A prototype scanning-slit X-ray imaging system with microchannel plate (MCP) detector has recently been developed and tested for potential applications in medical imaging. For this purpose, the quantum efficiency of the MCP needs to be calculated and verified experimentally for polychromatic X-ray beams. X-ray detection in the MCP is a cascade of several processes including the absorption of the photon in the MCP material, the escape of created fast (photo, Compton, or Auger) electrons from the MCP channel walls into the channels, triggering an avalanche of secondary electrons in the microchannels, and detecting the resulted charge pulse. Because electron transport in the material is involved in this process, Monte Carlo simulations were used to calculate the quantum efficiency of an MCP for photon beams used in mammography, computed tomography (CT), and chest radiography. The quantum efficiency of the MCP with 5-μm channel diameter was calculated to be 68%, 89%, and 81% at X-ray tube voltages of 45, 90, and 120 kVp, which are used in scanning slit mammography, breast CT, and chest radiography, respectively. The detection efficiencies of an MCP with 2 μm channel diameter was calculated to be 91%, 92%, and 86% at 45, 90, and 120 kVp tube voltages, respectively. The efficiency of a recently introduced silica MCP (pure SiO2) with 5 μm channel diameter was calculated to be 86% and 76% at 35 kVp and 45 kVp, respectively, and is rapidly decreased at higher energies. Results of the calculations show that the detection efficiencies of the MCPs are comparable to the currently available detectors in the medical X-ray imaging.