Thermogenic adipose tissue in energy regulation and metabolic health

The ability to generate thermogenic fat could be a targeted therapy to thwart obesity and improve metabolic health. Brown and beige adipocytes are two types of thermogenic fat cells that regulate energy balance. Both adipocytes share common morphological, biochemical, and thermogenic properties. Yet, recent evidence suggests unique features exist between brown and beige adipocytes, such as their cellular origin and thermogenic regulatory processes. Beige adipocytes also appear highly plastic, responding to environmental stimuli and interconverting between beige and white adipocyte states. Additionally, beige adipocytes appear to be metabolically heterogenic and have substrate specificity. Nevertheless, obese and aged individuals cannot develop beige adipocytes in response to thermogenic fat-inducers, creating a key clinical hurdle to their therapeutic promise. Thus, elucidating the underlying developmental, molecular, and functional mechanisms that govern thermogenic fat cells will improve our understanding of systemic energy regulation and strive for new targeted therapies to generate thermogenic fat. This review will examine the recent advances in thermogenic fat biogenesis, molecular regulation, and the potential mechanisms for their failure.

Effect of Propranolol on 18F-Fluorodeoxyglucose Uptake in Brown Adipose Tissue in Children and Young Adults with Neoplastic Diseases

PURPOSE

To evaluate the effectiveness of propranolol at mitigating FDG uptake in brown adipose tissue (BAT) of pediatric patients with known or suspected malignancies.

METHODS

PET/CT scans of 3 cohorts of patients treated from 2005 to 2017 were scored for the presence of FDG uptake by BAT at 7 sites: right or left neck/supraclavicular area, right or left axilla, mediastinum, posterior thorax, and abdomen/pelvis. Uptake was scored as follows: 0, none; 1, mild uptake < liver; 2, moderate uptake = liver; and 3, intense uptake > liver. Group 1 consisted of 323 patients (630 scans) who had no specific preparation to mitigate FDG uptake by BAT. Group 2 consisted of 345 patients (705 scans) who underwent only warming in an uptake room with a fixed temperature at 24 °C. Group 3 consisted of 622 patients (1457 scans) who underwent warming. In group 3, patients 8 years and older, 471 patients (1114 scans), were also pre-medicated with oral propranolol 60 min before injection of FDG. Generalized estimation equation, using the logit link method, was used to model the relationship between the incidence of BAT score > 0, in any site, as a function of age, sex, seasonal effect, and body surface area (BSA).

RESULTS

In patients aged 8 years or older, the incidence of BAT uptake was 35-44 % and declined to 15 % with propranolol. BAT was most frequent in the neck (26 %), axilla (18 %), posterior thorax (18 %), mediastinum (14 %), and abdomen/pelvis (8 %); BAT was less common in warm months (p = 0.001). No substantial benefit was shown with pre-injection warming alone. No significant effect was found for age, sex, or BSA separately. When BAT uptake was present, it was usually intense.

CONCLUSION

Propranolol preparation minimizes FDG uptake by BAT and should be considered routine for pediatric FDG PET/CT cancer-related protocols in children, adolescents, and young adults.

Effects of Nutrition/Diet on Brown Adipose Tissue in Humans: A Systematic Review and Meta-Analysis

BACKGROUND

Brown adipose tissue (BAT) provides a minor contribution to diet-induced thermogenesis (DIT)-the metabolic response to food consumption. Increased BAT activity is generally considered beneficial for mammalian metabolism and has been associated with favorable health outcomes. The aim of the current systematic review was to explore whether nutritional factors and/or diet affect human BAT activity.

METHODS

We searched PubMed Central, Embase and Cochrane Library (trials) to conduct this systematic review (PROSPERO protocol: CRD42018082323).

RESULTS

We included 24 eligible papers that studied a total of 2785 participants. We found no mean differences in standardized uptake value of BAT following a single meal or after 6 weeks of L-Arginine supplementation. Resting energy expenditure (REE), however, was increased following a single meal and after supplementation of capsinoid and catechin when compared to a control condition (Z = 2.41, p = 0.02; mean difference = 102.47 (95% CI = 19.28-185.67)).

CONCLUSIONS

Human BAT activity was not significantly affected by nutrition/diet. Moreover, REE was only increased in response to a single meal, but it is unlikely that this was due to increased BAT activity. BAT activity assessments in response to the chronic effect of food should be considered along with other factors such as body composition and/or environmental temperature.

Myocardial Positron Emission Tomography for Evaluation of Cardiac Sarcoidosis: Specialized Protocols for Better Diagnosis

Sarcoidosis is a multisystemic granulomatous disease of unknown etiology with various clinical presentations depending on the organs involved. Since cardiac sarcoidosis (CS) portends a higher risk of morbidity and mortality, early diagnosis and aggressive medical treatment are essential to improve the prognosis. ¹⁸F-Fluorodeoxyglucose (FDG) positron emission tomography (PET) has emerged as an important tool with practical advantages in assessing disease activity and monitoring the treatment response in patients with CS. While it has high sensitivity, it also has great variability in specificity, probably due to normal physiologic myocardial FDG uptake, which interferes with the evaluation and follow-up of CS using FDG-PET. This review details the technical aspects of FDG-PET imaging for evaluating and diagnosing CS, assessing disease activity, and monitoring therapeutic response.

Repeatability of Quantitative Brown Adipose Tissue Imaging Metrics on Positron Emission Tomography with 18F-Fluorodeoxyglucose in Humans

Brown adipose tissue (BAT) is a promising target for anti-obesity interventions. This prospective test-retest study assessed the repeatability of several important quantitative BAT metrics. After cold activation, 24 subjects underwent positron emission tomography (PET)/computed tomography (CT) and PET/magnetic resonance imaging (MRI), utilizing ¹⁸F-fluorodeoxyglucose. Repeat imaging occurred within 14 days per an identical protocol. BAT volumes were strongly correlated between sessions for PET/CT (intraclass correlation coefficient [ICC], 0.85) and PET/MRI (ICC, 0.82). BAT maximum lean-body-mass-adjusted standardized uptake values (SUL_max) were also strongly correlated between sessions for both PET/CT (ICC, 0.74) and PET/MRI (ICC, 0.83). Much longitudinal variability in BAT metrics was likely due to biological factors intrinsic to BAT, whole-body metabolic fluctuations, or temporal differences in cold-activation efficacy, rather than imaging factors. Future studies utilizing these imaging metrics to track the response BAT to interventions should incorporate this variation into sample-size considerations and response criteria.

