SU1610444A1 - Method of measuring heterogeneities of magnetic field - Google Patents

Description

This invention relates to magnetic field measurements and can be used to monitor the uniformity of a magnetic field in magnetic resonance spectrometers, NMR flow meters, etc.

The aim of the invention is to improve the accuracy of measuring the inhomogeneity of the magnetic field and to ensure the possibility of measuring local inhomogeneities of the magnetic field.

FIG. 1 shows a block diagram of a device implementing the described method; Fig. 2 shows experimental dependencies of the shape of the first derivative of the absorption signal of a decimolar solution in water for a fluid velocity of 10 cm / s (a is the longitudinal magnetic field gradient equal to 0; b is the longitudinal magnetic gradient lol equal to 0.03 G / cm; c - the longitudinal gradient of the magnetic field is 0.12 Gs / cm; g - the longitudinal gradient of the magnetic field is 0.12 Gs / cm, the sign of the gradient is reversed).

The method is carried out as follows.

The field acting on the element of the volume of a fluid moving in a magnetic field with a continuous gradient varies according to the law:

four

NJ

W grad B + 2nB fj cos () +

Ai

(one)

dBp dt

where W is the velocity of the fluid, cm / cj grad B is the longitudinal gradient of the magnetic field, Gs / cm (

B — modulation amplitude, Gs;

f is the modulation frequency, Hz;

dlip „„

 - linear sweep speed,

 Gs / s.

By appropriately selecting the frequency and amplitude of modulation, as well as the recording conditions of the line (field sweep speed — when the line passes through magnetic ttol or the frequency change rate of the resonant RF field when line is cooled by the frequency), it is possible that the precession frequency of the nuclear moments from the frequency of the resonant electromagnetic field will be determined mainly by the first member of formula (1), Toe. depends on the velocity of the fluid and the longitudinal gradient of the magnetic field. It is known that the shapes of the absorption and dispersion curves change when a non-stationary magnetic field is applied to a sample. Consequently, when registering the spectrum of a moving fluid in a magnetic field with a longitudinal gradient, the shapes of the curves will change, absorb and disperse, and the magnitude and distortion at the stationary velocity of the fluid is uniquely associated with the longitudinal magnetic field gradient. This dependence and is

0

five

0

0 35

five

It is used to measure the sign and magnitude of the longitudinal magnetic field gradient.

The device based on this method works as follows.

A transducer with a flowing fluid, consisting of a pipeline 2, a fluid motion system 3, a detection coil 4 and a modulation coil 5, are placed in the test field created by the magnets 1. The modulator 6 generates the modulation field needed to register the NMR signal. The line is passed by tuning the generation frequency of the spin detector 7. The first derivative of the absorption signal is recorded using a synchronous detector 8, to the signal input of which the output signal of the spin detector 7 is fed, and reference voltage from the output of the modulator 6. The output voltage of the synchronous detector is fed to the input of the processing and recording device 9.

FIG. 2 that the change in the longitudinal gradient of the magnetic field causes the asymmetry of the output signal of the synchronous detector 8, which is uniquely associated with the magnitude and sign of the gradient. The measure of the gradient can be the difference or the ratio of the areas under the positive and negative parts of the first derivative of the absorption curve, or the difference or ratio of the maximum deviations from zero of the positive and negative parts of the first derivative of the absorption curve. The value and sign of asymmetry are analyzed in processing and recording unit 9 and displayed.

Phage.2

Claims (1)

A METHOD FOR MEASURING THE INHOMOGENEITIES OF A MAGNETIC FIELD, including the effect of a magnetic field on a liquid