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Description  |
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The present invention is due to Mr. Guy Aubert, Director of the Service
National des Champs Intenses, and its object is a cylindrical permanent
magnet to produce a transversal and uniform magnetic field. It finds
application particularly in the medical field where magnets are used in
nuclear magnetic resonance imaging experiments. It can also find
application in all fields where such distributions of a magnetic induction
fields are required.
In the field of imaging by magnetic resonance, it is necessary to place the
objects to be imaged, the patients, in a high magnetic induction field
(usually of 0.1 to 1.5 Tesla) which is homogeneous and uniform (with a few
parts per million of variation) in a large volume of interest (commonly a
sphere of 50 cm diameter). Several classes of magnetic field generators
have been developed until now. The main ones are: superconductive magnets,
so-called resistive magnets and permanent magnets. Permanent magnets have
many advantages. In particular, they require no energy supply to produce
the field. They therefore do not run the risk of drift in their field
value due to the drift of their supplies, or possibly of the system for
discharging the dissipated heat. They therefore call for no cooling
systems in particular with sophisticated regulation techniques for the
flow of cryogenic fluids. Their working temperature is easily stabilized.
They are furthermore particularly suited to the making of structures or
systems producing a transversal main field, namely a field perpendicular
to the direction in which objects, patients, are introduced into the
magnet. This arrangement is highly favorable to the making of antennas
receiving highly uniform and high gain resonance signals. A major
difficulty in the use of permanent magnets is located, however, at the
level of their industrial-scale manufacture.
Permanent magnet structures producing a transversal, uniform magnetic field
in a relatively big volume have been described in the state of the art. In
particular, in an international patent application No. WO 84/01226 filed
on Sept. 23, 1983 and published on Mar. 29, 1984, D. Lee et al. have
described a magnet of this type. In it, the cylindrical structure
(theoretically of infinite length) is approximated by a stacking of a
certain number of annular sections each provided with a certain number of
magnetized blocks. The blocks are distributed on the rim of the rings in a
polygonal architecture which reproduces, as far as possible, the circular
appearance of a theoretical cylinder. To produce a field transversal to
the axis of the cylinder, the magnetization in each of the blocks is
constant as regards modulus and is oriented, with respect to the direction
of the induction field to be produced, with an angle equal to or twice
that which measures the positioning angle of the block in question with
respect to the axis of this cylinder. The blocks described are, in a
preferred way, prismatic volumes with trapezoidal sections.
The result of the distribution of magnetization thus recommended is that
the magnetization of certain blocks has to be oriented, with respect to
this block, in a direction which is parallel with none of the sides of the
trapezoidal section. The making of magnetic blocks of this type therefore
necessitates the use, industrially, of special magnetizers. While this
use, albeit more expensive than the use of standard magnetizers, is still
possible, the same is not the case for the forming of the blocks. In
effect, the distribution of the magnetization imposed in the cylinder
creates a demagnetizing excitation, the orientation of which is rarely
parallel, in each block, to that of the magnetization. This implies, for
the fabrication, the choice of so-called anisotropic magnetic materials.
Now, anisotropic magnetic materials which, as it happens, have the best
magnetic properties, have the drawback of being hard to machine in
directions that are oblique with respect to the direction of their
anisotropy. The above-mentioned patent application indicates, especially
in its FIG. 5 and in the associated text, that the making of the blocks
can be obtained by a stacking of elementary bricks. However, it is clear
that elementary bricks of parallelepiped shape have a favored direction of
magnetization which is parallel to one side of the parallelepiped. Hence
the fact remains that it is difficult, on the one hand, to cut the bricks
obliquely with respect to the sides of this parallelepiped or, on the
other hand, to efficiently magnetize the blocks formed in directions that
are oblique with respect to the sides of these parallelepipeds.
Consequently, in the structure presented, certain blocks, those in the
alignment of the bisectors of the four quadrants, cannot easily be
magnetized. The distribution of the magnetization in this magnet further
leads to a corresponding distribution of the demagnetizing excitation.
This is such therein that, at places, it may be sufficient to
substantially diminish the magnetization. Consequently, the theoretically
calculated magnet cannot be made and the performances of the real magnet
are quite removed from the ideal.
It is known by the U.S. Pat. No. 4,614,930 a structure with a radial
aimentation. But this structure does not have a null dipole moment.
A magnet of this type further has other drawbacks. In particular, there is
no equipment entrance possible into the interior of the zone of interest
apart from the axial entrance. Finally, as shall be seen from the reading
of the present description, for a homogeneity of the induction field to be
attained, the magnet disclosed in the above-mentioned patent application
has no optimum industrial-scale embodiment: the number of sections and the
distribution of the number of blocks in the sections must comply with
certain rules. And the quality of the homogeneity is lowered when they are
not followed.
