JPH01227745A - Chemical shift image forming method by nuclear magnetic resonance photographic device - Google Patents

Abstract

PURPOSE:To shorten scanning time by reducing the number of views of either or both of scans by means of an inverse pulse to be executed with a shift before and after an inverse pulse impressing time at the time of a normal scan. CONSTITUTION:The normal scan to impress a 180 deg. pulse after a prescribed time to impress a 90 deg. pulse is executed by a designated view. Next, either or both of the scans to be impressed in shifting the impressing period of the 180 deg. pulse before and after the 180 deg. pulse of the normal scan are executed in a small number of views, and the chemical shift image of water and fat, in which a phase error due to a static magnetic field ununiformity is corrected, is operated and prepared.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention separates and displays chemical shifts of water and fat while correcting phase errors caused by static magnetic field inhomogeneity of a nuclear magnetic resonance tomography device. This invention relates to a method for creating chemical shift images using a nuclear magnetic resonance tomography apparatus using an improved Dixton method.

(Prior Art) NMR-CT', which uses the nuclear magnetic resonance (hereinafter referred to as NMR) phenomenon to obtain a tomographic image of a subject focusing on a specific atomic nucleus, has been conventionally known. The outline of this NMR-CT principle will be briefly explained.

An atomic nucleus can be seen as a spinning top that is magnetic, but if it is placed in a static magnetic field Ho in the l-axis direction, for example, the atomic nucleus will precess at an angular velocity ω0 as shown by the following equation. . This is called Larmor precession.

ω0 - γHo However, γ: Nuclear gyromagnetic ratio Now, a high-frequency coil is arranged on an axis perpendicular to the Z-axis with a static magnetic field, for example, the x-axis, and the high-frequency rotating magnetic field with the above-mentioned angular frequency ω0 rotates in the xy plane. When applied, magnetic resonance occurs, and a population of atomic nuclei undergoing Zeeman splitting under the static magnetic field Ho undergoes a transition between levels due to the high-frequency magnetic field that satisfies the resonance condition, transitioning to a unit with a higher energy unit. do. Here, since the nuclear gyromagnetic ratio γ differs depending on the type of atomic nucleus, the atomic nucleus can be specified based on the resonance frequency. Furthermore, by measuring the strength of that resonance, we can determine the amount of that nucleus present. During a time determined by a time constant called the post-resonance relaxation time, the atomic nucleus excited to a higher level returns to a lower level and radiates energy.

Part 6 about the medium pulse method for observing this NMR phenomenon.
This will be explained with reference to the figures.

As mentioned above, a high frequency pulse (Hl
) is applied in the (X-axis) direction perpendicular to the static magnetism s<z-axis), the magnetization vector M changes in the zy-plane at an angular frequency of ω'-γH1 in the rotating coordinate system, as shown in Figure 6 (a). Start rotating. Now, if the pulse width is to, the rotation angle θ from Ha is expressed by the following equation.

θ=γH1to...(1)
In equation (1), a pulse having to that is θ-90° is called a 90' pulse. Immediately after this 90' pulse, the magnetization vector M moves along the xy plane at ω0 as shown in Figure 6 (b).
This means that it is rotating, and an induced electromotive force is generated in the coil placed, for example, on the z-axis. However, since this signal attenuates over time, this signal is called a free induction attenuation signal (hereinafter referred to as an FID signal). A signal in the frequency domain can be obtained by Fourier transforming the FED signal. Next, as shown in Figure 6 (c), after τ time from the 90' pulse, θ-180°
When a second pulse (180° pulse) is added with a pulse width such that τ
After a time the signal is observed again with focus in the -y direction.

This signal is called a spin echo (hereinafter referred to as SE multiplied signal). A desired image can be obtained by measuring this SE multiplied signal intensity. The resonance condition of NMR is ray γHo / 2π...(
2) is given by Here, ν is the resonant frequency, and t1o is the strength of the static magnetic field. Therefore, it can be seen that the resonance frequency is proportional to the strength of the magnetic field. For this purpose, a linear magnetic field gradient is superimposed on the static magnetic field to give a magnetic field of different strength depending on the position.
There is an NMR imaging method that obtains position information by changing the resonance frequency. Of these, the Fourier transform method will be explained. FIG. 7 shows a pulse sequence for applying a high frequency magnetic field and a gradient magnetic field used in this method =3=. (a) In the figure, X.

