FR2574981A1 - Solenoid magnet with uniform magnetic field - Google Patents

Abstract

Resistive solenoid magnet with uniform field, in particular for NMR imaging. According to the invention, the magnet includes several juxtaposed coils 13, 14, 15, preferably Bitter coils and certain parameters such as the lengths of the coils bi - bi-1, the thickness of the turns of the coils and their outer diameter a2 are chosen to obtain the required uniformity of field and preferably to moreover optimise the Power-Mass product of the magnet.

Description

SOLENOIDAL MAGNET WITH HOMOGENEOUS MAGNETIC FIELD
The invention, due to the collaboration of the National Service of
CNRS Intensive Fields (Director M. HAUBERT), refers to a solenordal magnet to produce a relatively intense magnetic field with great homogeneity in a predetermined region of space; the invention finds its field - of privileged application in an imaging installation - by Magnetic Rénonance
Nuclear (NMR) and also concerns, as an application, such an installation including this wound magnet.

 An NMR imaging facility includes, inter alia - a magnet capable of creating a relatively intense magnetic field (on the order of 0.15 to 1.5 tesla) and uniform in a test volume large enough to accommodate a patient; this magnetic field is called base field; a set of gradient coils arranged generally on the same cylindrical mandrel in order to be able to generate magnetic field gradients along predetermined directions in said examination volume, during successive sequences - radiofrequency transmission-reception means for exciting the spin of the atomic nuclei for which the NMR conditions are found.

 The magnet is traditionally a bulky and expensive set. - Until now, we have mainly focused on two different technologies, on the one hand the resistive coil magnets, without pole pieces, and on the other hand the magnets with superconducting coils.

 Superconducting magnets pose many problems.

In this system, it is essential to reduce the amount of cooling fluid (liquid helium) in thermal contact with the windings to keep them superconducting. To do this, the geometry of the coils is close to a cylinder. It is known that an infinitely long solenoid generates a uniform field in its internal space and that it is possible to homogenize the field inside a solenoid of finite length, for example by modifying the axial density of turns in the vicinity from its ends.

However, the size of the various cryogenic systems (liquid nitrogen and liquid helium) is large and the useful volume inside the coils is considerably reduced. In other words, for a given access, the diameter (and therefore the mass) of the windings is important. In addition, it is imperative to be able to cope with a failure of the cooling system. Indeed, any heating beyond a few Kelvin results in the disappearance of the phenomenon of superconductivity, and the current flowing without voltage drop in the superconducting winding, disappears suddenly as soon as said winding becomes resistive. The magnetic field also disappears very rapidly and the resulting variation in flux can lead to induction phenomena putting the patient's life in danger. To cope with this eventuality, a conductive cylinder (usually made of aluminum) is available between coils and the patient so that the energy released by the flow variation is dissipated as currents of
Foucault in this cylinder. In addition to the fact that this cylindrical envelope further reduces the useful volume, all things being equal, its presence considerably modifies the operation of the gradient coils. The establishment of the gradients is in fact nonnarded "at each sequence by the coupling between the -bobines of gradients and this protective envelope.To overcome this new disadvantage, it is necessary to oversize the feeds gradient systems.

 The most frequently used resistive coil magnets have sets of coils distributed in pairs on either side of a median plane of the examination volume. These coils often have diameters such that they are substantially distributed over a sphere. This design stems from the fact that it is known that a winding of coils properly wound on the surface of a sphere produces a uniform field in this sphere.

As a system embodying exactly this concept would be unusable because access to the internal volume of the sphere would be impossible, systems having such sets of coils of different diameters, spaced along a common axis and being substantially perceptible on a sphere, the homogenicity expressed in parts per million (ppm) is obtained in a sufficient volume of the sphere by adjusting various parameters such as the characteristics of the coils, their diameters and their locations along the axis. Such homogenous field magnets, for a purpose other than NMR imaging, were calculated by Garrett and the results of these calculations were published in an article in the Journal of Applied Physics in MAY 1967. However, these calculations were aimed mainly at the homogenization of the magnetic field, without proposing an industrially easy structure to manufacture with relatively little raw material for a given electric power, or vice versa.

