IMPROVED CONJUGATE VIEW QUANTITATION OF I-131 BY SUBTRACTION OF SCATTER AND SEPTAL PENETRATION EVENTS WITH A TRIPLE ENERGY WINDOW METHOD

The majority of radiation absorbed dose estimates for radioimmunothera py (RIT) with I-131 labeled antibodies-have been calculated based on i n vivo quantitation of activity using the conjugate view approach with planar Anger camera images. Scatter and septal penetration events con tributed by a small fraction of high-energy photons emitted by I-131 w ith an energy exceeding 600 keV lead to a significant degradation of I -131 images acquired with an Anger camera, which blurs the images of u ptake sites and complicates the definition of background regions. The objective of this study was to evaluate a triple energy window (TEW) s ubtraction method that has been used to remove these interfering event s from I-131 images. In the method, a primary photopeak image for I-13 1 is obtained after sequential subtraction of septal penetration and s catter events by using scatter multipliers derived from a photopeak wi ndow and two adjacent scatter window images. Qualitative improvement i n image contrast was demonstrated with this technique, together with m ore accurate and reproducible quantitation for I-131 in the organs of an abdominal phantom. This TEW scatter subtraction method can be used to provide more precise dosimetry estimates for radionuclide therapy a nd RIT with I-131.