JPS6288949A - Measuring method for magnetism distribution using nuclear magnetic resonance - Google Patents

Abstract

PURPOSE:To reconstitute an image at a high speed by generating a lateral magnetism signal for a desired area of a sample placed in a uniform magnetostatic field, applying a slanting magnetic field generated by the combination of a sine or consine wave function, and taking a measurement. CONSTITUTION:A magnetostatic field is produced by an electromagnetic 1 and a coil 3 detects a signal generated by the object body 2 while establishing a high frequency magnetic field. Slanting magnetic field producing coils 4X, 4Y, and 5 produce slanting magnetic fields in an X, a Y, and a Z direction and those magnetic fields vary in intensity according to commands from an object capacity measuring instrument 11. The high frequency magnetic field which excites nuclear spins of the body 2 is produced by modulating a high frequency wave generated by a synthesizer 12 and supplying the modulated wave to the coil 13. The lateral magnetism signal is obtained by applying a 90 deg. and a -180 deg. high frequency pulse and the slanting magnetic field in the Z direction and the coil 3 generates a magnetism signal indicating the spin density distribution of the body 2 by being applied with the X-directional or Y- directional slanting magnetic field of a sine or cosine wave function, thereby reconstituting a magnetism distribution image at a high speed.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention provides an inspection device for non-destructively determining the spin distribution, relaxation time distribution, etc. (hereinafter referred to as magnetization distribution) in a target object using nuclear magnetic resonance. In particular, the present invention relates to a device that quickly images the distribution of an object.

[Background of the invention]

The known technology closest to the present invention is, for example, Journal of Phys.

C), Solid Stay I-Nijix (Soli
dState Phys, ) Volume 10, Nos. 55-58
There is an echo planar method proposed by Mr. P. Mansfield, which is introduced on page 1. Typical RT-' and gradient magnetic field application timings for realizing this method are shown in FIG. The figure shows the sequence for selecting and measuring a cross section perpendicular to the Z-axis, where Gy in the figure is a gradient magnetic field that is constantly applied in one direction, and Gx in the figure is a gradient magnetic field that is perpendicular to this. The spin distribution is measured by applying an oscillating gradient magnetic field. Although this method requires switching the gradient magnetic field with large amplitude and high speed, in principle, an image can be constructed using NMR signals determined by 8111 by applying a 90° pulse once.

However, unless special post-processing is performed, a rectangular wave must be used as the imaging waveform of the oscillating gradient magnetic field, which places a heavy burden on the power source for generating the gradient magnetic field, and also requires a large frequency. If required, the gradient magnetic field generating coils also require special modifications.

[Object of the Invention] An object of the present invention is to use nuclear magnetic resonance to non-destructively determine the magnetization distribution in a target object, in which a rectangular gradient magnetic field is generated in a sinusoidal manner without switching. By using periodic waveforms, the following objects are provided: 1) A device 6 capable of inputting data necessary for image reconstruction at a speed of 6'.

[Summary of the invention]

Assuming two-dimensional imaging, the image plane is (χ + y)
Make it a face. The gradient magnetic field is Gy(t)=Gosinωt +GO(11tcosω
When driven with t, the obtained magnetic resonance signal is 5(t)
=, fp(x+y)exp l:1y(Gt(x
cosω is also +ysinωt) 3 dxdy.

The data string obtained from the above equation is obtained by sampling the spatial frequency region of ρ(x, y) in a spiral manner as shown in FIG. This spiral sampling in the Fourier domain is described in the Journal of Magnetic Resonance.
sonance), Vol. 54, pp. 338-343 (1
983), however, the above-mentioned document does not include any discussion beyond suggestions regarding methods such as image reconstruction techniques.

The present invention provides an image reconstruction method that inscribes spirally measured data, a data measurement method for applying this image reconstruction, and a data measurement method that is less burdensome to gradient magnetic field parameters derived from the image reconstruction method. Characterized by the fixed method.

The image reconstruction will be explained. Is the sampling interval of the signal meter & lII, that is, physically the total? The sampling interval Δt of the A n g converter which digitizes the converted signal and provides it to the computer.

