6.2. The Mask

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Functionality is available in the Pro edition
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The mask is a pattern beyond which the structure is not visible. A mask can be constructed in different ways:

by density: the mask includes voxels from a certain range of densities set manually;
by color table: the mask includes voxels corresponding to the color table;
by visible volume: the mask includes only voxels that are currently displayed on the screen.

The mask is not visible in the volume reconstruction window, and it is displayed in green in the MPR window. If necessary, the mask can be inverted. In this case, the tissues not under the mask are displayed in green. To invert the mask, set the flag Invert edit mask on MPR (see Section 15.5.3).

The structure can occupy an arbitrary volume that does not go beyond the base volume, or be empty.

The mask can be changed at any time. If the mask is reduced and the structure goes beyond the mask, then this part of the structure becomes invisible and is not taken into account in the calculations. However, it is not removed and when the mask is increased, it becomes visible again.

6.2.1 Creating a mask

Open the segmantation panel by clicking the Segmented structure panel button.

Click the Add a new segmented structure button. The dialog shown in Fig. 6.5 appears. Set the tissue density values for the mask. You can do it in one of the three ways:

using the Threshold mode. In this mode the mask is set for the tissue whose density is greater or less than the specified threshold value. To set the mask for the tissue with a density less than the threshold, select the Low intensity setting, and vice versa. Set the threshold;

using the Interval mode. In this mode the mask is set for the tissue whose density is in the range from minimum to maximum;

using color tables. In this mode the mask is set for the tissue displayed for the selected color table.
If nessesary, adjust the opacity of the mask in percentage.
To make visible the mask outline, check the Show edit mask outline box.

Figure 6.5: Set mask

6.2.2 Mask editing

You can edit the mask in several ways:

Using the mask settings dialog:

Select the target structure on the panel of segmented structures.
Click the Segmented structure editing button on the volume editing panel (only for the MPR window).
Click on the arrow on the right side of the Add a new segmented structure button, select the Segmented density config settings item.
Set values in the dialog that opens.

Using the Adjust W/L tool:

Select the target structure on the panel of segmented structures.

Click the Adjust W/L button on the toolbar.

Change the window width and level for the mask (see Section 2.13). The green mask changes in real time in the MPR window.
Selecting the color table from the dropdown list (see Section 2.17.1) or with the Custom W/L tool (see Section 2.13).
For the properties tree on the Segmented structure panel (Figure 6.6):
1. Unfold the list of the segmented structure properties by pressing "+" on the left-hand side of the structure name.
2. In the drop-down list, select the mask mode or the color table you need::
  - The High intensity and Low intensity mode provides the mask for tissues whose density is above or below the threshold value.
  - The Interval mode provides the mask for tissues whose density is within the set value range.
  - The CLUT mode provides the mask for the tissues displayed for the chosen color table.
3. To change the interval or the density threshold, double-click with the left mouse button on the values (see the red box in Figure 6.6). Provide the values you need and press Enter key on the keyboard.
Figure 6.6: Editing the mask

The mask parameters (the color table, the intensity threshold value and the interval) are changed for each segmented structure separately. The color table type and the mask mode for the segmented structure selected is shown on the visualization setup panel.

For the Volume Reconstruction window only: using the Set mask tool (see Section 6.2.3).

6.2.3 Set Mask Tool

By default, the base volume is the mask for all segmented structures. To set another structure as the mask for a structure or the base volume:

Select the structure (base volume) for which you want to change the mask.

Click on the Set Mask button. The dialog shown in Fig. 6.7 will be displayed.
Select the structure (base volume) which you want to set as the mask.
Click OK to set the mask and Cancel to calcel the action.

Figure 6.7: Select mask dialog