Patent Application
20080107238
The disclosure relates generally to X-ray imaging systems, X-ray apparatus and X-ray targets. The disclosure also relates to methods for manufacturing X-ray systems, X-ray apparatus and X-ray targets.
X-ray imaging systems typically include an X-ray apparatus operable to generate a beam of X-rays, a detection apparatus, and a control system connected to the X-ray apparatus and detection apparatus. The X-ray apparatus produces a beam of X-rays which interact with a subject and are detected by operation of the detection apparatus. One typical example of an X-ray imaging system is a high performance computed tomography (CT) X-ray imaging system, which accommodates a human subject for medical imaging. Medical X-ray imaging systems typically include a gantry which is movable in relation to the human subject.
X-ray apparatus typically include an X-ray tube which is operable to generate a beam of X-rays. A typical X-ray tube includes a housing which forms an evacuated chamber. The housing supports inside the chamber a cathode assembly with a cathode filament. A high voltage electrical circuit is formed between the cathode and an anode assembly supported inside the housing. The anode assembly includes an X-ray target spaced from the cathode filament. The X-ray target includes a generally disk-shaped target cap. The target cap is formed of a high conductivity refractory metal, such as an alloy of molybdenum. An annular focal track on the front surface of the target cap includes a suitable X-ray emitting material, such as a chemical species of high atomic weight, of a type which interacts with high energy electrons to emit X-rays. The X-ray target also includes a heat sink affixed to a rear surface of the target cap. The heat sink receives intense heat conducted away from the focal track and substrate. Typically, the heat sink is formed of an annular block of graphite brazed to the rear surface of the target cap. The target cap is supported for rotation about a longitudinal axis. High speed rotation of the X-ray target is driven by a rotor connected to a drive motor.
For an imaging scan, the electrical circuit energizes the cathode filament to generate high energy electrons which impinge upon the focal track of the X-ray target. Interactions between the electrons and high atomic weight species in the focal track emit high frequency electromagnetic waves, or X-rays. X-rays directed through a window in the chamber housing are focused on a subject for imaging purposes. The electron interactions release intense heat into the focal track and target cap. The X-ray target is rotated by the motor at high speed in order to avoid overheating. Heat is also conducted out of the focal track into the substrate, and then into the heat sink. Heat dissipates from the heat sink through evacuated space in the chamber and into the housing. The housing is cooled by immersion in an external fluid bath.
Conventional X-ray targets presently possess material densities ranging from about 90.0% to about 95.0% of theoretical density. X-ray targets possessing material densities ranging from about 90.0% to about 95.0% of theoretical density are hindered by remaining porosity and porosity variation. X-ray targets can be produced by a “PSF” method by cold pressing (P) a form of substrate material and X-ray emitting material, sintering (S) the cold pressed form, and forging (F) the sintered form to desired shape. X-ray targets produced by the PSF method can possess material densities ranging from about 90.0% to about 95.0% of theoretical density. X-ray targets produced by the PSF method can be hindered by limited density, density variations, remaining porosity, porosity variations, limited mechanical strength properties, variation of mechanical strength properties, limited thermal conductivity, limited thermo-mechanical properties, limited thermal loading capacity, limited mechanical loading capacity. Examples of specific properties limited by the foregoing include: resistance to creep, tensile strength, compressive strength, thermal conductivity, bulk modulus, yield strength, mass per unit diameter, X-ray target diameter, thermal durability per unit of mass, mechanical durability per unit of mass, fatigue resistance, resistance to fatigue crack growth, resistance to crack growth, focal track life, and focal track performance. X-ray apparatus including X-ray targets having the foregoing limitations are hindered by limited capacity to operate at peak power, limited X-ray target rotation speed, limited gantry rotation speed, limited X-ray output at peak power, limited frequency of exposures at peak power, longer cooling periods between exposures, and limited cycle rate.
The specified limitations of X-ray targets produced by the PSF method can worsen as diameter of the X-ray target increases. Targets produced by the PSF method can suffer CTE mismatched bending stress or warpage because of differences between material properties of the focal track and the substrate material supporting the focal track. X-ray targets produced by the PSF method are hindered by the limitation that microstructure of the substrate and focal track materials is not highly controlled and, thus, variations of material properties such as microstructure and variation of microstructure are not optimal and are subject to great variation.
For reasons stated above and for other reasons which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improved X-ray targets, X-ray apparatus, and X-ray imaging systems, and for improved methods of manufacturing the same.
