Miniaturized low power x-ray source - Patent Review 5153900

Claims

What is claimed is:

1. An x-ray source comprising:

A. a programmable power supply including drive means for establishing an output voltage having a peak value in the approximate range of 10 kV to 90 kV;

B. beam generator means responsive to said output voltage for emitting electrons to generate an electron beam along a beam path, said beam being characterized by a current in the approximate range of 1 nA to 100 .mu.A,

wherein at least one of the amplitude of said output voltage and the magnitude of said current can be varied over time in response to a programming operation; and further comprising:

C. a target assembly positioned in said beam path, said target including at least one x-ray emission element adapted to emit x-rays in a predetermined spectral range in response to incident electrons from said beam;

D. field distribution means for establishing an x-ray radiation pattern having a spatial distribution, said spatial distribution being at least in part external to said source, and

E. a controller including means for user-controlled adjustment of at least one of the amplitude of said output voltage and the magnitude of said current.

2. An x-ray source according to claim 1 wherein said target assembly includes at least one emission element.

3. An x-ray source comprising:

A. a power supply including drive means for establishing an output voltage having a peak value in the approximate range of 10 kV to 90 kV,

wherein the amplitude of said output voltage is a predetermined function of time;

B. beam generator means responsive to said output voltage for emitting electrons to generate an electron beam along a beam path, said beam being characterized by a current in the approximate range of 1 nA to 100 .mu.A,

wherein the magnitude of said current is a predetermined function of time;

C. a target assembly positioned in said beam path, said target including at least one x-ray emission element adapted to emit x-rays in a predetermined spectral range in response to incident electrons from said beam; and

D. field distribution means for establishing an x-ray radiation pattern having a spatial distribution, said spatial distribution being at least in part external to said source,

wherein said beam generator means includes a photocathode, an anode adapted to attract electrons emitted from said photocathode, and means responsive to said output voltage coupled across the photocathode and anode for establishing an accelerating electric field between said photocathode and said anode, and

wherein said anode is positioned between said photocathode and said target, said anode including an aperture through which said electrons pass.

4. An x-ray source comprising:

a power supply including drive means for establishing an output voltage having a peak value in the approximate range of 10 kV to 90 kV, wherein the amplitude of said output voltage is a predetermined function of time;

beam generator means responsive to said output voltage for emitting electrons to generate an electron beam along a beam path, said beam being characterized by a current in the approximate range of 1 nA to 100 .mu.A, wherein the magnitude of said current is a predetermined function of time;

a target assembly positioned in said beam path, said target including at least one x-ray emission element for emitting x-rays in a predetermined spectral range in response to incident electrons from said beam; and

field distribution means for establishing an x-ray radiation pattern having a spatial distribution, said spatial distribution being at least in part external to said source,

wherein said beam generator means includes a thermionic emitter, an anode and means responsive to said output voltage for establishing an accelerating electric field between said thermionic emitter and said anode, and

wherein said thermionic emitter includes a thermionic cathode having a first terminal and a second terminal, and said drive means comprises:

A. a voltage multiplier network having a control voltage terminal and having a high voltage terminal coupled to said first terminal of said thermionic emitter, said voltage multiplier network including first circuit means coupled between said control voltage terminal and said high voltage terminal and responsive to an applied control voltage at said control voltage terminal for establishing said output voltage at said high voltage terminal,

B. a thermionic emitter heater network having a current control terminal and including second circuit means capacitively coupled to said current control terminal for driving an rf ohmic heating current through said thermionic cathode in response to a current control signal applied at said current control terminal.

5. An x-ray source according to claim 4 wherein said beam generator includes a focussing electrode.

6. An x-ray source according to claim 5 wherein said first circuit means comprises:

i. a set of 2n series coupled diodes establishing a unidirectional dc path from said high voltage terminal and extending through the first diode, the second diode, and the remaining diodes in succession of said set of diodes, and then through a resistive element to a reference potential, where n is an integer,

ii. a first set of n series coupled capacitors coupled between the junction between said first and second diodes and said control voltage terminal, wherein each successive capacitor of the first n-1 capacitors of said first set is coupled across an associated successive pair of diodes of said set of diodes, starting with said second diode,

iii. a second set of n series coupled capacitors coupled between said high voltage terminal and a reference potential, wherein each successive capacitor of the first n-1 capacitors of said second set is coupled across an associated successive pair of diodes of said set of diodes, starting with said first diode, and

wherein said second circuit means comprises:

i. said first circuit means,

ii. a third set of n series coupled capacitors coupled between a current control terminal and said second terminal of said thermionic cathode, wherein each successive capacitor of said third set is associated with a correspondingly positioned capacitor of said second set, and the capacitor-to-capacitor junctions of said third set are resistively coupled to the correspondingly positioned capacitor-to-capacitor junctions of said second set, and

iii. an rf current source coupled to said current control terminal, said current source including means for driving said rf ohmic heating current through third set of capacitors, said thermionic cathode, and said second circuit means to said reference potential.

7. An x-ray source according to claim 6 further comprising a current feedback means for sensing the level of said rf current and for controlling said current level in response to said current control signal.

8. An x-ray source according to claim 7 further comprising a voltage feedback means for sensing the voltage level at said high voltage terminal and for controlling said voltage level in response to said control voltage signal.

9. An x-ray source according to claim 6 further comprising a voltage feedback means for sensing the voltage level at said high voltage terminal and for controlling said voltage level in response to said control voltage signal.

10. An x-ray source comprising:

A. a power supply including drive means for establishing an output voltage having a peak value in the approximate range of 10 kV to 90 kV,

wherein the amplitude of said output voltage is a predetermined function of time;

B. beam generator means responsive to said output voltage for emitting electrons to generate an electron beam along a beam path, said beam being characterized by a current in the approximate range of 1 nA to 100 .mu.A,

wherein the magnitude of said current is a predetermined function of time;

C. a target assembly positioned in said beam path, said target including at least one x-ray emission element adapted to emit x-rays in a predetermined spectral range in response to incident electrons from said beam; and

D. field distribution means for establishing an x-ray radiation pattern having a spatial distribution, said spatial distribution being at least in part external to said source,

further comprising a closed housing, wherein said beam generator means and said target assembly are disposed within said housing, said housing having a window on one outer surface thereof, whereby said emitted x-rays are emitted through said window, and

wherein said field distribution means comprises a shield assembly including means for restricting the x-rays emitted by said emission element, whereby said radiation pattern is restricted to have said spatial distribution.

11. An x-ray source according to claim 10 wherein said power supply further includes selectively operable control means including means for selectively controlling the amplitude of said output voltage.

12. An x-ray source according to claim 1 wherein said power supply further includes selectively operable control means further includes means for selectively controlling the amplitude of said beam generator current.

13. An x-ray source comprising:

A. a power supply including drive means for establishing an output voltage having a peak value in the approximate range of 10 kV to 90 kV,

wherein the amplitude of said output voltage is a predetermined function of time;

B. beam generator means responsive to said output voltage for emitting electrons to generate an electron beam along a beam path, said beam being characterized by a current in the approximate range of 1 nA to 100 .mu.A,

wherein the magnitude of said current is a predetermined function of time;

C. a target assembly positioned in said beam path, said target including at least one x-ray emission element adapted to emit x-rays in a predetermined spectral range in response to incident electrons from said beam; and

D. field distribution means for establishing an x-ray radiation pattern having a spatial distribution, said spatial distribution being at least in part external to said source,

further comprising a closed housing, wherein said beam generator means and said target assembly are disposed within said housing, said housing having a window on one outer surface thereof, whereby said emitted x-rays are emitted through said window, and

further comprising an elongated cup-shaped sheath and associated skin entry port, said sheath and port having a biocompatible outer surface, and wherein at least the portion of said housing, including said window, is insertable into the interior of said sheath.

14. An x-ray source according to claim 13 wherein said field distribution means comprises a shield assembly including means for restricting the x-rays emitted by said emission element, whereby said radiation pattern is restricted to have said spatial distribution.

15. An x-ray source according to claim 13 wherein said power supply further includes selectively operable control means including means for selectively controlling the amplitude of said output voltage.

16. An x-ray source according to claim 13 wherein said power supply further includes selectively operable control means further includes means for selectively controlling the amplitude of said beam generator current.

17. An x-ray source comprising:

A. a power supply including drive means for establishing an output voltage having a peak value in the approximate range of 10 kV to 90 kV,

wherein the amplitude of said output voltage is a predetermined function of time;

B. beam generator means responsive to said output voltage for emitting electrons to generate an electron beam along a beam path, said beam being characterized by a current in the approximate range of 1 nA to 100 .mu.A,

wherein the magnitude of said current is a predetermined function of time;

C. a target assembly positioned in said beam path, said target including at least one x-ray emission element adapted to emit x-rays in a predetermined spectral range in response to incident electrons from said beam; and

D. field distribution means for establishing an x-ray radiation pattern having a spatial distribution, said spatial distribution being at least in part external to said source,

further comprising a closed housing, wherein said power supply, said beam generator means, and said target assembly are disposed within said housing, said housing having a window on one outer surface thereof, whereby said emitted x-rays are emitted through said window, and wherein at least a portion of said closed housing has a biocompatible outer surface.

