, a 90 ° RF pulse capable of selectively exciting only protons contained in water and a gradient magnetic field for slicing a cross section thereof are simultaneously applied, By applying a 180 ° RF pulse for inverting the spin excited by the 90 ° RF pulse after a predetermined time after such application, simultaneously with the gradient magnetic field having a magnetic field strength larger than the previous gradient magnetic field, Only water protons can be selectively excited. Further, since the front and rear gradient magnetic fields have different intensities, multi-slice can be performed. As a result, a high-quality MR image in which the level of the MR signal from fat is suppressed can be obtained without using the pre-pulse and without generating the MTC effect and without increasing the repetition time TR.

Hereinafter, a first embodiment of the present invention will be described with reference to FIGS.

FIG. 1 shows a schematic configuration of the magnetic resonance imaging apparatus according to this embodiment. This magnetic resonance imaging apparatus includes a magnet unit for generating a static magnetic field, a gradient magnetic field unit for adding positional information to the static magnetic field, a transmitting and receiving unit for selective excitation and reception of MR signals, system control and image reconstruction. And a control / arithmetic unit that performs the functions.

Magnet portion, for example, a magnet 1 of a superconducting type, and a static power supply 2 for supplying a current to the magnet 1, a static magnetic field H ₀ in the Z-axis direction of the cylindrical diagnostic space which the subject P is inserted generate. The magnet section is provided with a shim coil 14 for primary shimming, and primary shimming can be performed by adjusting the current supplied to the shim coil 14.

The gradient magnetic field unit includes three sets of gradient magnetic field coils 3x to 3z in the X, Y, and Z directions incorporated in the magnet 1, a gradient magnetic field power supply 4 for supplying current to the gradient magnetic field coils 3x to 3z, And a gradient magnetic field sequencer 5a in the sequencer 5 for controlling the control unit 4. The sequencer 5a includes a computer, and receives a signal for instructing an acquisition sequence according to the FE method, the high-speed SE method, or the like from a controller 6 (including a computer) of the entire apparatus. Thus, the gradient sequencer 5a is, X in accordance with the commanded sequence, Y, and controls the application and its intensity of each gradient magnetic field in the Z axis direction, those of the gradient magnetic field is enabled superposed on the static magnetic field H ₀ I have. In this embodiment, a slice gradient G _S the gradient magnetic field in the Z-axis direction of the three mutually orthogonal axes, its X-axis direction and a readout gradient field G _R, further Y-axis direction that the phase and encoding gradient field _{G E.}

The transmitting and receiving unit includes a high-frequency coil 7 disposed near the subject P in the imaging space in the magnet 1, a transmitter 8T and a receiver 8R connected to the coil 7, and a transmitter 8T and a receiver An RF sequencer 5b (computer mounted) for controlling the operation timing of the 8R. Under the control of the RF sequencer 5b, the transmitter 8T and the receiver 8R supply a pre-pulse for exciting nuclear magnetic resonance (NMR) or an RF current pulse of the Larmor frequency to the high-frequency coil 7, while supplying the high-frequency coil 7 Receives the received MR signal (high-frequency signal) and performs various kinds of signal processing to form a corresponding digital signal.

The control / arithmetic unit further includes, in addition to the controller 6 described above, an arithmetic unit 10 for inputting digital data of an MR signal formed by the receiver 8R and calculating image data, and a storage unit for storing the calculated image data 11, a display 12 for displaying an image, and an input unit 13. The arithmetic unit 10 specifically performs processing such as arrangement of measured data in a two-dimensional Fourier space, which is a memory space, and Fourier transform for image reconstruction. The controller 6 controls the operation contents and operation timing of the gradient magnetic field sequencer 5a and the RF sequencer 5b while synchronizing the two.

FIG. 2 shows a pulse sequence executed by the sequencer 5 (the gradient magnetic field sequencer 5a and the RF sequencer 5b).

わ か る As can be seen from the figure, the sequencer 5 executes a first sequence for applying a binomial pulse and a second sequence for acquiring MR image data. The second sequence is preferably performed by, for example, the FE method as shown in FIG.

