This disclosure presents systems and methods that synchronize an x-ray imaging system with the heartbeat of a patient. A patient's heartbeat is sensed with a cardiac monitoring unit, and a processing unit generates x-ray pulses that are synchronized with the patient's heartbeat. Based on the real-time heartbeat information, an x-ray imaging device can be operated to obtain x-ray images at various states of the cardiac cycle. The x-ray images taken over several cardiac cycles can be combined based on the relative state of the cardiac cycle in which the images were obtained to achieve high temporal resolution of a cardiac cycle. Additionally, x-ray images obtained at common relative states of the cardiac cycle can be combined to provide higher quality cardiac image or images.