External Radiation
The specific gamma ray constant for I-123 is 1.6 R/hr-mCi at 1 cm. The first half-value thickness of lead (Pb) for I-123 is 0.005 cm. A range of values for the relative attenuation of the radiation emitted by this radionuclide that results from the interposition of various thicknesses of Pb is shown in Table 2. For example, the use of 1.63 cm of lead will decrease the external radiation exposure by a factor of about 1,000.
Table 2. Radiation Attenuation by Lead Shielding Shield Thickness (Pb), cm | Coefficient of Attenuation |
0.005 0.10 0.88 1.63 2.48 | 0.5 10-1 10-2 10-3 10-4 |
Note that these estimates of attenuation do not take into consideration the presence of radionuclidic contaminants.
To correct for physical decay of I-123, the fractions that remain at selected intervals after the time of calibration are shown in Table 3.
Table 3. Iodine I-123 Decay Chart: Half-Life 13.2 Hours Hours | Fraction Remaining | | Hours | Fraction Remaining |
*Time of Calibration
|
0* 3 6 9 12 15 | 1.000 0.854 0.730 0.623 0.533 0.455 | | 18 21 24 27 30 | 0.389 0.332 0.284 0.242 0.207 |
CLINICAL PHARMACOLOGY
Sodium Iodide I-123 is readily absorbed from the upper gastrointestinal tract. Following absorption, the iodide is distributed primarily within the extracellular fluid of the body. It is trapped and organically bound by the thyroid and concentrated by the stomach, choroid plexus and salivary glands. It is excreted by the kidneys.
The fraction of the administered dose which is accumulated in the thyroid gland may be a measure of thyroid function in the absence of unusually high or low iodine intake or administration of certain drugs which influence iodine accumulation by the thyroid gland. Accordingly, the patient should be questioned carefully regarding previous medications and/or procedures involving radiographic media. Normal subjects can accumulate approximately 10 to 50% of the administered iodine dose in the thyroid gland, however, the normal and abnormal ranges are established by individual physician's criteria. The mapping (imaging) of Sodium Iodide I-123 distribution in the thyroid gland may provide useful information concerning thyroid anatomy and definition of normal and/or abnormal functioning of tissue within the gland.
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