Background: We evaluated the effect of LV positional differences on detection of serial perfusion changes. Methods: We used the NCAT software phantom to simulate three different shaped LVs based on actual patient studies. A small (∼10% of the myocardium) single defect was simulated for each of the three patients; the severity of each defect was varied in 5% increments from .5 of LV maximum perfusion for 6 steps, with each severity repeated. This provided a total of 12 simulations for each patient. Random LV rotations (stdev=2.5°), translations (stdev=0.5cm), and small scaling factors (< 5%) were added to each of the 36 simulated LVs. Simulated projections included attenuation, noise, and detector response, and were reconstructed using filtered backprojection. Transaxials were reoriented manually for each simulation, with the operator blinded as to the differences between the simulated studies. Perfusion quantification was performed using the Emory Cardiac Toolbox; abnormally perfused regions were used to define areas of interest.. . Changes in perfusion values in these areas were statistically evaluated. Results: In the repeated studies, where no difference in defect severity should be detected, a mean difference of 1.3% of LV maximum was seen; this difference was significant (p<.05) in 27% of comparisons (n=18). For 5% changes in defect severity (e.g., a change in defect severity from 50 to 55% of maximum perfusion), we computed a mean difference of 2.3% of LV maximum; however, differences were statistically significant in only 40% of the individual comparisons (n=30). For 10% changes in defect severity, we quantitated a mean difference of 4.7%; the change was significant in 70% of the comparisons (n=24). For 15% changes in defect severity, we detected a mean difference of 7.7%; the change was significant in 72% of the cases (n=18). A trend toward improved accuracy was seen with increasing defect size. Conclusion: Current perfusion quantification methods may underestimate actual perfusion changes in serial studies. Noise, detector response, and orientation differences may make small changes in small defects difficult to detect with statistical significance.