Activation of Human Brown Adipose Tissue (BAT): Focus on Nutrition and Eating

Brown adipose tissue activation occurs most effectively by cold exposure. In the modern world, we do not spend long periods in cold environment, and eating and meals may be other activators of brown fat function. Short-term regulation of brown fat functional activity by eating involves most importantly insulin. Insulin is capable to increase glucose uptake in human brown adipose tissue fivefold to fasting conditions. Oxidative metabolism in brown fat is doubled both by cold and by a meal. Human brown adipose tissue is an insulin-sensitive tissue type, and insulin resistance impairs the function, as is found in obesity. Body weight reduction improves cold-induced activation of human brown adipose tissue.

Oncologic 18F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography: What All Physicians Need to Know

Fluorine-18 fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography (PET) hybrid examinations (PET/computed tomography, PET/magnetic resonance imaging) have become the most common PET imaging tools in the evaluation of the oncologic patient. Therefore it is of paramount importance that physicians who take care of oncology patients in any capacity are familiar with the basics of when these examinations are indicated, know how to best prepare the patients, and understand the benefits and limitations of the procedure. Additionally, it is important to understand which medical conditions and medications need to be controlled to maintain the diagnostic accuracy of these tests. In this article we aim to explain what ¹⁸F-FDG is, how to best prepare our patients, what PET is, and how these examinations are interpreted. Finally, we discuss some of the limitations of these examinations.

Regulation of Brown Adipose Tissue Activity by Interoceptive CNS Pathways: The interaction between Brain and Periphery

To maintain thermal homeostasis, specific thermogenic tissues are under the control of central thermoregulatory networks that regulate the body's response to thermal challenges. One of these mechanisms involves non-shivering thermogenesis in brown adipose tissue (BAT), which is activated in cold environments in order to defend the body against physical damage as a result of hypothermia. The objective of our study was to assess the interaction between CNS thermoregulatory pathways and sympathetic innervation in BAT during a cold exposure paradigm. Our results show that an innocuous whole-body cooling paradigm induces significant differences in fMRI BOLD signal at the location of the right anterior insula and the red nucleus/substantia nigra region, between lean subjects with high levels of sympathetic innervation in supraclavicular BAT (BAT+ group), and subjects with low levels of sympathetic innervation (BAT− group). Specifically, results indicate significantly larger fMRI BOLD signal changes between periods of cooling and warming of the skin in the BAT+ (as compared to BAT−) group at the location of the right anterior insula. In contrast, the BAT+ group showed significantly smaller fMRI BOLD signal changes in the midbrain between periods of skin cooling and warming. Our findings are consistent with a hierarchical thermoregulatory control system that involves the initiation of inhibitory signals from the right anterior insula toward midbrain areas that normally exert tonic inhibition on the medullary raphe, from where BAT is directly innervated. Our data suggests that exposure to cold elicits differential neuronal activity in interoceptive regulatory centers of subjects with high and low level of sympathetic innervation. As a result, the variability of cold-activated BAT mass observed in humans might be, in part, yoked to different sensitivities of interoceptive cortical brain areas to skin temperature changes.

Preparation methods prior to PET/CT scanning that decrease uptake of 18F-FDG by myocardium, brown adipose tissue, and skeletal muscle

BACKGROUND

The hypermetabolic environment of the myocardium, brown adipose tissue (BAT), and muscle will have an effect on the diagnostic accuracy of 18F-fluorodeoxyglucose (18F-FDG) positron-emission tomography (PET)/computed tomography (CT). A low carbohydrate, high fat, and protein-permitted diet before PET/CT scanning can reduce the degree of 18F-FDG uptake by the myocardium, brown adipose tissue, and skeletal muscle.

PURPOSE

To determine the effect of a low carbohydrate, high fat and protein-permitted diet on 18F-FDG uptake by myocardium, BAT, and muscle during PET/CT.

MATERIAL AND METHODS

A total of 126 patients who adhered to two meals before PET/CT scanning (that were prepared using a low carbohydrate, high fat, and protein-permitted diet), i.e. the diet group, were compared with 126 patients who fasted for at least 12 h prior to scanning (i.e. the fasting group). The degree of 18F-FDG uptake within the myocardium, BAT, and muscle were stratified into four grades (range, 0-3) with 0 for negligible uptake, and 3 for intense uptake. Correlations between the diet and fasting groups with respect to degree of 18F-FDG uptake within the myocardium, BAT, and muscle were analyzed.

RESULTS

The degree of 18F-FDG uptake within the myocardium, BAT, and muscle in the diet group was significantly lower compared with the 18F-FDG uptake within myocardium, BAT, and muscle in the fasting group (P < 0.001, P = 0.001, P < 0.001).

CONCLUSION

A low carbohydrate/high fat diet before 18F-FDG injection can suppress uptake of 18F-FDG within the myocardium, BAT, and skeletal muscle.

Brown Adipose Reporting Criteria in Imaging STudies (BARCIST 1.0): Recommendations for Standardized FDG-PET/CT Experiments in Humans

Human brown adipose tissue (BAT) presence, metabolic activity, and estimated mass are typically measured by imaging [18F]fluorodeoxyglucose (FDG) uptake in response to cold exposure in regions of the body expected to contain BAT, using positron emission tomography combined with X-ray computed tomography (FDG-PET/CT). Efforts to describe the epidemiology and biology of human BAT are hampered by diverse experimental practices, making it difficult to directly compare results among laboratories. An expert panel was assembled by the National Institute of Diabetes and Digestive and Kidney Diseases on November 4, 2014 to discuss minimal requirements for conducting FDG-PET/CT experiments of human BAT, data analysis, and publication of results. This resulted in Brown Adipose Reporting Criteria in Imaging STudies (BARCIST 1.0). Since there are no fully validated best practices at this time, panel recommendations are meant to enhance comparability across experiments, but not to constrain experimental design or the questions that can be asked.

