Final answer:
During an adenosine stress test, a radiotracer like technetium-99m is injected at peak stress or after adenosine administration. It is important to perform the scan quickly due to the radiotracer's short half-life, which allows for optimal imaging of cardiac areas affected by reduced blood flow.
Step-by-step explanation:
In the context of an adenosine protocol for a cardiac stress test using imaging, the radiotracer is typically injected while the patient is at peak stress or shortly after the adenosine is administered. Adenosine causes vasodilation and simulates the effect of exercise on the heart. This helps to highlight areas of reduced blood flow. The most common radiotracer used in this procedure is technetium-99m, a radioactive isotope which emits gamma rays detectable by the camera used in the imaging process. It is vital to perform the Positron Emission Tomography (PET) scan soon after ingestion of the radiopharmaceutical because the isotope has a short half-life, meaning it decays relatively quickly, and damaged tissues preferentially absorb some of these compounds, such as compounds of technetium-99, which then allows for detailed imaging of the affected areas. Technetium-99m is ideal due to its short half-life of approximately six hours, which permits ample time for imaging while limiting patient exposure to radiation.