The above-mentioned shortcomings, disadvantages and problems are addressed herein, as will be understood by those skilled in the art upon reading and studying the following specification.
In one aspect, systems, apparatus, and methods are provided through which X-ray imaging systems, X-ray apparatus, X-ray tubes, anode assemblies, and X-ray targets include a target cap formed of substrate material and a focal track layer formed of emitting material, and at least one of the substrate material and the emitting material has a density greater than about 95.0% of theoretical density.
In one aspect, systems, apparatus and methods are provided through which an X-ray target includes a target cap formed of substrate material and a focal track layer of emitting material, and at least one of the substrate material is dense substrate material having a final density greater than an intermediate density, or the emitting material is dense emitting material having a final emitting material density greater than an intermediate emitting material density.
Apparatus, systems, and methods of varying scope are described herein. In addition to the aspects and advantages described in this summary, further aspects and advantages will become apparent by reference to the following drawings, detailed description and claims.
In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments and disclosure. It is to be understood that other embodiments may be utilized, and that logical, mechanical, electrical, and other changes may be made without departing from the scope of the embodiments and disclosure. In view of the foregoing, the following detailed description is not to be taken as limiting the scope of the embodiments or disclosure.
Illustrated in
X-ray imaging system 100 includes a control system 120. X-ray imaging system 100 also includes an X-ray apparatus 140. X-ray apparatus 140 is connected to control system 120 and is operable to generate a beam of X-rays for imaging a subject (not shown). X-ray imaging system 100 also includes a detection apparatus 160. Detection apparatus 160 is connected to control system 120 and is operable to detect X-rays which can interact with the subject (not shown). In some specific arrangements, the X-ray apparatus 140 may include a movable gantry (not shown) connected to the control system 120 and operable for movement along a prescribed path.
Illustrated in
The X-ray tube 200 includes a glass or metal envelope or housing 210. Inside the housing 210 exists a vacuum or evacuated space having a reduced pressure of about 10.sup.-5 to about 10.sup.-9 torr. A cathode assembly 220 including a cathode filament 230 is supported inside the housing 210. The cathode filament 230 is connected to a selectively operable electrical circuit (not shown). The electrical circuit is connected to an anode assembly 240 supported inside the housing 210. The anode assembly 240 includes an X-ray target 250 spaced a fixed distance from the cathode assembly 220 along a longitudinal axis 255 (see
According to one embodiment, the substrate material 486 is dense substrate material 488. According to an embodiment, dense substrate material 488 has a density greater than or equal to about 95.0% of theoretical density. According to one embodiment, dense substrate material 488 has a density greater than or equal to about 96.0% of theoretical density. According to one embodiment, dense substrate material 488 has a density greater than or equal to about 97.0% of theoretical density. According to one embodiment, dense substrate material 488 has a density greater than or equal to about 98.0% of theoretical density. According to one embodiment, dense substrate material 488 has a density greater than or equal to about 99.0% of theoretical density. As used herein, “density” means the minimum density within the subject material.
Referring to
The focal track 480 is formed of emitting material 490 in a focal track layer 620 on the front surface 400 of the substrate material 486. Focal track layer 492 extends between the inner focal track edge 484 and outer focal track edge 482 in an annulus on the front surface 400. The focal track layer 492 of emitting material 490 is formed on the front surface 400 of the dense substrate material 488 in a suitable manner. In one embodiment, the focal track layer 492 is formed by depositing the emitting material 490 on the substrate material 486 by powder coating, plasma spraying, electroplating, chemical vapor deposition or physical vapor deposition.
According to one embodiment, the emitting material 490 is dense emitting material 494. According to one embodiment, dense emitting material 494 has a density greater than or equal to about 95.0% of theoretical density. According to one embodiment, dense emitting material 494 has a density greater than or equal to about 96.0% of theoretical density. According to one embodiment, dense emitting material 494 has a density greater than or equal to about 97.0% of theoretical density. According to one embodiment, dense emitting material 494 has a density greater than or equal to about 98.0% of theoretical density. According to one embodiment, dense emitting material 494 has a density greater than or equal to about 99.0% of theoretical density. As used herein and specified above, “density” means the minimum density within the subject material.
