Cocktail 18F-FDG/18F-NaF PET/MR imaging: feasibility, preclinical protocol optimization and potential benefits in MR-guided attenuation correction (2016)

1415 Objectives PET imaging can utilize the availability of multiple radiotracers to deliver multiple diagnostic in-vivo images by tracking complementary mechanisms of the same disease in a single subject. Therefore, the combined administration of complementing tracers to the same subject may allow for multi-parametric imaging and potentially more complete diagnostic assessments. Recently, dual-tracer PET/CT studies have combined temporally separate administrations of 18F-Fluorodeoxyglucose (18F-FDG) and 18F-sodium fluoride (18F-NaF) tracers at a single oncological PET scan session to enhance diagnostic value, while alleviating the need for additional radiation exposure due to multiple consecutive PET/CT scans. In this study, we are further proposing the simultaneous administration of the two tracers as a single cocktail PET tracer of optimal composition and scan time window, based on their measured pharmacokinetic distinct attributes. Our aim is to demonstrate feasibility and benefits of the proposed method particularly for PET/MR lesion quantification. Methods The proposed dual-tracer PET protocol has been currently validated and optimized through an extensive series of 90min dynamic scan sessions conducted on 2 healthy control rabbits (3-5kg). Initially, a series of dynamic PET/CT and PET/MR rabbit scans was conducted on Siemens Biograph © mCT and mMR scanners, respectively, each involving administration of 1mCi/kg of a single radiotracer each time. Thus, the individual spatiotemporal distribution of 18F-FDG and 18F-Fluoride tracers from 0-90min post-injection time was measured to assess the independent tracer kinetics and determine the optimal acquisition time window and concentration ratio for the proposed dual tracer protocol. Subsequently, a series of cocktail dynamic (0-90min p.i.) PET/CT and PET/MR scans were performed on the same cohort of rabbits to demonstrate feasibility and allow comparison between the mixed and individual tracer kinetics at a range of cocktail composition ratios. In particular, the same total amount of dosage as previously was administered, while three different FDG:NaF component ratios were evaluated (9:1, 4:1, 1:1). The time-activity curves (TACs) were then extracted from all acquired dynamic scans at selected regions of interest (ROIs) in the liver, the myocardium, the left-ventricle blood pool and the spinal cord of the rabbits. Results Our quantitative analysis on the acquired dynamic PET/CT rabbit data using gold standard CT-based attenuation correction has shown a higher bone tissue (spinal cord) uptake rate for NaF-only relative to FDG-only acquisitions, resulting in an uptake ratio of 1:7 (FDG:NaF) at 90min post-injection (p.i.). On the other hand, the individual input functions are not affected by the tracer composition itself, while the uptake in the liver is lower for NaF-only scans. Both tracers kinetics appear to stabilize after 60min p.i., except from PET NaF bone uptake which exhibits positive uptake rates even close to 90min p.i., thus suggesting an optimal acquisition window 60-90min p.i for static PET studies. The same kinetic behavior for the 2 tracers is confirmed by our PET/MR studies. Meanwhile, the distinctly different uptake properties of the two tracers may help in differentiating the two tracers and recovering the two component TACs from a dynamic cocktail study. In addition, the minimum evaluated FDG:NaF mixture ratio, for which bone tissues can be segmented and be used for more accurate MR-guided attenuation correction was found to be 4:1. Conclusions Simultaneous 18F-FDG/18F-NaF PET/CT and PET/MR multi-parametric imaging may be feasible in clinical routine and an optimal 60-90min p.i. time-window is recommended for static protocols. Moreover, PET/MR lesion quantification may be substantially enhanced by utilizing a 4:1 FDG:NaF cocktail to reliably segment the bone tissues from PET initial data.
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