What is the role of CT scanning in the differentiation of phleboliths from urinary tract stones in nephrolithiasis?

Updated: Sep 16, 2021
  • Author: Chirag N Dave, MD; Chief Editor: Bradley Fields Schwartz, DO, FACS  more...
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Phleboliths are often confused with calcific ureteral stones. On a KUB radiograph, the characteristic lucent center of a phlebolith is often visible; this is not present in a true calculus. Unfortunately, CT scans usually fail to reveal this central lucency or a bifid peak if a central lucency cannot be identified. Why this finding of a central transparency is so uncommon with CT scanning is unclear, but it may involve the orientation of the veins that form the phleboliths.

The "rim sign," originally reported by Smith in 1995, is described as a rim, ring, or halo of soft tissue visible on CT scans that completely surrounds ureteral stones. [33] The effect is enhanced by the local inflammation a stone produces in the ureteral wall, with subsequent edema at the site of the calculus. The rim sign is generally missing or incomplete with phleboliths.

While not absolutely definitive, the rim sign is strong evidence that the calcific density it surrounds is a stone and not a phlebolith. In several studies, more than 75% of all ureteral stones demonstrated a rim sign, while only 2-8% of phleboliths demonstrated it. The rim sign is more likely to be present in small or medium stones up to 5 mm in diameter. Larger stones (>6 mm) tend to lose the rim sign, presumably from stretching and thinning of the ureteral wall around a relatively large calculus.

Another way to differentiate a phlebolith from a calculus is to find a comet’s tail or comet sign, which is the noncalcified portion of a pelvic vein that is contiguous with the phlebolith. It appears as a small linear area of soft tissue that seems to pass obliquely through the CT scan section and attaches to the calcific density at one end. This is not observed in ureteral stones, although a ureter can mimic this sign to some degree. The comet sign is found in less than 20% of phleboliths, so its absence helps little, and its reliability is still unproved.

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