Natural gallium (₃₁Ga) consists of a mixture of two stable isotopes: gallium-69 and gallium-71. Twenty-nine radioisotopes are known, all synthetic, with atomic masses ranging from 56 to 86; along with three nuclear isomers, ^64mGa, ^72mGa and ^74mGa. Most of the isotopes with atomic mass numbers below 69 decay to isotopes of zinc, while most of the isotopes with masses above 71 decay to isotopes of germanium. Among them, the most commercially important radioisotopes are gallium-67 and gallium-68.

Gallium-67 (half-life 3.3 days) is a gamma-emitting isotope (the gamma ray emitted immediately after electron capture) used in standard nuclear medical imaging, in procedures usually referred to as gallium scans. It is usually used as the free ion, Ga³⁺. It is the longest-lived radioisotope of gallium.

The shorter-lived gallium-68 (half-life 68 minutes) is a positron-emitting isotope generated in very small quantities from germanium-68 in gallium-68 generators or in much greater quantities by proton bombardment of ⁶⁸Zn in low-energy medical cyclotrons,^[4][5] for use in a small minority of diagnostic PET scans. For this use, it is usually attached as a tracer to a carrier molecule (for example the somatostatin analogue DOTATOC), which gives the resulting radiopharmaceutical a different tissue-uptake specificity from the ionic ⁶⁷Ga radioisotope normally used in standard gallium scans.

Gallium-67 (⁶⁷
Ga
) has a half-life of 3.26 days and decays by electron capture and gamma emission (in de-excitation) to stable zinc-67. It is a radiopharmaceutical used in gallium scans (alternatively, the shorter-lived gallium-68 may be used). This gamma-emitting isotope is imaged by gamma camera.

Gallium-68 (⁶⁸
Ga
) is a positron emitter with a half-life of 68 minutes, decaying to stable zinc-68. It is a radiopharmaceutical, generated in situ from the electron capture of germanium-68 (half-life 271 days) owing to its short half-life. This positron-emitting isotope can be imaged efficiently by PET scan (see gallium scan); alternatively, the longer-lived gallium-67 may be used. Gallium-68 is only used as a positron emitting tag for a ligand which binds to certain tissues, such as DOTATOC, which is a somatostatin analogue useful for imaging neuroendocrine tumors. Gallium-68 DOTA scans are increasingly replacing octreotide scans (a type of indium-111 scan using octreotide as a somatostatin receptor ligand). The ⁶⁸
Ga
is bound to a chemical such as DOTATOC and the positrons it emits are imaged by PET-CT scan. Such scans are useful in locating neuroendocrine tumors and pancreatic cancer.^[8] Thus, octreotide scanning for NET tumors is being increasingly replaced by gallium-68 DOTATOC scan.^[9]