Boundary Cut: Upper airway volume + Lower airway volume does not equal Total airway volume

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Hello,

I am working on a research project where I need to measure upper, lower and total airway volumes from CBCT scans using 3D Slicer (version 5.10.0).

My workflow is the following:

  1. Segment the complete upper airway in ITK-SNAP.
  2. Import the segmentation into 3D Slicer.
  3. Convert the segmentation to a model.
  4. Create three anatomical planes using Markups Plane:
    • Superior plane (ENP-Ba)
    • Intermediate plane (C2)
    • Inferior plane (C4)
  5. Use Dynamic Modeler → Boundary Cut to create:
    • Upper airway (between ENP-Ba and C2)
    • Lower airway (between C2 and C4)
  6. Measure the volume of each model in Models → Information.

The anatomical definition is:

  • Upper airway: between ENP-Ba and C2
  • Lower airway: between C2 and C4
  • Total airway: between ENP-Ba and C4

Therefore, mathematically I would expect:

Total airway volume = Upper airway volume + Lower airway volume

However, I consistently obtain smaller summed volumes.

For example:

Example 1

  • Total airway = 14,084 mm³
  • Upper airway = 4,856 mm³
  • Lower airway = 2,789 mm³
  • Sum = 7,645 mm³

Example 2

  • Total airway = 16,622 mm³
  • Upper airway = 12,894 mm³
  • Lower airway = 3,039 mm³
  • Sum = 15,933 mm³

I also tried:

  • using Seed Points,
  • recreating the models,
  • repeating the cuts,

but the discrepancy remains.

My questions are:

  1. Is Boundary Cut intended to generate complementary sub-volumes?
  2. Should I expect the sum of the upper and lower airway volumes to equal the total airway volume?
  3. If not, what is the recommended workflow in Slicer to partition an airway model between anatomical planes while preserving volume?

The goal is to obtain reproducible airway volume measurements for scientific research.

Thank you very much for your help.