JPS6368151A - Magnetic resonance imaging method - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention relates to magnetic resonance imaging using the NMR (Nuclear Magnetic Resonance) phenomenon, particularly a chemical method that can separate water and fat images having different chemical shifts. It concerns the shift imaging method.

[Prior art] Figure 3 shows, for example, the 25th Japan ME Society Conference (198
'HNMR chemical shift imaging with reduced influence of transverse relaxation', Medical Electronics and Bioengineering, Vol. 24, Special Issue, 3-P^-12, No. 56, published in April 2016).
A pulse sequence diagram for conventional chemical shift imaging, described on page 1, is shown.

In the figure, Rf is a high-frequency magnetic field pulse, and (1) is 9
The 0° pulse and (2) are the 180° pulses, which rotate the nuclear spins by 90° or 180°, respectively. Gx, Gy
are gradient magnetic field pulses in the X-axis direction and the Y-axis direction, respectively, (3) is a pulse that imparts phase modulation (phase encoding), and (5) is a pulse that imparts frequency modulation (frequency encoding) to the signal, respectively. Further, (4) is applied to compensate for phase disturbance. NMR is the NMR signal, (6
).

In Figure 3, the pulse (
4) and the diagonally shaded area S 1 of the frequency modulation pulse (5)
, S 2 become equal, that is, from the time when the maximum value of the 180° pulse (2) is given, τb? &ni, N M
Let τa be the time between the peak values of the 90° pulse (1) and the 180° pulse (2) when the R signal (6) is re-imaged and has its maximum value, and let τ = τb - τa. If the chemical shifts are σ3, σ3, the nuclear gyromagnetic ratio is γ, and the phase difference between spins with two chemical shifts is θ, then θ=γ(σ, −σ,)τ, so that θ−90°. By choosing τ for
Time resonance images with different chemical shifts can be separated into real and imaginary parts of the reconstructed image. This method is described in detail in "Separation of NMR Chemical Shift Image into Real Part Image and Imaginary Part Image", Institute of Electronics and Communication Engineers Technical Report (IEICE Technical Report) MBE85-44, pages 33 onwards. The value of τ is extended when the static magnetic field strength B0 is low. For example, when separating the chemical shift images of water and fat with a chemical shift difference of 3.5 pp-, when B = 0.5 T (Tesla), τ = 3.5 m5ec, B, -0,
In the case of IT, τ = 17.5 m5ec, and the time until receiving the signal (echo time) becomes longer. By extending the echo time in this manner, a chemical shift image with emphasis on relaxation time can be obtained.

Originally, when you want to obtain a chemical shift density image, this is not desirable, and to overcome this, it was announced at the 25th Japan ME Society Conference that θ should be smaller than 90° [ Reducing the effects of transverse relaxation is described in 'HNMR Chemical Shift Imaging'. That is, the reconstructed image S (x, y) after correcting the phase error due to the non-uniformity of the static magnetic field with respect to the appointed officer's θ is as follows.

S (x, y) = p + (x, y) +
p2(x,y)exp(jθ)=ρI(xly)
+ρ2(x,y)cosθ+jρz(x,y)sin
It is given by θ.

,′, ρ2(X, y) = (I Il[S (x
,y)])/sinθp+(xly)=Rel:
S (x, y)] ppz (x, y) cos θ where Re[S (x, y)], I n[s (x, y)]
represent the real part and imaginary part of S (x, y), respectively. θ=9
Compared to 0°, when θ==30°, τ can be reduced by 30/90 times, that is, 33%. However, at this time, S (
The imaginary part image of
Therefore, the image intensity becomes 5 in 30", or 1/2. Since the background noise is constant, this means that the image quality is degraded.

[Problems to be solved by the invention] Since the conventional magnetic resonance imaging method for separating chemical shifts is configured as described above, it is easily affected by noise in a low static magnetic field, resulting in poor image quality. There were problems such as a decrease in

This invention has been made to solve the above-mentioned problems, and aims to provide a magnetic resonance imaging method that is less susceptible to the effects of noise and the like and that does not degrade image quality.

