The worldwide prevalence of Non-alcoholic Fatty Liver Disease (NAFLD) raises concerns about associated risk factors, such as obesity and type 2 Diabetes Mellitus, for leading causes of disability and death. Besides Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS), functional imaging with Positron Emission Tomography (PET) could contribute to a deeper understanding of the pathophysiology of NAFLD. Here we describe a novel approach using the PET tracer [¹⁸F]FTHA, which is an analog of long-chain free fatty acids (FFA) and is taken up by tissues to enter mitochondria or to be incorporated into complex lipids for further export as very-low-density lipoprotein (VLDL). Male Sprague Dawley rats, after 6 weeks on a high-fat diet (HFD), were used as a model of diet induced NAFLD, while a standard diet (SD) served as a control group. Liver fat was estimated by MR spectroscopy at a 9.4 T system for phenotyping. To measure hepatic FFA uptake, rats underwent 60 min dynamic [¹⁸F]FTHA-PET scans after unrestricted access to food (HFD: n = 6; SD: n = 6) or overnight (≤16h) fasting (HFD: n = 6; SD: n = 5). FFA removal was assessed from incorporated ¹⁸F-residual in de novo synthesized VLDL out of plasma. MRS of the liver confirmed the presence of NAFLD (>5.6% fat). Under non-fasting conditions, hepatic [¹⁸F]FTHA uptake was significantly increased in NAFLD: SUVmean (p = 0.03) within [0; 60] min interval, SUVmean (p = 0.01) and SUVmax (p = 0.03) within [30; 60] min interval. SUVs for hepatic uptake under fasting conditions were not significantly different between the groups. Analysis of FFA removal demonstrated elevated values of ¹⁸F-residue in the VLDL plasma fraction of the healthy group compared to the NAFLD (p = 0.0569). Our novel approach for assessing FFA metabolism using [¹⁸F]FTHA demonstrated differences in the hepatic FFA uptake and FFA incorporation into VLDL between healthy and NAFLD rats. [¹⁸F]FTHA-PET could be used to study metabolic disturbances involved in the progression of NAFLD.

14-(R,S)-[¹⁸F]fluoro-6-thia-heptadecanoic acid ([¹⁸F]FTHA) is a radiocompound for imaging the fatty acid circulation by positron emission tomography. A revived interest in imaging of lipid metabolism led us to a constant tracer production over three years, initially using a conventional vessel-based synthesizer and later transitioning to the cassette-based Elixys synthesizer. On the Elixys module, the radiochemical yield of [¹⁸F]FTHA could be increased by more than two times, reaching 13.01 ± 5.63% at the end of the synthesis, while maintaining necessary quality control results.

[¹¹C]Acetate-PET, widely applied in cardiology and oncology, offers unique potential as a non-invasive imaging method for assessing oxidative metabolism in metabolic disorders. In this study, we investigated its ability to detect early metabolic changes in the diet-induced model of metabolic dysfunction-associated fatty liver disease (MAFLD). Sprague Dawley rats were maintained on either standard diet or high-fat diet for 10 weeks before undergoing dynamic [¹¹C]acetate-PET/CT measurement. Four weeks later, radiometabolite analysis was performed in half of the animals, while the remainder underwent non-radioactive blood gas measurements. Uptake of [¹¹C]acetate at 60 minutes after administration was quantified in the kidneys, myocardium, and liver (SUV_mean and SUV_max), followed by analysis of time activity curves (AUC), alongside monoexponential clearance rates (k_mono) and kinetic modeling using 1- and 2-tissue compartment models (V_t). MAFLD animals exhibited altered [¹¹C]acetate metabolism, with [¹¹C]CO₂ excretion patterns validated by non-radiative blood gas analyses. While healthy rats showed a radiometabolite peak at ∼30 min post-injection, MAFLD rats displayed an earlier maximum at 5 min and a secondary peak at 40 min, indicating a shift in longitudinal oxidative metabolism. Despite contradictions between SUVs and k_mono, compartmental modeling demonstrated a clear separation of healthy from MAFLD groups, solely in hepatic volume of distribution (V_t). These results establish repurposing of [¹¹C]acetate-PET, particularly when combined with metabolite correction, as a sensitive approach for phenotyping and identifying metabolic alterations in MAFLD and also hold translational promise for understanding and monitoring of other obesity-related liver dysfunctions.

Stress impacts our health and triggers physiological adaptations, yet the metabolic programs engaged during stress remain incompletely understood. To fill this knowledge gap, we utilize total-body positron emission tomography (PET), multi-OMICS, and endocrine profiling to assess how various murine stress models affect systemic metabolic remodeling. We find that acute immobilization and surgery activate brown adipose tissue as part of the stress response, independently of hypothermia, thereby acting as a highly stress-sensitive metabolic hub. Additionally, we identify stress-specific hypo- and hypermetabolic signatures in different brain regions, and distinguish networks between brain and adipose tissue depots across the different stress groups. Our work presents a novel perspective on stress and its mobilization of metabolic resources and identifies PET imaging of brain and adipose tissue as a valuable, minimally invasive technique for tracking metabolic stress responses in mice, with relevance for animal welfare and disease models, and translational impact for mental health studies and preventive medicine.