In the first sequence, the center frequency of the 12 1 pulse P ₁₂₁ ^* is applied with a predetermined offset amount offset to the high frequency side (the symbol * indicates a pulse that can be offset as necessary). Indicating that). At this time, the offset amount is determined in advance by pre-scanning or the like, and is preferably set to 1.0 ppm to 4.0 ppm, and more preferably, 1.5 ppm to 3.0 ppm. In this case, for example, the frequency is 50 Hz to 250 Hz and 100 Hz to 200 Hz under a magnetic field of 1.5 Tesla, respectively.

To perform MR imaging using the magnetic resonance imaging apparatus of the present embodiment, first, a first sequence including a binomial pulse P ₁₂₁ ^* is executed. Due to the offset of the binomial pulse P ₁₂₁ ^* at this time, protons contained in fat and protons contained in the polymer are excited.

FIG. 3 is a graph showing a magnetization Mz curve when the binomial pulse P ₁₂₁ ^* is offset toward the high frequency side by, for example, 150 Hz.

わ か る As can be seen from the figure, the vicinity of the resonance frequency of water is a peak, so it is not excited. However, the resonance frequency of protons contained in fat, ie, about −220 Hz, is near a valley and is excited by a predetermined amount. Further, protons contained in the polymer are excited at the valleys.

Next, the second sequence is executed to collect MR image data. At the time of this collection, the fat signal level obtained in the second sequence will be lower because the fat and macromolecules have already been excited in the first sequence. Similarly, the signal level of the polymer is reduced. In contrast, protons contained in water are not excited in the first sequence, so that a strong signal is obtained in the second sequence.

As described above, the magnetic resonance imaging method and the magnetic resonance imaging apparatus of the present embodiment set the center frequency of the binomial pulse to water so that protons contained in fat and protons contained in the polymer are excited. Since the binomial pulse is applied by offsetting the resonance frequency of the contained protons to the high frequency side by a predetermined offset amount, the signal level of fat can be significantly reduced. Therefore, it is possible to image a thin blood vessel hidden in fat and invisible.

FIG. 4 shows experimental data obtained by measuring signal intensities coming from water and fat, and shows the offset amount on the horizontal axis.

わ か る As can be seen from the figure, when the offset amount is 1.0 ppm to 3.8 ppm, the signal intensity of water is about 100% to 50%, and the signal intensity of fat is 90% to 45%. When the offset amount is 1.5 ppm to 3.0 ppm, the signal intensity of water can be secured at about 96% to 84%, while the signal intensity of fat can be reduced to 84% to 63%.

In addition, the magnetic resonance imaging method and the magnetic resonance imaging apparatus according to the present embodiment can reduce not only the signal level of fat but also the signal level of a polymer, thereby obtaining an MTC effect. Particularly, in this embodiment, since the binomial pulse is offset, the frequency band in which the polymer is excited becomes close to the resonance frequency of water, and a higher MTC effect can be obtained than in the conventional sequence without such offset. .

Such a fat suppression effect and an MTC effect can be easily obtained by performing the first sequence prior to a normal imaging sequence, and it is not necessary to substantially increase the repetition time (TR).

Therefore, the magnetic resonance imaging method and the magnetic resonance imaging apparatus according to the present embodiment are extremely effective means for a pulse sequence using a short TR, for example, ultrafast scan or angiography.

、 In addition to the angio, it is possible to obtain the MTC effect while suppressing fat.

The magnetic resonance imaging method and the magnetic resonance imaging apparatus according to the present embodiment have another effect that the MR signal from the vein can be suppressed and the signal from the artery can be enhanced.

In the above embodiment, although the binomial pulse and 1 2 1 pulse, it is not limited thereto, it may be 1 3 3 1 or 1 4 6 4 1 pulse as necessary. With this configuration, the degree of separation between water and fat is further improved.

Next, a second embodiment of the present invention will be described with reference to FIGS. Here, the same reference numerals are used for the same or equivalent components or processes as in the first embodiment, and the description is simplified or omitted (the same applies to the third and fourth embodiments described later).

The magnetic resonance imaging apparatus according to the second embodiment relates to prevention of deterioration of the effect of suppressing an MR signal from fat in multi-slice imaging.

