JPH0620434B2 - Magnetic resonance imaging method - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention utilizes a magnetic resonance (MR) phenomenon to provide information on the density and relaxation time of specific atomic nuclei in an arbitrary tomographic plane in a subject. The present invention relates to a magnetic resonance imaging method in which information that enables medical diagnosis is displayed as a tomographic image by setting it non-invasively from the outside, and in particular, by using a two-dimensional Fourier transform, the density distributions of specific nuclei of two substances in the cross section of an object are measured. The present invention relates to a magnetic resonance imaging method that can be obtained simultaneously.

[Background Art and its Problems] In the magnetic resonance imaging method for hydrogen nuclei,
For example, complex images of water and lipid cannot be obtained independently. However, the resonance frequencies of hydrogen nuclei existing in water and lipids can be obtained by decomposing the images of water and lipids by effectively using different phenomena under the same magnetic field, so-called chemical shift phenomenon.

If the difference between the Larmor frequency of water and lipid is Δν [Hz], the above method can be explained as follows. As shown in FIG. 4, the subject is irradiated with a frequency-selective 90 ° RF pulse in a static magnetic field maintained along the first axis of the subject, and an RF magnetic field is applied along the second axis to cause nuclear spin. Of the RF pulse from the generation of τa + Δτ (where τ
a is a half of the time from the 90 ° RF pulse to the spin echo), and the subject is irradiated with a 180 ° RF pulse (4th time).
(Shown as 180 ° 'in the figure) to dephase the nuclear spins, and then add a phase return gradient having the same direction as the phase shift gradient to add the 180 ° RF pulse (180 °).
A nuclear spin echo occurs after (.tau.a + .DELTA..tau.), Which is a predetermined elapsed time from .degree. '), And a composite MR signal generated by this spin echo is collected.

In this case, a normal 180 ° RF in which the time from the generation of 90 ° RF pulse to the generation of 180 ° RF pulse is τa
Both the pulse sequence using the pulse and the pulse sequence using the 180 ° 'RF pulse in which the time from the invention of the 90 ° RF pulse to the generation of the 180 ° RF pulse is τa-Δτ as in the above method are π pulses. Since the applied amounts (GR 0 · TG) of the gradient magnetic fields (GR 0) on both sides are the same,
Collection of nuclear spin echoes with a gradient magnetic field is complete. Hereafter, we ignore this effect and consider only the phase shift due to chemical shift. Now, considering a rotating coordinate system that considers the Larmor frequency of water, the magnetization of water is stopped, but the magnetization of lipid rotates at Δν [Hz], so the chmical shift
The movement of the magnetization caused by is as shown in FIGS. 6 (a) and 6 (b), and the 180 ° RF pulse (180 °) shown in FIG.
In comparison with the movement of the magnetization in the sequence by (°), the movement of the magnetization in the sequence by the 180 ° RF pulse (180 ° ') shown in FIG. 5 (b) finally causes the phase of water and lipid by 2Δτ · Δν. Will be shifted. At this time, 2Δτ
If Δν = 1/2, the phase of the water and lipid signals is 180
Deviate. When a two-dimensional Fourier transform is performed by collecting MR signals while sequentially changing the intensity of the gradient magnetic field by adding a gradient magnetic field for phase encoding as shown in Fig. 1, the density of water is displayed as positive and the density of lipid is displayed as negative. To be done. A mixed image of water and lipids (water-lipid image) is obtained. Similarly, when MR signals are collected under the condition of Δτ = 0 and the two-dimensional Fourier transform is executed, an image (water + lipid image) showing the sum of the density distributions of water and lipid which is usually obtained is obtained.

That is, it is as in the following equation.

(Water + lipid) image + (water-lipid) image = water only image (water + lipid) image− (water-lipid) image = lipid only image However, when the above 180 ° RF pulse is used, If there is magnetic field inhomogeneity (maximum value ΔHmax), 2Δτ
Since a phase shift of rΔHmax occurs in the image, there is a problem that it is necessary to use it in an extremely long region of uniformity or to accurately measure and correct the distribution of nonuniformity. To obtain an image in which water and lipids are decomposed, Δ
The scan must be executed twice under the conditions of τ = 0 and ¼Δν.

