Calyceal (Dunbar) Crescents

Calyceal crescents are crescent-shaped collections of contrast seen on the early phase of intravenous urography that fade on delayed images as the contrast is excreted into the pelvicalyceal system.

They are seen in the setting of chronic obstruction with hydronephrosis: Calyceal dilatation causes compression, effacement, and eventual eversion of the papillae, leading to reorientation of the papillary ducts (ducts of Bellini). At the same time, increased pressure in the renal pelvis delays transit of urine, resulting in an increased concentration of the urine in these distorted and reoriented collecting ducts.

References

• Dunbar JS, Nogrady MB. The calyceal crescent--a roentgenographic sign of obstructive hydronephrosis. Am J Roentgenol Radium Ther Nucl Med. 1970 Nov;110(3):520-8.
• Khanna G. The calyceal crescent sign. Radiology. 2005 Aug;236(2):653-4.