Task-relevant brain networks identified with simultaneous PET/MR imaging of metabolism and connectivity

Abstract

Except for task-specific functional MRI, the vast majority of imaging studies assessed human brain function at resting conditions. However, tracking task-specific neuronal activity yields important insight how the brain responds to stimulation. We specifically investigated changes in glucose metabolism, functional connectivity and white matter microstructure during task performance using several recent methodological advancements. Opening the eyes and right finger tapping had elicited an increased glucose metabolism in primary visual and motor cortices, respectively. Furthermore, a decreased metabolism was observed in the regions of the default mode network, which allowed absolute quantification of commonly described deactivations during cognitive tasks. These brain regions showed widespread task-specific changes in functional connectivity, which stretched beyond their primary resting-state networks and presumably reflected the level of recruitment of certain brain regions for each task. Finally, the corresponding white matter fiber pathways exhibited changes in axial and radial diffusivity during the tasks, which were regionally distinctive for certain tract groups. These results highlight that even simple task performance leads to substantial changes of entire brain networks. Exploiting the complementary nature of the different imaging modalities may reveal novel insights how the brain processes external stimuli and which networks are involved in certain tasks.

Keywords: Continuous task performance; Functional PET, fPET; Functional connectivity; Glucose metabolism; White matter microstructure.

Fig. 1

Schematic diagram of the PET/MR acquisition and the task. Subjects opened their eyes (10–20 and 60–70 min) or tapped the right thumb to the fingers (35–45 and 85–95 min), with eyes closed and no finger movement otherwise. At baseline, a T1-weighted structural image (red) was acquired. Functional (green) and diffusion weighted MRI (yellow) was obtained at baseline as well as during the continuous task performance. PET (blue) was acquired simultaneously with the MRI. Images partly taken from pixabay.com under the CC0 creative common license

Fig. 2

Task-specific changes in glucose metabolism, functional connectivity and the corresponding white matter fiber tracts. a Significant changes in glucose metabolism during task performance served as seed regions for functional connectivity (p < 0.05 FWE-corrected, seeds marked by crosshair). Common changes of both conditions were assessed by conjunction analysis. b Compared to rest, widespread changes in functional connectivity were observed beyond the primary resting-state networks (p < 0.05 FWE-corrected). c White matter fiber tracts as modeled by probabilistic tractography exhibited task-related changes in radial and axial diffusivity during finger tapping and when eyes were opened, respectively

Fig. 3

Average changes during task performance as compared to rest for a glucose metabolism (µmol/100 g/min), b functional connectivity (z-score) as well as c axial (black) and radial diffusivity (white, µm²/s). For b and c brain regions indicate the connection with the corresponding seed region for each task (central left, occipital cortex and vmPFC). Asterisks denote significant task-related changes at *p < 0.05 or (*) p < 0.06