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:

Parameter	Description
Shell Thickness	Thickness of the hollow shell (mm).
Shell Mode	Where the shell is created relative to the surface.

Shell Mode options:

Mode	Description
Inside Surface	Shell extends inward from the original surface.
Medial Surface	Shell is centered on the original surface.
Outside Surface	Shell extends outward from the original surface.

Grow Mask

Expand the mask outward by a specified amount (dilation):

Parameter	Description
Growth Amount	Distance to expand the mask (mm).

Shrink Mask

Contract the mask inward by a specified amount (erosion):

Parameter	Description
Shrink Amount	Distance to shrink the mask (mm).

Parameters

Target Objects

Option	Description
Active mask	Apply to the active mask only.
Selected masks	Apply to all selected masks.
All masks	Apply 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).

info

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:

$$\text{Grown Mask} = M \oplus S_r$$ $$\text{Shrunk Mask} = M \ominus S_r$$

Where $S_r$ is a spherical structuring element with radius $r$.

See Also

• Morphology Operations — Detailed morphological operations.
• Cavity Fill — Fill hollow structures.
• Boolean — Create shells using Boolean difference.
• Segmentation Tab Overview — Overview of all segmentation tools.