[Object of the Invention] The present invention has been made in view of the above circumstances, and water and lipid with less magnetic field inhomogeneity by applying a planned planar pulse sequence to a planar imaging method such as two-dimensional Fourier transform imaging. It is intended to be able to obtain the separated images of 1 in one scan.

[Summary of the Invention] In order to achieve the above object, the present invention provides a magnetic resonance imaging method for obtaining a distribution of spin densities of specific nuclei of two substances in a cross section of a subject by a two-dimensional Fourier transform. A static magnetic field is maintained along the first axis of the object, and the subject is irradiated with a selective 90 ° RF pulse to excite nuclear spins in a predetermined region to generate a second axis during the period of the first predetermined region. The RF pulse for a second predetermined time period by applying a phase-determined amount of nuclear spins excited by applying two (2nd axis, 3rd axis) phase-shifted gradient magnetic fields along the Τa + Δτa (where τa is ½ of the time from the 90 ° RF pulse to the spin echo) from the average generation point of γ, and the phase of the excited nuclear spin is inverted by irradiating the subject with a 180 ° RF pulse. And the third after the second period
Of the phase of the nuclear spin phase-shifted by the phase-shifted gradient magnetic field (second axis) after τa-Δτ from the 180 ° RF pulse by applying the phase-shifted gradient magnetic field (second axis) during In order to make the nuclear spin echo generated by the return the spin echo of the specific nuclei of the two substances, Δτ is calculated as follows: Δτ = 1/8 · 1 / Δν (where Δν is the resonance frequency difference between the specific nuclei of the two substances) ), This spin echo generates a composite MR signal,
This composite MR signal is collected and a two-dimensional Fourier transform is performed to independently obtain distribution images of two substances in one scan.

Embodiments of the Invention Hereinafter, details of the present invention will be described with reference to the drawings.

As shown in FIG. 1, a static magnetic field is maintained along the first axis of the subject, and the subject 1 is irradiated with a frequency-selective 90 ° RF pulse for a period q ¹ to excite nuclear spins in a predetermined region. For at least 1 along the second axis during the first scheduled period q ^2.
Phases of the excited nuclear spins are phase-shifted by applying two phase-shifted RF magnetic fields, and τa + Δτ (where τa is a 90 ° RF pulse) from the average generation point of the RF pulses during the second scheduled period q ^2. (1/2) of the time τ _E from the spin echo to the spin echo, the object is irradiated with a 180 ° RF pulse to start the phase return of the excited nuclear spins, and after the second period q ³ . during the period q ⁴ 3 appointment, at least one rephasing after .tau.a-.DELTA..tau added a gradient from the 180 ° RF pulse, the nuclear spins by shifting the phase shifting the phase by a gradient with the same direction as the phase shift gradients The nuclear spin echo caused by the phase reversal of is set to be the spin echo of the hydrogen nuclei of water and oil.
τ is set to the following equation Δτ = 1/8 · 1 / Δν (where Δν is the resonance frequency difference between the hydrogen nuclei of water and oil), and this spin echo generates a composite MR signal,
This composite MR signal is acquired. 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 of the 180 ° RF pulse and the spin echo. Considering the resonance frequency of hydrogen nuclei contained in water as a reference, lipids are 2Δτ ・
A phase difference of Δν is generated, and when the phase difference is 90 ° (π / 2) (2Δτ · Δν = 1/4), the amplitude (or application time) of the gradient magnetic field for phase encoding is sequentially changed and collected. Among the complex images formed by performing the complex two-dimensional Fourier transform of the MR signal, the lipid image becomes the imaginary part image, while the water image becomes the real part image. In this case, Δτ = 1/8 · Δν. The phase difference of 90 ° between water and lipid can be retained as described above, but in actual MRI the absolute phase of water does not always become zero. There is Δψ. Then, in order to obtain this Δφ, a water phantom whose position is determined at the same time as the subject is set (see FIG. 2 (c)) and scanned to form an image. In this case, the image Ir '(x,
y) and the image Ii '(x, of the imaginary part shown in FIG. 2 (b).
In y), the real part of the complex image obtained by the two-dimensional Fourier transform for the phase Δψ and Ir ′ (x, y) and Ii ′
Since the position of the image of the water phantom is formed at (x, y) (3 and 4 in FIGS. 2 (a) and 2 (b)), the complex image value of the water phantom 3 (FIG. 2 (a), 3 of (b)
And 4) are taken out and Δψ is calculated by the following equation.

