Using Contour VOIs

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Using Contour VOIs

In PMOD a contour VOI is defined as a stack of planar, closed polygons which are named ROIs (region-of-interest). The contours are manually or semi-automatically outlined on the loaded images, and the pixels contained within the contour boundaries are considered for the VOI statistics.


There are a few principles to bear in mind when working with contour VOIs.

The coordinates of the contour vertexes are defined as the (x/y/z) triples. The x and y offsets are in [mm] from the image origin, whereas z represents the slice number. This makes the definition independent of the planar image resolution and the zoom factor. However, note that modification of the origin coordinate results in a shift of all VOIs defined for the image study.

A contour VOI can be defined in each of the orthogonal directions. Naturally however, a single VOI can only consist of contours in the same direction. There exists a tool for changing the direction.

Contour VOIs consist of one ROI per slice. This ROI can have one or multiple independent contours.

A contour is a closed polygon which appears in PMOD as below with vertexes marked by squares and connected by straight lines.
For each contour the user can define, whether the included pixels should be included into the statistics or excluded from it. By defining contours with exclusion, VOIs with hollow parts can be created.

Single pixels are a special case of contours. They can be easily added to or removed from the contour VOI.

For dynamic data the VOI can be extended to the temporal dimension as well. This means that the VOI may have a varying shape or location at different acquisitions over time, allowing to track moving organs over time. As long as the VOI has only been defined at one time point, it is applied equally at all times (static VOI). When defining the VOI differently at different times (dynamic VOI), care must be taken that a VOI is defined at all times.

Many studies can be loaded and processed in parallel. Hereby it is possible to have individual VOIs for the different studies, but the VOIs can also be shared among spatially matched images.