Luminescence occurs when….
materials absorb energy and emit light
types of luminescence
- fluorescence
- phosporescence
fluorescence emission
- light is emitted very quickly - in milliseconds
- light production from the valance band is virtually instantaneous
- this creates ‘real-time’ imaging - this avoids the image degradation caused by lag (image blurring, image persistence)
- sharp high resolution images
phosphorescence emission
- light is emitted more slowly
- light production in the valence band is slower and occurs over a longer period of time
- this can result in slower, blurred or unsharp imaging
- is not ideal for interventional work, where mm movements can have serious consequences
resolution in fluoroscopy
- can be performed with image intensifiers or flat-panel detectors.
- ## does not require high-resolution performance
luminescence explanation
- x-ray exposure passes through the patient, hitting material of phosphor
- the energy interacts with the phosphor and promotes electrons within the phosphor to a higher energized state and are able to move around in valence bands
- they drop back quickly or slowly and light is released
how does fluoroscopy work (image intensifier)
the x-ray image intensifier is an electronic device that converts the x-ray beam intensity pattern into a visible image suitable for capture by a video camera and displayed
how does fluoroscopy image intensifier work (input phosphor)
the caesium iodide input phosphur converts the x-ray image into visible light photons
- composed of caesium iodide activated with sodium
how does fluoroscopy image intensifier work (photocathode)
the photocathode is placed in close proximity to the input phosphor and releases electrons in direct proportion to the visible light from the input phosphor that is incident on its surface.
- wide curved surface - so that all points between input phosphor and photocathode are the same distance
how does fluoroscopy image intensifier work (electrostatic focusing lenses)
- they control the directional flow of electrons to the output phosphor, so that the resultant image accurately depicts the input phosphor information
- there is an electronic potential within the Image intensifier approx.30kV between the photocathode and positive anode
how does fluoroscopy image intensifier work (accelerating anode)
- the electrons are attracted to the anode by electric potential and focussed onto the output phosphor by the anode
how does fluoroscopy image intensifier work (output phosphor)
- converts the elecrtronic information back into visible light
- these are captured by a number of imaging devices
- made of zinc cadmium sulphide
- converts electron strikes into light photons
- -very thin (4-8 micrometers) - better resolution, due to being thin
Artifacts fluoroscopy
- pincushion distortion
- vignetting
- veiling glare
- S distortion
pincushion distortion
- this occurs when you project an image from a curved structure onto a flat structure
- only the middle of the image is truly represented and as you move further away, the image is distorted
- large FOV increasing the effect of pincushion distortion
Vignetting
- the apparent ‘drop off’ in image intensity/signal at the edges of the image
- consequence if the uniformity of scatter
veiling glare
- occurs inside the image intensifier between the photocathode and the anode
- caused by electrons, x-rays and light bouncing off and rebounding from the anode
- scattering of information within the housing of the x-ray tube itself
- electrons have travelled through the anode and reached the output phosphor (where light is also produced). this light can bounce off the output phosphor and go back towards the photocathode.
- since the photocathode is sensitive to light, it creates another response and releases more electrons. therefore there will be an increase of signal in areas where structures are less dense.
S distortion
- caused by external electromagnetic sources. e.g. MRI scanners located near by, or the earth’s magnetic field.
- this affects the electron paths at the periphery of the image more than the centre
solution to pincushion distortion
- using a smaller image intensifier or FOV
vignetting solution
- use a smaller image intensifier or switch to FPD
S distortion solution
- removing interfering magnetic field
