Abstract: An apparatus for determining MTF and DQE of an ionizing radiation imaging system/detector is provided, comprising a box having aligned windows transparent to an ionizing radiation beam. When the box is placed in front of the detector, the beam passes through the box. The apparatus further comprises: a KERMA module for measuring incident free-air KERMA; a backscatter baffle for preventing backscatter of the beam from the detector into the KERMA module; a scatter baffle for preventing scatter of the beam into the KERMA module, and to reduce backscatter from the backscatter baffle; at least one MTF module for enabling acquisition of at least one edge image. Each module and the at least one backscatter baffle are independently moveable in and out of the beam, such that open, dark and edge images may be independently acquired, and KERMA module measurements may be performed independent of image acquisition, to determine DQE.