Abstract
ObjectiveThe tumour-to-normal ratio (T/N) is a representative index reflecting brain tumour activity by 18F-fluorodeoxyglucose (FDG) and 11C-methionine (MET) PET. We proposed a new automated method of calculating the normal reference value (N-value) for use as the denomination of T/N. This method uses voxel-based analysis of FDG- and MET-PET images. We compared the results of this method with those of the standard region-of-interest (ROI) method.MethodsData sets were obtained from 32 patients with newly diagnosed glioma and 13 patients with recurrent brain tumour. Our methods were as follows: (1) FDG-PET and MET-PET images were co-registered. (2) The areas where the FDG uptake was higher than a set threshold were selected. (3) For the corresponding areas of MET-PET images, mode and mean voxel values were calculated as tentative MET N-values. (4) Applying the same coordinates to FDG-PET, the voxel values were averaged and used as tentative FDG N-values. (5) The threshold of FDG-PET and whether to use the mode or the mean voxel values were computationally optimized using learning data sets. (6) Applying the optimal threshold and either the mode or mean, N-values of FDG and MET were finally determined.ResultsN-values determined by our automated method showed excellent agreement with those determined by a manual ROI method (ICC(2,1) > 0.78). These values were significantly correlated with mean manual N-values (p < 0.001).ConclusionsOur new method shows sufficiently good agreement with the standard method and can provide a more objective metabolic index.
Highlights
The combination of 18F-fluorodeoxyglucoe (FDG) and 11C-methionine (MET) has been the most effective PET examination for evaluating brain tumours [1,2,3,4]
We propose a new automated method in which the voxels corresponding to the normal cortex are identified using characteristics of both FDG-PET and MET-PET, and in which the normal reference values (N-values) are calculated through voxel-based analysis
Scatter plots of the N-values obtained from the automated voxel-based method and those obtained from each of the three operators using the manual ROI method are shown in Figs. 5 and 6
Summary
The combination of 18F-fluorodeoxyglucoe (FDG) and 11C-methionine (MET) has been the most effective PET examination for evaluating brain tumours [1,2,3,4]. FDG uptake increases with the degree of malignancy in common brain tumour types [1, 4,5,6,7]. FDG uptake in the normal cortex is altered depending on neuronal activities, which can be affected by various factors, such as subclinical epileptic discharge, tumour invasion, and tissue damage from past treatment. These factors should be considered when identifying the reference area in the normal cortex. MET-PET overcomes these difficulties, because MET distribution in the normal cortex is
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