The rediscovery of BAT in adult humans using imaging

Human brown adipose tissue has been acknowledged in newborns and children but in adults the first printed publications are from the beginning of 20th century. Further evidence of the existence of adult brown fat was published throughout the century but only very recently the functionality of active brown adipose tissue in vivo in adulthood was confirmed. This was contributed mainly by advanced imaging technology, namely hybrid positron emission tomography (PET) and computed tomography (CT), being able to combine functional and anatomical imaging data. Functionality is most commonly measured with glucose analog, 18F-fluoro-2-deoxy-d-glucose (FDG) but other tracers for other functions than glucose uptake have been introduced as well. Growing body of evidence has increased the knowledge of the role of brown adipose tissue in human metabolism and energy expenditure, providing a promising option for the management of body weight balance and disturbed glucose and lipid metabolism.

Diagnostic Accuracy of PET, SPECT, and Arterial Spin-Labeling in Differentiating Tumor Recurrence from Necrosis in Cerebral Metastasis after Stereotactic Radiosurgery

BACKGROUND AND PURPOSE

Radiographic assessment of cerebral metastasis after stereotactic radiosurgery remains a major challenge in neuro-oncology. It is often difficult to distinguish tumor progression from radiation necrosis in this setting using conventional MR imaging. The objective of this study was to compare the diagnostic sensitivity and specificity of different functional imaging modalities for detecting tumor recurrence after stereotactic radiosurgery.

MATERIALS AND METHODS

We retrospectively reviewed patients treated between 2007 and 2010 and identified 14 patients with cerebral metastasis who had clinical or radiographic progression following stereotactic radiosurgery and were imaged with arterial spin-labeling, FDG-PET, and thallium SPECT before stereotactic biopsy. Diagnostic accuracy, specificity, sensitivity, positive predictive value, and negative predictive value were calculated for each imaging technique by using the pathologic diagnosis as the criterion standard.

RESULTS

Six patients (42%) had tumor progression, while 8 (58%) developed radiation necrosis. FDG-PET and arterial spin-labeling were equally sensitive in detecting tumor progression (83%). However, the specificity of arterial spin-labeling was superior to that of the other modalities (100%, 75%, and 50%, respectively). A combination of modalities did not augment the sensitivity, specificity, positive predictive value, or negative predictive value of arterial spin-labeling.

CONCLUSIONS

In our series, arterial spin-labeling positivity was closely associated with the pathologic diagnosis of tumor progression after stereotactic radiosurgery. Validation of this finding in a large series is warranted.

Rifaximin suppresses background intestinal 18F-FDG uptake on PET/CT scans

BACKGROUND

Identification of cancer or inflammatory bowel disease in the intestinal tract by PET/computed tomography (CT) imaging can be hampered by physiological uptake of F-fluorodeoxyglucose (F-FDG) in the normal colon. Previous work has localized this F-FDG uptake to the intestinal lumen, predominantly occupied by bacteria. We sought to determine whether pretreatment with an antibiotic could reduce F-FDG uptake in the healthy colon.

PATIENTS AND METHODS

Thirty patients undergoing restaging PET/CT for nongastrointestinal lymphoma were randomly selected to receive rifaximin 550 mg twice daily for 2 days before their scan (post-rifaximin). Their PET/CT images were compared with those from their prior study (pre-rifaximin). Cecal maximum standard uptake value (SUVmax) and overall colonic F-FDG uptake were compared between scans. All PET/CT images were blindly scored by a radiologist. The same comparison of sequential scans was also undertaken in 30 patients who did not receive antibiotics.

RESULTS

Thirty post-rifaximin scans were compared with 30 pre-rifaximin scans in the same patients. SUVmax in the cecum was significantly lower in the patient's post-rifaximin scans than in their pre-rifaximin scans (P=0.002). The percentage of scans with greater than grade 1 colonic F-FDG uptake was significantly lower in the post-rifaximin scans than in the pre-rifaximin scans (P<0.05). In contrast, there was no significant difference in the paired sequential scans from control patients, nor a reduction in the percentage of scans with greater than grade 1 colonic F-FDG uptake.

CONCLUSION

This pilot study shows that treatment with rifaximin for 2 days before PET/CT scanning can significantly reduce physiological F-FDG uptake in the normal colonic lumen.

Review of overlap between thermoregulation and pain modulation in fibromyalgia

Fibromyalgia (FM) syndrome is characterized by widespread pain that is exacerbated by cold and stress but relieved by warmth. We review the points along thermal and pain pathways where temperature may influence pain. We also present evidence addressing the possibility that brown adipose tissue activity is linked to the pain of FM given that cold initiates thermogenesis in brown adipose tissue through adrenergic activity, whereas warmth suspends thermogenesis. Although females have a higher incidence of FM and more resting thermogenesis, they are less able to recruit brown adipose tissue in response to chronic stress than males. In addition, conditions that are frequently comorbid with FM compromise brown adipose activity making it less responsive to sympathetic stimulation. This results in lower body temperatures, lower metabolic rates, and lower circulating cortisol/corticosterone in response to stress--characteristics of FM. In the periphery, sympathetic nerves to brown adipose also project to surrounding tissues, including tender points characterizing FM. As a result, the musculoskeletal hyperalgesia associated with conditions such as FM may result from referred pain in the adjacent muscle and skin.

Activating brown adipose tissue for weight loss and lowering of blood glucose levels: a microPET study using obese and diabetic model mice

Purpose

This study aims at using ¹⁸F-FDG microPET to monitor the brown adipose tissue (BAT) glucose metabolism in obese and diabetic mouse models under different interventions, and study the therapeutic potential of BAT activation for weight loss and lowering of blood glucose in these models.

Methods

Obese mice were established by a high-fat diet for eight weeks, and diabetes mellitus(DM) models were induced with Streptozocin in obese mice. ¹⁸F-FDG microPET was used to monitor BAT function during obese and DM modeling, and also after BRL37344 (a β₃-adrenergic receptor agonist) or levothyroxine treatment. The BAT function was correlated with the body weight and blood glucose levels.

