Chapter 2- from 29 to 49.pdf
Transcript
Physical half-life The half-life (t1/2) of a radionuclide is the time taken for its activity to decay to half of its original value. For example: * Two successive half-lives reduce the activity of a radionuclide by a factor of 2 X 2 = 4. * Ten half-lives reduce the activity by a factor of 210 ≈...
Physical half-life The half-life (t1/2) of a radionuclide is the time taken for its activity to decay to half of its original value. For example: * Two successive half-lives reduce the activity of a radionuclide by a factor of 2 X 2 = 4. * Ten half-lives reduce the activity by a factor of 210 ≈ 1000 29 Physical half-life what is the def with biological and Pusical Radioactive decay reduces the number of radioactive nuclei over time. In one half-life (t1/2), the number decreases to half of its original value. Half of what remains decay in the next halflife, and half of those in the next, and so on. This is an exponential decay, as seen in the graph of the number of nuclei present as a function of time. 30 Physical half-life This half-life is more properly called the physical half-life. It is a fixed characteristic of the radionuclide, cannot be predicted for a given radionuclide in any way. The physical half-life is unaffected by any other factors, such as heat, pressure, electricity or chemical reactions. It can range from fractions of a second (useless in imaging) to millennia in the case of 99Tc (also useless in imaging). 31 Physical half-life Physical half-lives of some radionuclide used in medical imaging: Half-life - Half-life Radionuclide 67 h Molybdenum-99 Rubidium-82 - 67 h Indium-111 10 min Nitrogen-13 73 h Thallium-201 20 min Carbon-11 78 h Gallium-67 5 days Xenon-133 8 days Iodine-131 211 000 years Technetium-99 13 s D 1 min D 68 min 110 min 6h 13 h - Radionuclide Krypton-81m & Gallium-68 & Fluorine-18 Technetium-99m P Iodine-123 & > important on NM 32 Effective half-life When a radionuclide is used in medical gamma imaging, it usually forms part of a salt or organic compound, the metabolic properties of which ensure that it concentrates in the tissues or organ of interest. A pharmaceutical that has been labelled with a radionuclide is referred to as a radiopharmaceutical. 33 Effective half-life If the pharmaceutical alone is administered, it is gradually eliminated from the tissue, organ and whole body by the usual metabolic processes of turnover and excretion. Such a process can be regarded as having a biological half-life (tbiol). If the radionuclide is stored in a bottle, its activity decays with its physical half-life (tphys). 34 Effective half-life If the radiopharmaceutical is administered to a patient, the radioactivity in specific tissue, an organ or the whole body decreases because of the simultaneous effects of radioactive decay and metabolic turnover and excretion. The radiopharmaceutical can be regarded as having an effective half-life (teff). 35 Effective half-life The effective half-life is shorter than either the biological or physical half-lives. teff < tbiol , tphys 1/teff = 1/tbiol + 1/tphys The effective half-life depends on the radiopharmaceutical used and the organ involved, and it can vary from person to person, depending on their disease state. 36 Radiopharmaceuticals 37 - Radiopharmaceuticals Desirable properties of a radionuclide for imaging are as oh half-lif follow: the best Techa H 1. A physical half-life of a few hours, similar to the time from preparation to injection. If the half-life is too short, much more activity must be prepared than the actually injected. cuse have - 2. Decay to a stable daughter or at least one with a very long half-life (e.g. 99Tc has half-life of about 211000 years). S = contaminating 38 Radiopharmaceuticals Desirable properties of a radionuclide for imaging are as follow: 3. Emission of gamma rays (which produce the image), but not alpha or beta particles nor very low energy photons, which have a short range in tissue and deposit unnecessary dose in the patient. 4. Emission of gamma rays of energy 50-300 keV and ideally about 150 keV – high enough to exit the patient but low enough to be easy collimated and easily detected. = looker 39 Radiopharmaceuticals Desirable properties of a radionuclide for imaging are as follow: energy sing) Has 5. Ideally, emission of monoenergetic gamma rays, so that scattered radiation can be eliminated by energy discrimination with a pulse height analyser (PHA). sing 6. Easily and firmly attached to the pharmaceutical at room temperature, but has no affect on its metabolism. 40 Radiopharmaceuticals Desirable properties of a radionuclide for imaging are as follow: 7. Readily available at the hospital site. 8. A high specific activity, i.e. high activity per unit volume. 41 Radiopharmaceuticals Desirable properties of a radiopharmaceutical are as follow: 1. localise largely and quickly in the target (i.e. the tissue of interest) 2. be eliminated from the body with a effective half-life similar to the duration of the examination, to reduce the dose to the patient 3. have a low toxicity 42 Radiopharmaceuticals Desirable properties of a radiopharmaceutical are as follow: 4. form a stable product both in vitro and in vivo 5. be readily available and inexpensive per patient dose The decay during transport and storage of a short-lived radionuclide is reduced if it can be supplied with its longerlived parent in a generator. 43