 The invention firstly proposes a new type of solenoldal magnet with high magnetic field homogeneity.

 In this spirit, the invention relates to a solenoidal magnet, particularly for NMR applications, of the type comprising an arrangement of coils arranged along a common longitudinal axis, symmetrically with respect to a median transverse plane of a zone of interest in which the magnetic field prevails, characterized in that all the coils have substantially the same inner and outer diameter, form a continuous stack, and are connected to be traversed by the same current, in that the turns of any two adjacent coils are of different sections and in that these sections, the lengths of the coils and possibly their outer diameter are determined to give said field homogeneity prescribed in said area of interest.

The invention further provides such a solenoldal magnet with high magnetic field homogeneity having the additional quality of a reduction of its PM product, compared to the resistive magnet systems proposed to date, M being the conductor mass used to produce the magnetic field and P being the electrical power consumed. Of all the possible choices of coil sections, coil lengths and outer diameter meeting the requirements of homogeneity of the magnetic field, the invention particularly relates to those which optimize the product.
PM

 Indeed, no coil magnet distributed over a sphere or other convex surface of revolution can be optimum from the point of view of the PM product. The fact of posing the problem of the optimization of this product has led to the choice of a cylindrical structure and to search among all the possible combinations of coil lengths and current densities in these coils, those which achieved both the required homogeneity (typically 1 to 10 ppm in a sphere 50 cm in diameter around the center of symmetry of the coil). magnet) and the optimum PM product.

 In addition, Garrett's article mentioned above is not concerned with practical conditions for making windings. In particular, it is difficult to produce reels without defects as soon as it becomes necessary to superpose several layers of turns. Such defects have repercussions on the homogeneity of the field so that the practical results are often rather far from the theoretical calculations. It is therefore often necessary to compensate for these defects experimentally by adjusting currents in correction windings independent of the windings.

These settings are laborious and require additional power supplies adjustable independently of each other. In addition, the need to cool the magnet to maintain it at a desired operating temperature involves studying accordingly the geometry and the dimensions of the coils and possibly to associate them with cooling means.

 The invention further aims to provide a solenoldal magnet of the type defined above and having the following additional qualities: - efficient cooling avoiding drift; a winding structure making it possible to avoid defects as much as possible in order to eliminate or at least considerably simplify the corrections.

 These problems are solved by the fact that the aforementioned coils are Bitter coils.

Bitter coil means any structure composed of metal annular disks (typically copper or aluminum) defining turns, these turns being connected end to end to materialize a solenoid, while the disks are stacked with interposition of insulation and that holes are made in the discs to define channels, extending substantially parallel to the longitudinal axis of the coil thus formed, in which circulates a cooling fluid. A structure corresponding to the above definition was first described by F. Bitter in an article by Review of Scientific
Instruments of Dec. 1936.

 In addition, the fact of using Bitter coils all having the same inside and outside diameter (which corresponds to a PM product optimization condition) considerably simplifies the technological realization of the magnet given the nature of the means. "heart" cooling of this type of coil.

The invention also relates to any imaging installation by
NMR incorporating the magnet described above and in particular any such installation in which the gradient coil system is arranged outside the coils of the magnet.

 The invention will appear more clearly in the light of the following description of a wound magnet conforming to its principle, the process of calculating the structural characteristics of this magnet and an NMR imaging installation incorporating such a magnet, given only by way of example, and with reference to the accompanying drawings in which: - Figure 1 shows schematically a wound magnet according to the invention; FIG. 2 illustrates the vector decomposition of a magnetic field created by a rotationally symmetrical current system around an axis Oz with a plane of symmetry perpendicular to Oz at O; and FIG. 3 schematically illustrates an NMR imaging installation incorporating the magnet of FIG. 1.

