WO1984000611A1

WO1984000611A1 - Adjustable magnet suitable for in vivo nmr imaging and method of adjusting the same

Info

Publication number: WO1984000611A1
Application number: PCT/US1983/001175
Authority: WO
Inventors: William H Oldendorf
Original assignee: William H Oldendorf
Priority date: 1982-08-04
Filing date: 1983-08-02
Publication date: 1984-02-16
Also published as: EP0114889A1

Abstract

An NMR imaging System especially adapted for in vivo human NMR imaging utilizing magnets which are adjustable to achieve a desired field configuration within a preselected zone located within the gap between the poles of the magnet. Preferably the source of magnetic lines of flux is a permanent magnet with movable magnetic material (40, 42) positioned on and/or carried by each of the poles to vary distribution of the flux lines to obtain a field strength which is substantially uniform throughout the entire volume which would be occupied by the target area of a human being. To obtain the uniformity of field strength necessary, the shape of the pole faces of the permanent magnet is changed responsive to measurements of field strength at discrete points within the gap (48) to obtain the desired flux distribution.

Description

ADJUSTABLE MAGNET SUITABLE FOR IN VIVO NMR IMAGING AND METHOD OF ADJUSTING THE SAME

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

This invention relates to NMR (nuclear magnetic resonance) imaging systems and more particularly to a novel system employing adjustable poles for generating the desired field configuration for establishing the main magnetic field used to polarize the sample.

PRIOR ART

Practical NMR imaging systems for in vivo examination of the human body are currently known. These systems operate by applying suitable combinations of magnetic fields to the body being examined through the utilization of coil systems and then detecting induced currents in one or more detector coil systems.

Satisfactory operation of the sequence of magnetic fields to the body being examined depends upon providing fields which precisely conform to desired conditions.

O.'PI The basic concept underlying all techniques to achieve spatial resolution in the body being examined is that the resonant frequency of the nuclei in the sample being examined is proportional to magnetic field strength. This is explicitly stated by the Lar or relation where f= γ x H where γ is the magnetogyric ratio of the nucleus and is 42.58 MHz/Tesla for hydrogen and H is the magnetic field strength in Tesla. Therefore by making the magnetic field in an object vary with position, one makes the resonant frequency of the nuclei vary with position. By knowing the position of dependence of the magnetic field, the position of resonating nuclei is defined by their resonant frequency.

To accurately define the known variance in the magnetic field one must start with the main magnetic field having a uniform density of lines of flux on the order of 10~3 or better. Thereafter a separate magnetic field is applied to establish a known gradient for purposes of accomplishing the desired imaging.

In the prior art, to establish the main field in systems of sufficient size for in vivo human NMR imaging, complex electromagnets of the resistive or superconducting types have been utilized. Where resistive electromagnets are utilized substantial expense is involved in power consumption, temperature control and the like to operate the imaging system. On the other hand, excessive power requirements are eleminated through the utilization of the superconducting electromagnets.

However, such superconducting electromagnets are extremely expensive to construct and they require regular filling with liquid nitrogen and/or helium. In either event, such prior art electromagnetic systems for NMR imaging are extremely expensive and thus are beyond the reach of the average clinician. The best known prior art is found in NMR Imaging, Proceedings of an International Symposium on Nuclear Magnetic Resonance Imaging held at the Bowman Gray School of Medicine of Wake Forrest University, Winston-Salem, North Carolina, October 1 through 3, 1981; the book entitled "Nuclear Magnetic Resonance Imaging in Medicine" published by Igaku-Shoin Medical Publishers, Inc., 50 Rockefeller Plaza, New York, New York, 1981; and United States Letters Patent 2,722,636.

SUMMARY OF THE INVENTION

A system including magnet means for establishing a magnetic field between the poles thereof and movable means of magnetic material disposed on at least one pole for changing the magnetic flux lines in the gap between the poles responsive to movement of the magnetic means to obtain a predetermined configuration of the magnetic field within a preselected zone within the field.

