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: Relation of perfusion defects observed with myocardial contrast echocardiography to the severity of coronary stenosis : correlation with thallium - 201 single - photon emission tomography .
### Output Text:
Relation of perfusion defects observed with myocardial contrast echocardiography to the severity of coronary stenosis : correlation with <span class="chemical">thallium</span> - 201 single - photon emission tomography . 