US3894239A

US3894239A - Monochromatic x-ray generator

Info

Publication number: US3894239A
Application number: US393773A
Authority: US
Inventors: Martin Braun
Original assignee: Raytheon Co
Current assignee: Varian Medical Systems Inc
Priority date: 1973-09-04
Filing date: 1973-09-04
Publication date: 1975-07-08
Anticipated expiration: 1992-07-08
Also published as: DE2441968A1; CA1003579A; JPS5448580U; DE2441968B2; JPS594439Y2; GB1478318A; DE2441968C3; NL7411735A; JPS5056196A; CH575698A5

Abstract

An x-ray generator in which a moving target is illuminated by a beam of electrons having a predetermined energy. The target is in the form of a thin foil of a material which emits fluorescent xradiation when illuminated by electrons having the predetermined energy. The target is sufficiently thin so that a substantial amount of the fluorescent radiation is emitted on the side opposite to the side being illuminated while bremsstrahlung radiation, to which the target is relatively opaque, is attenuated thereby providing for a greater percentage of radiation from the fluorescent portion of the spectrum. In addition, the target is inclined at an angle relative to an exit port of the generator for still greater attenuation of the bremsstrahlung with substantially no additional attenuation of the fluorescent radiation to which the target is substantially transparent. Highly monochromatic x-rays are thereby produced.

Description

United States Patent 1191 Braun [11] 3,894,239 [451 July 8,1975

[ MONOCHROMATIC X-RAY GENERATOR [75] Inventor:

[73] Assignee:

Martin Braun, Stamford, Conn.

Raytheon Company, Lexington,

Mass.

[22] Filed: Sept. 4, 1973 [21] Appl. No.: 393,773

[52] US. Cl. 250/493; 313/330 [51] Int. Cl. H01j 35/10 [58] Field of Search 250/403, 419, 503, 493,

[56] References Cited UNlTED STATES PATENTS 2,531,583 11/1950 Ott 1. 250/403 2,901,631 8/1959 Hansen et a1. 250/503 3,171,030 2/1965 Foster et al 250/403 3,239,706 3/1966 Farrell et al. 3131330 3,243,636 3/1966 Nineuil 3131330 3,662,210 5/1972 Maximou 313/31 1 3,697,798 10/1972 Machenschalk 313/330 3,719,854 3/1973 Sedlatschek et a1... 313/330 3,737,699 6/1973 Gager 313/330 3,790,838 2/1974 Baum 313/330 Primary Examiner-Archie R. Borchelt Assistant ExaminerB. C. Anderson Attorney, Agent, or Firm-David M. Warren; Joseph D. Pannone; Milton D. Bartlett [57] ABSTRACT An x-ray generator in which a moving target is il1uminated by a beam of electrons having a predetermined energy. The target is in the form of a thin foil of a material which emits fluorescent x-radiation when illuminated by electrons having the predetermined energy. The target is sufficiently thin so that a substantial amount of the fluorescent radiation is emitted on the side opposite to the side being illuminated while bremsstrahlung radiation, to which the target is relatively opaque, is attenuated thereby providing for a greater percentage of radiation from the fluorescent portion of the spectrum. In addition, the target is inclined at an angle relative to an exit port of the generator for still greater attenuation of the bremsstrahlung with substantially no additional attenuation of the fluorescent radiation to which the target is substantially transparent. Highly monochromatic x-rays are thereby produced.

8 Claims, 6 Drawing Figures POWER SUPPLY PkiTEl-ITFDJUL :3 1975 SHEET SHEEI FILAMENT HQ 2 E PHOTON ENERGY IN kev WAVELENGTH {N A P 2 I o 1 MONOCI-IROMATIC X-RAY GENERATOR BACKGROUND OF THE INVENTION A standard form of x-ray generator utilizes a rotating anode or target, typically of a heavy element such as tungsten or gold, which is illuminated by a beam of electrons accelerating through a difference of potential between a cathode and the anode. The beam of electrons is focussed to impinge upon a small portion of the target to provide the generator with the characteristics of a point source of x-radiation. As is well known. a point source of radiation produces distinct roentgenographs or photographs while a broader source of radiation having a lower spatial frequency bandwidth provides less clear images.

A problem arises in that the beam of electrons impinging upon the target excites a broad spectrum of frequencies of x-radiation with the attendant less-thanoptimal clarity of the roentgenographs. Ideally, roentgenographs should be taken with x-radiation of a single frequency, or monochromatic radiation, since the differing frequencies of a broad spectrum radiation interact differently with the material of a subject such as the flesh and bones of a human subject.

