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 revealed cortical laminar necrosis in arterial border zones in both cerebral hemispheres , ischemic changes in subcortical white matter of left cerebral hemisphere , and in the left putamen .
### Output Text:
Magnetic Resonance imaging revealed cortical laminar necrosis in arterial border zones in both cerebral hemispheres , ischemic changes in subcortical white matter of left cerebral hemisphere , and in the left putamen . 