2.1. CT acquisition

Chest CTA were performed on a 64-detector CT scanner (Revolution EVO; GE Medical Systems), with an automated tube current modulation at 100 kV. Collimation, pitch, rotation time and CT dose index volume (CTDIvol) were respectively 64 × 0.625 mm, 1.531:1, 0.4 s and 32 mGy (mean value). Slice thickness was 1.25 mm. All but one patient were examined supine. The enhanced scanning was performed during the intravenous bolus injection of a non-ionic contrast medium (∼50−70 mL; flow rate at least 3.5 mL/s; iomeprol 350 mg/mL; Bracco Imaging), followed by a rapid saline solution flush (∼30 mL, flow rate at least 3 mL/s). The acquisition of the arterial pulmonary phase relied on bolus tracking technique: a circular region of interest was manually set on the main pulmonary trunk, with a standard threshold of 100HU and a minimum scanning delay guaranteed by the CT scanner (7 s).