We have recently developed methodologies to assess the local distributions of alveolar ventilation and pulmonary perfusion using positron emission tomography (PET) with 13NN gas as a tracer. In order to quantify the true regional heterogeneity in lung function from these images, it was important to assess the contributions of noise caused by finite count statistics and by imaging artifacts. To characterize these artifacts we collected multiple images with different total number of counts from a uniform phantom labeled with 11CO2 and assessed their heterogeneity as the mean normalized variance of the pixel by pixel data. We developed a novel disc phantom made of open cell foam with a density comparable to that of the lungs. Images of this phantom were reconstructed with a Hanning filter set for different resolution lengths (L). The mean normalized variance of these images was found to closely follow a linear relationship with the inverse of the average number of counts per pixel and L-3 having an intercept that represented the heterogeneity caused by imaging and reconstruction artifacts.