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: Using magnetic resonance imaging ( MRI ) and new computational brain - mapping techniques , we determined the pattern of structural brain alterations associated with chronic MA abuse in human subjects and related these deficits to cognitive impairment .
### Output Text:
Using magnetic resonance imaging ( MRI ) and new computational brain - mapping techniques , we determined the pattern of structural brain alterations associated with chronic <span class="chemical">MA</span> abuse in human subjects and related these deficits to cognitive impairment . 