Evaluation of Dixon Sequence on Hybrid PET/MR Compared with Contrast-Enhanced PET/CT for PET-Positive Lesions

Hybrid positron emission tomography and magnetic resonance (PET/MR) imaging performs a two-point Dixon MR sequence for attenuation correction. However, MR data in hybrid PET/MR should provide anatomic and morphologic information as well as an attenuation map. We evaluated the Dixon sequence of hybrid PET/MR for anatomic correlation of PET-positive lesions compared with contrast-enhanced PET/computed tomography (CT) in patients with oncologic diseases.

Twelve patients underwent a single injection, dual imaging protocol. PET/CT was performed with an intravenous contrast agent (85 ± 13 min after ¹⁸F-FDG injection of 403 ± 45 MBq) and then (125 ± 19 min after injection) PET/MR was performed. Attenuation correction and anatomic allocation of PET were performed using contrast-enhanced CT for PET/CT and Dixon MR sequence for hybrid PET/MR. The Dixon MR sequence and contrast-enhanced CT were compared for anatomic correlation of PET-positive lesions (scoring scale ranging from 0 to 3 for visual ratings). Additionally, standardized uptake values (SUVs) for the detected lesions were assessed for quantitative comparison.

Both hybrid PET/MR and contrast-enhanced PET/CT identified 55 lesions with increased FDG uptake in ten patients. In total, 28 lymph nodes, 11 bone lesions, 3 dermal nodules, 3 pleural thickening lesions, 2 thyroid nodules, 1 pancreas, 1 liver, 1 ovary, 1 uterus, 1 breast, 1 soft tissue and 2 lung lesions were present. The best performance was observed for anatomic correlation of PET findings by the contrast-enhanced CT scans (contrast-enhanced CT, 2.64 ± 0.70; in-phase, 1.29 ± 1.01; opposed-phase, 1.29 ± 1.15; water-weighted, 1.71 ± 1.07; fat weighted, 0.56 ± 1.03). A significant difference was observed between the scores obtained from the contrast-enhanced CT and all four coregistered Dixon MR images. Quantitative evaluation revealed a high correlation between the SUVs measured with hybrid PET/MR (SUVmean, 2.63 ± 1.62; SUVmax, 4.30 ± 2.88) and contrast-enhanced PET/CT (SUVmean, 3.88 ± 2.30; SUVmax, 6.53 ± 4.04) in PET-positive lesions (SUVmean, ρ = 0.93; SUVmax, ρ = 0.95), although hybrid PET/MR presented a decrease of SUVs compared with contrast-enhanced PET/CT (mean reduction; SUVmean, 32.44 ± 15.64 %; SUVmax, 35.16 ± 12.59 %).

Despite different attenuation correction approaches, the SUV of PET-positive lesions correlated well between hybrid PET/MR and contrast-enhanced PET/CT. However Dixon MR images acquired for attenuation correction were insufficient to provide anatomic information of PET images because of low spatial resolution. Thus, additional MR sequence with fast and higher resolution may be necessary for anatomic information.