Presentation on theme: "Equilibrium and Generators"— Presentation transcript:
1Equilibrium and Generators Stephen M. Karesh, Ph.D.Nuclear Medicine DepartmentLoyola University Medical CenterMaywood, IL 60153
2Parent-Daughter Equilibria Equilibrium is established in a parent/daughter mixture when the daughter’s half-life is shorter than that of the parent.
3Transient Equilibrium Secular Equilibrium TYPES OF EQUILIBRIATransient EquilibriumSecular Equilibrium
4Secular Equilibriuma condition reached when the tphys of the parent is many times greater than the tphys of the daughter, e.g., times greater. To keep things in perspective, during 10 half-lives of the daughter, decay of the parent is negligible. Decay of the parent is represented by the flat line in the diagram below.
6Relationship Between Half-lives In secular equilibriumapparent physicalt = tdaughter parent
7Transient Equilibrium a condition reached when the tphys of the parent is approximately 10 times greater than the tphys of the daughter. A classical example is the 99Mo/99mTc Generator. During the 67 hr period representing 11 half-lives of 99mTc , 50% of the 99Mo has disappeared, as noted in diagram.diagram here
9Relationship Between Half-lives In transient equilibriumapparent physicalt = tdaughter parent
10Time to Reach Equilibrium: Rule of Thumb For transient equilibrium: reached in ~4 t½ of daughter. For Tc-99m, 23 hrFor secular equilibrium: reached in ~6 t½ of daughter
11RADIOISOTOPE GENERATORS A generator is a self-contained system housing a parent/daughter mixture in equilibrium. Designed to produce the daughter for some purpose usually separate from the parent.
12RADIOISOTOPE GENERATORS Generators produce certain short-lived radioisotopes on-site which 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 recipient.
13Ideal Generator Systems 1. If intended for clinical use, 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. Separations are usually performed by affinity or ion exchange chromatography.
14Ideal Generator Systems 3. Generator should 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 (tphys = hrs-days)
15Ideal Generator Systems 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.
16Ideal Generator Systems 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 those using it.
17Ideal Generator Systems 9. Generator is easily recharged (we do NOT recharge Mo/Tc generators, but store them in decay areas after their useful life is over).
18DESIGN OF Ra/Rn GENERATOR COLUMN 222Rn gasstopcock226RaCl2 solutionstopcockN2 gas
20A Dry Column Mo-99/Tc-99m Generator: Internal Construction 0.9%NaCllead shieldingTcO4 inevac vialfilter needle99MoO40.2 mm filteralumina column
21Mo/Tc Generator: Principles of Operation 1. Prior to shipping the generator to the Nuclear Medicine Department, 99Mo 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 Na pertechnetate, the daughter of 99Mo decay, is eluted from the column due to its almost total lack of affinity for alumina.
22Mo/Tc Generator: Principles of Operation 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.
23Mo/Tc Generator: Principles of Operation 5. When eluting the generator, the elution volume should be carefully controlled so a relatively constant radioconcentration is obtained every day.
24CLINICALLY USEFUL RADIONUCLIDE GENERATORS Enter Type of equilibriumeb+, e, g68Ge> 68Ga> 68Zn275 d1.14 hrb+, eg81Rb> 81mKr> 81Kr4.7 hr13 secb+, e, ge82Sr> 82Rb> 82Kr25d75 secb-, gg99Mo> 99mTc> 99Tc67 hr6 hreg113Sn> 113mIn> 113In118 d1.7 hr
26Decay Scheme for 99Mo 99Mo 99mTc 99Tc 99Ru (Stable) b-, 67 hr g, 6 hr b-, 2.2 X 105 yr99Ru(Stable)
27POP TEST99mTc-sulfur colloid never clears the liver. All of it will ultimately decay into 99Tc, which has a tphys of 2.2 X 105 yr. Why are you not concerned for your patient’s safety if 10 mCi of 99mTc sulfur colloid is injected into a patient?
28AnswerFor an isotope with a 220,000 yr half life:= decay constant = 0.693/t1/2= /220,000 yr = yr-1Since only 3 millionths decays per year, the number of decays in a person’s lifetime would be negligible
29What does this pattern represent? Does the slope of the line representparent or daughter?
30This pattern is an elution profile and represents daily elution for 1 week; elutions are spaced evenly since generator is eluted at the same time each morning. The slope of the line represents the parent’s decay.Mon Tue Wed Thu Fri
31Contact InformationStephen Karesh, Ph.D.Loyola University Medical Center708/