The intrinsic resolution (FWHM) of coincident bismuth germanate detector pairs was measured for geometries consistent with positron emission computed tomography systems designed for brain imaging. The resolutions were measured as a function of the position in the field of view (FOV) for single slice and multislice configurations for both the image and axial resolutions. System diameters of 66 to 42 cm were simulated, and intrinsic resolution variation for image and axial dimensions for a 24 cm FOV were 14 and 32% and 24 and 39%, respectively. Measurement of axial resolutions of interplane slices of multiplane systems employing interplane septa showed resolution variations for the 24 cm FOV of as large as a factor of three. A simple model was shown to predict the dominant features of the line shapes. Images of phantoms with small axial dimensions indicate that the variations in resolution, particularly in interplane data, seriously compromise the validity of point to point comparison of information within an image or between single plane and interplane images. Serious variations in axial resolution were shown to create artifacts and degrade the image resolution. Index Terms: Emission computed tomography--Image quality.