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Digital Radiography System Explained (step-by-step) - YouTube
Channel: Clover Learning
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In this video, we'll be
covering cassette-less digital
[3]
radiography systems.
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Cassette-less
digital radiography
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is different from CR radiography
in that there is no cassette
[12]
or imaging plate reader.
[13]
Cassette-less
digital radiography
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can be broken down into either
direct or indirect systems.
[21]
Indirect radiography can
be further broken down
[25]
into either thin flat-panel
transistors called
[29]
TFTs or charged-coupled
device systems called CCDs.
[36]
Indirect TFT systems are
composed of three main layers--
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the top scintillation layer,
a middle photodiode layer,
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and the bottom TFT array.
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The top scintillation
layer or light layer
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is where the incoming
incident X-ray photons are
[56]
converted into light photons.
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A scintillation is
nothing more than a burst
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of light, which is
why this layer is
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called the scintillation layer.
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Each photon that strikes
the scintillation layer
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creates a small burst
of light energy.
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Unfortunately, one downfall
of using a scintillation layer
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is that it produces light
isotropically or in all
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directions.
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The isotropic
nature of this layer
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thus acts to decrease
spatial resolution,
[88]
since the converted
light photons
[90]
are of a greater size than
the incoming incident X-ray
[93]
photons.
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Basically, the
scintillation layer
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create a small amount
of blur on each image.
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The scintillation
layer is comprised
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of either the ionic
compound cesium
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iodide or the chemical
element gadolinium.
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Cesium iodide is
preferred over gadolinium,
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since it can be formed
into very small rods.
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These small cesium
iodide rods are
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able to produce a smaller,
more focused light
[124]
beam, which in turn serves to
increase spatial resolution.
[129]
After the scintillation layer
comes the photodiode layer.
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The photodiode
layer is comprised
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of a semiconducting material
known as amorphous silicon.
[140]
The photodiode layer works by
converting the incoming light
[144]
photons from the scintillation
into electric charge.
[149]
The electric signal
or electrons produced
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by the photodiode layer
are then transferred
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to the thin-film
transistor or TFT layer.
[160]
The TFT layer is comprised
of an array or matrix
[165]
of very small detector
elements, or DELs for short.
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Each DEL is comprised of
a capture element or pixel
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detector, which is the active
element within each DEL,
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a storage capacitor,
which stores
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the electric charge produced
by the capture element,
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and a thin-film
transistor or switch that
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acts to open and close the
release of electric charge
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leaving each DEL, thus
producing the digital image.
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The ability of
each DEL to produce
[197]
a high spatial
resolution radiograph
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is designated by the percentage
of the active pixel area
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within each DEL.
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The percentage of
DEL to active pixel
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is known as the fill factor.
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There is a direct relationship
between fill factor
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and spatial resolution in that
as the active area increases--
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that is, takes up a
greater percentage
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of space within the DEL--
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spatial resolution increases.
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The charged-coupled
device, or CCD system,
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is the other form of
cassette-less digital
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radiography.
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CCD systems differ
from the TFT systems
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in that, with the CCD system,
there is no photocathode or TFT
[247]
layer.
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The scintillation layer
within CCD systems
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are optically coupled
to each CCD sensor
[255]
chip by either lenses
or fiber optics.
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The CCDs are able to both
convert light photons
[263]
into electrical signals as well
as send the electrical signals
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to the computer for processing.
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The final cassette-less
digital radiography system
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is the direct conversion
radiography system
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or non-scintillation system.
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This system is comprised of
an amorphous selenium-based
[285]
semiconductor and a TFT.
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This system works by applying
a high-voltage charge
[293]
to the top surface nanoseconds
before the extra exposure is
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made.
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The interaction between
the incident X-ray photons
[301]
and the high-voltage charge
causes the selenium atoms
[305]
to release their
electrons, which
[308]
are then able to be collected
and processed by the TFT layer.
[312]
This final image gives
a visual illustration
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of each type of cassette-less
digital radiography system.
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