Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

In all medical x-ray imaging, we want to reduce the radiation dose to thepatient to a minimum level, and we saw that there is a trade-off between SNR andradiation dose. Because of this concern, it is very important that every x-ray photonthat reaches the detector be counted and contribute toward the SNR. Perfect x-raydetector systems are ones that are "x-ray quantum limited," that is, where the imageSNR is dictated by only the SNR of the x-ray quanta used. While this is the goal,real-world detectors are not perfect and they generally do not absorb (detect) all thex-rays incident upon them. For example, a screen-film detector system may detect(depending on x-ray energy) about 60% of the x-ray photons incident on it. Thequantum detection efficiency (QDE) is the ratio of the number of detected photons(photons absorbed by the detector) to the number of incident photons for a givendetector system:QDE = NabsorbedNincidentIn terms of the image SNR, it is the number of detected photons, not incident photons,that are used in the SNR calculation, such thatEquation 10-22 includes the influence of the DQE, but does not include the influenceof other soutces of noise in the detector system. This will he discussed in thenext section.An x-ray imaging system employing an intensifying screen and a CCD camera isillustrated in Fig. 10-29A. As a single x-ray photon stimulates the imaging system,various numbers of quanta are used at different stages in the system. The stages aredescribed in Table 10-2, and the number of quanta propagating through two dif-100I \.B 10cIII::::Jạ •..0'II:0.10.011 2 3 4 5A Stage BFIGURE 10-29. A: An imaging system is illustrated with different stages, 1 through 5(see Table 10-2 for a description of the different stages). B: The quantum accountingdiagram is illustrated for two different imaging systems, P and Q. In the quantumaccounting diagram, if at any point the number of quanta is lower than the numberof detected x-ray photons, a secondary quantum sink occurs. System Q has a lowergain at stage 3 than system P, and a secondary quantum sink occurs at stage 4 as aresult.