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Calyceal crescent sign

The calyceal crescent sign (Dunbar's crescents) refers to the early IVP appearance of markedly dilated renal calyces. It is formed by early contrast opacification of the dilated collecting ducts and ducts of Bellini with the characteristic shape as a result of the associated enlarged calyces. 

This appearance can be appreciated on IVP and CT contrast excretory phase and becomes isoattenuating to the parenchyma as the kidney opacifies.

It is a marker of elevated pressure within the renal pelvis and is an important indicator as to whether the renal function is recoverable.

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