A state is shown in which gradient magnetic fields of Gx, Gy, and Gz are applied to the y- and z-axes, respectively, and a high-frequency magnetic field is applied to the x-axis.

(B) The figure shows the timing of applying each m field. In the figure, RF is a high frequency rotating magnetic field at 90°.
Apply a pulse and a 180° pulse to the z-axis. Gx is a fixed gradient magnetic field applied to the X axis called the lead axis, Gy is a gradient magnetic field whose amplitude changes depending on time applied to the z axis called the warp axis, and Qz is a fixed gradient applied to the z axis called the slice axis. It is a magnetic field. The signal shows the SE multiplied signal after 1800 pulses. The period is provided to indicate the timing of the signal of the gradient magnetic field applied to each axis. In period 1, the 90' pulse and the gradient magnetic field G'l+ selectively excite the spins in the slice plane that lies in the cross section 811 and shadow perpendicular to the two axes centered on 2=0. GX+ in period 2
is for disturbing the phase of the spins and inverting them with a 180' pulse, and is called a day phase gradient.

Gl- is for restoring the phase of spins disturbed by Gz+. In period 2, the phase encoding gradient Gy
n is also applied. This is to shift the phase of the spin in proportion to the position in the y direction, and its intensity is controlled to be different every cycle.

In period 3, a 180° pulse is applied to align the magnetic moments again, and the SE multiplied signal that appears thereafter is observed. G×+ in period 4 is a gradient magnetic field for aligning the disturbed phases and generating an SE multiplied signal, which is called a readout gradient, and S
E times signal appears.

In such NMR imaging, protons NM of hydrogen nuclei, which are the main target nuclide of current imaging,
Consider R.

Generally, in a substance, the resonance frequency ν of NMR often deviates from equation (2), which is the resonance conditional equation due to the externally applied static magnetic field Ho. This shift is extremely large in the case of atomic nuclei in metallic substances, but even in the case of protons, the resonance frequency changes sensitively depending on the chemical structure. One of the causes of this is due to the magnetic field (diamagnetic field) generated in the opposite direction to t1o due to the orbital motion of the electrons, which is called the shielding effect of the nucleus on the electrons. Therefore, the resonance frequency is given by the following equation using the constant σ due to this shielding effect.

ν=(γ/2π) t-to' (1-σ) Such a shift in resonance frequency is called a chemical shift, and resonance is observed on the lower magnetic field side. The magnitude of this chemical shift changes as the distribution of electron density and orbital motion of electrons change due to chemical bonds. Therefore, the resonance conditions differ depending on the compound even for the same proton. That is, the Larmor frequency differs by about 3.5 ppm between the protons in the fat 120 and the protons in the fat. With this degree of deviation, it is not possible to distinguish between the two using normal imaging.

The Dixon method is a method for imaging both components. The Dixon method will be explained with reference to FIG. The figure is a time sequence chart similar to FIG. 7.

In the figure, parts equivalent to those in FIG. 7 are given the same reference numerals.

(a) The figure is a diagram of scan 0 in which Te=TE/2 is selected, where To is the time interval between 90' pulse 1 and 180' pulse 2. Here, TE is the time interval between the 906 pulse and the SE signal.

At this time, the magnetization vectors of the proton components of water and fat are in phase. (b) The figure shows 180 with the time interval Ta from 90' pulse 1 set to Te - (TE/2) 1 ε
It is a diagram of scan 1 when pulse 4 is applied. At this time, the phases of the magnetization vectors of water and fat are opposite to each other. The phase relationship of the magnetization vectors in the excitation plane is as shown in FIG. In the figure, (a) shows the phase relationship between water and fat in the case of FIG. 4 during scan O with Ta=TE/2, and the magnetization vectors of water and fat are in the same phase.

(B) Figure 4 (B) shows Ts = (TE/2) - ε.
The phase relationship between water and fat in scan 1 is opposite. Here, ε is a value at which the phases of water and fat are opposite to each other, and is determined by determining the strength of the magnetic field. For example, when the magnetic field strength is O:3T, ε=5.6+ns, and when the magnetic field strength is 0.6T, ε=2.81118.

