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TFT/CCD/CMOS EXAM QUESTIONS AND ANSWERS
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TFT/CCD/CMOS EXAM QUESTIONS ANDANSWERS
True or false. Flat panel detectors have an active matrix of electronic components. -
ANSWER True. TFTs are built-in layers to accept X-ray photons and convert them
to electrical charges for storing and reading.
Describe the three levels of a flat-panel detector. - ANSWER First layer: the x-ray
converter.
The second layer: the TFT array.
Third layer: Glass substrate.
What's another term for the X-ray converter? - ANSWER The absorber
The type of x-ray absorption material used influences whether the detector is a
________ or _______ conversion system. - ANSWER: Direct or indirect
Describe the direct conversion system using TFT plates - ANSWER 1-step process.
The coating material absorbs X-ray photons and converts them instantaneously into
an electrical signal.
What is the common radiation-conversion material or photoconductor used in
direct conversion systems? - ANSWER Typically composed of amorphous
selenium (a-Se). This is the substance that absorbs and converts X-rays to
electrons, which are then stored in TFT detectors.
What absorbs X-rays and generates electric charges? - Answer: A photoconductor.
, Direct TFT plates can scan and convert more than ___ million pixels into a
composite digital image in less than ___ seconds. - Answer: 1 million in one
second.
Describe the indirect conversion mechanism using TFT plates - ANSWER 2 stage
process. X-ray photons are turned into light, which is then converted into an
electrical signal.
In which layer are X-rays transformed to light? - ANSWER The phosphor, or
scintillation layer
Is the scintillation layer structured or not? - ANSWER It could be structured or
unstructured.
What are the two kinds of phosphor structures? - ANSWER Turbid (unstructured
or random), and Needle/Columnar (structured).
What is turbid phosphor made of? - ANSWER Gadolinium oxysulfide (Gd2O2S).
What are the disadvantages of the turbid phosphor structure? - ANSWER It causes
light to escape laterally before to reaching the surface. This diminishes the
phosphor's efficiency (DQE) and degrades spatial resolution.
What are the needle/columnar phosphors made out of? - ANSWER Cesium iodide
(CsI
What are the benefits of employing structured phosphors versus unstructured
phosphors? - ANSWER They function as light directing tubes, with more
absorption and less light spread. Structured phosphors provide better DQE and
spatial resolution.
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