JPS6234549A - Magnetic resonance imaging method - Google Patents

Abstract

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

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention utilizes magnetic resonance (MR) phenomena to obtain information about the density and relaxation time of specific atomic nuclei within an arbitrary cross-sectional plane within the subject. Regarding the magnetic resonance imaging method, which is set non-invasively from the outside and displays information that enables medical diagnosis as a tomographic image, in particular, it is possible to calculate the density of each specific atomic nucleus of two substances in the cross section of the object using two-dimensional Fourier transform. i [i at the same time
Imaging method of one energy and one sound) Leap at people.

[Previous day's technology and its problems 1. In the normal magnetic imaging method of water mulberry atomic cough, for example, double water images of water and lipids cannot be obtained independently. 1. The hydrogen ratio j′ present in water and lipids is the resonant frequency of cough (J, the same) - different current τ2 under the magnetic field, the so-called ct
1emical 5hift] To use gx effectively J: So, water and fat Y! It can be obtained by decomposing the image of 1.

Here, the vagina of water and lipid t-at''mo' frequency is Δν[
1-17-1, the above method can be explained as follows.

As shown in Figure 4, an RF magnetic field is applied within the static magnetic field maintained along the 1441st line of the subject (at 6, the subject is irradiated with a frequency-selective 90° RF pulse to the second axis). to shift the phase of the nuclear spin, and after the generation of the RF pulse τa + △τ (1st position, τa is 1/2 of the time from the 90″ RF pulse to the spin echo), the object is rotated by 180°.
The phase of the variable spin is returned by irradiation with an RF pulse (indicated by IJ, 180 = doji- in Fig. 4), and then a phase-returning gradient having the same direction as the above-mentioned phase-shifting gradient is added. C
After the scheduled elapsed time (τa + △τ) from the 180° RF pulse (1B○°″), a post-cough spin “-” occurs;
A composite MR multiplied signal generated by this spin echo is collected.

In this case, the time from the generation of the 90° RF pulse to the generation of the 180° RF pulse is τa, which is the normal 180° RF pulse.
7. Pulse sequence using pulses and the time from generation of 90° RF pulse to generation of 180° RF pulse as in the above method. In all pulse sequences using a 180'-RF pulse that is a-△τ, the applied gradient magnetic field (GRO) m(GRO・]゛G) on both sides of the π pulse is the same, so the nuclear spin knee due to the gradient magnetic field 1 - collection is complete. From now on, ignore this effect and use chemica.
Only the change in phase due to l5hift will be considered. Now, if we consider the rotating coordinate system based on the Larmor frequency of water, the magnetization of water is either stopped or the magnetization of lipids rotates at △ν [Hz], so the movement of magnetization caused by chemical 5hift is shown in Figure 6. As shown in (a) and (b), Figure 5 (a)
Compared to the movement of magnetization in the sequence caused by the 180° RF pulse (180') shown in Figure 5(b), the 180° RF pulse (180') shown in Figure 5(b)
In the sequence of magnetization movement caused by RF pulses (180° -), the phase of water and lipid will eventually shift by 2Δτ・Δν1. At this time, if 2Δτ・△ν=1/2, The phase of the lipid signal 8 is shifted by 180°. As shown in Figure 1, a gradient magnetic field for phase encoding is applied, the intensity of the gradient magnetic field is sequentially changed, MR multiplied signals are collected, and a two-dimensional Fourier transform is performed. The density of water is positive, and the density of lipids is negative. is displayed. A mixed image of water and lipids (water-lipid image) is obtained. Similarly, by collecting MR multiplied signals under the condition of Δτ-0 and performing two-dimensional Fourier transformation, an image (water molecule lipid image) showing the sum of the density distributions of water and lipid, which is normally submerged, can be obtained.

That is, as shown in the following equation.