The invention also includes the method of establishing the field of predetermined configuration by measuring the field between the poles and adjusting the movable means of magnetic material to change the field responsive thereto until the field of desired predetermined configuration is attained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a schematic diagram of a system employing an adjustable magnet means; FIGURE 2 is a perspective view of one form of adjustable magnet constructed in accordance with the principles of the present invention;

FIGURE 3 is a perspective view of a part of the magnetic means illustrated in FIGURE 2 showing one pole thereof in greater detail;

FIGURE 4 is a cross sectional view of the pole shown in FIGURE 3 taken about the lines 4-4; and

FIGURE 5 is a schematic representation further illustrating the principles of the present invention.

DETAILED DESCRIPTION

The present invention provides a magnet means having poles which may^' be effectively tuned through manipulation of movable members of magnetic material to achieve a magnetic field of desired configuration. The invention has a multitude of uses in various types of applications. For example, one use may be to provide an appropriate field of using in cathode ray tube focusing. Another use is to appropriately shape the magnetic field in a particle accelerator. A further use is in NMR imaging. Even though the invention may be used in various applications, for purposes of ease and clarity of description the present specification will be limited to NMR imaging. All methods of NMR imaging fall into one of four categories, point scanning, line scanning, planar imaging and three dimensional imaging. Each of these methods and various systems of accomplishing information output utilizing various excitation signals have been published and are well understood in the prior art.

Each of the systems requires a main excitation field of

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OM?I uniform density throughout the area occupied by the specimen being imaged. FIGURE 1 is a block diagram generally illustrating a system for NMR imaging in accordance with the present invention. As is therein shown a computer 10 is utilized to control the entire system and to process the signal information which is thereafter displayed upon a display unit 12 which may be of any type presently known in the art. A receiver means 14 is utilized for positioning the specimen to be imaged within the desired magnetic field. Although the receiver means may be of any type desired and the present invention may be utilized in any system wherein precision or custom shaping of a magnetic field is required, the present description will be given in conjunction with a system used for NMR in vivo imaging of the human body. The receiver means 14 therefore is of sufficient size to receive all or portions of the human body within a uniform magnetic field. The magnet which forms a part of the receiver means is adjustable to provide the desired configuration of the magnetic field within the gap within which the human body is positioned. Preferably with a system used in accordance with the in vivo NMR imaging, the main magnetic field has a uniform field strength of about one part in one thousand or better.

As is well known, the main field must have a gradient applied thereto so that it linearly varies across the field within which the specimen is positioned. Additional gradients are applied in order to obtain the required signal information after excitation of the nuclei as will be more fully referred to hereinafter. To accomplish the required gradients, a gradient control circuit 16 is coupled to the computer 10. Gradient power supply(s) 18 are controlled by the gradient control circuits to apply energizing currents to the gradient coil (s) 20. ^^jRE

OMPI The nuclei within the specimen are caused to resonate by applying a radio frequency field thereto through the use of transmitter (Tx) coils 22 which are connected to a power amplifier 24 which in turn receives signals from a rc_dio frequency (R.F.) transmitter 26. As the resonating nuclei return to a stable state, radio frequency signals emanate from them and are detected by receiver (Rx) coil (s) 28. The signals from the detector coils are applied to an amplifier 30 and then to a receiver 32.

The functioning of the receiver and the transmitter is controlled by a transmitter/receiver (Tx/Rx) control circuit 34 which in turn is controlled by the computer 10.

Those skilled in the art will thus understand that through the utilization of the computer the nuclei in the specimen positioned within the receiver means 14 are caused to resonate and the signals generated by the relaxation of the nuclei constitute the signals which will be used to provide the desired output display whether the system be utilized for point scanning, line scanning, planar imaging or three dimensional imaging. Although the principles of the present invention can be practiced utilizing any type of structure (either electromagnet or permanent magnet) for creating the magnetic field, so long as the structure is adjustable, it is preferable to use a permanent magnet to create the desired main field. FIGURE 2 illustrates one generalized form which may be utilized for such a structure. As is therein shown there is provided a pair of magnets 40 and 42 having a flux return path provided by the ordinary soft steel bars 44, 44A. Provision may be made for adjusting the distance between the magnets 40 and 42 such for example as by the series of bolts 46 which hold the straps 44 together as well as permit vertical movement between the magnets 40 and 42. The purpose of the movement between the magnets 40 and 42 is to provide a desired air space 48 between the poles of the magnets 40 and 42 of sufficient size to allow the specimen to be examined to readily be placed therein.

