JPS59182351A - Nmr tomographic image pickup device - Google Patents

Abstract

PURPOSE:To reduce the time for sampling signals to half with the same resolving power as the resolving power of the conventional NMR tomographic image pickup device by giving a magnetic field gradient of twice the ordinary magnetic field gradient in the Y direction of a tomographic plane. CONSTITUTION:The sectional position in a body axis (Z-axis) direction is determined by impressing a magnetic field gradient Gz simultaneously with impression of a 90 deg. RF pulse. A frequency modulation is performed in the X direction by applying a magnetic field gradient Gx therein. A magnetic field gradient Gy is applied for each of lines (L=0, 1, 2-N-1) in the Y direction as shown in the equation ( I ) so that the gradient is made twice the conventional magnetic field gradient. Generated nuclear magnetic resonance signals are mixed with the resonance frequency at x=0 and are then passed through a low pass filter, by which the signals are converted to frequency signals o-f and are sampled at a 2N point at 1/2f time interval. N lines of signals are obtd. by repeating such procedure as L=0, 1,...N-1. NXN pieces of the resulted signals are subjected to a two-dimensional Fourier transform and a sectional image is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Industrial Application Field This invention relates to an NMR* image capturing device, and in particular, NMR imaging is performed using Fourier transform to obtain NMR signals from N rows of nuclear magnetic resonance signals consisting of 2N' sampling points. The present invention relates to an apparatus for obtaining a tomographic image having pixels of .

(B) Prior Art In FIG. 1, when the body axis direction is the Z direction, the longitudinal direction is the Y direction, and the lateral direction is the X direction, in the conventional NMR fragment imaging device, the magnetic field gradient Gx is By adding, the high frequency of a constant frequency applied to the subject is frequency modulated, and on the other hand, by adding a magnetic field gradient Gy in the Y direction, the high frequency is phase modulated.
A method of taking a tomographic image of a subject by performing computer calculations
The two-dimensional Fourier transform method of t or the spin warp method, which is an improved method thereof, is hereinafter referred to as the two-dimensional Fourier transform method. ).

Therefore, when calculating a tomographic image using the two-dimensional Fourier transform, 2N rows of nuclear magnetic resonance signals are required, each row having 2N sampling points, in order to obtain NXN pixel images within one tomographic plane. Met.

Therefore, since the longitudinal relaxation time of the nuclear magnetic moment is approximately 1 second, the time required to collect one row of nuclear magnetic resonance signals is approximately 1 second, which is approximately equal to the longitudinal relaxation time of the nuclear magnetic moment.

Therefore, there is a drawback that it takes a long time to acquire the required number of nuclear magnetic resonance signals. Furthermore,
The calculation time to obtain the tomographic image is correspondingly long (as a result, the time from the start of signal acquisition to the projection of the tomographic image, that is, the total time of the signal acquisition time and the calculation time) becomes longer. There were drawbacks.

(c) Objective The object of the present invention is to provide an NMR tomography apparatus that can reduce the signal acquisition time and computer calculation time to half compared to conventional NMR tomography apparatuses.

(d) Configuration This invention is based on giving a magnetic field gradient ay twice as large as the normal one in the Y direction (vertical direction). The device is configured to obtain nuclear magnetic resonance signals of 2N rows) and obtain a cross-sectional image of N×N”, which has the same resolution as the conventional device.

(E) Embodiment The above-mentioned FIG. 1 is also used to explain the present invention, and in FIG. Moreover, the same figure (b) shows the said cross section 2 itself taken out.

Here, the procedure for obtaining nuclear magnetic resonance signals will be explained.

That is, the cross-sectional position in the body axis (Z-axis) direction is determined by applying a magnetic field gradient Gz simultaneously with the application of a 9°' RF pulse (for example, 6 Mz).

Next, frequency modulation is performed by applying a magnetic field gradient Gx in the X direction (however, X=O and
The magnetic field gradient Gx is determined so that the resonance frequency difference becomes f at the position of =Lx. ) Therefore, the magnetic field gradient Gy in the Y direction is added for each row (L=0, 1.2, . . . N-1), as shown in equation (1).

