Cr/dr Final Exam Review

Front 1

photostimulable phosphor

Back 1

lower k edge compared to intensifying screens making them more sensitive to scatter radiation

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photostimulabe luminescence

Back 2

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

Front 3

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

Front 4

1/2 time image decay

Back 4

19 hours
6% less after 10 min
10% after 4 hours

Front 5

what happens in the phosphor layer

Back 5

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

Front 6

DR flat pannel detectors

Back 6

2 types: scintillation (DIRECT) and ionization (INDIRECT)

Front 7

direct FPD's

Back 7

use huge area AMA integrated circuits made of semiconducting elements called TFT's

Front 8

ama

Back 8

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)

Front 9

tft

Back 9

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

Front 10

scintillation detectors

Back 10

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

Front 11

INDIRECT FPD's

Back 11

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

Front 12

flat panel detector with scintillator system in indirect FPDs

Back 12

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

Front 13

disadvantage of indirect system FPD's

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loss of spatial resolution because of light divergence

Front 14

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

Back 15

higher DQE = lower pt dose

Front 16

fill factor

Back 16

the % of pixel area sensitive to the image signal
80%-- not 100 because some of the area devoted to TFT's and electric conductors

Front 17

comparing CR to DR

Back 17

same resolution- ~5 lp/mm

Front 18

DQE detector quantum efficiency

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SNR^2 out/ SNR^2 in

greater the DQE = better the detector

Front 19

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

Front 23

errors with histograms

Back 23

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

Front 24

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

Front 28

image resolution:

Back 28

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

Front 29

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

Front 30

noise

Back 30

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