Results

Compared with the controls, the obese mice and DM mice showed successively lower ¹⁸F-FDG uptake in the interscapular BAT (P = 0.036 and <0.001, respectively). After two-week BRL37344 treatment, the BAT uptake was significantly elevated in both obese mice (P = 0.010) and DM mice (P = 0.004), accompanied with significantly decreased blood glucose levels (P = 0.023 and 0.036, respectively). The BAT uptake was negatively correlated with the blood glucose levels in both obese mice (r = −0.71, P = 0.003) and DM mice (r = −0.74, P = 0.010). BRL37344 treatment also caused significant weight loss in the obese mice (P = 0.001). Levothyroxine treatment increased the BAT uptake in the control mice (P = 0.025) and obese mice (P = 0.013), but not in the DM mice (P = 0.45).

Conclusion

The inhibited BAT function in obese and DM mice can be re-activated by β₃-adrenergic receptor agonist or thyroid hormone, and effective BAT activation may lead to weight loss and blood glucose lowering. Activating BAT can provide a new treatment strategy for obesity and DM.

The clinical use of PET/CT in the evaluation of melanoma

Positron emission tomography combined with computed tomography (PET/CT) has emerged in the last decade as a dominant imaging modality used for staging, monitoring response and surveillance of various cancers, including melanoma. Using 2-deoxy-2-((18)F)fluoro-D-glucose ((18)F-FDG) as the radiopharmaceutical, PET/CT has demonstrated its efficacy and its utility in the management of patients with advanced melanoma. Nonetheless, challenges remain in the early stage evaluation of melanoma and in the development of novel radiotracers to better characterize lesions found on PET/CT. This chapter focuses on the advantages and limitations of this imaging modality in melanoma. We also detail and describe the approach to perform (18)F-FDG PET/CT, the methods to accurately quantify lesions, as well as the pearls/pitfalls of image interpretation. Finally, an overview of preclinical and investigational clinical radiopharmaceuticals is presented.

The prevalence and predictors of active brown adipose tissue in Chinese adults

OBJECTIVE

Previous studies have shown that active brown adipose tissue (BAT) is present in adults and may play important roles in the regulation of energy homeostasis. However, nearly every study has been carried out in patients undergoing scanning for cancer surveillance (CS), whose metabolism and BAT activity may not reflect those of healthy individuals. The objective of this study was to investigate the prevalence and predictors of active BAT in Chinese adults, particularly in healthy individuals.

DESIGN

A total of 31,088 consecutive subjects aged ≥18 years who had undergone positron emission tomography/computed tomography (PET/CT) scanning of BAT were evaluated in this study.

METHODS

We measured BAT activity via (18)F-fluorodeoxyglucose PET/CT in subjects who had undergone scanning for either a routine medical checkup (MC) or CS in Shanghai. Then, we investigated the predictors of active BAT, particularly in healthy individuals.

RESULTS

In both groups, the prevalence of BAT was higher in women than in men. Using a multivariate logistic analysis, we found age, sex, BMI, and high thyroid glucose uptake to be significant predictors of BAT activity in the MC group. Similarly, we found age, sex, and BMI to be significant predictors of BAT activity, but not thyroid high glucose uptake, in the CS group.

CONCLUSIONS

In Chinese adults, BAT activity inversely correlates with BMI and thyroid high glucose uptake, which reinforces the central role of brown fat in adult metabolism and provides clues to a potential means for treating the metabolic syndrome.

Molecular imaging of brown adipose tissue in health and disease

PURPOSE

Brown adipose tissue (BAT) has transformed from an interfering tissue in oncological (18)F-fluorodeoxyglucose (FDG) positron emission tomography (PET) to an independent imaging research field. This review takes the perspective from the imaging methodology on which human BAT research has come to rely on heavily.

METHODS

This review analyses relevant PubMed-indexed publications that discuss molecular imaging methods of BAT. In addition, reported links between BAT and human diseases such as obesity are discussed, and the possibilities for imaging in these fields are highlighted. Radiopharmaceuticals aiming at several different biological mechanisms of BAT are discussed and evaluated.

RESULTS

Prospective, dedicated studies allow visualization of BAT function in a high percentage of human subjects. BAT dysfunction has been implicated in obesity, linked with diabetes and associated with cachexia and atherosclerosis. Presently, (18)F-FDG PET/CT is the most useful tool for evaluating therapies aiming at BAT activity. In addition to (18)F-FDG, other radiopharmaceuticals such as (99m)Tc-sestamibi, (123)I-metaiodobenzylguanidine (MIBG), (18)F-fluorodopa and (18)F-14(R,S)-[(18)F]fluoro-6-thia-heptadecanoic acid (FTHA) may have a potential for visualizing other aspects of BAT activity. MRI methods are under continuous development and provide the prospect of functional imaging without ionizing radiation.

CONCLUSION

Molecular imaging of BAT can be used to quantitatively assess different aspects of BAT metabolic activity.

Arterial and fat tissue inflammation are highly correlated: a prospective 18F-FDG PET/CT study

PURPOSE

There is evidence that the link between obesity and cardiovascular disease might relate to inflammation in both fat tissue and the arterial wall. (18)F-FDG uptake on PET is a surrogate marker of vessel wall inflammation. The aim of the study was to measure FDG uptake in both regions using PET and identify links between adipose and arterial inflammation.

METHODS

Included in the study were 173 cardiovascular patients who were prospectively imaged with FDG PET/CT. Arterial FDG uptake was measured in the carotid arteries and ascending aorta. The same was done in fat tissue in the neck, the presternal region (both subcutaneous) and the pericardium. FDG uptake was quantified as average maximal target-to-background ratio (mean TBR max). Multivariate regression analyses were performed to identify significant associations between arterial and adipose tissue FDG uptake and clinical variables as given by the standardized correlation coefficient (β).