Referring to
The rear surface 277 of target cap 260 is generally parallel and in spaced opposition to front surface 400. Heat sink 270 is integrally affixed to rear surface 277 in thermal communication with dense substrate material 488. The heat sink 270 receives intense heat conducted away from the focal track 480 and front surface 400 through the dense substrate material 488. In one embodiment, the heat sink 270 is formed of an annular block of graphite 275. In one embodiment, the heat sink 270 is formed of suitable material having sufficiently high heat capacity and thermal emission to rapidly dissipate intense heat and sufficient mechanical strength to endure high speed rotation through repeated heating and cooling cycles. In one embodiment, the heat sink 270 is integrally affixed to the rear surface 277 by brazing. In one embodiment, the heat sink 270 is integrally affixed to the rear surface 277 by diffusion bonding.
Embodiments of the disclosure provide an X-ray imaging system 100, X-ray apparatus 140, X-ray tube 200, anode assembly 240, X-ray target 250 and target cap 260 as follows. An embodiment provides an X-ray target including a target cap having increased mechanical strength without decreased thermal conductivity. An embodiment provides an X-ray target including a target cap having increased mechanical strength and increased thermal conductivity. An embodiment provides an X-ray target including a target cap having increased tensile strength. An embodiment provides an X-ray target including a target cap having increased resistance to creep. An embodiment provides an X-ray target including a target cap having reduced porosity. An embodiment provides an X-ray target including a target cap having reduced variations of porosity. An embodiment provides an X-ray target including a target cap having increasingly consistent mechanical properties. An embodiment provides an X-ray target including a target cap having improved thermal and mechanical life per unit of mass. An embodiment provides an X-ray target including a target cap having improved capacity to endure increased thermal and mechanical loading. An embodiment provides an X-ray target including a target cap having reduced mass per unit diameter. An embodiment provides an X-ray target including a target cap having increased capacity to operate at increased peak power, and thus to produce an increased output of X-rays at peak power. An embodiment provides an X-ray target including a target cap having increased capacity to operate with more frequent exposures at peak power and shorter cooling periods between exposures. An embodiment provides an X-ray target including a less massive target cap capable of enduring increased rotation speeds and potentially being of greater diameter. An embodiment provides an X-ray target including a target cap capable of enduring increased gantry rotation speeds. An embodiment provides an X-ray target including a target cap of improved bulk modulus. An embodiment provides an X-ray target including a target cap of increased yield strength. An embodiment provides an X-ray target including a target cap of increased fatigue resistance. An embodiment provides an X-ray target including a target cap of increased resistance to fatigue crack growth. An embodiment provides an X-ray target including a target cap having emitting material of increased resistance to fatigue crack growth in the focal track layer. An embodiment provides an X-ray target including a target cap having substrate material of increased resistance to fatigue crack growth in the substrate material. An embodiment provides an X-ray target including a target cap having emitting material of increased resistance to fatigue crack growth in the axial direction in the focal track layer. An embodiment provides an X-ray target including a target cap having substrate material of increased resistance to fatigue crack growth in the axial direction in the substrate material. An embodiment provides an X-ray target including a target cap of increased resistance to crack growth. An embodiment provides an X-ray target including a target cap having emitting material of increased resistance to crack growth in the focal track layer. An embodiment provides an X-ray target including a target cap having substrate material of increased resistance to crack growth in the substrate material. An embodiment provides an X-ray target including a target cap having emitting material of increased resistance to crack growth in the axial direction in the focal track layer. An embodiment provides an X-ray target including a target cap having substrate material of increased resistance to crack growth in the axial direction in the substrate material. An embodiment provides an X-ray target including a target cap of increased thermal conductivity. An embodiment provides an X-ray target including a target cap having increased focal track life. An embodiment provides an X-ray target including a target cap having increased focal track performance. An embodiment provides an X-ray target including a target cap having reduced radiation output losses over the life of the X-ray target. An embodiment provides an X-ray target including a target cap having reduced surface roughening over the life of the X-ray target.
