WO1999050882A1

WO1999050882A1 - Multiple wavelength x-ray tube

Info

Publication number: WO1999050882A1
Application number: PCT/US1999/005852
Authority: WO
Inventors: G. Yale Eastman; Donald M. Ernst; Martin Fruitman
Original assignee: Thermal Corp.
Priority date: 1998-03-27
Filing date: 1999-03-15
Publication date: 1999-10-07

Abstract

The invention is an x-ray tube which has multiple wavelength capabilities. The x-ray tube anode (56) has several sections constructed as thin layers (52) of different materials. The sections can be circumferential bands, radial wedges or rectangular shaped sections attached to the anode. Variation in wavelength is then accomplished by either rotating the anode or scanning the electron beam (60) so that the electron beam bombards different sections. The rotation and the scanning can be done rapidly to yield a varying beam or more slowly to furnish different wavelengths for individual exposures. In one embodiment the tube anode is constructed as thin layers of material upon a high temperature heat pipe (74) which transfers heat away from the x-ray generating spot on the anode rapidly enough to prevent melting of the x-ray source layer.

Description

MULTIPLE WAVELENGTH X-RAY TUBE

BACKGROUND OF THE INVENTION This invention deals generally with x-ray tubes and more specifically with an x-ray tube which can generate radiation of more than a single wavelength.

X-ray tubes function on the basis of an electron beam being generated by a cathode within the tube, and the electron beam bombarding a very small spot on an anode which is also within the tube. The bombardment of the anode, which is constructed of a suitable x-ray generating material, creates the x-rays along with a great deal of heat. Until now all x-ray tubes have generated radiation of a single wavelength, so that different machines, with different tubes, would be required if there was a need to generate radiation at different wavelengths. Yet all such machines would likely be quite similar except for the different x-ray tubes installed within them.

Clearly, there would be significant improvements in medical diagnosis and treatment along with savings in the cost of equipment if an individual x-ray tube could generate more than a single wavelength of radiation.

SUMMARY OF THE INVENTION The present invention is an x-ray tube which can generate radiation at several different wavelengths. To accomplish this the tube anode is divided into several sections constructed as thin layers on the anode, with each section made from a different x-ray generating material, and each section capable of generating x-rays of a different wavelength.

These sections can be circumferential bands, radial wedges, or rectangular shaped sections of x-ray generating material. The sections are attached to a heat dissipating anode

- 1 - structure, with the anode cooled sufficiently so that the thin layers will not deteriorate from electron bombardment. The variation in wavelength is then accomplished by either rotating the anode or scanning the electron beam. Both the rotation and the scanning can be done rapidly to yield a varying beam or more slowly to furnish different wavelengths for individual

exposures.

The preferred embodiment of the invention has layers of pie or wedge shaped sections of x-ray generating material arranged on a surface, with the materials of adjacent sections being different. The materials in the sections either alternate between two different materials or sequence through several different materials. Conventional x-ray tube anodes are frequently constructed as rotating discs to essentially move the bombardment point of the anode out from under the electron beam before the anode material is damaged by heat. Such an arrangement spreads the heat over the surface of the anode, not by conduction of heat, but by moving the heated portion of the anode away from the source of heat. When the preferred embodiment of the invention is rotated in such a manner, the result is that the electron beam hits the different materials in the wedge sections as the wedge shaped sections move under the electron beam. The electron beam thus generates radiation of a different wavelength as it hits each section. The x-ray beam is therefore composed of a sequence of pulses of radiation of different wavelengths, which creates an exposure of two or more wavelengths, depending upon the number of different materials in the wedge sections. Furthermore, the different wavelengths are created rapidly because of the high rotation speed of the disk.

However, when the electron beam is itself pulsed and the pulses are synchronized with the rotation speed of the disc, the beam only hits wedge sections of the same material,

- 2 - and the same structure thereby generates a conventional single wavelength exposure.

Moreover, a simple change in the timing of the relationship of the electron beam pulse to the rotation of the disc then produces any of the single wavelengths for which the layers on the anode have been chosen. In an alternate embodiment of the invention, the layers of different material are arranged as concentric bands located at different radii from the center of a disc shaped anode.

In such a system the electron beam is magnetically deflected to fall on any one of the bands, and thus the tube generates the particular wavelength produced by the specific band selected.

In this embodiment the combination of electron beam pulsing and deflection can even be used to generate a nonrepetitive pattern of pulsed wavelengths for the exposure. The selection of wavelengths available is only limited by the number of bands of material which can be attached to the anode.

Another embodiment has a grid pattern of different materials attached to a stationary anode structure, a pattern which resembles a checker board. With such a structure the choice of wavelengths generated is completely controlled by the deflection of the electron beam. Although the invention can be used with the conventional rotating anode, another type of anode, a stationary one, can. also be constructed. Such an x-ray tube anode is constructed as a thin layer of x-ray generating material upon a high temperature heat pipe, such as a heat pipe with a tungsten casing and lithium or sodium heat transfer fluid. This structure transfers the intense heat at the x-ray generating spot on the anode away rapidly enough to prevent melting of the X-ray source layer, and the use of such a high performance heat pipe eliminates the need to rotate the anode. The present invention thereby furnishes a unique x-ray tube which is capable of generating multiple wavelengths of radiation, and also furnishes an x-ray tube which does not require a complex arrangement to rotate the anode within a vacuum enclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified top view of the x-ray tube anode of the preferred embodiment of the invention shown with a pattern of wedge shaped sections around a center axis.

FIG. 2 is a simplified top view of the x-ray tube anode of an alternate embodiment of the invention shown with a pattern of concentric band sections around a center axis. FIG. 3 is a schematic representation of a partial side cross section view of an x-ray tube within which the bombarded surface of the anode is cooled by a heat pipe.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a simplified top view of x-ray tube anode 10 of the preferred embodiment of the invention with base disc 11 having a pattern of wedge shaped sections 12, 14, 16, 18, and 20 around center axis 22 formed by attaching thin wedge shaped sections to the structure of base disc 11. The wedge shaped sections on anode 10 continue fully around and back to section 24 adjacent to section 12, and at least alternate wedge shaped sections are constructed of different materials, so that such different materials generate x-rays of different wavelengths. A conventionally generated electron beam bombards disc 11, at beam spot 26 on section 12, and as is well known in the art, generates an x-ray beam (not shown) by bombarding the material in the section upon which it impinges.

Disc 11 rotates in the direction shown by arrow A, and therefore, in the preferred embodiment, section 12 determines the wavelength for approximately one-eighth of the rotation time of disc 11, after which, section 14, section 16, section 18 and so forth determine the wavelength. As can be appreciated, with such a structure, the rotation of anode 10 establishes a circular path 28 on anode 10 through which beam spot 26 always falls. However, the radial distance between center axis 22 and beam spot 26 can be varied by either mechanical adjustment of the electron beam generator, or, preferably, by the electronic deflection of the electron bombardment beam. As the distance between beam spot 26 and axis 22 increases the effective beam spot speed increases which reduces the peak temperature to which the material beneath the spot is raised.

The wavelength of the resulting x-ray beam will vary with the speed of rotation of disc 11. Thus, if the rotation of anode 10 is 12,000 rpm, which is among the higher conventional rotation speeds, and the wedge shape sections of anode 10 include only two alternating materials, the x-ray wavelength will vary at 800 Hz. Typically Anode 10 is constructed as specified below. Base disc 11 - material - tungsten diameter - 1.0 inch thickness - .060 to .125 inch Wedge section 12 - material - copper thickness - .001 inch Wedge Section 14 - material - platinum thickness - .001 inch

Typical electron beam diameter - .010 to .030 inch Typical rotation speed - 10,000 rpm

- 5 This structure and operation yields an x-ray output varying between wavelengths of 1.381 Angstrom (generated by copper at 8.977 KEV) and 0.158 Angstrom (generated by platinum at 78.341 KEV).

It is important to recognize that the structure of FIG. 1 can also be used to generate a conventional single wavelength x-ray exposure. Conventional technology makes it a simple matter to modulate an electron beam, so that the beam only bombards beam spot 26 at certain regular intervals. It is then only necessary, again by conventional methods, to synchronize the rotation of anode 10 to the modulation of the electron beam so that beam spot 26 is only generated, for instance, on alternate wedge shaped sections 12, 16, 20 and so forth. The resulting x-ray exposure would then be composed of pulses of a single wavelength.

Furthermore, when a complex pattern of synchronized electron beam modulation is used along with multiple materials on all the sections, the structure of FIG. 1 can generate any pattern of pulses of different wavelengths, since each section is available to generate x-rays once during each rotation of anode 10. FIG. 2 is a simplified top view of x-ray tube anode 30 of an alternate embodiment of the invention with a pattern of concentric band sections 32, 34, 36, and 38 around center axis 40 of base disc 42. These bands are of different materials and can be used in the same manner as the sections of FIG. 1 to generate multiple wavelength x-rays, except that the electron beam is deflected radially among the bands to change wavelengths. Such electron beam deflection can be accomplished by either mechanical, electrostatic, or magnetic means. Magnetic deflection coils conventionally accomplish such electron beam deflection in every television receiver.

Thus, when electron beam spot 44 is held at one radius and disc 42 is rotated in direction B, anode 38 generates radiation of a single wavelength. However, when the electron beam is deflected, the wavelength generated will vary based upon the material of the other bands which the beam bombards. Such deflection can be mechanically controlled if the goal is simply to change the radiation output to a constant single wavelength. However, when a varying wavelength is desired the electron beam is electronically deflected radially on path D onto different bands, and the wavelength will vary based upon whatever pattern of deflection is used.