The invention has, as its object, the overcoming of the drawbacks referred
to. It concerns a cylindrical permanent magnet to produce a transversal
and uniform induction field, of the type comprising magnetized blocks with
a permanent magnetization, distributed along the cylinder in annular
sections arranged along the cylinder and distributed on the rim of each
section, characterized in that the magnetization in the blocks is oriented
radially and/or tangentially with respect to the cylinder.
The invention also has, as its object, a cylindrical permanent magnet to
produce a uniform and transversal induction field, of the type comprising
blocks magnetized with a permanent magnetization, distributed in a number
n.sub.a of annular sections arranged along the cylinder and distributed on
the rim of each section in n.sub.b blocks characterised in that n.sub.b is
greater than or equal to 2(n.sub.a +1).
The invention also has, as its object, a cylindrical permanent magnet to
produce a transversal and uniform induction field; of the type comprising
magnetized blocks with a permanent magnetization, distributed in a number
n.sub.a of annular sections arranged along the cylinder and distributed on
the rim of each section in a number n.sub.b of blocks, said induction
field being homogeneous to an order h if n.sub.a is greater than or equal
to (h+1)/2 and if n.sub.b is greater than or equal to (h+3). The order h
is defined further below in the description.
The invention will be better understood from the reading of the following
description and the examination of the figures that accompany it. These
figures are given purely by way of indication and in no way restrict the
scope of the invention. In the different figures, the same references
designate the same elements. The dimensions and proportions of the parts
shown have not been maintained. The figures show:
FIGS. 1a and 1b: schematic representation of magnet structures in
accordance with the invention;
FIG. 2: another possible embodiment of a magnet according to the invention;
FIGS. 3 to 6: steps in the assembly of blocks made of magnetic material to
obtain a magnet in accordance with that of FIG. 2.
FIG. 1a schematically represents a magnet structure according to the
invention. The shape of the magnet 1 shown is cylindrical, with an axis z,
and is designed to produce a uniform induction field B.sub.0 oriented
along an axis x orthogonal to z. The structure 1 has a certain number of
annular sections numbered 2 to 6. It is provided, on the rim of the
annular sections, with a number of magnetized blocks, such as those
numbered 7 to 18. According to an essential characteristic of the
invention, all the magnetized blocks have the particular feature wherein
their magnetization M, represented by small arrows, is radial to the
cylinder with an axis z. The result thereof is that the fabrication of the
magnetized blocks thereby gets facilitated. In effect, these blocks
arranged tangentially to the volume of interest of the magnet always have
faces oriented tangentially or radially to the cylinder. Furthermore,
whereas in the state of the art cited, to obtain the transversal uniform
field, the magnetization had to have an orientation which was variable
(with respect to each block) and a constant value, it was discovered that
a transverse field of this type could be obtained with magnetizations
according to the invention if, in each block, the modulus of the
magnetization varied as the cosine of the angle .PHI. of reference of the
block with respect to the plane x O z. This angle .PHI. is the angle of
the radial median plane of the block with the plane x O z. Since the
blocks. are contiguous, equal and with a same number n.sub.b per section,
the positioning angle is equal to k2.pi./n.sub.b (k whole number).
In this first configuration, where the direction of magnetization is always
radial, the directions of magnetization of the different blocks have a
radial aspect which is divergent on one side (to the right) and convergent
on the other side, to the left. In the block 10, as well as in the blocks
19 to 22 (it is the same thing in each section), the magnetization should
be nominal. In the blocks such as 9 and 11, it is reduced; it is even more
so in the blocks 8 and 12. On the blocks 7 and 13, the magnetization
should be null, since the positioning angle is equal to 90.degree.. In
this case, it suffices not to position blocks 7 and 13. This has the
advantage, on the one hand, of reducing the cost of the structure and, on
the other hand, of making a equipment entrance from the top and/or the
bottom of a magnet of this type. In the preferred embodiment shown in FIG.
1, there are 12 blocks per section, of which two blocks are fictitious.
These blocks have magnetizations which are respectively nominal (in the
blocks 10 and 16), proportionate to .sqroot.3/2 times the nominal
magnetization (in the blocks 9, 11, 15, 17), to half of the nominal
magnetization (in the blocks 8, 12, 14, 18), and to 0 (in the blocks 7 and
13).
In a structure which is a counterpart of the preceding one (shown with the
same dimensions by FIG. 1b) the magnetization is tangential to the
cylinder. Its value is proportionate to the sine of the angle .PHI. of
reference of the blocks. The field lines of magnetization follow the
reverse trigonometric direction in the blocks 15, 14, 13, 12 and 11, and
follow the trigonometric direction in the blocks 17, 18, 7, 8 and 9. The
magnetization is nominal in modulus in the blocks 7 and 13, it is null in
the blocks 16 and 10. In this configuration, the equipment entrance into
the magnet, in addition to the main longitudinal entrance, can be achieved
by the blocks 16 and 10 which are not present.
