11.1 Chemical Characteristics
Thallous Chloride Tl 201 Injection is supplied in an isotonic solution as a sterile, non-pyrogenic diagnostic radiopharmaceutical for intravenous administration. Each milliliter contains 37 MBq (1 mCi) Thallous Chloride Tl 201 at calibration time, made isotonic with 9 milligrams sodium chloride and preserved with 0.9% (v/v) benzyl alcohol. The pH is adjusted to between 4.5 to 7.0 with hydrochloric acid and/or sodium hydroxide. Thallium Tl-201 is cyclotron produced. At the time of calibration it contains no more than 1.0% thallium Tl-200, no more than 1.0% thallium Tl-202, no more than 0.25% lead Pb-203, and no less than 98% thallium Tl-201 as a percentage of total activity. No carrier has been added.
It is recommended to administer Thallous Chloride Tl 201 Injection close to calibration time to minimize the effect of higher levels of radionuclidic contaminants present at pre- and post-calibration dates. The concentration of each radionuclidic contaminant changes with time. Figure 1 shows maximum concentration of each radionuclidic contaminant as a function of time.
Figure 1. Radionuclidic Contaminants
11.2 Physical Characteristics
Thallium Tl-201, with a physical half-life of 72.9 hours, decays by electron capture to mercury Hg-201. Photons that are useful for detection and imaging are listed in Table 3. The lower energy x-rays obtained from the mercury Hg-201 daughter of thallium Tl-201 are recommended for myocardial imaging, because the mean percent disintegration at 68.9 to 80.3 keV is much greater than the combination of gamma-4 and gamma-6 mean percent disintegration.
Table 3. Principal Radiation Emission Data
From: Stabin MG, da Luz CQPL. New Decay Data for Internal and External Dose Assessment, 2002, Health Phys. 83(4):471-4753.
11.3 External Radiation
The specific gamma ray constant for thallium Tl-201 is 4.64 R/mCi-hr at
1 cm. The first half-value thickness of lead (Pb) is 0.0006 cm. A range of values for the radiation emitted by this radionuclide with the corresponding exposure rate at 1 cm that results from interposition of various thicknesses of lead is shown in Table 4. For example, the use of 0.21 cm of lead will decrease the external radiation exposure by a factor of about 1,000.
Table 4. Radiation Attenuation by Lead Shielding
Data supplied by Oak Ridge Associated Universities, Radiopharmaceutical Internal Dose Information Center, Oak Ridge, TN, 1994. Includes 10 keV x-rays4.
To correct for physical decay of the radionuclide, the fractions that remain at selected intervals after calibration time are shown in Table 5.
Table 5. Thallous Chloride Tl 201 Decay Chart; Half-Life 72.9 Hours
* Calibration Time
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