Detectability limits for a dual head coincidence imaging system

Phantom studies were performed to investigate detection limits for a dual head coincidence imaging system. Detection is defined as: D=contrast/noise (CNR)=(⟨sph⟩-⟨back⟩)/σ_back Where ⟨sph⟩ and ⟨back⟩ are average pixel values for sphere and background regions of interest (ROIs), respectively, and σ_back is the standard deviation of the counts in background regions of the same size as the ROI used for the sphere. Spheres of 2.8, 2.2, 1.6, 1.2, 1.0, 0.8 cm inner diameter were placed at a radius of 5.75 cm in a 22 cm diameter cylindrical phantom. Contrast to noise values were determined for three different sphere to background ratios (10:1, 5:1, and 3:1); for three different background count densities (high, medium, and low), and for three different energy window requirements (photopeak only, photopeak-Compton, and all combinations). Based upon the authors´ visual observation, the smallest detectable spheres were 0.8 cm, 1.2 cm and 1.6 cm for a 2.7 million event image set with sphere to background ratios of 10:1, 5:1 and 3:1, respectively