Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/385—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using gradient magnetic field coils
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/3806—Open magnet assemblies for improved access to the sample, e.g. C-type or U-type magnets

Definitions

• 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.
• 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.
• 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.
• 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;
• FIGURE 5 is a schematic representation further illustrating the principles of the present invention.
• 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.
• FIGURE 1 is a block diagram generally illustrating a system for NMR imaging in accordance with the present invention.
• 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.
• 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.
• the main magnetic field has a uniform field strength of about one part in one thousand or better.
• 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.
• 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.
• Tx transmitter
• R.F. rc_dio frequency
• Rx receiver
• the signals from the detector coils are applied to an amplifier 30 and then to a receiver 32.
• Tx/Rx transmitter/receiver
• 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.
• 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 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 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.
• each of the magnets may be formed from any permanent magnet material.
• the magnets are, however, formed of ceramic, Alnico or a rare earth cobalt.
• 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.
• the base plate 48 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.
• 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 "
• the pole 50 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
• the ring 52 is threaded internally thereof as shown at 66 and is threadably received on the outer threaded surface of the pole 50.
• FIGURE 5A By reference to FIGURE 5 the utilization of the ring 52 and the slugs 58 will become more apparent.
• 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.
• the permanent magnet of the present invention provides the uniform field through the utilization of the ring 52 and the slugs 58.
• 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.
• 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.
• an appropriate non-magnetic tool may be utilized to raise or lower the slugs 84 and 86 to thereby change the field.
• 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.
• 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.
• 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.
• 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
• 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.
• 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.
• the pole 90 would define the openings within which are received the movable magnetic members or slugs for the fine tuning.
• 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.
• 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.
• 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
• 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
• control circuits applying the current in the desired direction can be those readily known in the art.
• 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.

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• 1983
  • 1983-08-02 WO PCT/US1983/001175 patent/WO1984000611A1/en unknown
  • 1983-08-02 EP EP19830902771 patent/EP0114889A1/de not_active Withdrawn

Non-Patent Citations (1)

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