In order to achieve this object, the magnetic resonance imaging apparatus according to the second embodiment employs the same hardware configuration as the first embodiment, while the controller 6, the sequencer 5, and the arithmetic unit 10 cooperate with each other, as shown in FIG. The processing of FIG. 6 according to the sequence of (a) and (b) is performed.

That was carried out using shimming pulse sequence shown in first in step 30 X, Y, 1-order shimming the Z-axis direction (volume shimming), for example, FIGS. 5 (a) of FIG. 6, a uniform static magnetic field H ₀ strive to reduction, pre-pulse of each slice plane (here 1 3 3 1 binomial pulse _{P 1331} ^* to use: see FIG. 5 (b)) determine the frequency offset Δf of.

Specifically, of a plurality of slice planes constituting a three-dimensional slice area, a first slice plane (for example, a plane at the center in the slice direction corresponding to the isocenter) is excited by a 90 ° pulse to generate an MR signal (echo signal or echo signal). FID signal) is obtained (see step ₃₀ 1, 30 _2). Then, the MR signal is Fourier transform, for example, as shown in FIG. 7, the water and fat resonant frequency (center _frequency) f _WAT, spectrum comprising _{f FAT} is obtained (see step 30 _3). Therefore, the half widths W _WAT , W _FAT of the resonance curves C _WAT , C _FAT of water and fat are respectively calculated, and the half-widths W _WAT , W _FAT are supplied to the shim coil 14 (see FIG. 1) so as to minimize the half widths. current is adjusted (see step ₃₀ 4-30 _6). As a result, the water and fat resonance curves C _WAT and C _FAT are well separated from each other.

When the primary shimming is over the volume portion of the thus diagnosed sequentially executes the processing of step 30 _{7-30 10.} Among them, in step ₃₀ 7-30 _9, similarly to Step _{301 to 303,} the MR signals collected by performing scanning for each slice plane, and Fourier transform the collected MR signal. As a result, a spectrum distribution including resonance curves C _WAT and C _FAT of water and fat is obtained for each slice plane (see FIG. 7).

Further, in step _{30 10,} 1 3 3 1 pre-pulse _{P 1331} ^* the fat suppression effect becomes highest when the set frequency position prepulse _{P 1331} ^* center frequency _f 0 _{(= f d0)} and the resonance frequency of water The deviation from f _WAT is calculated and stored as a frequency offset amount Δf (= f _d0 −f _WAT ). However, in this embodiment, since the center slice plane (coincident with the isocenter position) is used as a reference, the frequency offset amount Δf = 0 of the center slice plane.

Operation of such offset amount is performed individually for all the slice planes (see step 30 _11). As a result, as shown in the example in FIG. 8, the frequency offset amount for 1 3 3 1 prepulse _{P 1331} ^* for each of a plurality of slice planes _{S 1 ~S n Δf = (Δf} 1, ..., Δf n) is obtained.

Subsequently, a 1 3 3 1 pre-pulse _{P 1331} ^*, by applying as shown in FIG. 5 (b) before the sequence depends on the SE method, the multi-scan imaging is performed (see step 31). In performing this series of sequences, the 1 3 3 1 pulse _{P 1331} ^* for fat suppression, the frequency offset Δf calculated in step _{30 10} is added, 1 corresponding to the addition result 3 3 1 pulse _{P 1331} ^* is determined (see step ₃₂ 1, 32 _2). When the corrected pulse _P1331 ^* is applied from the RF coil 7, the entire frequency range of magnetization by the prepulse _P1331 ^* moves to the water resonance curve _CWAT on the frequency axis for each slice plane. As a result, for example, in each of the spectra in Figure 9 shows a slice plane S ₁ to S n _{(corresponding} to FIG. 8), and due to higher turbulence of the magnetic field component of the static magnetic field H _0, the spectrum of each slice plane curve _{C WAT,} even when _{the C FAT} is moved on the frequency axis (misaligned) as for each slice plane, 1 3 3 1 pulse _{P 1331} ^* center frequency _{f 0} of the resonance curve _{C WAT} water It is controlled to match the center. At this time, the excitation range by the RF pulse is fixed even if the slice plane changes.

