Thallous Chloride Tl-201 Injection

DRUG DESCRIPTION

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

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