Δψ = tan ⁻¹ imaginary part value (4 in FIG. 2 (b)) real part value (3 in FIG. 2 (a)) Using Δψ obtained by the above formula, the following formula is applied to the image of the subject ( When the calculation is performed for each pixel in FIGS. 2 (a) and 2 (b), 1), images of water 1 and lipid 2 are separately obtained as shown in FIGS. 3 (a) and 3 (b).

Ir (x · y) = Ir ′ (x · y) cos Δψ + Ii ′ (x · Y) sin Δψ Ii (x · y) = − Ir ′ (x · y) sin Δψ + Ii ′ (x · Y) cos Δψ Further, , Water + lipid image is Between images or the following formula It is also possible to calculate for each pixel of.

Therefore, in the present invention, the movement amount of the 180 ° RF pulse is from Δτ = 1/4 · 1 / Δν to Δτ = 1/8 · 1 / Δν
Therefore, not only is the influence of the static magnetic field inhomogeneity reduced to 1/2, but the distribution image of water and lipid can be obtained separately by one scan.

It is needless to say that the present invention is not limited to the above-mentioned embodiments and includes various modifications within the scope of the gist of the present invention. For example, in the above-mentioned embodiment, the case of a complex image of hydrogen nuclei of water and oil has been described, but the characteristics thereof may be appropriately selected.

[Advantages of the Invention] The present invention is based on the average generation point of 90 ° RF pulse or τa + Δ
After τ, irradiate the subject with a 180 ° RF pulse.
Since Δτ is set to the following equation Δτ = 1/8 · 1 / Δν so that a nuclear spin echo is generated after τa + Δτ from a 0 ° RF pulse, a complex image of water and oil can be obtained by one scan. In addition, the influence of static magnetic field inhomogeneity is reduced to 1/2, and a clear complex image can be obtained by one scan, which is excellent in diagnosis.

[Brief description of drawings]

FIG. 1 shows M used in the two-dimensional Fourier transform of the present invention.
Graphs showing the R pulse sequence, FIGS. 2 (a) and 2 (b) show a complex image before correction processing, FIG. 2 (c) shows the relationship between the positions of the subject and the water phantom, and FIG. a), (b)
Shows the separated complex image after correction, and FIG.
Graphs showing the MR pulse sequence used for the three-dimensional Fourier transform, and FIGS. 5A and 5B are explanatory diagrams of the proton spin vectors of water and lipid. 1 ... water, 2 ... lipid, 3 ... water phantom.

Claims (1)

[Claims] 1. A magnetic resonance imaging method for obtaining a distribution of spin densities of specific nuclei of two substances in a cross section of a subject by a two-dimensional Fourier transform.
Maintains a static magnetic field along the axis of and selects the object 90 ° R
Two (2nd axis, 3rd axis) phase-shifted gradient magnetic fields along the second axis during the period of the first planned area by irradiating with F pulse to excite nuclear spins in the planned area. By a fixed amount of the phase of the excited nuclear spins during the second scheduled period from the average generation point of the RF pulse by τa
+ Δt (where τa is the spin
After the half of the time until the echo), the subject is RF 180 °
The phase of the excited nuclear spins is inverted by irradiating with a pulse, and the phase-shifted gradient magnetic field (second axis) is applied for a third predetermined period after the second period to obtain the 180 ° After τa−Δτ from the RF pulse, the nuclear spin echo generated by the phase reversal of the nuclear spin phase-shifted by the phase-shifted gradient magnetic field (second axis) becomes the spin echo of the specific nuclei of the two substances. Δτ is set to the following equation Δτ = 1/8 · 1 / Δν (where Δν is the resonance frequency difference between specific nuclei of two substances), and this spin echo generates a composite MR signal,
A magnetic resonance imaging method characterized in that a distribution image of two substances is independently obtained by one scan by collecting this composite MR signal and performing a two-dimensional Fourier transform.