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Which of the following are true regarding the differences between indirect and direct conversion detectors? a. The detective quantum efficiency (DQE) of direct conversion detectors is greater than that of indirect conversion detectors b. The difference in DQE is due to the difference in the K-edges of caesium iodide and seleniumc. Lower patient doses are possible with direct than with indirect conversion detectors d. For the same patient dose, a greater signal-to-noise ratio is possible with indirect than with direct detectors e. Spatial resolution is greater with direct than indirect conversion detectors |
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Answer» a. False. The DQE of indirect conversion is greater. b. True. The K-edge of selenium is only 13 keV, which is less than the mean photon energy range used in general radiography. Much less of the incident X-ray beam will therefore be absorbed than with caesium iodide, which has K-edges of 36 keV and 33 keV. c. False. For the reasons mentioned in (b), the opposite is true. To achieve the same image quality, a higher patient dose is required with direct detectors. d. True. Again, this is related to differences in the DQE. e. True. Another measure of this is the modulation transfer factor, which is greater with direct than indirect conversion detectors. The differences relate to the direct transfer of electrical charge within the selenium, driven by the potential difference applied. With indirect detectors there is the possibility of light scatter within the scintillator, which increases unsharpness. |
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