X-ray detector offering an improved light yield

Details X-ray detector offering an improved light yield X-ray detector offering an improved light yield

2001-07-13

2003-12-02

Gagliardi, Albert (Department: 2878)

Sheet-material associating

Associating or disassociating

Signature associating

C250S370090, C250S370110

Reexamination Certificate

active

06655675

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to an X-ray detector for converting X-rays into electric charges, including a photosensor arrangement and a scintillator arrangement that is situated on a first side of the photosensor arrangement.
BACKGROUND OF THE INVENTION
Dynamic X-ray detectors are used in the medical diagnostic field. They are considered to be universal detector components that can be employed in various application-specific X-ray apparatus. An important feature in this respect is the possibility of acquisition of X-ray images and X-ray image sequences with low doses. Therefore, the aim is to achieve an as high as possible signal-to-noise ratio for the X-ray detector.
X-ray quanta emitted by a radiation source are absorbed in contemporary X-ray detector technology by a scintillator arrangement (for example, CsI) which is arranged on a photosensor matrix consisting of thin-film electronics. The light arising in the scintillator arrangement is detected by a respective photosensor for each matrix cell (pixel). The photodiode does not occupy the entire surface area of the pixel. A part thereof is occupied by a thin-film transistor as well as by signal leads and control leads; moreover, openings of a width of approximately 10 &mgr;m exist between the pixels and leads for the purpose of insulation. Light going past the photodiodes via said openings does not contribute to the generation of signals. On the basis of the geometrical relationships this light can be estimated to be approximately 10% of the total amount of light applied in the direction of the photosensor arrangement by the scintillator layer. A coarse measurement has revealed a light loss of approximately 8% through the openings. This light penetrates the region underneath the photodiodes via a substrate plate. The uncontrolled scattering of light must be prevented in this region, because light that is detected in pixels that are situated far from its origin has an adverse effect on the resolution.
Underneath the substrate plate, being situated at the second side of the photosensor arrangement and being realized, for example as a glass plate, there are situated illumination means which emit light for the optical resetting of the pixels. This resetting light is also referred to as “backlight” or “reset light”. Such reset light passes through the openings between the photodiodes from the direction of the second side of the photosensor arrangement, is reflected or scattered by the scintillator layer and is picked up by the photodiodes which are thus reset.
U.S. Pat. No. 5,936,230 describes a sensor array in which a phosphor converter converts incident X-rays into visible light which is converted into an electric charge by photosensors arranged underneath said phosphor converter. Reflector strips are arranged between the phosphor converter and the photosensors. The light reflected in the openings between the photosensors is thus reflected back to the phosphor converter so as to be reflected to the photocells. This results in an improved light detection. It has been found that the fitting of such reflection or reflector strips between the photosensors and the phosphor converter is a very intricate operation.
OBJECTS AND SUMMARY OF THE INVENTION
Therefore, it is an object of the invention to provide an X-ray detector arrangement in which the light entering the openings can be effectively retroreflected and which can be simply manufactured.
This object is achieved in accordance with the invention in that a reflector arrangement is situated on the second side of the photosensor arrangement that is remote from the first side.
The X-ray detector includes a scintillator arrangement which is situated over a photosensor arrangement. The photosensor arrangement has two opposed sides as usual. The scintillator arrangement is arranged on a first side of the photosensor arrangement and the reflector arrangement is situated on the second side of the photosensor arrangement which is remote from the first side.
In the normal case it is to be assumed that the X-rays are incident on the scintillator arrangement from above in the direction of the first side of the photosensor arrangement, that the photosensor arrangement which detects the electric charges is arranged therebelow, and that the reflector arrangement is arranged underneath the photosensor arrangement on the second side thereof.
The openings between the photosensors enable light portions generated by the scintillator arrangement to travel past the photosensor arrangement without being used. In order to make use of these light portions for signal generation, the X-ray detector includes a regular reflector arrangement which is provided on the second side, that is, underneath the photosensor arrangement.
In an attractive embodiment of the invention the reflector arrangement which is situated underneath the photosensor arrangement is transparent to light that is incident from below or from the direction of the second side of the photosensor arrangement. The reflector arrangement is realized in such a manner that it reflects the light emitted by the scintillator layer as well as possible. At the same time the reflector arrangement is sufficiently transparent to light which is incident on the reflector arrangement from the direction of the second side of the photosensor arrangement, thus allowing passage to the reset light which is incident from below.
The specific influencing of the reflectivity and transmissivity of the reflector arrangement is realized by appropriately choosing the layer thickness and the material, that is, its refractive index. The optical properties of such a reflector arrangement in the form of a thin layer are based on interference. The wave selectivity is increased in general by using a plurality of layers of different materials. Customary materials in this respect are high melting metal fluoride (MgF
2
, CaF
2
) and oxide (Ta
2
O
5
, SiO
2
, ZrO
2
). The layer thicknesses are of the order of magnitude of from the wavelength of light to a few micrometers. Large-area homogeneous layers can be readily formed. For use in a dynamic X-ray detector the optical properties should be adjusted as follows. The reflectivity should be as high as possible (>90°) in a large spectral range of from approximately 450 nm to approximately 600 nm. At the same time the layer of the reflector arrangement should be as transparent as possible to light of other wavelengths so as to allow passage to the reset light from, for example, reset light emitting diodes that are arranged underneath the photosensor arrangement. The first requirement, however, has priority because an as large as possible amount of light should be reflected by the reflector arrangement so as to increase the signal. Given losses can be tolerated in the transmission of the reset light from the reset light emitting diodes, because such reset light emitting diodes can be driven with adequate intensity.
In a further advantageous embodiment of the invention a reflector arrangement is advantageously arranged underneath a substrate supporting the X-ray detector, said substrate preferably being realized in the form of a glass layer. The substrate is then situated between the photosensor layer and the reflector arrangement. This offers the advantage that the reflector arrangement cannot exert any electrical effect whatsoever on the photosensor arrangement.
The reflector arrangement in a further embodiment of the invention is constructed as a dielectric layer. Dielectric layers constitute electrical insulators and there will be no additional capacitances which would occur, for example, in the case of metallic reflector arrangements.
From a technological point of view it is advantageous to provide the dielectric reflector arrangement on the lower side of the substrate plate. Unlike when the dielectric reflector arrangement is provided directly underneath the photosensor arrangement, the processes for providing the photodiodes and the necessary thin-layer electronic circuitry for reading out the detected charge

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