THE EFFECT OF PHANTOM WALL THICKNESS ON VOLUME DETERMINATION IN SPET

Assessment, using phantoms, of 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 th ickness. With a 30% background activity level, the volume of a 600-cm( 3) cylinder with wall thickness similar to 0.4 mm was measured to an a ccuracy of similar to 1%, whereas for an almost identical cylinder wit h a wall thickness of similar to 3 mm the measured volume was underest imated by about 14%. To further investigate this 'wall-thickness' effe ct, theoretically generated SPET data have been produced, simulating a set of low-contrast cylindrical phantoms with identical internal dime nsions and wall thicknesses ranging from 0 to 6 mm. These image data h ave demonstrated a 10% reduction in the calculated volume for wall thi cknesses 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 u nderlying cause behind this 'wall-thickness' effect is undoubtedly the perturbation in the count profile across the surface boundary, which results from the cold region of the phantom wall. Thus, phantom wall t hickness will have an effect on the performance of most automated quan titation techniques, both two- and three-dimensional, since the majori ty must incorporate some form of analysis on this count profile.