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摘要
Aim: To accurately quantify the radioactivity concentration measured by PET, emission data need to be corrected for photon attenuation; however, the MRI signal cannot easily be converted into attenuation values, making attenuation correction (AC) in PET/MRI challenging. In order to further improve the current vendor implemented MR-AC methods for absolute quantification, a number of prototype methods have been proposed in the literature. These can be categorized into three types: template/atlas-based, segmentation-based, and reconstruction-based. These proposed methods in general demonstrated improvements compared to vendor implemented AC, and many studies report deviations in PET uptake after AC of only a few percent from a gold standard CT-AC. Using a unified quantitative evaluation with identical metrics, subject cohort, and common CT-based reference, the aims of this study were to evaluate a selection of novel methods proposed in the literature, and identify the ones suitable for clinical use. Methods: In total, 11 AC methods were evaluated: two vendor-implemented (MR-AC(DIXON) and MR-AC(UTE)), five based on template/atlas information (MR-AC(SEGBONE) (Koesters et al., 2016), MR-AC(ONTARIO) (Anazodo et al., 2014), MR-AC(BOSTON) (Izquierdo-Garcia et al., 2014), MR-AC(UCL) (Burgos et al., 2014), and MR-AC(MAXPROB) (Merida et al., 2015)), one based on simultaneous reconstruction of attenuation and emission (MR-AC(MLAA) (Benoit et al., 2015)), and three based on image-segmentation (MR-AC(MUNICH) (Cabello et al., 2015), MR-AC(CAR-RiDR) (Juttukonda et al., 2015), and MR-AC(RESOLUTE) (Ladefoged et al., 2015)). We selected 359 subjects who were scanned using one of the following radiotracers: [(18)FDDG (210), [C-11]PiB (51), and [F-18] florbetapir (98). The comparison to AC with a gold standard CT was performed both globally and regionally, with a special focus on robustness and outlier analysis. Results: The average performance in PET tracer uptake was within +/- 5% of CT for all of the proposed methods, with the average SD global percentage bias in PET FDG uptake for each method being: MR-AC(DIXON) (-11.3 +/- 3.5)%, MR-AC(UTE) (-5.7 +/- 2.0)%, MR-AC(ONTARIO) (-4.3 +/- 3.6)%, MR-AC(MUNICH) (3.7 +/- 2.1)%, MR-AC(MLAA) (-1.9 +/- 2.6)%, MR-AC(SEGBONE) (-1.7 +/- 3.6)%, MR-AC(UCL) (0.8 +/- 1.2)%, MR-AC(CAR-RiDR) (-0.4 +/- 1.9)%, MR-AC(MAXPROB) (-0.4 +/- 1.6)%, MR-AC(BOSTON) (-0.3 +/- 1.8)%, and MR-AC(RESOLUTE) (0.3 +/- 1.7)%, ordered by average bias. The overall best performing methods (MR-AC(BOSTON), MR-AC(MAXPROB), MR-AC(RESOLUTE) and MR-AC(UCL), ordered alphabetically) showed regional average errors within +/- 3% of PET with CT-AC in all regions of the brain with FDG, and the same four methods, as well as MR-AC(CAR-RiDR), showed that for 95% of the patients, 95% of brain voxels had an uptake that deviated by less than 15% from the reference. Comparable performance was obtained with PiB and florbetapir. Conclusions: All of the proposed novel methods have an average global performance within likely acceptable limits (+/- 5% of CT-based reference), and the main difference among the methods was found in the robustness, outlier analysis, and clinical feasibility. Overall, the best performing methods were MR-ACBOSTON, MRACMAXPROB, MR-ACRESOLUTE and MR-ACUCL, ordered alphabetically. These methods all minimized the number of outliers, standard deviation, and average global and local error. The methods MR-ACMUNICH and MR-ACCAR-RiDR were both within acceptable quantitative limits, so these methods should be considered if processing time is a factor. The method MR-ACSEGBONE also demonstrates promising results, and performs well within the likely acceptable quantitative limits. For clinical routine scans where processing time can be a key factor, this vendor-provided solution currently outperforms most methods. With the performance of the methods presented here, it may be concluded that the challenge of improving the accuracy of MR-AC in adult brains with normal anatomy has been solved to a quantitatively acceptable degree, which is smaller than the quantification reproducibility in PET imaging.
- 出版日期2017-2-15