As shown in Figure 2, k8
Measurement points are lined up at equal intervals on a straight line that makes an angle θ with the axis. (In the figure, sampling points are indicated by circles.

) The time when the spiral intersects these straight lines is ω
It is expressed as ω. Substituting into equation (L), s (t n)=, fp (x e y) exp[i
y (Gotn'(xcosθ+ysjnθ))
]dxdy. If we consider tn' to be a continuous variable and write it as t6.

S (t') = < 1 + sgn (t -")
)J”/l (XI y) expcy (Gnt
, 6(xcoSO+ySinθ))]dxdy
(3) where sgn(t)=1 t>0-1 to.

It is. Here, S (t') in FIG. 2 refers to a data group arranged on a radius vector that is an angle. From now on, performing calculations regarding t# means converting the data on the helix into θ
This means that the data are grouped into data groups on different radius vectors. To make this clear, data on the radius vector of angle O is expressed as 5a(t). Taking the Fourier transform of both sides of equation (3), we get π↑ Here, the skewer means convolution. Also Pe
(f)=J−p(x+y)δ(f−−Go(
xcos 02π +ysinθ))dxdy (5
), and this Pθ(f) is ρ given to the frequency axis
This is a projection of (x*y) in the O direction. (:3) and (4
) formula, 2+111! P, (f) = RQ (c' ζ(s, (t')l
) Obtain equation (6). Therefore, the projection at angle 0 is obtained from equation (6), and from P (f) of 0 to 360'', ρ(
x, y).

Reconstruction is performed as follows.

Fourier transform P#(f) with respect to f. 1r of this
It is written as a(R).

F*(R)=J"'P#(f)e"lH'df (
6L), and by performing the following calculation on this FlI(R), ρ(x, y) can be reproduced. That is, +
ysinO)] RdRd fl (6-
2). By the way, 1. The calculation of equation (6-2) is usually performed in two parts. In other words, the method is to calculate and find . In particular, the calculation of equation (6-4) is called back projection. Here, (6).

Even if W, (u) can be obtained. This (6-
The method using formula 5) groups the data on the spiral into data on the radius vector of each angle, then weights it in the radial direction and then performs Fourier transformation.

This is a method to directly obtain We(u), and (6), (e −
1) Compared to the method using equation s(6-3), the calculation of BeFourier transform can be saved twice for each angle worth of data.

[Embodiments of the invention]

An embodiment of the present invention will be explained below using screens.

Figure 3 shows an inspection device using nuclear magnetic resonance according to an embodiment of the present invention! i't (hereinafter referred to as the "inspection device")
FIG.

In FIG. 3, 1 is an electromagnet that generates a static magnetic field Ho;
2 is the target object, 3 is the object that generates a high frequency magnetic field, and at the same time,
a coil for detecting a signal originating from the target object 2;
x g 4 y+ and 5 are gradient magnetic fields in the X direction, Y direction, and Z direction, respectively! '; This is a gradient magnetic field generating coil for generating light. As the gradient magnetic field generating coil 5, circular wire rings are used which are wired so that currents flow in opposite directions. 6 and 7°8 are the gradient magnetic field generating coils 4x, respectively.

This is a V@ device for supplying current to 4y and 5.

9 is a computer, ] 0 is a power source for a 1° C. magnet for generating a static magnetic field, and 11 is an object volume measuring device. The strength of the gradient magnetic field generated by the gradient magnetic field generating coils 4x, 4y, and 5 is determined by the above-mentioned object volume measurement'! It can be changed by a command from the A position 11.

Next, the operation of this inspection device will be explained using a schematic diagram.

The high frequency magnetic field that excites the nuclear spins of the target object 2 is generated by shaping and power amplifying the high frequency wave generated by the synthesizer 12 with the modulator 13 and supplying current to the coil 3 . A signal from the target object 2 is received by the coil 2, passes through the amplifier 14, is subjected to orthogonal detection by the detector 15, and is input to the computer 9. After signal processing, the computer 9 displays an image corresponding to the nuclear spin density distribution or relaxation time distribution on the CRT display 6.

Reference numeral 17 is a memory for storing data in the middle of calculation or final data.

