Fractal branching quantifies vascular changes and predicts survival in pulmonary hypertension: a proof of principle study.

OBJECTIVES: To develop a non-invasive method of assessing disease severity in pulmonary hypertension by quantifying the overall degree of vascular pruning using fractal geometry.
DESIGN: A retrospective analysis of ECG-gated CT pulmonary angiograms.
SETTING: A single national referral centre for the investigation and treatment of children with pulmonary hypertension. PATIENTS: Consecutive CT pulmonary angiograms in children and young adults (mean age 10.3 years, range 0.7-19.1) with pulmonary arterial hypertension assessed between January 2007 and April 2009. MAIN OUTCOME MEASURES: The fractal dimension (FD) of skeletonised CT pulmonary angiograms was calculated using the box counting method. The FD was compared with pulmonary vascular resistance, the percentage of predicted 6-min walk distance, WHO functional class and survival.
RESULTS: Diagnostic plots confirmed that the pulmonary artery angiograms were all fractal. The FD correlated negatively with the pulmonary vascular resistance index (r=-0.55, p=0.01, n=21) and with WHO functional class (p<0.01, n=31) while it correlated positively with the percentage of predicted 6-min walk distance (r=0.43, p=0.04, n=24). A lower FD was associated with poorer survival (HR 5.6; 95% CI 1.2 to 25; p=0.027) for every SD reduction in FD.
CONCLUSION: The FD derived from CT can be used to quantify vascular changes in pulmonary hypertension. This non-invasive technique may be useful in monitoring disease progression and response to therapy.