Presentation on theme: "Equilibrium and Generators Stephen M. Karesh, Ph.D. Nuclear Medicine Department Loyola University Medical Center Maywood, IL 60153."— Presentation transcript:
Equilibrium and Generators Stephen M. Karesh, Ph.D. Nuclear Medicine Department Loyola University Medical Center Maywood, IL 60153
Parent-Daughter Equilibria Equilibrium is established in a parent/daughter mixture when the daughter’s half-life is shorter than that of the parent.
TYPES OF EQUILIBRIA Transient Equilibrium Secular Equilibrium
a condition reached when the t phys of the parent is many times greater than the t phys 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.
Secular Equilibrium activity time 10 t phys parent daughter
Relationship Between Half-lives In secular equilibrium apparent physical t= t daughter parent
Transient Equilibrium a condition reached when the t phys of the parent is approximately 10 times greater than the t phys of the daughter. A classical example is the 99 Mo/ 99m Tc Generator. During the 67 hr period representing 11 half-lives of 99m Tc, 50% of the 99 Mo has disappeared, as noted in diagram. diagram here
Transient Equilibrium activity time 10 t phys daughter parent A daughter /A parent = ~1.05
Relationship Between Half-lives In transient equilibrium apparent physical t= t daughter parent
Time to Reach Equilibrium: Rule of Thumb For transient equilibrium: reached in ~4 t ½ of daughter. For Tc-99m, 23 hr For secular equilibrium: reached in ~6 t ½ of daughter
RADIOISOTOPE 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.
RADIOISOTOPE 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.
Ideal 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.
Ideal 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 (t phys = hrs-days)
Ideal 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.
Ideal 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.
Ideal 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).
DESIGN OF Ra/Rn GENERATOR COLUMN 226 RaCl 2 solution 222 Rn gas N 2 gas stopcock
A Dry Column Mo-99/Tc-99m Generator: Internal Construction filter needle 0.9% NaCl TcO 4 in evac vial lead shielding 0.2 m filter alumina column 99 MoO 4
Mo/Tc Generator: Principles of Operation 1. Prior to shipping the generator to the Nuclear Medicine Department, 99 Mo sodium molybdate is immobilized on a column of alumina (Al 2 O 3 ; aluminum oxide) due to its very high affinity for alumina % saline solution (the eluant) is passed through the column and Na pertechnetate, the daughter of 99 Mo decay, is eluted from the column due to its almost total lack of affinity for alumina.
Mo/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.
Mo/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.
CLINICALLY USEFUL RADIONUCLIDE GENERATORS 68 Ge> 68 Ga +, 275 d > 68 Zn 1.14 hr 81 Rb> 81m Kr 4.7 hr > 81 Kr +, 13 sec 82 Sr> 82 Rb +, 25d > 82 Kr 75 sec 99 Mo> 99m Tc 67 hr > 99 Tc 6 hr 113 Sn > 113m In 118 d > 113 In 1.7 hr Enter Type of equilibrium
Decay Scheme for 99 Mo 99 Mo 99m Tc 99 Tc 99 Ru (Stable) 67 hr hr 2.2 X 10 5 yr
POP TEST 99m Tc-sulfur colloid never clears the liver. All of it will ultimately decay into 99 Tc, which has a t phys of 2.2 X 10 5 yr. Why are you not concerned for your patient’s safety if 10 mCi of 99m Tc sulfur colloid is injected into a patient?
Answer For an isotope with a 220,000 yr half life: = decay constant = 0.693/t 1/2 = 0.693/220,000 yr = yr -1 Since only 3 millionths decays per year, the number of decays in a person’s lifetime would be negligible
What does this pattern represent? Does the slope of the line represent parent or daughter?
This 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
Contact Information Stephen Karesh, Ph.D. Loyola University Medical Center 708/