Photon attenuation and scatter are the most important factors degrading the quantitative accuracy of single photon emission tomography (SPET). Simultaneous SPET and transmission tomographic (TT) scans with dual isotopes have been reported to correct attenuation and scatter. However, there is cross-contamination of different energies (scatter from emission data to transmission data and from transmission data to emission data). A method has been proposed to acquire emission (functional) and transmission (structural) data simultaneously with a single isotope scan. A 99Tcm transmission line source at the focal distance is attached to the rotating drum plate of a triple-headed gamma camera equipped with fan-beam collimators. The transmission source has the same energy spectrum as the emission source because 99Tcm is also used for SPET. The triple-headed SPET system allows the transmission and emission data to be acquired by one detector (D1), while the other two detectors (D2, D3) simultaneously acquire emission data. The transmission data can be calculated by subtracting D2 (D3) from D1 after correction for time-decay. A transmission-dependent method for scatter correction was implemented with the transmission and emission data. In principle, there is no energy cross-contamination using 99Tcm as the transmission-emission source. The results of scatter correction demonstrate a clear improvement in spatial resolution. The contrasts were increased for different sized 'hot' regions both in SPET and brain phantoms. The results indicate that the proposed method can overcome the difficulty associated with simultaneous dual-isotope acquisition. They further support the feasibility of simultaneous SPET and TT scanning using a single isotope.