The acquisition of positron-emission-tomography (PET) transmission information after tracer injection using ring and rotating pin transmission sources is discussed. A combined transmission/emission scan was acquired, followed by an emission scan used to substract the emission counts from the transmission/emission data. The ratio of emission count rate for brain scans to transmission count rate is 50-100% for a 5-mCi ring source and less than 5% for a 5-mCi pin source. Windowing of the sinogram, which rejects most random and scattered coincidences, also eliminates most emission counts. The magnitude and effects of residual random and scattered coincidences as well as increases in variability from transmission/emission scans were studied. The results of combined transmission/emission scans for a high-contrast emission source distribution using ring and pin sources are also described. >