FIG. 4 shows the measurement sequence of the embodiment of the present invention, and shows the RF
is the high frequency magnetic field waveform, Gz, Gx, Gy are each Zy
Xr'/force direction gradient magnetic field generation, AD indicates the signal sampling period) -0 First, 90° high frequency pulse and gradient magnetic field G z Q) t4
By adding J, spins within a desired slice of the subject are induced. Next, when time τ has elapsed from the peak of the 90°C high-frequency pulse, an 80° high-frequency pulse is applied, so that the transverse magnetization signal of the spin at the point where time τ has elapsed (t = 0) is obtained again. Make it.

From the point in time when 1=0, the gradient magnetic field Gx, with the waveform shown in equation (1),
Apply Gy and start sampling the signal.

As mentioned above, the data with a sampling interval Δ<Ar) are divided into groups. That is, the data on different radius vectors of the corner J skin are grouped. Data on the radius vector of angle O1 is expressed as bag. Here ' a I
n Δ 0 2 n π and = −+
− ω ω and AD = 2 tc/M.

For each m, the data sequence Sθm (self'') is subjected to discrete Fourier transform for n, and after phase correction according to equation (6), the real part is taken to obtain pg, (f). Performed for each m, Pa at each angle
, (f) to (6-4)-(6-4), or (6
-5) The image is reconstructed by the back projection method described in the equation.

With the above sequence, magnetization that shows the spin density distribution of the subject is obtained by applying a 6J field with a sinusoidal periodic waveform, without oscillating the rectangular M oblique magnetic field as in the conventional echo planar method. A distribution image is obtained. Note that in order to obtain a magnetization distribution image (T'1-weighted image) that includes the effect of the distribution of the longitudinal relaxation time TI of the spins of the specimen, it is well known that the 90° high-frequency pulse is °The spins are reversed in advance by applying a high-frequency pulse, and at the time when 1=0, the transverse magnetization signal including the effect of longitudinal relaxation becomes 1! ), the angle Os (m=0.1.
. . , M-1), and then performs Fourier transformation. Several modification methods can be considered for the image reconstruction method. First, instead of using equation (6), 5a(t') is weighted and the projection after convolution is directly obtained.

The frequency characteristic of the convolution weight is ψ(+,)
Then, the projection W e (f
) is W (f)=Re(e”””ζ (Y(t)S,
(t')) ) ('7). This equation (7) corresponds to the above equation (6-5).

Here, 'P (t, ) is the transverse relaxation of spin (T 2
If we consider the influence of decay), the average Tz of the sample
By setting 'f' (t, )ωteT2 to 〒2, the influence of lateral relaxation can be reduced to some extent.

By the way, when considering the non-uniformity of the static magnetic field within the field of view, this influence can be corrected by devising back projection. This will be explained below.

Let E(x, y) be the inhomogeneity of the static magnetic field within the field of view. When this is taken into consideration, equation (1) becomes S' (t) =
f/) (X q y)exp[iy (Get(xc
osωt+ysjnωt+E(x+y)t))kodx
dy (1)', and by similar derivation, Pa' (f)==Re (e''/+ζ[8'
(tn') ko), and we get . Equation (3)' shows that E (x t y ) causes a frequency shift in the projection data, which can be corrected by selecting the frequency shift in consideration of h (x + y) at the time of backprojection. This correction will be explained below.

The relationship between the image plane and the frequency axis at the time of back projection is shown in the 61st figure, where fc is the resonance frequency at the center of the field of view. In normal backprojection, the backprojection data from angle 0 to point (x+y) is taken from the next frequency position denoted by iB, as shown in the figure.

JR= γG(xsir+1?+ycosO)/
27C+f.

However, the resonance frequency of the point (x, y) is not f8 in the above equation, but fB' = fn + fe due to the inhomogeneity of the static magnetic field.
It is given by fBI. Therefore, if the magnetic field inhomogeneity is known, the influence of the static magnetic field inhomogeneity can be corrected by using this J11' in place of one mouth in the back projection coordinate calculation.

Now, in the following explanation, P at O ~ 360°, (f)
We have explained the method of image reconstruction from Images can be reproduced with Pa(f) from 0° to 180°. However, as shown in FIG. 2, 56(t, n'
) v 5tray (tn"') Since the measurement points are irregularly spaced near the origin, equally spaced data points must be estimated by interpolation.

