JPS63212337A - Water/fat separating 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] [Industrial application field]

The present invention relates to imaging using nuclear magnetic resonance (so-called MRI), and particularly to a method of separating water and fat in a living body (human body) to obtain an image of only water or an image of only fat.

[Conventional technology]

Conventionally, the Dixon method is known, which uses the phase difference between the NMR signals of water and fat in a living body and separates these signals to obtain an image of water or fat (W, T,
Dixon: Radiology, 153.189-
194 (1984)).

[Problems to be solved by the invention]

However, although the 1xon method is certainly a very effective method, what is directly observed is a difference image between water and fat, and to obtain a separated image of only water or fat, it is necessary to use a normal image ( It is necessary to create an image of only water or an image of only fat by performing inter-pixel calculations between the sum image of water and fat) and the above-mentioned difference image. An object of the present invention is to provide an imaging method that can directly obtain an image of only water or only fat without using a roundabout method such as the Dixon method and without performing inter-pixel calculations.

[Means to solve the problem]

The water/fat separation imaging method according to this invention uses a high-frequency signal whose carrier frequency is the nuclear magnetic resonance frequency of either the proton nucleus of water molecules or the proton nucleus of fat as an excitation pulse, and first gives a 90° pulse. After that, the intensity ratio is 1:3
:3:1, the flip angles are 22.5°, -67, respectively.
Four hard pulses of 5°, 67.5°, and -22,5° are applied at a time interval τ such that the nuclear magnetic resonance frequency difference between the proton nucleus of water molecules and the proton nucleus of fat is 1/(2τ).
It is characterized in that data is collected from the spin echo signals generated thereafter.

[For production]

After giving a 90° pulse, the intensity ratio is 1:3:3:1 and the flip angle is 2, arranged with equal time interval τ.
When four hard pulses are given at 2.5°, -67.5°, 67.5°, and -22.5°, for a nuclide whose offset frequency with respect to the carrier frequency of these pulses is zero, The total effective flip angle due to the pulse train consisting of these four pulses is zero, which is essentially the same as irradiating no pulses. On the other hand, for a nuclide that rotates exactly half a revolution in the rotating system during the time τ, that is, for a nuclide whose offset frequency with respect to the carrier frequency is 1/(2τ), the total effective flip angle is 22. 5°+67.5°+6
7.5°+22.5°=180°, producing an effect equivalent to a 180° pulse. In the human body, the proton nucleus of water molecules and the proton nucleus of fat have a chemical shift difference of about 3 ppm, and the NMR resonance frequencies when no gradient magnetic field is applied differ by about 3 ppm. Therefore, by setting the pulse interval τ so that 1/(2τ) exactly corresponds to this 3 ppm frequency difference, and setting the carrier frequency of the high-frequency excitation pulse to one of the NMR resonance frequencies of water and fat, it is possible to Spin echo signals of only the other fat can be obtained. Therefore, since the spin echo signal of only one of water and fat can be directly captured, by collecting data from this, it is possible to directly capture an image of only water or fat.

【Example】

FIG. 1 shows a pulse sequence according to an embodiment of the present invention. As shown in this diagram, first, 90" pulses are applied while a gradient magnetic field for slice selection is applied (see FIG. 1). (See A and D). This 90° pulse is a so-called soft pulse in which a high frequency signal with a frequency corresponding to the NMR resonance frequency of water or fat in the human body (in this case, the NMR resonance frequency of water) is amplitude-modulated in a 5-inch shape. By irradiating this 90° pulse, the transverse magnetization in the slice selected by the slice selection gradient magnetic field is excited.Thereafter, as shown in Fig. 1B and C, the gradient magnetic field for imaging, that is, frequency coding, is excited. A gradient magnetic field for slice selection, a gradient magnetic field for frequency coding, and a gradient magnetic field for phase coding are usually applied. These gradient magnetic fields for slice selection, gradient magnetic field for frequency coding, and gradient magnetic field for phase coding are usually magnetic fields whose magnetic field strengths are gradient in three orthogonal axes directions. Next, when TE/2 hours have elapsed since the 90° pulse, a 180° pulse is applied to generate a spin echo signal (Fig. 1A).This 180° pulse is shown in Fig. 2. It consists of four so-called hard pulses with narrow spectral width arranged at equal time intervals τ, with flip angles of 22.5°, -67.5°, and 6, respectively.
The intensity ratio is 1:3:3 so that it is 7.5° and -22.5°.
:1, the phase relationship between the first and third ones and the second and fourth ones is reversed. The time interval τ is set such that the difference between the NMR resonance frequency of the proton nuclei of water molecules in the human body and the resonance frequency of the proton nuclei of fat is 1/(2τ). The frequency characteristics of the total effective flip angle with respect to the offset frequency due to this pulse train consisting of four pulses are as shown in FIG. That is, for a nuclide whose offset frequency with respect to the carrier frequency (here the NMR resonance frequency of water as above) is zero, here the proton nucleus of a water molecule, the total effective flip angle due to the four pulses described above is: 22 .5°-67,5°+67.5°-22,5°=0
°, which is actually the same as not irradiating any excitation pulse. On the other hand, a nuclide whose offset frequency with respect to the carrier frequency (NMR resonance frequency of water) is 1/(2τ), that is, which rotates exactly half a revolution in the rotating system in time, that is, the proton nucleus of fat in this case. For, the total effective flip angle due to the four pulses above is 22.5” +67.5°+67.5°+22.5°=
The angle becomes 180°, and it essentially functions as a 180° pulse. Therefore, a spin echo signal of the fat proton nucleus is generated when TE/2 hours have elapsed from the 180° pulse, and this is sampled and data collected while applying a frequency coding gradient magnetic field. An image of only fat can be obtained by repeating this sequence while changing the gradient magnetic field strength for phase coding, collecting data, and performing two-dimensional Fourier transformation as usual. In this example, an image of only fat is taken by setting the carrier frequency of the high-frequency excitation pulse to the NMR resonance frequency of water. However, if the carrier frequency is set to the NMR resonance frequency of fat, an image of only water can be taken. Can be done. Further, in the above embodiment, the phase coding gradient magnetic field is applied before the 180° pulse, but it may be applied at a position after the 180° pulse. The frequency coding gradient field before the 180° pulse may have its sign reversed and placed at a position after the 180° pulse.

【Effect of the invention】

According to the MRI imaging method of the present invention, images of only water or fat can be directly captured, eliminating the roundaboutness of the DixOn method.

[Brief explanation of the drawing]

FIG. 1 is a time chart showing a pulse sequence according to an embodiment of the present invention, FIG. 2 is a time chart showing a pulse train constituting a 180° pulse, and FIG. 3 is a graph showing frequency characteristics of the pulse train in FIG. 2. It is.

Claims (1)

[Claims] (1) A high-frequency signal whose carrier frequency is the nuclear magnetic resonance frequency of either the proton nucleus of water molecules or the proton nucleus of fat is used as an excitation pulse, and after first giving a 90° pulse, the intensity ratio is 1:3. : Four hard pulses with flip angles of 22.5°, -67.5°, 67.5°, and -22.5° at a ratio of 3:1 were applied to the proton nuclei of water molecules and the proton nuclei of fat. A water/fat separation imaging method characterized in that the magnetic resonance frequency difference is given at a time interval τ such that the difference is 1/(2τ), and data is collected from spin echo signals generated thereafter.