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Positron emission tomography (PET) is potentially useful for the quantitative imaging of radiolabeled antibodies, leading in turn to improved dosimetry in radioimmunotherapy. Iodine-124 is a positron-emitting nuclide with appropriate chemical properties and half-life (4.2 days) for such studies since the radiolabeling of antibodies with iodine is well understood and the half-life permits measurements over several days. Unfortunately, I-124 has a complex decay scheme with many high-energy gamma rays and a positron abundance of only 25%. It has therefore been largely ignored as a PET-imaging nuclide. However, measurements made with phantoms and animals under realistic conditions using a BGO-based PET scanner have shown that satisfactory imaging and quantitation can be achieved. Investigations of spatial resolution, the linearity of regional observed count rate versus activity in the presence of other activity, and the visualization and quantitation of activity in spheres with different surrounding background activities were carried out with phantoms up to 22 cm in diameter. Compared with F-18, spatial resolution was only slightly degraded (13.5 mm FWHM vs 12 mm FWHM) while linearity was the same over a 10:1 activity range (0.015 to 0.15 MBq/ml for I-124). The visualization and quantitation of spheres was also slightly degraded when using similar imaging times. Increasing the imaging time for I-124 reduced the difference. To verify that the technique would work in vivo, measurements were made of human neuroblastoma tumors in rats which had been injected with I-124 labeled 3F8 antibody. Although the number of samples was small, good agreement was achieved between image-based measurements and direct measurements of excised 4-g tumors. Thus quantitative imaging of I-124 labeled antibodies appears to be possible under realistic conditions.

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The present status and future directions of research and development on radionuclide generator technology are reported. The recent interest to develop double-neutron capture reactions for production of in vivo generators; neutron rich nuclides for radioimmunotherapeutic pharmaceuticals; and advances with ultrashort lived generators is highlighted. Emphasis is focused on: production of the parent radionuclide; the selection and the evaluation of support materials and eluents with respect to the resultant radiochemical yield of the daughter, and the breakthrough of the radionuclide parent; and, the uses of radionuclide generators in radiopharmaceutical chemistry, biomedical and industrial applications. The 62 Zn -62 Cu, ^Ni -""Cu, ,03m Rh -• ,03 Rh, ,88 W -• ,88 Re and the 225 Ac -22, Fr -2l3 Bi generators are predicted to be emphasized for future development.