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: The perfusion defect produced in 16 open chest dogs was compared with the anatomic area at risk measured by the postmortem dual - perfusion technique and with thallium - 201 single - photon emission tomography ( SPECT ) .
### Output Text:
The perfusion defect produced in 16 open chest dogs was compared with the anatomic area at risk measured by the postmortem dual - perfusion technique and with <span class="chemical">thallium</span> - 201 single - photon emission tomography ( SPECT ) . 