What causes the halo sign on CT of the chest?
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Answer:
The halo sign is a rim of ground-glass opacity surrounding a pulmonary nodule or mass on CT. In a profoundly neutropenic patient, most often during induction chemotherapy for acute leukaemia or after haematopoietic stem cell transplantation, it is an early and important indicator of angioinvasive aspergillosis, appearing within the first few days of infection and often before the classical air crescent sign, which develops later during neutrophil recovery. Because it can be an early sign of a treatable and rapidly fatal infection, it is one of the imaging criteria in the EORTC/MSGERC definitions of invasive fungal disease and should trigger galactomannan testing, bronchoalveolar lavage and pre-emptive antifungal therapy in the right host. The sign itself is not specific: a systematic review of the CT halo sign lists other angioinvasive moulds such as mucormycosis, which is more often associated with the reversed halo and with a larger number of nodules, other infections including candidiasis, herpes and cytomegalovirus, and non-infectious causes such as haemorrhagic metastases from vascular primaries, Kaposi sarcoma, granulomatosis with polyangiitis, organising pneumonia and adenocarcinoma in situ. Interpretation therefore depends on the host, and the same appearance means something entirely different in an immunocompetent patient.
Why is it called so?
The ring of ground-glass around the dense nodule looks like a halo encircling it.
Pathophysiology
Angioinvasive Aspergillus hyphae invade small and medium pulmonary arteries, causing thrombosis and infarction of the lung distal to the occluded vessel. The infarcted core becomes a dense nodule of necrotic tissue and organisms, while blood leaking from the damaged vessels haemorrhages into the surrounding alveoli, partially filling them and producing the ground-glass rim. The mechanism explains the timing: the halo requires vascular invasion in a host unable to contain the organism and lasts only days before evolving, and once neutrophils return, the necrotic core retracts from the surrounding lung and air enters the resulting crescentic space, giving the air crescent sign. In non-fungal causes the same rim is produced by peritumoural haemorrhage or by a peripheral inflammatory infiltrate.
Alternative names: CT halo sign
Other associated named signs: Air crescent sign, the reversed halo or atoll sign and the vessel occlusion sign of angioinvasive infection
References
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