The poles for the magnets are more clearly illustrated in the perspective view of FIGURE 3 to which reference is hereby made. As is therein illustrated the permanent magnet 40 has affixed thereto a base plate 48 from which extends a slug matrix 50. The base plate 48 and the slug matrix 50 form a pole for the permanent magnet 40. A ring 52 of magnetic material is positioned around the pole and extends upwardly from the face ^"thereof as is illustrated. As will be more fully described below, the ring 52 is adjustable. The pole 50 has defined in the face 54 thereof a plurality of openings such as those shown at 56. The openings 56 preferably are threaded and receive threaded slugs 58. The ring 52 and the slugs 58 are used to shape or tune the magnetic field existing in the gap between the poles of the magnets 40 and 42 to have a desired configuration within a specific zone within the gap depending upon the particular application.

The structural details of the permanent magnet are more fully illustrated in FIGURE 4 to which reference is hereby made. As is shown in FIGURE 4, the magnet 40 generally includes a permanent magnet 60 which preferably is constructed from a plurality of small permanent magnets, each of which is individually magnetized and are then brought together to form the desired structure.

Typically each of the magnets may be formed from any permanent magnet material. Preferably the magnets are, however, formed of ceramic, Alnico or a rare earth cobalt. Preferably the ceramic magnets are considered to be the best since they are the least expensive, relatively easy to fabricate and do not demagnetize readily. It is presently contemplated that the small ceramic magnets produced by calcining ferrites of barium, strontium or lead will be cast into "bricks" approximately one inch by three inches by six inches, magnetized, and then assembled in a side by side relationship utilizing an epoxy adhesive to hold them together to provide a magnet 60 which is 27 inches by 27 inches by 12 inches deep. Such a magnet would provide sufficient size to generate a magnetic field sufficiently large and uniform to provide in vivo NMR imaging of the human body.

A casing of non-magnetic material 62 may be placed around the exterior of the magnet. This material may be constructed from plastic, a non-magnetic metallic material, wood or other structural matter as may be desired.

Positioned upon the permanent magnet 60 is the base plate 48. The base plate is constructed of magnetic material such as soft steel and functions to pre-homogenize the magnetic lines of force. As above noted, since the magnet 60 is made up of a plurality of small magnetic bricks, the upper surface of the magnet will not be perfect. There will be gaps between the bricks and also there will be gaps at the corners of the bricks since in the casting process the corners are not precisely sharp. The base plate 48 therefore conducts the field through it to effectively smooth out the magnet lines of flux. The base plate 48 should be of a constant thickness and for a magnet 27 x 27 x 12 should be approximately one-half inch in thickness. The thickness must be sufficient to offer physical support without distortion under the forces of the weight of the magnet, particularly when it is suspended at the top of the air gap. In addition, the thickness must be sufficient for the field to distribute itself equally and evenly throughout. ^c"

Extending from the base plate 48 is the pole 50. The pole 50 concentrates the magnetic field into an approximately circular symmetry so as to provide the desired general overall configuration for the magnetic field extending through in the gap 48. The pole 50 for a magnet of the size above indicated should be approximately 2 inches thick so as to maintain rigidity and to properly shape the magnetic field. The pole 50 includes a face

64 which defines the bores or openings 56 which preferably are threaded. As above indicated the movable magnetic members or slugs 58 are disposed and threadably received within the threaded bores 56. Around the periphery of the pole 50 there is received the ring 52. As illustrated in FIGURE 4, the ring is threaded internally thereof as shown at 66 and is threadably received on the outer threaded surface of the pole 50.

