Why are two administrations of tracer required in stress-rest myocardial imaging with 99mTc sestamibi?

In stress-rest myocardial imaging using 99mTc sestamibi, the requirement for two administrations of the tracer stems primarily from the radiopharmaceutical's properties regarding retention in the myocardium. Specifically, 99mTc sestamibi does not redistribute once it has been taken up by the myocardial tissue.

When this tracer is injected during the stress phase, it accumulates in the myocardium based on perfusion at that moment. However, the tracer remains fixed in the myocardial cells and does not wash out or redistribute when the stress is removed. When imaging is performed after rest, a second dose of the tracer is necessary to assess perfusion under resting conditions. This two-step process allows for a clear comparison between the increased demand during stress and the perfusion at rest, giving valuable insights into areas of ischemia or infarcts.

This aspect of the tracer's behavior necessitates a different injection for the rest phase, allowing for an accurate evaluation of myocardial blood flow and identifying any perfusion defects effectively.