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: Dipyridamole - thallium - 201 imaging is often performed in patients unable to exercise because of peripheral vascular disease .
### Output Text:
<span class="chemical">Dipyridamole</span> - <span class="chemical">thallium</span> - 201 imaging is often performed in patients unable to exercise because of peripheral vascular disease . 