Comet sign | Radiology Signs

The comet sign (also called the “tail sign“) is seen on nonenhanced helical CT of the abdomen and pelvis and helps distinguish a pelvic phlebolith from a true ureteral calculus. A phlebolith characteristically shows a small soft-tissue “tail” projecting eccentrically from one pole of the calcified focus, representing the adjacent collapsed pelvic vein, whereas a genuine ureteral stone typically lacks this tail and instead may show a circumferential “rim sign” of edematous ureteral wall, lying in the expected course of the ureter and often associated with proximal hydroureter or hydronephrosis.

Why is it called so?

It is named the “comet sign” or “tail sign” because the small linear soft-tissue density trailing from the calcified phlebolith resembles the tail of a comet extending from a fixed “head.” This is a different entity from the chest “comet tail sign” of rounded atelectasis, even though both borrow the same descriptive comet-tail terminology for an unrelated appearance.

Pathophysiology

Phleboliths are calcified thrombi that form within pelvic veins, most often along the internal iliac venous plexus. As the thrombus calcifies within the vein, the adjacent vein wall or perivenous soft tissue can collapse eccentrically at one pole, producing the comet-like tail on cross-sectional imaging. A true ureteral calculus, in contrast, sits within the ureteral lumen and is surrounded circumferentially by edematous, thickened ureteral wall (the rim sign) rather than an eccentric tail, and it lies along the expected retroperitoneal course of the ureter rather than adjacent to a pelvic vein.

Alternative names: Tail sign.

Other associated named signs: Rim sign (soft-tissue rim of edematous ureteral wall surrounding a true ureteral stone, used alongside the comet/tail sign to differentiate stones from phleboliths).

References

1. Boridy IC, et al. Ureterolithiasis: value of the tail sign in differentiating phleboliths from ureteral calculi at nonenhanced helical CT. Radiology. 1999.
2. Guest AR, et al. Assessment of the clinical utility of the rim and comet-tail signs in differentiating ureteral stones from phleboliths. AJR Am J Roentgenol. 2001.