Final answer:
The isotope not used for imaging the heart is Tc99m diphosphonate, as it is commonly used for bone marrow imaging, whereas Tl-201 and Tc99m, including Tc99m RBCs, are used for cardiac imaging.
Step-by-step explanation:
The isotopes commonly used for imaging the heart include thallium-201 (Tl-201) and technetium-99m (Tc-99m). Thallium-201 gets concentrated in healthy heart tissue, which makes it useful for cardiac imaging. Technetium-99m, with its short 6-hour half-life and single gamma-ray emission, is used in a number of compounds for imaging various organs, including the heart, which allows doctors to observe blood flow and look for damaged tissue. The incorrect option in the provided list is diphosphonate tagged with technetium-99m (Tc99m diphosphonate), as this is commonly employed for bone marrow imaging, not specifically for the heart. It is important to note that Tc99m-labeled red blood cells (Tc99m RBCs) are indeed used in cardiac imaging to evaluate the heart's blood-pumping function. Therefore, of the given options, Tc99m diphosphonate is not used to image the heart.