Radionuclide generator yield

Figure 2. "Tube method" for determining radionuclide generator yield.

Studies of short-lived radionuclide generators (4-6) do not adequately treat the quantitative problems of the daughter nuclide elution or those specific to their optimal clinical use. Two essential physical characteristics of a generator are the yield of the daughter nuclide and its radiochemical and radionuclidic purity. To realize the full potential of a short-lived radionuclide generator for medical studies requires that these two characteristics are optimized and are compatible with parameters important to clinical use such as total perfused volume and duration of the scintigraphic examination. 

The fundamental conclusion of this study is that the optimal clinical elution flow rate in the case of continuous elution of a short-lived radionuclide from a perfusion generator does not necessarily correspond to the conditions for maximum elution yield. This conclusion was confirmed by experimental studies with various short-lived radionuclide generators such as the Hg-195m/Au-195m and Sr-82/Rb-82 systems. 

Production of Sr-82. An important consideration in the development of radioisotope generators is the availability, cost, and radionuclidic purity of the long-lived parent. In the case of Sr-82, the 25 day radionuclide is needed in 100-200 mCi amounts in order to provide adequate elution yields of Rb-82 from one loading of Sr-82 every three months. Initially the Sr-82 for the generator was produced at the Lawrence Berkeley Laboratory (LBL) 88-inch cyclotron by the Rb-85 (p,4n) Sr-82 nuclear reaction (12). However, because of the long irradiation time required to produce... 

A third focus has been directed toward incorporation of conventional PET radionuclides nC or 18F into existing substrates or inhibitors known to interact with Pgp [113-115]. This strategy has been employed to produce various PET agents, including 1 -colchicine, 11C-verapamil, nC-daunomycin, and uC/18F-paclitaxel[115-123]. While promising data have been generated, some of these PET agents suffer from modest radiochemical yields and others from complex pharmacokinetics in vivo mediated, at least in part, by rapid metabolism of the radiolabeled compounds. 

A generator should ideally be simple to build, the parent radionuclide should have a relatively long half-life, and the daughter radionuclide should be obtained by a simple elution process with high yield and chemical and radiochemical purity. The generator must be properly shielded to allow its transport and manipulation. 

Deuterium is in very low concentration. Lithium has an atomic weight of 6.94 and the abundance of Li is around 7% in natural Li. The main reaction product of B is Li which does not generate but there are other, minor reactions that do. Except in boron steels, the activation of Li predominates. Another source of in fission reactors is the low yield, ternary fission of fuel (-130 x 10 atoms per fission product pair). In Magnox gas-cooled reactors, from ternary fission is mainly retained in the metallic uranium fuel and its cladding but some is released into the coolant circuits, where it may possibly diffuse into structures within the primary vessel. Tritium is a low energy /5 emitting radionuclide of low radio-toxicity and with a half life of 12.3 years. 

Mound Facility Low-Risk Waste. Approximately 3 x 105 l of low-level waste are generated weekly at the Mound Facility. This particular waste stream is essentially local hard water which has been demineralized and then used in various chemical processes involving the radionuclides 238Pu and 233U. The first step in decontaminating the low-risk wastes entails addition of small amounts of calcium and iron salts followed by addition of NaOH to pH 11.5 to precipitate iron and calcium hydroxides and carbonates. The clarified effluent from the precipitation step is then passed through a 200 micrometer sand filter to yield a solution containing, typically, 0.1+ d/min/m alpha activity. 

Chemical separations may be specific for the analyte of interest (see Chapter 3), such as liquid or gas chromatography, or scavenging (such as by precipitation) to remove the major interfering substances. Addition of carrier, as practiced in radioanalytical chemistry to assist in purifying radionuclides, usually is not appropriate for mass spectrometric analysis. Such addition undermines the isotopic ratio measurements that are often at the heart of this procedure, and also overloads the system for ion generation and peak resolution (but carrier addition is used for accelerator mass spectrometry). Addition of tracers, known as isotope dilution, is often employed for yield determination (see Section 17.2.9). Interferences are distinctly different in radiometric and MS analyses of radionuclides, and may be the deciding factor in selecting one method versus the other. 

The enzyme fluorinase (5 -fluoro-5 -deoxyadenosine synthase) was applied to convert S-adenosyl-L-methionine (SAM) and [ F]fluoride directly into 5 -[ F]fluoro-5 -deoxyadenosine (5 -[ F]FDA) (O Fig. 42.30a) and L-methionine, which was the first example where a radionuclide was enzymatically introduced into an organic molecule (Martarello et al. 2003). In this approach, the native wildtype fluorinase deriving from Streptomyces cattleya was employed in low concentrations (pg ml ) and generated only very low radiochemical yields of - 1%. Better conversions of -20-25% were achieved by the use of higher fluorinase concentrations (mg/ml), which became available by fluorinase over-expressing cloned Escherichia coli (Deng et al. 2006). 

The most important isotope of plutonium is Pu = 24,200 years). It has a short half-life so only ultra traces of plutonium occur naturally in uranium ores, and most plutonium is artificial, being an abundant byproduct of uranium fission in nuclear power reactors. The nuclear reactions involved include the radiative capture of a thermal neutron by uranium, U( , y) U the uranium-239 produced is a beta-emitter that yields the radionuclide Np, also a beta-emitter that yields Pu. To date, 15 isotopes of plutonium are known, taking into account nuclear isomers. The plutonium isotope Pu is an alpha-emitter with a half-life of 87 years. Therefore, it is well suited for electrical power generation for devices that must function without direct maintenance for time scales approximating a human lifetime. It is therefore used in radioisotope thermoelectric generators such as those powering the Galileo and Cassini space probes. 

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