(water-soil-lipid) image + (water-lipid) image = water-only image (water + lipid) image - (water-lipid) image - lipid-only image 1
However, when using the above 180° RF pulse,
If magnetic field inhomogeneity (maximum value △Hmax) exists, 2
Since a phase shift of Δτ・rΔl−1max occurs in the image, it has the problem that it must be used in an extremely long region of uniformity, or that it is necessary to accurately measure and compensate for the nonuniformity. was. Furthermore, in order to obtain an image in which water and lipid are separated, scanning must be performed twice under the conditions of Δτ=O and 1/4Δν.

[Purpose of the Invention] The present invention has been made in view of the above-mentioned circumstances, and is to apply a predetermined planar pulse order to a planar imaging method such as two-dimensional Fourier transform imaging to obtain water and lipids with less magnetic field inhomogeneity. 1 separated image in one scan? The purpose is to make it possible to

[Summary of the Invention 1 In order to achieve the above object, the present invention provides a magnetic resonance imaging method that obtains the distribution of spin densities of specific atomic nuclei of two substances in a cross section of an object by two-dimensional Fourier transformation. Maintaining a static magnetic field aligned with the first axis of the specimen, the specimen is irradiated with a selective 90'' RF pulse to excite the nuclear spins in the predetermined region and generate a second predetermined region for the duration of the first predetermined region. By applying two (second axis, third axis) phase-shifting gradient magnetic fields along the axis, only the phase of the excited nuclear spins is determined, and during the second predetermined period, the RF From the average generation point of the pulse τa + Δτa (however, τa is R of 90'
1/2 of the time from the F pulse to the spin echo), the subject is irradiated with a 180° RF pulse to invert the phase of the excited nuclear spins, and
During the scheduled period of time, the phase shifting gradient magnetic field (second axis) is applied, and after τa−Δτ from the 180° RF pulse, the phase of the nuclear spins shifted by the phase shifting gradient magnetic field (second axis) The above Δτ is set to the following formula % (where Δν is the resonant frequency difference of the specific atomic nuclei of the two materials) so that the nuclear spin echo generated by the return becomes the spin echo of the specific atomic nucleus of the two materials, This spin echo generates one MR signal, and this composite MR signal is collected and subjected to two-dimensional Fourier transformation, thereby obtaining distribution images of two substances independently in one scan.

[Embodiments of the invention] Hereinafter, details of the present invention will be explained using the drawings.

As shown in Figure 1, a static magnetic field is maintained along the first axis of the subject, and the open subject 1 is irradiated with a frequency-selective 90° RF pulse during period q1 to excite cough spins in the predetermined region. at least 1 along the second axis during the first scheduled period q2.
The excited nuclear spins are phase-shifted by applying two phase-shifting RFla fields, and during a second scheduled period q3, τa + Δτ (where τa
is the time from 90” RF pulse to spin echo τE
1/2), the subject is irradiated with a 180° RF pulse to start rephasing the excited nuclear spins, and the second
during a third predetermined time period q4 after period q3 of , by adding at least one phase-returning gradient having the same direction as the phase-shifting gradient τa−Δτ after the 180° RF pulse, by the phase-shifting gradient. The above Δ
τ is set to the following formula % formula % (where Δν is the resonance frequency difference between the hydrogen nuclei of water and oil), and this spin echo generates a composite MR multiplied signal,
This composite MR multiplied signal is collected. therefore,
Time interval τa between 90° RF pulse and 180° RF pulse
There is a time difference of 2Δτ between +Δτ and the time interval between the 180° RF pulse and the spin echo, and considering the resonance frequency of hydrogen nuclei contained in water as a standard, lipids have a time difference of 2Δτ・
A phase difference of Δν is generated, and this phase difference is 90° (π/2)
When (2Δτ・Δν=1/4) MR multiplied signal complex collected by sequentially changing the amplitude (or application time) of the gradient magnetic field for phase encoding A lipid image among the complex images created by two-dimensional Fourier transformation is an image of the imaginary part, but the image of water is an image of the real part.

