Quantification and accuracy of clinical [11C]-PiB PET/MRI: the effect of MR-based attenuation correction

Abstract

Aims: The Dixon-Water-Fat segmentation (DWFS) method (1) is a standard attenuation correction (AC) method in PET/MRI and has demonstrated a systematic and spatially variable quantitative bias in [18F]-FDG-PET/MRI studies of the brain (2) compared to PET/CT. The Aim of this study was to measure the regional and absolute bias of DWFS-AC in combined PET/MR images of the brain using [11C]-PiB for cerebral amyloid binding and evaluate the impact on the clinical reading of these images. Methods: Twenty-one healthy volunteers, 1 MCI patient and 3 patients with clinical Alzheimers disease (14 M, 10 F, age: 51-78 y, median: 66 y) underwent a simultaneous PET/MRI (Siemens mMR) acquisition 40 min pi of (201-838) MBq [11C]-PiB. A single 30 min frame was reconstructed (OSEM-3D, 21it4sub, 5 mm Gauss) with the only difference being the AC calculation. AC was performed using either DWFS or a head CT scan acquired independently on the same day and co-registered to DWFS using rigid body transformation. AC-PET images were fused with T1w-MRI and activity concentration (Bq/mL) was sampled from symmetrically delineated cortical ROI's in lateral frontal, lateral and medial parietal, lateral temporal, lateral occipital cortical areas, deep white matter, and the caudate nuclei. Ratios of region-to-cerebellar grey matter (SUVr) were calculated. Amyloid uptake was considered abnormal at cortical SUVr ≥1.5 or increased amyloid uptake in two or more grey matter regions on visual evaluation. Bland-Altman analysis was used to compare AC Methods. Results: The overall activity concentration in the average of all ROI's was biased by -23 % (95% CI: -14%; -37%) in DWFS-AC compared to CT-AC. The SUVr were spatially variable from the center of the brain to the periphery. SUVr using DWFS-AC compared to CT-AC was biased by 0.12 (95% CI: -0.03;0.26) in the caudate nuclei and 0.04 (95% CI: -0.05; 0.12) in lateral cortical ROI's. Although the visual categorization of both, amyloid-positive (n=5) and -negative (n=20) scans was unaffected by DWFS-AC, 2 healthy subjects were quantitatively reclassified as amyloid positive. Conclusion: The visual and quantitative consequences of MR-AC using DWFS in the brain with [11C]-PiB resemble those described for [18F]-FDG (2). Following DWFS-AC [11C]-PiB images exhibit a noticeable radially variable bias. The quantitative diagnostic criteria using this biomarker with DWFS-AC may need to be modified.