[Means for Solving the Problems] The magnetic resonance imaging method according to the present invention applies a high-frequency magnetic field pulse under a first gradient magnetic field without using a 180° pulse to obtain images within a slice of a certain thickness. After exciting the spins, when applying the first, second, and third gradient magnetic fields and then inverting the third gradient to reimage the signal, the phase difference of the spins with respect to the two chemical shifts is The image is re-formed at the time when the image becomes 90''.

[Function] In the magnetic resonance imaging method of this invention, tilt! i
! Invert the field and generate NMR signal (spin echo signal)
This was used to separate each chemical shift.

[Embodiment] Hereinafter, an embodiment of the present invention will be explained with reference to the drawings.1 As a device for implementing the present invention, a conventional device can be used. in the direction perpendicular to the fault plane of
That is, a static magnetic field B0 is obtained along the Z-axis, and high-frequency pulses are transmitted and received along the Y-axis or the Y-axis perpendicular to the Z-axis.

The reception method is Q D (Quadrature
Performing signal processing using the D etection method,
Furthermore, with respect to the static magnetic field B0, X, Y, which are orthogonal to each other,
Gradient magnetic fields G x , G y , G z in the Z direction, respectively
A coil is provided for forming the. This type of device is described in, for example, Medical Informatics (1985), Volume 5, No. 3,
It is described on page 245.

At the 1st t21, R" is a high frequency magnetic field pulse,
(1) is a 90° pulse. Gz, Gx, and Gy are the first, second, and third gradient magnetic fields in the Z-axis, Y-axis, and Y-axis directions, respectively, and the first gradient magnetic field is the pulse (7) and (8 ), the second gradient magnetic field is pulse (3), and the third magnetic field gradient is pulse (9) and (10
). Also, NMR1 and NMR2
are the absorption and dispersion components of the NMR signal, indicated by pulses (6a) and (6b), respectively. τ is the peak time of the 90” pulse (1) and N M
It is the time (echo time) between the peak times of pulse (6a) indicating the absorption component of the R signal, and pulse (7)
The area of the shaded part of (8), (9) and (1o) is S 3-
3 +, S s = S 4.

Next, the operation will be explained according to the flowchart shown in FIG. The first gradient field G indicated by pulse (7)
90” pulse (1) of high frequency 1ifl field Rr under z
Apply. This makes it possible to excite nuclear spins within a slice having a certain thickness in the Z direction (step S
1) The first gradient magnetic field Gz is reversed and the phase disturbance in the Z direction is corrected by the pulse (8).
Since the phase disturbance is proportional to the area of the applied gradient magnetic field,
S 3 = 34, and at the same time, pulse (3)
A second gradient magnetic field Gx for phase modulation shown by is applied. As a result, a phase is given to the NMR signal to be detected later by vA. In addition,.

The amount of phase applied is proportional to the area of pulse (3).

