Patent Application
20070007874
The present invention relates generally to an x-ray tube with reduced focal spot motion and more particularly to a CT x-ray tube generating a correction moment to partially counter the eccentric moment generated by the cathode assembly.
Modern medical imaging assemblies, such as x-ray tubes, are becoming increasingly powerful as their respective technologies advance. The increase in power and complexity introduces physical characteristics to the assemblies such as thermal growth that can effect the performance characteristics of the imaging assembly. Often these performance altering characteristics are related directly to increase power usage and may negatively impact image quality.
One such known effect of many x-ray tube assemblies is lateral drift of the focal spots. Under typical operating conditions, it is desirable for focal spot drift to be minimized. In addition, with the use of large and small focal spots in combination, it is often desirable for their drifts to be in opposite directions. Yet as cathode assemblies become more powerful and complex, the thermal properties in combination with the mechanical configurations make such drift control difficult. This is highly undesirably and can reduce the ability of the installation team to achieve a high level of image quality during tube calibrations.
One approach to minimizing focal spot drift would be through the development and introduction of active magnetic or electrostatic controls into the cathode assembly or directly onto the electron beam. However, such an approach would add undesirable cost increases to the system in addition to undesirable complexity. In addition, such complex solutions may be economically or structurally unfeasible for implementation onto existing imaging assemblies. Therefore, the use of magnetic control of the electron beam to steer and control the motion of the focal spots may be undesirable in many circumstances.
It would, therefore, be highly desirable to have an x-ray tube assembly that minimized the drift of focal spots without necessitating the introduction of complex and costly electronic or magnetic controls. It would additionally be highly desirable to have an x-ray tube assembly that harnessed the natural physical properties of existing tube structures to reduce the focal spot drift.
An x-ray tube assembly is provided comprising a tube casing assembly including a plurality of vertical mount posts. An insulator plate is mounted to the plurality of vertical mount posts such that the insulator plate can translate vertically on the posts. A cathode assembly is mounted to the insulator plate and generates both an eccentric moment and a vertical expansion in response to a cathode power load. A semi-compressible element is positioned between at least one of the vertical mount posts and the insulator plate. The semi-compressible element becomes incompressible at a cathode power threshold such that the vertical expansion is translated into a correction moment countering the eccentric moment.
Other features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.
Referring now to
Within the tube casing 12 is mounted a tube insert 26 including an anode backplate 28 and a target assembly 30 mounted thereto. A first tube insert wall 32 and a second tube insert wall 34 are mounted to the anode backplate 28 and are utilized to support for a plurality of flex mounts 36 acting as a flexible connection between the tube insert 26 and a cathode assembly 38. Although a variety of cathode assemblies 38 are contemplated, one embodiment contemplates a CT type cathode assembly. The cathode assembly 38 includes a tube element 40 preferably having a large filament element 42 and a small filament element 44. These filament elements 42, 44 generate a corresponding large filament focal spot 46 and small filament focal spot 48 on the target assembly 30.
When the cathode assembly 38 is powered with a cathode power load 50 in order to generate the focal spots 46,48, the cathode assembly 38 generates considerable thermal energy which generates thermal expansion 52 in addition to an eccentric moment 54 (skewing moment). The eccentric moment 54 if left unchecked can result in undesirable drift of the focal spots 46,48 and thereby result in a negative impact of imaging. The present invention, however, provides a unique mounting structure for the cathode assembly 38 that minimizes the undesirable drift without requiring complex electronic or magnetic controls.
The present invention's mounting structure includes an insulator plate 56 mounted to the plurality of vertical mount posts 24 such that the insulator post 56 can translate vertically along these posts 24. The cathode assembly 38 is mounted to the lower insulator surface 58. An insulator element 60 may be mounted to the upper insulator surface 62. A motion resistance spring 64 is mounted in between the insulator plate 56 and one of the vertical mount posts 24 on the right side of the tube casing 12 relative to the eccentric moment direction 54 (see
In contrast, the present invention includes a semi-compressible element 66 positioned between the insulator plate 56 and a vertical mount post 24 opposite the motion resistance spring 64. The semi-compressible element 66 is designed to become incompressible at a predetermined cathode power threshold. In one embodiment, this is accomplished by the use of a semi-compressible spring or a plurality of washers 66 which form an incompressible cylinder 68 at the cathode power threshold. When the semi-compressible element 66 becomes the incompressible cylinder 68, the thermal expansion 52 of the cathode assembly 38 is translated into a correction moment 70 that opposes the eccentric moment 54 generated naturally by the cathode assembly 38. Thus the correction moment 70 counteracts the eccentric moment 54 and drift is thereby minimized.
As is illustrated in
While particular embodiments of the invention have been shown and described, numerous variations and alternative embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.
