Performance characteristics of the 3-D OSEM algorithm in the reconstruction of small animal PET images

Rat brain images acquired with a small animal positron emission tomography (PET) camera and reconstructed with the three-dimensional (3-D) ordered-sub sets expectation-maximization (OSEM) algorithm with resolution recovery hav e better quality when the brain is imaged by itself than when inside the he ad with surrounding background activity. The purpose of this study was to c haracterize the dependence of this effect on the level of background activi ty, attenuation, and scatter, Monte Carlo simulations of the imaging system were performed. The coefficient of variation from replicate images, full-w idth at half-maximum (FWHM) from point sources and image profile fitting, a nd image contrast and uniformity were used to evaluate algorithm performanc e. A rat head with the typical levels of five and ten times the brain activ ity in the surrounding background requires additional iterations to achieve the same resolution as the brain-only case at a cost of 24% and 64% additi onal noise, respectively, For the same phantoms, object scatter reduced con trast by 3%-5%. However, attenuation degraded resolution by 0.2 mm and was responsible for up to 12% nonuniformity in the brain images suggesting that attenuation correction is useful, Given the effects of emission and attenu ation distribution on both resolution and noise, simulations or phantom stu dies should be used for each imaging situation to select the appropriate nu mber of OSEM iterations to achieve the desired resolution-noise levels.