Abstract
In brain PET/MRI, the major challenge of zero-echo-time (ZTE)-based attenuation correction (ZTAC) is the misclassification of air/tissue/bone mixtures or their boundaries. Our study aimed to evaluate a sinus/edge-corrected (SEC) ZTAC (ZTAC(SEC)), relative to an uncorrected (UC) ZTAC (ZTAC(UC)) and a CT atlas-based attenuation correction (ATAC). Methods: Whole-body (18)F-FDG PET/MRI scans were obtained for 12 patients after PET/CT scans. Only data acquired at a bed station that included the head were used for this study. Using PET data from PET/MRI, we applied ZTAC(UC), ZTAC(SEC), ATAC, and reference CT-based attenuation correction (CTAC) to PET attenuation correction. For ZTAC(UC), the bias-corrected and normalized ZTE was converted to pseudo-CT with air (-1,000 HU for ZTE < 0.2), soft-tissue (42 HU for ZTE > 0.75), and bone (-2,000 × [ZTE - 1] + 42 HU for 0.2 ≤ ZTE ≤ 0.75). Afterward, in the pseudo-CT, sinus/edges were automatically estimated as a binary mask through morphologic processing and edge detection. In the binary mask, the overestimated values were rescaled below 42 HU for ZTAC(SEC) For ATAC, the atlas deformed to MR in-phase was segmented to air, inner air, soft tissue, and continuous bone. For the quantitative evaluation, PET mean uptake values were measured in twenty 1-mL volumes of interest distributed throughout brain tissues. The PET uptake was compared using a paired t test. An error histogram was used to show the distribution of voxel-based PET uptake differences. Results: Compared with CTAC, ZTAC(SEC) achieved the overall PET quantification accuracy (0.2% ± 2.4%, P = 0.23) similar to CTAC, in comparison with ZTAC(UC) (5.6% ± 3.5%, P < 0.01) and ATAC (-0.9% ± 5.0%, P = 0.03). Specifically, a substantial improvement with ZTAC(SEC) (0.6% ± 2.7%, P < 0.01) was found in the cerebellum, in comparison with ZTAC(UC) (8.1% ± 3.5%, P < 0.01) and ATAC (-4.1% ± 4.3%, P < 0.01). The histogram of voxel-based uptake differences demonstrated that ZTAC(SEC) reduced the magnitude and variation of errors substantially, compared with ZTAC(UC) and ATAC. Conclusion: ZTAC(SEC) can provide an accurate PET quantification in brain PET/MRI, comparable to the accuracy achieved by CTAC, particularly in the cerebellum.