Hybrid PET- and MR-driven attenuation correction for enhanced 18F-NaF and 18F-FDG quantification in cardiovascular PET/MR imaging

Abstract

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 Ki expected from 18 F-Sodium Fluoride (18 F-NaF) to segment bones from PET data and support 5-class hybrid PET/MR-driven AC for 18 F-NaF and 18 F-Fluorodeoxyglucose (18 F-FDG) PET/MR cardiovascular imaging. Methods: We introduce 5-class Ki/MR-AC for (i) 18 F-NaF studies where the bones are segmented from Patlak Ki images and added as the 5 th tissue class to the MR-Dixon 4-class AC map. Furthermore, we propose two alternative dual-tracer protocols to permit 5-class Ki/MR-AC for (ii) 18 F-FDG-only data, with a streamlined simultaneous administration of 18 F-FDG and 18 F-NaF at 4:1 ratio (R4:1), or (iii) for 18 F-FDG-only or both 18 F-FDG and 18 F-NaF dual-tracer data, by administering 18 F-NaF 90 min after an equal 18 F-FDG dosage (R1:1). The Ki-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 18 F-NaF SUVmean scores when applying 5-class AC with Ki-segmented bone (5-class Ki/CT-AC) versus CT-segmented bone (5-class CT-AC) tissue. Considering the PET data corrected with continuous CT-AC maps as gold-standard, the percentage SUVmean bias was reduced by 17.6% (18 F-NaF) and 15.4% (R4:1) with 5-class Ki/CT-AC versus 4-class CT-AC. In humans without prior CAD indication, we reported 17.7% and 20% higher 18 F-NaF target-to-background ratio (TBR) at carotid bifurcations wall and vertebral bones, respectively, with 5-versus 4-class AC. In the R4:1 human cohort, the mean 18 F-FDG TBR increased by 12.2% at carotid bifurcations wall and 19.9% at vertebral bones. For the R1:1 cohort of human without CAD indication, mean TBR increased by 15.3% (18 F-FDG) and 15.5% (18 F-NaF) at carotid bifurcations and 21.6% (18 F-FDG) and 22.5% (18 F-NaF) at vertebral bones. Similar TBR enhancements were observed whe applying the proposed AC method to human subjects with prior CAD indication.. Conclusions: Ki-driven bone segmentation and 5-class hybrid PET/MR-driven AC is feasible and can significantly enhance 18 F-NaF and 18 F-FDG contrast and quantification in bone tissues and carotid walls.

Assessment of different quantification metrics of [18F]-NaF PET/CT images of patients with abdominal aortic aneurysm

“…TBR 50 gave the best trade-off between increased CNR and reduced DTBR the tracer is taken up) to the adjacent aneurysm. 9,10 Our previous study 17 extensively investigated the spill-in effect in [ 18 F]-NaF PET imaging of AAA, and it was shown that the spill-in effect depends on the activity uptake in the bone, proximity of the aneurysm to the bone, as well as ROI delineation criteria. This effect poses a great challenge to the quantification accuracy at the aneurysm site and it may adversely affect AAA disease prediction and patient management.…”

“…However, it is important to note that a major challenge and an inherent limitation of using [ 18 F]-NaF for AAAs is that [ 18 F]-NaF is taken up by the vertebrae because it is mainly a bone radiotracer. 9,10 The anatomical site of the vertebrae is close to the posterior wall of the aorta, which results in an increased signal from this region because of the spill-in effect, leading to inaccurate quantification results. 11 Thus, to increase the accuracy of the results, it is essential to either correct for the spill-in effect or else to identify the most appropriate quantification metrics which are less affected by the spill-in effect.…”

Diagnostic performance of a whole-body dynamic 68GA-DOTATOC PET/CT acquisition to differentiate physiological uptake of pancreatic uncinate process from pancreatic neuroendocrine tumor

“…In this point of view, development of PET/CT acquisition with a dual-tracer simultaneous administration of 18 FDG and 68 Ga-DOTApeptides to assess high G2/G3 NETs or neuroendocrine carcinoma would be useful and could allow differentiating non- 68 Ga-DOTApeptides-avid NETs from Ups. [ 23 ]…”

Clinical perspectives for the use of total body PET/CT

“…However, it has not been implemented in routine practice due to several limitations such as the lack of consensus for dose proportion and the difficulty in separating the signal of both tracers. Recently, a feasibility study has suggested that WB-dynPET could quantitatively extract the two separate tracer kinetics and also analyze different processes [25].…”