Therefore, the conventional problem that the spectrum and the pre-pulse are shifted as the slice plane moves away from the center position in the slice direction is surely eliminated, and the peaks and valleys of the magnetization characteristics of the pre-pulse _P1331 ^* are changed to water and water regardless of the slice plane. Good agreement with the fat resonance curves C _WAT and C _FAT . For this reason, the effect of fat suppression is reliably exerted on any slice surface, and a high quality image is obtained, and a uniform and stable image with very little variation between slice surfaces in terms of the fat suppression effect is obtained, and the reliability of the apparatus is also improved. improves. In addition, since the shim coil 14 of this embodiment only needs to be used for the primary shimming, the hardware configuration of the shim coil does not become particularly large. Further, a long scan preparation time such as when performing higher-order shimming is not required.

Next, a third embodiment of the present invention will be described with reference to FIGS.

As in the second embodiment, the magnetic resonance imaging apparatus according to the third embodiment also relates to prevention of deterioration of the effect of suppressing MR signals from fat in multi-slice imaging. In both the second and third embodiments, the technique of suppressing MR signals from fat using a pre-pulse is adopted. In the second embodiment, as described above, the frequency band of the pre-pulse is adjusted for each slice plane. In contrast to this, the third embodiment adjusts the frequency itself of the high frequency signal of the RF pulse for each slice plane, while suppressing the MR signal from fat.

The magnetic resonance imaging apparatus according to the third embodiment employs the same hardware configuration as that of the first embodiment, and the controller 6, the sequencer 5, and the arithmetic unit 10 cooperate with each other to form the magnetic resonance imaging apparatus shown in FIGS. ) Are performed according to the sequence shown in FIG.

In a first step 40 in FIG. 11, carried out using X, Y, shimming pulse sequence showing first-order shimming the (volume shimming) in FIG. 10 (a) for the Z axis, achieve uniform static magnetic field H ₀ At the same time, the offset amount Δf with respect to the frequency of the RF pulse applied to the slice plane is calculated for each slice plane.

Among them, the steps ₄₀ 1 to 40 ₆ shows an overview of a process for the first-order shimming, the same processing contents as steps ₃₀ 1 to 30 ₆ of FIG. 6 described above. Note that higher-order shimming is also possible as shimming.

When the shimming is completed, the process of step ₄₀ 7-40 ₉ (scan, MR signal acquisition, the Fourier transform) sequentially performed to obtain water and fat resonances curve _{C WAT,} the spectral distribution of each slice plane including the _{C FAT} (See FIG. 12). Thereafter, the process proceeds to step _{40 10,} 1 3 3 center frequency _f o _{(= f do)} and the reference peak of one of the pre-pulse _{P 1331} when the set becomes highest frequency position is fat suppression effect prepulse _{P 1331} frequency shift of the resonant frequency _{f WAT} of water as a calculates a frequency offset _{Δf (= f do -f WAT)} , and stores. In this embodiment, as shown in FIG. 12, the frequency position of the pre-pulse P ₁₃₃₁ is set with reference to the center slice plane (slice plane including the isocenter position). Δf = 0. The calculation of the offset amount Δf is individually performed for all slice planes (step 40 ₁₁ ). As a result, the frequency offset amount Δf (= Δf ₁ ,..., Δf _n ) with respect to the frequency of the RF pulse is obtained for each of the plurality of slice planes of the volume part (eg, abdomen) to be diagnosed.

Thereafter, at step 41, executed as showing the sequence of SE method using 1 3 3 1 pre-pulse _{P 1331} for suppressing MR signals from fat in FIG. 10 (b), the multi-scan imaging I do. During this sequence, the frequency of the RF pulse for excitation is corrected for each slice plane S ₁ (... S _n ).

のように調整される。 Specifically, in step 42 _1, after calling step ₄₀ the frequency offset has been precomputed / storage at ₁₀ Delta] f = Delta] f ₁ a (... Δf _n) from the memory, in step 42 _2, advance as a reference value The frequency offset amount Δf is added to the set frequency (for example, at 64 MHz: 1.5 T), and the center frequency of the RF pulse: RF ₁ (... RF _n ) is corrected and calculated. Thus, for example, a slice plane _S 1 ... _{S n} of the RF pulses: the center frequency _f 1 ... _{f n} of the _RF 1 ... RF _n (where, at the time 1.5T)

It is adjusted as follows.

