Hybrid PET- and MR-driven attenuation correction for enhanced F-NaF and F-FDG quantification in cardiovascular PET/MR imaging.

BACKGROUND: The standard MR Dixon-based attenuation correction (AC) method in positron emission tomography/magnetic resonance (PET/MR) imaging segments only the air, lung, fat and soft-tissues (4-class), thus neglecting the highly attenuating bone tissues and affecting quantification in bones and adjacent vessels. We sought to address this limitation by utilizing the distinctively high bone uptake rate constant K expected from F-Sodium Fluoride (F-NaF) to segment bones from PET data and support 5-class hybrid PET/MR-driven AC for F-NaF and F-Fluorodeoxyglucose (F-FDG) PET/MR cardiovascular imaging.

METHODS: We introduce 5-class K/MR-AC for (i) F-NaF studies where the bones are segmented from Patlak K images and added as the 5th tissue class to the MR Dixon 4-class AC map. Furthermore, we propose two alternative dual-tracer protocols to permit 5-class K/MR-AC for (ii) F-FDG-only data, with a streamlined simultaneous administration of F-FDG and F-NaF at 4:1 ratio (R4:1), or (iii) for F-FDG-only or both F-FDG and F-NaF dual-tracer data, by administering F-NaF 90 minutes after an equal F-FDG dosage (R1:1). The K-driven bone segmentation was validated against computed tomography (CT)-based segmentation in rabbits, followed by PET/MR validation on 108 vertebral bone and carotid wall regions in 16 human volunteers with and without prior indication of carotid atherosclerosis disease (CAD).

RESULTS: In rabbits, we observed similar (< 1.2% mean difference) vertebral bone F-NaF SUV scores when applying 5-class AC with K-segmented bone (5-class K/CT-AC) vs CT-segmented bone (5-class CT-AC) tissue. Considering the PET data corrected with continuous CT-AC maps as gold-standard, the percentage SUV bias was reduced by 17.6% (F-NaF) and 15.4% (R4:1) with 5-class K/CT-AC vs 4-class CT-AC. In humans without prior CAD indication, we reported 17.7% and 20% higher F-NaF target-to-background ratio (TBR) at carotid bifurcations wall and vertebral bones, respectively, with 5- vs 4-class AC. In the R4:1 human cohort, the mean F-FDG:F-NaF TBR increased by 12.2% at carotid bifurcations wall and 19.9% at vertebral bones. For the R1:1 cohort of subjects without CAD indication, mean TBR increased by 15.3% (F-FDG) and 15.5% (F-NaF) at carotid bifurcations and 21.6% (F-FDG) and 22.5% (F-NaF) at vertebral bones. Similar TBR enhancements were observed when applying the proposed AC method to human subjects with prior CAD indication.

CONCLUSIONS: K-driven bone segmentation and 5-class hybrid PET/MR-driven AC is feasible and can significantly enhance F-NaF and F-FDG contrast and quantification in bone tissues and carotid walls.