Editing the visual appearance of the volume...

Sometimes visual artifacts appear, especially if you assign a sharp boundary in the tranfer function. Usually such artifacts can be reduced by adjusting the sampling rate. A higher sampling rate however influences the rendering performance, so adjusting the sampling rate carefully is important for both the image quality and the interactivity.
Open the volume editor and select the Sampling tab.
Move the upper slider to the left. The number of slice images that is used to display the volume now decreases. The image quality is low, the rendering speed is high.
Now move the upper slider to the right. The number of slice images increases and thus the quality of the image. You may have to adjust the opacity curve of transfer function to adapt it to the new sampling rate.
If you switch on LowRes on Motion in the Volume menu, you can assign a lower sampling rate for user interaction. If this feature is enabled, the lower slider determines the sampling rate, that is used during user interaction. Using different sampling rates for still images and interactive renderings, we are able to effectively trade quality for speed.

SIGGRAPH'02 IEEE VIS'02 EG 2002 SourceForge.net