The effect of phantom wall thickness on volume determination in SPET

Assessment, using phantoms, of a a three-dimensional, second-derivative, surface-detection algorithm for accurately determining the volumes of structures in single photon emission tomography (SPET) has shown the performance of the algorithm to be highly dependent on phantom wall thickness. With a 30% background activity level, the volume of a 600-cm3 cylinder with wall thickness approximately 0.4 mm was measured to an accuracy of approximately 1%, whereas for an almost identical cylinder with a wall thickness of approximately 3 mm the measured volume was underestimated by about 14%. To further investigate this 'wall-thickness' effect, theoretically generated SPET data have been produced, simulating a set of low-contrast cylindrical phantoms with identical internal dimensions and wall thicknesses ranging from 0 to 6 mm. These image data have demonstrated a 10% reduction in the calculated volume for wall thicknesses as thin as 1 mm. A less acute dependence is also demonstrated for a threshold-based quantitation algorithm, where a wall thickness of 4-5 mm is required to produce an effect of similar magnitude. The underlying cause behind this 'wall-thickness' effect is undoubtedly the perturbation in the count profile across the surface boundary, which results form the cold region of the phantom wall. Thus, phantom wall thickness will have an effect on the performance of most automated quantitation techniques, both two- and three-dimensional, since the majority must incorporate some form of analysis on this count profile.