A solenoidal magnet according to the invention, as shown, consists of several coils, which are in the example described Bitter coils according to the above definition, arranged along a common longitudinal axis Oz symmetrically with respect to a transversal plane median PO of an area of interest II where the magnetic field prevails, this area of interest being the one where the patient takes place in the case of the application to an NMR imaging installation. The coils form a continuous stack along the axis
Oz so that the length and position of the ith coil from the center of symmetry O are entirely defined by the distance b i defining the distance from O of its axial end furthest from the center of symmetry 0. Due to the symmetry with respect to the median transverse plane Pg, the number n of coils is necessarily odd, which means that the magnet comprises a central Bitter coil that is n = 2q - 1.

The example represented is that where n = 7, ie q = 4 with three pairs of coils 13a-13b, 14a-14b, 1Sa-15b, symmetrical with respect to the plane Po and a central coil 16. According to an important characteristic of the In the invention, all coils are selected with the same inner radius a1 and the same outer radius a2. This has two essential advantages. It is on the one hand the configuration which makes it possible to optimize the product PM and it is, on the other hand, that which makes it possible to simplify as much as possible the technological realization of the magnet, in particular with regard to acting as Bitter coils) the structure of the cooling fluid circulation circuit through the coils, the cooling channels defined by the holes of the Bitter disks extending from one coil to the other. On the other hand, all the coils are crossed by the same current; they are connected in series. In addition, the turns of any two adjacent coils
(eg 16 and 13a, 14a and 15a) are of sections (i.e.
here: of different thicknesses) but the turns of two coils
symmetrical are of the same section.

We will now show how to determine some
parameters: bi, a2, the section of the turns of the different coils
after fixing some others:
al, n, M or P to obtain a magnetic field of uniformity required (1 to 10 ppm) in a sufficient volume of the area of interest II around
of center of symmetry 0. If we consider FIG. 2, we know that the field created at a point M by a system of currents with symmetry of revolution around the axis Oz with a plane of symmetry Po perpendicular to Oz in 0, can be expressed by its longitudinal components Hz and radial Hp as follows:

est le polynome de Legendre d'ordre 2p

polynome de Legendre associé du même ordre. D'autre part, r est la distance OM et & l'angle entre OM et Oz.Ce développement est valable à l'intérieur d'une sphère de rayons rmax, où rmax est la distance à O du point le plus proche de O où il passe effectivement du courant (autrement dit rmax = al dans l'exemple décrit) et ro est une longueur de référence quelconque, par exemple le rayon de la sphère d'intérêt dans laquelle on cherche à obtenir un champ homogène.

is the Legendre polynomial of order 2p

associated Legendre polynomial of the same order. On the other hand, r is the distance OM and the angle between OM and Oz.This development is valid inside a sphere of rmax rmax, where rmax is the distance to O from the point closest to O where it actually passes current (in other words rmax = al in the example described) and ro is any reference length, for example the radius of the sphere of interest in which one seeks to obtain a homogeneous field.

The H2p coefficients have the size of a field and characterize the homogeneity of the magnet. To homogenize the field to a certain order 2po is thus to cancel all the coefficients H2, H4 ... H2po. We then say that the field is homogeneous up to the order 2po, that is to say:
Ho Hz #
Hf 0
The residual inhomogeneity essentially depends on the values of the terms of order greater than 2p0. Therefore, the more we succeed in canceling H2p terms, the greater the homogeneity of the field is large. We prove that for a magnet with n coils, there is no solution canceling the H2, H4 ... H2po if pO n.

 Therefore for NMR Imaging applications, a six or seven Bitter coil magnet can be calculated to provide a required homogeneity field.

