Starting with version 4.4, PMOD implements a new mask-based VOI functionality for rapid segmentation of the high-resolution data such as micro-CT. The result are labeled masked called raster VOIs.
In principle, the raster VOI is a mask and therefore observes the mask properties: overlaps are not allowed and dynamic format is not supported. The mask-based VOIs tool is taking advantage of the flexible Segmentation interface and allows editing in 3D.
The definition is started via the dedicated button 
in the VOI tools panel as illustrated below:
Two options are available: Open Segmentation interface and contour the segments for creating VOIs and Create/Edit Mask VOI. They are described in detail below.