ACR Codes: 1.3
Although commonly thought of as tumors, intracranial lipomas are, in fact, malformations of the brain. They are the result of abnormal differentiation of the meninx primitiva, a substance that usually degenerates to become the subarachnoid space.
When lipomas occur adjacent to the corpus callosum ("lipomas of the corpus callosum"), the corpus callosum is always hypoplastic. Most commonly, the splenium and posterior body are absent, as in this case. When such hypogenesis is seen adjacent to a stripe of tissue with a very short T1 relaxation time, the diagnosis of lipoma is nearly certain. As a verification, look for chemical shift artifacts. Chemical shift artifact results from the fact that the MR scanner is tuned to the Larmour frequency of water hydrogen protons. The scanner uses this frequency to spatially map protons within the imaging plane. Fat has a slightly different chemical frequency shift than water. Therefore, when signals from fat protons are received, they are spatially mismapped by a small distance, which is proportional to field strength. The result is a signal void at the junction of the fat and the adjacent tissue.
Lipomas also occur in other locations. Some of the more common sites are the supravermian/inferior collicular region, the hypothalamic region, and the cerebellopontine angle cistern. Film .3 illustrates a hypothalamic lipoma in a patient who is imaged because of hydrocephalus. Notice that the fat is above the normal high signal of the posterior pituitary.
There are two very important concepts pertaining to intracranial lipomas. First, because these lesions are in fact malformations, and not tumors, there is no place for surgical management. In fact, blood vessels and nerves very frequently course through the lipomas, making surgery extremely dangerous. Second, these lesions are usually incidental findings and are only a problem if they are not recognized as lipomas and are not left alone.
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Disease Topic 1522 
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