SUMMARY OF THE INVENTION The aforementioned problem is overcome and other features are provided by an x-ray generator in accordance with the invention which provides increased monochromaticity to x-radiation emitted by a target illuminated by a beam of high energy electrons. The increased monochromaticity is attained by utilizing a target in the form of a thin foil or deposition, on the order of IO to 40 micrometers, of a material such as cerium or molybdenum which have a pronounced fluorescent line in their radiation spectrum. The target is relatively opaque to bremsstrahlung having photon energies above the absorption edge while being relatively transparent to the fluorescent radiation and bremsstrahlung at lower values of photon energy. This difference in the relative attenuations of the radiations at frequencies above the fluorescent frequency as compared to radiations at and below the fluorescent frequency is utilized to provide improved monochromaticity to the radiation emitted from the generator as follows. The depth of the target is sufflciently thin to serve as a transmissive target which, when illuminated on one side thereof by the electrons, emits a substantial portion of the radiation from the surface of the target opposite the side which is illuminated by the electrons. A movable support is provided for orienting the target at an angle relative to an exit port of the generator so that radiation is emitted at a glancing angle to the surface of the target on the side of the target opposite the side illuminated by the electrons. Due to this oblique orientation of the target, radiation emitted in the direction of the exit port must traverse a greater distance within the material of the target, with the result that bremsstrahlung is severely attenuated while the fluorescent radiation emerges with substantially no attenuation. Soft x-rays having frequencies substantially lower than the fluorescent radiation are attenuated by a window in the exit port which is substantially opaque to these soft x-rays. Thus, the emergent radiation has a greatly reduced spectral width with substantial intensities of radiation being produced at and near to the fluorescent frequency. The supporting structure for the target is both thermally and electrically conductive to provide for the cooling of the target as well as the establishment of a difference of electrical potential between the target and a cathode.

BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned aspects and other advantages of the invention are explained in the following description taken in connection with the accompanying drawings wherein:

FIG. I shows a sectional view of an x-ray generator which comprises a target inclined at an oblique angle to a viewing port in accordance with the invention for providing monochromatic x-radiation;

FIG. 2 shows an embodiment of the invention where a source of x-rays is positioned off to the side of a rotating target in accordance with the invention;

FIG. 3 shows a third embodiment in which the beam of x-rays is parallel to the axis of rotation of the target while the exit port is positioned off to the side of the target;

FIG. 4 is a typical spectrum of x-radiation of a material exhibiting fluorescent radiation; and

FIGS. 5 and 6 show the angular dependency of the purity and relative intensity of the emitted radiation of molybdenum and cerium targets for use in the generator of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I there is seen a generator 20 which provides monochromatic x-rays 22 in accordance with the invention. The generator 20 comprises a housing 24 of a metal such as lead which is opaque to x-radiation and which has a window 26 which serves as an exit port for the x-radiation and is composed of a material such as aluminum or beryllium. An anode assembly 28 includes a circular dish-shaped member 30 of a material such as beryllium or graphite which is transparent to xradiation and is rotatably supported upon a shaft 32 which is, in turn, rotated by a motor 34. The member 30 is enclosed by an evacuated envelope 36 of a material such as glass which is transparent to xradiation. A thin target 38 in the form of a band is deposited upon the inner surface of the member 30 adjacent the periphery thereof and, upon rotation of the member 30, is passed through a beam 40 of electrons which are accelerated from a cathode 42 to the target 38 under a difference of electrical potential established by a power supply 44 which is coupled via the

terminals

46 and 48, respectively. to the anode assembly 28 and the cathode 42, the power supply 44 being shown diagrammatically.

The anode assembly 28 is electrically conducting for coupling the voltage of the power supply 44 to the target 38. In addition, the dish-shaped member 30 is thermally conducting for cooling the target 38. The cathode 42 is supported and positioned by a strut 50 which also contains electrical conductors (not shown) for coupling electrical voltage from the power supply 44 to the cathode 42. In addition, the cathode 42 encloses a heater or filament (not shown) which is, in turn, energized by a filament supply (not shown).

