Improved conjugate view quantitation of 1-131 by subtraction of scatter and septal penetration events with a triple energy window method

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