Projection data (302) acquired during a tomographic examination of a periodically moving object is used to reconstruct a plurality of image layers (308). The image layers (308) are combined to generate image data at a desired phase of motion. To generate a weighting function used to combine the image layers, a reference weighting function (512) is generated at the desired phase. The image layers (308) are weighted to approximate the first weighting function (312). The number of image layers and the size of a sub-region of interest are advantageously selected so that the various image layers can be stored in a relatively high speed memory portion of a computer.