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The use of positronemitting radionuclides offers unique ■ advantages for radionuclide scintigraphy and also presents unique problems. Positron scintigraphy is of interest for three reasons.First, the detection of the annihilation radiation results in high ■detection efficiency and high spatial resolution. The reason for this is that absorbing collimators are not an intrinsic part of the imaging system. As such collimators provide resolution for gammaray scintigraphy devices primarily by absorbing a large frac ■ .tioh of the gamma rays, it is clear that, in principle, positron scintigraphy efficiencies may be much higher than corresponding gammaray scintigraphy efficiencies (Brownell and Sweet, 1953;Brownell, 1964;Burnham, Aronow and Brownell, 1970).The second factor is the attenuation characteristics of the annihilation radiation. Since for' a given patient thickness the sum of the distance traversed by the two quanta is constant, the attenuation is independent of location of a source positioned between the two detectors.. This may aid in visualizing certain details of a radionuclide distribution and offers the possibility of obtaining absolute activity distributions by the. use of an external standard.

9 hybrid positron scanner, which combines the simplicity of a mechanical scanner with the increased sensitivity of a large area array, has been designed utilizing nine crystal pairs and 25 coincidence circuits. This technique has the advantage over single-pair positron systems in that greatly increased source activity may be viewed. The instrument functions reliably in clinical studies and has proven useful in supplying data for computer studies of radioisotope scan data processing. Valuable experience with multi-detector systems and fast timing techniques has been gained.