By reference to FIGURE 5 the utilization of the ring 52 and the slugs 58 will become more apparent. As is shown in FIGURE 5A, when a pair of poles 70 and 72 are provided to define a magnetic field within an air gap, the lines of force are not evenly distributed. As is well known, at the center of the air gap the greatest concentration will occur. As the edges of the magnet are reached, the lines of force tend to bulge outwardly. Thus if one were to plot the concentration of the lines of magnetic force across the field a curve as shown at 74 would result. Such a field distsribution is useful in NMR imaging only at the very center wherein there is a relatively uniform field. To provide a field of the required uniformity, that is 10~³ or better, utilizing permanent magnets of the type shown in FIGURE 5A for in vivo imaging of the human body, would require permanent magnets several yards in diameter and weighing many tons. Such would be effectively impractical to construct.

The permanent magnet of the present invention provides the uniform field through the utilization of the ring 52 and the slugs 58. As is shown in FIGURE 5B, by utilization of a ring 76, 78 on the upper and lower poles 70 and 72, respectively, the distribution of flux lines within the gap is made more uniform. Such occurs as is illustrated by the flux lines at 80 and 82 which occur between the rims of the rings 76 and 78. As is shown the traditional pattern of bowing out of the flux lines occurs between the edges of the rings 76 and 78. These flux lines effectively are "robbed" from the flux lines normally appearing between the faces of the poles 70 and 72. In addition, by the bowing out of the flux lines at 80 and 82 inwardly toward the center of the pole 70 and 72, causes the flux lines appearing between the pole faces of the poles 70 and 72 to straighten somewhat and become more uniform. This occurs simply because the flux lines bowing toward the center of the poles from the edges of the rings 76 and 78 will repel the flux lines tending to bow outwardly between the poles 70 and 72. Such magnetic flux line redistribution through the use of the rings 76 and 78 will give a uniformity of approximately 10~2 to 10""3. if the slug matrix or pole 50 is properly machined and made perfectly flat along the pole face thereof, it is conceivable that a usable uniform field could be obtained with the ring only. It has, however, been determined that the likelihood of having an appropriate finish on the surface of the poles 70 and 72 to provide such is not great. Furthermore, the -poles may be tilted somewhat from the ideal and therefore an additional movable magnetic means to provide adjustment is required. Therefore to finally tune the permanent magnets to obtain the required uniformity of field between the poles 70 and 72, slugs such as shown at 84 and 86 disposed within the surfaces of the poles 70 and 72, respectively, are utilized. By raising and/or lowering the slugs 84 and 86 and at the same time by measuring the field strength in the gap between the magnets 70 and 72 a substantially uniform field can be obtained between the poles 70 and 72 of the magnet. The procedure followed in obtaining the desired uniform field is to measure the flux appearing at various points throughout the desired specimen imaging portion between the poles 70 and 72. The variation in uniformity can thus be determined. After such a measurement an appropriate non-magnetic tool may be utilized to raise or lower the slugs 84 and 86 to thereby change the field. As the slugs are thus moved, a measurement is taken to determine variations in uniformity of the field. As such is done, further fine tuning using the slugs and/or the rings is effected. Such continuous measurement and adjustment continues until the desired field uniformity is obtained. Although the description of the process has been given with respect to obtaining a uniform field strength, those skilled in the art will readily understand that a field strength of any desired configuration within a particular zone between the poles 70 and 72 may be obtained in like manner. The measurements of flux may be made through the utilization of a Hall probe or by the measurement of the resonance of the hydrogen molecule in a body of water or by measuring the differences in the resonant frequencies between the hydrogen molecule in water and the resonance of lithium 7; all of which are well known in the art.

To provide the full range of adjustment needed in accordance with the principles of the present invention, the bores 56 within which the slugs are received are of sufficient depth to permit the slugs 58 to be positioned flush with the pole face 64 or to extend outwardly therefrom into the gap thereby to fine tune the magnetic lines of force appearing therein.

It will be understood by those skilled in the art that although the preferred method for adjustment of the slugs 58 is to provide a cylindrical slug which is threaded externally thereof and is recessed to receive a non-magnetic tool for adjustment purposes, such need not be the case. The slugs 58 may take any geometric shape desired as may the bores or recesses 56 and the slugs may be positioned in any manner desired and may be held in place once positioned in any manner desired, such as by friction, an adhesive, or the like.

