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Morphology of numerical phantom used: (a) Transaxial cross-section (slice 17) through the centers of all hot spheres; (b)Transaxial cross-section (slice 49) through the centers of all cold spheres; (c) Sagittal cross-section through the centers of the two largest spheres. The mean activity ratios of hot:background:cold areas are 40:10:1, respectively. The spheres radii are counterclockwise: 3, 6, 4, 5, 7, 9 and 14 (center) pixels, respectively. The pixel size used is 0.172 cm.
