Nuclear Magnetic Resonance Apparatus Including Permanent Magnet Configuration

1. Field of the Invention

The present invention pertains to nuclear magnetic resonance apparatus for scanning and imaging, and more particularly to nuclear magnetic resonance apparatus which includes a primary magnetic field produced by a permanent magnet configuration, formed of a combination of relatively low energy flux magnetic material and relatively high energy flux magnetic material.

2. Description of the Prior Art

Nuclear magnetic resonance (NMR) apparatus has been employed only relatively recently for scanning and imaging of biological tissue as an aid in the practice of medicine. In 1974, U.S. Pat. No. 3,789,832, issued to the inventor of the present invention, for use of NMR scanning and imaging for medical applications. The primary components of NMR apparatus presently known for imaging of biological tissue, and more particularly whole body imaging of live biological tissue, include a primary magnetic field, an rf field, and a detector for detecting a nuclear magnetic resonance signal. The tissue to be imaged is subjected to the primary magnetic field and simultaneously an rf field of a particular frequency. Reradiated energy is detected by a detector and the signal therefrom is processed as desired to provide imaging.

In order to effectively image living biological tissue, a magnet must be constructed of sufficient size to accommodate the human form therebetween. In addition, the primary magnetic field generated by the magnet must be of sufficient strength and also must provide a substantially flat field over a relatively large area in order to be useful for its intended purpose.

NMR apparatus for such applications employ air core solenoids or resistive air core electromagnets. In order to obtain stronger magnetic fields a shift was later made to air core super conducting solenoids. These superconducting solenoids or electromagnets must be maintained by a vacuum system at approximately 1/1,000,000,000 of a vacuum and must be maintained at a temperature of approximately 4.degree. above absolute zero or -269.degree. C. The necessary vacuum systems are expensive, difficult to manufacture and maintain, and are extremely vulnerable. Cooling is accomplished by a combination of liquid helium and liquid nitrogen. These substances boil off in a vacuum and constantly must be replenished. Costs of $100,000 a year in liquid helium and nitrogen to keep an air core superconducting solenoid in operation are not uncommon. While vacuum systems and cooling systems have been designed which will provide functional NMR apparatus for use in imaging biological tissue, it is apparent that these devices are largely undesirable because of the precise atmosphere these superconducting electromagnets must be kept in and because of the cost of maintaining the proper atmospheric conditions.

Furthermore, magnetic lines of flux generated by air core solenoids cover an immense area and, since interference with these lines of magnetic flux can cause inappropriate NMR responses, elaborate sanctuaries uninvaded by materials or objects which would affect the magnetic field must be constructed.

Because of the relatively large flat working field of magnetic flux needed and because of the relatively high field strengths needed for NMR imaging of biological tissue, it has been heretofore thought impractical to employ a permanent magnet for such an application. Presently available low energy flux material, such as ceramic magnets, although inexpensive, do not produce sufficient field strength per unit volume so that a magnetic field produced by an aggregation of such magnets must be unreasonably large making this material impractical for use. Therefore, the use of low energy flux magnetic material is largely impractical since such magnets would have to be of an enormous size. The suggestion of using a permanent magnet in an NMR apparatus was made in the present inventor's U.S. Pat. No. 4,354,499. The present invention moves such a construction from the possible to the commercially practical.

High energy magnetic flux material is available for use and sufficient concentrations of magnetic field can be produced when these materials are aggregated but the cost of these materials is prohibitive and therefore their use has been ignored. Use of high energy magnetic flux materials such as rare earth magnets therefore is entirely impractical because of the cost involved in production and assembly of such a magnet.

The present invention overcomes the problems associated with the prior art by providing a permanent magnet configuration for use in nuclear magnetic resonance scanning and imaging apparatus for generation of a primary magnetic field which incorporates a combination of low energy flux magnetic material, high energy flux magnetic material, and flux conductors in a configuration which concentrates the magnetic flux over a region of sufficient area, sufficient uniformity and at a sufficient strength to produce a usable magnetic field for imaging of biological tissue and in particular for imaging of living whole body biological tissue. In addition, the area in which the permanent magnet configuration of the present invention can be employed is reduced, relative to air core type magnets, through magnetic shielding accomplished by placement of magnets of particular polarity at selected areas about the permanent magnet to reduce the magnetic radiation therefrom and to concentrate the magnetic field. Such a combination has not been heretofore known for use in generating a primary magnetic field for nuclear magnetic resonance scanning and imaging.