The ring 52 in order to accomplish the desired adjustment in accordance with the principles of the present invention is preferably, as shown, cylindrical with the internal surface threaded. Such, however, is not required and so long as the ring is adjustable so as to extend from the surface 64 of the pole 50 upwardly into the gap between the poles to a distance of approximately

10% of the gap width, the appropriate concentration of the flux lines at the edge of the field as shown in

FIGURES 5B and 5C, thus robbing from the center of the field as above referred to, can be accomplished thereby generating the first order of field uniformity.

By referring to FIGURE 6 an alternative arrangement utilizing a pole" constructed in accordance with the present invention having the adjustable magnetic elements therein is shown. As is therein illustrated the pole 90 may be of a polygonal construction and be surrounded with a peripheral member 92 which is in contact with the outer edges of the pole 90. Appropriate adjustment members (set screws or the like) 94 extend from the lower surface of the ring shaped member 92 to provide elevation of the member 92 to the desired position from perfectly even with the face of the pole 90 to extend upwardly as above referred to approximately 10% of the gap width. As is also illustrated the pole 90 would define the openings within which are received the movable magnetic members or slugs for the fine tuning.

As a further alternative to obtaining the edge coarse tuning of the magnetic field the ring may be totally eliminated and in its place substituted a peripheral ring of the slugs as clearly shown in FIGURE 3 at 96. The peripheral ring of slugs 96 may provide even more refined tuning around the outer edge to compensate for misalignment of the poles, one with respect to the other.

In any event, the outer peripheral movable magnetic member for adjustment of the field should be adjustable through the predetermined range of from 0 to 10% of the air gap in any manner which is desirable to provide the desired tuning.

The adjustable magnet if subjected to temper- ature changes will cause flux field changes. For example, as the temperature of the iron in the magnet increases.

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OMPI the iron in the magnet as well as the return path becomes less efficient. It is therefore important that the _c magnet be temperature compensated. Such compensation can be accomplished by placing the magnet in a shroud 98 (FIGURE 1 ) of any of the types currently known to the art. The shroud 98 may then be subjected to standard air conditioning to maintain the magnet at substantially

1_Q constant temperature. Alternatively, a coil 98 (Fig. 2) may be placed on the return path 44 for the flux between the magnets. The coil can then be subjected to appropriate electrical current to either buck or enhance the magnetic field as may be required to compensate for variations in

15 temperature. The control circuits applying the current in the desired direction can be those readily known in the art.

There has thus been disclosed a system for providing NMR in vivo imaging of the human body which

20 includes an adjustable magnet, preferably a permanent magnet which can provide a uniform field over the area within which the specimen to be imaged is placed.

25

30

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Claims

WHAT IS CLAIMED IS:

1. A rπclear magnetic resonance apparatus for examining a body including a display, radio frequency transmitter and receiver means for creating and detecting radio frequency resonance signals in said body, means for generating a steady magnetic field of substantially uniform field strength, and means for producing a gradient in said uniform magnetic field within said body; the improvement comprising: magnet means for generating said steady magnetic field between a pair of opposed spaced apart poles defining an air gap therebetween; and movable means of magnetic material disposed on at least one pole for adjusting the magnetic lines in said gap responsive to movement of said movable means to obtain substantial uniformity of field strength within at least that portion of said gap occupied by said body.

2. The nuclear magnetic resonance apparatus as defined in claim 1 wherein said magnet means is a permanent magnet.

3. The nuclear magnetic resonance apparatus as defined in claims 1 or 2 wherein said movable means of magnetic material is disposed on each of said poles.

O PI -, 4. The nuclear magnetic resonance apparatus

₂ as defined in claims 1 or 2 wherein said poles each define a plurality of bores extending inwardly from the

4 face thereof and said movable means includes a plurality of slugs received within said bores for movement therein g to achieve said uniform magnetic field.

