X-ray tube and method for generating x-rays in the x-ray tube

Abstract: A method for generating X-rays in an X-ray tube (10), comprises the steps of rotating an X-ray target (17) of a rotating anode (16), the X-ray target having a metal coated layer (23) thereon, of applying electron beams (18) emitted from a cathode (15) onto the metal coated layer (23) of the X-ray target, and of offsetting thermal deformation of the X-ray target (17) due to the application of said electron beams by deformation of the X-ray target (17) due to centrifugal force, thereby maintaining a position of the X-ray target (17) in a direction of the application of the electron beams, at a room temperature of the X-ray target, thus generating the X-rays.

Description: The present invention relates to a method for generating X-rays in an X-ray tube used for X-ray computed tomographs and the like, and to an X-ray tube for performing such a method. In order to reduce diagnosis times using an X-ray computed tomograph and to improve the resolution of the image obtained by the tomograph, it has been necessary to use an X-ray target, on a rotating anode in an X-ray tube, of sufficient size to increase the heat capacity of the X-ray tube. For example, there has been a need for an X-ray target operable at an average temperature of about 1,200°C. In a conventional X-ray tube, there was a problem that, if the X-ray target was so large as to increase the heat capacity of the tube, the X-ray target was considerably deformed by heat at its side opposite to the side against which electron beams were applied, with the result that the effective amount of X-rays obtained by the X-ray tube was not increased even though its heat capacity was being increased. Further, if the heat capacity of the X-ray tube is increased, the cooling capacity of the X-ray target must also be increased; to this end, the X-ray target must be rotated at a high speed, of the order of about 10,000 r.p.m. If a conventional X-ray target, being reasonable heavy, is rotated at such high speeds, the bearings supporting the target are subjected to a considerable load, causing wear of the bearings and resulting in eccentric rotation of the X-ray target. This inconvenience also gives rise to the problem that the effective amount of X-rays obtained by the X-ray tube is not increased as its heat capacity is increased. Further, in this case, there also arises a problem that the voltage which the X-ray tube is capable of withstanding is reduced due to metal powder worn from the bearings. An X-ray target having a base made of graphite, to reduce its weight, is already known. However, such a target has previously been used at rotation speeds less than about 5,000 r.p.m.; thus, since high speed rotation of such a conventional light-weight X-ray target has never been considered, the target is likely to suffer from cracking, etc ... if rotated at higher speeds of about 10,000 r.p.m. Such an X-ray target cannot therefore be used in safety. Other prior art includes EP-A-168736, in which a rotating anode is described for use in an X-ray tube, the anode comprising an X-ray target which has a base including parallel upper and lower surfaces, a metal coated layer, a central hole, an inclined surface and a recess in the base. JP-A-61,66349 describes a rotary anode target for an X-ray tube, the target being made from a composite material. In EP-A-37,956, there is shown a rotating anode of which the body may comprise a two-layer construction of molybdenum and graphite, the body also including a recess in its base. An object of the present invention is to provide a method for generating X-rays in an X-ray tube wherein the heat capacity of an X-ray target of a rotating anode in the X-ray tube can be increased. Another object of the present invention is to provide an X-ray tube for carrying out the method of the present invention, wherein the effective amount of X-rays can be increased in proportion to the increase in the heat capacity of the tube, without undue reduction of the voltage which the tube is able to withstand. A further object of the present invention is to provide an X-ray target for use as part of a rotating anode in an X-ray tube, which can increase the heat capacity of the X-ray tube and can be rotated at a high speed corresponding to the increase in heat capacity. A method for generating X-rays in an X-ray tube, according to the present invention, comprises the steps of rotating an X-ray target of a rotating anode, said X-ray target having a metal coated layer thereon, and of applying electron beams emitted from a cathode onto said metal coated layer of the X-ray target, said X-ray target having a base and a metal coated layer for generating X-rays when it receives electron beams, said base including an upper surface, a lower surface substantially parallel to said upper surface, a central hole formed in a central portion of said base, and an annular inclined surface formed on said upper surface, the inclined surface being coaxial with said central hole and inclined toward an outer periphery of said base so as to reduce the thickness of said base at said outer periphery, the method being characterised in that: said base of said X-ray target includes a recess formed