Ferromagnetic compensation rings for high field strength magnets

A reduced length, high field strength magnet, comprised of a number of superconducting magnet coils has ferromagnetic compensation rings positioned coaxially around the magnet bore axis to reduce axial inhomogeneities. The rings are positioned symmetrically around the bore axis centerpoint. Both the position of the magnet coils and the position of the ferromagnetic compensation rings are determined by an iterative optimization process checked by finite element modeling.

BACKGROUND OF THE INVENTION 
The present invention relates to the construction of high field D.C. 
magnets and, more particularly, to a means for reducing the axial length 
of such magnets while preserving a high degree of field homogeneity. 
DISCUSSION OF THE PRIOR ART 
High field strength magnets are used in a variety of applications including 
particle accelerators, NMR spectroscopy equipment and magnetic resonance 
imaging equipment. In magnetic resonance imaging equipment, high 
uniformity (homogeneity) of the magnetic field is required because small 
deviations in magnetic field strength may distort or produce artifacts in 
the resulting image. Typically such magnets are superconducting: 
constructed of solenoid coils wound of superconducting wire and immersed 
in a bath of liquid helium held within a cryogenic container or cryostat. 
The highly uniform magnetic fields of such magnets are realized through the 
use of multiple pairs of solenoid coils whose number and volume may 
require an extended magnet bore length. As a general rule, highly uniform 
magnet fields require many coil pairs over long bore lengths. 
Magnets with long bore lengths have certain drawbacks. The weight and cost 
of the magnet rises as the length increases because of the need for 
additional support structure. In the case of self-shielded magnets, a 
longer bore length increases the size and cost of the required shielding. 
Patient comfort, during magnetic resonance imaging in which the patient is 
positioned within the bore of the magnet, may be less in longer bore 
length magnets which create a sense of being "closed in". Finally, space 
restrictions at many institutions preclude the use of longer bore length 
magnets. 
SUMMARY OF THE INVENTION 
In the present invention, a reduced bore length magnet comprises one or 
more magnet coil pairs positioned coaxially about a bore axis and 
symmetrically opposed about a bore axis center point. One or more 
ferromagnetic compensation ring pairs are positioned coaxially about the 
magnet bore axis with each ring also symmetrically opposed about the bore 
axis center point. The dimensions of the coils: radius, axial position and 
ampere-turns, and the dimensions of the compensation rings: radius and 
axial position, are selected to reduce the magnetic field inhomogeneities. 
Initial values are chosen for the number of coils and ferromagnetic rings 
and the limits of the magnet's dimensions. A target flux density is chosen 
for certain points within a volume of interest within the magnet bore and 
the dimension of the coils and rings, within the predetermined limits, are 
determined by means of an iterative process based on this target flux 
density. The optimization process assumes the rings have a saturation flux 
and initially the contribution from any shield is ignored. A finite 
element model is then constructed to determine the field contributions, at 
the target flux points, resulting from the shield and the actual flux of 
the compensation rings. If the homogeneity of the field, as determined by 
the finite element model, is not at the level desired, the field 
contribution from the shield and rings is held constant and the iteration 
is repeated adjusting only the coil dimensions until the desired 
homogeneity is reached. 
It is an object of the present invention, therefore, to produce a magnet 
with a shorter bore length, and yet with a field homogeneity comparable to 
longer bore length magnets. The ferromagnetic rings alter the local 
magnetic field structure to reduce inhomogeneities resulting from the 
shorter bore length or fewer magnet coil pairs. 
It is a further object of the invention to provide a method of positioning 
the magnet coils and ferromagnetic rings so as to reduce magnetic field 
inhomogeneities for a given number of magnet coils in a magnet of a given 
length. 
Other objects and advantages besides those discussed above shall be 
apparent to those experienced in the art from the description of a 
preferred embodiment of the invention which follows. In the description, 
reference is made to the accompanying drawings, which form a part hereof, 
and which illustrate one example of the invention. Such example, however, 
is not exhaustive of the various alternative forms of the invention, and 
therefore reference is made to the claims which follow the description for 
determining the scope of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring FIG. 1 an MR magnet assembly 10 is contained in a vacuum vessel 
cylinder 12, and end plates 16 and bore cylinder 14 centered around a bore 
axis 36. The vacuum cylinder 12 and the end plates 16 may be formed of a 
ferromagnetic material such as mild rolled steel plate for shielded 
magnets or non-magnetic material for unshielded magnets while the bore 
cylinder 14 is formed of a non-magnetic stainless steel. These elements 
are welded together to form a vacuum tight annular chamber 15. For 
shielded magnets, the end plates 16 fit tightly against the vacuum vessel 
cylinder 12 to conduct magnetic flux therebetween and to reduce the 
magnetic field outside of the magnet bore cylinder 14 as is generally 
known in the art. The bore cylinder 14 enables the magnetic field to 
penetrate the magnet bore where a patient is situated during NMR scanning. 
