Thallous Chloride Tl-201 Injection


Thallous Chloride Tl-201 Injection (thallous chloride) is supplied in an isotonic solution as a sterile, non-pyrogenic diagnostic radiopharmaceutical for intravenous administration. Each milliliter contains 37 megabecquerels (1 millicurie) 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 that Thallous Chloride Tl-201 be administered 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

Radionuclidic Contaminants - Illustration

Physical Characteristics

Thallium Tl-201, with a physical half life of 73.1 hours, decays by electron capture to mercury Hg 201.1 Photons that are useful for detection and imaging are listed in Table 1. 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 1. Principal Radiation Emission Data

Radiation MeanPercent/ Disintegration Energy (keV)
Gamma-4 2.7 135.3
Gamma-6 10.0 167.4
Mercury x-rays 94.4 68.9-80.3

External Radiation

The specific gamma ray constant for thallium Tl-201 is 4.7 R/mCi-hr2 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 2. For example, the use of 0.21 cm of lead will decrease the external radiation exposure by a factor of about 1,000.

Table 2. Radiation Attenuation by Lead Shielding

cm of Lead(Pb) Coefficient of Attenuation
0.0006 0.5
0.015 10-1
0.098 10-2
0.21 10-3
0.33 10-4

To correct for physical decay of the radionuclide, the fractions that remain at selected intervals after calibration time are shown in Table 3.

Table 3. Thallium Tl-201 Decay Chart; Half-Life 73.1 Hours

Hours Fraction Remaining Hours Fraction Remaining
0* 1.00 66 0.53
6 0.94 72 0.51
12 0.89 78 0.48
18 0.84 84 0.45
24 0.80 90 0.43
30 0.75 96 0.40
36 0.71 108 0.36
42 0.67 120 0.32
48 0.63 132 0.29
54 0.60 144 0.26
60 0.57    
*Calibration Time

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