GB2184243A

GB2184243A - Electromagnet arrangements

Info

Publication number: GB2184243A
Application number: GB08628580A
Authority: GB
Inventors: John Vincent Mario Mcginley
Original assignee: Picker International Ltd
Current assignee: Philips Design Ltd
Priority date: 1985-12-09
Filing date: 1986-11-28
Publication date: 1987-06-17
Also published as: GB2184243B; GB8628580D0

Abstract

A coil arrangement for producing a magnetic field of high homogeneity, such as is required in magnetic resonance imaging, comprises a single pair of identical annular coils (17A, 17B) disposed coaxially in spaced relationship and a pair of annular members (19A, 19B) of ferromagnetic material disposed coaxially with the coils and symmetrically with respect to the plane which perpendicularly intersects the axis of the coils centrally 19 between them. The coils and ferromagnetic members have dimensions and are relatively positioned so that, with the coils carrying equal energising currents, the more significant spherical harmonic coefficients are eliminated. <IMAGE>

Description

SPECIFICATION Electromagnet arrangements This invention relates to electro mag net arra ngements.

More particularly the invention relates to electromagnet arrangements for producing a field of high homogeneity in a volume such as is required, for example, in a nuclear magnetic resonance imaging apparatus, to establish an equilibrium axis of magnetic alignment in a body being imaged.

In orderto be usable for magnetic resonance imaging an electromagnet arrangement must produce afield in which the axial harmonic coefficients describing the field are substantially zero up to at leastthesixth order.

Two known electromagnet arrangements capable of providing such a field will now be described with reference to Figures 1 and 2 of the accompanying drawings which are respectively schematic diagrams of the two arrangements; Referring to Figure 1the firstarrangement comprises a solenoid 1 with an annular correction coil 3 or5 at each end, the coils 1,3 and 5 being disposed coaxiallywith respectto an axis 7 parallel to the direction ofthe required magnetic field.

Referring to Figure 2, the second arrangement comprises two or more identical pairs ofannularcoils disposed coaxially with respectto an axis 9 parallel to the direction of the required magneticfield so thatthe coils lie on the surface of a sphere indicated by dotted line 11 in Figure 2, two pairs of coils 13 and 15 being shown in Figure 2.

Both these arrangements suffer from disadvantages in respect of transverse access to the homogeneous magnetic field, i.e. in a direction perpendicularto the axis7 or9 ofthefield. In the case ofthe arrangementof Figure 1 such access is completely closed offwhilst in the case of the arrangement of Figure 2 the gap between the central coils, coils 1 in Figure 2, is too small to provide satisfactory access. In the case of magnetic resonance imaging of a patient e.g. forthe purposes of medical diagnosis, this rules outthe possibility of imaging with the patient's longitudinal axis transverse to the axis of the magneticfield produced by the coil arrangement.Furthermore, in such an application, the closed nature of the coil arrangements of Figures 1 and 2 can induce claustrophobicfeelings in some patients.

It is an object ofthe present invention to provide an electromagnet arrangement wherein the above described disadvantages are alleviated.

According to the present invention an electromagnet arrangement comprises a single pair of substantially identical annular coils disposed coaxially in spaced relationship, and a pair of annular members of ferromagnetic material disposed coaxially with the coils symmetrically with respectto a plane which perpendicularly intersects the axis ofthe coils at a position centrally between the coils, the dimensions and relative positions of the coils and rings being such that, with the coils carrying substantially equal currents, at least two of the spherical harmonic field coefficients are substantially eliminated.

Preferably the dimensions and relative positions of the coils and rings are such that the second, fourth and sixth spherical harmonic field coefficients are substantially eliminated.

In one such preferred arrangementthe coils and rings are dimensioned and relatively positioned so asto satisfy the relationship

where R is the mean radius of said rings; Ro is the radius ofthesphere on which the mean centres ofthecross sections of said coils lie; and PE, PD, OE and OD are dimensionless quantities as hereinafter defined.

One electromagnet arrangement in accordance with the invention will now be described, by way of example, with reference to Figures 3 and 4 of the accompanying drawings in which: Figure 3 is a schematic diagram of the arrangement; and Figure 4 illustrates, on a reduced scale, a modification of the arrangement of Figure 3.

