JP2916929B2 - MRI equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention uses a magnetic resonance phenomenon to image a subject.
More particularly, the present invention relates to an MRI apparatus capable of simultaneously obtaining a plurality of images having different contrasts.

(Prior Art) When an nucleus is placed in a static magnetic field, the nucleus precesses at an angular velocity proportional to a constant that varies depending on the strength of the magnetic field and the type of the nucleus. When a high-frequency rotating magnetic field having the above frequency is applied to an axis perpendicular to the static magnetic field, magnetic resonance occurs, and a group of specific nuclei having the constant causes transition between levels by a high-frequency magnetic field satisfying the resonance condition, and energy Transit to the higher level. Nuclei excited to a higher level after resonance return to a lower level and emit energy. MR
I is a device for observing a nuclear magnetic resonance (hereinafter referred to as NMR) phenomenon caused by this specific nucleus and capturing a tomographic image of the subject.

FIG. 4 shows a pulse sequence of applying a high-frequency magnetic field and a gradient magnetic field used in the Fourier transform method in MRI. In period 1, z = 90 ° pulse 1 and slice gradient 2
Spins in a slice plane perpendicular to the z direction centered at = 0 are selectively excited. The rephase gradient 3 in the period 2 is for restoring the phase of the spin disturbed by the slice gradient 2. The dephase gradient 4 in the same period 2 is for giving a phase difference according to the place to the spin so that the center of the SE signal 5 coincides with the time center of the data read period 4. In period 2, a warp gradient 6 for shifting the phase of the spin in proportion to the position in the y direction is applied, and the warp gradient 6 is applied with its intensity changed every period. Thereafter, a 180 ° pulse 7 is given to align the magnetic moments, and the SE signal 5 appearing thereafter is observed. Period 4
Then, a read gradient 8 is applied to the x-axis. Thus, the phase difference given by the dephase gradient 4 is canceled at the time center of the read gradient 8 in the period 4, and the SE signal 5 appears. This sequence is called a view, and after the pulse repetition period TR, 90 ° pulse 1 is added again to start the next view.

In the above-mentioned MRI, it takes about 4 to 5 minutes for one scan when performed in a normal sequence. However, in the imaging of a moving organ or in the case where it is difficult to stop the movement of the subject itself, imaging is performed. In order to reduce the influence of the movement in the part where the pulse is repeated, a high-speed scanning method for shortening the pulse repetition period is used. For example, FAST (Fourier Acquired
Steady-state Technique), FISP (Fast Imaging with
Steady Precession), CE-FAST (Contrast Enhansed F)
AST). Each of these scan sequences uses one type of RF pulse with a flip angle.
The sequence observes the FID signal immediately after the RF pulse, and the CE-FAST sequence observes the SE signal 5 immediately before the RF (the echo from the previous RF pulse appears immediately before the next RF pulse). It is completely different.

(Problems to be Solved by the Invention) By the way, each of the above-mentioned scanning methods can obtain only an image having one type of contrast per scan.
On the other hand, FADE (Fast Acquisition Double Ec) is used as a sequence for simultaneously obtaining the above two types of contrast.
A method called ho) has been proposed, but this method also involves only repeated application of RF pulses with the same flip angle, so there is little freedom in how to apply image contrast, and only images with two types of contrast can be obtained. Absent.

The present invention has been made in view of the above points, and an object of the present invention is to realize an MRI apparatus capable of simultaneously obtaining images having various types of contrast.

(Means for Solving the Problems) According to the present invention for solving the above problems, spins in a subject placed in a static magnetic field are set to a steady state, and each of a slice direction, a warp direction, and a read direction of the subject is set. In a high-speed MR imaging method in which a plurality of gradient magnetic fields and RF pulses are applied during one repetition cycle according to a predetermined sequence in the direction to detect a plurality of MR signals from a slice plane and obtain a tomographic image of a subject, Apply any combination of RF pulses with the same flip angle or different flip angles among multiple RF pulses with different flip angles to be applied during the cycle,
Each of the gradient magnetic fields in the slice direction and the warp direction is applied to the RF
In addition to applying so that the time integration becomes zero within the pulse application cycle, readout gradient magnetic fields of different waveforms for different high-speed scan sequences in the read direction are maintained so that their areas are kept equal for each RF pulse application cycle. The readout gradient magnetic field is applied by arbitrarily combining gradient magnetic fields having different waveforms or gradient magnetic fields having the same waveform.

