Abstract
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Objectives Ammonia N13 injection is prepared as a carrier free solution for cardiac imaging. We questioned whether 13NH3 is carrier free. The assessment of carrier NH3 is an important quality assurance test. Under air-equilibrated irradiation conditions, we found an amount of carrier that exceeded the limits of NH3 in sterile water for injection (SWI) and sought to determine the source of this carrier.
Methods We prepare 13NH3 by the irradiating SWI containing 5 mM ethanol. Our target body is Al (~3 mL) with a Ti window; the target is cooled with circulating 54°F water. We typically irradiate for 10 min x 30 µA of 11 MeV H+. The assay for carrier NH3 uses ion chromatography (Dionex IonPac C14 column) with a conductivity detector and anion suppression and was done prior to making it isotonic.
Results Under normal irradiation conditions with 5 mM ethanol solution sterile-filter vented to air, the NH3 in product is ~1.5 µg/mL or ~ 4 GBq/µmol at EOB, an amount well below theoretical specific activity and above the USP limit for NH3 in SWI. The mass correlated with the amount of beam (µA x min) with a near-zero intercept. 5 mM ethanol held in the target for the time of a typical bombardment but without beam produced < 0.05 µg/mL of carrier. Water contains ~18 µg/mL of dissolved N2 at 1 atm and RT. Sterile sparging with inert gas reduced the level of carrier to <0.5 µg/mL after irradiation, well within the USP limit.
Conclusions Nucleogenic 13N atoms from the 16O(p,α)13N nuclear reaction are formed with high energy and charge and, in pure H2O produce oxidized products. However, the highly reducing environment caused by radiation chemistry in the target converts nitrate and nitrite to NH3 and this conversion is aided by addition of ethanol. Our results showed that radiation chemistry also converts N2 to NH3, a reaction that has not been previously reported in the cyclotron targetry literature. Water solutions for making 13NH3 at high beam current should be degassed prior to irradiation, especially for low E accelerators where the GBq yields require a large fraction of the target product for an individual patient dose.
Research Support Supported by NIH grants P01 CA42045 and RR17229.