Pulmonary target sign
The target sign in the context of the lungs, or pulmonary target sign, has been defined as a central high attenuation focus surrounded by a dense ring like consolidation in the form of one or two circles within the lung parenchyma. This feature was almost always described on non-contrast Chest CT in patients with COVID-19 pneumonitis .
The sign appears similar to the reverse halo sign but there are some imaging differences.There is a focal central dense opacity dot rather than ground glass opacity (GGO), and there is no internal reticulation as compared to the necrosis associated with the reverse halo sign . The surrounding layer is thin unlike one in the atoll sign . These findings may be prone to misinterpretation .
One possible pathophysiology of COVID-19 lung CT target opacities is that of organizing pneumonia (OP) . 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 infectious consolidation.
The Target sign is currently only reported in SARS-Cov2. This sign usually occurs along with other typical OP features including the halo or reverse halo sign, although the mechanism of its correlation with these features is not clear .
This sign usually includes the following radiologic characteristics :
- Peripheral or pleural based location
- With one or two dense surrounding rings
- Sometimes multiple such lesions occur
- Nearby broncho-vascular bundles may be involved
- Usually adjacent to ground glass opacities
Target sign with the mentioned features, a dense central focal opacity without central GGO or calcified nodule , has not been reported in non-COVID-19 conditions, but as radiologists become familiar with this finding, it may be reported in some other diseases in the future.
As the target sign is better identified and understood regarding its relationship to COVID-19 pneumonitis, radiologists may begin to use this sign to suggest COVID-19 testing and rapid follow-up. The target sign appears to be supportive of COVID-19 pneumonitis.
The term target sign is used in a number of other contexts: see target sign (disambiguation). Radiologists unfamiliar with this sign may use different descriptive language in their reports e.g. bull eye sign , target shape reverse halo and rings of Saturn but pulmonary target sign or simply target sign seems to be the consensus on the correct term for the associated findings.