RESULTS

FDG uptake values in all fat tissue regions were highly predictive of vascular FDG uptake in both the carotids (β 0.262, p < 0.0001, in the neck subcutaneous region) and aorta (β 0.22, p = 0.008, in the chest pericardial region; β 0.193, p = 0.019, in the chest subcutaneous region). Obesity was significantly associated with elevated FDG uptake in adipose tissue (β 0.470, p < 0.0001, in the neck subcutaneous region; β 0.619, p = 0.028, in the chest subcutaneous region; β 0.978, p = 0.035, in the chest pericardial region).

CONCLUSION

FDG uptake in diverse fat tissue regions was significantly associated with arterial FDG uptake, a reasonable surrogate of inflammation. Increasing body weight significantly predicted the level of fatty inflammation. FDG PET therefore provides imaging evidence of an inflammatory link between fat tissue and the vasculature in patients with cardiovascular disease.

Differences in the metabolic status of healthy adults with and without active brown adipose tissue

BACKGROUND

Previous studies have proven the existence of active brown adipose tissue (BAT) in adults; however, its effect on systematic metabolism remains unclear.

AIM

The current study was designed to investigate the differences in the metabolic profiles of healthy adults with and without active BAT using positron emission tomography-computed tomography (PET-CT) scans in the un-stimulated state.

METHODS

A cross-sectional analysis was performed to assess the health of adults using PET-CT whole-body scans at Huashan Hospital Medical Centre between November 2009 and May 2010. A total of 62 healthy adults with active BAT were enrolled in the BAT-positive group. For each positive subject, a same-gender individual who underwent PET-CT the same day and who had no detectable BAT was chosen as the negative control. Body composition was measured, and blood samples were collected for assays of metabolic profiles and other biomarkers.

RESULTS

In both the male and female groups, BAT-positive individuals were younger and had lower body mass indexes, fasting insulin, insulin resistance, and leptin, but a greater level of high-density lipoprotein cholesterol compared with the negative controls. In the male group, body fat content and levels of tumor necrosis factor-α were significantly lower in the BAT-positive than in the negative control group.

CONCLUSIONS

The healthy adults with active BAT in an un-stimulated state had favorable metabolic profiles suggesting that active BAT may be a potential target for preventing and treating obesity and other metabolic disorders.

Brown adipose tissue in adult humans: a metabolic renaissance

Brown adipose tissue (BAT) plays a key role in energy homeostasis and thermogenesis in animals, conferring protection against diet-induced obesity and hypothermia through the action of uncoupling protein 1 (UCP1). Recent metabolic imaging studies using positron emission tomography computerized tomography (PET-CT) scanning have serendipitously revealed significant depots of BAT in the cervical-supraclavicular regions, demonstrating persistence of BAT beyond infancy. Subsequent cold-stimulated PET-CT studies and direct histological examination of adipose tissues have demonstrated that BAT is highly prevalent in adult humans. BAT activity correlates positively with increment of energy expenditure during cold exposure and negatively with age, body mass index, and fasting glycemia, suggesting regulatory links between BAT, cold-induced thermogenesis, and energy metabolism. Human BAT tissue biopsies express UCP1 and harbor inducible precursors that differentiate into UCP1-expressing adipocytes in vitro. These recent discoveries represent a metabolic renaissance for human adipose biology, overturning previous belief that BAT had no relevance in adult humans. They also have implications for the understanding of the pathogenesis and treatment of obesity and its metabolic sequelae.

Noninvasive identification and assessment of functional brown adipose tissue in rodents using hyperpolarized ¹³C imaging

OBJECTIVE

The recent identification of functional depots of brown adipose tissue (BAT) in adult humans has potential implications for the treatment of obesity. In order to evaluate new therapies aimed at inducing the production of more BAT or activating BAT in humans, it will be important to develop noninvasive methods to assess the functional state of the tissue in vivo. In this study, we investigate the feasibility of using hyperpolarized (13)C imaging to noninvasively identify functional, activated BAT in an in vivo rodent model, in less than 1 min, following an infusion of pre-polarized [1-(13)C] pyruvate.

DESIGN

Hyperpolarized (13)C imaging was used to monitor BAT metabolic conversion of pre-polarized [1-(13)C] pyruvate in rats during baseline and norepinephrine (NE)-stimulated conditions.

RESULTS

Activated BAT, stimulated by NE injection, can be detected in rats by increased conversion of pre-polarized [1-(13)C] pyruvate into its downstream products (13)C bicarbonate and [1-(13)C] lactate. The colocalization of the (13)C signal to interscapular BAT was validated using hematoxylin-eosin histological staining.

CONCLUSION

The radiation-free nature and recent translation into the clinic of the hyperpolarized (13)C-imaging test may potentially facilitate trials of therapeutics targeting BAT activation in humans.

Stability in brain glucose metabolism following brown adipose tissue inactivation in chinese adults

BACKGROUND AND PURPOSE

The thermogenesis of BAT is believed to be controlled through some pathways initiated in the brain, though the changes in brain activity among different states of BAT-positive subjects are still unclear. We hypothesized that some significant differences of regional cerebral metabolism between various groups were related to the BAT activities regardless of temperature changes.

MATERIALS AND METHODS

Relative regional cerebral glucose metabolism was compared between 15 healthy subjects with activated BAT and 30 healthy controls without activated BAT by using a brain FDG-PET scan. A follow-up PET scan was performed to assess metabolic changes of the brain when BAT activity was eliminated by heat exposure.

RESULTS

Compared with controls, BAT-positive subjects exhibited lower activity in the inferior parietal lobule, limbic system, and frontal lobe and higher activity in the precuneus before heat exposure. Compared with the BAT elimination status, subjects with activated BAT showed a decreased metabolism in the parietal lobe, frontal lobe, culmen, cingulate gyrus, and sublobar region. Compared with controls, BAT-positive subjects after BAT inactivation had significant hypometabolic areas in the temporal lobe and limbic lobe and hypermetabolic areas in the parietal lobe.

CONCLUSIONS

Our findings illustrate that changes of regional cerebral metabolism are related to BAT activities regardless of temperature changes. This before-after controlled study supports the finding that the brain responses appear to be active in modulating the metabolic function of BAT activity.

The relationship between patients' serum glucose levels and metabolically active brown adipose tissue detected by PET/CT

PURPOSE

To compare blood glucose levels in patients with or without "detectable" brown adipose tissue (BAT) using 2-deoxy-2-[(18)F]fluoro-D-glucose positron emission tomography/computed tomography (FDG PET/CT).

