In this post we go through the Region Growing tools which allow you to segment a specific area of an image.

What is Region Growing?

Region Growing is a way of segmenting anatomical structures of interest which has two key elements: 

  1. A seed voxel point inside the structure to be segmented. 
  2. A span of possible voxel grey-scale intensity values that the region can attain. 

Once it has a seed and a span, the region grows from the seed point to include all the contiguous voxels having intensity values contained in the span, and keeping the whole as a single connected component. The algorithm stops when there is no more voxels to include that fulfill these conditions. 

In Rhino3DMedical, Region Growing Segmentation is a subsection located in the Segmentation tab.

Region Growing tools are in the Segmentation tab.

How do I Define a Region?

There is one way of providing the seed voxel point (through the button Pick Seed Point), and one way of giving the span: using the threshold range. This will take the intensities defined by the threshold (subsection Set Threshold in Image Analysis tab).
Let us do an example where we want to segment the lung in the above image.


In the tab Image Analysis, we adjust the Threshold so that we have the region of interest properly segmented.

Image Threshold

In the tab Segmentation, we press button Pick Seed Point, select a slice and pick a point inside the region we want to segment. Note that we see the grey-scale intensity of the voxel we hover the mouse over.

Pick Seed Point

After picking the point, its 3D coordinates and intensity value are displayed in the Region Growing Segmentation subsection in tab Segmentation

Picked Seed Point

We can then extract the segmented region as a mesh, by pressing the button Create Surface from Region Growing in tab Segmentation. Note that the extracted region will not correspond to the whole colored area: for that we already have the Create Surface from Threshold button. Region Growing will start at the seed point and progressively add as many colored voxels as it’s possible while keeping a single connected component (the region will be a single object and not a set of smaller components).

Create Surface from Region Growing