In stable patients whose symptoms have resolved by the time of evaluation, technetium (99mTc) sestamibi (i.e. a "MIBI scan") or thallium-201 chloride can be used in nuclear medicine to visualize areas of reduced blood flow in conjunction with physiological or pharmacological stress. Thallium may also be used to determine viability of tissue, distinguishing whether nonfunctional myocardium is actually dead or merely in a state of hibernation or of being stunned. Medical societies and professional guidelines recommend that the physician confirm a person is at high risk for myocardial infarction before conducting imaging tests to make a diagnosis. Patients who have a normal ECG and who are able to exercise, for example, do not merit routine imaging. Imaging tests such as stress radionuclide myocardial perfusion imaging or stress echocardiography can confirm a diagnosis when a patient's history, physical exam, ECG, and cardiac biomarkers suggest the likelihood of a problem.

What purpose does technetium serve in terms of identifying tissue?