4 - Image receptors Flashcards
what are the two types of image receptors for film x-ray?
- direct action film
- indirect action film
what are the two types of image receptors for digital x-ray?
- solid state sensor
- photo stimulable phosphor
direct action film: what type of xrays do they take? and what do the x-rays act on?
- intra oral radiographs
- xrays act directly on silver halide crystals in film emulsion
indirect action film: what type of xrays? light from what acts on which substance in the film emulsion?
- extra oral radiographs
- light from intensifying screen acts on silver halide crystals in the film emulsion
example of phosphor plate?
PSP - photostimulable phosphor plates
example of solid state image receptors?
CCD - charge-coupled device
CMOS - complement metal oxide semiconductor
difference between solid state receptors and phosphor plates? (think in terms of mechanism, compatibility, price and maintenance)
- phosphor plates absorb x-rays, image plate is then scanned and the stored x-ray energy is released as light, which is then detected and converted into the image. solid state receptors are similar to digital cameras in image capture
- phosphor plates can be used with existing film holders and are well tolerated by patients. solid state receptors can only be used with its own holders and are not tolerated well by patients as they are larger than film.
- phosphor plates are relatively expensive but solid state sensors are very expensive
- phosphor plates easily damaged, are sensitive to background radiation. must be cleaned/scanned every 24hours if not used.
photostimulable phosphor plate system (PSP): where does the phosphor layer absorb and store the x-ray energy?
in the electrons of the phosphor crystals
photostimulable phosphor plate system: how is the digital image produced?
- phosphor layer absorbs and stores xray energy in the electrons of the phosphor crystals
- plate inserted into scanner
- plate scanned by red laser
- light is released, and detected by the photo-multiplier tube
- converted to electric signal and is relayed to computer to produce digital image
photostimulable phosphor plate system: what happens to the residual trapped energy after scanning?
image on plate is erased by exposing phosphor to bright light. this releases any remaining trapped energy in phosphor electrons
PSP: describe the state of the phosphor plate as it goes through the process of receiving an x-ray to when it is converted to an image
1) imaging plate (xray) 2) stable excited state (plate scanned by red laser) 3) unstable excited state (emits blue light, scanner collects blue light and converts to image) 4) back to imaging plate
solid state sensors:
CCD - charge-coupled device
what are the main components and what does each do? from front to back cover
- front cover
- scintillator layer: converts xray energy to light
- silicon wafer: converts light to electrical signal
- back cover: incorporating cable to carry signal to PC
solid state sensors: CCD - charge-coupled device: how does it produce an image?
what makes up the pixels of the image? how is each pixel related to its neighbouring one? how is the signalling like?
- x-ray hits scintillator layer within sensor
- this gives off light, which hits photosensitive cells within the thin piece of silicon
- electrons released within each cell and these forma charge which is converted to a voltage -> electric signal which the computer software will convert into an image
- the thin piece of silicon makes up pixels of the image
- each pixel is connected to its neighbour
- signal comes from lines of pixels
solid state sensors: CMOS - complementary metal-oxide semiconductor. how is it different from CCDs?
CMOS is similar to CCD, the only difference is that photocells are electrically isolated therefore charge to voltage signal is from each individual cell (pixel) rather than lines of cells (pixels)
CCD vs CMOS? in terms of: - image quality - price to manufacture - reliability - power usage
- CCD better quality image (high quality digital camera) vs CMOS (mobile phone camera). however CMOS is catching up
- CMOS cheaper to manufacture
- CCD been around longer, more reliable
- CMOS uses less power
