Polo mint sign | Radiology Signs

Answer:

The polo mint sign is a central filling defect surrounded by a rim of contrast within a pulmonary artery imaged in cross-section, so that the vessel looks like a ring-shaped mint. It indicates an acute non-occlusive pulmonary embolus lying free in the lumen with contrast passing around it. When the same clot is imaged along the length of the vessel it produces the railway track sign, contrast running on either side of a longitudinal defect. The distinction from chronic thromboembolic disease matters: chronic thrombus is eccentric, forms obtuse angles with the vessel wall, may calcify, is associated with webs, bands and abrupt vessel narrowing or complete occlusion with a tapered end, and with mosaic attenuation and right heart strain when pulmonary hypertension has developed, whereas acute clot is central, forms acute angles and expands the artery. The commonest pitfalls that mimic a filling defect are respiratory motion artefact, poor contrast timing with flow-related mixing artefact at the level of the pulmonary trunk, beam-hardening streak from dense contrast in the superior vena cava, partial volume averaging with adjacent lymph nodes or unopacified veins, and mucus-filled or fluid-filled bronchi paralleling the artery. Reporting should include clot burden, the most proximal level involved, and the right ventricular to left ventricular diameter ratio, since a ratio above 1.0 indicates right heart strain.

Why is it called so?

The ring of contrast around the central clot resembles a Polo mint, the confection with a hole in the middle.

Pathophysiology

Thrombus that has embolised from a deep vein of the leg or pelvis lodges in a pulmonary artery whose calibre it approximately matches. In the acute phase the clot is fresh, soft and not yet adherent to the endothelium, so it sits centrally in the flowing blood column and contrast surrounds it completely, giving the ring appearance in cross-section. Over weeks an unresolved clot organises: it retracts against the vessel wall, becomes endothelialised and fibrotic, and is remodelled into eccentric mural thickening, webs and bands, which is why chronic disease loses the central position and the acute angles. The persistent obstruction and the accompanying small-vessel arteriopathy raise pulmonary vascular resistance and lead to chronic thromboembolic pulmonary hypertension.

Alternative names: Doughnut sign of pulmonary embolism

Other associated named signs: Railway track sign in the longitudinal plane and the vessel cut-off sign of complete occlusion