The present application relates to the x-ray tube arts. The invention finds particular application in x-ray tube assemblies for large bore computed tomography scanners. It is to be appreciated, that the present invention finds further application in other x-ray devices where it is desirable to incorporate beam shapers and off-focal radiation vanes into the x-ray tube assembly itself and in those where a wider scan area is desirable while maintaining overall device size.
Some computed tomography (CT) scanning applications require a large opening in the center of the gantry A central bore with a diameter of 85-95 cm provides flexibility in the placement of a patient within the gantry. The diameter of the reconstruction field of view (FOV) may be increased to 60 cm or greater. CT gantries used current in medical practice generally have a 70 cm bore diameter and a reconstruction field size of 50 cm.
Future interventional and robotic surgery applications will require space for access to the patient inside the bore. Room to manipulate various probes and apparatus will be necessary. The scanner may be required to exhibit excellent low contrast image quality without excessive radiation dose to the patient.
X-rays from conventional rotating anode x-ray tubes are typically emitted from the edge of the anode nearest the patient. When the gantry bore diameter is 80 cm or larger, the available space between the x-ray source and patient, normally used for beam shaping filters, is reduced or is unavailable. A reduction of size or elimination of beam shaping filters may result in higher radiation dose to the patient.
Also, the distance between the focal spot and the primary collimator is shortened. The shorter focal spot-to-collimator distance results in a more diffuse x-ray projection on the patient and the radiation detectors because of the increased penumbra. Narrow slice thickness scan options are not available and the radiation dose to the patient will be greater.
Undesirably, accommodations for these deficiencies results in physically larger CT gantries. A large bore, narrow slice, high performance CT scanner requires that the focal spot is optimally positioned with respect to the scanner""s iso-center and pre-patient collimator. One consequence of moving the x-ray tube further from iso-center is an increase in the diameter of the scanner. A commercial scanner will be unable to fit through a standard hospital door. Another consequence in a fourth generation CT scanner is to require a larger and more expensive ring of detectors. Similar consequences are true of third generation CT scanners designed with large central openings.
Another detracting feature is increased off-focal radiation projection of the detectors. If not corrected, the off-focal radiation causes blurring of the reconstructed tissue near boundaries of high contrast objects. For example, brain tissue near the skull can be improperly reconstructed.
The present invention contemplates an improved method and apparatus which overcomes the above-referenced problems and others.
In accordance with one embodiment of the present invention, an x-ray tube assembly includes a housing with a vacuum envelope and an x-ray permeable exit window. An anode is positioned within the vacuum envelope oriented such that one side is nearer to the exit window than the other or far side. A cathode assembly is also positioned within the vacuum envelope which directs an electron beam toward a focal spot on the far side of the anode.
One advantage of the present invention resides in the focal spot being located further from the CT scanner""s iso-center without increasing the diameter of the detector ring or increasing the size of the scanner.
Another advantage of the present invention resides in a larger diameter rotating anode usable to permit higher instantaneous x-ray loading.
Another advantage of the present invention resides in the off-focal vane restricting the view of the anode by a detector, hence the viewed off-focal contribution of the radiation from the x-ray tube is reduced.
Yet another advantage of the present invention resides in increased space within the bore of the CT apparatus.
Still another advantage of the present invention resides in the ability to position x-ray attenuating filters as close as possible to the focal spot reducing the penumbra effect.
Still another advantage of the present invention resides in the easy removal of heat generated by back scattered electrons landing on the rear wall of the tube housing.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.