ENERGY-BASED SCATTER CORRECTIONS FOR SCINTILLATION CAMERA IMAGES OF I-131

The use of high-dose I-131 antibody therapy requires accurate measurem ent of normal tissue uptake to optimize the therapeutic dose. One of t he factors limiting the accuracy of such measurements is scatter and c ollimator septal penetration. This study evaluated two classes of ener gy-based scatter corrections for quantitative I-131 imaging: window-ba sed and spectrum-fitting. Methods: The window-based approaches estimat e scatter from data in two or three energy windows placed on either si de of the 364-keV photopeak using empirical weighting factors, A set o f images from spheres in an elliptical phantom were used to evaluate e ach of the window-based corrections. The spectrum-fitting technique es timates detected scatter at each pixel by fitting the observed energy spectrum with a function that models the photopeak and scatter, and wh ich incorporates the response function of the camera. This technique w as evaluated using a set of Rollo phantom images, Results: All of the window-based methods performed significantly better than a single phot opeak window (338-389 keV), but the weighting factors were found to de pend on the object being imaged. For images contaminated with scatter, the spectrum-fitting method significantly improved quantitation over photopeak windowing. Little difference, however, between any of the me thods was observed for images containing small amounts of scatter. Con clusion: Most clinical I-131 imaging protocols will benefit from quali tative and quantitative improvements provided by the spectrum-fitting scatter correction. The technique offers the practical advantage that it does not require phantom-based calibrations. Finally, our results s uggest that septal penetration and scatter in the collimator and other detector-head components are important sources of error in quantitati ve I-131 images.