Abstract
BackgroundPositron emission tomography (PET) is increasingly applied for in vivo brown adipose tissue (BAT) research in healthy volunteers. To limit the radiation exposure, the injected 18F-FDG tracer dose should be as low as possible. With simultaneous PET/MR imaging, the radiation exposure due to computed tomography (CT) can be avoided, but more importantly, the PET acquisition time can often be increased to match the more extensive magnetic resonance (MR) imaging protocol. The potential gain in detected coincidence counts, due to the longer acquisition time, can then be applied to decrease the injected tracer dose. The aim of this study was to investigate the minimal 18F-FDG dose for a 10-min time-of-flight (TOF) PET/MR acquisition that would still allow accurate quantification of supraclavicular BAT volume and activity.MethodsTwenty datasets from 13 volunteers were retrospectively included from a prospective clinical study. PET emission datasets were modified to simulate step-wise reductions of the original 75 MBq injected dose. The resulting PET images were visually and quantitatively assessed and compared to a 4-min reference scan. For the visual assessment, the image quality and artifacts were scored using a 5-point and a 3-point Likert scale. For the quantitative analysis, image noise and artifacts, BAT metabolic activity, BAT metabolic volume (BMV), and total BAT glycolysis (TBG) were investigated.ResultsThe visual assessment showed still good image quality for the 35%, 30%, and 25% activity reconstructions with no artifacts. Quantitatively, the background noise was similar to the reference for the 35% and 30% activity reconstructions and the artifacts started to increase significantly in the 25% and lower activity reconstructions. There was no significant difference in supraclavicular BAT metabolic activity, BMV, and TBG between the reference and the 35% to 20% activity reconstructions.ConclusionsThis study indicates that when the PET acquisition time is matched to the 10-min MRI protocol, the injected 18F-FDG tracer dose can be reduced to approximately 19 MBq (25%) while maintaining image quality and accurate supraclavicular BAT quantification. This could decrease the effective dose from 1.4 mSv to 0.36 mSv.
Highlights
Positron emission tomography (PET) is increasingly applied for in vivo brown adipose tissue (BAT) research in healthy volunteers
Visual assessment The visual assessment showed that the reference and the 35%, 30%, and 25% activity reconstructions have good image quality (Fig. 2a) and no artifacts (Fig. 2b)
The 20% activity reconstructions have a statistically significant reduction (p = 0.02) in image quality compared to the reference, the image quality was still acceptable with no artifacts
Summary
Positron emission tomography (PET) is increasingly applied for in vivo brown adipose tissue (BAT) research in healthy volunteers. The growing interest for a better understanding of brown adipose tissue (BAT) was triggered by the discovery of active BAT on 18F-fluoro-2-deoxy-D-glucose (18F-FDG) positron emission tomography (PET) scans in adult oncology patients [2]. In contrast to white adipocytes, a very high amount of mitochondria, have multilocular lipid droplets, and contribute to thermogenesis by directly dissipating chemical energy as heat [3]. The high glucose consumption of activated BAT leads to a large number of studies, using 18F-FDG PET as a surrogate marker for its metabolic activity [5,6,7]. Most of these studies, looking into the prevalence and factors that correlated with high BAT activity, were done on retrospective data and showed that active BAT was more commonly seen on 18F-FDG PET scans in winter than in summer, in women than in man, and in normal weight vs obese patients [5, 8,9,10,11,12]
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