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Performance characteristics of a new design of positron tomograph with automatically retractable septa for brain imaging have been studied. The device, consisting of block BGO detectors (8* 8 elements per block), has a ring diameter of 76 cm and an axial FOV of 106.5 mm. The in-plane resolution is on average 5.8 mm and 5.0 mm (FWHM) for stationary and wobble sampling, respectively, over the central 18 cm of the transaxial FOV. Its unique feature is the capability of data acquisition both in the'conventional'2D mode (with septa) or 3D mode (septa retracted) where coincidences between any of the 16 detector rings are acquired. In spite of the increase in scatter when septa are retracted, the increased efficiency in the 3D mode of acquisition yields distinct advantages, particularly in the many studies where tracer concentration is low and consequently where dead time and random rates are less important.