GB2094057A - X-ray generator - Google Patents

Abstract

An X-ray tube has a tubular envelope with a cathode (40) for directing an electron beam (44) onto a focal spot area (46) of a spaced anode target (54) to generate X-rays. The target is mounted for axial rotation on one end of a rotor disposed in an end portion of the envelope and encircled by a stator (20) of an alternating current induction motor (22). An annular shield (24) of high permeability magnetic material extends transversely between the electron beam and the stator of the induction motor for shunting stray or fringe electromagnetic fields established by the stator away from the electron beam to avoid consequent lateral defections of the electron and corresponding lateral movements of the focal spot area. <IMAGE>

Description

SPECIFICATION X-ray generator 1. Field of the invention This invention relates generally to X-ray generator systems and is concerned more particularly with an X-ray tube of the rotating anode type provided with magnetic shielding means for reducing distortion in X-ray tomography imaging systems and enhancing X-ray beam resolution in radiographs.

2. Discussion of the prior art An X-ray generator system may include an x-ray tube of the rotating anode type having a tubular envelope wherein an electron emitting cathode is disposed to beam electrons onto an aligned focal spot area of a rotatable anode target. In operation, the beamed electrons impinge on the focal spot area with sufficient energy to generate X-rays which emanate therefrom in a divergent beam. This divergent beam of X-rays, which passes through a window in the tube envelope, may be directed through beam collimating means of the system and then through internal structure of a patient. The variations in X-ray intensity, thus produced, may be conveyed to a detector or detectors to produce equivalent electrical signals which can be suitably processed, as in computerized tomography, for example, to produce a corresponding image.

Accordingly, the resulting image may be enhanced by maintaining the focal spot area in as fixed a position as possible relative to the collimating means.

In order to avoid damage due to overheating, the target material in the focal spot area aligned with the electron beam is continually changed by rotating the anode target relative to the electron beam. Consequently, the anode target generally is mounted on a rotor which extends into an end portion of the envelope encircled by a stator of an induction type motor. In operation, an alternating current energizes the stator to produce a rotating magnetic field having flux lines which extend diametrically through the encircled end portion of the envelope. As a result, the rotating magnetic field rotates the rotor within the encircled end portion of the envelope and rotates the attached anode target correspondingly.

However, it has been found that the magnetic field established by the stator also has stray or fringe flux lines which extend transversely through the electron beam emanating from the cathode and impinging on the focal spot area of the anode target. Consequently, the electron beam is deflected laterally from its true position relative to the window in the tube envelope and to the collimating means of the system. Also, the electron beam, moves annularly about this true position due to the rotational movement of the magnetic flux lines. As a result, the focal spot area is enlarged; and resolution of the divergent X-ray beam emanating from the enlarged focal spot area is degraded accordingly.Furthermore, the variations in position of the focal spot area relative to the window in the tube envelope and the collimating means of the system will produce small variations in the X-ray beam intensity which may be significant in some X-ray techniques, such as computerized tomography scanning where variations of less than one percent are detected, for example.

Attempts have been made in the prior art two shield the electron beam from the magnetic field established to rotate the target of a rotating anode type X-ray tube. However, some of these attempts have been frustrated by the requirements that the encircled end portion of the envelope be made of non-magnetic material so that the magnetic flux lines established by the stator extend diametrically through the encircled end portion of the envelope.

Other attempts have resulted in magnetic permeable members being disposed such that magnetic field lines established by the stator are actually directed toward the electron beam region of the tube.

Accordingly, these and other disadvantages of the prior art are overcome by this invention which provides an X-ray generator system comprising a rotating anode type of X-ray tube having a portion encircled by a stator of an induction type motor, and a magnetic shield member disposed to extend radially between an end portion of the stator and an electron beam region ofthetube.

The rotating anode type of X-ray tube includes a tubular envelope having therein a cathode disposed to direct an electron beam onto a focal spot area of a rotatable anode target which is secured to a rotor.

The rotor is rotatably supported in an end portion of the tube envelope encircled by a stator of an induction type motor, which is energized by alternating current to produce a rotating magnetic field having diametrically extending flux lines. The magnetic shield member is made of material having high magnetic permeability, such as greater than ten thousand, for example, and preferably has an annular configuration for encircling a portion of the tube.

