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: Findings of initial and follow - up MRIs were retrospectively evaluated by 2 neuroradiologists by consensus , to analyze the presence of abnormal signal intensities , their locations , and signal changes on follow - up images .
### Output Text:
Findings of initial and follow - up MRIs were retrospectively evaluated by 2 neuroradiologists by consensus , to analyze the presence of abnormal signal intensities , their locations , and signal changes on follow - up images . 