Can CT pulmonary angiography replace ventilation-perfusion scans as a first line investigation for pulmonary emboli?

A prospective study was performed to determine efficacy of diagnosis of pulmonary emboli by computed tomographic pulmonary angiography (CTPA) in patients who underwent both CTPA and ventilation-perfusion (V/Q) scanning. The results were compared with the Prospective Investigation of Pulmonary Embolism Diagnosis (PIOPED) study in which conventional pulmonary angiography had been performed instead of CTPA. Forty-two of 161 (26%) patients had a positive CTPA compared with a 27% prevalence in the PIOPED population. Fourteen of 16 patients (87.5%) with high-probability V/Q scans also had a positive CTPA compared with 87% in PIOPED. Twelve of 40 patients (30%) with intermediate probability V/Q scans also had a positive CTPA compared with 34.7% in PIOPED, while 12 of 80 patients (15%) who had low-probability V/Q scans had positive CTPA compared with 14.5% in PIOPED. Four of 25 patients (16%) with normal V/Q scans had positive CTPA compared with 0% in PIOPED. While the present study size was relatively small, the results compared favourably with PIOPED, suggesting that equivalent prevalence of clot was being detected using CTPA. This result, together with the cost considerations, has led us to replace V/Q scanning with CTPA for investigation of the majority of cases of suspected, acute pulmonary emboli.