The arrangement comprises two identical annularcoils 17Aand 17B each of which has a rectangular cross-section of width 2a, thickness 2b and mean radius Rm. The coils 17 are dsposed coaxiallyabouta common axis Z and are symmetrically placed about a plane of sym metry V which intersects the axis Zata point 0 hereinafter referred to as the origin. The mean centre ofthe cross-section of each ofthe coils 17 subtends an angle tothe Z-axis at the origin.

The arrangementfurther includes two identical annular rings 1 9A and 19B of mean radius R made of ferromagnetic material. The two rings 19 are disposed coaxially with the coils 17 symmetricallyaboutthe plane V. Each ring 19 has a rectangular cross-section ofarea Awhose mean centre subtends an angleatothe Z-axis at the origin.

In operation the coils 17Aand 17B carry equal currents, and are thus suitably energised by being connected in series across a suitable DC supply.

The net magnetic field ata point P having spherical polarcoordinates r, 6 with respect to the origin isthe sum ofthefield duetothe coils 17 alone and the field dueto rings 19 alone. The coils and rings 17 and 19are dimensioned and positioned sothatthe coefficients of the spherical harmonicfields due to the coils 17 balance the coefficients of the spherical harmonicfields due to the rings 19so that the first non-zero spherical harmonic coefficient is the eighth order coefficient.

The required constraints to achieve this may be determined asfollows: The intensityofthe magnetic field in the Z direction Hz produced by the arrangement at point P is given by: Hz = So + S2r2P2(x) + S4r4P4(x) + S6reP6(x) + where P2(x) etc. are different order Legendre Polynomials;So etc. are different order net spherical harmonic coefficients of the arrangement; and x = cos 6 It will be appreciated thatthe objectofthe design isto eliminate S2, S4 and Se Thefield in the Z-direction Hc due to the coils 17 alone is given by:

where H, is the field atthe origin duetothe coils 17 alone; Ro is the radius ofthe sphere on which the mean centres of the cross-sections of the coils 17 lie; and E2 etc. are coefficients which are dimensionless functions ofthe coil dimensions.

H1 is given by 0.4 #J JRo Ho whereJ is the current density in the coils 17, and the coefficients E2 etc. are defined by the relation E2n = C2n/HO where C2 and Ho are defined in formulae 3 to 11 of an article by K.

Kaminishi and S. Nawata appearing inthe March 1981 issue of Review of Scientific Instruments 52 (3) at pages 447 to 453 and entitled "Practical Method of improving the uniformity of mag neticfields generated by single and double Helmholtz coils", which article is hereby incorporated by reference.

Thefield in the Z direction dueto the rings 19 alone is given by:

where Mis the mean magnetisation of the rings in the Z direction; and Do etc. are dimensionless coefficients as defined in an article, which is hereby incorporated by reference, by F. Romeo and D. I. Hoult appearing in Magnetic Resonance Imaging in Medicine 1,1984, pages44to 6 entitled "Magnetic Field Profiling: Analysis and correcting Coil Design" from which article it may be deduced that Dn= (n+1) (n+2) (sin"a)n+3) Pn + 2 (COS) (4) From equations (1), (2) and (3) expressionsforthe net coefficients describing the magnetic field can be obtained, as follows:

wheres MA'H1 The aim isto produce a magnet arrangement in which S2=S4=S6=0.Ifthis is so then from equation (6):

andfromequation (7):

Substituting equation (9) into equation (10):

Further, from equation (8):

Now substituting equation (9) into equation (12):

Therefore, summarising the results of equations (11) and (13), in order to eliminate simultaneously S2, S4,and S6we require:

To further condense and separate the requirements sfthe coils and rings the following relations are defined:

Thus the coil pair can be characterised by two numbers PE and QE and the ring pair can be characterised by P0 and QD Clearly, conditions (14) are now reduced to::

This allowsforthe magnitude ofthe components only. Constraints must also be placed on the signs ofthe D and E coefficients. It can be seen from equations (6), (7), and (8) that since W, Rand Ro are positive quantities the following restrictions must be imposed to eliminate S2, S4 and S6.

