From the case:
Arc of Riolan
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This patient had severe proximal SMA stenosis.
An enlarged Arc of Riolan (red arrow) can be seen connecting the proximal IMA (not opacified in this image) with the proximal SMA (poorly opacified due to the stenosis). During the angiographic run, the Arc of Riolan filled retrogradely from the IMA to the SMA to provide collateral flow, illustrating the hemodynamic significance of the SMA stenosis.
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