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A gamma camera system which is able to acquire simultaneous single-photon emission tomographic (SPET) data and gamma ray transmission computed tomography (TCT) data for brain study using external rod sources and fan-beam collimators was developed and evaluated. Since the three external rod sources were located at the focal points of fan-beam collimators, which also happened to be the apexes of the equilateral triangle defined by the three detectors, simultaneous SPET and TCT scan could be performed using a 120° shared scan. Therefore, the proposed system required less than one third of the scanning time of a single-head system. Since the combination of rod sources and fan-beam collimators decreased the scatter component in transmission data without a slit collimator for each rod source, the radioactivity of the rod source was less than one-tenth of the previous investigations. For evaluation, we used two isotopes, thallium-201 for TCT and technetium-99m for SPET. The cross-contamination of transmission and emission was well compensated using the triple energy window (TEW) method. In a separate TCT scan, the measured attenuation coefficient of201Tl for water was 0.19±0.01 cm-1, while in a simultaneous scan, it was 0.20±0.01 cm-1. The measured attenuation coefficient for water agreed well with the narrow-beam (theoretical) value of 0.187 cm-1. In SPET images, scatter compensation was also performed using the TEW method and attenuation compensation was done using the measured attenuation map. The results showed the feasibility of simultaneous SPET and TCT scanning using the TEW method to obtain quantitative SPET images.