(Operation) A plurality of RF pulses having different flip angles or equal flip angles are appropriately combined in one repetition period, and a readout gradient magnetic field is used to change a waveform of a high-speed scan sequence that is different for each RF pulse period or a waveform of the same high-speed scan sequence. When used in an appropriate combination, a plurality of images having different contrasts can be obtained.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram of a pulse sequence according to one embodiment of the present invention. In the figure, reference numeral 11 denotes a flip angle of α ₁ , α ₂ , α ₃ ,
.., Α _n are RF pulses whose values are equal or different, and the interval between each RF pulse 11 is τ.
(≦ T ₁ , T ₂ : T ₁ is the vertical relaxation time, T ₂ is the horizontal relaxation time) and n
One RF pulse constitutes one repetition period (1TR). Therefore, TR = n · τ. Reference numeral 12 denotes a slice gradient for determining a plane for selectively exciting spins. The integral of the gradient magnetic field per 1τ time applied in the slice direction by the rephase gradient 13 of the negative pulse applied before and after the slice gradient is set to zero, and the spin is determined. It is in a steady state. A warp gradient 14 that changes the amplitude by one step for each TR is applied to the warp axis, and gives phase information in the warp direction. By applying a warp gradient 15 having the same amplitude and the opposite direction immediately before the next RF pulse 11 to the warp gradient 14, the integral value of the gradient magnetic field area per 1τ time applied in the warp direction is made zero, Spin is in steady state.

16 is a readout gradient for observing the echo signal,
R ₁ , R ₂ ,..., R _n are applied between the RF pulses 11 by arbitrarily selecting equal values or different waveform gradients.
FIG. 2 shows an example of the waveform of the readout gradient 16. In the figure, (a) is the readout gradient in the FAST sequence.
The FID signal 18 immediately after the RF pulse 11 is observed in the form of a field echo with the 16 waveform a. In order not to generate the SE signal appearing immediately before the RF pulse, the spoiler 17 may be inserted after data sampling for adjusting the area with another gradient waveform. (B) The figure shows the readout gradient waveform b in the case of the CE-FAST sequence, in which the SE signal 5 immediately before the RF pulse 11 is observed in the form of a field echo. Also in this case, a spoiler 17 may be inserted to adjust the area with another gradient waveform so as not to generate the FID signal 18 immediately after the RF pulse 11. (C) The figure shows the read gradient waveform c of the FISP sequence and the above FID signal 18 and SE
The signals 5 are superimposed and observed simultaneously. It is essential that this waveform c has a total area of zero, and therefore cannot be used with other gradient waveforms.
R ₁ = R ₂ = R ₃ =... = R _n = c in the figure. That is, only the set of RF pulses 11 can be changed. (D)
The figure shows the readout gradient waveform d of FADE and the FI immediately after RF pulse 11.
The D signal 18 and the SE signal 5 immediately before the RF pulse 11 are simultaneously observed in the form of a field echo. A spoiler 10 may be inserted to more clearly separate the front and rear echoes or to adjust the area with another gradient waveform.
(E) The figure shows the waveform e of the read gradient of the ME-FAST (multi-echo FAST) sequence. By increasing the amount of dephase after data sampling, the SE signal 5, stimulated echo (3 times RF) (Obtained by a pulse) or more historical echoes in the form of field echoes. At this time, RF pulse 11
In order not to generate the FID signal 18 immediately after, a spoiler 17 may be inserted for adjusting an area with another gradient waveform.

Returning to FIG. 1, R _1, R ₂ of the readout gradient 16, ..., the second to R _n
Readout gradient of various scan sequences shown in the figure 16
May be used. The readout gradient 16 is the second
The waveform is not limited to the waveform of the readout gradient 16 shown in the drawing, and any waveform may be used as long as the waveform applied between the RF pulses 11 has the same area. The slice gradient 12, rephase gradient 13, and warp gradient 14 applied during 1TR are
The gradient area is constant between the RF pulses 11. However, the warp gradient is applied in increments of one step for each TR. Normally, the gradient area preferably has its integral value set to zero, but a spoiler 17 may be inserted if necessary.