18. An x-ray source according to claim 17 wherein said field distribution means comprises a shield assembly including means for restricting the x-ray radiation emitted by said emission element, whereby said radiation pattern is restricted to have said spatial distribution.

19. An x-ray source according to claim 17 wherein said field distribution means comprises said x-ray emission element and a beam steering assembly, wherein said x-ray emission element has a predetermined shape and said beam steering assembly includes means for steering said electron beam to selected surface regions of said emission element whereby said emission element emits an x-ray radiation pattern having said spatial distribution.

20. An x-ray source according to claim 17 further comprising temporal control means for establishing an x-ray pattern having a predetermined temporal intensity variation.

21. An x-ray source according to claim 20 wherein said temporal control means includes a programmable means for controlling time variation in the amplitude of said output voltage.

22. An x-ray source according to claim 20 wherein said temporal control means includes a programmable means for controlling time variation in the amplitude of said beam generator current.

23. An x-ray source according to claim 17 further comprising:

A. means for generating a signal representative of a desired x-ray radiation pattern;

B. means associated with said power supply and responsive to said signal to control said power supply to generate said output voltage.

24. An x-ray source according to claim 23 wherein said power supply further includes selectively operable control means including means for selectively controlling the amplitude of said output voltage.

25. An x-ray source according to claim 23 wherein said selectively operable control means further includes means for selectively controlling the amplitude of said beam generator current.

26. An x-ray source comprising:

A. a power supply including drive means for establishing an output voltage having a peak value in the approximate range of 10 kV to 90 kV,

wherein the amplitude of said output voltage is a predetermined function of time;

B. beam generator means responsive to said output voltage for emitting electrons to generate an electron beam along a beam path, said beam being characterized by a current in the approximate range of 1 nA to 100 .mu.A,

wherein the magnitude of said current is a predetermined function of time;

C. a target assembly positioned in said beam path, said target including at least one x-ray emission element adapted to emit x-rays in a predetermined spectral range in response to incident electrons from said beam; and

D. field distribution means for establishing an x-ray radiation pattern having a spatial distribution, said spatial distribution being at least in part external to said source,

wherein said target assembly includes a plurality of emission elements, and the x-ray emission characteristic of at least one of said emission elements is selectively controllable independent of each other of said emission elements.

27. An x-ray source according to claim 26 wherein each of said elements have predetermined shape characteristics.

28. A method of treating tumors in a patient, comprising the steps of:

A. identifying and locating a tumor in vivo;

B. implanting at least a portion of an adjustable x-ray radiation source in said patient proximate to said tumor; and

C. controlling said source to generate an x-ray radiation pattern, characterized by a spatial and temporal distribution, to selectively irradiate said tumor.

29. A method according to claim 28 further comprising the step of controlling said temporal distribution of said x-ray radiation pattern.

30. A method according to claim 28 further comprising the step of controlling said spatial distribution of said x-ray radiation pattern.

31. An x-ray source according to claims 1 or 3 or 4 or 10 or 13 or 17 or 26 wherein said power supply further includes selectively operable control means for selectively controlling the amplitude of said output voltage.

32. An x-ray source according to claims 1 or 3 or 4 or 10 or 13 or 17 or 26 wherein power supply further includes said selectively operable control means for selectively controlling the amplitude of said beam generator current.

33. An x-ray source according to claims 1 or 3 or 4 or 10 or 13 or 17 or 26 further comprising a diamond window allowing passage of said x-rays to a region to be irradiated.

BACKGROUND OF DISCLOSURE

The present invention relates to a miniaturized, low power, programmable x-ray source for use in delivering low-levels of substantially constant or intermittent x-rays to a specified region

Conventional medical x-ray sources are large, fixed position machines. Generally, the head of the x-ray tube is placed in one room and the control console in an adjoining area, with a protective wall, equipped with a viewing window, separating the two. The x-ray tube typically is approximately 20 to 35 centimeters (cm) long, and approximately 15 cm in diameter. A high voltage power supply is housed within a container located in a corner of the room containing the x-ray tube. Patients are brought to the machine for diagnostic, therapeutic, or palliative treatment.

