Several papers also reported that either SSD or MIP has the problem of either overestimation or underestimation. Overestimation of stenosis is possible when low subject contrast is present between the vessel and background tissue, due to reduction of vessel attenuation. Also, vessel attenuation is reduced when the vessel diameter is less than the effective section thickness. Elevation of background attenuation occurs naturally in MIP images, because the MIP process selects the statistical outliers for every projection ray through a uniform background. This effect is proportional to the number of voxels through which the projection rays are passed. Background attenuation elevation is accentuated when the background is not uniform, as occurs when soft tissue such as the psoas muscle is included in the edited volume. Under these conditions, a stenotic vessel may appear occluded if subject contrast falls below the low contrast resolution of the imaging system. Besides, because of sub-optimal bolus timing, reduced flow distal to stenoses, and noise and partial volume effects, the attenuation in vessels may not be uniform. For these reasons, a recently successful technique of volume rendering, Direct Volume Rendering, which does not require choice of a threshold value, and which retains complete volume data, probably will solve this problem.