In accordance with the invention, the target 38 is po sitioned by the member 30 at an oblique angle relative to the window 26. Thereby, a portion of the radiation emitted by the target 38 in response to bombardment by the electron beam 40 is emitted in the direction of the window 26. The depth of the target 38 is on the order of ID to 40 micrometers which is a sufficient depth to stop the electrons of the beam 40. X-radiation is produced at the surface of the target 38 facing the cathode 42, with a portion of that radiation propagating through the target 38 to exit on its far side in the direction of the window 26. Due to the oblique orientation of the target, which orientation is at typically an angle of 70-85 depending on the particular material utilized in the target 38, the rays emitted through the window 26 must pass through a distance wihtin the target 38 much greater than the depth of the target 38. The target 38 is formed, in one embodiment of the invention, of molybdenum which is a source of strong fluorescent x-radiation. Upon illumination by the electron beam 40, the target 38 provides both bremsstrahlung and fluorescent x-radiation, a portion of both of these radiations being emitted in the direction of the window 26. Thus, both these radiations traverse a distance within the target 38 which is substantially greater than its depth, and, since the molybdenum is significantly more opaque to bremsstrahlung of photon energies greater than the photon energies of the fluorescent radiation, the bremsstrahlung is severely attenuated during its passage through the target 38 while the fluorescent radiation is not so attenuated. Softer x-rays having photon energies substantially below the energies of the fluorescent radiation are attenuated and substantially stopped by the aluminum window 26. Thus, the x-rays 22 which have propagated through the window 26 are substantially monochromatic in that only the radiations at the fluorescent frequency and frequencies approximating the fluorescent frequency are passed through the window 26.

Referring now to FIG. 2 there is seen a sectional view of a portion of an alternative embodiment of the generator of FIG. I, here indicated by the reference numeral 20A. X-rays 52 are seen exiting from a window 26A in a housing 24A which are similar to the window 26 and housing 24 of FIG. 1. A target 54 is supported on a member 56 which is rotated about a shaft 58 with the aid of a suitable motor such as the motor 34 of FIG. I. The shaft 58 and member 56 are electrically conducting to couple voltage from the power supply 44A to the target 54. Electron optics 60 comprising a cathode 62 and grid 64 which is shown as an annular grid that does not intercept electrons of an electron beam 66 has provided for directing electrons of the electron beam 66 from the cathode 62 to the target 54. A filament 68 or heater is energized by a filament supply 70 and heats the cathode 62 for emission of the electrons. A difference of potential is established between the cathode 62 and the target 54 by the power supply 44A and a grid pulser circuit 72 provides a difference of potential between the grid 64 and the cathode 62 for pulsing ON and pulsing OFF the electron beam 66.

In this embodiment of the invention, the target 54 is oriented at an oblique angle relative to the window 26A in a manner analogous to the orientation of the target 38 relative to the window 26 of FIG. 1. As in the embodiment of FIG. I, the axis of the electron beam is normal to the target. Radiation emitted from the target 54 of FIG. 2 propagates at a glancing angle relative to the surface of the target 54 thereby providing for attenuation of the bremsstrahlung as was explained with reference to FIG. 1. Thus, it is seen that the x-rays S2 of 4 FIG. 2 are monochromatic as are the x-rays 22 of FIG. 1.

Referring now to FIG. 3 there is shown a partial view of a third embodiment of the generator 20 of FIG. 1, here indicated by the legend 20B. A member 74 is rotated about a shaft 76 and supports a target 78 positioned such that a normal to the surface of the target 78 is slightly angled relative to the axis of the shaft 76. Electron optics 60 are positioned for directing an electron beam 66 normal to the surface of the target 78. A housing 80, partially shown, has a window 82 to serve as an exit port for x-rays 84 emitted at a glancing angle relative to the surface of the target 78. And, for the same reasons as explained with reference to the em bodiment of FIG. 2, the x-rays 84 are highly monochromatic.

Referring now to FIG. 4 there is shown the spectrum of molybdenum as measured at an exit angle of relative to a normal to the surface of the target such as the target 38 of FIG. 1. As indicated on the figure, measurements are shown for target thicknesses of IO micrometers and 20 micrometers as well as the back scatter situation corresponding to an infinite target thickness. The x-ray yield is plotted on the vertical axis while the photon energy of the x-rays is plotted on the horizontal axis. The fluorescent radiation occurs in a pair of lines at approximately I8 keV (kiloelectron volts). The highest purity is obtained for the measurement at a target thickness of 20 micrometers while the lowest purity is obtained for the back scatter condition where the purity is the ratio of the intensity of the radiation at the fluorescent frequencies to the total intensity of the radiation throughout the spectrum. A spectrum for cerium is similar to that for molybdenum.