in said lower surface, coaxial with said central hole and having a depth such that the ratio (Tm/T), where Tm is the average thickness of a target portion situated below said annular inclined surface and T is the depth of said central hole, has a value of between 1.2 and 1.6; and the method further comprises the step of offsetting thermal deformation of the X-ray target, due to the application of said electron beams, by deformation of the X-ray target due to centrifugal force, thereby maintaining the room-temperature position of said X-ray target in the direction of application of the electron beams. This method makes it possible to increase the heat capacity of the X-ray tube. An X-ray tube according to one aspect of the present invention comprises a sealed envelope, an X-ray bulb arranged in said sealed envelope, a cathode arranged in said X-ray bulb, an X-ray target arranged in said X-ray bulb, and a rotating mechanism for rotating said X-ray target, and wherein said X-ray target has a base and a metal coated layer for generating X-rays when it receives electron beams, said base including an upper surface, a lower surface substantially parallel to said upper surface, a central hole formed in a central portion of said base, and an annular inclined surface formed on said upper surface, the inclined surface being coaxial with said central hole and inclined toward an outer periphery of said base so as to reduce the thickness of said base at said outer periphery, characterised in that: said base includes a recess formed in said lower surface, coaxial with said central hole and having a depth such that the ratio (Tm/T), where Tm is the average thickness of a target portion situated below said annular inclined surface and T is the depth of said central hole, has a value of between 1.2 and 1.6. According to another embodiment of the present invention, the X-ray tube comprises an annular disc fixed to the lower surface, the disc being coaxial with said central hole and having a thickness such that the ratio (Tm/T), where Tm is the average thickness of a base portion situated between an inner diameter and an outer diameter of said annular inclined surface and T is the depth of said central hole, has a value of between 1.2 and 1.6, said recess being defined at least in part by an aperture provided in the disc. An X-ray target for use as part of a rotating anode in an X-ray tube, according to another aspect of the present invention, comprises a base and a metal coated layer for generating X-rays when it receives electron beams, wherein said base includes an upper surface, a lower surface substantially parallel to said upper surface, a central hole formed in a central portion of said base, and an annular inclined surface formed on said upper surface, the inclined surface being coaxial with said central hole and inclined toward an outer periphery of said base so as to reduce the thickness of said base at said outer periphery, characterised in that: said base comprises a recess formed in said lower surface, coaxial with said central hole and having a depth such that the ratio (Tm/T), where Tm is the average thickness of a base portion situated below said annular inclined surface and T is the depth of said central hole, has a value of between 1.2 and 1.6. According to one embodiment of this aspect of the present invention, the X-ray target comprises an annular disc fixed to the lower surface, the disc being coaxial with said central hole and having a thickness such that the ratio (Tm/T), where Tm is the average thickness of a base portion situated between an inner diameter and an outer diameter of said annular inclined surface and T is the depth of said central hole, has a value of between 1.2 and 1.6, said recess being defined at least in part by an aperture provided in the disc. In the drawings: Fig. 1 is a schematic sectional view of an X-ray tube according to the present invention; Fig. 2 is a schematic sectional view of a rotating anode used with the X-ray tube of Fig. 1; Fig. 3 is a sectional view of the right half of an X-ray target, for explaining a method for generating X-rays according to the present invention; Fig. 4 is a graph showing the relationship between the ratio of the depth T of a central hole of an X-ray target according to the present invention to an average thickness Tm of an annular inclined portion of the target, and the stress distribution along the central hole of the X-ray target; Fig. 5 is a sectional view of an X-ray target according to an embodiment of the present invention; Fig. 6 is a graph showing the stress distribution along the central hole of the X-ray target shown in Fig. 5; Fig. 7 is a sectional view of an X-ray target according to another embodiment of the present invention; Fig. 8 is a graph showing the stress distribution along the central hole of the X-ray target shown in Fig. 7; Fig. 9 is a sectional view of an X-ray target according to a further embodiment of the present invention; and Fig. 10 is a sectional view of an X-ray target according to yet another embodiment of the present invention. The present invention will now be explained with reference to the drawings. As shown in Figs. 1 and 2, an X-ray tube 10 includes an X-ray bulb 12 arranged