Diagnostic x-ray machines are typically operated at voltages below 150 kilovolts (kV), and at currents from approximately 25 to 1200 milliamps (mA). By contrast, the currents in therapeutic units typically do not exceed 20 mA at voltages which may range above 150 kV. When an x-ray machine is operated at nominal voltages of 10 to 140 kV, the emitted x-rays provide limited penetration of tissue, and are thus useful in treating skin lesions. At higher voltages (approximately 250 kV), deep x-ray penetration is achieved, which is useful in the treatment of major body tumors. Supervoltage machines, operable in the 4 to 8 megavolt (MV) region, are used to ablate or destroy all types of tumors, except superficial skin lesions.

A conventional x-ray tube includes an anode, grid, and cathode assembly. The cathode assembly generates an electron beam which is directed to a target, by an electric field established by the anode and grid. The target in turn emits x-ray radiation in response to the incident electron beam. The radiation absorbed by a patient generally is that which is transmitted from the target in the x-ray tube through a window in the tube, taking into account transmission losses. This window typically is a thin section of beryllium, or other suitable material. In a typical x-ray machine, the cathode assembly consists of a thoriated tungsten coil approximately 2 mm in diameter and 1 to 2 cm in length which, when resistively heated with a current of 4 amps (A) or higher, thermionically emits electrons. This coil is surrounded by a metal focussing cup which concentrates the beam of electrons to a small spot on an opposing anode which also functions as the target. In models having a grid, it is the grid which both controls the path of the electron beam and focuses the beam.

The transmission of an electron beam from cathode to anode is influenced by electron space charge forces which tend to become significant in conventional x-ray machines at currents exceeding 1 A. In such conventional machines, the beam is focussed on the anode to a spot diameter ranging anywhere from 0.3 to 2.5 millimeters (mm). In many applications, most of the energy from the electron beam is converted into heat at the anode. To accommodate such heating, high power medical x-ray sources often utilize liquid cooling and a rapidly rotating anode, thereby establishing an increased effective target area, permitting a small focal spot while minimizing the effects of localized heating. To achieve good thermal conductivity and effective heat dissipation, the anode typically is fabricated from copper. In addition, the area of the anode onto which an electron beam is incident requires a material of high atomic number for efficient x-ray generation. To meet the requirements of thermal conductivity, effective heat dissipation, and efficient x-ray generation, a tungsten alloy typically is embedded in the copper.

In use, the total exposure from an x-ray source is directly proportional to the time integral of the electron beam. During relatively long exposures (e.g. lasting 1 to 3 seconds), the anode temperature may rise sufficiently to cause it to glow brightly, accompanied by localized surface melting and pitting which degrades the radiation output. However, thermal vaporization of the tube's coiled cathode filament is most frequently responsible for conventional tube failure.

While the efficiency of x-ray generation is independent of the electron beam current, it is highly dependent on the acceleration voltage. Below 60 kV, only a few tenths of one percent of the kinetic energy from an electron is converted to x-rays, whereas at 20 MV that conversion factor rises to 70 percent. An emitted x-ray spectrum is composed in part of discrete energies characteristic of transitions between bound electron energy levels of the target element. The spectrum also includes an x-ray energy continuum, known as bremsstrahlung, which is caused by acceleration of the beam electrons as they pass near target nuclei. The maximum energy of an x-ray cannot exceed the peak energy of an electron in the beam. Further, the peak of the bremsstrahlung emission curve occurs at approximately one-third the electron energy.

Increasing the electron current results in a directly proportional increase in x-ray emission at all energies. However, a change in beam voltage results in a total x-ray output variation approximately equal to the square of the voltage, with a corresponding shift in peak x-ray photon energy. The efficiency of bremsstrahlung radiation production increases with the atomic number of the target element. The peak output in the bremsstrahlung curve and the characteristic spectral lines shift to higher energies as the atomic number of the target increases. Although tungsten (Z=74) is the most common target material used in modern tubes, gold (Z=79) and molybdenum (Z=42) are used in some specialty tubes.

X-rays interact in several ways with matter. For biological samples, the following two types of interactions are most important: Compton scattering of moderate-energy x-rays with outer shell electrons; and, photoionizing interactions of inner shell electrons. In these processes, the probability of atom ionization decreases with increasing photon energy in both soft tissue and bone. For the photoelectric effect, this relationship follows an inverse third-power law.

One disadvantage of present x-ray devices used for therapy is the high voltage required when directed to soft tissue within or beneath bone. One example is in directing x-rays to areas of the human brain, which is surrounded by bone. High energy x-rays are required to penetrate the bone, but often damage the skin and brain tissue. Another example in radiation therapy is in directing the x-rays to soft tissue located within the body cavity, couched among other soft tissue, or within an internal calciferous structure. Present high-voltage machines are limited in their ability to selectively provide desired x-ray radiation to such areas.

