Following intravenous injection of contrast material the outlines of the kidneys and collecting systems are demonstrated. This helps to determine if a suspected stone is within the collecting system.
In this case the renal stone (calculus) - which is less dense than the surrounding contrast material - appears as a filling defect in the renal pelvis.
Nephrocalcinosis is an uncommon form of renal calcification. Calcified material is seen in the renal parenchyma itself rather than in the collecting system.