in a sealed envelope 11. Around the X-ray bulb 12, the interior of the sealed envelope 11 is filled with cooling medium 13. The sealed envelope 11 has an X-ray emission window 14 formed therein, through which the X-rays are emitted. In the X-ray bulb 12, there are arranged a cathode 15 for emitting an electron beam 18 and a rotating anode 16 onto which the electron beam 18 is applied. The rotating anode 16 has an X-ray target 17 and a rotor 19 for rotating the X-ray target 17. A stator 20 is arranged around the X-ray bulb 12 in a position opposed to the rotor 19. An open end of the sealed envelope 11 is sealed closed by a rubber lid 21. The X-ray target 17 includes a base 22 and a metal coated layer 23 which can emit X-rays when it receives the electron beam 18. The base 22 is mainly made of graphite, and the metal coated layer 23 comprises tungsten or a rhenium/tungsten alloy. The base 22 includes an upper surface 24, a lower surface 25 substantially parallel to the upper surface, and a central hole 26 into which a rotatable shaft 27 is inserted, the base 22 being fixed to the rotatable shaft 27 by means of an appropriate fastening means such as nut 28. An annular inclined surface 29 is formed on the upper surface 24 of the base, coaxial with the central hole 26. The annular inclined surface is inclined toward an outer periphery of the base 22 so as to reduce the thickness of the base towards the outer periphery. Preferably, the annular inclined surface is inclined at an angle of 8° - 12°. The metal coated layer 23 is deposited on the annular inclined surface 29 by means of a chemical vapour deposition process or the like. If the thickness of the coated layer 23 is more than 0.6 mm, the number of revolutions (of the target) at which failure occurs becomes less than 15,000 r.p.m.; thus, in this case, a sufficient safety factor cannot be ensured at a practical revolution speed of 10,000 r.p.m. On the other hand, if the thickness of the coating layer 23 is less than 0.2 mm, excessive heat will be transmitted to the base; thus, in this case, the service life of the rotatable shaft 27 is considerably reduced. Accordingly, the thickness of the metal coated layer 23 is preferably in the range of 0.2 mm - 0.6 mm. A method for generating X-rays according to the present invention will now be explained with reference to Fig. 3. When the X-ray target 17 is operated at an average temperature of about 1,200°C, as shown by a broken line in Fig. 3, the X-ray target is deformed by heat towards the side (the lower side in Fig.3) opposite to the side (of the target) to which the electron beams 18 are applied. Consequently, the inclination angle of the metal coated layer 23 changes, so that the effective amount of X-rays emitted from the X-ray emission window 14 is reduced. In the present invention, by positively utilizing a centrifugal force created by the high speed rotation of the target, the X-ray target 17 is deformed towards the side (the upper side in Fig. 3) to which the electron beams 28 are applied, in order to cancel or offset the thermal deformation of the target, thus maintaining the room-temperature position of the X-ray target 17, in the direction of application of the electron beam 18. In this manner, the metal coated layer 23 is maintained at a proper inclination, so that it is possible to increase the effective amount of X-rays emitted in proportion to an increase in the heat capacity of the X-ray target. The offsetting of the thermal deformation of the X-ray target by centrifugal deformation (due to the centrifugal force) may be effected by adjusting the rotational speed of the X-ray target 17. Alternatively, such offsetting may be effected by adjusting the conditions for applying the electron beam 18 onto the metal coated layer 23. An X-ray target for carrying out the above-mentioned method will now be described. In order to deform the X-ray target using centrifugal force towards the side on which the electron beams are applied, a recess 30 is formed in the lower surface 25 of the base 22, coaxial with the central hole 26. Fig. 4 shows measurement results or data obtained by measuring the distribution of resultant stress, consisting of the circumferential thermal stress acting on the base 22 and the centrifugal stress created when the X-ray target 17 is rotated at a speed of 10,000 r.p.m. along the central hole 26, at different values of the ratio Tm/T, where Tm is the average thickness of a base portion 22a situated between an inner diameter and an outer diameter of the annular inclined surface 29 (i.e., a base portion situated under the annular inclined surface 29) and T the depth of the central hole 26, in order to determine the depth of the recess 30. In Fig. 4, for the ratio Tm/T = 1.2, the relationship is shown by a one-dot chain line; for Tm/T = 1.4, the relationship is shown by a solid line; and for Tm/T = 1.6, the relationship is shown by a broken line. Although it is preferable that the magnitude of the resultant stress is uniformly distributed along the central hole 26, the inventor of the present invention has found that it is permissible for the distribution of the resultant stress to have a value within ±10% of the average value of the resultant stress. As is apparent