log,,, Quantum Fluence

Fig. 4.12. NEQ for a screen-film mammography system plotted versus spatial frequency and the radiation exposure incident on the receptor (Bunch, 1999).

a more complete characterization of imaging performance (Figure 4.13). Determination of DQE requires three pieces of data: the MTF of the imaging system, its noise power spectrum, and a measure of how many x-ray quanta are incident on the receptor. Note that, NEQ describes the quality of the image while DQE characterizes the efficiency of the imaging system in producing that quality.

Currently, measurement of noise power spectrum, MTF, NEQ, and DQE on screen-film systems require sophisticated microdensi-tometers to digitize the films and are, therefore, done primarily in manufacturers' laboratories. Because digital mammography systems will provide data in digital form directly, it is much more straightforward to carry out these measurements in the field and to incorporate them in routine QC programs for digital systems.

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