Curved x-ray filter elements are arranged circularly around a drum (40). Alternate circumferentially adjacent elements have different attenuation properties. Diametrically opposite elements have the same properties. An x-ray source (11) provides a sequence of alternate low and high energy x-ray beam pulses. A multicell x-ray detector (18) produces x-ray attenuation data for each pulse and the data is switched to altemate memories (1, 2). A drum drive motor (55) is energized at power line frequency. A pinion on the motor shaft turns a gear. A lug that is mounted for angular adjustment on the gear drives the drum (40). A multiple blade shutter is also mounted on the gear for angular adjustment and rotation with the gear. The shutter alternately interrupts and restores a light beam to a photosensitive device which produces corresponding signal state changes or trigger signals (95). The signals are used to synchronize the low and high energy x-ray pulses, the filter elements angular locations and memory switching with power line frequency.