Elephant on a flagpole | Radiology Signs

Answer:

The elephant-on-a-flagpole sign describes a rounded extrarenal pelvis sitting atop a normal-calibre ureter: the bulbous pelvis is the elephant and the ureter below it the flagpole. It is seen on intravenous urography and on the excretory phase of CT urography. The important point is that an extrarenal pelvis is a normal anatomical variant, in which the pelvis lies largely outside the renal sinus and is therefore free to balloon without being constrained by sinus fat. It must not be mistaken for hydronephrosis. The features that argue for a variant rather than obstruction are non-dilated calyces with preserved sharp fornices, a normal-calibre ureter below, symmetrical and prompt excretion, and no cortical thinning or perinephric stranding. When the distinction is genuinely uncertain, a diuretic renogram settles it. The term is a descriptive one from the animal-sign compendia rather than a diagnostic sign in its own right.

Why is it called so?

The rounded pelvis perched on the thin straight ureter resembles an elephant balanced on top of a flagpole.

Pathophysiology

During development the renal pelvis may lie predominantly within the renal sinus (intrarenal) or predominantly outside it (extrarenal). An extrarenal pelvis is not enclosed by sinus fat and parenchyma, so it distends readily with urine during diuresis and assumes a rounded, sac-like profile that abruptly narrows into a normal ureter. Because the calyces drain freely and no obstruction exists, calibre change stops at the pelviureteric junction and the calyces stay sharp. In true pelviureteric junction obstruction the pelvis dilates for a different reason and the pressure is transmitted back to the calyces, which blunt and dilate as well.

Alternative names: Extrarenal pelvis appearance

Other associated named signs: Calyceal crescent sign of obstruction, drooping lily sign, and the flower vase appearance of a horseshoe kidney

References

1. Ridley LJ, Xiang H, Han J, Ridley WE. Animal signs in Radiology: method of creating a compendium. J Med Imaging Radiat Oncol. 2018;62 Suppl 1:7-11. PMID: 30309150.