When PET image volumes are reconstructed with ordered subset expectation-maximization (OSEM) and subjected to filtration with a 3D Gaussian filter the effective spatial resolution is a function of both the intrinsic scanner resolution and the user-selectable spatial width of the filter. A method was developed to derive the effective spatial resolution from such volumes obtained after acquisitions with a line source on a Siemens Biograph 64 PET/CT scanner. Assuming Gaussian distributions, the full widths at half maximum (FWHM) were derived from probit plots of cumulative spatial distributions across the line source. The effective FWHM values were also used to estimate the FWHM of the intrinsic resolution by extrapolation to a zero filter width.