Scintillator arrays for CT imaging and other applications

Details Scintillator arrays for CT imaging and other applications Scintillator arrays for CT imaging and other applications

2000-11-08

2002-11-12

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

With or including a luminophor

Reexamination Certificate

active

06479824

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates generally to methods and apparatus for detecting radiation in CT imaging and other radiation imaging systems, and more particularly to scintillator arrays having a cast reflector mixture that includes at least one filler material selected to enhance performance.
In at least some computed tomography (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent the scintillator.
One or more rows of scintillator cells are provided in a detector array configured to acquire projection data from which one or more image slices of an object are reconstructed. One known detector array includes a two-dimensional array of scintillator cells, with each scintillator cell having an associated photodetector. An epoxy material is used to cast the scintillator cells into a block having specified dimensions for easier handling. To maximize reflectivity and to prevent cross-talk between adjacent detector cells, the cast reflector mixture includes a material having a high refractive index, such as TiO
2
. Thus, light generated in the scintillating material by impinging x-rays is confined to the detector cell in which it is generated. However, neither the epoxy, the TiO
2
, nor their mixture are particularly absorptive of x-rays. Thus, neither the photodetectors nor the cast reflector mixture itself is protected from damage caused by impinging x-rays.
In one known cast reflector mixture, a small amount of an oxide of chromium is also incorporated in the cast reflector mixture to further reduce cross talk between cells. However, inclusion of this material reduces the efficiency of the detector, because the absorbed portion of the generated visible light is never detected by the photodetectors.
In one known CT imaging system, a post-patient collimator is used. This collimator comprises tungsten wires perpendicular to a series of plates that are suspended above cast gaps between scintillator elements. Such post-patient collimators are used to prevent x-rays from significantly penetrating cast reflector mixture in gaps between scintillator elements, from entering the sides of scintillator elements that are not directly facing the x-ray source, and from entering photodetectors associated with scintillator elements. Post-patient collimators have also been used because the focal spot of the x-ray source of the CT imaging system is not perfectly stable, and its movement would result in a change in apparent projected detector cell aspect ratio were it not for the presence of the post-patient collimator. A typical post-patient collimator is required to be about 0.008″ thick at the gaps between scintillator elements because of alignment tolerances of the CT imaging system. This thickness is high enough to reduce the quantum efficiency of the scintillator elements because of excess shadowing.
It would therefore be desirable to provide a detector that inherently provides protection for photodetectors and cast reflector mixture. It would also be desirable if the detector had improved x-ray quantum efficiency. Ideally, it would also be desirable if the detector provided reduced manufacturing costs by eliminating the need for a post-patient collimator in front of the detector array.
BRIEF SUMMARY OF THE INVENTION
There is therefore provided, in one embodiment, a detector for detecting x-rays of an imaging system. The detector has a plurality of photodetectors; and a plurality of scintillator elements optically coupled to the plurality of photodetectors. The scintillator elements have sides separated from adjacent scintillator elements by gaps; and a cast reflector mixture in the gaps between the sides of the scintillator elements. The cast reflector mixture include a first powdered material having a higher Z and a higher density than titanium dioxide, and a refractivity index sufficient to effectively scatter and reflect light within the cast reflector mixture.
The above-described embodiment inherently provides protection for photodetectors and cast reflector mixture, and is capable of providing improved x-ray quantum efficiency and of eliminating the need for a post-patient collimator in front of the detector array.


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