The environmental density artifact: a beam-hardening effect in computed tomography.

The environmental density artifact is an effect due to beam hardening in computed tomography (CT), and is the small but definite change in the CT number of a central region when surrounded by a larger region of different attenuation coefficient. This effect was measured in concentric cylindrical phantoms. The outer cylinder contained different solutions of iodinated contrast medium or isopropyl alcohol, and the central cylinder contained water. When the CT number in the outer cylinder increased by about 100 HU, that of the central water-filled cylinder increased by about 5 HU. This artifact may be the cause of erroneous diagnosis when contrast enhancement is employed in computed tomography.