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Accuracy of rigid CT–FDG-PET image registration of the liver

Diagnostic and surgical strategies could benefit from accurate localization of liver malignancies via CT–FDG-PET image registration. However, registration uncertainty occurs due to protocol differences in data-acquisition, the limited spatial resolution of positron emission tomography (PET) and the low uptake of 18F-fluorodeoxyglucose (FDG) in normal liver tissue. To assess this uncertainty, methods were presented to estimate registration precision and systematic bias. A semi-automatic, organ-focused method was investigated to minimize the uncertainty well beyond the typical uncertainty of 5–10 mm obtained by commonly available methods. By restricting registration to the liver region and by isolating the liver on computed tomography (CT) from surrounding structures using a thresholding technique, registration was achieved using the mutual information-based method as implemented in insight toolkit (ITK). CT and FDG-PET images of 10 patients with liver metastases were registered rigidly a number of times. Results of the organ-focused method were compared to results of three commonly available methods (a manual, a landmark-based and a ‘standard’ mutual information-based method) where no dedicated image processing was performed. The proposed method outperformed the other methods with a precision (mean ± s.d.) of 2.5 ± 1.3 mm and a bias of 1.9 mm with a 95% CI of [1.0, 2.8] mm. Unlike the commonly available methods, our approach allows for robust CT–FDG-PET registration of the liver, with an accuracy better than the spatial resolution of the PET scanner that was used.