To prevent patients from being overexposed or underexposed, it has been attempted to modulate either voltage or current in conventional single energy CT systems. The voltage modulation causes incompatibility in projection data among the views while the current modulation reduces only noise. To solve these and other problems, dual energy CT is combined with voltage modulation techniques to improve the dosage efficiency. Furthermore, dual energy CT has been combined with both voltage modulation and current modulation to optimize the dosage efficiency in order to minimize radiation to a patient without sacrificing the reconstructed image quality.