Gy (L)=Gy (L-N/2)...
(1) However, as shown in Figure 2, Gy is the center Y=
The magnetic field gradient in the Y direction becomes zero at Ly/2, where Y=0, γcy=-n, Y=LV, and r G'y-.

Here, T is the gyromagnetic ratio. That is, the above means that the magnetic field gradient in the Y direction is doubled compared to the conventional one.

Under the above conditions, the generated nuclear magnetic resonance signals are mixed at the resonance frequency at X=0 as shown in the block diagram of FIG. 0 to 10KHz), and further convert this into a signal of 1 that satisfies the sampling theorem.
2N points are sampled at a time interval of /2f.

Such a procedure is performed when L=0. By repeating the sequence l, 2, . . . N-1, N rows of signals consisting of 2N sampling points are changed. FIG. 3 is an explanatory diagram showing sampling points obtained by the NMR tomography apparatus according to the present invention.

Next, a cross-sectional image consisting of NXN pixels is obtained by performing two-dimensional Fourier transform on the NXN signals obtained as above and calculating the absolute value of the left half plane (or right half plane). be.

The above principle will be explained below using mathematical formulas.

FIG. 4 shows an object (a triangle in the figure) 21 to be photographed in a cross section of the subject. First, the static magnetic field in the Z-axis direction applied to the object 21 is Ho. Then, if the resonance frequency at the position of X = L x / 2 (hereinafter referred to as the center resonance frequency) is f', then r' = 2nω' = 2n7H0... (2) Now,
When the above sequence is applied to an object having a density of ρ(x, y) as shown in FIG. 4, the nuclear magnetic resonance signal S (t, L
) is a reflex plus signal r (t) −〇O3(2n (f ′−f, /2)t
), M (t, L)=S (t, L)-r (t)Lx
Ly (L-N/2)) ・・・・・・・・・・ (5)
Here, if we pass <M (t, L) through a low-pass filter as described above and remove the first term in (5), the signal S in equation (6)
' (t, L) can be obtained.

That is, (L-N/2)) ・・・・・・・・・ (6)
This is an interval that satisfies the sampling theorem, Δt=1/2
When sampling 2N points at f0, -/2f0 (
k=0.1,2. ...2N-1), (7)
The formula holds true. That is, X (L-N/2))
(7) Here, k=o, 1, 2.
...2N-IL=0. 1. 2.・
...N-'1.

Furthermore, as shown in FIG. 4, x (0=Lx) and y (
If the object is discretized by dividing 0~Ly) into N, x=mLx/N, m=0.1. ...N-1, y=
nLy/N, n=0.4.・・N-1・ (8)
It is expressed as in equation (9) using .

(L-N/2)) ・・・・・・・・・・・・ (9)
However, k=o, L2. ...2N-1L=0. L
2. ...N-1... (10) Note that generally, N is a number of about 128 or 256.

nn j)) ) ・
・ ・ (12) Here, the following function Φ (m, n)
and φ(m.

n) from ρ(m, n).

Furthermore, the functions Φ(rn, n) and φ(rn, n) are assumed to be periodic functions having a period of 2N for m and N for n outside the defined area below. That is, Φ(m, n)=
0 Here, N≦m≦2N-1, 0≦n≦N-1φ(m, n
)-Φ(2N-m, N-n) (14) Here, 0≦m≦2N-1, 0≦n≦N-1 Then, (1
2) (゛ is transformed as follows using the two-dimensional discrete Fourier transform formula.

N ZJ2N points, L represents a discrete two-dimensional Fourier transform of N points, 2N points, and n represents a discrete two-dimensional inverse Fourier transform of N points.

Here, if we use the Fourier transform shift formula, (15)
Obtain the following formula from someone.

N/2)-φ (m, n-N/2)) ・ ・ (1
6) FIG. 5 is a diagram showing the values of the above equation (16) in absolute values.