Another disadvantage of the high voltage output of present x-ray sources is the damage caused to skin external to the affected organ or tissue. Therefore, high voltage devices of present systems often cause significant damage not only to the target region or tissue, but also to all surrounding tissue and surface skin, particularly when used for human tumor therapy. However, since present devices apply x-ray radiation to target regions internal to a patient from a source external to the target region, such incidental tissue damage is practically unavoidable.

An alternative form of tumor therapy involves implanting encapsulated radioisotopes in or near the tumor to be treated. While such use of radioisotopes may be effective in treating certain types of tumors, introduction of the isotopes requires invasive procedures which have potential side-effects, such as the possibility of infection. Moreover, brain swelling may occur in some applications because the emission from the isotope cannot be controlled. Further, there is no ability to provide selective control of time dosage or radiation intensity. Handling and disposal of such radioisotopes involves hazards to both the individual handler and the environment.

In another application, x-ray radiation is often used to inspect materials in support of structural analysis and manufacturing processes, particularly in the semiconductor chip manufacturing industry. X-ray machines for such applications are large, fixed-position machines often incorporated into the manufacturing assembly line. These cumbersome machines not only take up much physical space, but make the use of x-ray imaging impractical in many, otherwise useful, applications.

In view of the above requirements and limitations to the use of x-rays from present machines in therapeutic, diagnostic, palliative, or evaluative environments, there remains a need for a relatively small, easily manipulated, low-energy, x-ray device. Such a device operating at low energy and power will be suitable for many of the applications described herein.

Thus, it is an object of the present invention to provide an easily manipulated, low-power x-ray device.

It is another object of the invention to provide a relatively small, low-power x-ray device having a controllable, or programmable, power supply.

It is another object of the invention to provide a relatively small, low-power x-ray device which is implantable into a patient for directly irradiating a desired region of tissue with x-rays.

It is yet another object of the invention to provide a relatively small, surface-mountable, low-power x-ray device for affecting a desired surface region with x-rays.

It is yet another object of the invention to provide a relatively small, low-power x-ray device which is partially implantable into a patient for directly irradiating a specified region with x-rays.

SUMMARY OF THE INVENTION

Briefly, the invention is an easily manipulated apparatus having a low-level, electron beam (e-beam) activated x-ray source of preselected, or adjustable, duration and intensity. In medical applications, the apparatus may be fully or partially implanted into, or surface-mounted onto a desired area of a host to irradiate a preselected region with x-rays.

The apparatus operates at a relatively low voltage, for example, in the range of approximately 10 kV to 90 kV, with small electron currents, for example, in the range of from approximately 1 nA to 100 .mu.A. To achieve a desired radiation pattern over a desired region, while minimally irradiating other regions, x-rays may be emitted from a nominal, or effective "point" source located within or adjacent to the desired area to be affected. A low dose rate of x-rays irradiates any part of the desired region, either continually or periodically, over extended periods of time.

The apparatus may include a controllable, or programmable, power supply located outside the desired region to enable variations in voltage, current, and timing of x-ray radiation. The target, or x-ray emitting, material may be tailored in its composition and/or geometry to provide a customized pattern of x-rays. Shielding at the emission site, or around the target, further enables control of the energy and spatial profile of the x-ray emission to match the preselected distribution of radiation throughout the desired region.

The present invention further provides a method of treating malignant cells, such as found in tumors, in vivo. utilizing the apparatus described above. Generally, the method involves identifying and locating malignant cells with a device generally available in the art, such as by computer-aided tomography (CAT) scan or magnetic resonance imaging (MRI). Then, a low-power electron beam source and a selectively shaped x-ray radiation pattern generating target and shield assembly are positioned proximate to the malignant cells, the target and shield assembly geometry and materials being shaped and selected in accordance with the characteristics of the malignant cells. A programmable power supply is provided, which may be used to vary the voltage, current, and duration of the electron beam source to establish a desired electron beam which is directed to the target. Finally, x-rays emitted from the target and shield assembly are introduced into the malignant cells for selective destruction of the cells.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which:

FIG. 1 is a perspective view of a low power x-ray source embodying the present invention;

FIG. 2 is a schematic representation of a sheath adapted for use with the apparatus of FIG. 1;

FIGS. 3A and 3B are a perspective view and sectional view, respectively, of a surface-mountable apparatus embodying the present invention;

FIG. 4 is a schematic block diagram of the embodiment of FIG. 1;

FIGS. 5A and 5B are graphical representations of the x-ray emission spectrum of tungsten- and molybdenum-targets, respectively;

FIG. 6 is a detailed block diagram of the representative power supply of the embodiment of FIG. 1;

FIG. 7 is a detailed sche