SU1436039A1 - Method of magnetic resonance introscopy - Google Patents

Abstract

Ie puffed-up relates to magnetic resonance radiospectroscopy and its use for qualitative and quantitative analysis of a substance and can be used in diagnostics when disturbing the integrity of the internal structure of objects is unacceptable. The goal is to increase the sensitivity and resolution. The method consists in affecting the object under study by a constant magnetic field, excitation of particles contained in the object, when exposed to a pulsed radio frequency field at the frequency of the spin resrnance of particles with a given pulse frequency, and simultaneously acting on the object under investigation with a spatially inhomogeneous alternating magnetic field parallel to the main one. The spin indication signal is then detected by a spin induction signal, the values of the magnetic resonance parameters associated with the structure of the object are determined by the average signal magnitude, and the effect of an additional spatially non-uniform magnetic field is produced synchronously with a pulsed radio frequency field at the initial phase shift between the frequency changes in the spatially inhomogeneous magnetic field and the pulse frequency of the radio frequency field in the range 27-33. 5 il. (About W 00 05 o 00

Description

The invention relates to magnetic resonance radiospectroscopy and its application for qualitative and quantitative analysis of a substance and can be used in diagnostics in cases where the integrity of an object is unacceptable, in particular, in the study of living organisms.

The purpose of the invention} is an increase in sensitivity and resolution.

Fig. 1 shows the calculated dependence of the contribution to the constant component of the induction signal with the coincidence of the frequency of the radio pulse and the frequency of variation of the non-uniform magnetic field; Figs 2 and 3 show models of the objects under study; in FIG. 4, 20 pair-wise nuclear induction, depending on, and 5, experimental dependencies on the dimensionless coordinate fl in

signal values in the presence of a phase shift between the frequency of the radio pulse and the frequency of an alternating inhomogeneous magnetic field .25

The system of spins, in the object being studied, creates the magnetization M, whose behavior in magnetic fields is described by the Bloch equations.

The spin system can be 30 in a constant and inhomogeneous alternating magnetic fields, and the gradient of the inhomogeneous magnetic field is directed along the X axis, periode occurs when the frequency of the high-frequency radio pulses coincides with the frequency of the spatial non-uniform magnetic field and the end of the radio pulse corresponds to the phase 30 ° variable inhomogeneous magnetic field having a cosine-law law of variation over time. The allocated volume is conditionally bounded by the values of x, and k - x, corresponding to the first zero io, Bessel function 1о (р).

In this case, the undesirable contributions from various elements that are not related to the total volume, have opposite signs depending on the position of the element. Due to this, in the case of exactly the volume of the object under study compared with the r allocated, the contributions from the individual elements are mutually compensated, with the exception of the contribution from the allocated volume

the pulse rate of the high-frequency field, and the instants of the appearance of high-frequency pulses are strictly related to a certain phase of the variable field, then the solutions of the Bloch equations for a certain point of the sample follow the expression dp of the local averaged spin induction signal under the condition of magnetic resonance

g (x) (b)

where d is the value of the phase of an alternating non-uniform magnetic field at the moment of its termination

high frequency pulse

floor;

dimensionless coordinate

equals

,) / Q

8x

- hygromagnetic ratio;

- amplitude of the magnetic field gradient along the x axis;

X is the current coordinate;

Xd is the coordinate of the position of the plane in which the non-uniform magnetic field vanishes;

O. - circular frequency of an alternating magnetic field; The IO (P) Bessel function is of order 0. From the expression it can be seen that the contribution to the averaged signal of the spin information from the entire object depends both on the spatial location of the volume element and on the phase cf of the alternating magnetic field at the moment of the end of the RF field pulse. Fig. I shows the calculated dependence of the contribution V to the constant component of the quasi-stationary nuclear induction signal as a function of the dimensionless coordinate fl in

five

0

five

0

five

0

five

when the frequency of the high-frequency radio pulses coincides with the frequency of the spatial non-uniform magnetic field and the end of the radio pulse corresponds to the 30 ° phase of the alternating non-uniform magnetic field, which has a cosine change in time. The allocated volume is conditionally bounded by the values of x, and k - x, corresponding to the first zero io, Bessel function 1о (р).

In this case, the undesirable contributions from various elements that are not related to the total volume, have opposite signs depending on the position of the element. Due to this, in the case of exactly extended, in Comparison with r allocated, the volume of the object under study, the contributions from individual elements are mutually compensated, except for the contribution from the selected volume,

It is possible to choose the phase cf of the variable inhomogeneous magnetic field so that the total contribution from the volume elements not related to the selected one is minimal.

