1. Technology Field
The present invention generally relates to x-ray tubes. In particular, embodiments of the present invention are directed to x-ray tube configurations that reduce the distance between the focal spot of an anode and an adjacent end of the evacuated enclosure in which the anode is disposed.
2. The Related Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, and then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device. The x-ray tube generally comprises a vacuum enclosure, a cathode, and an anode. The cathode, having a filament for emitting electrons, is disposed within the vacuum enclosure, as is the anode that is oriented to receive the electrons emitted by the cathode.
The vacuum enclosure may be composed of metal such as copper, glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. Aside from a window region that allows for the passage of x-rays, the outer housing is typically covered with a shielding layer (composed of, for example, lead or similar x-ray attenuating material) for preventing the escape of x-rays produced within the vacuum enclosure. In addition a cooling medium, such as a dielectric oil or similar coolant, can be disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure. Depending on the configuration, heat can be removed from the coolant by circulating it to an external heat exchanger via a pump and fluid conduits.
In operation, an electric current is supplied to the cathode filament, causing it to emit a stream of electrons by thermionic emission. An electric potential is established between the cathode and anode, which causes the electron stream to gain kinetic energy and accelerate toward a target surface disposed on the anode. Upon impingement at the target surface, some of the resulting kinetic energy is converted to electromagnetic radiation of very high frequency, i.e., x-rays.
The specific frequency of the x-rays produced depends at least partly on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (“Z numbers”) are typically employed, and are usually selected based on the application and characteristic x-ray that is desired. The resulting x-rays can be collimated so that they exit the x-ray device through predetermined regions of the vacuum enclosure and outer housing for entry into the x-ray subject, such as a medical patient.
One challenge encountered with the operation of x-ray tubes, particularly tubes employed in the field of mammography, relates to the optimum positioning of the tube with respect to the patient's body (and in particular, the portion of the patient's body that is of interest) during x-ray imaging. For example, when performing a mammography, it is beneficial to position the focal spot of the x-ray tube, i.e., the point on the anode target surface where the electrons emitted and focused by the cathode impinge, as close to the chest wall as possible. Such positioning is desirable to overcome “heel effect”—a characteristic of anode-based x-ray imaging that produces non-uniformity in the imaging x-ray beam—in order to acquire as precise an image of the breast tissue as is possible. Conversely, should the focal spot be located a relatively large distance away from the chest wall, image quality will consequently suffer.
The above notwithstanding, known tube designs are not configured to minimize spacing between the chest wall and the focal spot of the anode. In particular, known tube designs are typically configured with part or all of the cathode assembly being interposed between the anode and the nearest end wall of the vacuum enclosure. This configuration, while beneficial in some respects, nonetheless prevents placement of the focal spot desirably close to the chest wall.
The above imaging challenges present with known tube designs are exacerbated when the breast or other subject to be imaged is relatively large, thereby requiring a correspondingly large anode target surface focal track angle to be employed. Use of large focal track angles undesirably increases the size of the focal spot, and therefore is undesirable for many mammography applications.
Moreover, high voltage tubes, i.e., tubes having operating voltages greater than 50 kV, may increase chest wall-to-focal spot spacing. Specifically, as operating voltage of an x-ray tube increases, the anode-to-cathode spacing requirements necessarily also increase to provide adequate voltage standoff This increased separation of the cathode from the anode target surface correspondingly increases the distance from the focal spot on the target surface to the nearest end of the x-ray tube, and thus the chest wall of the patient, thereby producing the undesirable effects discussed above.