Thoracic Imaging 2003 - Society of Thoracic Radiology

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One type of direct-readout system is a large-area, solid-state
x-ray detector consisting of a structured Cesium Iodide (CsI)
scintillator directly coupled to an array of amorphous silicon
thin-film photodiodes and readout electronics. The multi-layered
detector is manufactured on a glass substrate, approximately 41
cm square. When exposed to x-rays, the visible light is channeled
within the CsI crystal matrix directly to the photodiode
array where the electric charge is collected and digitized, forming
the digital image. Because of its direct-readout design,
structured scintillator, and the use of very-low-noise electronics,
this flat-panel detector is anticipated to provide exceptionally
high image quality.
Another style of direct-readout digital detectors utilizes the
same type of TFT array for charge collection and readout, but
the active detector element is amorphous selenium instead of a
scintillator and photodiode. Because selenium is an x-ray photoconductor,
x-ray photons striking the detector are converted
directly to electrical charge with no intermediate (visible light)
stage. This direct conversion eliminates one step in image production,
and thus removes one opportunity for noise to enter the
imaging chain.
Image quality from digital acquisition systems is influenced
by detector materials, design, and electronics, and it is unknown
at this time which style of direct-readout digital x-ray receptor
will offer superior performance. It is clear, however, that largearea,
direct-readout digital detectors offer expanded opportunities
in the radiology department.
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