Generally, the use of a permanent magnet in an apparatus employing NMR principles is shown in U.S. Pat. No. 4,240,439 issued Abe et al on Dec. 23, 1980. The apparatus of Abe teaches a probe for observing NMR phenomenon for small surface areas of a living body or areas disposed therein by transcutaneous insertion of the probe. The use of a permanent magnet configuration which generates a field sufficient to permit whole body scanning is not shown or suggested.

Similarly, the use of permanent magnets in nonbiological NMR apparatus is shown in U.S. Pat. Nos. 2,894,199 issued to Kirchner on July 17, 1959 and 4,350,955 issued to Jackson on Sept. 21, 1982. However, generation of a magnetic field of the type produced by the present invention through the use of a combination of high and low magnetic flux magnets, and flux conductors in combination is not shown or suggested.

Aggregation of magnets is known as shown in U.S. Pat. Nos. 3,059,156 to Moriya on Oct. 16, 1962 and 3,889,220 issued to Spodig on June 10. 1975, however neither of these references shows or suggests a manner for combining magnets in a form suitable for producing a magnetic field of sufficient strength and of sufficient flatness for use in imaging of biological tissue, and more particularly, to whole body imaging.

Furthermore, the shielding of permanent magnets to concentrate the magnetic field generated by a permanent magnet in an NMR device is not shown in the prior art. While teaching for shielding of magnets is shown in U.S. Pat. Nos. 2,398,653 issued to Linlor on Apr. 16, 1946; 2,993,152 issued to L. Pierce on July 18, 1961; 3,768,054 issued to Neugebauer on Oct. 23, 1973; 3,168,686 issued to King on Feb. 2, 1965; and 3,205,415 issued to Seki on Sept. 7, 1965, none of these references show or suggest the use of such shielding for NMR apparatus and none show the use of shielding on a magnet which physically even remotely approaches the size and complexity magnet necessary for whole body NMR imaging.

Maintaining a flat field is important in NMR apparatus. A technique for accomplishing this is shown in U.S. Pat. No. 2,930,966 issued to Bell on Mar. 29, 1960.

Therefore, a primary object of the present invention is to provide nuclear magnetic resonance scanning and imaging apparatus which employs a permanent magnet to produce the primary magnetic field thereof.

A further object of the present invention is to provide a permanent magnet for use in conjunction with nuclear magnetic resonance scanning and imaging apparatus wherein a sufficiently strong magnetic field is produced to permit scanning and imaging of selected portions of a human body.

A still further object of the present invention is to provide a permanent magnet for use in nuclear magnetic resonance scanning and imaging apparatus wherein the magnetic field produced is of adequate uniformity.

Another still further object of the present invention is to provide nuclear magnetic resonance scanning and imaging apparatus which includes a permanent magnet for generating the primary magnetic field thereof, the permanent magnet employing a combination of low energy magnetic flux material, high energy magnetic flux material, and selected flux conductors.

An additional object of the present invention is to provide nuclear magnetic resonance scanning and imaging apparatus which employs a permanent magnet that can be constructed in a manner wherein the cost of construction is no greater than the cost for producing a supercooled electromagnet structure.

A still additional object of the present invention is to provide nuclear magnetic resonance scanning and imaging apparatus which includes means for stabilizing the magnetic field thereof regardless of thermal atmospheric changes.

A still further and additional object of the present invention is to provide a permanent magnet construction for use in nuclear magnetic resonance scanning and imaging apparatus which is simple in design, reliable in construction and efficient in operation.

These objects as well as further objects and advantages in the present invention will become readily apparent after reading the ensuing description of a nonlimiting illustriative embodiment and viewing the accompanying drawing.

