Comparison of mutual information-based warping accuracy for fusing body CT and PET by 2 methods: CT mapped onto PET emission scan versus CT mapped onto PET transmission scan

This article assesses the resulting accuracies of 2 registration methods using the same multimodal mutual information registration algorithm. In the indirect, fusion method, the CT dataset is warped onto the PET transmission scan, and then the patient's attenuation-corrected emission dataset is substituted for the transmission dataset. In the direct, fusion method, the CT is warped directly onto the attenuation-corrected emission data-set. Methods: CT and ¹⁸F-FDG PET image datasets from 14 subjects with malignant lesions in the thorax were registered. In both CT and PET imaging acquisitions, the patient's arms were at the patient's side, resting on the scanning couch in a manner similar to that of routine PET acquisition procedures. The accuracy of the 2 warping registrations was assessed by measuring the distance between lesion centroids on CT and PET emission after fusion. Results: The indirect method has a statistically smaller mean error, 6.2 mm, than the direct method, 10.6 mm. Conclusion: The indirect method appears to be the more accurate/reliable choice for fusing body CT and FDG PET.