As an example of the use of the generator 20 of FIG. 1, it is convenient to consider the situation where angiography is utilized for examining a human subject. In angiography, a dye such as iodine is commonly administered to the patient for purposes of absorbing xradiation to better define the shadow cast by an organ or blood vessel as compared with the shadow of other tissue which has absorbed a differing amount of the iodine dye. In this situation, cerium or an oxide thereof known as ceria is chosen for use in the target 38 based on the fact that the x-ray emission spectrum of cerium advantageously matches the absorption spectrum of iodine. The fluorescent emission lines of cerium (resulting from an electron of an outer shell of the cerium atom falling into a void in an inner shell due to bombardment by electrons from the cathode 42) occur at essentially the peak of the x-ray absorption curve for iodine in the well-known graphical portrayal of the x-ray absorption of iodine as a function of the energy in electron volts of the radiation to be absorbed. In this way, the choice of cerium in the target 38 and iodine in the subject cooperate to provide a well-defined image of the patient, the definition being due to the monochromaticity of the radiation incident upon the patient and the choice of the energy or frequency of the incident radiation to be equal to the energy or frequency at the peak of the absorption spectrum of the dye which has been administered to the patient.

The target 38 comprises a material composed of the lower atomic numbered x-radiating elements such as cerium or molybdenum which produce a more pronounced K emission line than a high atomic numbered element such as tungsten. This results in a higher intensity of the softer x-rays, typically 34 keV as used in angiography, being produced directly in response to electron bombardment of the target. For example, in the case of a 20 micron thick layer of nolybdenum, the k emission lines are at 17.5 keV. With illumination by 35-40 keV electrons, more than 95% of the total radiation is concentration in the range l4-2O keV photon energy. Illumination of a 40 micron thick layer of cerium with 70 keV electrons generates a spectrum, as viewed at an angle of 80 relative to a normal to the surface of the cerium layer, which contains 70% of its energy in the range 33-40 keV. This cerium emission spectrum corresponds to the maximum absorption region of the iodine spectrum thus making iodine an ideal radiographic dye for use with a cerium x-ray source.

Referring now to FIGS. 5 and 6 there are seen graphs portraying the purity of the x-ray spectrum obtained from an x-ray target, such as the target 38 of FIG. 1, as a function of the viewing angle by which the emitted radiation is observed, the viewing angle being measured from the normal to the surface of the target. FIG. 5 represents data obtained for a target comprised of a 20 micron (micrometer) thick layer of molybdenum while FIG. 6 shows a similar graph obtained for a 40 micron thick layer of cerium. Also shown in FIGS. 5 and 6 are the intensity of the radiation of the k or lines and the bremsstrahlung. The curve representing the purity represents the intensity measured at the ka lines divided by the total intensity of the bremsstrahlung plus the ka line radiations. It is noted that the purity curve peaks up in the range of approximately 70 to 85 this peaking effect on the purity curve being one of the reasons for inclining the surface of the target 38 relative to the axis of the x-ray tube. Thus, the purity curve is a measure of the monochromaticity of the emitted radiation.

It is noted that the depth of the target has been selected to be thin enough to be substantially transparent to its own fluorescent radiation while having sufficient depth to attenuate the bremsstrahlung. In this film or foil-like target the width thereof is many times greater than its depth. By orienting the target at an oblique angle relative to the exit port, a thinner target may be utilized since those rays of radiation which are directed towards the exit port travel through a greater distance in the inclined target than do rays directed normally to the surface of the target. Improved monochromaticity has been observed in the situation in which a normal to the target surface is directed towards the exit port; however, still greater monochromaticity has been observed with the inclined target, apparently because bremsstrahlung directed normally to the surface and having energies above the fluorescent energy excite further sites of fluorescent radiation on or near the side of the target opposite the electron bombardment. Radiation emanating from these further sites experiences less attenuation than does the fluorescent radiation from the side illuminated by the electrons with the result that the obliquely oriented target provides still further monochromaticity.

A further advantage of the oblique orientation of the target is the relatively small projected area of the target as viewed from the exit port. Thus a relatively large 6 area of the target may be illuminated by the electron beam providing greater power to the resulting radiation while the radiation exiting through the exit port appears to emanate from a relatively small source approximating a point source.

