Introduction: Filtered backprojection (FBP) reconstructions for transmission (Tx) data can be used for attenuation correction (AC) in phantom data, however impact on body mass index (BMI) has not been adequately demonstrated. We performed a paired comparison of Tx maps and AC myocardial perfusion images (Em) in patients with clinically relevant range of BMI (20–45 kg/m 2) processed with the Bayesian Iterative Transmission Gradient Algorithm (BITGA) and FBP. Methods: We examined 50 rest/stress Tc-99m Sestamibi patient studies (5 male and 5 female in five BMI groups: <25, 25–30, 30–35, 35–40, >40 kg/m 2). with a low-likelihood (<5%) of coronary artery disease. Attenuation coefficients were measured in a 5X5 ROI centered in the mediastinum. Uniformity of the Em data was measured as relative uptake in the septal, lateral, anterior, inferior and apical walls. Data Acquisitions: Data was acquired on a Philips Cardio system using VantagePro™ and ExSPECT II™ software using LEHR collimation, 30s/25s (rest/stress) per stop, 64 angles. Results: Tx maps appeared less accurate and less uniform when reconstructed with FBP (see figures). For Em data, BITGA reduced the wall to wall variability from 4.8% (BITGA) to 4.1% (FBP) (p=0.02). Furthermore, the number of patients whose septal-lateral wall ratios (SL) differed from unity by more than ten percent was reduced from 12% to 4% (p<0.05). Patients with BMI>30 were more likely to have a divergent SL (83.3% vs 16.7%). Visual inspection of FBP based images indicate this may be due to poor definition of the lung/heart boundary. Conclusion: Use of BITGA produced more accurate and uniform TX results for all BMI groups. Also, patients with BMI>30, use of FBP Tx maps introduced artifacts that can compromise Em quality.