An apparatus for nuclear magnetic resonance scanning and imaging which employs a primary magnetic field, an rf field, and a detector for detecting a nuclear magnetic resonance signal the improvement in accordance with the principles of the present invention for producing the primary magnetic field which comprises, in a broad sense, a magnetically conductive frame forming an opening therein; an assembly of relatively low energy flux magnetic material disposed within the opening adjacent to the frame, magnetic flux conductor means disposed adjacent to the assembly of relatively low energy flux magnetic material; the magnetic flux conductor means and the frame sandwiching therebetween the assembly of relatively low energy flux magnetic material; an assembly of relatively high energy flux magnetic material disposed within the opening adjacent to the magnetic flux conductor means, the magnetic flux conductor means being disposed between the assembly of the low energy flux magnetic material and the assembly of high energy flux magntic material, the assembly of relatively high energy flux magnetic material being of a small volume relative to the assembly of relatively low energy flux magnetic material; and pole piece means being disposed adjacent to the assembly of relatively high energy flux magnetic material, the pole piece means and the magnetic flux conductor means sandwiching therebetween the assembly of relatively high energy flux magnetic material, the primary magnetic field being formed between the pole piece means and the magnetically conductive frame.

More specifically, the preferred embodiment of the present invention is an apparatus for nuclear magnetic resonance scanning and imaging which employs a primary magnetic field, an rf field and a detector for detecting a nuclear magnetic resonance, the improvement according to the principles of the present invention for producing the primary magnetic field comprising a magnetically conductive frame forming an opening therein, the opening being formed by opposed first and second spaced apart surfaces of the frame; a first assembly of relatively low energy flux magnetic material disposed within the opening adjacent to the first surface of the frame; first magnetic flux conductor means disposed adjacent to the first assembly of relatively low energy flux magnetic material, the first magnetic flux conductor means and the first surface of the frame sandwiching therebetween the first assembly of relatively low energy flux magnetic material; a first assembly of relatively high energy flux magnetic material disposed within the opening adjacent to the first magnetic flux conductor means, the first magnetic flux conductor means being disposed between the first assembly of low energy flux magnetic material and the first assembly of high energy flux magnetic material, the first assembly of relatively high energy flux magnetic material being of smaller volume than the first assembly of relatively low energy flux magnetic material; first pole piece means being disposed adjacent to the first assembly of relatively high energy flux magnetic material, the first pole piece means and the first magnetic flux conductor means sandwiching therebetween the first assembly of relatively high energy flux magnetic material; a second assembly of relatively low energy flux magnetic material disposed within the opening adjacent to the second surface of the frame; second magnetic flux conductor means disposed adjacent to second assembly of relatively low energy flux magnetic material, the second magnetic flux conductor means and the second surface of the frame sandwiching therebetween the second assembly of relatively low energy flux magnetic material; a second assembly of relatively high energy flux magnetic material disposed within the opening adjacent to the second magnetic flux conductor means, the second magnetic flux conductor means being disposed between the second assembly of low energy flux magnetic material and the second assembly of high energy flux magnetic material, the second assembly of relatively high energy flux magnetic material being of a smaller volume than the second assembly of relatively low energy flux magnetic material; and second pole piece means disposed adjacent to the second assembly of relatively high energy flux magnetic material, the second pole piece means and the second magnetic flux conductor means sandwiching therebetween the second assembly of relatively high energy flux magnetic material, the first and second pole pieces being spaced apart and forming therebetween the primary magnetic field.

In order that the present invention may be more fully understood it will now be described, by way of example, with reference to the accompanying drawing in which:

FIG. 1 a partially broken away perspective view of NMR apparatus showing the permanent magnet configuration of the present invention;

FIG. 2 is a cross sectional view of the apparatus of FIG. 1; and

FIG. 3 is a partially broken away cross sectional view taken along the lines 3--3 of FIG. 2.