It is the main objective of the magnet assembly 10 to provide a 
high-strength and homogeneous magnetic field within this bore of the 
magnet. 
Referring to FIG. 2, the vacuum tight annular chamber 15 encloses a first 
heat shield comprised of cylindrical, concentric outer and inner walls 26 
and 24, respectively, disposed about the bore axis 36. The spaces between 
the ends of the outer wall 26 and inner wall 24 are closed by end plates 
28. 
A second heat shield, comprised of cylindrical, concentric outer and inner 
wall 32 and 30, disposed about the bore axis 36, is enclosed within the 
first heat shield. The spaces between the ends of the outer wall 32 and 
inner wall 30 are closed by end plates 34. 
Within the space enclosed by the outer and inner wall 32 and 30 of the 
second heat shield is a liquid helium vessel comprised also of cylindrical 
concentric outer and inner walls 20 and 18 which are also co-axial with 
the bore axis 36. The spaces between the ends of the outer wall 20 and 
inner wall 18 are closed by end plates 22. 
The first and second heat shield and helium vessel are constructed out of a 
non-magnetic material such as aluminum. 
Within the space contained by the helium vessel is a cylindrical coil form 
38, which is also co-axial with the bore axis 36. It holds a series of 
magnet coils 40 spaced apart along the bore axis 36. The coils 40 are 
positioned coaxially with the bore axis 36 and are symmetrically paired 
about a center point 44 along the bore axis 36. The construction of one 
such a coil form, suitable for use in the present invention is described 
in in co-pending application serial number 07/278,124 which was filed on 
Nov. 30, 1988 and entitled:"Support Structure for High Field Magnet Coils" 
and is assigned to the same assignee as the present invention and is 
hereby incorporated by reference. 
During operation of the magnet 10, the liquid helium vessel is filled with 
liquid helium 46 so as to cool the magnet coils 40 into a superconducting 
state. The magnetic fields generated by the coils 40 cause the coils 40 to 
repel or attract each other and, therefore, the coils 40 must be 
restrained against axial movement by a series of retention surfaces 39 
which ring the circumference of the coil form 38. These retention surfaces 
39 also serve to accurately position the magnet coils 40 with respect to 
each other. 
The construction of a superconducting magnet suitable for use with the 
present invention is further described in detail in the following U.S. 
patents assigned to the same assignee as the present invention and hereby 
incorporated by reference: 4,771,256, "Integral Shield for MR Magnet"; 
4,724,412, "Method of Determining Coil Arrangement of an Actively Shielded 
Magnetic Resonance Magnet"; 4,800,354, "Superconducting Magnetic Resonance 
Magnet and Method of Making Same"; 4,721,934, "Axial Strap Suspension 
System for a Magnetic Resonance Magnet. 
A pair of ferromagnetic compensation rings 52 are welded to the outer 
surface of the bore cylinder 14 coaxially with the bore axis 36, and 
symmetrically disposed about the bore axis center point 44. The 
compensation rings 52 may be fabricated of low carbon steel or ingot iron 
or other material with a high saturation flux density. 
For the 0.5 Tesla magnet of the present invention having three coils pairs 
A, B, and C, of ampere turns 216834.8, 90650.0, and 51489.2 respectively, 
the following dimensions have been used successfully to reduce the 
inhomogeneities within a spherical volume of 40 cm dia centered at the 
bore centerpoint 44 to less than 18 ppm.: 
______________________________________ 
Dimensions Inches 
______________________________________ 
Average radius of coil A 
20.182 
Axial distance from center of A 
22.710 
to bore centerpoint 
Average radius of coil B 
20.328 
Axial distance from center of B 
8.946 
to bore centerpoint 
Average radius of coil C 
20.289 
Axial distance from center of C 
2.744 
to bore centerpoint 
Inside radius of compensation ring 
16.920 
Radial thickness of compensation coil 
0.127 
Axial length of compensation coil 
2.0475 
______________________________________ 
The magnet assembly 10 has a vacuum vessel cylinder 12 of iron which serves 
as a magnetic shield; the cylinder 12 has an outer radius of 32.967" and 
thickness of 1.865". The end plates 16 have a bore radius of 16.732" and 
thickness of 1.865". 
