Fractal analysis of nuclear medicine images for the diagnosis of pulmonaryemphysema: Interpretations, implications, and limitations

OBJECTIVE. The purpose of this study was to investigate, on images obtained in nuclear medicine examinations, the physical meanings and consequent imp lications of fractal analysis developed in a recent study that was reported to be effective in quantifying the heterogeneous distribution of carbon pa rticle radioaerosol in the lungs. MATERIALS AND METHODS. Fractal dimensions were computed for 108 sets of rad ionuclide imaging data from 28 patients according to the methods in a previ ous report, and were then correlated with the ratio of tissue areas segment ed at two thresholds (15% and 35% of maximal radioactivity). RESULTS. Fractal dimension was found to linearly correlate with the ratio n atural logarithm of tissue areas segmented at two different threshold level s (n = 108, r = 0.999), with re regression slope accurately predicted (erro r = 0.06%). Bland-Altman analysis showed that fractal dimensions ranging fr om 0.2 to 1.9 can be explained by this area ratio with disagreement of only 5.13% at two standard deviations; thus, fractal dimension seems to be an o versimplified parameter unrelated to spatial heterogeneity of radioaerosol distribution. CONCLUSION. The analysis of this study suggested that the fractal dimension defined in a previous report was limited to the indication of the percenta ge area of low-radioactivity regions with respect to total tissue area in t he image. Because the fractal dimension partially reflects, but is not spec ific to, a certain degree of focal spots of low radioactivity, we suggest u sing fractal analysis in clinical practice only with careful control and th orough understanding of the physical meanings.