1. An X-ray imaging apparatus comprising: an X-ray emitter that emits X-rays to an object at a plurality of positions; a detector that detects X-rays having passed through the object and converts the detected X-rays into electric signals; and an image processor that is configured to generate X-ray images at the plurality of positions by reading out the electric signals, acquire volume data of the object using the X-ray images, and reproject the acquired volume data by using different bands of energy spectrums to acquire reconstructed reprojection images of different energy bands.