Figure 1. Fractional dimensions and self-similarity are defining properties of fractal geometry.

(A) Two-dimensional airway trees of varying fractal dimensions (Df) ranging from 1.09 to 2. The airway geometry becomes more space filling as Df approaches 2, the Euclidean dimension of a plane (24). (B) Three-dimensional model of the human airway tree showing self-similarity at different scales (25).