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30 Cards in this Set
- Front
- Back
photostimulable phosphor
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lower k edge compared to intensifying screens making them more sensitive to scatter radiation
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photostimulabe luminescence
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release of stored energy within the phosphor by stimulating emission of trappend ENErgy in imaging plate + visible light emitted. light is released and collected and strikes PMT to produce electric signal which is digitized and stores then plate is erased by exposure to fluorscent light
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what happens in photostimulable phosphor
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electrons excite when their hit by radiation and then they deexcite and produce fluorsecence. PSPs contain impurities acting as e- traps. e-s de-excite when exposed to specific wavelengths of light, randomly. then they will release the visible light and it can strike the PMT
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1/2 time image decay
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19 hours
6% less after 10 min 10% after 4 hours |
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what happens in the phosphor layer
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electrons excite from valence to conduction band
during exposure- some trapped latent image exists in the phosphor layer as e' trapped in high energy states, during reading laser light stims e-s back to valence band and blue-green light emitted. intensity of light emitted proportional to radiation absorbed |
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DR flat pannel detectors
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2 types: scintillation (DIRECT) and ionization (INDIRECT)
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direct FPD's
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use huge area AMA integrated circuits made of semiconducting elements called TFT's
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ama
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large circuit component, size of largest field size in both directions
made up of many TFTs image display shows in < 10 sec matrix size- ~ 3000^2 dynamic range ~ 14 bits (<16000 Gy) |
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tft
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made of light sensitive element (photodiode) and switching component
light proportional to # of X-rays free charges --> image data off during exposure and turned on row by row signal amplified-> ADC -> stored in memory of image processor |
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scintillation detectors
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radiation creates light by striking fluorescent material (OSI)
light strikes layer of amorphous (non crystaline) silicon -> converts to electric charge charge is moved out of detectors by TFTs |
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INDIRECT FPD's
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convert x-ray energy into light energy, then to electronic signal (indirect!)
do not use fluorscent phosphors but rather photoconductors availible in 2 kinds- flat panel detector with a scintillator or a charged-coupled detector |
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flat panel detector with scintillator system in indirect FPDs
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involves coupling scintillator (cesium iodide usually) with amorphous silicon (noncrystaline state) which does not absorb x-rays well so a phosphor layer or cesium iodide is used to absorb them and emit light, which activates a-Si photodetectors creating an electric signal that is stored by the TFT until readout
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disadvantage of indirect system FPD's
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loss of spatial resolution because of light divergence
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DIRECT system of FPD's
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use photoconducting material like a-Se, not phosphors
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advantage of direct FPDs
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higher DQE = lower pt dose
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fill factor
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the % of pixel area sensitive to the image signal
80%-- not 100 because some of the area devoted to TFT's and electric conductors |
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comparing CR to DR
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same resolution- ~5 lp/mm
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DQE detector quantum efficiency
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SNR^2 out/ SNR^2 in
greater the DQE = better the detector |
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image processing (3 stages)
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1. exposure recognition
2. histogram analysis 3. automatic histogram rescaling |
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1. exposure recognition
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identifies filed margins and discards signal outside collimation margins
s1 and s2 are lowest and highest signal tail is peak of black |
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2. histogram analysis
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program compares histogram with a preprogrammed histogram for the same body part/projection
shape determines info in exposure field + exposure level |
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3. automatic histogram rescaling
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changes brightness and contrast
provides with uniform display over mid exposure range errors are corrected by algorithms, greyscale values allocated |
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errors with histograms
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lead to decrease in image quality
causes --- pathology, led apron, contrast, prosthesis, positioning error, too large field size more common in CR because entire plate scanned |
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algorithms
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agfa uses MUSICA for decrease in post processing
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high pass filtering
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amplify some signals and supress others
create edge enhancement |
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low pass filtering
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averaging pixels (smoothing)
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raw data image
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the unprocessed image
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image resolution:
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CR- depends on nyquist limit = max # lp/mm that can be recorded
depends on IR size but some use 2000x2000 pixels for all cassettes so smaller cassette is better resolution in this case exposure--> smaller pixel size = lower quanta contribute to the image signal from each pixel = more noise/pixel DR- depends on detector element size (DELS) smaller DEL = better resolution |
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exposure indicatiors
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values to assess exposure to IR fomr histogram
inaccurate if collimation margin not detected # inversely prop to exposure (linear) for Fiji agfa uses LgM (old) which was logirithmic but now uses linear model |
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noise
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quantum nosie, structure of IP, electric all cause noise
mottle noise at low exposures-- not acceptable image is exposure at 1/4-1/2 normal value |