Background Brown adipose tissue (BAT) has been proposed as a target to treat obesity and metabolic disease. Currently, 18F‐Fluordeoxyglucose positron emission tomography (FDG‐PET) is the standard for BAT‐imaging. MRI might be a promising alternative, as it is not associated with ionizing radiation, offers a high resolution, and allows to discriminate different types of soft tissue. Purpose We sought to evaluate whether supraclavicular BAT (scBAT) volume, fat‐fraction (FF), and relaxation rate (R2*) determined by MRI can predict its metabolic activity, which was assessed by measurement of cold‐induced thermogenesis (CIT). Study Type Prospective cohort study. Subjects Twenty healthy volunteers (9 female, 11 male), aged 18–47 years, with a body mass index (BMI) of 18–30 kg/m2. Field Strength/Sequence Multiecho gradient MRI for water–fat separation was used on a 3T device to measure the FF and T2* of BAT. Assessment Prior to imaging, CIT was determined by measuring the difference in energy expenditure (EE) during warm conditions and after cold exposure. Volume, FF, and R2* of scBAT was assessed and compared with CIT. In 11 participants, two MRI sessions with and without cold exposure were performed and the dynamic changes in FF and R2* assessed. Statistical Tests Linear regression was used to evaluate the relation of MRI measurements and CIT. P‐values below 0.05 were considered significant; data are given as mean ± SD. Results R2* correlated positively with CIT (r = 0.64, R2 = 0.41 P = 0.0041). Volume and FF did not correlate significantly with CIT. After mild cold exposure EE increased significantly (P = 0.0002), with a mean CIT of 147 kcal/day. The mean volume of scBAT was 72.4 ± 38.4 ml, mean FF was 74.3 ± 5.8%, and the mean R2* (1/T2*) was 33.5 ± 12.7 s‐1. Data Conclusion R2* of human scBAT can be used to estimate CIT. FF of scBAT was not associated with CIT. Level of Evidence: 2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:1160–1168.

Background: Muscles from patients with cerebral palsy (CP) are often spastic and form contractures that limit the range of motion. Injections of botulinum toxin A (BTX) into the calf muscles are an important treatment for functional equinus; however, improvement in gait function is not always achieved. BTX is also used to test muscle weakening for risk evaluation of muscle lengthening surgery. Our aim was to assess the effect of BTX over time on calf muscle properties in pediatric CP patients with MRI.Material and Methods: Six toe-walking CP patients (mean age 11.6 years) with indication for lengthening surgery were prospectively enrolled and received BTX injections into the gastrocnemius and soleus muscles. MRI scans at 3T of the lower legs and clinical examinations were performed pre-BTX, 6 weeks (6w), and 12 weeks (12w) post-BTX. A fat-suppressed 2D multi-spin-echo sequence was used to acquire T2 maps and for segmentation. Fat fraction maps were calculated from 3D multi-echo Dixon images. Diffusion tensor imaging (DTI) with a 2D echo-planar imaging (EPI) sequence yielded maps of the mean apparent diffusion coefficient (ADC) and of the fractional anisotropy (FA). Hyperintense regions of interest (ROIs) on the T2-weighted (T2w) images at 6w were segmented in treated muscles. Mean values of T2, fat fraction, ADC, and FA were calculated in hyperintense ROIs and in reference ROIs in non-treated muscles.Results: Hyperintensity on T2w scans and increased T2 (group mean ± standard deviation: 35 ± 1 ms pre-BTX, 45 ± 2 ms at 6w, and 44 ± 2 ms at 12w) were observed in all patients at the injection sites. The T2 increase was spatially limited to parts of the injected muscles. FA increased (0.30 ± 0.03 pre-BTX, 0.34 ± 0.02 at 6w, and 0.36 ± 0.03 at 12w) while ADC did not change in hyperintense ROIs, indicating a BTX-induced increase in extracellular space and a simultaneous decrease of muscle fiber diameter. Fat fraction showed a trend for increase at 12w. Mean values in reference ROIs remained unchanged.Conclusion: MRI showed limited spatial distribution of the BTX-induced effects in pediatric CP patients. It could be a promising non-invasive tool for future studies to test BTX treatment protocols.

Background: Brown adipose tissue (BAT) is a thermogenic tissue which can generate heat in response to mild cold exposure. As it constitutes a promising target in the fight against obesity, we need reliable techniques to quantify its activity in response to therapeutic interventions. The current standard for the quantification of BAT activity is [ 18 F]FDG PET/CT. Various sequences in magnetic resonance imaging (MRI), including those measuring its relative fat content (fat fraction), have been proposed and evaluated in small proof-of-principle studies, showing diverging results. Here, we systematically compare the predictive value of adipose tissue fat fraction measured by MRI to the results of [ 18 F]FDG PET/CT. Methods: We analyzed the diagnostic reliability of MRI measured fat fraction (FF) for the estimation of human BAT activity in two cohorts of healthy volunteers participating in two prospective clinical trials (NCT03189511, NCT03269747). In both cohorts, BAT activity was stimulated by mild cold exposure. In cohort 1, we performed [ 18 F]FDG PET/MRI; in cohort 2, we used [ 18 F]FDG PET/CT followed by MRI. Fat fraction was determined by 2-point Dixon and 6-point Dixon measurement, respectively. Fat fraction values were compared to SUV mean in the corresponding tissue depot by simple linear regression. Results: In total, 33 male participants with a mean age of 23.9 years and a mean BMI of 22.8 kg/m 2 were recruited. In 32 participants, active BAT was visible. On an intra-individual level, FF was significantly lower in high-SUV areas compared to low-SUV areas (cohort 1: p < 0.0001 and cohort 2: p = 0.0002). The FF of the supraclavicular adipose tissue depot was inversely related to its metabolic activity (SUVmean) in both cohorts (cohort 1: R 2 = 0.18, p = 0.09 and cohort 2: R 2 = 0.42, p = 0.009). Conclusion: MRI FF explains only about 40% of the variation in BAT glucose uptake. Thus, it can currently not be used to substitute [ 18 F] FDG PET-based imaging for quantification of BAT activity.

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