The first group includes Ga-68, Kr-81m, Rb-82, Tc-99m, and In-113m, all of which are generator-produced radionuclides. Of particular note is Tc-99m. Due to its ideal imaging energy and physical half-life as well as the ability to bind to so many compounds, approximately 85% of all imaging procedures in the United States are performed following administration of Tc-99m.
A generator is a self-contained system housing a parent/daughter mixture in equilibrium, which is designed to yield the daughter for some purpose usually separate from the parent. The principal utility is to produce certain radioisotopes on-site which, because of their short half-lives, cannot be shipped by commercial sources. To be useful, the parent's half-life must be long compared to the travel time required to transport the generator to the recipient.
IDEAL GENERATOR SYSTEMS
1. If intended for clinical use, the output of the generator must be sterile and pyrogen-free.
2. The chemical properties of the daughter must be different than those of the parent to permit separation of daughter from parent. Most often, separations are performed chromatographically.
3. Generator should ideally be eluted with 0.9% saline solution and should involve no violent chemical reactions. Human intervention should be minimal to minimize radiation dose.
4. Daughter isotope should be short-lived gamma-emitting nuclide (physical half-life = hrs-days)
5. Physical half-life of parent should be short enough so daughter regrowth after elution is rapid, but long enough for practicality.
6. Daughter chemistry should be suitable for preparation of a wide variety of compounds, especially those in kit form.
7. Very long-lived or stable granddaughter so no radiation dose is conferred to patient by decay of subsequent generations.
8. Inexpensive, effective shielding of generator, minimizing radiation dose to users.
9. Easily recharged (we do NOT recharge Mo/Tc generators, but store them in decay areas after their useful life is over).
Mo/Tc GENERATOR: PRINCIPLES OF OPERATION
1. Prior to shipping the generator to the Nuclear Medicine Department, Mo-99 sodium molybdate is immobilized on a column of alumina (Al2O3; aluminum oxide) due to its very high affinity for alumina.
2. 0.9% saline solution (the eluant) is passed through the column and sodium pertechnetate, the daughter of Mo-99 decay, is eluted from the column due to its almost total lack of affinity for alumina.
3. The pertechnetate is collected in a shielded, evacuated sterile vial and calibrated prior to use. It is referred to as the eluate.
4. Quantitative removal of pertechnetate is attributed to the lack of affinity of pertechnetate for alumina, whereas molybdate is essentially completely and irreversibly bound to the alumina.
5. When eluting the generator, the elution volume should be carefully controlled so a relatively constant radioconcentration is obtained every day.
The Sn-113/In-113m GENERATOR
1. tphys of the daughter: 100 min; gamma energy: 393 keV.
2. Requires replacement every 6 months.
3. Eluted with dilute hydrochloric acid; requires aseptic pH adjustment
4. Chemistry available for all commonly performed imaging procedures.
|Stephen Karesh, PhD.||
Last Updated: August 14, 1996