This Dixon method contains a phase error due to static magnetic field inhomogeneity, and it is difficult to separate water and fat with a slight difference in resonance frequency. An improved Dixon method has been proposed to remove this phase error. This method will be explained with reference to FIG.

(A) Figure shows the same 90° pulse 1 and 180° pulse 2 as in scan 0 in Figure 4 applied, (B) Figure shows the same 90° pulse 1 and 1806 pulse 4 as in scan 1 in Figure 4 applied. 1 is a diagram of scan 1. (c) The figure is a diagram of scan 2 in which 90' pulse 1 and 180' pulse 5 were applied.
8o6 pulse 5 has a time interval of (TE/
2) With a pulse of '+ε, the phase of the magnetization vector of fat is also in the opposite phase to that of water.

Now, the image data in scans 0 and 1.2 are So,
Let Sl and S2 be the amplitude of water, F9 the amplitude of fat, α the phase offset, and θ the phase of the error due to static magnetic field inhomogeneity.
Then, So = (W1F> −eXp (i α)
・= (3)81 = (WF)-exp
(i θ) −exp (i a>...
(4) 82 = (WF) -exp (-i
θ)・cxp(i α)・
...(5). Find the phase error 〇 due to static magnetic field inhomogeneity.

Dividing equation (3) by equation (4) gives 82/81 = eXl) (-i θ)/exp(
i θ) = eXp (-i2θ) If the magnetization vector after eliminating the non-uniformity correction error is 83, then 83 = St XeXD (Sat x -12θ) = 'SIX
2 't=Cdr3T= (W-F)exp
(i α) −(6) From equations (3) and (6), (1/2 > − (So +83 > +t−W*”exp (i (Z) = Wr'+i
'w'l ++ (7) (1/2) (So-83
>=F−eXI) (i α)=Fr +i −F+
-(8) From equation (7), W-r 10 From equation (8), r= Fv-+F-['-In this way, the improved Dixon method eliminates the static magnetic field phase error and the phase offset j~ Can be done.

(Problem to be solved by the invention) By the way, in the improved Dixon method, Skiton O, Ski 17 and 1. It is a serious problem that it is necessary to perform three scans of 11 scans and 2 scans, which requires 318 hours in NMR-CT, which already takes a long scan time.

The present invention has been made in view of the above problems, and its purpose is to shorten the scan time in an improved version of the Dixon method that separates water and fat and eliminates phase errors caused by static magnetic field inhomogeneity. The objective is to realize a method for creating chemical shift images that can be used to create chemical shift images.

(Means for Solving the Problems) The present invention solves the above-mentioned problems by correcting the phase -L error caused by the non-uniform static magnetic field of a nuclear magnetic resonance tomography apparatus, while correcting the chemical shift of water and fat. In a chemical shift image creation method using a nuclear magnetic resonance imaging 1iWI imager using an improved Dixon method that separates and displays, one of the scans using inversion pulses performed at different times before and after the application time of the inversion pulse during normal scanning. Or, it is characterized by reducing the number of views for both.

(Operation) A normal scan in which a 180° pulse is applied at TE/2 after the 90° pulse is performed in a specified view, and then a scan in which the application timing of the 180° pulse is shifted before or after the 180° pulse of the normal scan. Either or both are performed with a small number of views, and chemical shift images of water and fat corrected for phase errors due to static magnetic field inhomogeneity are extracted and created in a short time.

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

The hardware of the NMR-CT of the present invention is the same as usual. Figure 1 is a diagram of the RF pulse sequence used in the method of the present invention, and (a) is a diagram of the pulse sequence of the modified Dixon method similar to Figure 3, and the same symbols as in Figure 3 are used. be.

