SU1732246A1 - Device for magnetic field construction - Google Patents

Abstract

The invention relates to a device for creating a magnetic field, used, for example, in medical equipment using nuclear magnetic resonance. The aim of the invention is the expansion of the magnetic field with a given gradient linearity value. The device includes plates 1, 2, coils 3, 4, 5, 6. A feature of the invention is the arrangement of the turns of the coils along the contour of a part of a circular ring, the center of which is the geometric center of the plate, and the choice of an angular aperture of 120 °. 1 tab., 1 Il.

Description

The invention relates to magnetic field devices (gradient systems) used, for example, in medical equipment using nuclear magnetic resonance.

In NMR tomographs, where electromagnets or permanent magnets are used to create the main magnetic field, the gradient systems are located on the flat tips of the main magnet to provide access to the working volume.

A gradient system is known, containing four identical saddle-shaped coils arranged symmetrically about the geometric center on a cylindrical surface. Each coil contains two arc sections at distances of 0.38R and 2.57R from the center of the system, where R is the radius of the cylindrical surface. Such a gradient system in the described tomographs cannot be used, since the coils of the gradient system overlap the working area.

A flat gradient system is known, consisting of four identical coils arranged in pairs in two mutually parallel planes. The turns of each coil of such a system are located along the contours of several rectangles, the three sides of which coincide, and the fourth sides are arranged evenly within the contour of the coils. The gradient of the magnetic coil created by this system has insufficient linearity, since it does not use the ability to compensate for the nonlinearity of the gradient.

The closest in technical essence to the present invention is a flat gradient system consisting of four identical coils arranged in pairs in two mutually parallel planes. The dimensions and arrangement on the planes of the turns of the coils made along the contour of the rectangle are chosen so that the third order decomposition term in the Taylor series of the magnetic field induction component of the gradient system directed perpendicular to the coil planes is carried out.

The decomposition of this function for the coil system, taking into account the compensation of the third order term, can be written as follows: | s

, () + G5 () 5

where / and

i is the current in the coils;

Gi, Gs are the Taylor p and t coefficients of the first and n order, respectively;

0

five

0

five

0

five

0

five

0

five

X is the coordinate of the observation point;

2.0 is half the distance between the coil planes.

From this expression it follows that the non-linearity of the gradient is determined by a member of the nth order of the Taylor series. Transform get

(Ј) ".

The nonlinearity of the gradient system is estimated by the nonlinearity coefficient Kn On / Off, which for the system in question is equal to 0.625.

A disadvantage of the known gradient system is the low linearity of the induction gradient created by its magnetic field, which leads to a deterioration of the technical characteristics of NMR tomography.

The purpose of the invention is the expansion of the magnetic field of space with a given magnitude of the linear gradient.

This goal is achieved by the fact that in a device for creating a magnetic field, including four coils, arranged in pairs symmetrically in two planes, which are mutually parallel and symmetrical, the turns of coils arranged in one plane are wound oppositely, the direction of their winding in each of the planes. opposite, the coils are located along the contour of a part of a circular ring whose center is the geometric center of the plane, the angular aperture is 120 °, and the inner and outer arcs are respectively 0,1946 Zn 1.2843Z, where Z is the distance tion between planes.

The drawing schematically shows a device for creating a magnetic field.

The device contains two mutually parallel plates 1 and 2 of electrically insulating material. The coils of coils 3-6 are arranged on them; they are located along the contour of a part of a circular ring, the center of which is the geometric center of the plate, and the angular opening is 120 °. In the geometric center of the coil system, there is a center of the Cartesian coordinate system O. The Z axis of the coordinate system is perpendicular to the planes of the plates 1 and 2, and the OXZ plane is the plane of symmetry of each of the coils. The coils of coils located in the same plane are wound opposite, and the direction of their winding in each of the planes is opposite. The described gradient system creates a magnetic field with a linear gradient along the X axis.

The following method can be used to determine the size of the coils. We expand in the Taylor series the function of induction of a magnetic field created by an arc of a circular coil with current I and radius R

(|) + G3 (f) 3 +

+ G5 (f) 5 + ...

where is c

I is the current in the coils;

P Q

 Dz

Gn

- coefficients p yes (n

N x 1,3,5), X - coordinate along the X axis of the observation point.

The expression for GI, converted with respect to the angular size of the arc 120 °, has the form

в, - Ј2-е

A7tR (1 + Ј2) 5/2 where e Z / 2R;

Z is the distance between the planes. Differentiator, we get

3 / d 4Ј4-27Ј2 + 4

327TR

9/2

(1 + Ј2) VG 270-4545Ј2 + 3225Ј4-366E6

384 JTR

(1 + Ј2)

11/2

Equating Cz to zero, we obtain the values of the radii of the arcs of the circular coil by the drain RI 1.2843Z, and Rz 0.1946Z, at which the GS member is compensated.

For the specified values of the radii of the arcs, the nonlinearity coefficient is Kn Gs / Gi 0.471, i.e. less than the prototype.

The table shows a comparative assessment of areas with a given nonlinearity of the gradient of the induction vector magnetic field of the prototype and the proposed device.

Claims (1)

Comparison of these gradient systems shows that the proposed device has an extended linear gradient area of induction of the magnetic field in comparison with the prototype. This improves the performance of NMR instrumentation. The invention The device for creating a magnetic field, comprising four coils, arranged in pairs symmetrically in two planes, which are mutually parallel and symmetrical, with the turns of coils arranged in one plane wound oppositely, and the direction of their winding in each of the planes is opposite to, Apparently, in order to expand the magnetic field with a predetermined magnitude of the linear gradient, the turns of the coils are located along the contour of a part of a circular ring whose center is the geo The metric center of the plane whose angular opening is 120 °, and whose inner and outer arcs are respectively 0.1946Z and 1.2843Z, where Z is the distance between the planes.