A hybrid emission tomograph, combining a single photon emission tomograph (SET) and a positron emission tomograph (PET) in one system, has been developed. The system was designed as a high speed, high accuracy neurological tomograph capable of measuring cerebral blood flow with 133Xe, cerebral glucose metabolism with 18F-fluorodeoxyglucose, etc. The detector array consists of 64 Nal crystals in a 42 cm diameter circle. Two collimator sets can slide axially on the inner face of the crystal ring to change from SET to PET imaging. The SET collimator consists of 64 units, each containing a 2 mm thick, 30 × 70 mm tungsten fin plus five sub-fins. All the fins are driven synchronously to swing back and forth over a 60° angle, covering an image field of 21 cm diameter. Linear sampling is selectable from 2.5 to 20 mm by changing the ring rotation increments. Slice thickness is also selectable with interchangeable slice collimators of 16, 20, and 24 mm gap. SET performance with 99mTc showed a resolution of 7.5 and 10 mm full width at half maximum (FWHM) at the periphery and at the center, respectively. System sensitivity with the 20 mm slice collimator is 10,000 counts/sec/μCi/ml for a 20 cm diameter uniform solution of 133Xe and 21,000 counts/sec/μCi/ml for 99mTc. The fastest scan time is 2 sec and provides a spatial resolution of 20 mm FWHM, which is sufficient for dynamic studies of 133Xe clearance. The PET collimator consists of two doughnut shaped lead plates that define the slice plane. The gap is selectable at 12, 20, and 28 mm. Linear sampling in PET is about 20 mm with a stationary ring, 10 mm employing the ring rotation, and 3 mm with the ring wobble. The wobble diameter is 7.3 mm. The PET resolution with a S8Ga line source and a nine point wobble is 11 mm FWHM at the center and 13 mm at a 10 cm radius. System sensitivities were 7,000, 20,000, and 30,000 true events per μCi/ml for 12, 20, and 28 mm slice gap, respectively. Presently, the system is under clinical use for cerebral blood flow studies using 133Xe inhalation or 8lmKr intracarotid continuous infusion, and for blood-brain barrier studies using 99mTeO4 or 68Ga-EDTA, and has yielded many fruitful results.