from Fig. 4, when Tm/T is 1.2 - 1.6, the thickness of the central hole 26 (in other words, the depth of the recess 30) permits the magnitude of the resultant stress along the central hole 26 to fall within the permissible range. Fig. 5 shows an embodiment of the X-ray target according to the present invention. The base 22 of the target is made of graphite, the depth of the recess 30 is 8 mm, and the ratio Tm/T is 1.2. The distribution of the circumferential resultant stress (of the base 22) along the central hole 26 is shown in Fig. 6. As seen in Fig. 6, the distribution of the resultant stress is in the range of ±10% of the average value of the resultant stress and is uniformly distributed. This means that the base 22 is merely deformed in a radial direction and the inclination of the metal coated layer 23 is maintained at its original level (i.e. its inclination at room temperature), thus emitting X-rays from the X-ray tube effectively. Fig. 7 shows another embodiment of the X-ray target wherein the base 22 is made of graphite, the depth of the recess 30 is 20 mm and the ratio Tm/T is about 1.5. The distribution of the circumferential resultant stress (of the base 22) along the central hole 26 is shown in Fig. 8. As seen in Fig. 8, the distribution of the resultant stress is in the range of ±10% of the average value of the resultant stress, as intended by the inventor. Fig. 9 shows a further embodiment of the X-ray target according to the present invention, wherein the base 22 comprises an upper layer made of a composite material including ceramics 32 of silicone carbide and graphite 31, and an annular disc 33 of graphite fixed to an undersurface of the upper layer. The annular disc 33 has a thickness such that the ratio Tm/T, where Tm is the average thickness of the base portion 22a situated between the inner diameter and the outer diameter of the annular inclined surface 29 and T the depth of the central hole 26, is 1.2- 1.6. According to this embodiment, since the base is made of a composite material, its strength can be greater than that of a graphite base. Fig. 10 shows another embodiment of the X-ray target of the invention, wherein the base 22 comprises a lower layer 35 made of graphite and an upper thin layer 34 made of molybdenum. According to this embodiment, the weight of the base 22 is slightly increased by the provision of the thin molybdenum layer 34; however, the strength of the base is still larger than that of a graphite base.

Claims: A method for generating X-rays in an X-ray tube, comprising the steps of rotating an X-ray target (17) of a rotating anode (16), said X-ray target having a metal coated layer (23) thereon, and of applying electron beams (18) emitted from a cathode (15) onto said metal coated layer (23) of the X-ray target, said X-ray target (17) having a base (22) and a metal coated layer (23) for generating X-rays when it receives electron beams (18), said base including an upper surface (24), a lower surface (25) substantially parallel to said upper surface (24), a central hole (26) formed in a central portion of said base, and an annular inclined surface (29) formed on said upper surface (24), the inclined surface being coaxial with said central hole (26) and inclined toward an outer periphery of said base (22) so as to reduce the thickness of said base (22) at said outer periphery, the method being characterised in that: said base (22) of said X-ray target (17) includes a recess (30) formed in said lower surface (25), coaxial with said central hole (26) and having a depth such that the ratio (Tm/T), where Tm is the average thickness of a target portion (22a) situated below said annular inclined surface (29) and T is the depth of said central hole, has a value of between 1.2 and 1.6; and the method further comprises the step of offsetting thermal deformation of the X-ray target (17), due to the application of said electron beams, by deformation of the X-ray target (17) due to centrifugal force, thereby maintaining the room-temperature position of said X-ray target (17) in the direction of application of the electron beams. A method according to claim 1, wherein said step of offsetting the thermal deformation of the X-ray target (17) by deformation of the X-ray target due to centrifugal force is effected by adjusting the rotational speed of said X-ray target (17). A method according to claim 1, wherein said step of offsetting the thermal deformation of the X-ray target (17) by deformation of the X-ray target due to the centrifugal force is effected by adjusting an application condition of the electron beams (18). A method according to claim 1, wherein said step of offsetting the thermal deformation of the X-ray target (17) by deformation of the X-ray target due to the centrifugal force is effected by deforming said X-ray target (17), by centrifugal force, toward a side of the X-ray target to which said electron beams are applied. An X-ray tube (10) comprising a sealed envelope (11), an X-ray bulb (12) arranged in said sealed envelope (11), a cathode (15) arranged in said X-ray bulb (12), an X-ray target (17) arranged in said X-ray bulb (12), and a rotating mechanism (19,20) for rotating said X-ray target, and wherein said X-ray target (17) has