MR properties of brown and white adipose tissues

Human Brown Adipose Tissue Temperature and Fat Fraction Are Related to Its Metabolic Activity

BACKGROUND AND AIM

The metabolic activity of human brown adipose tissue (BAT) has been previously examined using positron emission tomography (PET). The aim of this study was to use proton magnetic resonance spectroscopy (1H MRS) to investigate whether the temperature and the fat fraction (FF) of BAT and white adipose tissue (WAT) are associated with BAT metabolic activity determined by deoxy-2-18F-fluoro-d-glucose (18F-FDG)-PET.

MATERIALS AND METHODS

Ten healthy subjects (four women, six men; 25 to 45 years of age) were studied using PET-magnetic resonance imaging during acute cold exposure and at ambient room temperature. BAT and subcutaneous WAT 1H MRS were measured. The tissue temperature and the FF were derived from the spectra. Tissue metabolic activity was studied through glucose uptake using dynamic FDG PET scanning during cold exposure. A 2-hour hyperinsulinemic euglycemic clamp was performed on eight subjects.

RESULTS

The metabolic activity of BAT associated directly with the heat production capacity and inversely with the FF of the tissue. In addition, the lipid-burning capacity of BAT associated with whole-body insulin sensitivity. During cold exposure, the FF of BAT was lower than at room temperature, and cold-induced FF of BAT associated inversely with high-density lipoprotein and directly with low-density lipoprotein cholesterol.

CONCLUSION

Both 1H MRS-derived temperature and FF are promising methods to study BAT activity noninvasively. The association between the lipid-burning capacity of BAT and whole-body insulin sensitivity emphasizes the role of BAT in glucose handling. Furthermore, the relation of FF to high-density lipoprotein and low-density lipoprotein cholesterol suggests that BAT has a role in lipid clearance, thus protecting tissues from excess lipid load.

Water and lipid diffusion MRI using chemical shift displacement-based separation of lipid tissue (SPLIT)

PURPOSE

To obtain water and lipid diffusion-weighted images (DWIs) simultaneously, we devised a novel method utilizing chemical shift displacement-based separation of lipid tissue (SPLIT) imaging.

MATERIALS AND METHODS

Single-shot diffusion echo-planar imaging without fat suppression was used and the imaging parameters were optimized to separate water and lipid DWIs by chemical shift displacement of the lipid signals along the phase-encoding direction. Using the optimized conditions, transverse DWIs at the maximum diameter of the right calf were scanned with multiple b-values in five healthy subjects. Then, apparent diffusion coefficients (ADCs) were calculated in the tibialis anterior muscle (TA), tibialis bone marrow (TB), and subcutaneous fat (SF), as well as restricted and perfusion-related diffusion coefficients (D and D*, respectively) and the fraction of the perfusion-related diffusion component (F) for TA.

RESULTS

Water and lipid DWIs were separated adequately. The mean ADCs of the TA, TB, and SF were 1.56±0.03mm²/s, 0.01±0.01mm²/s, and 0.06±0.02mm²/s, respectively. The mean D*, D, and F of the TA were 13.7±4.3mm²/s, 1.48±0.05mm²/s, and 4.3±1.6%, respectively.

CONCLUSION

SPLIT imaging makes it possible to simply and simultaneously obtain water and lipid DWIs without special pulse sequence and increases the amount of diffusion information of water and lipid tissue.

The Relationship Between Brown Adipose Tissue Content in Supraclavicular Fat Depots and Insulin Sensitivity in Patients with Type 2 Diabetes Mellitus and Prediabetes

BACKGROUND

The evaluation of brown adipose tissue (BAT) and its role in metabolism and obesity remains an important topic in the recent literature. This study evaluated the influence of the BAT triglyceride content measured by proton magnetic resonance (MR) spectroscopy in patients with type 2 diabetes mellitus (DM2) and prediabetes on insulin sensitivity.