The calculation of the total contribution from an inhomogeneous object, the length of which corresponds to the values of pa-. of size f lying in the interval j-lOQ:. 100, it is possible to determine the optimum phase shift range between the frequency of variation of a non-uniform magnetic field and the pulse frequency of the radio frequency field, which is in the range of 24-35 °.

3

As an example of the experimental implementation of the proposed method, let us consider obtaining the projection of the distribution of the density of nuclear spins in model objects in a certain direction, taken beyond the x axis. The spin density projection is obtained by placing an object in a constant magnetic field, exposing the object to a sequence of high frequency pulses, applying an alternating non-uniform magnetic field, then synchronously detecting and averaging the spin induction signal and changing the non-uniform alternating magnetic field.

The experiment was carried out on an NMR spectrometer in intermediate fields modified to obtain: the possibility of observing pulsed .. NMR signals and localizing the region,

from which the NMR signal is recorded,

I

The measuring system consists of

an NMR spectrometer operating at a frequency of 300 kHz, a high frequency pulse unit, a variable variable field control unit, and coils for producing a variable inhomogeneous gradient field along one of the coordinate axes, use pairs of symmetrical catzpek, each one of the coils constituting a pair , they feed with alternating current of different amplitude and in the opposite phase.

The degree of imbalance of the modulating currents in the coils, creating a 1X non-uniform magnetic field with a gradient along the X axis (modulating coils) determines the position of the plane in which the amplitude of the modulating field vanishes. Change the degree of imbalance, i.e. The ratio of the currents in the modulating coils changes, moves the position of such a plane and, therefore, changes the value of the XQ coordinate. The value of the XQ coordinate is unambiguously related to the values of the amplitudes of the currents in the modulating coils. The values of the coordinates are experimentally determined from the values of the modulating currents in the modulating coils,

Either two capillaries filled with an aqueous solution of NiSO paramagnetic salt (model 1) or a cylinder filled with the same solution and separated by a partition parallel to the axis are used as model objects.

36039

(model 2), The cross sections of models 1 and 2 are shown in Figures 2 and 3, respectively.

Figures 4 and 5 show the experimental dependences of the averaged spin induction signal on the plane position, for models 1 (curves 1 and 2) and 2 (curves 3 and 4) in the presence of

0 synchronization between the repetition frequencies of the high-frequency pulses and the variation of the non-uniform magnetic field at the phase shift value tf 30 without synchronization. In Fig. 4, dependences for model 1 are obtained with the value of the amplitude of the gradient of an alternating inhomogeneous magnetic field 2.5 10. According to curve 2, the background value for XD is 14 mm,

0 where the spin density in the model is zero and is 50% in the absence of synchronization, whereas in the presence of synchronization at the same point, the background is completely absent with accuracy up to the level of ga5mov. The enhancement of the contour of the resulting distribution at half the height of the relative true is about 30 and 75% for the curves and, respectively, that

0 characterizes a significant effective increase in resolution, in the case of model 2, with an amplitude gradient of 7 JH, the background is not present for curve 3 and is 25% 2g for curve 4 (figure 5),

Thus, by introducing synchronization between the repetition frequency of the high-frequency pulses and the frequency

40i changing a non-uniform magnetic field when choosing the phase of an alternating non-uniform magnetic field at the time of the end of the high-frequency pulse in the range of 24-35 ° according to the proposed method

45 achieve a decrease in the background noise to the level of the noise and, as a result, an increase in the resolving power and sensitivity,

Claims (1)

50 claims The method of magnetic resonance introscopy, which involves exposing the object under investigation to a constant magnetic field, exciting spin induction signals of the object particles by applying a pulsed radio frequency field at the particle spin resonance frequency and a non-uniform alternating magnetic field parallel to the constant magnetic field, as well as synchronous detection of the spin induction signal, the magnitude of which determines the values of the magnetic resonance parameters associated with the structure of the object, which distinguishes so that the ability of an external variable magnet is synchronized with the radio frequency and initial phase shifts between the alternating non-uniform field and the frequency of the tracks : RF field of sensitivity enhancement and resolution 2A-35. capacity, an inhomogeneous alternating magnetic field ni) O is induced synchronously with an impulse RF field at an initial phase shift between the alternating non-uniform magnetic field frequency and the pulse frequency range sov radio frequency floor 2A-35. hell /./7Х - - ... .1and. ||.,, 1. L 9U.Z.2 .five tc ) j f N (y 7fV // Offf / 3OC / if} U JOffff / J / fJU / f //) VT) J. f-) v (f fffff / 7off / J 9 Jff // fffJJffju //// i