A further embodiment of the invention contemplates the fabrication of the target by diffusing the target material into the material of the supporting structure or substrate. For example, with respect to FIG. 1, the target 38 would consist primarily of rhenium which would be diffused in an oven at 2500C into the dish-shaped member 30 which would consist primarily of graphite. A major portion of the target would then be a mixture of rhenium in a graphite matrix which, because of the relatively low atomic number of a major constituent, the graphite, would permit penetration of the bom barding electrons to a greater depth than pure rhenium. The heat produced by the stopping of the electrons is thus distributed over a greater volume thus providing increased cooling capacity and permitting a higher intensity electron flux. The diffusion technique provides a strong bond between the target and substrate.

The dish-shaped member 30 of FIG. I, as has been described previously, is fabricated from a low atomic numbered material. Suitable materials are beryllium, boron, carbon, aluminum and combinations of these elements with each other and with other low atomic number elements. With respect to the fabrication of the target 38, elements from zirconium (atomic number 40) to palladium (atomic number 46), or mixtures thereof, which produce radiographs of good contrast of soft tissue may be utilized. The lanthanides (atomic numbers 57 to 71), or mixtures thereof, may also be utilized. These elements have fluorescent lines in their spectra which are readily absorbed in iodine or barium used in contrast agents.

It is understood that the above-described embodiments of the invention are illustrative only and that modifications thereof will occur to those skilled in the art. Accordingly, it is desired that this invention is not to be limited to the embodiments disclosed herein but is to be limited only as defined by the appended claims.

What is claimed is:

1. An x-ray generator comprising:

means for generating a beam of electrons having a predetermined energy;

a target comprising a material which emits fluorescent x-radiation when illuminated on a first side thereof with electrons of said predetermined energy, the width of said target being many times greater than its depth, at least a portion of said radiation being emitted from a second side of said target opposite said first side;

a housing opaque to said radiation enclosing said target, said housing having a window transparent to said radiation;

means for moving said target through said beam, said moving means including means for orienting said target such that said second side thereof is angled relative to said window to permit only those rays of said radiation emitted at a glancing angle from said second side to exit said housing.

2. A generator according to claim 1 wherein the depth of said target is in the range of approximately [0 to 40 micrometers.

3. A generator according to claim 2 wherein the material of the target includes an element having an atomic number drawn from the range of atomic numbers of approximately 40-46 and 57-7 l.

4. In combination:

a target comprising a material which emits fluorescent x-radiation when illuminated on a first side thereof with electrons of a predetermined energy, at least a portion of said radiation being emitted from a second side of said target opposite said first side;

means for moving said target through said beam of electrons; and

window means for transmitting the portion of the rays of said radiation emitted in a glancing direction from said second side of said target.

5. A combination according to claim 4 wherein said selecting means comprises a housing opaque to said radiation enclosing said target. said housing having a window transparent to said radiation emitted in said direction.

6. A combination according to claim 5 wherein said moving means includes means for positioning said target such that said second side thereof is oriented obliquely relative to said window.

7. A combination according to claim 6 wherein said positioning means is in the form of a substrate for supporting said target, said target being formed by diffusing said fluorescent material into said substrate.

8. A combination according to claim 7 wherein said oblique orientation is an angle in the range of to between a normal to the surface of said second side of said target and a normal to the surface of said window. 1

Claims

1. An x-ray generator comprising: means for generating a beam of electrons having a predetermined energy; a target comprising a material which emits fluorescent xradiation when illuminated on a first side thereof with electrons of said predetermined energy, the width of said target being many times greater than its depth, at least a portion of said radiation being emitted from a second side of said target opposite said first side; a housing opaque to said radiation enclosing said target, said housing having a window transparent to said radiation; means for moving said target through said beam, said moving means including means for orienting said target such that said second side thereof is angled relative to said window to permit only those rays of said radiation emitted at a glancing angle from said second side to exit said housing.

2. A generator according to claim 1 wherein the depth of said target is in the range of approximately 10 to 40 micrometers.

3. A generator according to claim 2 wherein the material of the target includes an element having an atomic number drawn from the range of atomic numbers of approximately 40-46 and 57-71.

4. In combination: a target comprising a material which emits fluorescent x-radiation when illuminated on a first side thereof with electrons of a predetermined energy, at least a portion of said radiation being emitted from a second side of said target opposite said first side; means for moving said target through said beam of electrons; and window means for transmitting the portion of the rays of said radiation emitted in a glancing direction from said second side of said target.

5. A combination according to claim 4 wherein said selecting means comprises a housing opaque to said radiation enclosing said target, said housing having a window transparent to said radiation emitted in said direction.

8. A combination according to claim 7 wherein said oblique orientation is an angle in the range of 70* to 85* between a normal to the surface of said second side of said target and a normal to the surface of said window.