Final answer:
Nuclear Medicine is the medical imaging technique used to visualize heart function and blood flow during a cardiac stress test, often involving the administration of substances like thallium-201.
Step-by-step explanation:
The medical imaging technique used to capture detailed images of blood flow in the heart during a cardiac stress test is Nuclear Medicine. During a stress test, substances like thallium-201 may be administered to a patient to visually analyze heart function and blood flow. This technique, which includes practices such as PET scans, uses radiopharmaceuticals to create images of the active blood flow and physiological activity of the heart.
Ultrasound, which is another imaging modality mentioned, is widely utilized in sensitive scenarios such as monitoring pregnancies and examining heart function but is not the primary technique for cardiac stress tests focusing on blood flow imagery.