SU1603264A1 - Method of construction of spin image - Google Patents

Abstract

The invention relates to the field of radio-spectroscopy, and more specifically to magnetic resonance-based introscopy. The purpose of the invention is to eliminate the phase distortion of the image. The method is based on the 2D Fourier-tomography method, which involves exposing a layer of excited spins to a sample with orthogonal gradients in the plane of the layer and then recording magnetic resonance signals at a different value of one of the gradients. The method uses a sequence of pulses acting on the layer of excited spins of the sample, which allows to obtain a magnetic resonance reference signal, calculate the phase correction, correct the signals carrying information about the spin density, and then build an image free of phase distortions. 6 Il.

Description

The invention relates to radio-spectroscopy, in particular to magnetic resonance-based introscopy, and can be used in constructing a picture of the spatial distribution of resonant spin magnetic moments in a sample under study.

The purpose of the invention is to eliminate phase distortions,

FIG. Figure 1 shows the sequence of high-frequency excitation pulses by voltage And acting on the sample; in fig. 2 - magnetic field gradient pulses in the direction of the Z axis - in FIG. 3 - magnetic field gradient pulses in the direction of -. The x-axis is gyj in FIG. 4 - magnetic field gradient pulses in the Y direction - § „; in fig. 5 - spin echo signals received from the sample as a result of the action of the indicated pulses; in fig. 6 is a block diagram of a device implementing this method.

The method is carried out in the following manner.

A sample located in a uniform polarizing magnetic field is affected in time interval 1 by high-frequency spleen and a magnetic field gradient, for example, along the Z axis. As a result, magnetic resonance is excited in the sample layer by rotating the spin magnetization vector in the layer by 90. Then on

O5

about

00 ND

ABOUT)

a layer of vacant spins (nuclear or electron) acts as a frequency-coding gradient of the field along the Y axis. After the sign of the gradient changes in the sample, a spin echo signal appears, which is recorded.

After that, in the time interval 2, a layer of spin magnetic moments reappears in the sample. The layer is influenced by a frequency-coding gradient along the X axis, and along the Y axis by a phase-gradient gradient. The sequence of pulses shown in time interval 2 is repeated N times with N values of the amplitude phase-coded. gradient. Receiving N spin echo signals recorded by t.

The magnetic resonance signal received in time interval 1 is described by the expression

Jr l -b olg

dxdy, (1)

Sv (t) e jjp (x.y) e where C) is the signal phase};

(x, y) is the spin density distribution in the excited layer; If is the gyromagnetic spin ratio; gu - magnetic field gradient in

 Y direction; t, is time;

ip is the time instant corresponding to the maximum of the spin echo signal.

The signals recorded in the time interval 2 form a two-dimensional matrix:

S (t, .p) (x.y) dxdy .. (2)

where Li (p) is the phase increment with each of the values of the phase-encoding g.

 Nn

dienta; , -.,. .., -1,0,1, ...,

S (tJ, p) e

icf-iuqJCP

Comparison of expressions (1) and (3) shows that at time 35 they differ in the presence of a phase multiplier, which is determined from the relation:

1Sh (S. (to,) - SCtiTpT (4)

Then the two-dimensional signal, determined by the extrusion (2), is multiplied by the value of the result

two-dimensional Fourier transform, thereby obtaining an image free from phase distortion.

A block diagram of a device implementing the method is shown in FIG. 6. The device contains a control electronic computer (ZIM) 1, a receiver-CEC 2 magnetic resonance signals, a transmitter 3, power sources of gradient coils 4-6, a display 7,. , receiving coil 8, transmitting coils 9, system of gradient coils 10, sample 11, electromagnet 12, source-Shatani electromagnet .13.

N, N m

t rl 9 about the time corresponding to the maximum of the echo signal.

Fix value, we get

thirty

JJ p (x, y) e dxdy (3)

five

0

The device operates under the control of an electronic computer 1. As follows.

A signal with a frequency equal to the resonant frequency of the spins of sample 11, located in the field of electromagnet 12, from transmitter 3 enters the transmitting coils 9, creating a high-frequency magnetic field orthogonal to the field of electromagnet 12. A pulse is applied from the power source 4 to the gradient coils 10, current s-gradient. As a result, a spin layer in the X, Y plane is excited in the sample 11. Then, from the power source 5, a current pulse is applied to the coils of the Gradient of the gradient coils system 10, which after some time changes its sign to anti-drop. In the receiving coil 8 thereafter, a spin echo signal is induced, which is amplified in receiver 2 and recorded in the computer memory.

Then the layer of spins of sample 11 is again excited, and the c-power source of the gradient coils 6 is fed to

coils 8x current pulse gradient, forming an 11th echo signal. In this case, prior to the formation of an echo, a current pulse, creating a gradient of p, m, is supplied for a short time to the excited spin layer from the power source of the gradient coils 5 of the system 10. The echo signal is then recorded, the amplitude of the Y-gradient is changed, the sequence of pulses (in time interval 2 of Fig. 1) is repeated. The resulting spin echo signal is recorded in the memory of the computer 1, and so on for N different values of the amplitude of the current pulse in the coils of the Y-gradient. From the received signals with a computer, a phase correction is calculated, a two-dimensional data array is corrected, after 2 Lurie transformations of which a picture of the spin density i-distribution in the sample layer displayed on the display 7 is obtained.

in magnetic resonance-based introscopy, since in most cases the phase instability

completely destroys the image.

Claims (1)

Invention Formula A method for constructing a spin Q-image, consisting in that a spin M2 resonance occurs in the sample layer using a 90 HF pulse and a magnetic field with a gradient along the Z-gg axis, then a 15 magnetic field is applied, with gradients along the Xg axis and along the y-gu axis, they form a spin echo signal, which is recorded at a fixed gy value and repeats this sequence 20 for various values of gu and after the mathematical processing of the spin echo signals, a spin image is obtained, differing in that. that, in order to eliminate phase images, the proposed method of constructing image spans, additionally, allows us to eliminate the spin echo signal during phase image distortions, the resulting fixed value of gij, determine the kaursch as a result of instability. The phase of the spin echo signal and taking into account The magnetic field and the parameters of the elec-Phase are mathematically processed. equipments. This is an entity JO; ku signal ,, ), 1603264. in magnetic resonance-based introscopy, since in most cases the phase instability completely destroys the image. Invention Formula A method for constructing a spin image, consisting in that a spin m2 resonance is excited in the sample layer using a 90 HF pulse and a magnetic field with a gradient along the Z-gg axis, then affected by a magnetic field, with gradients along the Xg axis and along the axis Y-gu, form a spin echo signal, which is recorded at a fixed value of gy and repeats this sequence at various values of gu, and after mathematical processing of the spin echo signals, a spin image is obtained, differing in that. (Rig.y.