An X-ray CT-assisted method for radionuclide activity measurement using planar views

Accurate in vivo measurement of activity is required for dosimetry of internally-administered radionuclides. The authors have developed and demonstrated a technique for the absolute measurement of radionuclide activity that uses registered X-ray CT images to provide physical information for the automated evaluation of planar nuclear images. The registered CT images provide an estimate of the object-specific attenuation with sufficient resolution for the determination of anatomical volumes of interest (VOIs). The CT-derived VOIs are transferred and projected onto the acquired planar nuclear scans using the known geometry of the imaging system and are used to estimate object and background activities on the planar scans. Attenuation is determined directly from the registered and scaled X-ray CT images and is factored into the calculation of the activity. Comparisons of this CT-assisted measurement method to the conjugate-view approach of using a single point source for attenuation compensation have been made experimentally. Both methods provide similar accuracy in studies using small activity-filled objects placed in phantoms. However, an automated, CT-assisted method would reduce the inter-operator variability that arises in clinical practice.