Fig. 3
From: Algorithms for joint activity–attenuation estimation from positron emission tomography scatter
One iteration of joint estimation: an electron density estimate is used for attenuation correction in MLEM scatter-to-activity (step 1) and subsequent trues-to-activity (step 2) updates. Then, the activity distribution estimate is used as a source term in MLGA scatter-to-attenuation (step 3) and MLTR trues-to-attenuation (step 4) updates. In low-attenuation cases, only steps 2 and 3 are used in each iteration, while all steps are used in high-attenuation cases