1. A method comprising: emitting a first plurality of radiation beams from respective ones of a first plurality of locations along a first arc; acquiring a first plurality of projection images of a target based on the first plurality of radiation beams; emitting a second plurality of radiation beams from respective ones of a second plurality of locations along a second arc; acquiring a second plurality of projection images of the target based on the second plurality of radiation beams; and generating a three-dimensional image of the target based on the first plurality of projection images and the second plurality of projection images, wherein a density of the first plurality of locations along the first arc is less than the density of the second plurality of locations along the second arc.