Transaxial cross‐sections of a phantom with: (a) six cold (no activity) piecewise constant spheres with radii of 4, 5, 6, 7, 8, and 9 mm, (b) eight point sources with maximum‐activity‐to‐mean‐background ratio of 100:1 at different radial distances from the central axis of the phantom, (c) six hot Gaussian blobs with radii (FWHM) of 4, 5, 6, 7, 8, and 9 mm with maximum‐activity‐to‐mean‐background ratio of 3:1 and (d–f) reference phantom containing warm Gaussian blobs only. Both phantoms were of the size 128 × 128 × 128 voxels, with voxel size set to 2.2 × 2.2 × 2.2 mm3.