Which radioactive substance is injected into the blood?

The radioactive substance commonly injected into the blood for medical imaging is a radiotracer, such as Technetium-99m. It is used in various imaging techniques, including positron emission tomography (PET) and single-photon emission computed tomography (SPECT), to help visualize internal organs and tissues. These radiotracers emit gamma rays, which can be detected by imaging equipment to assess physiological functions and detect diseases.