What causes the melting ice cube sign on serial CT of the chest?
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Answer:
The melting ice cube sign is the characteristic way a pulmonary infarct resolves on follow-up imaging: the wedge-shaped peripheral opacity shrinks progressively while retaining its shape and pleural base, becoming smaller but remaining geographic, until it disappears or leaves a linear scar. This behaviour separates infarction from pneumonia, which resolves in a patchy, fragmented fashion from within, and it is a useful retrospective confirmation when the initial imaging was equivocal. The initial appearance is the Hampton hump, a pleura-based wedge with its apex directed to the hilum, often with the vessel sign, a feeding pulmonary artery leading to its apex, and with central lucencies that reflect preserved aerated secondary lobules and help distinguish it from consolidation. Resolution is slow, typically over several weeks to a few months, and the residual scar may retract the adjacent pleura. Infarction complicates only a minority of pulmonary emboli because of the dual bronchial and pulmonary arterial supply, and it is more common with distal emboli and in patients with impaired bronchial collateral flow such as those with cardiac failure. A cavitating or enlarging opacity in this setting instead suggests superinfection with abscess formation or septic embolism and warrants reassessment rather than reassurance.
Why is it called so?
The opacity shrinks while keeping its shape, in the way an ice cube melts down without changing its outline.
Pathophysiology
When a pulmonary artery branch is occluded, the alveoli of the affected segment fill with blood from the intact bronchial circulation, producing a haemorrhagic wedge of consolidation whose boundaries are those of the vascular territory. If the tissue is not truly necrotic, the alveolar haemorrhage is cleared by macrophages and the lung re-expands from the periphery of the wedge inward, so the lesion contracts along its own outline rather than breaking up. Where true infarction with necrosis has occurred, healing is by fibrosis and the residuum is a permanent linear or wedge-shaped scar with pleural retraction. Bacterial pneumonia by contrast resolves through patchy clearance of inflammatory exudate across the affected airspaces, which is why its appearance fragments as it improves.
Alternative names: Melting sign; vanishing wedge
Other associated named signs: Hampton hump, the vessel sign of the feeding artery, and the Westermark sign in the same disease
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