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An Activity Quantification Method Based on Registration of CT and Whole-Body Scintillation Camera Images, with Application to ¹³¹I

Department of Radiation Physics, Jubileum Institute, University Hospital, Lund University, Lund, Sweden

This article presents a new method for conjugate view activity quantification for ¹³¹I-labeled monoclonal antibody distribution. Methods: The method is based on the combined use of images from 3 modalities: whole-body (WB) scintillation camera scanning, WB transmission scanning using ⁵⁷Co, and CT. All images are coaligned using a recently developed program for the registration of WB images. Corrections for attenuation, scatter, and septal penetration are performed in image space. Compensation for scatter and septal penetration is performed by deconvolution, using point-response functions determined from Monte Carlo simulations. Attenuation correction is performed by applying a patient-specific 364-keV narrow-beam attenuation map obtained by combining information from the CT and the transmission scan. A relationship is presented for the conversion of the CT numbers to mass density. The attenuation- and scatter-compensated image is converted from counts to activity using a sensitivity value that was determined for 364-keV photons in air. This activity projection image is then analyzed for the activity of volumes of interest (VOI) using 2-dimensional regions of interest (ROIs) that are determined from the CT study. The CT is first resliced into coronal slices, and a maximum-extension ROI is outlined that encloses the VOI. Compensation for background activity and overlapping organs is performed on the basis of total patient thickness in the projection line, and on precalculated organ- background thickness fractions. Results: Method evaluation was performed using data from both experimental measurements and Monte Carlo simulations. The use of an attenuation map derived directly from the CT study was also evaluated. For organ activity quantification, an accuracy of 10% was obtained. For small-diameter tumors, deviations were larger because of lack of correction for the background-dependent partial-volume effect. Conclusion: Registration of CT and WB scintillation camera images was successfully applied to improve activity quantification by the conjugate view method.

Key Words: image registration • conjugate view • activity quantification • ¹³¹I • attenuation correction • scatter correction

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