Fusion or Apposition? A Case of a Broken Horseshoe Kidney

A Tc-99m DMSA renal study in a child showed a horseshoe pattern of uptake with upper pole “fusion.” T2-weighted MRI images showed a parenchymal bridge joining the upper poles. However, a repeat MRI study performed 2 months later with T1-weighting showed fatty tissue separation of the upper poles. This unusual case shows apposition of the upper poles, but it does not represent a true horseshoe kidney because of the absence of a parenchymal or fibrous bridge. The occurrence of fusion of the upper poles is exceptionally rare; it occurs in only 5% of horseshoe kidneys, which have an overall prevalence of about 1:600.