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PACS Chp. questions and answers verified 2024
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PACS Chp. questions and answers verified 2024
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PACS Chp. 6charge-coupled device (CCD) - correct answer ✔the oldest indirect
conversion digital radiography systems used (CCDs) to acquire the digital
image.
in a CCD system, x-ray photons interact with a scintillation material and the
signal is transmitted by lenses or fiber optics to the CCD. during the
transmission process, the lenses reduce the size of the projected visible light
image and transfer the image to one or more small capacitors that convert the
light into am electrical charge.
this charge is stored and released line by line and send to an *analog digital
converter (ADC)*
structure and function - correct answer ✔a photosensitive receptor and
electronics embedded into a substrate material in a silicon chip make up a
CCD.
incident light from a scintillator strikes the detector and electron/hole pairs are
produced in the silicon. the number of electron/hole pairs is related to the
amount of light that is absorbed. the electrons are held by electrostatic forces
in the array until the charge is read out to form the image.
the chip is made up of a polysilicon layer, a silicon dioxide layer, and a silicon
substrate.
the *polysilicon layer* is coated with a photosensitive material and contains
the electrostatic gates.
the *silicon dioxide layer* is an insulator, and the *silicon substrate* contains
the charge storage area.
, each pixel or *detector element (del)* contains three electrodes that hold the
electrons in a electrical potential well. the dels are formed by voltage gates
that, at readout, are opened and closed like gates to allow the flow of
electrons.
to collect the charge on the silicon chips, the voltage sign is changed on the
electrodes within each del, moving the electrons by rows down the columns
until the readout row is reached. a process known as the *"bucket brigade
scheme"*
the data are sent sequentially to the sense amplifier and are then digitized. the
readout process requires that the movement of the electrons is timed, and
multiple voltage changes occur.
issues with overfill in the dels, which causes excess electrons to spill out of
the del into an adjacent del. this is handled by building overflow drains into the
array to reduce or prevent this *"blooming"* effect.
these drains allow the electrons to flow away from the imaging plate to ground
image-related characteristics - correct answer ✔high cost of a CCD chip, not
practical to manufacture CCD chips any larger than 5cm X 5cm
most chips range from 2 to 4 cm, the image has to be matched to the size of
the CCD which implies that the image must be reduced in size.
the CCD technology uses lenses or fiber optics so that the image matches the
receptor size.
these factors include the scintillator, light collection components, and noise
the scintillator - correct answer ✔the type of scintillator and the way it is
constructed will determine how much of the incident x-ray photons are
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