A 4-dimensional digital tomosynthesis system includes an x-ray source, an x-ray detector and a processor. The x-ray source is suitable for emitting x-ray beams to an object with a periodic motion. The periodic motion is divided into (n+1) time intervals, n being a positive integer. Each of the (n+1) time intervals is associated with a time instance t.sub.i, i=0, 1, 2, . . . , n. The x-ray detector is coupled to the x-ray source for acquiring projection radiographs of the object at each time instance t.sub.i for each scan angle based on the x-ray beams. The processor is communicatively coupled to the x-ray source and the x-ray detector for controlling the x-ray source and processing data received from the x-ray detector such that all projection radiographs acquired from all scan angles for each time instance t.sub.i are reconstructed and (n+1) sets of cross sections of the object are obtained. The cross section is either a coronal cross section or a sagittal cross section. Each of the (n+1) sets of cross sections is for a different time instance t.sub.i.