Referring now to the figures, and more particularly to FIG. 1 thereof there is illustrated therein a permanent magnet construction 10 of nuclear magnetic resonance scanning and imaging apparatus. The permanent magnet construction 10 includes a housing 12 which encloses the components of the magnet 10. The supporting structure of the magnet 10 is a frame 14 constructed of a plurality of magnetizable steel or iron girders 16 and 18. The steel girders 16 and 18 are assembled with the aid of corner pieces 20 to form a plurality of substantially rectangular structures which are joined together side by side to form the frame 14. The frame 14 defines an opening 22 formed by the girders 16 and 18.

Although the girders 16 and 18 are illustrated in a particular configuration, it is to be understood that other assemblies or structures can be employed to fabricate the desired frame structure and, for instance, the structure could be constructed of four rectangular plates or other similar configurations to produce a frame having an opening therein for the mounting of the balance of the components of the magnet construction 10.

With reference to FIGS. 1, 2, and the permanent magnet construction 10 can be seen to include a first magnet structure 24 disposed adjacent to an interior first surface 26 of the frame 14 within the opening 22 thereof and a second magnet structure 28 disposed adjacent to an interior second surface 30 of the frame 14.

Further shown in FIGS. 1, 2, and as best illustrated in FIG. 3, the first magnet structure 24 and the second magnet structure 28 are essentially identical as will be hereinafter described. The first magnet structure 24 includes an assembly 32 of relatively low energy flux magnetic material which comprises a plurality of low energy flux magnets such as ceramic magnets 34 or the like. The ceramic magnets 34 are arranged in rows and columns in a stacked matrix configuration, with appropriate polarity orientation so that the magnetic strength thereof is aggregated.

A plate 36 which serves as means for conducting magnetic flux is disposed adjacent to the first assembly 32 of relatively low energy flux magnetic material such that magnetic material thereof is sandwiched between the plate 36 and the first surface 26 of frame 14 The plate 36 is magnetically conductive and can be constructed of magnetic steel or the like. The plate 36 serves to concentrate the magnetic flux of the first assembly 32 of relatively low energy flux magnetic material therein. This flux is conducted through the plate 36 and concentrated into an auxiliary conducting plate 38 in contact with a first assembly 40 of relatively high energy flux magnetic material. The plates 36 and 38 can be of integral or unitary construction. The first assembly 40 of relatively high energy flux magnetic material is bounded on one side thereof by the auxiliary conducting plate 38 and on the other side by a first pole piece 42.

The first assembly 40 of relatively high energy flux magnetic material comprises a plurality of rare earth magnets 44 which are arranged in rows and columns in a stacked matrix configuration. Any suitable rare earth material can be employed and, for this particular application, rare earth magnet bricks of a material called MISH metal has been employed.

The ceramic magnets 34 and the rare earth magnets 44 are each formed of bricks approximately four inches by six inches by one inch. In order to mount the magnets of the assemblies of relatively high and low energy flux magnetic material, suitable nonmagnetic material such as aluminum can be employed. The polarity of each of the rare earth magnets 44 is observed in assembling the first assembly 40 of relatively high energy flux magnetic material of relatively low energy flux magne material and the first assembly 40 of relatively high energy flux magnetic. By virtue of the combination of the first assembly 32 of relatively low energy flux magnetic material and the first assembly 40 of relatively high energy flux magnetic material with the plates 36 and 38, the magnetic flux of both the assemblies 32 and 40 is concentrated at the pole piece 42.

In order to reduce leakage from the plate 36 and the auxiliary conducting plate 38, stacks 46 of ceramic magnets 48, with their polarity reversed relative to the first assembly 32 of relatively low energy flux magnetic material, are provided.

The polarity of the magnets in the assemblies 32 and 40 and in the stacks 46 is such that the upper face of each of the bricks is one pole of the magnet and the lower face of each of the bricks is another pole of the magnet. The bricks are stacked in a manner known in the art, such that the negative pole of one brick contacts the positive pole of the next brick in each stack. When the bricks are of reverse polarity, each stack is still in the same configuration, but the upper and lowermost poles of the stacks are reversed relative to the other stacks. For purposes of illustration, plus and minus signs have been placed at the top and bottom of each stack of magnets illustrated in FIG. 3 of the drawing to show the 30 relationships of the poles thereof. In order to further reduce leakage of magnetic flux from the plate 36, a band 50 of stacked ceramic magnets is disposed about the peripheral edge thereof.

