digital fluoroscopy fluoro 2 Flashcards
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
what does high voltage (ON) and low voltage (OFF) represent
high voltage: digit 1
low voltage: digit 0
the voltage at each point determines…
grey level
how many grey levels available
256
(0=black)
(255=white)
what is a look up table (LUT)
- a mathematical computer processing
- converts an input value to output value
- i.e. converts charge on a transistor to density in image pixel
features of flat panel fluorography
- higher detective quantum efficiency (DQE) gives reduced dose
- small pixels
- large dynamic range
- magnified images without the loss of bright + resolution
- no image distortion
what does detective quantum efficiency (DQE) measure
efficiency of an imaging systems transfer of XR signal to image signal
- and of incoming signal to noise ratio to outgoing signal to noise ratio
what does a high DQE mean
gives better ability to detect fine details in image
- high contrast and resolution
‘perfect imaging system’
- a DQE of 100%
- not achievable as noise always added
the greater the level of bit depth …
the greater the contrast resolution and smoother the image
contrast resolution
ability to demonstrate subtle differences in XR attenuation
spatial resolution
ability to discriminate and detect small adjacent objects (fine details)
describe sampling of a video signal
conversion of analogue signal to digital signal via periodic measurement (over time)
what does a low sample rate result in
coarse digital signal
what would happen in sampling rate doubled
- pixel size would be halved
- matrix size doubled
- spatial resolution improved by factor of 2
- BUT requires more memory
the larger the sampling matrix …
the greater the spatial resolution
features of a digital spot image
- high resolution
- short exposure at high mA = freezing movement
- video system becomes inoperable and camera writes it in computer memory
how can greyscale numbers be processed and manipulated before reading out an image from memory
- windowing
- noise reduction (averaging or smoothing
- edge enhancement
- data shifting
- quantification
what is data subtraction aniography
if two images are almost the same theyre subtracted from each other so resulting image only shows whats different
- technique regularly used to show contrast flowing through blood vessels
how to produce a DSA image
- sequence of pulsed fluoroscopic images of relevant body part is recorded as contrast injected into vessel
- pre-contrast image then substracted from post contrast image to give DSA
how to average/ reduce noise
- FRAME ADDITION
- image of same subject are added together + averaged
- useful signals added
- patient must be immobile
