We investigated combined scatter and attenuation correction with maximum likelihood expectation maximization (ML-EM) algorithm for projections obtained with simultaneous transmission (99nrTc) and emission (201T1) data acquisition in 201T1 SPECT. An energy window 15% in width centered at 140 keV was used for 999'C transmission photons. Measured linear attenuation coefficient was included in probability factor, cii, of ML-EM algorithm. Emission data acquisition was performed with three energy windows: a 30% photopeak energy window set symmetrically over 73-keV peak of 201T1 and a 7.3% energy window set over 54.5-keV peak and a 4.4% energy window set over 91.0-keV peak. Latter two scatter windows were placed one full width half maximum (FWHM) below and above the photopeak centerline. The scatter fraction in the primary peak was estimated using trapezoid approximation and scatter was compensated intrinsically within ML-EM algorithm. Ordered subsets expectation maximization (OSEM) algorithm was used to accelerate ML-EM algorithm.201T1 myocardial perfusion images and quantitative accuracy for activity concentration in uniform attenuating medium was reported.
