SU518712A1 - The method of obtaining kinematic spin echo signals in an external inhomogeneous magnetic field - Google Patents

Description

(54) METHOD OF OBTAINING SIGNALS

KINEMATIC SPIN ECHO IN AN EXTERNAL INHOMOGENEOUS MAGNETIC FIELD 12 of the bone that flows inside the coil of the NMR sensor. This circumstance limits the possibility of applying the method for practical purposes. For example, as measured by phase shift. D F spin echo signals, the flow rate of the fluid inside the NMR sensor coil will always differ from the flow rate of the fluid in the free flow, and this difference is difficult; and take into account the full-height,., exclude (it depends on the viscosity of the liquid, the speed of the state of the pipe surface and its clogging, etc.). In addition, as can be seen from (1), there is a linear dependence of the phase shift of the LF on the gradient (j y and squares on T. .1 Formula (1) connecting the signal parameters (US and t) with the flow rate of fluid V is valid only for a linear gradient l along the direction of the flow (along the axis of the coil). Therefore, these parameters need to be stabilized for measurement accuracy V. This apparently eliminates the use of the method when installing an NMR sensor on a movable platform. To do this, one successively polarizes a non-uniform electromagnetic field, identical in its geometry to the field of the receiving coil, moving in the outer area of the sensor coils and the fluid volume, then in different places along its motion, it is affected by a 180-degree radio frequency pulse through time T determined by t L (ev) where L is the distance between the polarizing and receiving coils, V is the velocity of the fluid relative to the NMR sensor, 2 T after the start of the free precession of the controlled liquid is recorded with Signal of kinematic spin echo (CSE); Invention is illustrated by drawings. Fig. 1 is a diagram of the magnetic fields of the NMR sensor coils, where Hj is the polarizing field created by the coil L1, H is the magnetic field along the spine; the receiving coil L created by all magnetic dipoles of the L of the controlled liquid (the dotted curves show the power lines, the M field lines of the coil L, H are the field created by the coil Lj when a 180-degree radio frequency pulse passes through it: current, V n &, is the distance between the coil axes, v is the velocity of the fluid relative to the NMR sensor along 51 x and oh (the coordinate system l is connected to the sensor, the coordinate origin is in the frontal part of the sensor, and the axis is OH passes through the middle of the vertical sides all coils), HQ is an external magnetic field oriented along the axis o, - Fig. 2 shows the dependence of the amplitude of the NMR signal on the time b measured from the moment of the beginning of the free precession of the controlled fluid, where A is the signal of the free precession der, in, produced in the coil L (shortened due to the movement of Sdcock), B - the SSC signal induced in the coil in the case of a homogeneous external magnetic field, the G signal of the free precession of the nucleus - induced. in the coil L (shortened due to the movement of the fluid and the effect of the magnetic field gradient, D is the SSC signal induced in the coil L in the case of a nonuniform magnetic field, 180 - And - 180-degree RF pulse, - black) 3 SSC signals, the time interval between the beginning of the free precession of the cores and the moment of applying the 180-degree pulse. The method of obtaining SSC signals in an external inhomogeneous magnetic field consists of the following. It is known that in order to obtain SSC signals, NMR sensors are used, coils of the second (two or more) by passing an electric current through them create in the Exterior region a non-uniform periodically symmetric magnetic field. If, for example, a B NMR sensor uses two coils L and Lj identical in their geometrical and electrical parameters, the configuration of the magnetic fields of which is shown in FIG. 1, then; SSC signals are obtained as follows. When a direct current is passed through the polarizing coil L., the opiate-IJ of the nuclear magnetization vectors M of the elements of the sample volume L Vj along the magnetic field lines HjT is created. .. vfijil i4f Ljiji .. tt4, ti.ftfi, l l1: dl. DLG1 l iJ. MifZh L If then non-adiabatically fast switching off the polarizing field H j, then the vectors M will begin to precess freely around the direction of the external homogeneous magnetic field HQ with frequency. will be determined by the orientation of the latter, given by the field H (the field M1 can be turned off slowly, and the rotation of the vectors in the xy plane can be done using a 90-degree current pulse fed to the coil L immediately after the polarization current is turned off). At this moment the oscillating components of precessing vectors tangent-i

SILOSH.1M. line m coil

to magnetic

q (. MJ}, the synphaqs of L will change,

J, n the transference they create in coil L, and the magnetic flux created by them in coil L, will be maximal. ; Moving elements of the sample volume; . , leading to a change in the angle between the tangent to the power line of the coil L at the point DU- and the plane; Changes in the phase of shshill

M; in time (cX, Ct) -th O1.I).

IX at 1

These changes are even in the case of a uniform.

sample movement relative to the NMR sensor will occur at an effective rate for uV fluid volumes. moving at different distances from the x axis, and causing different detuning of the oscillation frequencies of the M components,

The resulting dispersion of duu detunings will increase as the fluid moves along the path | , and then in the region e - to decrease (in the case of uniform movement of the sample C ,. "".). It determines its minimum value (dtt) Cu O) when each AV: travels along a path parallel to the mid-plane of the OX sensor, to distance 2. In this case, in the coil L j, the total magnetic flux from the synAase-reclining components is M j; will reach

maximum value and it will induce a kinematic spin echo signal.