Now, if we introduce T such that ω=2 FC/T and let the radius of the field of view be r, we have the relationship 2π 2R. When the image matrix is IX, J, the resolution Δγ=2R/J and at the same time, Δγ= −(8) γGotsax, so from these formulas, J t, □= −r (9)
get. At this time, the maximum amplitude G++ax of G8 and G
is Gmax= GO(11t max= πJGo
It is given by (10). For example, R = 15cm, T = 0.25m5゜J =
1.28, Go=0. O]6 Gauss/Cm.

G, , = 6.4 Gauss/am. This T and 611
It is very difficult to realize a combination of 8 values in an actual device.

To avoid this difficulty, use an oblique magnetic field Gy, Gy.

Assume that it can be expressed by the following formula.

Here, the sine wave is the element of this gradient magnetic field waveform.

The total i! of the sequence shown in Figure 4 is repeated multiple times by changing the phase φ of the cosine wave at equal intervals. +'l is repeated to obtain a set of data on multiple equally spaced spirals. Next, in the same manner as described in the embodiment shown in FIG. 4, the data on the radius vector of each angle may be grouped, and the image may be reconstructed by Fourier transform. FIG. 5 shows an example in which signals of four spirals with different φs by - are suppressed.

Now, 1] T! ! If the image is reconstructed using the A-ray, 1, ■ t wax ” T' D Gsax = -πJG.

It can be seen that the conditions for Gmax are relaxed by a factor of D.

In addition, the present invention can be used as an NMR microscope or a pI microscope.

This is particularly easy to implement if the subject can be rotated.

That is, such a case is shown in FIG. In the figure, the subject is rotating within the field of view, and the center of rotation and the center of the field of view coincide. The coordinate system fixed in the laboratory is (x, y
). In place of the rotating magnetic fields Gx, Gy shown in FIG. 4, as shown in FIG. 8, the oblique magnetic field G K ,
Apply Gy.

In other words, two gradient magnetic fields perpendicular to the field.

One is applied constantly and one is applied proportionally to time.

The proportionality constant is 11'H, which is determined by the magnitude of the gradient magnetic field that is constantly applied! 11 Rotate the body with an angular velocity S. In this way, if the PP and drift fibers fixed to the subject rotating once are (x r + y r ), then the following equation is obtained. So, by substituting into equation (14), we get 5(t)=fp(xr, yr)cxp[jy(Gnxr
, f'(coswt,'-ut'sinωt')
dt' + GoyJ"(sinωt' + tas tcos (11t') dt)
x rd y r (1
6) and calculate the integral.

5(t)=J'ρ(xrpyr)exp[1y(xrG
otcosωt+yrGotsinωt)ldxrdy
r (17). That is, spiral sampling similar to that shown in FIG. 2 can be performed by rotating the subject and applying the gradient magnetic field shown in FIG. 8.

Therefore, by setting the signal sampling interval to 1/M of the rotation period of the object and performing the grooving and image reconstruction processing of the obtained sampling signals in the same manner as explained in FIG. The spin density distribution of the specimen is obtained. Furthermore, if the signal sampling interval is not an integer fraction of the rotation period of the subject, the grooving and image reconstruction processing may be performed after calculating a signal corresponding to an integer fraction of the rotation period by interpolation.

Furthermore, if measurements are performed multiple times by changing the application timing of the gradient magnetic fields G x and G y shown in FIG. Sampling data on the spiral can be obtained. From this data, the image plane configuration can be performed in the same way as in the embodiment shown earlier.6 [Effects of the Invention] According to the invention shown above, a new method of sampling the Fourier space in a spiral pattern is achieved. By incorporating this method, data necessary for image reconstruction can be captured at high speed without requiring high-speed vibration of a rectangular gradient magnetic field, and the spin distribution or relaxation time distribution in the target object can be captured without destroying the spin distribution or relaxation time distribution. equipment constraints can be relaxed.

ff of the drawing? 1@Explanation FIG. 1 shows a pulse sea fence that realizes the echo prechie method, which is a known technique similar to the present invention, FIG. 2 shows an example of how measurement points are arranged according to the present invention, and FIG. 3 shows an example of an embodiment of the present invention. An example of the apparatus 49, FIG. 4 is a sea fence for applying RF and gradient magnetic fields according to an embodiment of the present invention, FIG. 5 is another example of how measurement points are arranged according to the present invention, and FIG. 6 is an image at the time of back projection. The relationship between the plane and the frequency axis, FIG. 7 shows the seat system of another embodiment of the present invention in which the subject is rotated simultaneously, and FIG. 8 shows the pulse sequence of the embodiment of FIG. 7.