It is possible to accomplish one of the two solutions at choice. However,
all of magnets, including the permanent magnets, should preferably have a
null dipole moment. If this is not so, it becomes difficult to approach a
control desk provided with a display panel and cathode ray tube screen in
the immediate environment of the magnet. It is well known that the
indications displayed on this screen are distorted because of the external
magnetic field of the magnet. This moreover, prohibits the use of color
display panels. To avoid this problem, the control desk should be far away
from the magnet and, hence, an operator directing an NMR experiment is
obligatorily far from the patient who undergoes an examination such as
this. This is rather harmful to the psychological comfort of this patient.
Moreover, practitioners who possess cardiac assistance machines sensitive
to the spurious induction fields cannot perform NMR experiments. In order
to create a weak, spurious field external to the magnet (it is said that
the magnet has a null total dipole moment), the two corresponding
structures can be combined together.
To this end, it is possible to make two concentric structures, one with
radial magnetization, the other with tangential magnetization. In a
preferred way, a variegated structure, which is described further below,
is made. It has been discovered that a variegated solution requires each
solution to take part in the variegation with magnetizations in each of
the rings respectively proportionate to a.sub.1 cos .PHI. and to a.sub.2
sin .PHI. times a nominal magnetization common to both solutions. In this
case, a.sub.1 and a.sub.2 should be such that (a.sub.1).sup.2
+(a.sub.2).sup.2 is smaller than or equal to 1. Furthermore, the null
dipole moment is obtained when a.sub.1 is equal to a.sub.2. The resolution
of these two equations indicates that, in taking proportions a.sub.1 and
a.sub.2 equal to 1/.sqroot.2, we obtain the maximum intensity of the
induction field compatible with a null dipole moment. If, for each of the
three solutions (sine, cosine, variegated), we choose the same materials,
capable therefore of having one and the same nominal magnetization, the
solutions leading to the smallest mass of material to achieve a given
field, in a given volume, are, in a substantially equivalent way, the
cosine solution and the variegated solution. This variegated solution
further has the advantage of leading to a null dipole moment, but also
causes the benefit of equipment entrance perpendicular to the axis to be
lost. The sine solution is appreciably more unwieldy.
The choice of the orientation of the radial or tangential magnetization
resolves, as recalled previously, the problem of the magnetization of the
blocks. In effect, the blocks are always magnetized according to their
structure and orientations (and not with any orientations with respect to
these structural orientations). FIG. 2 shows how it is further possible in
another way to resolve the problem of the oblique cutting of the magnetic
blocks of FIG. 1. In FIG. 1, the magnetic blocks have a trapezoidal
section. The distance from the small bases and the large bases of the
trapezoids to the axis z of the cylinder contribute to determining, with
the value of the nominal magnetization, the value of the induction
B.sub.0. In FIG. 2, the trapezoidal sections have been replaced by
rectangular sections, one side of which is equal to the small base of the
trapezoid. The loss of contribution to magnetization, provided by the
lateral triangular sections of the trapezoids, is compensated for by a
increase in correspondence of the thickness of the blocks measured
radially to the cylinder. The blocks 100 and 190 to 220 respectively
replace the blocks 10 and 19 to 22 of FIGS. 1 a and 1B. However, if
non-parallelepiped blocks are used, it is appropriate to perform the
oblique cutting before magnetization.
FIGS. 3 to 6 show the different steps of a general method of construction
of a magnet according to the invention. One of the first problems to be
resolved consists in having to magnetize the blocks with a magnetization
for which the modulus can be set. FIG. 3 shows that this result is
achieved simply by attaching plates (bar could also be placed) made of
magnetic materials 23 to plates 24 made of non-magnetic material. The
proportion of the magnetic material with respect to the general volume is
adjusted by bringing into play the thickness e.sub.1 of the magnetic
plates with respect to the thickness e.sub.2 of the non-magnetic plates.
It can be easily shown that the macroscopically equivalent magnetization
of a block thus made is equal to the product of the intrinsic
magnetization of the parts made of magnetic materials by the proportion
(e.sub.1 /(e.sub.1 +e.sub.2)) of these magnetic materials in the whole.
Failing the availability of plates 23 of adequate dimension, it is also
possible to use slabs 25 to 27, of permanently magnetizable materials,
that are joined end to end with one another with a very fine contact, and
in such a way that their magnetic anisotropy A is aligned and continuous,
on the one hand, with one another and, on the other hand, with the
magnetization direction subsequently imposed on the blocks. The brick thus
formed is of standard dimension. It is calculated to be capable of being
magnetized in a standard magnetizer such as the magnetizer 28 of FIG. 4.
