Pulmonary target sign

The pulmonary target sign has been described in the lung parenchyma as a central high attenuation focus surrounded by one or more dense complete or incomplete ring-like consolidation, forming one or more circles. This sign has been predominantly reported on non-contrast chest CT in patients with COVID-19 pneumonia ¹.

Terminology

Synonyms used for this finding include the bulls-eye sign ¹⁰, target shape reverse halo sign ^12,13 and rings of Saturn sign ¹⁴ but pulmonary target sign (or just target sign) has become the consensual term for this appearance.

For other uses of the term, see target sign (disambiguation).

Pathology

A hypothesized pathophysiology of the target sign of COVID-19 on lung CT is that it is a manifestation of organizing pneumonia (OP) ^7,8.  It seems that the central nodule-like opacity may correspond to a bronchiole-vascular bundle which is occluded with exudates or clot, while the peripheral ring-shaped airspace opacities corresponds to pulmonary tissue filled with the fluid of infective air space opacity.

• Practical points

• the target sign usually includes the following radiologic characteristics ¹¹:
  • peripheral or subpleural location
  • one or more dense surrounding rings
  • surrounding rings could be complete or incomplete   
  • multiple lesions may occur
  • close-by bronchovascular bundles may be involved
  • usually adjacent ground-glass opacities
• mimics:
  • reverse halo sign with key differences
    • focal central dense opacity rather than ground glass opacity
    • no internal reticulation vs necrosis associated with the reverse halo sign ²
  • atoll sign, however, the surrounding layer is thin ³
  • rings of saturn sign¹⁴
  • ring of fire sign,¹⁶ if central core was absent
  
   

The pulmonary target sign usually coexists with other typical organizing pneumonia features including the halo or reverse halo sign, although the mechanism of its correlation with these features is not clear ^9,10.