Soap-bubble nephrogram | Radiology Signs

Answer:

The soap-bubble nephrogram is the appearance of multiple rounded lucencies within a dense, persistent nephrogram, produced when contrast opacifies the parenchyma around dilated, initially non-opacified calyces in severe or high-grade urinary obstruction. It belongs to the family of abnormal nephrograms that indicate obstructed drainage, together with the obstructive or delayed dense nephrogram, which becomes progressively denser and persists for hours rather than fading, and the rim nephrogram of a chronically obstructed kidney with thinned parenchyma. Recognising it identifies the kidney as obstructed rather than simply dilated, which is the central question in a dilated collecting system. Modern practice answers that question differently: non-contrast CT identifies calculi and the secondary signs of obstruction such as perinephric stranding and ureteric dilatation to a transition point, CT urography with an excretory phase demonstrates the level and cause, and diuretic renography quantifies drainage and differential function when the anatomy alone is inconclusive. Other abnormal nephrogram patterns should be distinguished: the striated nephrogram of pyelonephritis, contusion or reflux, the spotted nephrogram, and the persistent nephrogram of contrast-induced or other acute kidney injury. Reporting should always describe the level of obstruction, the cause and the state of the parenchyma, because they determine whether decompression is urgent.

Why is it called so?

The rounded lucent calyces against dense opacified parenchyma resemble a cluster of soap bubbles.

Pathophysiology

When the collecting system is obstructed, intratubular pressure rises and glomerular filtration falls, so contrast is filtered slowly and is not washed out into the calyces. Contrast therefore accumulates in the tubules and the parenchyma retains it, producing an increasingly dense nephrogram that persists long after the normal side has cleared. The calyces, distended with unopacified urine, appear as lucent rounded spaces within that dense parenchyma. If imaging is delayed for several hours, contrast eventually enters the dilated system and the lucencies fill in, which is the basis of the classical delayed film in obstruction. In chronic obstruction the parenchyma atrophies, so only a thin rim of enhancing cortex remains around a hugely dilated system.

Alternative names: Soap bubble appearance of the nephrogram

Other associated named signs: Rim nephrogram, the striated nephrogram and the obstructive dense nephrogram