Thus, the magnetc shield member may encircle a portion of the tube such that it extends radially between an adjacent end portion of the stator and the electron beam extending from the cathode to the focal spot area of the target. As a result, stray or fringe flux lines extending from the adjacent end portion of the stator are diverted away from the electron beam by the high magnetic permeability material of the shield member providing a low reluctance path. Consequently, electrons in the resulting undeflected beam are permitted to impinge on the true focal spot area of the anode target.Thus, the focal spot area may be maintained as small as possible, and fixed in relation to a window portion of the tube envelope and collimating means of the system thereby enhancing resolution qualities and uniformity of intensity in an X-ray beam emanating from the focal spot area and passing through the window portion of the tube envelope.

For a better understanding of this invention, reference is made in the following detailed description to the accompanying drawings wherein: Figure 1 is a schematic view of an X-ray generator embodying the invention; Figure 2 is an elevational view, partly in section, of the X-ray generator shown in Figure 1 but with the outer wall thereof broken away; Figure 3 is an elevational view, partly in section, of the support sleeve and magnetic shield member shown in Figure 2; and Figure 4 is a plan view of the magnetic shield member shown in Figure 3.

Figures a schematic fragmentary axial view of the generator shown in Figure 2 but without the magnetic shield member; Figure 6 is a schematic fragmentary axial view similar to Figure 5 but including the magnetic shield member to illustrate its effect on a representative magnetic field; and Figure 7 is a schematic plan view of the magnetic shield member shown in Figure 6 to illustrate further its effect on the representative magnetic field.

Referring to the drawings wherein like characters of reference designate like parts, there is shown in Figure 1 an X-ray generator system 10 comprising a conventional X-ray shielded housing 12 having a generally hollow cylindrical configuration for enclosing a longitudinally disposed X-ray tube 14. The tube 14 is of the rotating anode type, and has an electron beam region 16 disposed in alignment with a conventional type port 18 extending outwardly from a central portion of the housing. Mounted over the port 18 is a conventional collimator 19, such as shown in U.S. Patent No. 3,581,094 granted to Leonard F. Peyser and assigned to the assignee of this invention.

Within housing 12, one end portion of X-ray tube 14 is encircled by a generally cylindrical stator 20 of an induction type motor 22. The stator 20, when energized electrically, produces rotating electromagnetic fields having flux lines which extend transversely through the encircled end portion of tube 14. However, since the electromagnetic fields may have stray or fringe flux lines which extend transversely through the electron beam region 16 of tube 14, there is transversely disposed between the region 16 and the adjacent end portion of stator 20 a magnetic shield member 24 made of material having a high magnetic permeability, such as greater than ten thousand, for example.

As shown more clearly in Figure 2, the housing 12 is provided with a conventional lining 26 of X-ray absorbent material, such as lead, for example, which generally is maintained at electrical ground potential. The housing 12 also is provided with a conventional pair of horn-type electrical receptacles, 28 and 30, respectively, which extend outwardly from respective opposing end portions of the housing.

Receptacles 28 and 30 are disposed for receiving therein, in a well-known manner, plug-in terminal end portions of respective electrical cables (not shown) whereby electrical power is applied to the electrodes of tube 14.

X-ray tube 14 includes a tubular envelope 32 having disposed adjacent the receptacle 28 a cathode reentrant end portion which is peripherally sealed to a cathode support cylinder 34. The cylinder 34 extends axially within envelope 32 and has an inner end portion closed by a transversely disposed cap 36. Extending radially from the cap 36 is a hollow arm 38 which supports on a distal end portion thereof a conventional cathode 40. Cathode 40 may comprise a helically wound filament of electron emitting material, such as tungsten, for example, which is longitudinally disposed in an elongated opening of a stepped focussing cup (not shown).The cathode 40 is supplied with a filament current and is maintained at a predetermined electrical potential with respect to electrical ground by a plurality of electrical conductors (not shown) extending through hollow arm 38 and hermetically out of envelope 32 for electrical connection through a cable 42 to respective terminals in electrical receptacle 28. Thus, the cathode 40 is disposed for emitting electrons into electron beam region 16 of tube 14, and for directing the emitted electrons into a beam 44 which passes through the region 16.

Electrons in the beam 44 are accelerated through the region 16 and impinge on an aligned focal spot area 46 of a radially sloped focal track 48 which is made of efficient X-ray emissive material, such as rhenium-tungsten alloy, for example. As a result, the impinging electrons generate X-rays which emanate from the focal spot area 46 in a divergent beam 50.

