Cross-validation stopping rule for ML-EM reconstruction of dynamic PET series: effect on image quality and quantitative accuracy

A major shortcoming of the maximum likelihood expectation maximization (ML-EM) method for reconstruction of dynamic positron emission tomography (PET) images is to decide when to stop the iterative process for image frames with largely different statistics and activity distributions. A widespread practice to overcome this problem involves overiteration of an image estimate followed by smoothing. Here, the authors investigate the qualitative and quantitative accuracy of the cross-validation procedure (CV) as a stopping rule, in comparison to overiteration and post-filtering, for the reconstruction of phantom and small animal dynamic ¹⁸F-fluorodeoxyglucose PET data acquired in two-dimensional mode. The CV stopping rule ensured visually acceptable image estimates with balanced resolution and noise characteristics. However, quantitative accuracy required some minimum number of counts per image. The effect of the number of ML-EM iterations on time-activity curves and metabolic rates of glucose extracted from image series is discussed. A dependence of the CV defined number of iterations on projection counts was found that simplifies reconstruction and reduces computation time