In this improved Dixon method, as already explained, scan 1 and scan 2 are used for static magnetic field inhomogeneity error calculation and correction, and the gap 1? Scan 0 and Scan 1 are used to calculate the respective water and fat components. Here, the number of views for water-fat (scan 1) is 70% to 80% of the number of views for water + fat (scan 0).
It has been experimentally confirmed that there is very little deterioration in image quality even when the value is reduced to %. By reducing the pico number of Skillen 1, the degree of separation of high spatial frequency components of water and fat deteriorates, but most of the information contained in the image is distributed in relatively low spatial frequencies, and the scan O By obtaining high frequency components from scan data, the number of views for scan 1 can be reduced. In addition, since the static magnetic field non-uniformity changes slowly, the number of scan lines used only for correcting phase errors due to static magnetic field non-uniformity can be further reduced, and even if it is reduced to about 25%, there will be no problems. Does not occur. FIG. 1(B) is a diagram showing the conditions of each scan, and shows the scan conditions in which the number of views of the warp gradient of scan 0.1.2 is 256 and 192.64, respectively. The lead sampling is 256 in both cases. In this case, the data of SE signal 3 is S
o, St.

It is S2.

FIG. 2 shows a flowchart for implementing the method of the invention. The data of 256 views of scan O is subjected to Fourier transformation a
do. Scan 1. After the data of scan 2 has been Fourier transformed, the number of views is smaller than that of scan O, so in order to create a frequency plane of the same matrix, the surrounding data plane is zero-filled with O and image reconstruction is performed (
Step 1). Divide scan 2 data S2 by scan 1 data S1 to obtain phase error eXp(-+2θ)
(Step 2). The data $1 of scan 1 is multiplied by the square root of the phase error to obtain data S3 from which the phase error has been eliminated (step 3). Scan 0 data So
Separated data of water and fat is obtained from the sum and the difference of 1/2 of the data S3 obtained in step 3 (step 4). This data includes a phase offset error. Next, take the absolute values of the complex data of water and fat to obtain data that does not include phase offset errors (Step 5)
. The calculations in the above flowchart have been explained in the conventional improved Dixon method, so the explanation here will be omitted.

As explained above, according to the method of the fifth embodiment, water and fat can be separated by correcting the phase error caused by static magnetic field inhomogeneity in a shorter time than the conventional method using the improved Dixon method.

Note that the present invention is not limited to the above embodiments.

(2) The number of views for scan O is not limited to 256, and may be the number of views specified by the user. Similarly, scan 1. The number of views for scan 2 can also be increased or decreased accordingly.

■The method of this invention can be applied to all water and fat separation imaging techniques that have the same principle as the Dixon method.

(2) In this embodiment, the number of samples in the read direction was not reduced, but it may be reduced if necessary.

(Effect of the invention) Even if the number of views for water-fat (scan 1) is reduced to 70% to 80% of the number of views for water-fat (scan O), the image quality deteriorates extremely for the reasons mentioned above. Since the number of views is small, the number of views for scan 1 can be reduced. Furthermore,
Since the phase error due to static magnetic field inhomogeneity changes slowly, scan 2 can be reduced to 64 to 32 views.

This makes it possible to use the improved Dixon method with a shortened scan time, which has a great practical effect.

[Brief explanation of the drawing]

1 is a diagram of the sequence of RF pulses of an embodiment of the method of the present invention; FIG. 2 is a flowchart for implementing the method of the present invention; FIG. 3 is a diagram of the pulse sequence of RF pulses of the modified Dixon method; Figure 4 is a diagram of the pulse sequence of the Dixon method, Figure 5 is an illustration of the phases of water and fat according to the Dixon method, Figure 6 is an illustration of the principle of the NMR-CT pulse method, and Figure 7 is an illustration of the NMR-CT pulse sequence. It is a figure which does not show the pulse sequence of CT. 1...90″ pulse 2...1806 pulse 3
... S, E signal 4 ... 180' pulse of 10 epsilon 5 ... 180° pulse of - epsilon Patent applicant Yokogawa Medical Systems Co., Ltd. 16) 1 Figure 3 Figure 4 Figure 7e=TE/2 - (B) Scan 1 Figure 5 (A) (B) Scan 1

Claims (1)

[Claims] In a method for creating a chemical shift image using a nuclear magnetic resonance tomography device using an improved Dixon method that separates and displays chemical shifts of water and fat while correcting phase errors due to static magnetic field inhomogeneity of the nuclear magnetic resonance tomography device, 1. A method for creating a chemical shift image using a nuclear magnetic resonance tomography apparatus, characterized in that the number of views of one or both of the scans using the inversion pulses are decreased before and after the application of the inversion pulses during a normal scan, respectively.