What is CCD?
CHARACTERISTICS OF CCD:
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Its physically small-typical 2-4 cm2, which is
much smaller than typical projected x-ray areas.
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Cost-effective
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Include some means of optical coupling to reduce the size of
the projected visible light image and transfer the image to the face of one or
more CCDs.
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Some CCD-based system have an image intensifier that reduces
the large x-ray field to the size of one CCD.
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Other systems are based on an array of CCD cameras, each of
which is coupled to a scintillator by a lens or a fibreoptic taper.
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The lens system:
Ø
Produces the number of photons that reach the CCD.
Ø
This increases the appearance of image noise (for example,
quantum mottle) and degrades image quality.
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In addition, optical coupling with lenses can introduce
geometric distortions, optical scatter and reduced spatial resolution.
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Thermal noise within the CCD itself also can degrade image
quality, although this is less of a factor with modern, cooled CCDs.
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| Figure 5a. CCD-based indirect conversion DR system. (a) Drawing illustrates a lens-coupled CCD-based system. The incident x-ray energy is converted into light by a scintillator. The emitted light has to be bundled by an optical lens to fit the size of the CCD chip, which subsequently converts the light energy into electrical charges. (b) Drawing illustrates a slot-scan CCD-based system. The patient is scanned with a fan-shaped beam of x-rays. A simultaneously moving CCD detector of the same size collects the emitted light and converts the light energy into electrical charges. |