The beam 50 passes through a window portion 52 of envelope 32, the port 18 of housing 12, and the collimator 19 which are radially aligned with the focal spot area 46 on sloped target track 48. As a result, the X-ray beam 50 appears to be emanating from a radial projection of the focal spot area 46 commonly called the "effective" focal spot. Accordingly, in order to enhance resolution of fine detail structure in images produced by the X-ray beam 50, it is necessary to maintain the focal spot area 46 as small as possible so that the "effective" focal spot approximates a point source for the X-ray beam 50, and to maintain the position of focal spot area 46 fixed with respect to the window portion 52 of envelope 32 and the collimator device 19 in order to reduce intensity fluctuations.

The focal track 48 comprises an outer annular portion of a target disc 54 which is tranversely disposed in envelope 32 and has a central portion secured by conventional means to one end portion of an axially extending stem 56. The opposing end portion of stem 56 is affixed by suitable means to a closed end of a cup-shaped rotor 58 which extends axially in a small diameter anode end portion of envelope 32. The rotor 58 is rotatably supported by well-known bearing means on a fixed shaft 60, which is attached to the adjacent end of envelope 32 and has an external end portion extended into a dielectric support cup 62. The external end portion of shaft 60 has journalled therein a screw 64 which extends through a central aperture in the closed end of support cup 62. Screw 64 supports the anode end portion of tube 14 and electrically connects the target disc 54 through a cable 66 to a high voltage terminal in electrical receptacle 30. Thus, a high positive voltage, such as seventy-five kilivolts, for example, with respect to electrical ground may be applied to the target disc 54 for accelerating electrons in the beam 44 onto a suitably small focal spot area 46 of focal track 48.

In order to avoid damaging the target material in focal spot area 46 due to overheating, the target disc 54 is rotated by having the stator 20 encircling the anode end portion of envelope 32. Stator 20 encircles a sleeve 68 made of dielectric, non-magnetic material, such as glass, for example, and having an outwardly flared end portion 69 through which the anode end portion of envelope 32 is passed to extend into the dielectric support cup 62. The stator 20 comprises a plurality of wire coils wound around pole-pieces (not shown) of a laminated sheet metal core to produce a generally hollow cylindrical structure. Stator 20 is energized with a suitable alternating current for establishing a rotating electromagnetic field having magnetic flux lines which extend transversely through the encircled end portion of envelope 32.Consequently, the rotor 58 is made of suitable electrically conductive material, such as copper, for example; and the envelope 32 is made of nonmagnetic material, such as lead-free glass, for example.

However, it has been found that stray magnetic flux lines of fringe magnetic fields may extend transversely through the electron beam region 16 from the adjacent end portion of stator 20. As a result, the electron beam 44 may be deflected laterally and rotated about the center of focal spot area 46 due to the rotation of the electromagnetic field produced by stator 20. Thus, the resultant focal spot area will be enlarged in comparison to focal spot area 46 and will not be fixed in position relative to the window portion 48 of envelope 32 and the collimator 19. Consequently, the "effective" focal spot from which X-ray beam 50 appears to emanate will not approximate a point source as closely as possible; and the resolution in an image produced by beam 50 will be degraded accordingly.

As shown more clearly in Figures 3 and 4, the magnetic shield 24 may be annular and provided with a central aperture 25 of suitable diameter for sliding over the small diameter portion of sleeve 68.

Also, magnetic shield 24 may be provided with a generally frusto-conical configuration for fitting against the outwardly flared end portion 69 of sleeve 68 and being secured thereto by any convenient means, such as epoxy resin adhesive, for example.

The magnetic shield 24 is made of a material having high magnetic permability, such as greater than ten thousand, for example, and has sufficient thickness, such as sixty thousandths, for example, to provide a low reluctance path for magnetic flux lines of the field established by stator 20. Also, the annular magnetic shield 24 is provided with an outer diameter sufficient for diverting transversely away from electron beam region 16 stray or fringe magnetic flux lines extending outwardly of the stator 20.

As shown in Figure 5, the stator 20, when energized, produces a rotating magnetic field having flux lines, such as 71, for example, which extend diamet ricallythrough the rotor 58. Without the magnetic shield 24 in place, however, the magnetic field also has stray or fringe magnetic flux lines, such as 72-76, for examples, which extend outwardly of stator 20.

Some of these stray or fringe flux lines, such as 74, for example, pass transversely through the electron beam region 16 and have components which are directed substantially perpendicular to the electron beam 44. As a result, the electron beam 44 will be deflected laterally, and rotated about the center of focal spot 46 since the magnetic field produced by stator 20 is rotating.

