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PCT system design is complex and involves optimization and tradeoffs in numerous design parameters. A design approach or system evaluation in terms of FWHM resolution and gross efficiency is insufficientand mist. leading. There are many sources of noise in PCT other than statistical which must be minimized. When they become competitive with statistical noise, then further increases in efficiency will not produce significant improvements in performance and image signal/noise. At this time, circumferential systems are more optimal than planar arrays for PCT. The advantages and disadvantages of the hexagon and circle need to be analytically determined in more detail. The importance of positron emission computed tomography (PCT) has grown over the past five years. The principle objectives of this technique is to combine the noninvasive, analytical measurement of tissue radionuclide concentration of labeled compounds which trace a physiologic process in a known manner, and physiologic models that allow the calculation of physioloqic variables of interest. This approach is referred to as Physiologic Tomography, PT1 PT requires tomographs capable of analytical measurements in man rather than qualitative grey scale imaging of activity distributions. This requires more careful and detailed system design to both control the fundamental factors which can compromise this objective, and to maximize the image signal-to-noise ratio (S/N). This paper will discuss some of the physical aspects, sources of noise, design factors, and design alternatives in PCT.