For the calculation of the characteristics of the magnet, H0 and H2p must be expressed as a function of the structural parameters of the magnet, that is to say the bi, al and a2 taking into account the particular expression of the magnet. current density revolving around 0z which for a Bitter coil is of the form
ji a1 # where Ç is the distance from a considered point of a Bitter disk to the Oz axis and ji the maximum current density (on the cylindrical radius α). In these conditions. we have:

le polynôme de Legendre en x d'ordre 2p-1 (x: variable muette).

the Legendre polynomial in x of order 2p-1 (x: dummy variable).

où m est la masse volumique du matériau tandis que la puissance

On the other hand, the mass of driver M is expressed:

where m is the density of the material while the power

<tb> dissipated <SEP> by <SEP> the magnet <SEP> is:
<tb><SEP> P <SEP> = <SEP> a, <SEP> batch <SEP> 4 tAcy
<tb> with bo = o where <0 0 is the resistivity of the conductor used.

All these expressions are given in units of the international system, that is to say, the lengths are expressed in meters, the current density ii in ampere per square meter and Ho in ampere per meter. The magnetic induction field BO is expressed in Tesla with Bg = 0 0 and 0 = 4 7 7
According to the invention, certain parameters are fixed which are dictated by the needs of the user. For example, we choose: al in relation to the desired access
Ho in relation to the desired magnetic induction field n: the number of Bitter coils 2po in the desired homogenous order.

 We also choose the mass M (where the power P).

We can then determine the other parameters representative of the desired magnet (namely bi, ji, a2) by looking for the minimum of a function of these variables, in this case the power P (where the mass M, respectively) with the following constraints: - values of Ho, M, al, n

 The values of the variables giving the minimum of the function P while respecting these constraints determine the dimensional characteristics bi of the desired optimum magnet and the thicknesses of the turns to be provided in the different coils, deduced from the ji, since the coils are connected in series . The problem of minimizing a non-linear function of several variables with constraints is classical and perfectly solved by iterative calculations by means of a computer. Programs called "routines" capable of carrying out such minimization processes are available in the most "libraries" associated with a scientific computing center. One can use for example the "routine" EO4UAF of the library "Numerical Algorithms Group" of British origin.

 The programming of the analytical expressions expressed above, Hg, H2p, M and P, in order to implement the "routine", is within the abilities of those skilled in the art.

 An additional verification calculation consisting of then calculating the nonpull H2p (p) po) with the values of the parameters determined by the calculation previously described makes it possible to know the theoretical homogeneity of the magnet in any sphere of radius less than al.

The optimization calculation process may furthermore be subject to several variants. It is indeed possible to minimize other functions than the power P or the mass M, such as, for example, the first non-zero H2p, that is to say H2 (po + 1) or any other function of the non-zero H2p. We can finally minimize a function representative of the power P and of one or more non-zero H2p (p> PO)
Figure 3 schematically illustrates an NMR imaging installation incorporating a Bitter coil magnet 12 according to the foregoing description.

 The magnet comprises the six Bitter coils 13a-13b, 14a-14b, 15a-15b and the central coil 16 mentioned above and connected in series. The coils are traversed by fluid of a cooling fluid circulation circuit 25 further comprising a pump 26 and the secondary circuit 27b of an exchanger 27 the primary circuit 27a of this exchanger being traversed by another colder fluid. The Bitter coils are traversed by a direct current delivered by a power supply 29.

 The installation also comprises a system of correction coils (known as "shims") known per se and not shown, essentially intended to compensate for the disturbing effects of the environment on the magnet. The installation is completed by a system of gradient coils 30, known per se, by a radiofrequency antenna system 31 housed inside the zone of interest 11 of the magnet, by a radiofrequency generator 32 and by a computer 33. The gradient coils are connected to a set of DC power supplies Gx, Gy and Gz which are controlled by a cycle of sequences programmed in the computer 33 to feed the gradient coils so as to superimpose the field base generated by the magnet magnetic field gradients of different intensities and orientations predetermined during said sequences.On known that these gradients allow, among other things, to select the cutting plane whose image is to be reconstructed. The radiofrequency generator 32, connected to the antenna system 31, is also controlled by the computer 33 to generate calibrated pulses of radiofrequency signal during said sequences. The NMR signals re-transmitted by the subject under examination are picked up by the same antenna system and operated by a computing unit of the computer 33, applying known algorithms to reconstitute an image. This image is for example displayed on a cathode ray tube of a television receiver 35. According to the prior art, the gradient coils, arranged on a cylindrical mandrel, were placed inside the zone of interest 1 1 which forced the builders to size the magnet accordingly (choice of the minimum access al) so that there is sufficient room to accommodate the patient.

