digital fluoroscopy fluoro 2 Flashcards
(44 cards)
problems with image intensifiers
- big and bulky
- prone to image un-sharpness, distortion and noise
- vacuum tube can break down
- limited image post-processing
what is the order of the image intensifier chain
x rays –> light –> electrons –> light
what goes between x-rays and light
input phosphor
what goes between light and electrons
photocathode
what goes between electrons and light
electron optics and output phosphor
what do image intensifiers provide that is difference to digital flat panel detectors
a smaller dynamic range of image densities than digital detectors
what type of response do image intensifiers give
a non-linear response to exposure and narrow dynamic range
what happens during image acquisition
- x-ray beam attenuated through patient normally
- x-ray photons incident to detector plate (same as DR)
- sequence of static images taken during screening (at regular intervals)
- images are post processed + can be played back as video
advantages of digital fluoroscopy
- more compact equipment
- less radiation dose
- less noise
- reduced artefacts
- image magnification without increased dose/ loss of brightness + resolution
- PACS + image distribution
image of image intensifier
image of flat panel detector
density of analogue images
continuous varying density
density of digital images
has discrete pixels with numerical density values
what is signal to noise ratio
the ratio of the amplitude of the desired signal to the amplitude of noise signals at a given point in time
two ways digital output can be obtained
- by taking output from image intensifier (old fashioned)
- by using flat panel solid state detector (more modern)
what type of digital technology do many detectors in fluoro use
indirect
what absorbs x-rays and emits light in flat panel detectors
- caesium iodide scintillator layer
- gives more signal per XR dose received compared to image intensifiers
why are CsI in needle form
channels light downwards towards silicon diode
how do flat panel detectors work
- light strikes silicon photodiode which emits electrons
- electrical charge collected in thin film transistor (TFT) array
- signal passes through analogue to digital converter (ADC)
what type of digital technology do SOME flat panel fluoro detectors use
direct
direct digital technology
- a photoconductor that absorbs x-rays
- amorphous selenium, a-Se
- XR converted into directly to electrical signal
- photoconductor generates electrical charge
- electrical charge collected in thin film transistor (TFT) array
- signal passes through ADC
what are flat panel direct detectors
- detectors are direct imaging matrices using thin film sensors
- detectors very similar to those used in plain radiographic imaging
what does an ADC determine
the digital values allocated to each pixel
what does ADCs consist of
- integrated circuits containing ‘silicon chips’ that perform complex electronic processes