1 5. The nuclear magnetic resonance apparatus

2 as defined in claim 4 wherein said bores and said slugs

3 are threaded and said adjustment is achieved by threadably moving said slugs within said bores.

1 6. The nuclear magnetic resonance apparatus

2 as defined in claims 1 or 2 wherein said movable means

3 includes peripherally disposed means on said pole face.

1 7. The nuclear magnetic resonance apparatus

2 as defined in claim 8 wherein said poles are substantially

3 cylindrical and said movable means includes a ring

4 disposed about the periphery of said pole.

1 8. The nuclear magnetic resonance apparatus

2 as defined in claim 7 wherein said ring is threaded on

3 the internal surface thereof and said pole is threaded

4 on the external surface thereof thereby to threadably

5 receive said ring.

9. The nuclear magnetic resonance apparatus as defined in claim 8 wherein said movable means further includes a plurality of slugs received in bores defined by said pole and extending inwardly from the face thereof.

10. The nuclear magnetic resonance apparatus as defined in claim 9 wherein said bores and said slugs are threaded.

11. The nuclear magnetic resonance apparatus as defined in claim 6 wherein said poles are polygonal and said movable means includes peripherally disposed means surrounding said poles.

12. The nuclear magnetic resonance apparatus as defined in claim 4 wherein said bores are of sufficient depth to permit retraction of said slugs within said bores by an amount so that said slugs do not extend outwardly beyond the surface of the face of said poles.

13. The nuclear magnetic resonance apparatus as defined in claim 12 wherein said slugs are disposed in such a manner as to be substantially evenly distributed over the surface of the face of each of said poles. 1

14. The nuclear magnetic resonance apparatus

2 as defined in claim 6 wherein peripheral means includes

3 a continuous ring-like member in intimate contact with

4 the outer surface of said pole and further includes means

5 for adjusting the position of said ring-like member with

6 respect to said pole face.

1 15. A nuclear magnetic resonance apparatus

2 for examining a body including a display, radio frequency transmitter and reciver means for creating and detecting ^ radio frequency resonance signals in said body, means

5 for generating a steady magnetic field of substantially

*> uniform field strength, and means for producing a gradient

' in said uniform magnetic field within said body; the- improvement comprising:

^ a permanent magnet defining an air gap between

10 opposed spaced apart poles; ϋ a plate of magnetic material in intimate l ^A ~ contact with and extending substantially over the opposed

1³ surface of each of said poles; a generally cylindrical shaped member of

15 magnetic material extending outwardly from each of said 16 magnetic plates and having a substantially flat surface 17 defining a face for each of said poles; and 18 a movable member of magnetic material on each 9 of said poles, said member being adjustable to protrude 0 into said air gap.

16. A nuclear magnetic resonance apparatus as defined in claim 15 wherein said movable member includes a plurality of slugs received within a plurality of openings defined in the face of each pole member.

17. A nuclear magnetic resonance apparatus as defined in claim 16 wherein said movable member further includes a ring-shaped member of magnetic material substantially surrounding and in contact with the outer periphery of said poles and movable with respect thereto.

18. A nuclear magnetic resonance apparatus as defined in claim 17 wherein said movable members are each threaded to provide independent adjustment of each of said members with respect to said face.

19. Apparatus for obtaining a magnetic field of predetermined configuration within a preselected zone comprising: (a) magnet means having positionally fixed opposed spaced apart poles to define an air gap within which said zone resides; (b) movable means of magnetic material disposed on at least one pole for changing the magnetic lines in said gap responsive to movement of said means in said gap to obtain said desired configuration; and means for holding said movable means in the position resulting in said desired configuration.

20. Apparatus as defined in claim 19 wherein said magnet means is a permanent magnet means.

21. Apparatus as defined in claims 19 or 20 wherein said movable means includes a plurality of slugs received in bores defined by the face of said pole.

22. Apparatus as defined in claims 19 or 20 wherein said movable means is disposed on each of said poles.

23. Apparatus as defined in claims 19 or 20 wherein said movable means includes peripherally disposed means on said pole.

