Comparison of Segmentation-Based Attenuation Correction Methods for PET/MRI: Evaluation of Bone and Liver Standardized Uptake Value with Oncologic PET/CT Data

Abstract

For attenuation correction (AC) in PET/MRI systems, segmentation-based methods are most often used. However, the standardized uptake value (SUV) of lesions in the bone and liver, which have higher attenuation coefficients than other organs, can be underestimated, potentially leading to misinterpretation of clinical cases. Errors in SUV estimation are also dependent on the segmentation schemes used in the segmentation-based AC. In this study, this potential bias in SUV estimation using 4 different segmentation-based AC methods was evaluated for the PET/CT data of cancer patients with bone and liver lesions. Methods: Forty patients who had spine or liver lesions and underwent 18 F-FDG PET/CT participated (18 women and 22 men; 20 spine lesions and 20 liver lesions; mean age (6SD), 60.5 6 11.4 y; mean body weight, 57.7 6 10.4 kg). The patient body region was extracted from the CT image and categorized into 5 tissue groups (air, lungs, fat, water, and bone) using Hounsfield unit thresholds, which were determined from the CT histogram. Four segmentation-based AC methods (SLA [soft-tissue/lung/ air], WFLA [water/fat/lung/air], SLAB [soft-tissue/lung/air/bone], and WFLAB [water/fat/lung/air/bone]) were compared with CTbased AC. The mean attenuation coefficient for each group was calculated from 40 CT images and assigned to the attenuation maps. PET sinograms were reconstructed using segmentationand CT-based AC maps, and mean SUV in the lesions was compared. Results: Mean attenuation coefficients for air, lungs, fat, water, and bone were 0.0058, 0.0349, 0.0895, 0.0987, and 0.1178 cm 21 , respectively. In the spine lesions, the SUVs were underestimated by 16.4% 6 8.5% (SLA AC) and 14.7% 6 7.5% (WFLA AC) but not to a statistically significant extent for SLAB and WFLAB AC relative to CT AC. In the liver lesions, the SUVs were underestimated by 11.1% 6 2.6%, 8.1% 6 3.0%, 6.8% 6 3.8%, and 4.1% 6 3.8% with SLA, SLAB, WFLA, and WFLAB AC, respectively. Conclusion: Without bone segmentation, the SUVs of spine lesions were considerably underestimated; however, the bias was acceptable with bone segmentation. In liver lesions, the segmentation-based AC methods yielded a negative bias in SUV; however, inclusion of the bone and fat segments reduced the SUV bias. The results of this study will be useful for understanding organ-dependent bias in SUV between PET/CT and PET/MRI. Hybri d imaging devices such as PET/CT and SPECT/CT are now widely used in clinical and preclinical studies. Morphologic information provided by CT is particularly useful for the localization of abnormal uptake of radiotracers and for g-ray attenuation correction (AC) (1-3). Inaccurate AC of PET and SPECT images can lead to incorrect quantification and misinterpretation of lesions (4-9).MRI is another important morphologic imaging tool that can be combined with PET or SPECT. MRI has several advantages over CT, including better soft-tissue contrast, no additional radiation hazard, and true 4-dimensional simultaneous imaging (9-11). It is likely that combined PET/MR...

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