The volume of the first assembly 32 of 35 relatively low energy flux magnetic material is much greater than the volume of the first assembly 40 of relatively high energy flux magnetic material. Essentially, the assembly 30 provides a base level magnetic strength and the assembly 40 adds to that base level in a cost effective manner.

The second magnet structure 28 is virtually identical to the first magnet structure 24 and includes a first assembly 52 of relatively low energy flux magnetic material, a plate 54, an auxiliary conducting plate 56, a second assembly 58 of relatively high energy flux magnetic material, and a second pole piece 60. The plate 54 sandwiches the second assembly 52 of relatively low energy flux magnetic material between itself and the second surface 30 of the frame 14. The auxiliary conducting plate 56 is mounted adjacent to the plate 54 and the second assembly 58 of relatively high energy flux magnetic material is sandwiched between the auxiliary conducting plate 56 and the second pole piece 60. Shielding bricks similar to stacks 46 and band 50 of the first magnet structure 24 are also provided.

The magnetic flux developed between the first and second pole pieces 42 and 60 in an apparatus constructed in accordance with the principles of the present invention is approximately 3,000 gauss and is stable to one part in 10.sup.-4 over a radius of eight inches. Of course, if a desired flatness is not acheived, for instance, a flatness better than one part in 10.sup.-3 over a ten inch diameter, through the configuration of the magnets alone, it is to be understood that electromagnetic shim coils can be used to further flatten the field by placement of these coils over the pole pieces. The hereinbefore discussed combination of relatively low energy flux magnetic material and relatively high energy flux magnetic material permits the achievement of the noted magnetic strength and flatness at an economically feasible cost and in an area the size of which is practical.

Of course, in addition to the provision of a primary magnetic field between the first and second pole pieces 42 and 60, an rf field source and a detector for detecting nuclear magnetic resonance must also be provided. However, since these are well known in the art, they have not been illustrated.

When a body is to be imaged, it is moved into the magnetic field created between the first pole piece 42 and the second pole piece 60 by any suitable means well known in the art. When the permanent magnet construction 10 is employed in an NMR apparatus it has been found to be desirable to work at a frequency of 12 MHz. It is known that permanent magnets drift as a result of changes in temperature. The drift of the magnet construction 10 has been held to less than 10 Hz per hour by temperature regulation of the interior of the housing 12 This temperature regulation is accomplished by a heater 62, of the forced air type, disposed within the housing 12. The heater senses the temperature via a thermistor disposed in one of the bricks, not illustrated, and circulates heated air within the housing to maintain a desired temperature. For instance, a temperature of 77.degree. F. has been employed.

In addition to maintaining the temperature within the housing 12, further stability has been achieved by wrapping of coils around the rare earth brick magnets and inducing current in these coils. For instance, a coil 64 is illustrated disposed around the first assembly of relatively high energy flux magnetic material 40. The coil 64 is coupled to a power supply 66 which in turn is controlled by a control means 68. Control means 68 monitors NMR sample 70 within the primary magnetic field created by the first and second pole pieces 42 and 60. The frequency of the NMR sample is determined by the control means 68. Depending upon shift in this frequency, the power supply 66 is activated to induce current in the coil 64. As a result, the coil 64 creates a supplementary magnetic field which can be added or subtracted to the primary magnetic field depending upon the polarity of the current in the coil, a factor controlled by the control means 68. The control means preferably employs a relatively standard microprocessor to process data and issue control instructions, such instrumentation control being well known to those of ordinary skill in the art.

It should be understood that although a particular permanent magnet configuration has been illustrated, that such a permanent magnet configuration can be combined with an electromagnet configuration to achieve a desired field strength within the principles and scope of the invention.

It should further be understood that various changes in the details, materials, arrangements of parts, and operational conditions which have been herein described and illustrated in order to explain the nature of the invention may be made by those skilled in the art within the principles and scope of the invention.