Imaging isotopes emit radiation that can be detected outside the body, usually gamma radiation or positrons.
Gamma emissions are high-energy photons with a short wave-length, which allows them to penetrate tissue and transmit radiation to the outside of the body. There they are detected by gamma cameras that either display planar images (scintigraphies) or 3D computed tomographies (called SPECT: single photon emission computed tomography).
Positrons are the antiparticles to electrons. Depending on their energy, they travel a short distance within the tissue until they collide with an electron and annihilate. When positrons from imaging isotopes annihilate, two photons of the same energy are emitted at an exact 180° angle. These two photons are detected in PET imaging (PET = positron emission tomography).
Typical SPECT isotopes are 99m-Technetium (6 h half-life), 111-Indium (2.8 day half-life), and 123-Iodine (13.2 h half-life).
Typical PET isotopes are 18-Fluorine (108 min half-life), 68-Gallium (68 min half-life), 64-Copper (12.7 h half-life), and 89-Zirconium (3.3 day half-life).
Therapy isotopes emit high energy beta– or alpha particles which lead to the formation of toxic radicals and cause DNA single or double strand breaks. The higher the mass of the particle, the higher the impact on the surrounding matter upon collision; therefore, alpha particles (consisting of two protons and two neutrons) are more cytotoxic than beta– particles (i.e. electrons).
Typical beta– emitters are 90-Yttrium (2.7 day half-life), 177-Lutetium (6.6 day half-life), and Iodine-131 (8 day half-life).
Typical alpha emitters are 225-Actinium (10 day half-life), 212-Lead (10.6 h half-life) and 211-Astatine (7.2 h half-life).