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3 elements of digital radiography
capture
coupling
collection
that in which the x-ray is captured
capture element
that which transfers the x-ray generated signal to the collection element
coupling element
examples of a collection element
photodiode
charge-coupled device (CCD)
a thin-film transistor (TFT)p
photodiode and CCD collect
light
TFT collects
electrons
capture element of CR
BaF PSP
coupling element of CR
lens/fiber optics
collection element of CR
photodetector
capture element of SPR
Nal/Csl
coupling element of SPR
none
collection element of SPR
photodetector
capture element of indirect DR
Csl, GdOS
coupling element of indirect DR
fiber optics
contact layer
collection element of indirect DR
CCD/CMOS
TFT
capture element of direct DR
a-Se
coupling element of direct DR
none
collection element of direct DR
TFT
Used in CT to help with patient positioning image volume
scanned projection radiography
how is scanned projection radiography obtained in CT?
maintaining the energized x-ray tube
early versions of scanned projection radiography had
patient motion blur
scatter
new versions of scanned projection radiography are used with
digital radiographic tomosynthesis
The light-sensing element for most digital cameras, silicon based
charge coupled device (CCD)
3 principal advantageous imaging characteristics of CCD
sensitivity
dynamic range
size
ability to detect and respond to low levels of light
sensitivity
ability of the CCD to respond to a wide range of light intensity, from very dim to very bright
dynamic range
size of CCD is (large/small)
very small
when CCD responds to low level of exposure this means
lower patient dose
CCD contrast (related/unrelated) to IR exposure
unrelated
indirect DR process by which x-rays are
converted first to light and then to an electronic signal.
Csl/CCD
CCD are tiled, so exposure time is
short
because Light from a CsI phosphor is transmitted through fiber-optic bundles to the CCD array
high x-ray capture efficiency
good spatial resolution
Because of the edges of each CCD, pixel values must be
interpolated at interfaces
an indirect DR process by which x-rays are converted first to light and then to an electronic signal.
Csl/a-Si
collection element of Csl/a-Si
silicon and sandwiched as TFT
Amorphous silicon is __ that can be
fluid painted
spatial resolution in DR is
pixel limited
Large portion of the face of the pixel is covered by
electronic components and wires that are not sensitive to the light emitted by the CsI
percentage of pixel face sensitive to x-rays
fill factor (80%)
how much of beam in (CsI/a-Se) beam does not contribute to image
20%
what happens to spatial resolution when pixel size decreases?
increases
why when pixel size decreases, patient dose increases?
fill factor is reduced, but x-ray intensity must be increased to maintain signal strength
a direct DR process by which x-rays are converted directly into an electronic signal
amorphous selenium (a-Se)
incoming x-ray beam interacts with a-Se. producing a
charged pair
charged pair from a-Se is stored by capacitor until signal is read by
TFT
(making a-Se capture and coupling element)