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A new transmission source-based attenuation correction system, c.clear, has been developed for the Siemens c.cam cardiac SPECT camera. The transmission sources are two arrays of 14 Gd-153 line sources placed in cassette wings affixed to the detectors. The source strengths decrease geometrically away from the array center. The absorbed radiation dose received from these sources is about 0.01% of that received from a conventional Tl-201 myocardial scan. The line sources are stationary relative to the detector during the SPECT scan. The emission and transmission radiation are acquired simultaneously and are inherently registered spatially and temporally, so avoiding misregistration artifacts. Transmission reconstruction is performed using an ordered subset maximum likelihood algorithm with gradient descent. It models the geometry of the sources and the radiation emanating from these sources as narrow fan beams. The parameters of this model are estimated from near and far blank calibration images. It includes correction for the Compton emission down-scatter into the transmission energy window that occurs during the simultaneous Tc-99m and Gd-153 imaging. The down-scatter is estimated from down-scatter measured for a short interval when the line sources are obstructed by shutters in the cassette wings. The down-scatter is estimated by linear filtering using the measured short-interval down-scatter and the emission data. The transverse FOV of the camera can truncate the transmission radiation, particularly for obese or improperly positioned patients. Compensation for such truncations is achieved by employing robust data extrapolation techniques and extending the model of the source geometry beyond the measured FOV. This attenuation correction method has been investigated using both phantom and clinical studies and has been shown to produce corrected emission reconstructions of excellent quality.