An embodiment of the disclosure provides various improvements, benefits, advantages, features and solutions which will be described in further detail, as follows. X-ray targets in X-ray imaging systems such as computed tomography (CT) systems can be formed with a relatively large diameter target cap and focal track in order to accommodate increased peak power loads and thus provide increased X-ray output and image resolution. The diameter of X-ray targets can be limited by mechanical factors, such as limitations of the mechanical strength, thermal conductivity, and thermo-mechanical durability of the target cap substrate material and emitting material. In X-ray imaging systems such as computed tomography (CT) systems, a gantry rotates at approximately three revolutions per second around a patient and an anode assembly including the X-ray target rotates at approximately 100 to 200 revolutions per second. These rotations create large forces on the X-ray target and target cap that increase exponentially as the diameter and mass of the target cap and X-ray target increase. X-ray targets in X-ray imaging systems can also have a limiting mechanical factor in the thermal conductivity of the target cap substrate material and emitting material. The target cap substrate material and emitting material must be able to conduct heat at specified rates in order to be capable of emitting X-ray energy at a related minimum rate. Limits on the rate of emitting X-ray energy in turn limits the maximum number of imaging scans per unit of time, or usage rate, at which X-ray images can be made by the X-ray imaging system, and thus limits the usefulness of such X-ray imaging systems. During periods of continuous usage of some systems, the maximum usage rate at peak power can also be limited by the length of time required between exposures to adequately dissipate heat from the anode assembly. Operating an X-Ray system repeatedly or continuously at or in excess of the maximum usage rate can cause premature failure of X-ray tube components, especially the X-ray target. Temperatures reached in adjoining components decreases as those components are located increasingly distant from the focal track. Additionally, in order to rapidly dissipate heat from the heat sink, it is effective to rotate the X-ray target at high speed. However, other limitations frequently are prohibitive of continuously rotating the X-ray target in order to dissipate heat. In ordinary use, if the X-ray target and rotor were allowed to continue to rotate between exposures, the bearings would wear rapidly and fail prematurely. Thus, under certain circumstances of ordinary use dictating an excessive time delay between exposures, the X-ray system control system rapidly slows or stops the rotor and X-ray target in a period of seconds. When ready to initiate a scan, the control system returns the rotor and X-ray target to operational rotation speed as quickly as possible. Rapid acceleration and rapid deceleration are utilized because, among other reasons, there are a number of resonant frequencies that must be avoided during acceleration and braking. During such rapid acceleration and rapid braking, mechanical stresses and thermal stresses impact the components of the anode assembly. Embodiments of the disclosure provide X-ray imaging systems, X-ray apparatus, anode assemblies, X-ray targets, target caps, and methods for producing the same, having improvements, benefits, advantages, features and solutions which address the foregoing issues.
In the previous section, apparatus embodiments were described. In the present section, and by reference to the accompanying series of flowcharts, are described methods for manufacturing X-ray targets according to embodiments of the disclosure. It is to be understood that embodiments other than those specifically described herein are possible. It is to be understood that methods according to embodiments provide X-ray imaging systems, X-ray apparatus, X-ray tubes, anode assemblies, and X-ray targets having the same features, improvements and benefits described above in reference to the apparatus embodiments. It will be understood by those skilled in the art that X-ray targets are readily manufactured using target caps produced by methods according to the embodiments. It is to be understood that methods according to the embodiments can readily be adapted by one skilled in the art to produce target caps, X-ray targets, anode assemblies, X-ray tubes, X-ray apparatus and X-ray imaging systems.
Method 500 also includes compacting 504 the intermediate target cap form of substrate material and the focal track layer of emitting material by application of gas pressure at elevated temperature for a time period to form a final target cap form of dense substrate material having a final density greater than the intermediate density and a focal track of dense emitting material having a final emitting material density greater than the intermediate emitting material density. According to an embodiment, at least one of the substrate material and the emitting material is densified. As used herein, “densified” means that the subject material has a final density greater than a preceding intermediate density. According to one embodiment, at least one of the substrate material is dense substrate material having a final density greater than the intermediate density or the emitting material is dense emitting material having a final emitting material density greater than the intermediate emitting material density. According to one embodiment, only one of the substrate material and the emitting material is present during compacting 504. As used herein, “dense substrate material” means a dense substrate material formed in compacting 504 and which has a final density greater than the intermediate density. As used herein, “dense emitting material” means a dense emitting material formed in compacting 504 and which has a final emitting material density greater than the intermediate emitting material density. Suitable gases are inert gases or reducing gases. The ranges of gas pressure, temperature and time period may vary, as further described below.
According to an embodiment, the final density of the substrate material and final emitting material density are greater than or equal to about 95.0% of theoretical density. According to an embodiment, the final density and final emitting material density are greater than or equal to about 96.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 97.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 98.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 99.0% of theoretical density. According to one embodiment, at least one of the substrate material and the emitting material has a respective final density or final emitting material density as specified in the preceding.
