Published online: 5 December 2003 Springer-Verlag 2003 How does the choice of KV affect patient dose, scattered radiation, and image quality in CT examinations? When the x-ray tube voltage (kV) in CT is reduced while keeping the mAs constant, the patient dose is decreased. Dropping the x-ray tube voltage from 120 to 80 kV at a constant current (mAs) typically reduces the patient dose by about 60% because of the large reduction in x-ray tube output. The choice of kV dose not affect the amount of ‘‘scatter’’—CT has negligible image scatter since the thin detector (up to 2–3 cm) and the data acquisition geometry ensure that most scattered photons miss the detector. The contrast of a lesion relative to the surrounding background generally increases as kV is reduced, but the noise (mottle) level also increases since there are fewer photons at the lower kV (hence the lower patient dose). The lesion contrast-tonoise ratio (CNR) generally gets worse as the kV is reduced at a constant mAs in CT—the increase in contrast is less than the corresponding increase in noise [1]. If one reduces the kV and increases the mAs, however, it may be possible to maintain image quality (CNR) while reducing patient doses. CT optimization with respect to x-ray tube voltage is being investigated by several researchers, with initial findings suggesting that the use of lower kV values is promising for CT performed using iodinated contrast material [2, 3]. To summarize, reducing the CT kV at a constant mAs will always reduce the patient dose, and would probably reduce the lesion CNR. When iodine is administered to a patient, reducing the kV is likely to reduce patient doses with no adverse effect on CNR.