In this case, Δτ=1/8·Δν is required. The phase difference of 90° between water and lipids can be maintained as described above, but in actual MRI
In this case, the absolute phase of water is not necessarily zero, and there is a phase difference Δψ unique to the device and sequence composed of the RF G magnetic field. In order to obtain this Δψ, a water phantom whose position is determined at the same time as the subject is placed (Fig. 2 (C)).
Reference) Scan and form an image. In this case, Figure 2 (
In the real part image Ir'' (X, V) of a) and the imaginary part image 1i' (x, y) shown in FIG. Real part Ir of the image
An image of the water phantom is created at - (X, V) and ll- (X, V) (3 and 4 in Figure 2 (a) and (b)) are fixed, so the complex image of water phantom 3 is created. Value (Figure 2 (a)
>, 3 and 4) in (b) are taken out and Δψ is determined by the following equation.

△ψ=tan-1 Imaginary part value (Figure 2 (b>(7) 4) Real part value (3 in Figure 2 (a))
Using the above equation (2), calculate the image of the subject using the following equation.
Figure 3 (a) l
), 1°fr (x-y)-Ir-(x-y)cosΔψ+I
i ” (X-Y)sinΔφ I i (X-V)=-I r-(x-y)
sin Δφ+I i −(X−Y)CO3ΔΦ Furthermore, the image of water and 10 lipids can be calculated using the following formula % formula % () or the following formula Ir”'2 ×Xy)+I i ′2 (X−
Even the calculation for each pixel of y> is given by the function b.

Therefore, in this invention, the movement loss of the 180° RF pulse becomes △τ = 1/8 1/Δ from Δτ-1/4 1/Δν, and the influence of static v7i lift non-uniformity is reduced by 1/2. Not only is it smaller, but it is also possible to separate the distribution images of water and lipids in a single scan.

This invention is not limited to the above embodiments.
It goes without saying that various modifications are included within the scope of the invention. For example, in the above embodiment, the case of a complex image of hydrogen nuclei in water and oil was explained, but the characteristics may be selected as appropriate. τa+Δ
After τ, a 180° RF pulse is irradiated to the subject, and this 18
Since the above △τ is set to the following formula % so that a nuclear spin echo is generated at τa + △τ1 from the 0° RF pulse, for example, 1q complex images of water and oil can be obtained in − scans. , the influence of static magnetic field inhomogeneity is 1/2
It is small in size, and a clear complex image can be obtained in a single scan, which has excellent diagnostic effects.

[Brief explanation of drawings]

Figure 1 shows M used in the two-dimensional Fourier transform of this invention.
Graphs showing the R pulse order, Fig. 2 (a>, (b) show complex images before correction processing, Fig. 2 (C) show the relationship between the positions of the subject and the water phantom, and Fig. 3 ( a), (b)
shows the separated root thread image after correction, and Fig. 4 shows the conventional two root thread images.
Graphs showing the MR pulse order used for dimensional Fourier transform, Figure 5 (a>, (b) are explanatory diagrams of proton spin patterns of water and lipids. Figure 5 (G) (b) Q4 q5

Claims (1)

[Claims] In a magnetic resonance imaging method in which the distribution of spin densities of specific atomic nuclei of two substances within a cross section of an object is obtained by two-dimensional Fourier transformation, a static magnetic field is applied along a first axis of the object. and irradiate the subject with a selective 90° RF pulse to excite the nuclear spins in the predetermined region along a second axis (second axis, Third
τa+Δt (where τa is 90 After 1/2 of the time from the 180° RF pulse to the spin echo, the subject is irradiated with a 180° RF pulse to invert the phase of the excited nuclear spins, and the second
During a third predetermined period after a period of The above Δτ is calculated by the following formula Δτ = 1/8・1/Δν (where Δν is 2 substances (resonance frequency difference of a specific atomic nucleus), this spin echo generates a composite MR signal,
A magnetic resonance imaging method characterized by collecting this composite MR signal and performing two-dimensional Fourier transformation to obtain distribution images of two substances independently in one scan.