Evaluation of sinus/edge-corrected zero-echo-time–Based attenuation correction in brain PET/MRI

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 ¹⁸F-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 (21,000 HU for ZTE, 0.2), soft-tissue (42 HU for ZTE . 0.75), and bone (22,000 · [ZTE 2 1] 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% 6 2.4%, P 5 0.23) similar to CTAC, in comparison with ZTAC_UC (5.6% 6 3.5%, P, 0.01) and ATAC (20.9% 6 5.0%, P 5 0.03). Specifically, a substantial improvement with ZTAC_SEC (0.6% 6 2.7%, P, 0.01) was found in the cerebellum, in comparison with ZTAC_UC (8.1% 6 3.5%, P, 0.01) and ATAC (24.1% 6 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.