Thallous Chloride Tl 201 Injection is supplied in isotonic solution as a sterile, non-pyrogenic diagnostic radiopharmaceutical for intravenous administration. The aqueous solution at the time of calibration contains 74MBq/ml (2mCi/ml) Thallous Chloride Tl 201. The pH is adjusted with hydrochloric acid and/or sodium hydroxide solution. It is made isotonic with 9mg/ml sodium chloride and is preserved with 9mg/ml benzyl alcohol.

Thallium Tl 201 is cyclotron produced with no carrier added and contains no less than 98% Thallium Tl 201 as a percentage of total activity with contaminants less than 0.3% Thallium Tl 200, 1.2% Thallium Tl 202, and 0.2% Lead Pb 203 expressed as a percentage of Tl 201 Injection activity at calibration.

It is recommended that Thallous Chloride Tl 201 Injection be administered close to calibration time to minimize the effect of higher levels of radionuclide contaminants pre-and postcalibration. The concentration of each radionuclide contaminant changes with time. Graph 1 shows maximum concentration of each radionuclide contaminant as a function of time.

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 Tl 201 are recommended for myocardial imaging, because the mean %/disintegration at 68-80.3 KeV is much greater than the combination of gamma-4 and gamma-6 mean %/disintegration.

EXTERNAL RADIATION

The specific gamma ray constant for Thallium Tl 201 is 33 micro-coulombs/Kg-MBq-hr (4.7R/mCi-hr.) at 1cm. The first half-value layer is 0.0006cm of lead. A range of values for the relative attenuation of the radiation emitted by this radionuclide that results from the interposition of various thicknesses of lead (Pb) is shown in Table 2. For example, the use of 0.21cm of lead will decrease the external radiation exposure by a factor of about 1,000.

To correct for physcial decay of this radionuclide, the fractions that remain at selected intervals before and after calibration are shown in Table 3.

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