In addition, in order to observe the spin echo signal under the third gradient magnetic field Gy for high frequency adjustment shown by pulse (10), a reverse magnetic gradient magnetic field shown by pulse (9) is applied in advance ( (corresponds to step s2), and then reverses the gradient magnetic field 4 Gy of the cylinder 3 (step S2).
N as a spin echo signal at time τ 6 (corresponding to 3)
Reimage the MR signal. At this time, the application timing of the second gradient magnetic field Gy and the pulse (
9) and (1o) are determined (step S4).
), select human body protons as the measurement target,
Consider as an example the distribution images of water and fat. Letting the chemical shifts of water and fat be σ1 and σ2, and the nuclear gyromagnetic ratio as γ, as shown in the conventional example, (2n+1)・π/2:γ1σ1−σ21τ(n=o,
1, 2. ...) By choosing τ such that the spin phase difference for the two chemical shifts can be 90'', wait for a zero recovery time of 0.5 to 2.0 sec, and then Measure the green delay of the spin echo signal by changing the amplitude of the pulse (3) to the gradient magnetic field G (corresponding to steps s5 and S6) 2 for the 0 time and the second gradient magnetic field Gx
A dimensional Fourier transform is performed to obtain an image S (x, y). At this time, S (x, y) = (ρ + (X, y) + jp2 (x,
y)) x exp[-j(r (E (x, y) + σ1
〉τ+θ, )]...(1) Here, ρ, (x, y),
ρ2(x, y) are the density distribution of water and fat, respectively, E
(x, y) is the non-uniform distribution of the static magnetic field, θ. is the phase off-cell I・ existing in the device, and in the 0th order with σ2=0, in equation (1), the phase term exp[−Nγ(E (x
,y)+σ1)τ10. )], we obtain 100(x,y)-ρ, (x,y>+jρ2(x,y).This elimination method can be used, for example, in the technical report of the Institute of Electronics and Communication Engineers mentioned above. , MBE85-44, pages 33 onwards. As a result, ρ + (x, y) = Re [no (x, y) ρz (x, y)
)=Id100(x,y)] is obtained. That is,'
The real part and imaginary part of ff(x, y) form the water and fat distribution image. Here, the value of τ is the static magnetic field strength B o ” 0, 17.5 m at IT, as in the conventional case.
5ec, but the echo time is 501 times faster than the conventional example.
The degree can be reduced. Therefore, the influence of relaxation time can be reduced. Furthermore, image quality is improved compared to conventional methods.

In the above embodiment, a case was explained in which a 90'' pulse was used as the high frequency pulse, but it is also possible to use a 30'' pulse or the like and set the recovery time for repetition to about 0.05 to 0.1 seconds. As a result, chemical shift images can be obtained quickly.

[Effects of the Invention] As described above, according to the present invention, after spin excitation, the gradient magnetic field is reversed to obtain a spin echo signal, and the phase difference of the spins for two chemical shifts is determined at the reimaging time (echo time). Since it was made to be 90 degrees, 0. IT
This method has the effect that chemical shift images with less influence of relaxation time can be obtained with high image quality even when the static magnetic field strength is low.

[Brief explanation of the drawing]

Fig. 1 is a pulse sequence diagram showing an embodiment of the magnetic resonance imaging method according to the present invention, Fig. 2 is a flowchart 1-diagram showing the operation of the embodiment of Fig. 1, and Fig. 3 is a conventional magnetic resonance imaging method. FIG. 3 is a pulse sequence diagram showing the method. In the figure, (1) is a 90” pulse of high-frequency magnetic field, (3
) is the second gradient magnetic field pulse, (6a) and <6b> are NM
In the R signal, (7) and (8) are the first gradient magnetic field pulses, and (10) are the third gradient magnetic field pulses. In the drawings, the same reference numerals indicate the same or corresponding parts. fever 2 figure

Claims (6)

[Claims] (1) A first step of exposing the object to a continuous static magnetic field and exciting nuclear spins with two or more chemical shifts in a certain volume, and applying first, second, and third gradient magnetic fields. and a third step of reimaging the nuclear spins under the third gradient field, and further performing the series of steps with the second gradient field having different gradient values. In magnetic resonance imaging, in which steps are repeated and the recovery period is between the repetitions of the series of steps,
The spin phase difference for the above two chemical shifts is 90°
A magnetic resonance imaging method characterized by re-imaging at a time when . (2) The magnetic resonance imaging method according to claim 1, wherein the first step is performed under a first gradient magnetic field. (3) The magnetic resonance imaging method according to claim 1 or 2, wherein the first to third gradient magnetic fields are orthogonal to each other. (4) The magnetic resonance imaging method according to any one of claims 1 to 3, characterized in that a 90° pulse is used as the high-frequency magnetic field pulse for exciting nuclear spins in the first step. . (5) As the high-frequency magnetic field pulse that excites the nuclear spins in the first step, a pulse that rotates by 90 degrees or less is used. magnetic resonance imaging. (6) The magnetic resonance imaging method according to claim 5, characterized in that a pulse giving a rotation of 30° is used as the high-frequency magnetic field pulse for exciting the nuclear spins in the first step.