The exact dimensions and composition of the compensation rings will vary 
depending on the particular configuration of the magnet including the 
ampere turns of the magnet coils, their radius and axial spacing. 
The desired length of the magnet and the number of coil pairs and the 
number of compensation rings are predetermined based on the magnet's 
application and the physical and cost constraints on the system. Generally 
there is a tradeoff between magnet length and number of coils, homogeneity 
and cost. As mentioned, the availability of space at the institution using 
the magnet may limit magnet length. 
Determining the position the coils and ferromagnetic compensation rings in 
the magnet is complicated by the nonlinear influence of the ferromagnetic 
compensation rings and magnetic shield on the magnetic field. This 
prevents the simple superposition of the effects of each coil and ring in 
determining the magnetic field within the magnet bore. Instead, a two-part 
iterative procedure is used as shown in FIG. 4. 
Referring to FIG. 4, and process block 60, the number of coils 40 and 
compensation rings 52 is selected as are the bounds for the coil and ring 
variables, specifically, the axial and radial dimensions and ampere-turns 
of the coils and the axial and radial dimensions of the rings. The bounds 
are limits outside of which the variables may not move. For example, the 
maximum magnet length is specified which limits the maximum axial position 
of the coils and rings. Similar bounds are imposed for minimum and maximum 
coil and ring radius and for maximum and minimum coil ampere-turns. These 
bounds are predetermined based on the tradeoffs discussed above. The 
dimensions of the magnetic shield, if any, are also chosen. 
Also at process block 60, a target flux density is established representing 
values of flux density at a series of field points within the volume of 
interest 54. The initial target flux values are selected to provide the 
desired level of magnetic field homogeneity. The number of field points in 
the target flux will be equal to the number of degrees of freedom in the 
coil structure to be discussed below. 
At process block 62 the dimensions of the coils and rings are determined 
using an iterative Newton-Raphson procedure to produce the target flux 
specified at the particular field points. This procedure is performed on a 
high speed computer such as the VAX manufactured by the Digital Equipment 
Corporation. The variables that may be changed and hence the degrees of 
freedom of the iteration are: the axial and radial position and 
ampere-turns of the coils, for each coil, and the axial and radial 
position of the rings for each ring. The flux density of the rings is 
assumed to be at their saturation level and the contribution of each ring 
and coil to the field at the field points is computed using the 
Biot-Savart law. 
The variables selected in process block 62 are entered into a finite 
element magnetic modeling program which may also be run on a VAX or 
similar computer. The finite element program calculates the actual effect 
of the magnetic shield and the rings on the magnetic field at the field 
points in the volume of interest, as indicated by process block 63. 
The homogeneity of the axial magnetic field within the volume of interest 
is checked at decision block 64 and if the desired homogeneity has been 
reached, the program is exited at process block 66 and the coil is 
fabricated with the variables determined in process block 62. 
If the desired homogeneity has not been reached, the contribution of the 
magnetic shield and compensation rings to the magnetic field at the field 
points is held constant in the Newton-Raphson iteration, per process block 
68 and the coil positions are recalculated by the iterative NewtonRaphson 
procedure per process block 70, but with the ring variables fixed and only 
the coil variables changed. The loop formed by the process and decision 
blocks 63-70 is repeated until the desire homogeneity has been obtained. 
Finite element programs suitable for calculating the homogeneity of 
magnetic fields generated by current carrying coils in the presence of a 
ferromagnetic shield and ferromagnetic rings, are commercially available 
from a number of suppliers of computer aided engineering products, 
including: The MacNeal-Schwendler Corporation of Milwaukee Wis., and the 
Magnus Software Corporation of Woodlands Tex., who both produce a finite 
element magnetic modeling program suitable for this iterative process. 
It will be apparent to one skilled in the art that the symmetry of a 
cylindrical magnet and the symmetry of the coil and ring placement about 
the bore centerpoint 44 will yield a savings in calculations. Only a two 
dimensional plane incorporating the bore axis need be considered, and only 
one fourth of that plane, as defined by the bore axis and a lateral axis 
intersecting the bore axis centerpoint 44 at right angles to the bore axis 
36. 
The above description has been that of a preferred embodiment of the 
present invention. It will occur to those who practice the art that many 
modifications may be made without departing from the spirit and scope of 
the invention. For example, Monte Carlo, steepest descent, or linear 
programming techniques may be used to determine the optimal coil and ring 
positions, with the appropriate constraints in magnet dimensions and 
ampere turns. Additional rings may be used with appropriate adjustments in 
ring and coil positions and dimensions. In order to apprise the public of 
the various embodiments that may fall within the scope of the invention, 
the following claims are made.