Claims (1)

[Claims] 1. A first step of preparing a transverse magnetization signal of a desired region of a subject placed in a substantially uniform static magnetic field.Measurement is performed under the application of the transverse magnetization signal and a regional magnetic field. Second
The process includes a third process of reconstructing a magnetization distribution image from the measured signals, and the gradient magnetic field is an oscillating waveform composed of a combination of sine waves or cosine wave functions. The method used to measure magnetization distribution. 2. The method for measuring magnetization distribution using nuclear magnetic resonance according to claim 1, wherein the integral value of the vibration waveform draws a spiral in a spatial frequency domain. 3. Claim 1, characterized in that the value obtained by dividing the period of the sine wave or cosine wave function that is a component of the vibration waveform by the sampling interval of the signal in the second process is an integer. A method for measuring magnetization distribution using nuclear magnetic resonance as described in . 4. The third step is a step of calculating, by interpolation from the signals sampled in the second step, signals at measurement points at intervals whose ratio to the period of the sine wave or cosine wave function is an integer. A method for measuring magnetization distribution using nuclear magnetic resonance according to claim 1, comprising: 5. The third process is a process of grouping the data of measurement points distributed on the spiral into one-dimensional data for each radial direction, a process of weighting the one-dimensional data and then performing Fourier transformation; Claim 2 comprising a step of reconstructing the image by back projecting the result of the Fourier transform.
A method for measuring magnetization distribution using nuclear magnetic resonance as described in Section 3. 6. The inspection apparatus according to claim 1, wherein data regarding static magnetic field distribution within the field of view is used for coordinate calculation of back projection data during the back projection. 7. Claims 1 or 2, characterized in that the first and second processes are repeated multiple times by changing the phase of the sine wave or cosine wave function that is a component of the vibration waveform. A method for measuring magnetization distribution using nuclear magnetic resonance as described in Section 1. 8. A first step of preparing a transverse magnetization signal of a desired region of the subject placed in a substantially uniform static magnetic field, rotating the subject and applying a predetermined gradient magnetic field to produce the transverse magnetization signal. and a third step of reconstructing a magnetization distribution image from the measured signals, and the gradient magnetic field includes a constant first gradient magnetic field component and a direction orthogonal to the first gradient magnetic field component. An inspection device using nuclear magnetic resonance characterized by comprising a second gradient magnetic field component that is applied to a magnetic field and whose magnitude changes in fixed increments. 9. The method for measuring magnetization distribution using nuclear magnetic resonance according to claim 8, wherein the gradient magnetic field draws a spiral line in a spatial frequency region related to the rotating subject. 10. Magnetization distribution using nuclear magnetic resonance according to claim 8, wherein the value obtained by dividing the rotation period of the object by the sampling interval of the signal in the second process is an integer. How to measure. 11. The third step includes a step of calculating, by interpolation from the signals sampled in the second step, signals at measurement points at intervals whose ratio to the rotation period of the object is an integer. 9. A method for measuring magnetization distribution using nuclear magnetic resonance according to claim 8. 12. The third process is a process of grouping the data of measurement points distributed on the spiral into one-dimensional data for each radial direction, a process of weighting the one-dimensional data and then performing Fourier transformation; 10. The method for measuring magnetization distribution using nuclear magnetic resonance according to claim 9, which includes a step of reconstructing an image by back projecting the result of Fourier transformation. 13. According to claim 8 or 9, the first and second steps are repeated a plurality of times by changing the application timing of the gradient magnetic field with respect to the rotation of the subject. A method for measuring magnetization distribution using nuclear magnetic resonance as described.