Positron emitters, generators

Our production parameters for this generator are presented. The Xe-122/l-122 combination, a convenient source of a short-lived (3.6m) positron emitting iodine, is also discussed. Recent developments in rapid iodination procedures will broaden the potential applications of this generator. Finally, preliminary investigations of another generator derived radionuclide that may have promise is described. Tellurium-118 (6d) is the parent of the 3.5 minute positron emitter Sb-118 which may be useful for first pass angiography. 

Radioisotope generators provide a desirable alternative to cyclotrons for the production of short lived positron emitters. A fully automated microprocessor controlled generator system for obtaining 76 sec rubidium-82 is described in detail. 

Ultra short lived radionuclides, with a half-life of a few seconds to a few minutes are readily available from long-lived parent radionuclides adsorbed to an organic or inorganic ion exchange support matrix (1-3). These radionuclide generator systems are an inexpensive alternative to an on-site cyclotron, especially for positron emitters used for positron emission tomography (PET). 

Automated radionuclide generators capable of providing precise dose delivery of multi-millicurie amounts of short-lived positron emitters on demand from a safe and easily operated system are an attractive alternative to on-site cyclotrons for positron emission tomography. The availability of curie quantities of parent radionuclides from national laboratories and the development of microprocessor automation makes it feasible to utilize these generators in the clinical setting. 

We have demonstrated the use of generator produced Rb-82 as a readily available supply of a positron emitter for PET studies. 

The short lived positron-emitter Rb-82 (t 1/2=1.26m) has potential application in cardiovascular diagnostic nuclear medicine. A generator system containing the parent Sr-82 has been developed that will provide an eluate of Rb-82 suitable for direct infusion. The Rb-82 is eluted by a syringe pump from a hydrous stannic oxide column in a continuous stream of physiological saline solution. The rate of elution (infusion) can be controlled from 10 to 100 ml/ min. At elution rates of 25, 50, and 75 ml/min,... 

The third means of radionuclide production involves target irradiation by ions accelerated in a cyclotron. One example of this approach is provided by the production of Ge, which decays with a 280 day half-life to the positron emitter Ga. Proton irradiation of Ga produces Ge in a (p,2n) reaction. After dissolution of the target material a solution of the Ge product in concentrated HCl is prepared and adsorbed on an alumina column which has been pre-equilibrated with 0.005 M EDTA (ethylenediaminetetraacetate) solution. The Ga daughter may then be eluted using an EDTA solution in a system which provides the basis of a Ga generator. Cyclotron production of radionuclides is expensive compared with reactor irradiations, but higher specific activities are possible than with the neutron capture process. Also, radionuclides with particularly useful properties, and which cannot be obtained from a reactor, may be prepared by cyclotron irradiation. In one example, cyclotron produced Fe, a positron emitter, may be used for bone marrow imaging while reactor produced Fe, a /3-emitter, is unsuitable. " ... 

It is important to consider how quickly the radionuclide can be delivered to the patient from the source (including the time taken for any complex or compound preparation). Unless the hospital has a cyclotron and the chemistry is simple, half-lives of the order of minutes are too short. There is an alternative, in that nuclides that are not positron emitters may produce them by radioacti-vative decay. Such nuclides, called generators, can have longer half-lives and, thus, provide a ready source of short-lived positron emitters. It is also important that the parent and daughter nuclides be easily separated, so that the parent can produce more daughter nuclides while the first batch is being used. Examples of such systems include the use of " Ce (half-life 3 days) to produce (half-life 7 min), and the use of Ge (half-life 271 days) to produce Ga (half-life 68 min). 

The generator produced positron emitters find application mostly in PET studies at centers without a cyclotron. A discussion of generator preparation is beyond the scope of this article (for a detailed discussion, c O Chap. 40 in this Volume). Here a very brief account is given of the production of the two long-lived parent radionuclides concerned (cf. Qaim 1987 Qaim et al. 1993). 

Generator-produced positron emitters with potential for positron emission tomography (PET) imaging... 

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