Résumé : Objective: To investigate the effect of a two-third reduction of the scanned length (i.e. 10cm) on diagnosis of both pulmonary embolism (PE) and alternative diseases. Methods: 247 consecutive patients suspected of acute PE had a CT pulmonary angiography (CTPA) of the thorax (standard length, L). Based on this acquisition, a second set of images was created to obtain a scan length of 10cm caudally to the aortic arch (l). Images were anonymized, randomized and interpreted by two independent readers. The quality of enhancement, the presence of PE and the possible alternative and/or complementary diagnoses were recorded. A McNemar exact test investigated differences in discrepancies between readers and between scan lengths. Results: 57 (23%) patients had an acute PE. Among l sets, PE was missed by both readers in one (1.8%) patient, because the unique clot was localized in a subsegmental artery out of the 10-cm range. There were discrepancies between L and l sets in 9 (3.6%) and 11 (4.5%) patients, by Readers 1 and 2 (p50.820), respectively. Discrepancies between the readers of L sets and those between both sets were not different regardless of the reader (p > 0.99). There were discrepancies between both sets for alternative and/or complementary diagnoses in 43 (17.2%) patients. Conclusion: Although its performance in diagnosing PE is maintained, CTPA should not be restricted to a range of 10cm centred over the pulmonary hilum, because alternative and/or complementary diagnoses could be missed. Advances in knowledge: (1) A 10-cm CTPA acquisition reduces the radiation dose by two-thirds as compared with a standard one, but does not impair the accuracy for the diagnosis of PE. (2) Significant alternative diagnoses are missed in 17.2% of patients when reducing the acquisition height to 10 cm. © 2014 The Authors. Published by the British Institute of Radiology.