We present a new high spatial resolution brain positron tomograph the ECAT 953B/31. The machine consists of two 76.5 cm diameter rings (patient port diameter : 36 cm) made of 24 8x8 BGO detector blocks each, each block being coupled to four 1 incha Hammamatsu phototubes. The machine has fifteen 9 cm x 1 mm motorized removable septa which allows reconstruction of 31 slices 3.375 mm apart.The transaxial resolution (FWHM) in the reconstructed image (with wobbling) is 4.5 mm at center, 5.2 mm (tangential) and 5.6 mm (radial) at 10 cm. The axial resolution is 4.4 mm and 5.8 mm at 10 cm from center. With septa out of the field of view (FOV), the axial resolution degrades to 5.5 mm at center and 6.3 mm at 10 cm from center. At 1 pCi/cc with a 250 keV threshold and a 12 nsec coincidence window, the sensitivity is 146,100 true events/sec, the ratio of random to trues is .lo, the scatter fraction is 17% and the dead time losses are 30%. With septa out of the FOV, the sensitivity is increased by a factor 3.6, while the scatter fraction reaches 41%.Images obtained with '"F-DG and Hi50 in human brains and '"F-DOPA in a baboon brain demonstrate that the ECAT 953B/31 will be particularly suitable both for high resolution and for low count rate brain studies.