VOI and Contour Properties

The List tab illustrated below is used to work with contour VOIs and define their properties.

VOI List Tab

The VOIs section contains the list of defined contour VOIs, and VOIs which are in the progress of outlining. New VOI creates a new list entry, with the plane direction P according to the currently selected plane (Z = axial for all VOIs in the example above). The selected VOI can be removed using the Remove button. The arrow buttons on top of the list allow changing the location of the selected VOI in the list.

VOI Properties

In addition to the plane orientation a VOI has several properties, which can be changed by the buttons to the right of the list. They have the following meaning.

Toggle /	Static VOI : This property is only relevant for dynamic series. If a VOI is static, the same VOI definition is applied and shown at all times. Dynamic VOI : This property is only relevant for dynamic series. For dynamic VOIs, a different VOI definition can be used at different times. Note that a dynamic VOI is only shown when viewing the images of the time frame it was defined on. As long as only one VOI is defined, statistics calculation will apply this VOI at all times as with a static VOI. However, as soon as VOIs have been defined at more than one time, it is required that VOIs are defined at all times. Here the VOI propagate functionality may be helpful.
Toggle /	Filled VOI : All pixels inside the contours belong to the VOI. Tape VOI : The contour is interpreted as the center line of a VOI which includes pixels inside a band along the contour. The number of pixels inside and outside can be specified. This type of VOI was introduced for myocardium VOIs.
Toggle /	Full VOI : Uses pixels from all directions. Sector VOI : Only the pixels in a defined angular sector will be considered. The sector angle can be defined in the VOI Action Mode using the handles at the angle corners. This type of VOI was introduced for myocardium VOIs.
	Color Palette. Allows changing the contour line color. All contours belonging to a VOI have the same color.

The illustration below illustrates the effect of the different properties on the pixels used for VOI statistics.

Sector VOI

Contour Properties

The Contours section contains the list of the contours defined in the current slice for the VOI selected in the VOIs list. The arrow buttons on top of the list allow changing the location of the selected contour in the list. Each contour has properties, which can be changed with the buttons on the right of the list.

Plus Contour: This buttons sets all contours selected in the list to + in the operation column O. The meaning of a + contour is that the pixels defined by the contour are included in the statistics. This operation mode is equivalent to the OR set operation.

Minus Contour: This buttons sets all contours selected in the list to -. The meaning of a - contour is that the pixels defined by the contour are excluded from the statistics. This operation mode is equivalent to the AND NOT set operation.

Cardiac example illustrating how a negative inner contour can be used to create a hollow VOI.

Toggle
/

Closed Contour : The last vertex is connected to the first vertex, so that a closed trajectory is formed.

Line Contour : Only the pixels along the trajectory from the first to the last contour vertex are included into the VOI.

Display Properties

In the lower right, there are some properties defining how the VOIs are displayed in the image overlay.

VOI Display Properties

The following properties can be configured in the option menu:

Show Names	Shows the VOI names next to the contours in the image overlay. Note: The location of the names can not be changed.
Hide Vertices	Removes the squares which indicate the polygon vertex locations from the image overlay, allowing a un-obstructed view of small contours.
Selected Only	Shows only the contours of the currently selected VOI to avoid confusion in the presence of multiple VOIs.
Data driven display	Determines the behavior of the image layout when switching between different series. If the box is checked, each image has its own properties such as layout, zoom, color etc. Otherwise, the images are shown with the same layout properties.

Usually contours are outlined on images displayed with interpolation, often combined with image zooming. In such a representation the images look smooth, even if the original pixels are big as with modalities suffering from low spatial resolution relative to the structure being outlined. To make sure that the right pixels are included in the VOI it is recommended - especially for small structures - to examine the VOI pixels as follows:

Check the Fill ROI box. The display now shows a raster outlining the VOI pixels (D). The raster can also be filled by setting the radio button to F (opaque filling) or T (transparent filling).
Disable image smoothing by disabling the

button in the image control section.
Modify the VOI contours while the Fill ROI box is still checked, for instance by moving a vertex. The raster is updated as soon as the changes are completed.

ROI Filling

Note: PMOD only considers the entire pixels for the statistics calculation which are visualized with the Fill ROI functionality.