PROCEDURES

Nine hundred eight patients had PET/CT scans and were previously identified as having, or not having, FDG uptake in BAT. The original database was retrospectively reviewed for blood glucose level and body mass index (BMI) at the time of imaging. Blood glucose levels were compared between patients with or without FDG uptake in BAT, adjusting for age, sex, and BMI.

RESULTS

Fifty-six patients (6.2%) had FDG uptake in BAT. In the univariate analysis, patients without FDG uptake in BAT had a higher risk of glucose ≥100 mg/dL (odds ratio 3.4, 95% CI = 1.6-7.3; P = 0.0007). After adjustment for age, sex, BMI, and significant interaction of sex and BMI, patients without BAT tended to have a higher risk of glucose ≥100 mg/dL, although not statistically significant (odds ratio = 1.6, 95% CI = 0.7-3.6; P = 0.268).

CONCLUSIONS

Although causal relationships are not specified, the data suggest that BAT uptake, glucose levels, BMI, sex, and age are inter-related and the possibility that presence of "detectable" BAT is protective against diabetes and obesity. FDG PET/CT may be a vital tool for further investigations of diabetes and obesity.

Determinants of physiologic 18F-FDG uptake in brown adipose tissue in sequential PET/CT examinations

PURPOSE

The aim of this study was to assess independent predictors of 2-deoxy-2-[(18)F]fluoro-D-glucose ((18)F-FDG) uptake in brown adipose tissue (BAT) in patients undergoing repeated positron emission tomography (PET)/computed tomography (CT) scans.

PROCEDURES

Eight hundred forty-eight (mean age 50.9 ± 16 years) patients in whom PET/CT scan was repeated (mean interval 5 ± 1.5 months) constituted the study group. (18)F-FDG uptake in characteristic areas of BAT, with CT density of adipose tissue, greater than background soft-tissue activity was considered as evidence of BAT uptake. Both distribution and maximum standardized uptake values (SUVmax) were registered. Clinical and anamnestic data were collected for each patient.

RESULTS

(18)F-FDG uptake in BAT was present in 8.6% patients at first scan. Independent predictors of presence of uptake were age (younger), gender (female), body mass index (lower), and maximum outdoor temperature (lower). Age was the only independent predictor of BAT (18)F-FDG uptake distribution, while SUVmax was related to both age and outdoor temperature. Independent determinants of persistence of BAT (18)F-FDG uptake at second PET/CT were outdoor temperature at time of second scan and extension of metabolically active BAT at first scan.

CONCLUSIONS

Age, body mass index, and outdoor temperature are significant determinants of BAT evidence at (18)F-FDG PET/CT. Moreover, extension of BAT and outdoor temperature are the strongest determinants of persistence of BAT evidence on (18)F-FDG PET/CT in repeated scan.

Review of Physiologic and Pathophysiologic Sources of Fluorodeoxyglucose Uptake in the Chest Wall on PET

The chest wall can be defined as the osseous and soft tissue structures that form the outer framework of the thorax and move during breathing. Topics discussed in this article include physiologic uptake of fluorodeoxyglucose, benign diseases of the chest wall, and malignant tumors of the chest wall.

A new era for brown adipose tissue: New insights into brown adipocyte function and differentiation

Until quite recently, brown adipose tissue was considered of metabolic significance only in small mammals and human newborns, since it was thought to disappear rapidly after birth in humans. However, nowadays this tissue is known to play a role in the regulation of energy balance not only in rodents, but also in humans. In this review we highlight new features regarding brown adipose tissue origin and function and revise old paradigms about brown adipocyte differentiation.

Brown adipose tissue 18F-FDG uptake in pediatric PET/CT imaging

Positron emission tomography (PET) using [F-18]2-fluoro-2-deoxyglucose (FDG) fused with CT ((18)F-FDG PET/CT) has been widely adopted in oncological imaging. However, it is known that benign lesions and other metabolically active tissues, such as brown adipose tissue (BAT), can accumulate (18)F-FDG, potentially resulting in false-positive interpretation. Previous studies have reported that (18)F-FDG uptake in BAT is more common in children than in adults. We illustrate BAT FDG uptake in various anatomical locations in children and adolescents. We also review what is known about the effects of patient-related physical attributes and environmental temperatures on BAT FDG uptake, and discuss methods used to reduce BAT FDG uptake on (18)F-FDG PET.

Brown adipose tissue can be activated or inhibited within an hour before 18F-FDG injection: a preliminary study with microPET

Brown adipose tissue (BAT) is emerging as a potential target for treating human obesity. It has been indicated that BAT is rich in innervations of sympathetic nerve control. Using (18)F-FDG microPET imaging, this study aims at evaluating how factors related to sympathetic activation/inhibition changed BAT metabolism of mice. BAT (18)F-FDG uptake were semiquantitatively evaluated in different groups of mice under temperature (cold or warm stimulus) or pharmacological interventions (norepinephrine, epinephrine, isoprenaline, or propranolol) and were compared with the corresponding controls. It was found that BAT activation can be stimulated by cold exposure (P = 1.96 × 10(-4)), norepinephrine (P = .002), or both (P = 2.19 × 10(-6)) within an hour before (18)F-FDG injection and can also be alleviated by warming up (P = .001) or propranolol lavage (P = .027). This preliminary study indicated that BAT function could be evaluated by (18)F-FDG PET imaging through short-term interventions, which paved the way for further investigation of the relationship between human obesity and BAT dysfunction.

Effective reduction of brown fat FDG uptake by controlling environmental temperature prior to PET scan: an expanded case series

INTRODUCTION

Brown fat uptake of 2-deoxy-2-[F-18]fluoro-D: -glucose (FDG) on a positron emission tomography (PET) scan may limit the ability to assess for cancer. Previously, Garcia et al. demonstrated in ten patients a significant decrease in brown fat uptake of 2-deoxy-2-[F-18]fluoro-D: -glucose (FDG) after controlling the patient's environmental temperature.