In one embodiment, compacting 504 includes hot isostatic pressing. As used herein, hot isostatic pressing means compacting a form of substrate material and emitting material by application of gas pressure, at homologous temperature, for a time period to form dense substrate material having a final density greater than an intermediate density and dense emitting material density having a final emitting material density greater than an intermediate emitting material density. As used herein, “homologous temperature” means the ratio, on an absolute temperature scale, of process temperature to the melting point of a material. According to one embodiment, only one of the substrate material and the emitting material is present during hot isostatic pressing. According to one embodiment, either or both of the substrate material and the emitting material are in the form of powder before compacting 504. According to one embodiment, compacting 504 includes: compacting the intermediate target cap form of substrate material and emitting material by application of gas pressure between about 35 MPa and about 500 MPa, at homologous temperature (Th) between about 0.3 of the lowest melting point component and about 0.8 of the highest melting point component, for a time period. In one embodiment, the time period ranges from at least about 1 minute to at least about 100 hours. In one embodiment, the time period ranges from at least about 1 minute to about 100 hours. In one embodiment, the time period ranges from at least about 30 minutes to about 100 hours. In one embodiment, the time period ranges from at least about 4 hours to about 100 hours. It is to be understood that the ranges of pressure, temperature and time period can vary in embodiments.
According to one embodiment, method 500 includes mechanically working 506 the final target cap form to impart work into the dense substrate material and the dense emitting material. Imparting mechanical work into the dense substrate material and dense emitting material forms or influences desired properties, such as desired grain size and more uniform grain size distribution.
According to one embodiment, method 500 also includes final machining 508 the final target cap form to predetermined dimensions.
According to an embodiment, the final density of the substrate material and final emitting material density are greater than or equal to about 95.0% of theoretical density. According to an embodiment, the final density and final emitting material density are greater than or equal to about 96.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 97.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 98.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 99.0% of theoretical density. According to one embodiment, at least one of the substrate material and the emitting material has a respective final density or final emitting material density as specified in the preceding.
According to an embodiment, the final density of the substrate material and final emitting material density are greater than or equal to about 95.0% of theoretical density. According to an embodiment, the final density and final emitting material density are greater than or equal to about 96.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 97.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 98.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 99.0% of theoretical density. According to one embodiment, at least one of the substrate material and the emitting material has a respective final density or final emitting material density as specified in the preceding.
According to an embodiment, the final density of the substrate material and final emitting material density are greater than or equal to about 95.0% of theoretical density. According to an embodiment, the final density and final emitting material density are greater than or equal to about 96.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 97.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 98.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 99.0% of theoretical density. According to one embodiment, at least one of the substrate material and the emitting material has a respective final density or final emitting material density as specified in the preceding.
According to an embodiment, the final density of the substrate material and final emitting material density are greater than or equal to about 95.0% of theoretical density. According to an embodiment, the final density and final emitting material density are greater than or equal to about 96.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 97.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 98.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 99.0% of theoretical density. According to one embodiment, at least one of the substrate material and the emitting material has a respective final density or final emitting material density as specified in the preceding.
According to an embodiment, the final density of the substrate material and final emitting material density are greater than or equal to about 95.0% of theoretical density. According to an embodiment, the final density and final emitting material density are greater than or equal to about 96.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 97.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 98.0% of theoretical density. According to one embodiment, the final density and final emitting material density are greater than or equal to about 99.0% of theoretical density. According to one embodiment, at least one of the substrate material and the emitting material has a respective final density or final emitting material density as specified in the preceding.
X-ray targets, X-ray apparatus, and X-ray imaging systems according to embodiments of the disclosure are described. Although specific embodiments are illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose can be substituted for the specific embodiments shown. This application is intended to cover any adaptations or variations of the embodiments and disclosure. For example, although described in terminology and terms common to the field of X-ray imaging systems, X-ray apparatus and X-ray targets, one of ordinary skill in the art will appreciate that implementations can be made for other systems, apparatus or methods that provide the required function.
In particular, one of ordinary skill in the art will readily appreciate that the names of the methods and apparatus are not intended to limit embodiments or the disclosure. Furthermore, additional methods, steps, and apparatus can be added to the components, functions can be rearranged among the components, and new components to correspond to future enhancements and physical devices used in embodiments can be introduced without departing from the scope of embodiments and the disclosure. One of skill in the art will readily recognize that embodiments are applicable to future X-ray imaging systems, X-ray apparatus, anode assemblies, X-ray targets, target caps, different substrate materials, and different emitting materials.
Terminology used in the present disclosure is intended to include all environments and alternate technologies which provide the same functionality described herein.
