A computer-implemented method of assessing the quality of a functional image for an object includes causing a computer to execute instructions for providing a signal distribution of values N generating a transformed distribution by calculating, for each value N, a transformed value X=√{square root over (N+¼)}, reconstructing the functional image from the signal distribution, deriving an expected distribution of expected values λ from the functional image, generating a residual distribution by calculating, for each value N, a residual values ΔX=X−√{square root over (λ)}, and outputting the residual distribution.