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We have developed a simple technique to gate positron emission tomography (PET) imaging in synchronization with respiratory motion to reduce image blurring caused by breathing and improve quantification of tracer uptake in lesions in the chest and abdomen, especially in very high-resolution PET systems. Taking advantage of the temperature differences in the air flow in a nostril due to inhalations and exhalations, a simple solid thermometer was used to construct a respiratory gating system to monitor the respiratory cycles. The gating system provided trigger signals synchronous with respiration and 40 Hz timing marks. These trigger signals and timing marks were inserted into the data stream in real-time while the PET camera was taking data. The gating trigger signals represent a particular phase of respiratory motion, and the evenly cycled timing marks were designed for motion-image frame registration. This gating system was implemented and tested with our MDAPET, a very high-resolution (2.7-mm resolution) PET camera developed at The University of Texas M. D. Anderson Cancer Center. A volunteer with 2 spherical lesion phantoms (diameters 3 mm and 5 mm) placed on the abdomen close to the navel was scanned in the gated mode. The respiration-gated images of the lesion phantoms were compared and studied. The simple respiratory gating system worked well in terms of detecting the breathing cycle and providing gating trigger signals and timing marks. Image blur and errors in the measurements of the lesions' volumes in the gated PET images were reduced, compared with those of the non-gated PET images.