The optimization of the PM product results in such a large reduction in the dimensions of the magnet (especially if using Bitter coils) that it becomes possible to place the gradient coil system 30 outside the magnet 12, as shown in Figure 3. The coupling between the gradient coils and Bitter coils of the magnet is further extremely low due to the structure "laminated" Bitter coils, perpendicular to the axis It should be noted that placing the gradient coil system 30 outside the magnet 12 makes it possible to calculate that by choosing a lower parameter al, which results, all things being equal. moreover, to a lower PM product, that is to say to a further reduction of the mass (and therefore of the bulk) of the magnet 12.

 On the other hand, it should be noted that Bitter coils as actually manufactured up to now were mainly intended to create intense magnetic fields, for applications where the homogeneity of the field was less important. In this spirit, the structure proposed by Bitter could not be considered by those skilled in the art as a means of easily obtaining a homogeneous field.

In particular, the best-known method of connecting the turns (using pressure contact of the flat faces of the adjacent discs by cutting the insulation) has been recognized as one of the causes of intrinsic inhomogeneity of the Bitter magnets. constructed so far, this structural feature not being taken into account in the equations. This is why it is desirable to modify this generally used structure, for example by making recesses at the ends of the radial cuts of the rings and welding these ends. It is also possible to advantageously use the connecting rods of the disks constituting a Bitter coil to reduce the current towards an axial end of the coil. All these improvements which are the subject of other patent applications filed by the
Applicants make it possible to improve the intrinsic qualities of the Bitter structure from the point of view of the homogeneity of the field produced.

Claims (6)

REVENDICAnONS  1. Solenoidal magnet, especially for applications in NMR, of the type comprising an arrangement of coils disposed along a common longitudinal axis and symmetrically with respect to a median transverse plane (Po) of an area of interest (11) in which the magnetic field prevails, characterized in that all the coils (13a13b, 14a-14b, 15a-15b, 16) have substantially the same inner and outer diameter, form a continuous stack, and are connected to be traversed by the same current, in that the turns of two adjacent coils Any of these are of different cross sections and in that these sections, the lengths of the coils and possibly their outside diameter are determined to give said field a homogeneity prescribed in said area of interest.  Solenoid magnet according to claim 1, characterized in that said sections, said lengths and said outside diameter are determined to obtain an optimized power-mass product.  3. Magnet according to claim 1 or 2, characterized in that said coils are coil-shaped windings of annular discs, Bitter coil type.  An NMR imaging apparatus including a solenoidal magnet (12) for creating a base field and a gradient coil system (30), said magnet consisting of an arrangement of coils disposed along an axis longitudinal axis (Oz) and symmetrically with respect to a median transverse plane of an area of interest (11) where the magnetic field prevails, characterized in that said coils (13a-13b, 14a-14b, 15A-15b, 16 ) have substantially the same inner and outer diameter and are connected to be traversed by the same - current, in that the turns of any two adjacent coils are of different sections and in that these sections, the lengths of the coils and possibly their diameter are determined to give said field homogeneity prescribed in said area of interest and in that said gradient coil system (30) is arranged outside said coils of said magnet ( 12).  5. The installation as claimed in claim 4, characterized in that said coils of said magnet (12) are annular-coil-shaped coil windings of the Bitter coil type.  6. Installation according to claim 4 or 5, characterized in that said sections, said lengths and said outer diameter are determined to obtain an optimized power-mass product.