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 scans and standard neuropsychological tests were performed > 2 . 6 years after the end of treatment .
### Output Text:
Magnetic resonance imaging scans and standard neuropsychological tests were performed > 2 . 6 years after the end of treatment . 