Clinicians have long known that the symptoms and signs of pulmonary embolism are non-specific and that, unless the index of suspicion is reasonably high, the diagnosis is frequently overlooked. Against this background, clinically unsuspected pulmonary emboli are increasingly being spotted by radiologists on CT scans. Clinicians not only need to be aware of this, but also need to know how to deal with such serendipity. The quality of CT examinations has improved unimaginably; image acquisition, particularly with the new generations of multidetector CT (MDCT) machines, is now astonishingly fast and access to CT scanning has increased. The entire thorax can now be covered in a single breath-hold, and image degradation due to respiratory and cardiac motion is no longer a major issue. Furthermore, because of narrow collimation, images with exquisite spatial resolution are almost the norm and visualisation of opacified peripheral pulmonary arteries (down to fifth order branches) is now possible.^1,2

The detection of unsuspected pulmonary emboli by CT scanning has been the subject of a number of previous reports.^3–8 The prevalence of incidental emboli in these studies has varied from 0.6% to 5% and has depended to a large degree on the tests used for detection (single-slice CT vs MDCT vs echocardiography), the manner in which images were reviewed (hard copy vs workstation analysis) and the demographics of the study population (cancer vs non-cancer or inpatient vs outpatient).^3–9

In the current issue of Thorax (see p 536), another study documents the prevalence of incidental pulmonary emboli in consecutive inpatients undergoing MDCT scanning.¹⁰ Most patients were imaged on a 16-channel CT machine and, following the routine report, all studies were reviewed by an experienced thoracic radiologist unaware of the original findings. In nine …