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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 to 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-state thermometer was used to construct a respiratory gating system to monitor the respiratory cycles. The gating system provided trigger signals indicating inspiration and expiration as well as periodic (about 40 Hz) timing-tick signals. These trigger and timing-tick signals were inserted into the data stream in real-time while the PET camera was taking data. The gating trigger signals represent particular phases of respiratory motion. 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 two 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 and timing-tick signals. Image blur and errors in the measurements of the lesions' volumes in the gated PET images were reduced, compared with those of the nongated PET images.