The current I which flows through this magnetizer has to be strong enough,
and produce sufficiently great excitation, for all the parts, made of
magnetizable materials, of the brick to be brought to their saturation
magnetization. If this saturation magnetization is called M.sub.s, the
brick will be magnetized with a value macroscopically equal to the product
of M.sub.s by the proportion of magnetic material.
The magnetized bricks are then assembled to form the magnetized blocks
described above now. In the approach using variegated magnetization, the
blocks comprise, juxtaposed with one another, bricks magnetized
orthogonally to one another. For example, a block 29 in FIG. 5 comprises
radial magnetization bricks 30 to 32 that are juxtaposed, or even stacked,
on bricks 33 to 35 with tangential magnetization. The assembling of the
bricks in the blocks can be done in the same way as the assembling of the
plates or of the slabs in the bricks: i.e. with epoxy resin based bonders.
The blocks formed are then placed in a crown 36 made of epoxy resin. If
they are rectangular, they are separated from one another by wedge-shaped
shims such as 37. In case of need, another crown (not shown) opposite the
crown 36 may hold the blocks in a sandwich to ensure the rigidity of the
annular section. During assembly, the crowns are housed (FIG. 6) in
cradles such as 38 on which they may lie by all their lower parts. In a
preferred way, the cradles 38 and the crowns 36 are provided with means to
ensure a slight rotation or a slight shift, in one direction or in the
other, in the crowns. For example, a threaded rod 39, lying on the cradle
38, may be screwed into a pommel 40, solidly joined to the crown, and may
cause the rotation of this crown when the rod is turned. In this way, a
simple means is available to make on-site corrections, at an industrial
level, in the homogeneity of the magnet built.
It is possible to give an analytic expression of the value of the field
B.sub.0 as a function of the coordinates of the location of the space
where this field prevails. In particular, if the field B.sub.0 is oriented
in parallel to the axis x, this value can be expressed in a series of
polynomial terms in x, y, z, of increasing power. The coefficients which
weight each of these terms may be assigned to an order equal to the power
of the concerned polynomial in x, y and z. It is known that the
homogeneity of the field B.sub.0 obtained is all the higher as the
coefficients may be considered to be null up to an order which is the
highest possible. It is said that a field is homogeneous to an order h, if
all the polynomial terms of power smaller than or equal to h have a null
coefficient. In the invention, it was realized that, for a given order h,
there was a minimum number of sections and a minimum number of blocks for
which this homogeneity was achieved. It was further realized that this
optimum is also applicable to a magnetization structure like that
indicated in the state of the art cited. It was thus discovered that the
number of sections n.sub.a had to be such that:
n.sub.a greater than or equal to (h+1)/2
As for the number of blocks per section n.sub.b, it should be such that:
n.sub.b greater than or equal to (h+3).
This leads to the following conclusions. Firstly, the optimization
advocated by the invention proposes a minimum structure, namely one where
the number of blocks to be made ultimately is the smallest possible to
obtain homogeneity to a given order. This minimum structure is such that:
n.sub.b =2(n.sub.a +1)
That is, a number of blocks equal to twice the number of sections plus one.
It will be observed, to this effect, that the magnetization represented up
to now is optimum from the viewpoint of homogeneity for it comprises five
sections (n.sub.a =5) and twelve blocks per section (n.sub.b =12=2(5+1)).
It is homogeneous to the order 9, namely that the polynomials of the
lowest degree taking part in the development of its field, are of degree
10. Thus the number of blocks to be manipulated is small, which
facilitates the fabrication and setting of the magnet. Secondly, the
installation and setting of the crowns will be as delicate as the
fabrication of the blocks and the crowns is relatively easy. Hence, and in
a preferred way, it is seen to it that the homogeneity sought is met with
a minimum number of crowns but, on the other hand, it is possible to
position a number of blocks greater than that necessary. Ultimately, in
the case of a variegated structure, the most used industrial solution is
such that the number of blocks per section is greater than or equal to the
number of sections plus one. On the other hand, the solution that leads to
the biggest lateral equipment openings in the case of a non-variegated
structure, is that where the blocks are the biggest and where their number
is therefore minimal.
In a preferred way, the magnetic materials used are either ferrites, of
strontium or barium, or alloys of samarium-cobalt or alloys of
iron-neodymium-boron. These different materials have intrinsic remanent
magnetizations with different saturations. Besides, they have different
volume mass and price. It is possible to choose in this way, depending on
the different specifications of the induction field to be produced, that
solution which is the most adapted one. The principle of computation of
the internal and external radii of the annular sections, as well as of the
heights of these sections, is the same in the case of the invention as
that leading to the determination of blocks in the state of the art
referred to. It is noted, however, that for constant internal and external
radii, irrespectively of the section, the thickness of these sections
along the axis should be increased with distance from the center of the
zone of interest of the magnet. This appears particularly in FIGS. 1 and
2.
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