In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: Magnetic resonance imaging ( MRI ) brain showed abnormal signal intensity involving both dentate nuclei of cerebellum and splenium of corpus callosum .
### Output Text:
Magnetic resonance imaging ( MRI ) brain showed abnormal signal intensity involving both dentate nuclei of cerebellum and splenium of corpus callosum . 