It can be seen that by calculating the left or right half and interchanging the upper and lower halves, an image of the object having the density distribution shown in FIG. 4 can be obtained.

FIG. 6 shows a block diagram of a circuit for extracting N rows of nuclear magnetic resonance signals and inputting them into a computer in order to realize the above-mentioned NMR tomography apparatus.

That is, the 90° RF pulse generating circuit 20 is driven by the reference clock generating circuit 10 to generate a 90° RF pulse, and the 90° RF pulse is sent to the transmission/reception switching circuit 40 via the RF amplifier 30.
and the transmission/reception switching circuit 40 connects to the RF
A 90° RF pulse is applied to the coil 50.

Based on the magnetic field gradient Gz, Gy (L) when L=0 in equation (1) is added, and Gx is further added, one line of nuclear magnetic resonance signals is obtained, and the transmission/reception switching is performed. The signal is input to a mixer circuit 70 via a circuit 40 and a preamplifier 60.

On the other hand, the reference clock from the reference clock generation circuit 10 is input to the mixer circuit 70 via the reference signal circuit 80, and the output from the mixer circuit 70 is input to the low-pass filter 90 as an analog signal including 2N sampling points. However, the output of the A/D circuit 100 is input to the computer 110 as the first row of data.

By performing the same operation as above, L=1.2...
N-1 rows of data are input to computer 110.

The computer 110 performs the above-mentioned two-dimensional Fourier transform on the N rows of data consisting of 2N sampling points, and separates the desired image having the distribution density of ρ(x,y) illustrated in FIG. Appears on the installed playback device.

Further, in the above-described embodiment, an image having a square shape with N×N pixels to be obtained has been described, but an image to be examined of an actual subject may be rectangular. Therefore, the number of data included in -scanning is not limited to the above-mentioned N, but can generally be handled as N'.

(F) Effect The NMR tomography apparatus according to the present invention is an apparatus that obtains nuclear magnetic resonance signals by frequency modulating the X direction of the tomographic plane and phase modulating the Y direction, and in which a predetermined signal is obtained in the Y direction. Based on the addition of the magnetic field gradient cy determined by the relationship, two-dimensional Fourier transform is performed from N rows of nuclear magnetic resonance signals each consisting of 2N sampling points, so compared to conventional NMR tomography equipment, signal acquisition time can be cut in half.

[Brief explanation of the drawings] Figure 1 is an explanatory diagram showing the positional relationship between the subject and its tomographic image when the body axis direction is the Z direction, the longitudinal direction is the Y direction, and the lateral direction is the X direction. 3 is an explanatory diagram showing the Y-direction distribution of Gy gradient in the NMR tomography apparatus according to the present invention, FIG. 3 is an explanatory diagram showing sampling points obtained by the NMR tomography apparatus according to the present invention, and FIG. represents the object (triangle in the figure) to be imaged in the cross section of the subject, Figure 5 is a diagram showing the value of the above equation (16) as an absolute value, and Figure 6 is a diagram showing the above-mentioned NMR In order to realize a tomographic imaging apparatus, a block diagram of a circuit for extracting N rows of nuclear magnetic resonance signals and inputting them into a computer is shown. Patent applicant Shimadzu Corporation Representative Patent attorney Takaharu Ohnishi

Claims (1)

[Claims] (1) By applying a magnetic field gradient Gx in the X direction of the subject, the high frequency of a constant frequency applied to the subject is frequency modulated, and on the other hand, by applying the magnetic field gradient GY in the Y direction, the high frequency is phase-modulated. In an NMR tomography apparatus that obtains a tomographic plane of a subject based on modulation, N rows each consisting of 2N' sampling points are created by applying a magnetic field gradient Gy determined by a predetermined relationship in the Y direction. An NMR tomography apparatus characterized in that a cross-sectional image consisting of NXN' pixels is obtained by obtaining nuclear magnetic resonance signals of N rows and performing two-dimensional Fourier transform on the N rows of nuclear magnetic resonance signals using a computer.