Half-life time
Updates to Article Attributes
Physical half-life time (Tp)
The time interval required for an amount of certain radioactive nuclei to decay to half of its original value. T p is always a constant for a particular radioactive isotope and is unaffected by changes in surrounding such as temperature or pressure.
Biological half-life time (Tb)
The time interval required for the body to eliminate 50% of any substance by normal routes of elimination: metabolic turnover and excretion. T b is affected by many external factors such as hydration, hepatic function and renal function.
Effective half-life time (Te)
The time interval required for the radioactivity of a certain amount of radioactive substance distributed in tissues and organs to decrease to half its original value due to radioactive decay and biological elimination.
In most cases T e is calculated using :
Te = (Tp x Tb) / (Tp + Tb) or simply 1/Te = 1/Tp+1/Tb
However, there are three special situations described below to understand Te better2.
1) Tp >>> Tb then Te ~ Tb
An. An example would be Xe-133 which is used for pulmonary ventilation studies. It has a Tp of 5.3 days and Tb of 15 seconds. Te of Xe-133 is equal to Tb which is 15 seconds.
2) Tb >>> Tp then Te ~ Tp
An. An example would be Tc-SC used-99m sulfur colloid used for liver scans which has a Tp of 6 hours and Tb which is indefinitely long. The Te for Tc-SC-99m sulfur colloid is therefore equal to Tp which is 6 hours.
3) Tp = Tb then Te = 1/2 T p +p = 1/2 Tb
.An example would be Tc-MAA -99m MAA (macroaggregated albumin)used for pulmonary perfusion studies where Tp = 6 hours and Tb = 6 hours and therefore Te = 3 hours
-<h5>Physical half-life time (T<sub>p</sub>)</h5><p>The time interval required for an amount of certain radioactive nuclei to decay to half of its original value. T p is always a constant for a particular radioactive isotope and is unaffected by changes in surrounding such as temperature or pressure.</p><h5>Biological half-life time (T<sub>b</sub>) </h5><p>The time interval required for the body to eliminate 50% of any substance by normal routes of elimination: metabolic turnover and excretion. T b is affected by many external factors such as hydration, hepatic function and renal function.</p><h5>Effective half-life time (T<sub>e</sub>)</h5><p>The time interval required for the <a href="/articles/radioactivity">radioactivity</a> of a certain amount of radioactive substance distributed in tissues and organs to decrease to half its original value due to radioactive decay and biological elimination.</p><p>In most cases T e is calculated using :</p><p>1/T<sub>e</sub> = 1/T<sub>p</sub>+1/T<sub>b</sub></p><p>However, there are three special situations described below to understand T e better.</p><p>1) T p >>> T b then T e ~ T b</p><p>An example would be Xe-133 which is used for pulmonary ventilation studies. It has a T p of 5.3 days and T b of 15 seconds. T e of Xe-133 is equal to T b which is 15 seconds.</p><p>2) T b >>> T p then T e ~ T p</p><p>An example would be Tc-SC used for liver scans which has a T p of 6 hours and T b which is indefinitely long. The T e for Tc-SC is therefore equal to T p which is 6 hours.</p><p>3) T p = T b then T e = 1/2 T p + 1/2 T b</p><p>An example would be Tc-MAA used for pulmonary perfusion studies where T p = 6 hours and T b = hours and therefore T e = 3 hours</p><p> </p><p> </p>- +<h5>Physical half-life time (T<sub>p</sub>)</h5><p>The time interval required for an amount of certain radioactive nuclei to decay to half of its original value. T p is always a constant for a particular radioactive isotope and is unaffected by changes in surrounding such as temperature or pressure.</p><h5>Biological half-life time (T<sub>b</sub>) </h5><p>The time interval required for the body to eliminate 50% of any substance by normal routes of elimination: metabolic turnover and excretion. T b is affected by many external factors such as hydration, hepatic function and renal function.</p><h5>Effective half-life time (T<sub>e</sub>)</h5><p>The time interval required for the <a href="/articles/radioactivity">radioactivity</a> of a certain amount of radioactive substance distributed in tissues and organs to decrease to half its original value due to radioactive decay and biological elimination.</p><p>In most cases T e is calculated using :</p><p>Te = (Tp x Tb) / (Tp + Tb) or simply 1/T<sub>e</sub> = 1/T<sub>p</sub>+1/T<sub>b</sub></p><p>However, there are three special situations described below to understand T<sub>e </sub>better <sup>2</sup>.</p><p>1) T<sub>p </sub>>>> T<sub>b</sub> then T<sub>e</sub> ~ T<sub>b</sub>. An example would be <a href="/articles/xenon-133-1">Xe-133</a> which is used for pulmonary ventilation studies. It has a T<sub>p</sub> of 5.3 days and T<sub>b</sub> of 15 seconds. T<sub>e</sub> of Xe-133 is equal to T<sub>b</sub> which is 15 seconds</p><p>2) T<sub>b</sub> >>> T<sub>p</sub> then T<sub>e</sub> ~ T<sub>p</sub>. An example would be <a href="/articles/tc-99m-sulfur-colloid-4">Tc-99m sulfur colloid</a> used for liver scans which has a T<sub>p</sub> of 6 hours and T<sub>b</sub> which is indefinitely long. The T<sub>e</sub> for Tc-99m sulfur colloid is therefore equal to T<sub>p</sub> which is 6 hours</p><p>3) T<sub>p</sub> = T<sub>b</sub> then T<sub>e</sub> = 1/2 T<sub>p</sub> = 1/2 T<sub>b</sub>.<sub> </sub>An example would be <a href="/articles/tc-99m-maa">Tc-99m MAA</a> (macroaggregated albumin)<strong> </strong>used for pulmonary perfusion studies where T<sub>p</sub> = 6 hours and T<sub>b</sub> = 6 hours and therefore T<sub>e</sub> = 3 hours</p>
References changed:
- 1. https://www.env.go.jp/en/chemi/rhm/basic-info/1st/02-01-05.html
- 2. https://nucmedtutorials.files.wordpress.com/2016/12/half-lives1.pdf
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