With further movement Vj | along the sensor, again in terms of $ (U and the SEC signal is exhausted (in this case, the appearance of the SEC signal will also be promoted by the transfer of a sample that is not uniformly magnetized by the Mil module, although the influence of this factor is less resonant).

The time of occurrence of the CSE signal after the onset of free precession in a uniform, external magnetic field H at VsConSt. is determined: by the formula

-one

L-V

SEC

xx

between os -

DG1-distance

: Mi LatusK; L.j and L -,.

If the precession is free of Hbix nuclear magnetizations M; will occur in an inhomogeneous external magnetic field H Cxvz-) dHUv4), the conditions for the occurrence of the SSC signal are violated, namely, phasing of the spin precessions due to the movement of the liquid relative to the NMR sensor will not occur and the SEC signal will not occur.

However, if the inhomogeneity of the magnetic field OH over the entire path L of the fluid movement is linear, then its dephasing effect on M can be reversed with the help of a 180-degree radar frequency pulse fed through time T after the start of the free precession of the der. Then, as is known, through time ET, the transverse component M of the precessing vector of the nuclear magnetization M, disappearing due to the action of dH, the reducing agent.

In the case under consideration, the effect on a moving sample by a 180-degree radio frequency pulse should be with the help of a coil L, the magnetic field geometry of which is the same as that of the coils L and L j,

E.SLI coil L is located exactly midway between coils L and L, then in the case of uniform motion (lSt 2 to restore the transverse component, nenta M at the time of occurrence of SEC signals 180-degree radio frequency current pulse into the coil L,

file at time.

li

. t (

Formula (2) expresses the condition | obtaining SSC signals in an inhomogeneous magnetic field with {inhomogeneous inhomogeneity of OH (or with a constant gradient (& Shu) “asop51) at V consfc. It is also valid for some nonlinear inhomogeneities9N in the field of effective action of the sensor coils, i.e. when in this area

GtXvz,) Ticonst.,

It is only necessary that these nonlinear H cause the same detuning dU- when uV moves both on the track i and on Eg, in each stream of flow, At the same time 6, and E may be non-equal,. T: may differ from X. The simplest of these inhomogeneities are, for example, those in which O.a dH .con & t, Ha trajectories of movement are each V

 different trajectories of nonV this field with the simplest are not identical linear heterogeneity of TG. "T and, therefore, i - 2.

Non-uniformity of the magnetic Lol 5H of the above types can be created artificially (for example, using two conductors with a current) or use natural JH at the location of the NMR sensor coils. In the latter case, they can be made, for example, linear with the help of compensators of heterogeneity.

the floor

To obtain a series of SSC signals (like the series of spin echo signals in the Carr-Purcell method), several pairs of coils are used in the same row.

which serve as. to populate the monitored fluid and to receive SSC signals. If it is necessary to do this in a non-uniform external magnetic field, then the coils of the recovering 180-degree radio frequency pulses must be located between the two coils in each pair.

Opportunity received cngs gsi gfi set rtch on NMR. In places with large natural homogeneity of the magnetic field, in which icKrean free precessions and nuclei attenuate quickly, it significantly expands the possibilities of using the NMR method in moving environments. Thus, Sh1fimer, it is possible to measure the flow rate and flow rate of the hydraulic structures of reinforced concrete structures or hydraulic structures close to pipelines, measure the speed of movement of a vessel related to water, or investigate the intrinsic nature of the turbulent movement in a water reservoir by installing a sensor in a space under a head of a horticopter under a headspace and in close proximity to a headspace and a distance of a distance from a distance of a head to the head of the sea. P.

Claims (1)

Invention Formula The method of obtaining signals of kinematic spin-echo echo in an external inhomogeneous magnetic field, which consists in preliminary liquidisation of the electromagnetic field, polarizing the NMR sensor coils, the effect of a 180-hp radar-impulse radio-frequency impulse signal and the registration of the signal of the spontaneous pulse and YM sensor that is, with the aim of expanding possibly Neither the use of the method, successively polarize with a non-detachable aptromagnetic field, the ideas of the t: 11th are different in geometry riemzhL coil dvnzhu1tse4s bo outer region kktushek fluid volume sensor, and then cut in the ground in the course of its movement are 18Ongradusiym radiochayutotnym impupsom through time t defined by a IK) fsfmule t-t-CEv) where i- is the distance between the polarizing u1 P {) k; removable coils; V is the velocity of the motion of the liquid with respect to the NMR sensor, after a time of 2 T nixyie of the beginning of the free precession of the nuclei (the liquid of the liquid is recorded, the signal of the kinematic spin echo is recorded. .one AT 5H4ff FIG. 2