FIG. 3 is a schematic representation of a partial side cross section view of x-ray tube 50 within which bombarded surface 52 of anode 54 is attached to and cooled by heat pipe 56 while generating x-ray beam 61. Such a tube is constructed as a conventional x-ray tube would be with cathode 58 which generates electron beam 60 mounted within evacuated envelope 62 and interconnected to suitable power supplies (not shown) by cathode connections 64 which penetrate envelope 62.

FIG. 3 also schematically depicts conventional structures which are used to control electron beam 60. Magnetic coil 66 is the device which deflects electron beam 60 in any direction along bombarded surface 52, as indicated by beams 60A and 60B. Control grid 68 is also mounted within evacuated envelope 62 and along electron beam 60 and is capable of modulating beam 60 to prevent it from bombarding surface 52, in effect to turn off electron beam 60. Control grid 68 is also a conventional device used within other electronic tubes, and is controlled by power supplies and pulse generators (not shown) to which it is attached by connection 70 which penetrates envelope 62.

Cylindrical heat pipe 56 penetrates envelope 62 and is sealed to it at vacuum seals 72 by conventional means. Heat pipe 56 eliminates the need to rotate anode 56 because heat pipe 56 is capable of cooling bombarding surface 52 well enough to prevent thermal damage to surface 52.

- 7 - In this embodiment, in order to sufficiently cool bombarding surface 52, heat pipe 56 is constructed with a tungsten casing, lithium fluid, and a niobium powder wick for high power density operation. Heat pipe 56 removes the heat generated at the spots at which electron beam 60 bombards surface 52 which is attached to heat pipe 56 in any manner which promotes heat conduction. Cooling coil 74, located at the condenser end of heat pipe 56, and through which a cooling fluid is pumped, then moves the heat from heat pipe 56 to a remote heat exchanger (not shown).

Elimination of the need to rotate anode 54 perfectly compliments the ability to deflect electron beam 60 because it permits full electronic control of the location of the spot generating x-ray beam 61. Thus, with either the structure of FIG. 1 or FIG. 2, the electron beam can be rotated around the center axis instead of rotating the base disc. Furthermore, with rotation of the anode eliminated, the invention is not restricted to circular layouts of x- ray generating sections, so it is quite practical to arrange the sections in a grid pattern, much like a checkerboard. Such a layout permits a vast number of x-ray pulse patterns to be generated, and, of course, permits a virtually unlimited number of wavelengths to be generated within a single x-ray tube, because each section of the grid can be constructed of a different material.

It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims.

Claims

What is claimed as new and for which Letters patent of the United States are desired to be secured is:

1. An x-ray tube comprising:

a means for generating an electron beam; an anode to which the electron beam is directed, the anode comprising; a base structure; at least two individual sections of at least two different x-ray generating materials attached to the base structure surface, with the electron beam bombarding a spot on a section and generating x-ray radiation; means for displacing the electron beam bombarding spot relative to the surfaces of the individual sections, so that the spot moves from one section to another.

2. The x-ray tube of claim 1 wherein the means for displacing the spot is rotating the

anode.

3. The x-ray tube of claim 1 wherein the means for displacing the spot is a means for deflecting the electron beam.

4. The x-ray tube of claim 3 wherein the means for deflecting the electron beam is a magnetic deflection coil.

5. The x-ray tube of claim 1 further including means for modulating the electron beam to stop the bombardment of the sections of x-ray generating material.

6. The x-ray tube of claim 1 wherein the base structure is a heat pipe.

7. An x-ray tube comprising: a means for generating an electron beam; an anode to which the electron beam is directed; a heat pipe to which the anode is attached, with the heat pipe transferring heat away

from the x-ray generating surface of the anode to prevent damage to the anode surface; and heat dissipating means to remove the heat from the heat pipe.

8. The x-ray tube of claim 7 further including a means for deflecting the electron beam.

9. The x-ray tube of claim 7 further including means for. modulating the electron beam.

10. The x-ray tube of claim 7 further including at least two individual sections of at least two different x-ray generating materials on the anode surface, with the electron beam bombarding a spot on a section and generating x-ray radiation; and means for displacing the electron beam bombarding spot relative to the surfaces of the individual sections, so that the spot moves from one section to another.

- 10

11. An x-ray tube comprising: a means for generating an electron beam; an anode base structure; a thin layer of material different from the material of the anode base structure with the thin layer attached to the anode base structure and with the electron beam bombarding a spot on the thin layer and generating x-ray radiation; and a cooling means to which the anode base structure is attached, with the cooling means transferring heat through the anode base structure and away from the thin layer on the anode to prevent damage to the thin layer.

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