Quantum Detection Efficiency (QDE):

1. This is a measure of how efficient your detection system is.
2. It is defined as the ratio of the number of photons detected to the number incident.
3. You want this to be as high as possible to keep the patient dose low and achieve low noise in your images. 

1. This is why phosphor screens and build-up materials are used for imaging.

Detective Quantum Efficiency (DQE):

1. 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).
2. SNR is the signal to noise ratio (out and in, respectively above).
3. 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:

1. Phosphor screens are used in film radiography as they function to amplify the number of photons striking a film.
2. 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.
3. Screens may be placed in front of and behind films to maximize their effectiveness.
4. 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. 

1. Consider in the early days of x-ray usage without a phosphor screen, the doses were at least 20X higher than they are now.