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.