Volume and activity quantitation with iodine-123 SPECT.

UNLABELLED: The goals of this study were to investigate the effect of septal penetration on 123I SPECT activity quantitation using low-energy, high-resolution collimators, and to evaluate a semi-automatic method for measuring volume and activity of 123I distribution with SPECT. METHODS: Data were acquired from experimental phantoms containing spheres filled with a high-purity 123I solution. The penetration study compared the reconstructed activity of a 3.4-cm diameter sphere with and without the presence of surrounding activity. In the study of volume and activity quantitation, three different size spheres (diameters of 1.8 cm, 2.8 cm and 3.4 cm) were imaged in three different sphere-to-background (S:B) 123I concentration ratios (2.5, 5 and 10) with low-energy collimators. The filtered backprojection reconstruction method was used with compensation for scatter, attenuation and detector response. Volume and activity measurements were obtained from the SPECT image using a semiautomatic gradient technique which estimates the location of the sphere/boundary in three dimensions. RESULTS: With the low-energy collimator, there was only a small (< 2%) increase in the measured activity of the sphere when surrounding activity was present. The measured volume for the two largest spheres was within 5% of the true volume for all S:B ratios. The activity measurement of these spheres was consistently underestimated by 20%-25% but suggested that the accuracy could be improved with calibration. For the smallest sphere, the volume was grossly overestimated and only at the 10 S:B ratio was the activity measured reasonably accurately (< 20%). CONCLUSIONS: The low-energy collimators used in this study are suitable for quantitative 123I SPECT. Accurate SPECT volume and activity quantitation of 123I distribution can be achieved by semiautomatic means at clinical count densities for objects as small as 2.8 cm in diameter and reasonable activity quantitation is possible for smaller objects with an S:B ratio of at least 10.

Duke Scholars

Author Timothy Garvey Turkington Radiology