Protocols and imaging agents Various protocols are used depending on the imaging agent, which include Radioactive thallium-201 (Tl-201) Technetium-99 m (Tc-99 m) markers (sestamibi, tetrofosmin, and teboroxime) Iodine-123 (I-123) - labeled fatty acids I-123 metaiodobenzylguanidine (MIBG) Radioactive thallium-201 (Tl-201), which acts as a potassium analog, was the original tracer used in stress testing.