When the corrected RF pulses RF ₁ ... RF _n are applied from the RF coil 7, the excitation range Rf itself moves on the frequency axis as shown in FIG. magnetization characteristics of 1 3 3 1 pulse for suppressing also moves on the frequency axis. That is, in each slice plane, the excitation range Rf and the magnetization characteristics move on the frequency axis integrally. The moving distance (frequency value) is offset Delta] f described above _{(= Δf 1, ..., Δf} n) matches, in the offset Delta] f is the frequency domain of the water resonance curve for each slice plane S ₁ ... S _n because are determined in accordance with the movement, in each slice plane, as shown in FIG. 13, the center frequency _{f o} of 1 3 3 1 pulse _{P 1331} matches favorably to the center frequency of the resonance curve _{C WAT} water.

In other words, even after the shimming is performed, for example, the disturbance of the higher-order static magnetic field components remains, and even if the water or fat spectral distribution of each slice plane moves due to this, any slice plane may be moved. In particular, the peaks and valleys of the magnetization characteristics of the pre-pulse P ₁₃₃₁ match well with the resonance curves C _WAT and C _FAT of water and fat.

As a result, the effect of suppressing the MR signal from fat by the CHESS (Chemical {Select-ive} Suppression) method is always exerted on any slice plane, and the unevenness due to the signal from fat for each slice plane is significantly reduced. The same effect as that of the second embodiment can be obtained.

Especially, even when the shift of the spectral distribution of a certain slice plane is larger than the chemical shift of water and fat, the frequency correction of this embodiment works effectively. In other words, as in the conventional case, when the magnetization characteristic of the pre-pulse for fat suppression is fixed on the frequency axis, the effect of fat suppression is effective on one slice plane, but the effect of fat suppression is reduced on another slice plane. Or there is a situation where there is almost nothing. However, according to the present embodiment, such a situation can be prevented, and the MR signal from fat can be significantly and uniformly suppressed in each slice plane.

Incidentally, the second, the pre-pulse in the third embodiment such as 1 2 1 pulse is not limited to 1 3 3 1 binomial pulse, may be another binomial pulse, sinc function, It may be a pulse of a Gaussian function. Further, the image data acquisition sequence performed after the pre-pulse is not limited to the above-described SE method, and a sequence such as the FE method and the FastSE method may be adopted.

In addition, in the second and third embodiments, the configuration in which the resonance curve of water is referred to as the reference peak on the spectrum distribution when calculating the frequency offset amount Δf has been described. However, as another example of the reference peak, , A fat resonance curve. In addition, since a part containing almost no water or fat may be a target of diagnosis, in such a case, the operator uses a reference reagent (eg, TMS: tetramethylsilene) outside the part to be diagnosed. Then, the offset amount may be obtained with reference to the resonance curve of this reagent.

Next, a fourth embodiment of the present invention will be described with reference to FIGS.

The magnetic resonance imaging apparatus according to the fourth embodiment does not use the prepulse described in the first to third embodiments, does not increase the repetition time TR, and suppresses the MR signal from fat. Can be performed.

Specifically, the sequencer 5 of the magnetic resonance imaging apparatus according to the present embodiment is configured to perform multi-slice imaging in the sequence shown in FIG. The sequence shown in the figure is a sequence to which the fast SE (Fast SE) method is applied, and is a sequence in which a slice plane of a subject is selected and only water in the plane is selectively excited (fat is not excited). Hereinafter, will be the imaging method using this sequence is referred to as a "Water Ch emic a l S elective E xcitation (Water-CHASE)" method.

The Water-CHASE method, first, as shown in FIG. 14, while applying a gradient magnetic field in the Z axis direction _{G S} = strength _{G S1} for slice selection. A 90 ° pulse is applied as an excitation RF pulse. The 90 ° pulse is set to have a pulse length of about 20 msec by increasing the number of π of the sink function by (−4, +1) π and taking one side lobe (left side in the figure) longer. Note that the side lobe on the other side (right side in the figure) is cut in this embodiment in order to shorten the echo time TE. Regarding this π number, there is an effect even if it is set to about (−1, + 1) π to (−10, + 10) π as necessary.

