INFLUENCE OF COLLIMATOR CHARACTERISTICS ON QUANTIFICATION IN SPECT

Accurate, absolute quantification of activity in SPECT depends on the counting sensitivity of the gamma camera collimator system and on the contribution of the tail of the point spread function (PSF) that conta ins both scattered radiation and septal penetration. Methods: These fa ctors were studied for the radionuclides Tc-99m, I-131 and I-123 on di fferent cameras and collimators. The ability of the geometric mean lin e source scatter function subtraction technique to correct for the eff ect of the tail of the PSF was investigated in a computer simulation s tudy with Tc-99m and a physical phantom study with I-123. Results: The sensitivities of Tc-99m collimators were relatively constant with dis tance in air whereas those of I-123 and I-131 decreased considerably. For these radionuclides the contribution of the PSF tail was also incr eased as compared to Tc-99m. The geometric mean line source scatter fu nction subtraction technique was confirmed as being able to correct ef fectively for this factor. Conclusion: Collimator design affects both the fall off of sensitivity in air and the tail of the PSF. Attention to correction of these factors enables accurate quantification.