Iodine-123

Iodine-123 (123I) is a radioactive isotope of iodine used in nuclear medicine imaging, including single-photon emission computed tomography (SPECT) or SPECT/CT exams.

[3] 123I for medical applications has also been produced at Oak Ridge National Laboratory by proton cyclotron bombardment of 80% isotopically enriched tellurium-123.

[4] The detailed decay mechanism is electron capture (EC) to form an excited state of the nearly-stable nuclide tellurium-123 (its half life is so long that it is considered stable for all practical purposes).

The energy of the photon, 159 keV, is ideal for the NaI (sodium iodide) crystal detector of current gamma cameras and also for the pinhole collimators.

Moreover, scanning a thyroid remnant or metastasis with 123I does not cause "stunning" of the tissue (with loss of uptake), because of the low radiation burden of this isotope.

This is administered to a patient by ingestion under capsule form, by intravenous injection, or (less commonly due to problems involved in a spill) in a drink.

Quantitative measurements of the thyroid can be performed to calculate the iodine uptake (absorption) for the diagnosis of hyperthyroidism and hypothyroidism.

Sequence of 123-iodide human scintiscans after an intravenous injection, (from left) after 30 minutes, 20 hours, and 48 hours. A high and rapid concentration of radio-iodide is evident in cerebrospinal fluid (left), gastric and oral mucosa , salivary glands , arterial walls , ovary and thymus . In the thyroid gland, I-concentration is more progressive, as in a reservoir (from 1% after 30 minutes, and after 6, 20 h, to 5.8% after 48 hours, of the total injected dose).(Venturi, 2011)