Thallous Chloride Tl 201

THALLOUS CHLORIDE TL 201- thallous chloride, tl-201 injection, powder, lyophilized, for solution
GE Healthcare, Medi-Physics, Inc.


Product Number: 2097, 2098



Thallous Chloride Tl 201 Injection is supplied in isotonic solution as a sterile, nonpyrogenic, diagnostic radiopharmaceutical for intravenous administration. Each unit dose contains 1 mL and each milliliter contains 37 MBq (1 mCi) of Thallous Chloride Tl 201 Injection at calibration time. The pH is adjusted to 4.5-7.5 with hydrochloric acid or sodium hydroxide. It is made isotonic with 9 mg sodium chloride/mL and is preserved with 0.009 mL benzyl alcohol/mL.

Thallium Tl 201 is cyclotron produced with no carrier added. The radionuclidic composition at calibration time, expressed as percent of total activity, is not less than 98 percent Thallium Tl 201 with not more than 0.3 percent Thallium Tl 200 , not more than 1.2 percent Thallium Tl 202, not more than 0.2 percent Lead Pb 203, and not more than 0.3 percent all others.

The concentration of each radionuclidic contaminant changes with time. Therefore, it is recommended that Thallous Chloride Tl 201 Injection be administered close to calibration time to minimize the effect of higher levels of radionuclidic contaminants pre and post calibration. Graph 1 shows maximum allowable concentration of each radionuclidic contaminant as a function of time.

Graph 1. Radionuclidic Contaminants

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Thallium Tl 201 decays by electron capture to Mercury Hg 201 with a physical half-life of 73.1 hours. 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 68.9-80.3 keV is much greater than the combination of gamma-4 and gamma-6 mean %/disintegration.

Table 1. Principal Radiation Emission Data *
Radiation Mean
Energy (keV)
Kocher David C, “Radionuclidic Decay Data Tables.” DOE/TIC-11026, 182 (1981)
Gamma-4 2.7 135.3
Gamma-6 10.0 167.4
Mercury x-rays 94.4 68.9-80.3


The specific gamma ray constant for Thallium Tl 201 is 3.21 microcoulombs/hr-kg-MBq (0.46 R/hr-mCi) at 1 cm. The first half-value layer is 0.023 cm 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 0.31 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
Method of calculation: Data dated 1984 supplied by Oak Ridge Associated Universities. Radiopharmaceutical Internal Dose Information Center.
0.023 0.5
0.081 10-1
0.19 10-2
0.31 10-3
0.44 10-4

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

Table 3. Thallium Tl 201 Decay Chart: Half-life 73.1 Hours
Hours Fraction
Hours Fraction
Hours Fraction
Calibration time
0* 1.00 42 0.67 84 0.45
6 0.94 48 0.63 90 0.43
12 0.89 54 0.60 96 0.40
18 0.84 60 0.57 108 0.36
24 0.80 66 0.53 120 0.32
30 0.75 72 0.51
36 0.71 78 0.48

At two and four days post calibration, Thallium Tl 201 concentrations amount only to about 63% and 40%, respectively, of their initial value. This condition would require use of proportionately larger volume doses.


Thallous Chloride Tl 201 Injection with no carrier added has been found to accumulate in viable myocardium in a manner analogous to that of potassium. Experiments employing labeled microspheres in human volunteers have shown that the myocardial distribution of Thallous Chloride Tl 201 Injection correlates well with regional perfusion.

In clinical studies, thallium images have been found to visualize areas of infarction as “cold” or nonlabeled regions which are confirmed by electrocardiographic and enzyme changes. Regions of transient myocardial ischemia corresponding to areas perfused by coronary arteries with partial stenoses have been visualized when thallium was administered in conjunction with an exercise stress test.

Intravenous administration of Thallous Chloride Tl 201 Injection is characterized by rapid biexponential clearance from the blood with about 91.5% of blood radioactivity disappearing with a half-life of approximately 5 minutes and the remainder with a half-life of about 40 hours. Maximal concentration by normal myocardium occurs at about ten minutes with sustained myocardial retention and adequate concentration in heart muscle to permit gated imaging. In addition, localization occurs in parathyroid adenomas and to a lesser extent in sites of parathyroid hyperplasia and other abnormal tissues such as thyroid adenoma, neoplasia (e.g. parathyroid carcinoma) and sarcoid. Biodistribution is generally proportional to organ blood flow at the time of injection. Blood clearance of Thallous Chloride Tl 201 Injection is primarily by the myocardium, kidneys, thyroid, liver and stomach with the remainder distributing fairly uniformly throughout the body. The dosimetry data in Table 4 reflect this distribution pattern and are based on a biological half-life of 11 days and an effective half-life of 2.4 days. Thallous Chloride Tl 201 Injection is excreted slowly and to an equal extent in both feces and urine.

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