Due to the 90 ° pulse that is longer in time than the conventional one, the excitation frequency range Rf becomes narrow as shown in FIG. This excitation frequency range Rf is adjusted so that only the resonance spectrum curve C _WAT of water is included in the plane serving as a reference for multi-slice imaging, for example, the central slice plane, and the resonance curve C _{FAT of} fat due to chemical shift does not enter. Is done. The slice gradient G _S while conventionally applies a 90 ° pulse of long pulse length than is set to a predetermined intensity G _S1.

Thereafter, with the readout gradient G _R, the gradient G _E is applied for phase encoding.

Thereafter, in accordance with the elapse of TE / 2 hours, inverted 180 ° pulse as a RF pulses ([pi number is less than 90 ° pulse) is applied together with a slice-selective gradient _{G S} = strength _{G S2.} The intensity of the gradient magnetic field _GS2 at this time is set to be larger than that at the time of applying the 90 ° pulse ( _GS2 > _GS1 ). As described above, since the two slice gradient magnetic fields G _S1 and G _S2 and the frequency bands of the 90 ° RF pulse and the 180 ° RF pulse are different, the plane sliced when the 180 ° pulse is applied is the slice plane when the 90 ° pulse is applied. And about the slice thickness.

Further, thereafter, after a lapse TE / 2 hours, while applying a readout gradient field G _R, echo signals are received.

Hereinafter, an amount corresponding similar 180 ° pulses of the number of determined echoes is applied (slice gradient strength at that time G _{S =} strength G _S2) image data is collected by the _high-speed SE method.

Thus, in the Water-CHASE method, the first long 90 ° pulse with the time axis slice gradient _G S _{(G S1)} is applied, the desired slice plane is selected, and, the slice plane As shown in FIG. 15 (only the resonance curve C _WAT of water is included in the excitation range Rf by the 90 ° pulse), the protons contained only in the water within are excited selectively. In other words, the protons of the water are always excited by a narrow-band 90 ° pulse (On-resonance). Thereafter, when a 180 ° pulse is applied, the intensity and frequency band of the slice gradient magnetic field G _S (= intensity G _S2 ) are different, so that the slice excitation positions differ by about the slice thickness. Even if there is a plane, the next 180 ° pulse excites a different plane, so that no fat MR signal is collected.

結果 As a result, the spin of water that has been tilted to the XY plane by the 90 ° pulse is then refocused by the 180 ° pulse and collected as an MR signal. However, since fat spin is not excited by a 90 ° pulse, even if it is excited by its wide excitation range Rf ′ (see FIG. 16), it is only tilted in the −Z-axis direction by a 180 ° pulse, and the fat spin MR is reduced. No signal is collected. Thereby, the fat does not contribute to the acquisition of the MR signal, and as a result, the MR signal from the fat can be greatly suppressed, and multi-slice imaging can be performed, so that good image quality can be obtained.

As described above, since only water protons can be selectively excited at the same time as slicing, it is not necessary to use a pre-pulse, and the repetition time TR can be shortened as compared with the case where a pre-pulse is used. In addition, since water is not selectively excited by a pulse such as a binomial pulse, the non-uniformity of the static magnetic field becomes stronger than when a binomial pulse is used. Further, unlike the method of exciting a frequency shifted from the resonance frequency of the protons of water, the MTC effect does not occur, so that the MR signal value of water does not decrease. Thus, it satisfactorily maintains the S / N ratio to that of the corresponding contrast of T ₁ weighted image. On the other hand, in fat molecules, there is spin J-coupling between adjacent protons, and the signal value decreases. However, in the Fast SE method, J-modulation occurs due to J-modulation. There is a problem that the signal from fat rises. However, in the Water-CHASE method, since fat is hardly excited, the problem of J-modulation hardly occurs. Therefore, the Water-CHASE method can be applied to the Fast SE method.

The sequence applied to the Water-CHASE method may be the SE method, and may be the fast SE (Fast SE) method.