OBJECTIVE

The objective of the current study is to validate the effectiveness of controlled environmental temperature (CET) to reduce physiologic brown fat (BF) FDG uptake on a PET scan in a larger series.

METHOD

A retrospective review was performed from January 2002 to October 2007 of patients who had (1) a pattern of FDG uptake on PET scan consistent with BF, (2) no evidence of cancer by computed tomography in the regions of interest noted below, (3) repeat scan with CET within 4 months of the 1st PET scan, and (4) no use of drugs reported to reduce BF FDG uptake (e.g., benzodiazepine, beta-blockers, reserpine) unless they were used identically prior to and during both studies. The FDG-PET and controlled environmental temperature-positron emission tomography (CET-PET) scans were performed as per protocol. The non-CET and CET-PET images were blinded/randomized, and three physicians assessed three regions (right neck, left neck, and paraspinal area) semiquantitatively using the following scale: "0" (background [bkgd]), 1 + (> bkgd < liver), 2 + (equal to liver), 3 + (> liver). Standard uptake value (SUV) data was recorded. Results were analyzed using a two-tailed t test.

RESULTS

Of 8,640 FDG-PET scans performed, 30 patients (four male, 26 female) met the above criteria. The median age was 36 years (range, 12-60 years). The mean (± 1 standard deviation) of differences in the scores between the two studies for right neck, left neck, and paraspinal regions, respectively, were for reader 1:(2.1 ± 1.37), (1.95 ± 1.43), and (1.85 ± 1.26); reader 2 (2.3 ± 1.40), (1.70 ± 1.13), and (1.77 ± 1.13); reader 3 (2.17 ± 1.17), (2.20 ± 1.18), and (0.50 ± 1.30); for maximum SUV score (3.4 ± 2.9), (3.3 ± 2.9), and (1.77 ± 1.13). All p values were <0.001.

CONCLUSION

In this larger series, CET effectively reduced the false-positive (18)FDG uptake in BF on PET scans without the use of drugs.

Brown adipose tissue in humans

PURPOSE OF REVIEW

Human brown adipose tissue (BAT) has recently found to be functionally active in adults. The purpose of this review is to chart the importance of BAT in the light of recent publications in humans.

RECENT FINDINGS

After publication of the direct evidence of functional BAT in human adults the original findings in human studies have been sparse. This indicates that intensive human data collection is ongoing.

SUMMARY

Brown adipocytes and myocytes share the same origin. Regardless of different origin, brown and white adipocytes transdifferentiate into each other's under specific exposure but in addition, another cell type is suggested to exist, 'brite' adipocyte, holding the capacity of transdifferentiation. This indicates the plasticity of adipose organ.Human BAT can be detected using PET. Cold exposure increases the probability to detect brown fat due to increased metabolic activity by induced thermogenesis.BAT glucose uptake rate is 10-15-fold higher in cold than in normal room temperature. Other factors that may increase the activity of sympathetic nervous system and uncoupling protein 1 are several including the complex network of hormonal and neuronal signals. Preliminary results suggest that BAT resembles skeletal muscle not only by origin but also by the effect of insulin on the tissue.

Human brown fat and obesity: methodological aspects

Much is known about brown adipose tissue (BAT) in rodents. Its function is to generate heat in response to low environmental temperatures and to diet or overfeeding. The knowledge about BAT in humans is still rather limited despite the recent rediscovery of its functionality in adults. This review highlights the information available on the contribution of BAT in increasing human energy expenditure in relation to obesity. Besides that methodological aspects will be discussed that need special attention in order to unravel the heat producing capacity of human BAT, the recruitment of the tissue, and its functionality.

Altered biodistribution on FDG-PET with emphasis on brown fat and insulin effect

(18)F-fluorodeoxyglucose (FDG) is the radiotracer used in the vast majority of positron emission tomography (PET) cancer studies. FDG is a powerful radiotracer that provides valuable data in many cancer types. Normal FDG biodistribution is easily identified. In the PET-only era, physiological uptake provided important anatomical landmarks. However, the normal biodistribution of FDG is often variable and can be altered by intrinsic or iatrogenic factors. Recognizing these patterns of altered biodistribution is important for optimal FDG-PET interpretation. Altered FDG uptake in muscles, brown adipose tissue, bone marrow, the urinary tract, and the bowel is demonstrated in a significant proportion of patients, which can hide underlying malignant foci or mimic malignant lesions. The introduction of PET/computed tomography revolutionized PET imaging, bringing much-needed anatomical information. This modality allowed better characterization of some types of uptake, particularly brown adipose tissue FDG uptake. Different approaches to minimize interference from altered FDG biodistribution should be considered when performing PET scans. Otherwise, careful review and correlation of metabolic (FDG-PET) and anatomical (computed tomography) data should be performed to accurately characterize the foci of increased FDG uptake.

Impact of age on the relationships of brown adipose tissue with sex and adiposity in humans

OBJECTIVE

Brown adipose tissue (BAT) regulates energy homeostasis and fat mass in mammals and newborns and, most likely, in adult humans. Because BAT activity and BAT mass decline with age in humans, the impact of BAT on adiposity may decrease with aging. In the present study we addressed this hypothesis and further investigated the effect of age on the sex differences in BAT activity and BAT mass.

RESEARCH DESIGN AND METHODS

Data from 260 subjects (98 with BAT and 162 study date-matched control subjects) who underwent (18)F-fluorodeoxyglucose positron emission tomography/computed tomography ((18)F-FDG PET/CT) under thermoneutral conditions were analyzed. BAT activity and BAT mass were determined in the upper body.

RESULTS

BAT activity and BAT mass were higher in female (1.59 +/- 0.10 and 32 +/- 5 g vs. 1.02 +/- 0.10 and 18 +/- 4 g, both P < or = 0.0006) than in male subjects. In multivariate analyses, sex (P < 0.0001), age (P < 0.0001), and BMI (P = 0.0018) were associated independently with BAT activity. Interestingly, only in male subjects was there an interaction between BMI and age in determining BAT activity (P = 0.008) and BAT mass (P = 0.0002); BMI decreased with increasing BAT activity and BAT mass in the lowest age tertile (Spearman rank correlation coefficient r(s) = -0.38, P = 0.015 and r(s) = -0.37, P = 0.017, respectively), not in the higher age tertiles. Furthermore, BAT activity and mass differed between female and male subjects only in the upper two age tertiles (all P < or = 0.09).

