A delayed nephrogram, commonly described on plain film urography, but also visible on CT urography, is when there is absence or reduction of the normal renal parenchymal enhancement on nephrographic phase images.
A delayed nephrogram is characteristically unilateral and is usually distinguished from bilateral delayed nephrograms, which are termed persistent nephrograms, and are usually caused by systemic hypotension.
Normal enhancement of the renal parenchyma involves contrast passing via the renal arterial bed to the glomeruli and nephrons before being finally concentrated into the collecting system. A temporal delay in this occurring can therefore result from any disruption of the pathways in this process.
Failure of the normal temporal progression of nephrographic contrast can result from a number of renal conditions:
- obstructive uropathy (most common)
- renal vein thrombosis
- renal artery stenosis
- extrinsic compression
The abnormal kidney shows nephrographic enhancement in the delayed or urographic phase which persists for a prolonged period of time. This is evident on delayed images as the normal kidney shows contrast within the collecting system and ureters while the abnormal kidney does not.
These features are distinct from non-enhancement of the renal parenchyma which can occur in cases of renal infarction.
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