What are the two types of pericallosal lipomas described, and which variety is this one?
Tubulonodular and curvilinear (this case).
How do curvilinear pericallosal lipomas differ from the tubonodular variety?
Smaller, and thin/elongated morphology. They are located posteriorly and associated with minor callosal abnormalities; usually splenial hypoplasia rather than true callosal dysgenesis.
How can you distinguish splenial hypoplasia from callosal dysgenesis?
By examining the rostrum of the corpus callosum, which is the last part to form. If it is normal formed, then the corpus callosum finished forming (thus not dysgenic) and a small size implied hypoplasia.
The irregular curvilinear left para-median mass extending over the superior margin of the corpus callosum body, curving over the posterior margin of the splenium through the retro-thalamic cistern, follows fat signal on all sequences. On the sagittal T1 images the rostrum of the corpus callosum is not terribly well imaged but appears normally formed.