CONCLUSIONS

Our data corroborate that, in general, BAT activity and BAT mass are elevated in female subjects and in younger people. Importantly, we provide novel evidence that the impact of BAT activity and BAT mass on adiposity appears to decline with aging only in male subjects. Furthermore, while BAT activity and BAT mass only moderately decline with increasing age in female subjects, a much stronger effect is found in male subjects.

FDG PET/CT of extranodal involvement in non-Hodgkin lymphoma and Hodgkin disease

The term extranodal disease refers to lymphomatous infiltration of anatomic sites other than the lymph nodes. Almost any organ can be affected by lymphoma, with the most common extranodal sites of involvement being the stomach, spleen, Waldeyer ring, central nervous system, lung, bone, and skin. The prevalence of extranodal involvement in non-Hodgkin lymphoma and Hodgkin disease has increased in the past decade. The imaging characteristics of extranodal involvement can be subtle or absent at conventional computed tomography (CT). Imaging of tumor metabolism with 2-[fluorine-18]fluoro-2-deoxy-d-glucose (FDG) positron emission tomography (PET) has facilitated the identification of affected extranodal sites, even when CT has demonstrated no lesions. More recently, hybrid PET/CT has become the standard imaging modality for initial staging, follow-up, and treatment response assessment in patients with lymphoma and has proved superior to CT in these settings. Certain PET/CT patterns are suggestive of extranodal disease and can help differentiate tumor from normal physiologic FDG activity, particularly in the mucosal tissues, bone marrow, and organs of the gastrointestinal tract. Familiarity with the different extranodal manifestations in various locations is critical for correct image interpretation. In addition, a knowledge of the differences in FDG avidity among the histologic subtypes of lymphoma, appropriate timing of scanning after therapeutic interventions, and use of techniques to prevent brown fat uptake are essential for providing the oncologist with accurate information.

Clinical utility of PET/CT in lymphoma

OBJECTIVE

The purpose of this review is to assist interpreting radiologists in becoming familiar with the role of PET/CT in baseline staging and therapeutic response assessment in the management of lymphoma, in becoming aware of imaging pitfalls, and in understanding the natural behavior of lymphoma and the therapeutic options.

CONCLUSION

Therapeutic strategies for the management of lymphoma are constantly being refined to improve long-term survival with the lowest risk of toxicity to the patient. PET/CT is accurate for baseline staging and yields important prognostic information for determining the most appropriate initial treatment. Used for evaluation of treatment response, PET/CT can depict residual viable malignant lesions with greater accuracy than can other imaging techniques. The findings thereby influence decisions about the need for additional or alternative treatment.

BAT: a new target for human obesity?

Two types of adipose tissue can be distinguished histologically and functionally: white (WAT) and brown adipose tissue (BAT). Whereas BAT is specialized in the production of heat, WAT stores excess energy as triacylglycerols. BAT is present throughout life in rodents, whereas in humans it was thought to involute rapidly postnatally, having essentially disappeared within the first years after birth. However, positron emission tomography has provided evidence that adults retain metabolically active BAT depots that can be induced in response to cold and sympathetic nervous system activation. These findings together with the recent identification of specific molecular determinants (PRDM16 and BMP7) activating brown adipogenesis highlights BAT as a potential relevant target for pharmacological and gene expression manipulation to combat human obesity.

Brown fat: atypical locations and appearances encountered in PET/CT

OBJECTIVE

The purpose of this article is to review the physiology and describe the typical and atypical presentations of brown fat on (18)F-FDG PET.

CONCLUSION

The presence of brown fat on FDG PET has the potential to lead to misinterpretation and unneeded invasive tests, which can be avoided by using measures such as ensuring the patient is warm, reducing FDG uptake in brown fat before the procedure, and correlating PET uptake to a specific anatomic location with PET/CT fusion imaging.

Identification and importance of brown adipose tissue in adult humans

BACKGROUND

Obesity results from an imbalance between energy intake and expenditure. In rodents and newborn humans, brown adipose tissue helps regulate energy expenditure by thermogenesis mediated by the expression of uncoupling protein 1 (UCP1), but brown adipose tissue has been considered to have no physiologic relevance in adult humans.

METHODS

We analyzed 3640 consecutive (18)F-fluorodeoxyglucose ((18)F-FDG) positron-emission tomographic and computed tomographic (PET-CT) scans performed for various diagnostic reasons in 1972 patients for the presence of substantial depots of putative brown adipose tissue. Such depots were defined as collections of tissue that were more than 4 mm in diameter, had the density of adipose tissue according to CT, and had maximal standardized uptake values of (18)F-FDG of at least 2.0 g per milliliter, indicating high metabolic activity. Clinical indexes were recorded and compared with those of date-matched controls. Immunostaining for UCP1 was performed on biopsy specimens from the neck and supraclavicular regions in patients undergoing surgery.

RESULTS

Substantial depots of brown adipose tissue were identified by PET-CT in a region extending from the anterior neck to the thorax. Tissue from this region had UCP1-immunopositive, multilocular adipocytes indicating brown adipose tissue. Positive scans were seen in 76 of 1013 women (7.5%) and 30 of 959 men (3.1%), corresponding to a female:male ratio greater than 2:1 (P<0.001). Women also had a greater mass of brown adipose tissue and higher (18)F-FDG uptake activity. The probability of the detection of brown adipose tissue was inversely correlated with years of age (P<0.001), outdoor temperature at the time of the scan (P=0.02), beta-blocker use (P<0.001), and among older patients, body-mass index (P=0.007).

CONCLUSIONS

Defined regions of functionally active brown adipose tissue are present in adult humans, are more frequent in women than in men, and may be quantified noninvasively with the use of (18)F-FDG PET-CT. Most important, the amount of brown adipose tissue is inversely correlated with body-mass index, especially in older people, suggesting a potential role of brown adipose tissue in adult human metabolism.