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Quantum Detection Efficiency (QDE):
- This is a measure of how efficient your detection system is.
- It is defined as the ratio of the number of photons detected to the number incident.
- You want this to be as high as possible to keep the patient dose low and achieve low noise in your images.
- This is why phosphor screens and build-up materials are used for imaging.
Detective Quantum Efficiency (DQE):
- DQE (not to be confused with QDE) is a measure of the whole imaging system’s efficiency in forming an image. (It lumps the electronic system’s noise in with errors from counting photons).
- SNR is the signal to noise ratio (out and in, respectively above).
- DQE is a great parameter to track for QA as it is a direct metric for the dose efficiency of the imaging system.
- Phosphor screens are used in film radiography as they function to amplify the number of photons striking a film.
- Screens are scintillating materials (CsI, CdWO4, etc.) and function to take the energy from an x-ray and split it into many photons in the optical spectrum which then strike the film.
- Screens may be placed in front of and behind films to maximize their effectiveness.
- In a screen-film cassette, x-rays are only responsible for about 5% of the exposure of the film. Whereas the other 95% is due to UV being emitted by the phosphor.
- Consider in the early days of x-ray usage without a phosphor screen, the doses were at least 20X higher than they are now.