Medullary nephrocalcinosis is the result of calcification in the medullary portions of the kidney in the distribution of the renal pyramids. This condition varies from a subtle distribution of the calcifications to a florid deposition of calcium throughout the pyramids. With minimal calcifications, there is no shadowing. Ultrasound is more sensitive than x-ray or CT in detecting the faint calcifications.
Ultrasound features of medullary nephrocalcinosis:
1. Early- hyperechogenicity in the periphery of the pyramids
2. Mid- diffusely hyperechoic pyramids, fine to coarse echotexture
3. Late: clumps of calcium, shadowing
The causes of medullary nephrocalcinosis are many, but the common thread is a high calcium state. Medullary sponge kidney is different in that it is a congenital abnormality (due to tubular ectasia or dilated collecting tubules).
See http://www.sonoworld.com/Client/Cases/CaseDetails.aspx?CaseId=85 for an example and discussion of medullary sponge kidney.
See http://www.sonoworld.com/Client/Cases/CaseDetails.aspx?CaseId=112 for an example of nephrocalcinosis in the adult.
See http://www.sonoworld.com/Client/Cases/CaseDetails.aspx?CaseId=111 for a case of cortical nephrocalcinosis due to renal tubular acidosis.