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: Repeated cerebral perfusion SPECT scans revealed decreased basal ganglia perfusion while the movement disorder was present , and a return to normal perfusion when the rabbit syndrome resolved .
### Output Text:
Repeated cerebral perfusion SPECT scans revealed decreased basal ganglia perfusion while the movement disorder was present , and a return to normal perfusion when the rabbit syndrome resolved . 