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Manipulate Mask

The Manipulate Mask tool modifies mask geometry by hollowing, growing (dilating), or shrinking (eroding) the mask boundaries. These operations change the extent of the mask without altering its overall shape.

Accessing the Tool​

  1. Navigate to the Segmentation tab in the ribbon.
  2. Click Manipulate Mask in the Operations section.

Operations​

Hollow Mask​

Create a hollow shell from a solid mask by removing interior voxels:

ParameterDescription
Shell thickness (mm)Thickness of the hollow shell. The tool reports the thickness it can actually produce at the mask's resolution, in millimetres and in voxels, and warns when the value is not feasible.
Shell modeWhich surface of the shell the original mask surface becomes.

Shell mode options:

ModeDescription
Inside surfaceThe original surface becomes the inside of the shell, so the shell extends outward by the thickness.
Medial surfaceThe original surface runs through the middle of the shell, half the thickness to each side.
Outside surfaceThe original surface becomes the outside of the shell, so the shell extends inward by the thickness.

The three shell modes and the Grow and Shrink operations on one shape: Inside surface builds the shell outward from the original surface, Medial surface centres it on the surface, Outside surface builds it inward; Grow moves the boundary outward by the margin, Shrink moves it inward

Grow Mask​

Move the mask boundary outward by a distance (dilation):

ParameterDescription
Margin size (mm)How far the boundary moves outward.

Shrink Mask​

Move the mask boundary inward by a distance (erosion):

ParameterDescription
Margin size (mm)How far the boundary moves inward.

Parameters​

Target Objects​

OptionDescription
Active maskApply to the active mask only.
Selected masksApply to all selected masks.
All masksApply to every mask.

Workflow​

  1. Open the Manipulate Mask tool from the Segmentation tab.
  2. Select the Target Objects.
  3. Choose the Operation (Hollow, Grow, or Shrink).
  4. Configure the operation parameters.
  5. Click Apply to modify the mask.

Use Cases​

Creating Hollow Structures​

Convert solid segmentations to shells for:

  • Lightweight 3D printing models
  • Wall thickness analysis
  • Sheath or casing design

Margin Creation​

Grow masks to create safety margins around structures:

  • Radiation therapy planning margins
  • Surgical planning buffers
  • Tolerance zones

Size Reduction​

Shrink masks to:

  • Create inner boundaries
  • Remove surface voxels
  • Compensate for partial volume effects

Tips​

  • Use physical units (mm) for consistent results across different image resolutions.
  • For Hollow operations, ensure shell thickness is larger than voxel spacing.
  • Combine Grow and Shrink operations to smooth boundaries (morphological opening/closing).
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The Grow and Shrink operations are equivalent to morphological dilation and erosion, respectively. Combining them creates opening (shrink then grow) or closing (grow then shrink) effects.

Technical Background​

Hollow Operation​

The hollow effect is created by:

  1. Computing a distance transform from the mask surface.
  2. Selecting voxels within the specified shell thickness.
  3. Removing voxels outside the shell zone.

Grow/Shrink Operations​

These operations use morphological structuring elements:

Grown Mask=M⊕Sr\text{Grown Mask} = M \oplus S_r Shrunk Mask=M⊖Sr\text{Shrunk Mask} = M \ominus S_r

Where SrS_r is a spherical structuring element with radius rr.

See Also​