DR systems Flashcards
what are some advantages of Digital Radiography
wide latitude
ability to window image to exploit this dynamic range
ability to process and store the images on the computer
the thin film transistor array (TFT) is common to both indirect and direct DR systems. True or False
True
what does the detector element consist of?
TFT
charge collection electrode
charge collection capacitor
what connects the TFT and the capacitor
the gate and drain lines connect the TFT and the capacitor
the TFT is ____ during exposure, allowing the charge that is produced to be stored in the ____
the TFT is closed during exposure, allowing the charge that is produced to be stored in the capacitor
when the X-Ray exposure is terminated, one gate line at a time is set to a ___ _____ which activates the ___ along that row
when the x-ray exposure is terminated, one gate line at a time is set to a high voltage which activates the TFTs along that row
charge flows from the ____ to the transistors and down the ____ in parallel
charge flows from the capacitors to the transistors and down the drain lines in parallel
what is the charge met with at the column of each matrix
output charge amplifiers
what effect does deactivating the gate line have?
deactivating the gate line will reset the TFTs for the next exposure
what is the fill factor
the percentage of the detector element that collects the charge from the energy deposited by the photons in the converter material
- basically the area of the detector that can take X-Rays
name some dead areas of the detector
the TFT
the capacitor
the gate and drain connections
what is the effect of a smaller detector element on fill factor and DQE
as detector element gets smaller, the fill factor and DQE gets smaller
what is a typical range for a fill factor
0.5-0.8
what is used as the detector material in direct conversion systems
amorphous selenium
in direct systems, where in relation to the TFT layer is the photoconductor
it is on top of the TFT
what does the large voltage placed between the two electrodes (between selenium) have
this reduces the spread of ion pairs and prevents recombination
advantages of direct conversion systems
simpler TFT structure with no photodiode
high intrinsic spatial resolution due to high voltage ion spread
selenium plates are easy to manufacture
disadvantages of direct conversion systems
charge trapping in thick selenium layer reducing absorption efficiency, increases signal retention and causes greater lag
can damage TFT due to over charging caused by high x-ray exposures
difference between direct and indirect DR systems
indirect DR systems have the addition of an amorphous silicon photodiode and a scintillator made of CsI on top of the TFT
what does the photodiode and scintillator do
converts x-ray photons to light photons proportional to the incident x-ray intensity
what happens to the light photons in indirect DR
The light photons are converted to an electric charge by the amorphous silicon photodiode
how is the light spread in the scintillator reduced
the light spread in the scintillator is reduced by structuring the scintillator CsI into crystal needles
spatial resolution is determined by ___ in DR systems
spatial resolution is determined by pixel size in DR systems
what determines pixel size in DR systems
pixel size is determined by the size of the detector element
what is the spatial resolution for DR systems
2.5-5lp/mm
DR vs CR
More rapid image production in DR, higher DQE, similar spatial resolution