As shown in Figures 6 and 7, when the magnetic shield member 24 is transversely disposed between the electron beam 44 and stator 20, the stray or fringe flux lines 72-76 enter the interposed arcuate portion of shield 24 and exit from opposing arcuate portion thereof to return to the stator 20. Thus, the stray or fringe flux lines of the magnetic field produced by stator 20 and extended toward electron beam 44 are shunted transversely away from the electron beam region 16 of system 10 to prevent lateral deflection of the electron beam 44 and consequent enlargment of focal spot area 46. As a result, the "effective" focal spot of system 10 is not deflected appreciably with respect to window 52 or collimating device 19; and the X-ray beam 50 has a more uniform intensity distribution than would be obtained from the X-ray generator system 10 without the magnetic shield 20.

Since the annular magnetic shield 24 is supported between the highly positive anode structure of tube 14 and the electrically grounded housing 12, the shield 24 may become charged up to an intermediate electrical potential. Accordingly, in order to prevent the shield 24 from acquiring such an electrical charge, the shield 24 is connected electrically, as by conductor 70, for example, to the housing 12 to maintain the magnetic shield 70 at electrical ground potential. The shield 24 preferably, is connected to housing 12 and secured to flared portion 69 of sleeve 68 in a removable manner so that the shield 24 may be removed readily when the X-ray generator system 10 is to be used for diagnostic techniques which do not require the high intensity uniformity desired for other diagnostic techniques, such as computerized tomography scanning, for example.

From the foregoing, it will be apparent that all of the objectives of this invention have been achieved by the structures shown and described herein. It also will be apparent, however, that various changes may be made by those skilled in the art without departing from the spirit of the invention as expressed in the appended claims. It is to be understood, therefore, that all matter shown and described is to be interpreted as illustrative rather than in a limiting sense.

Claims (13)

1. An X-ray generator comprising: means for producing an electron beam; means spaced from said electron beam for producing a magnetic field having flux line components directed transversely of said electron beam; and means extended transversely in spaced relationship between said electron beam and said magnetic field producing means for shunting flux lines of said magnetic field away from said electron beam.

2. An X-ray generator comprising: a tubular envelope having therein an axially disposed electron beam region; a motor stator disposed in encircling relationship with a portion of the envelope axially spaced from said electron beam region; and a magnetic shield member extended transversely in spaced relationship between said electron beam region and said stator.

3. An X-ray generator as set forth in claim 2 wherein said magnetic shield member is disposed externally of the envelope.

4. An X-ray generator as set forth in claim 2 wherein said magnetic shield member is annular and encircles a portion of the envelope between said electron beam region and said stator.

5. An X-ray generator as set forth in claim 2 wherein said magnetic shield member is electrically grounded.

6. An X-ray generator comprising: An X-ray tube including a tubular envelope having a non-magnetic portion, a cylindrical rotor rotatably supported within said non-magnetic portion of the envelope, and an anode target coupled to an end portion of said cylindrical rotor; motor stator means disposed externally of said envelope and in encircling relationship with said non-magnetic portion for producing a rotating magnetic field having magnetic flux lines extended transversely through said non-magnetic portion of the envelope; and magnetic shield means disposed externally of said envelope and extended in spaced relationship between said stator means and said anode target for shielding said target from fringe flux lines of said magnetic field.

7. An X-ray generator as set forth in claim 6 wherein said magnetic shield means comprises a magnetically permeable annulus disposed in encircling relationship with a portion of said envelope adjacent said stator means and extended radially outward from said envelope a greater distance than said stator means.

8. An X-ray generator as set forth in claim 6 wherein said generator includes housing means for enclosing said X-ray tube and supporting said motor stator means and said magnetic shield means.

9. An X-ray generator as set forth in claim 8 wherein said housing means includes a sleeve of nonmagnetic material encircling said non-magnetic portion of the envelope and supporting said stator means and said magnetic shield means in encircling relationship with said envelope.

10. An X-ray generator as set forth in claim 8 wherein said housing means comprises a hollow cylindrical housing enclosing said X-ray tube and including electrically conductive material maintained at electrical ground potential.

11. An X-ray generator as set forth in claim 9 wherein said magnetically permeable annulus is electrically connected to said electrically conductive material.

12. An X-ray generator comprising: a housing; cathode means disposed within the housing for producing an electron beam; anode target means rotatably mounted within the housing for receiving the electron beam and producing X-rays; motor means having a rotor connected to the anode target means and a stator disposed for producing a magnetic field and coupling to said rotor to rotate said anode target means within the housing; and magnetic shield means disposed within the housing between said motor means and said electron beam and having portion disposed transversely of the electron beam for shielding the electron beam from the magnetic field.

13. An X-ray generator substantially as hereinbefore described with reference to the accompanying drawings.