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: We used high - resolution MRI and surface - based computational image analyses to map regional abnormalities in the cortex , hippocampus , white matter , and ventricles in 22 human subjects who used MA and 21 age - matched , healthy controls .
### Output Text:
We used high - resolution MRI and surface - based computational image analyses to map regional abnormalities in the cortex , hippocampus , white matter , and ventricles in 22 human subjects who used <span class="chemical">MA</span> and 21 age - matched , healthy controls . 