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Ammonia N 13 Injection USP is indicated for diagnostic Positron Emission Tomography (PET) imaging of the myocardium under rest or pharmacologic stress conditions to evaluate myocardial perfusion in patients with suspected or existing coronary artery disease.
11.1 Chemical Characteristics
Ammonia N 13 Injection is a positron emitting radiopharmaceutical that is used for diagnostic purposes in conjunction with positron emission tomography (PET) imaging. The active ingredient, [13N] ammonia, has the molecular formula of 13NH3with a molecular weight of 16.03, and has the following chemical structure:
Ammonia N 13 Injection is provided as a ready to use sterile, pyrogen-free, clear and colorless solution. Each mL of the solution contains between 0.138 GBq to 1.387 GBq (3.75 mCi to 37.5mCi) of [13N] ammonia, at the end of synthesis (EOS) reference time, in 0.9% aqueous sodium chloride. The pH of the solution is between 4.5 to 7.5. The recommended dose of radioactivity (10-20 mCi) is associated with a theoretical mass dose of 0.5-1.0 picomoles (8.47-16.94 picograms) of ammonia.
11.2 Physical Characteristics
Nitrogen N13 decays by emitting positron to Carbon C13 (stable) and has a physical half-life of 9.96 minutes. The principal photons useful for imaging are the dual 511 keV gamma photons that are produced and emitted simultaneously in opposite direction when the positron interacts with an electron (Table 2).
| Radiation/Emission | %Per Disintegration | Energy |
|---|---|---|
|
||
| Positron(β+) | 100 | 1190 keV (Max.) |
| Gamma(±)* | 200 | 511 keV |
The specific gamma ray constant (point source air kerma coefficient) for nitrogen N13 is 5.9 R/hr/mCi (1.39 x 10-6 Gy/hr/kBq) at 1 cm. The half-value layer (HVL) of lead (Pb) for 511 keV photons is 4 mm. Selected coefficients of attenuation are listed in Table 3 as a function of lead shield thickness. For example, the use of 39 mm thickness of lead will attenuate the external radiation by a factor of about 1000.
| Shield Thickness (Pb) mm | Coefficient of Attenuation |
|---|---|
| 4 | 0.5 |
| 8 | 0.25 |
| 13 | 0.1 |
| 26 | 0.01 |
| 39 | 0.001 |
| 52 | 0.0001 |
Table 4 lists fractions remaining at selected time intervals from the calibration time. This information may be used to correct for physical decay of the radionuclide.
| Minutes | Fraction Remaining |
|---|---|
|
|
| 0* | 1.000 |
| 5 | 0.706 |
| 10 | 0.499 |
| 15 | 0.352 |
| 20 | 0.249 |
| 25 | 0.176 |
| 30 | 0.124 |