a base (22) and a metal coated layer (23) for generating X-rays when it receives electron beams (18), said base (22) including an upper surface (24), a lower surface (25), substantially parallel to said upper surface (24), a central hole (26) formed in a central portion of said base, and an annular inclined surface (29) formed on said upper surface (24), the inclined surface being coaxial with said central hole (26) and inclined toward an outer periphery of said base (22) so as to reduce the thickness of said base (22) at said outer periphery, characterised in that: said base (22) includes a recess (30) formed in said lower surface (25), coaxial with said central hole (26) and having a depth such that the ratio (Tm/T), where Tm is the average thickness of a target portion (22a) situated below said annular inclined surface (29) and T is the depth of said central hole (26), has a value of between 1.2 and 1.6. An X-ray tube according to claim 5, wherein said annular inclined surface (29) is inclined at an angle of 8° - 12°. An X-ray tube according to claim 5 or claim 6, wherein said metal coated layer (23) has a thickness of 0.2 mm - 0.6 mm. An X-ray tube according to any one of claims 5-7, wherein said base (22) of said X-ray target (17) is made of graphite. An X-ray tube according to any one of claims 5-7, wherein said base (22) of said X-ray target (17) is made of a composite material including ceramics (32) of silicone carbide and graphite (31). An X-ray tube according to any one of claims 5-7, wherein said base (22) of said X-ray target (17) has a two-layer construction including an upper layer (34) made of molybdenum and a lower layer (35) made of graphite. An X-ray tube (10) according to any one of claims 5-10, wherein said base (22) includes an annular disc (33) fixed to said lower surface (25), the disc being coaxial with said central hole (26) and having a thickness such that the ratio (Tm/T), where Tm is the average thickness of a base portion (22a) situated between an inner diameter and an outer diameter of said annular inclined surface (29) and T is the depth of said central hole (26), has a value of between 1.2 and 1.6, said recess (30) being defined at least in part by an aperture provided in the disc (33). An X-ray tube according to claim 11, wherein said annular disc (33) is made of graphite. An X-ray target (17) for use as part of a rotating anode (16), the target comprising a base (22) and a metal coated layer (23) for generating X-rays when it receives electron beams (18), wherein said base (22) includes an upper surface (24), a lower surface (25) substantially parallel to said upper surface (24), a central hole (26) formed in a central portion of said base, and an annular inclined surface (29) formed on said upper surface (24), the inclined surface being coaxial with said central hole (26) and inclined toward an outer periphery of said base (22) so as to reduce the thickness of said base (22) at said outer periphery, characterised in that: said base (22) comprises a recess (30) formed in said lower surface (25), coaxial with said central hole (26) and having a depth such that the ratio (Tm/T), where Tm is the average thickness of a base portion (22a) situated below said annular inclined surface (29) and T is the depth of said central hole (26), has a value of between 1.2 and 1.6. An X-ray target according to claim 13, wherein said annular inclined surface (29) is inclined at an angle of 8° - 12°. An X-ray target according to claim 13 or claim 14, wherein said metal coated layer (23) has a thickness of 0.2 mm - 0.6 mm. An X-ray target according to any one of claims 13-15, wherein said base (22) of said X-ray target (17) is made of graphite. An X-ray target according to any one of claims 13-15, wherein said base (22) of said X-ray target (17) is made of a composite material including ceramics (32) of silicone carbide and graphite (31). An X-ray target according to any one of claims 13-15, wherein said base (22) of said X-ray target (17) has a two-layer construction including an upper layer (34) made of molybdenum and a lower layer (35) made of graphite. An X-ray target according to any one of claims 13-18, wherein said base (22) comprises an annular disc (33) fixed to said lower surface (25), the disc being coaxial with said central hole (26) and having thickness such that the ratio (Tm/T), where Tm is the average thickness of a base portion (22a) situated between an inner diameter and an outer diameter of said annular inclined surface (29) and T is the depth of said central hole (26), has a value of between 1.2 and 1.6, said recess (30) being defined at least in part by an aperture provided in the disc (33). An X-ray target according to claim 19, wherein said annular disc (33) is made of graphite.

Inventors: BABA NOBORU, INAMURA ICHIRO, KAGOHARA HIROMI, MIYOSHI TADAHIKO, NAKAGAWA YUSAKU, NISHIHARA MOTOHISA, SHIMADA SATOSHI, TANAKA AKIRA, YAMADA KAZUJI, BABA, NOBORU, INAMURA, ICHIRO, KAGOHARA, HIROMI, MIYOSHI, TADAHIKO, NAKAGAWA, YUSAKU, NISHIHARA, MOTOHISA, SHIMADA, SATOSHI, TANAKA, AKIRA, YAMADA, KAZUJI
Applicants: HITACHI LTD, HITACHI MEDICAL CORP, HITACHI MEDICAL CORPORATION, HITACHI, LTD.
Application year: 1988
Main classification: H01J35
IPC codes: H01J35