The selection of the readout gradient 16 (R ₁ ,..., R _n ) is free as described above, and the order is also free. Further, they may be used repeatedly. For example, R ₁ = (a), R ₂ = (b), R ₃ =
(D), and R ₁ = (a), R ₂ = (a), R ₃ =
(B), R ₄ may be equal to (b). Further, R ₁ = R ₂ = R ₃ = ... = R
_The waveforms may all be the same as _n . However, if the waveform between the readout gradients 16 is different, it is necessary to adjust the size of the spoiler 17 to make the area the same.

The combination of the read gradient 16 is as described above,
A combination of flip angles of the RF pulse 11 can also be freely selected. That is, α ₁ ≠ α ₂ ≠ ... ≠ α _n may be used,
α ₁ = α ₂ ≠ α ₃ = α ₄ ≠ α ₅ = α ₆ ≠ ... α _n-1 = α _n
And α ₁ = α ₂ =... = Α _n
RF pulses 11 having the same flip angle may be used.
However, when the readout gradients 16 are all the same and the flip angles of the RF pulses are all the same, there is no change, so that any one of them must be different.

FIG. 3 is a diagram showing a specific example based on the above principle. In the figure, the same parts as those in FIG. 1 are denoted by the same reference numerals. In this example, two different RF pulses 11 having flip angles α and β are applied, and 1T is applied by the two RF pulses 11.
R (TR = 2τ), and the readout gradient 16 is a combination of the waveforms a and b in FIG. That is, this pulse sequence uses the waveform a of the readout gradient 16 in the first half and the waveform b in the second half. The signal obtained is the FID signal 18 immediately after the RF pulse 11 in the first half.
And the SE signal 5 immediately before the RF pulse 11 in the latter half is observed in the form of a field echo. In this pulse sequence, in the first half, the SE signal 5 immediately before the RF pulse 11 was not generated, and in the second half, the FID immediately after the RF pulse 11 was generated.
The spoiler 17 is inserted so as not to generate the signal 18, and the area is made equal between the first half and the second half. In this manner, two types of images having different contrasts in the first half and the second half can be obtained at the same time. Further, by adjusting the flip angles α and β of the RF pulse 11, the degree of freedom in changing the contrast can be increased.

As described above, according to the present embodiment, by appropriately selecting a combination of a plurality of RF pulses having an arbitrary flip angle and a combination of a plurality of gradient waveforms for observing different types of echoes, different contrasts are obtained. Multiple images can be obtained simultaneously. Further, by changing the magnitude of the flip angle of the RF pulse and by changing the combination thereof, the degree of freedom to change the contrast is greatly increased.

(Effects of the Invention) As described in detail above, according to the present invention, images having various types of contrast can be obtained at the same time, and the practical effect is great.

[Brief description of the drawings]

FIG. 1 is a diagram of a pulse sequence according to one embodiment of the present invention, FIG. 2 is a waveform diagram of a read gradient used for various sequences, (a) is a FAST sequence, and (b) is a CE.
-FAST sequence, (c) figure is FISP sequence,
(D) is a FADE sequence, (e) is a waveform diagram of the read gradient of the ME-FAST sequence, FIG. 3 is a pulse sequence of a specific example of the embodiment of the present invention, and FIG. 4 is a conventional MRI. It is a figure of a pulse sequence. 1… 90 ° pulse 2,12… Slice gradient 3,13… Rephase gradient 5… SE signal 6,14,15… Warp gradient 8,16… Readout gradient, 11… RF pulse 17… Spoiler, 18 …… FID signal

Claims (1)

(57) [Claims] An object is placed in a static magnetic field, and a plurality of objects are set in one repetition cycle according to a predetermined high-speed scan sequence.
Applying an RF pulse and a gradient magnetic field, a plurality of
An MRI apparatus for detecting an NMR signal and generating an image of the subject, applying the plurality of RF pulses including a flip angle different from the other, and applying a plurality of read gradients to the plurality of gradient magnetic fields. A magnetic field is included and at least one of the plurality of read gradient magnetic fields is provided with means for setting a fast scan sequence for applying a read gradient magnetic field corresponding to a different fast scan sequence from the other. An MRI apparatus characterized in that: