An algorithm to adjust a rigid CT-SPECT fusion so as to maximize tumor counts from CT VoI in I-131 therapies

The accuracy of the estimate of tumor I-131 activity in therapy patients using CT-SPECT fusion is difficult to establish. In our work, we have had the impression that a tumor volume of interest (VoI) from CT sometimes does not perfectly overlay the true position of the tumor in the SPECT image set. However, the magnitude of the difference between the activity estimate from the procedure and that from a perfect overlay is uncertain. Although we have not estimated that magnitude, we have investigated how much the activity estimate increases if we apply a last variation on the fusion to change the location of the tumor VoI so as to increase tumor counts. The results show that the algorithm can be effective in registering tumors in CT and SPECT locally as proved by visual checks of the optimized location of the tumor VoI in SPECT. For 14 tumors in seven patients the increases in tumor counts average 6.65%. The maximum increase is 26.7%.