E2 and D2 have opposite signs E4 and D4 have opposite signs E6 and D6 have opposite signs (18) It will be appreciated that the required constraints are effectively those which cause the contributions to the unwanted net spherical harmonic coefficients due to the coils to balance the contributions to those coefficients due to the rings.

In orderto satisfy the constraints of condition (18) it isfound in general thato must be in the range 58.5"to 70.5 and ss within the range 53.65to63.4 . Furthermorethe constraints are such thatthe rings 19 will be of smaller radius than the coils 17.

It will be understood that buy a curtailment or extension of the above procedure an electromagnet arrangement in accordance with the invention in which a different selection of two or more of the spherical harmonic field coefficients are eliminated may be provided.

The magnetisation M of rings 19 may arise solely from the field induced by coils 17, or alternatively, may arise partly by virtue ofthe rings 19 being permanently magnetised.

For either of these options the rings 19A and 19B are suitably each sandwiched between two thin annular auxiliary coils 21A and 23A, or21 B and 23B, one on the inside and the other on the outside radial surface ofthe associated ring, as shown in Figure 4. The two coils 21 and 23 associated with each ring 19 carry equal and opposite currents and are thus suitably all connected in series. The coils 21 and 23 are used forfine adjustment ofthe mean mangetisation M in the rings 19to assist in setting up ofthefield ofthearrangement.

It will be understood that an electromagnet arrangement according to the invention may be constructed either as a resistive arrangement, or a superconducting arrangement. Inthe latter case the coils are suitably housed in twin cryostats.

It will be appreciated that an arrangement in accordance with the invention, in addition to providing good transverse access to the centre of the magnet, and reducing claustrophobicieffects in the case of use for NMR imaging of patients, is potentially cheaper than conventional arrangements as illustrated in Figures 1 and 2, and simplerto set up than spherical arrangements as illustrated in Figure 2.

Claims

1. An electromagnetarrangementcomprising a single pair of substantially identical annular coils disposed coaxially in spaced relationship, and a pair of annular members offerromagnetic material disposed coaxially with the coils symmetrically with respect to a plane which perpendicularly intersects the axis of the coils at a position centrally between the coils, the dimensions and relative positions of the coils and rings being such that, with the coils carrying substantially equal currents, at leasttwo ofthe spherical harmonicfield coefficients are substantially eliminated.

2. An arrangement according to Claim 1 wherein the dimensions and relative positions of the coils and rings are such that the second, fourth and sixth spherical harmonicfield coefficients are substantially eliminated.

3. An arrangement according to Claim 2 wherein the coils and rings are dimensioned and relatively positioned so as to satisfy the relationship

where R is the mean radius of said rings; Ro is the radius ofthe sphere on which the mean centres ofthecross sections of said coils lie; and PE, PD, QE and OD are dimensionless quantities as hereinbefore defined.

4. An arrangement according to any one ofthe preceding claims wherein the mean centre ofthecross section of each ofthe coils subtends to the axis ofthe coils at said position where said plane intersects said axis an angle of between 53.6 and 63.4 and the mean centre of the cross-section of each of the ringssubtendsto said axis atsaid position an angle of between 58.5" and 70.5".

5. An arrangement according to any one ofthe preceding claims wherein the rings are of smaller radius than the coils.

6. An arrangement according to any one of the preceding claims wherein the rings have a magnetisation arising partly by virtue of the rings being permanently magnetised.

7. An arrangement according to any one of Claims 1 to 5 wherein the rings have a magnetisation arising substantially solely from the magnetic field produced by the coils.

8. An arrangement according to any one ofthe preceding claims wherein said coils are connected in series.

9. An arrangement according to any one ofthe preceding claims wherein the rings are associated with auxiliary coilswherebythe magnetisation ofthe rings can be adjusted.

10. An arrangement according to Claim 9 wherein each ring is sandwiched between a pair of annular auxiliary coils one on the inside radial surface and one on the outside radial surface of the associated ring.

11. An arrangement according to Claim 10 wherein said auxiliary coils are all connected in series.

12. An arrangement according to any one ofthe preceding claims wherein said coils and rings are of substantially rectangular cross-section.

13. An electromagnet arrangement substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.