24. Apparatus as defined in claims 19 or 20 wherein said movable means includes a plurality of slugs received in bores substantially evenly distributed over the surface of the face of said pole.

25. Apparatus as defined in claims 19 or 20 wherein said .poles are cylindrical and said movable means includes a ring disposed about the periphery of said pole.

26. Apparatus as defined in claims 19 or 20 wherein said movable means includes a plurality of slugs received in bores substantially evenly distributed over _^ the surface of the face of said pole and peripherally disposed means on said pole. ■_j_

27. Apparatus as defined in claims 19 or 20

- wherein said poles are cylindrical and said movable means

₃ includes a plurality of slugs received in bores substantially

. evenly distributed over the surface of the face of said

₅ pole, a ring disposed about the periphery of said pole.

1 28. Apparatus as defined in claims 19 or 20

2 wherein said poles are polygonal and said movable means

3 includes a plurality of slugs received in bores substantially

4 evenly distributed over the surface of the face of said

5 poles, peripherally disposed means surrounding said

6 pole.

1 29. Apparatus as defined in claim 21 wherein

2 said slugs are cylindrical and threaded and are received

3 within threaded bores defined by the face of said poles.

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30. Apparatus as defined in claim 29 wherein

2 said bores are of sufficient depth to permit retraction of said slugs within said bores by an amount so that

₄ said slugs do not extend outwardly beyond the surface c of the face of said poles.

1 31. Apparatus as defined in claim 30 wherein said slugs are disposed in each pole face and are substantially evenly distributed over the surface of each said pole face.

32. Apparatus as defined in claim 23 wherein said peripheral means includes a plurality of slugs received in bores defined by the face of said poles and disposed about the outer edge thereof.

33. Apparatus as defined in claim 23 wherein said peripheral means includes a continuous ring-like member in intimate contact with the outer surface of said pole and means for adjusting the position of said ring with respect to said pole face.

34. Apparatus as defined in claim 33 wherein said ring is cylindrical and said adjusting means includes threads on the inner surface of said ring and the outer surface of said poles.

35. Apparatus as defined in claim 33 wherein said adjusting means includes a plurality of threaded members carried by said ring-like member.

36. Apparatus for obtaining a magnetic field of predetermined configuration within a preselected zone comprising: (a) permanent magnet means defining an air gap between opposed spaced apart poles; (b) a plate of magnetic material in intimate contact with the opposed surface of each of said poles;

-£U_--_ CI^*.-1PI (c) a generally cylindrical shaped member of magnetic material extending outwardly from each of said magnetic plates and having a flat surface defining a face for each of said poles; (d) said face defining a plurality of openings therein; and (e) a movable member of magnetic material received within each of said openings.

37. Apparatus as defined in claim 36 further including a ring shaped member of magnetic material surrounding and in contact with the outer periphery of said cylidnrical shaped member and movable with respect thereto.

38. Apparatus as defined in claim 37 wherein said movable members are each threaded and said openings are threaded to provide independent adjustment of each of said members with respect to said face.

39. Apparatus as defined in claim 36 wherein said ring is threaded internally thereof and said σylindrically shaped member is threaded externally thereof to provide adjustment of said ring with respect to said face.

40. Apparatus as defined in claim 38 wherein said ring is threaded internally thereof and said cylindrically shaped member is threaded externally thereof to provide adjustment of said ring with respect to said face.

41. The method of establishing a steady state magnetic field of desired configuration between a pair of pole faces comprising the steps of: establishing a magnetic field between said poles; measuring said field at various positions between said gap; adjusting movable magnetic member means disposed on at least one of said poles to vary its position with regard to said gap; and continuing said adjusting and measuring until said magnetic field is of the desired configuration.

42. The method as defined in claim 41 wherein said adjustment step includes the step of adjusting a member of magnetic material disposed about the periphery of said poles.

43. The method as defined in claim 41 wherein said adjusting step includes positioning a plurality of slugs disposed within said pole.

44. The method as defined in claim 42 wherein the said adjustment step includes the second step of adjusting a plurality of slugs of magnetic material disposed in the face of said pole.

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