Combined PET/MR, utilizing functional positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), is an ideal tool to study functional and metabolic processes in the brain.¹ Here, we study rat brain function in response to a barrel-field stimulus using simultaneously [¹⁸F]FDG-PET, tracing changes in glucose metabolism, and functional MRI (fMRI) using the blood oxygen level dependent (BOLD) effect, assessing hemodynamic changes. A major criterion of preclinical brain activation studies applying functional imaging is the exact correlation of baseline and stimulation scans. Such a protocol could be also useful for human studies.² The purpose of this study was to establish a novel protocol to measure baseline as well as stimulation intervals using [¹⁸F]FDG-PET and BOLD-fMRI in one single scanning session. Thus, influences such as anesthesia depth, animal condition etc. could be eliminated to prevent confounding factors. Furthermore, the comparison of the results was expected to indicate similarities and differences of both modalities depicting the complementary nature of PET and fMRI.PET/MRI was performed on a 7 T small animal scanner with a 300 mT/m gradient system and a 2 x 2 rat brain receive coil combined with small animal PET insert. Male lewis rats (n = 4, 352 ± 5 g) were anesthetized (20 mg/kg/h α-chloralose and 1 mg/kg/h Pancuroniumbromide i.v.) and body temperature stabilized at 37.0 ± 0.5° C. During the measurements, animals were artificially ventilated (60 1/min, regular air). Blood glucose levels were measured before and after the scans. Besides anatomical images EPI BOLD sequences (TE: 18 ms, TR: 2000 ms, Voxel size: 0.5×0.25×1.25 mm³, FoV 32×32 mm, slice thickness: 1 mm, 8 slices) were acquired. Simultaneously to an 80 min EPI BOLD scan, rats were continuously infused with [¹⁸F]FDG (dose at infusion start 111 ± 7.4 mBq) i.v. and PET images were recorded. Four electrical stimulation intervals (3 mA, 3 Hz) of the left whisker pad were applied at 20-30 min, 40-45 min, 55-65 min and 75-80 min. Data were analyzed using statistical parametric mapping (SPM12, GLM) and region of interest analysis (PMOD).We found spatial and quantitative correlations between the PET and the fMRI activation data. According to a stimulation of the left whisker pad the fMRI analysis revealed significant BOLD signal (p ≤ 0.05; T = 25.11) in the contralateral (cl) barrel-field cortex S1BF. PET data showed also significant activation (p ≤ 0.05; T = 2.22) and a spatial shift of the activation centre compared to BOLD data (Euclidean distance = 2.48 mm). BOLD data revealed also ipsilateral S1BF activation in the left hemisphere and PET data pointed out additional activation centres in the cerebellum and cortical regions (Fig. 1). The PET time activity curve showed higher [¹⁸F]FDG uptake (ca. 20% increase of PET signal) in the contralateral S1BF compared to the control ROI (Fig. 2). Presence of the BOLD effect was confirmed before and after simultaneous PET/MR scans (Fig. 3). Venous blood glucose levels showed no significant difference pre and post [¹⁸F]FDG-infusion (pre: 125.25 mg/dL, post: 101.25 mg/dL, p = 0.15) (Fig. 4).Here, we report a spatial correlation but also in some areas discrepancies between brain activation measured by PET and BOLD-fMRI. The shift between peak activation centres of the same region can be explained by the fact, that T2* BOLD-fMRI results rather from the venous space, whereas [¹⁸F]FDG uptake occurs mainly in cellular areas. However, the additional regions observed in [¹⁸F]FDG-PET indicate a more complex mismatch between the two techniques, which could stem from the complex nature of the BOLD-fMRI response as an interplay of complex factors such as changes in oxygen consumption, blood flow and volume. Advantages of this study compared to the data available so far are simultaneous PET and MR scans as well as baseline and activation periods in one single scanning session.To our knowledge, this is the first small animal study reporting a novel animal scan protocol to allow spatial and quantitative correlation of PET and fMRI activation within one single scanning session. The established protocol has clear evidence for the characterization of functional and metabolic changes during brain activation on the basis of a stable experimental setup, since baseline and activation measurements were conducted at the exact same conditions. This paves the way also to studies using different metabolic tracers and is ultimately transferable to clinical settings.