Radiant energy – Invisible radiant energy responsive electric signalling – With or including a luminophor
Reexamination Certificate
1999-01-25
2001-06-26
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
With or including a luminophor
C250S367000
Reexamination Certificate
active
06252231
ABSTRACT:
TECHNICAL FIELD
The present invention relates to x-ray detector systems for computed tomography (CT) scanners.
BACKGROUND OF THE INVENTION
CT scanners are typically used to obtain images of internal anatomical structures of a patient, or of objects inside containers, such as luggage or packages for transport, which cannot easily be identified other than through time-consuming manual inspections. X-rays are projected into the object to be scanned in a fan-shaped beam, and x-rays passing through the object are detected by an x-ray detector system disposed on the opposite side of the object from the source of the x-rays. The intensity of the detected x-rays is inversely proportional to the density of the structures in the path of the x-rays. An image of the scan plane can be reconstructed from the x-ray intensity data. Reconstructed images of successive scan planes can be integrated to form a three-dimensional image of the object.
X-ray detector systems for CT scanners typically include a plurality of scintillating crystals which are responsive to x-rays, a corresponding plurality of photodiodes which receive light generated by the crystals in response to x-radiation thereof and convert the light to electrical signals, and a cable or other connector for transmitting the signals from the photodiodes to a data acquisition system for reconstructing an image from the x-ray intensity data.
For greatest accuracy and resolution in the reconstructed image, light generated within the scintillator crystals should all be transmitted to the corresponding photodiodes with minimum losses from absorption and scattering. The scintillating crystals in CT scanners generally are therefore covered with some type of light reflecting medium, such as, for example, a white- or light-colored paint containing, for example, titanium dioxide.
In addition to obtaining maximum light output from a scintillating crystal array, it is desirable to achieve maximum x-ray absorption by the spaces between adjacent crystals so as to prevent x-rays from passing directly to the photodiodes and generating random noise. Hence, the crystals may additionally be covered with an x-ray absorbing medium such as, for example, lead oxide. However, lead oxide is relatively dark in color and thus absorbs light instead of reflecting light. Therefore, its use with a light-reflective medium reduces the efficacy of the light-reflective medium and results in light losses which can be significant.
X-ray detector arrays typically are assembled with corresponding arrays of anti-scatter plates to reduce the amount of scattered x-rays which enter the scintillator crystals. The anti-scatter plates are aligned to be substantially parallel to x-ray beams emanating from the focal spot of the x-ray source and are typically disposed over the spaces between adjacent crystals. Positioned in this manner, the anti-scatter plates absorb scattered radiation and also shield the spaces between adjacent crystals, thereby minimizing x-ray passage into the photodiodes. However, in two-dimensional detector arrays, the crystals are arranged in multiple columns and rows, and the spaces between adjacent crystals in the direction normal to the plane of the fan beam are not shielded by anti-scatter plates and thus are exposed to x-rays. If light-reflective and x-ray absorbent media are used together, as known in the prior art, light absorption is increased and light reflection decreased, as discussed above. Because the light-reflective and x-ray absorbing medium between the crystals is generally much less dense than the material comprising the scintillating crystals, at least a portion of the x-rays that impinge on the spaces between adjacent crystals will likely pass through to the photodiodes below the crystals and generate random noise signals which reduce the sensitivity of the scanner and the accuracy of the reconstructed image.
It would therefore be an advantage to provide an x-ray detector system which maximizes light output by the scintillator crystals while preventing passage of x-rays into the photodiodes.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is provided an x-ray detector system, including a plurality of scintillating crystals arranged in an array, a corresponding plurality of photodiodes arranged so that each photodiode receives light energy from a scintillating crystal and generates a representative electrical signal, and an electrical connector for transmitting the electrical signals from each photodiode to a data acquisition system. The x-ray detector system further includes a light reflective, x-ray absorbent medium disposed in the spaces between adjacent scintillating crystals.
The light reflective, x-ray absorbent medium preferably comprises, in one embodiment, a mixture of an optically transparent curable vehicle and tantalum pentoxide. In a preferred embodiment, the substantially transparent curable vehicle is an epoxy.
The ratio of the amount of tantalum pentoxide to the amount of epoxy by weight in the mixture is preferably at least approximately 1:1.
The scintillating crystals can be arranged in any type of one- or two-dimensional array.
These and other objects and advantages of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, the scope of which will be indicated in the claims.
REFERENCES:
patent: 3936645 (1976-02-01), Iversen
patent: 4747973 (1988-05-01), Cusano et al.
patent: 4982096 (1991-01-01), Fujii et al.
patent: 5059800 (1991-10-01), Cueman et al.
patent: 5382798 (1995-01-01), Mouyen
patent: 5440129 (1995-08-01), Schmidt
Analogic Corporation
Hannaher Constantine
McDermott & Will & Emery
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