Ring of fire sign | Radiology Signs

Answer:

The ring of fire sign is intense circumferential colour Doppler flow around a rounded adnexal structure, reflecting the low-resistance, high-velocity vascularity of trophoblastic or luteal tissue. It is well known as a feature of an ectopic pregnancy, where it surrounds the tubal ring, but the critical teaching point is that it is equally common around a normal corpus luteum and therefore does not distinguish the two. The features that do distinguish them are anatomical rather than vascular: the corpus luteum lies within the ovarian parenchyma and moves with the ovary on transducer pressure, whereas an ectopic tubal ring lies outside the ovary and slides separately from it, and the wall of a tubal ring is typically more echogenic than the ovarian stroma while the corpus luteum wall is less echogenic. Diagnosis of ectopic pregnancy rests on the combination of a positive pregnancy test, an empty uterus above the discriminatory serum hCG level, an adnexal mass separate from the ovary, free fluid, particularly echogenic free fluid indicating haemoperitoneum, and serial hCG kinetics; a live embryo outside the uterus is definitive but uncommon. A pregnancy of unknown location requires structured follow-up rather than a premature diagnosis, and interstitial, caesarean scar and cervical implantations must be specifically excluded because their management differs.

Why is it called so?

The circle of colour signal around the lesion resembles a ring of fire on the screen.

Pathophysiology

Both the corpus luteum and implanted trophoblast are supported by angiogenesis producing dilated, thin-walled vessels with low impedance, so colour Doppler shows abundant flow with a low resistive index, typically below 0.4 to 0.5. The corpus luteum forms after ovulation under luteinising hormone stimulation, with vascular endothelial growth factor driving the intense peripheral neovascularity that maintains progesterone production. Trophoblast invading the tubal wall induces the same angiogenic response in order to establish its blood supply, which is why the two look identical on Doppler. Because the tubal wall cannot accommodate the growing gestation, invasion of the muscular layer eventually causes rupture and haemorrhage, which is the emergency the imaging is meant to prevent.

Alternative names: Ring of fire; tubal ring with peripheral vascularity

Other associated named signs: Tubal ring sign, the pseudogestational sac of ectopic pregnancy and the double decidual sac sign of an early intrauterine pregnancy

References

1. Stein MW, Ricci ZJ, Novak L, Roberts JH, Koenigsberg M. Sonographic comparison of the tubal ring of ectopic pregnancy with the corpus luteum. J Ultrasound Med. 2004;23(1):57-62. PMID: 14756354.