FIG. 1 is an overall block diagram of a magnetic resonance imaging apparatus common to embodiments. 9 is a pulse sequence illustrating a sequence of MR imaging according to the first embodiment. 9 is a graph showing a magnetization Mz curve when a binomial pulse is offset by 150 Hz to a high frequency side. Experimental data measuring the signal intensity coming out of water and fat. FIG. 7A shows a pulse sequence for shimming according to the second embodiment, and FIG. 7B shows a pulse sequence for MR imaging according to the second embodiment. 9 is a schematic flowchart for executing the sequence of the second embodiment. The figure which shows the separation of the resonance spectrum of water and fat by primary shimming. FIG. 6 is a diagram for explaining calculation of an offset amount for each slice plane. FIG. 9 is a diagram for explaining a pre-pulse offset for each slice plane according to the second embodiment. FIG. 7A shows a pulse sequence for shimming according to the third embodiment, and FIG. 7B shows a pulse sequence for MR imaging according to the third embodiment. 9 is a schematic flowchart for executing the sequence of the third embodiment. FIG. 6 is a diagram for explaining calculation of an offset amount for each slice plane. FIG. 13 is a diagram for explaining an RF pulse offset for each slice plane according to the third embodiment. 16 is a pulse sequence illustrating a sequence of MR imaging of the fourth embodiment. The figure which shows the state in which the resonance curve of only water is in the excitation range by 90 pulses. The figure which shows the relationship between the excitation range by a 180 degree pulse, and the resonance curve of water and fat. FIG. 3 is a spectrum diagram of protons contained in water, fat, and a polymer. The figure which showed the pre-pulse for selectively exciting a polymer proton. Curve represents the magnetization characteristics of the 1 2 1 pulse. Shows a 1 3 3 1 pulse. FIG. 7 is a diagram illustrating a frequency shift caused by disturbance of a higher-order component of a static magnetic field in a slice direction. FIG. 7A is a diagram showing a state in which the resonance curves of the pre-pulse and water and fat match, and FIG. 7B is a diagram showing a state in which the resonance curves of the pre-pulse and water and fat are shifted.

Explanation of reference numerals

Reference Signs List 1 magnet 2 static magnetic field power supply 3x to 3y gradient magnetic field coil 4 gradient magnetic field power supply 5 sequencer 6 controller 7 high frequency coil 8T transmitter 8R receiver 10 arithmetic unit 11 storage unit 12 display 13 input device 14 shim coil

Claims (6)

In a magnetic resonance imaging apparatus that excites spins of protons contained in water in a slice cross section of a subject placed in a static magnetic field and images an MR signal related to the spin excitation,
RF pulse applying means for applying a 90 ° RF pulse capable of selectively exciting only protons contained in the water,
A magnetic resonance imaging apparatus comprising: a gradient magnetic field applying unit that applies a gradient magnetic field for slicing the cross section simultaneously with the application of the 90 ° RF pulse. In a magnetic resonance imaging apparatus that excites the spin of protons contained in water of a plurality of slice cross sections of the subject placed in a static magnetic field, and collects and images an MR signal related to the spin excitation in a predetermined sequence,
First RF pulse applying means for applying a 90 ° RF pulse capable of selectively exciting only protons contained in the water, and simultaneously slicing each of the plurality of slice sections simultaneously with the application of the 90 ° RF pulse A first gradient magnetic field applying means for applying a first gradient magnetic field having a first magnetic field strength for applying the 90 ° RF pulse and the first gradient magnetic field after a predetermined time has elapsed after the first gradient magnetic field has been applied. Second RF pulse applying means for applying a 180 ° RF pulse for inverting the spin excited by the RF pulse, and simultaneously applying the 180 ° RF pulse, a second magnetic field intensity larger than the first magnetic field intensity And a second gradient magnetic field applying means for applying a second gradient magnetic field, the magnetic resonance imaging apparatus comprising: The magnetic resonance imaging according to claim 1, wherein the 90 ° RF pulse is set such that an excitation frequency region of the 90 ° RF pulse includes only a resonance frequency of protons contained in the water. apparatus. The magnetic resonance imaging apparatus according to claim 1, wherein the 90 ° RF pulse is a pulse obtained by modulating a high-frequency signal with a sine function having an increased π number. The magnetic resonance imaging apparatus according to claim 2, wherein the frequency bands of the 90 ° RF pulse and the 180 ° RF pulse are different from each other. 3. The magnetic resonance imaging apparatus according to claim 1, wherein the sequence is a sequence of a spin echo method.

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