A diagnostic imaging system (8) images a subject at a preselected phase point (46) which occurs in one or more successive pulmonary cycles. A breathing monitor (44) monitors a cyclic physiological parameter in the pulmonary cycle and generates a cyclic pulmonary phase indicative signal. A CT scanner (12) is disposed adjacent an examination region (28) to generate transmission radiation data. A data processor (60) reconstructs an attenuation map (96) from the transmission data by weighting the transmission radiation data such that each of the pulmonary phases contributes substantially equally to the attenuation map.