1. A system for tomosynthesis imaging comprising: an x-ray source configured to move to different imaging positions during tomosynthesis image acquisition; a detector configured to obtain images while the x-ray source is in motion; and a collimator configured to control dispersion of radiation from the x-ray source, the collimator being positionable so that the collimator moves in a direction opposite to the directional movement of the x-ray source during tomosynthesis image acquisition.