Spotted nephrogram | Radiology Signs

Answer:
A spotted nephrogram is caused by segmental and subsegmental areas of renal parenchymal nonenhancement due to small-vessel occlusion leading to infarctions within renal lobes. This pattern is typically seen in intrarenal vasculitis (such as polyarteritis nodosa, Wegener granulomatosis, systemic lupus erythematosus, scleroderma, and arteriolar nephrosclerosis), multiple emboli from cardiac or aortic sources, and severe pyelonephritis. The sign reflects patchy ischemic injury or infarctions causing uneven perfusion within the kidney.

Why is it called so?

It is named the “spotted nephrogram” because the normally uniform, homogeneous enhancement of the renal parenchyma on CT is replaced by multiple, discrete, patchy (spotted) areas of absent or diminished enhancement representing infarcted or nonperfused renal tissue. It was first described as a distinct CT nephrogram pattern in renal scleroderma.

Pathophysiology

The spotted nephrogram develops due to occlusion of small intrarenal vessels causing segmental ischemia and infarction. These infarctions create areas where contrast fails to reach the renal parenchyma during the nephrographic phase of CT imaging, resulting in multiple amorphous zones of decreased or absent enhancement interspersed among normally perfused areas. The heterogeneity arises from uneven blood flow caused by vascular inflammation, thrombosis, or embolic occlusion within the kidney.

Alternative names:
Mottled nephrogram

Other associated named signs:
Striated nephrogram (linear bands of alternating density, seen in acute pyelonephritis or tubular obstruction) and rim nephrogram (thin peripheral cortical enhancement, classically seen in acute renal artery occlusion/infarction) are other named nephrogram patterns representing different underlying pathophysiology.

References

1. Winograd J, Schimmel DH, Palubinskas AJ. The spotted nephrogram of renal scleroderma. AJR Am J Roentgenol. 1976;126(4):734-738.