Image analysis – Applications – Biomedical applications
Reexamination Certificate
1997-06-20
2001-05-15
Chang, Jon (Department: 2723)
Image analysis
Applications
Biomedical applications
C382S255000, C358S504000, C378S062000, C378S207000
Reexamination Certificate
active
06233349
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to apparata and methods which utilize image analysis for implementation of quality control measures, and specifically to imaging apparata and methods for quality control of X-ray tubes.
DESCRIPTION OF THE PRIOR ART
In X-ray machines and related apparata (e.g., computerized axial tomography or CAT scanners), X-ray photons are produced in an X-ray tube by directing a focused electron beam from a cathode to a rotating anode. The X-ray focal spot used to produce a diagnostic image is defined by the area of electron beam impingement on the rotating anode. As the electron beam strikes the anode, X-ray photons are emitted and collimated in a beam from the X-ray tube. The object to be imaged is placed between the X-ray tube and radiographic film or other detector device so that the film intercepts X-ray photons passing through the object. Good descriptions of the general state of the art in X-ray tube structure and operation may be found in U.S. Pat. Nos. 3,851,204; 4,052,640; 4,132,916; 4,953,190; and 5,422,527.
To produce images of high quality, a stable and properly-shaped focal spot is critical. This requires that the different parts of the X-ray tube be constructed and assembled to a high degree of precision; for example, the anode must have minimal surface irregularities and minimal unbalance about its axis of rotation, the cathode must be properly shaped and oriented toward the anode, etc. At present, the best way to determine whether an X-ray tube is properly made is to place it in use and determine the integrity of its focal spot. Standardized procedures for this determination have been developed by NEMA and the IEC, and generally involve directing the photon beam through an aperture having a particular size and configuration and onto radiographic film to obtain an image of the focal spot. Technicians can then review the focal spot image, generally with the assistance of an optical loupe, to determine whether it meets desired standards relating to its size, shape, radiation intensity, etc. If the standards are not met, the tube can be refurbished or discarded and the tube manufacturing process can be reviewed to determine whether a systematic manufacturing error is occurring.
Unfortunately, this method is unreliable in some respects. While some quality standards are easy to accurately apply to the focal spot images, the manual application of certain types of quality standards to the images is inevitably tainted by the reviewer's subjective opinion of the image quality: whether the degree of film exposure at a certain area indicates a particular photon density, where the precise edges of the image are within grey zones, etc. Further, it is difficult for technicians to adjudge multiple exposures in exactly the same manner, particularly where technician fatigue is a factor. The tendency for “drift” in the standards of image review results in erroneous rejection of quality tubes and/or acceptance of defective tubes, as well as misguided review and alterations to the tube manufacturing process.
SUMMARY OF THE INVENTION
The present invention is directed to apparata and methods for automated analysis of X-ray focal spot images and application of quality standards to their X-ray tubes. A film sample bearing an image of a focal spot created by a selected X-ray tube is imaged by imaging means, e.g., a slide scanner, to transform the focal spot image into a processable signal. Processing means, such as a personal computer and associated software, receive the processable signal and analyze it to determine parameters relating to the image. The parameters relate to at least one of the image's dimensions, its shape, and its optical density, and may include such parameters as the line spread function, the modulation transfer function, the background and/or peak optical density, the skew, and the loading of the image. After the parameters are determined, they may be compared to known parameter standards. Where the determined parameters do not conform to the known parameter standards, the X-ray tube can be rejected.
The quality of an X-ray tube can be even more thoroughly analyzed by obtaining two or more film samples, each bearing an image of the focal spot of an X-ray tube wherein the samples are obtained at different times. For example, the samples can bear images of the tube's focal spot (1) when its stator is unpowered and its anode is stationary, (2) when its stator is powered and its anode is rotating, and (3) when its stator is unpowered and its anode is coasting (i.e., its stator current was just cut off). The parameters calculated for each of these images may then be compared to ascertain other operating characteristics of the tube in question. To illustrate, a comparison of the parameters for images (1) and (3) will demonstrate how any rotational imbalance in the anode affects the parameters, and a comparison of the parameters of images (2) and (3) will demonstrate how any electromagnetic leakage between the stator and anode affects the parameters. The analysis of multiple film samples in this manner may be assisted by combining the imaging means with feeding means for supplying multiple film samples to the imaging means in succession, e.g., film feeders and slide feeders.
The primary advantages of the aforementioned apparata and methods are that the subjectivity of human image analysis is eliminated, allowing for consistent review of multiple images with quantifiable repeated error, and additionally that the use of electromechanical apparata and methods allows for a savings of time and manpower. Further features and advantages of the invention will be apparent from the following detailed description of the invention in conjunction with the associated drawings.
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Gravelle Stephen W.
Nagy Paul G.
Cabou Christian G.
Chang Jon
Dastouri Mehrdad
Dewitt Ross and Stevens
General Electric Company
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