Figure 1. 63-year-old patient with fever at aplasia following bone marrow transplant for acute leukaemia. Axial unenhanced CT-image (view from below) shows a large area of consolidation in the right upper pulmonary lobe. (a) Using a narrow "soft tissue" window setting, the central hypointensity (white arrow) is easy to delineate (width 120 HU; level 20 HU). (b) Note poor delineation of the infarcted area (white arrow) on the same image viewed by standard "soft tissue" window setting (width 450 HU; level 35 HU). (c) The same lung opacity is viewed by a lung window setting (documented two slices caudally from a,b). This window setting is not suitable for recognition of the central hypointensity, but it demonstrates the halo sign (white arrow).