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Volume and Activity Quantitation with Iodine-123 SPECT

Department of Radiology, Duke University Medical Center, Durham, North Carolina

Correspondence: For correspondence and reprints contact: David R. Gilland, PhD, Dept., of Radiology, Box 3949, Duke University Medical Center, Durham NC 27710.

ABSTRACT

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 ¹²³ 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.

Key Words: iodine-123 • SPECT • activity quantitation • volume quantitation • septal penetration

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