Hot sphere detection limits for a dual head coincidence imaging system

Phantom studies were performed to investigate detectability limits for a dual head coincidence imaging system. 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. The contrast to noise ratio (CNR) of the spheres to background was used as the authors´ metric of detectability. CNR 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 and 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 2.5 million event photopeak-Compton image set with sphere to background ratios of 10:1, 5:1 and 3:1, respectively. The CNR values for these spheres were 5.1, 7.8, and 4.8. Over all observations, spheres with a CNR greater than 4 were detectable.