METHODS

A total of 25 patients with DM2 and prediabetes (45.9 ± 10.1 years old, body mass index [BMI] of 31.6 ± 5.4 kg/m²) underwent anthropometric measurements (BMI), insulin sensitivity analysis (M value during euglycemic hyperinsulinemic clamp and homeostasis model assessment of insulin resistance), proton MR spectroscopy, and blood tests (total cholesterol, low-density lipoproteins, high-density lipoproteins, and triglycerides). The relationship between the triglyceride content in the supraclavicular fat depot and insulin sensitivity, anthropometric measurements, and blood test results was assessed.

RESULTS

The triglyceride content in the supraclavicular fat depot varied between 79.2% and 97.1% (mean: 92.6% ± 4.2%). The triglyceride content in the subcutaneous white adipose tissue of the neck was significantly higher (85.3%-99.3%; mean: 95.5% ± 2.9%; P = 0.0007). The triglyceride content in the supraclavicular fat depot exhibited a significantly moderate correlation with the BMI (r = 0.64; P = 0.0009). A significant weak negative correlation between the supraclavicular fat content and M value was revealed (r = -0.44; P = 0.002). Patients with high insulin resistance (IR) had a higher triglyceride content in the supraclavicular fat depot than patients with normal and lower IR (94.3% ± 2.0% vs. 90.4% ± 5.2%; P = 0.02).

CONCLUSIONS

Reducing the BAT content in the supraclavicular fat depot can influence the development of IR in patients with DM2 and prediabetes.

Relation between brown adipose tissue and measures of obesity and metabolic dysfunction in patients with cardiovascular disease

PURPOSE

To evaluate whether brown adipose tissue (BAT) is present in middle-aged patients with cardiovascular comorbidities and to quantify how BAT presence associates with obesity and metabolic dysfunction.

MATERIALS AND METHODS

Supraclavicular and subcutaneous adipose tissue fat-signal-fraction (FF) was determined with 1.5T water-fat magnetic resonance imaging (MRI) in 50 patients with coronary artery disease, cerebrovascular disease, or peripheral artery disease. The association between BAT presence, as measured by a higher FF difference between supraclavicular and subcutaneous adipose tissue, and obesity and metabolic dysfunction was quantified using multivariable linear regression.

RESULTS

Supraclavicular adipose tissue displays a lower FF of 82.6% (interquartile range [IQR] 78.8-84.3) compared to 90.2% (IQR 87.3-91.9) in subcutaneous white adipose tissue (WAT, P < 0.0001). BAT presence was associated with less obesity and metabolic dysfunction. For example, 1 SD lower waist circumference (11.7 cm), 1 SD lower triglycerides (1.0 mmol/L), and absence of metabolic syndrome and type 2 diabetes were associated with 1.1% (95% confidence interval [CI] 0.1; 2.0), 1.1% (95% CI 0.1; 2.0), 2.1% (95% CI 0.1; 4.1), and 4.1% (95% CI 0.1; 7.1) higher FF difference between supraclavicular adipose tissue and subcutaneous WAT, respectively.

CONCLUSION

Supraclavicular adipose tissue has BAT characteristics in adult patients with clinical manifest cardiovascular disease and BAT presence is associated with less obesity and a more favorable metabolic profile.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 2 J. MAGN. RESON. IMAGING 2017;46:497-504.

Differentiating brown and white adipose tissues by high-resolution diffusion NMR spectroscopy

There are two types of fat tissues, white adipose tissue (WAT) and brown adipose tissue (BAT), which essentially perform opposite functions in whole body energy metabolism. There is a large interest in identifying novel biophysical properties of WAT and BAT by a quantitative and easy-to-run technique. In this work, we used high-resolution pulsed field gradient diffusion NMR spectroscopy to study the apparent diffusion coefficient (ADC) of fat molecules in rat BAT and WAT samples. The ADC of fat in BAT and WAT from rats fed with a chow diet was compared with that of rats fed with a high-fat diet to monitor how the diffusion properties change due to obesity-associated parameters such as lipid droplet size, fatty acid chain length, and saturation. Feeding a high-fat diet resulted in increased saturation, increased chain lengths, and reduced ADC of fat in WAT. The ADC of fat was lower in BAT relative to WAT in rats fed both chow and high-fat diets. Diffusion of fat was restricted in BAT due to the presence of small multilocular lipid droplets. Our findings indicate that in vivo diffusion might be a potential way for better delineation of BAT and WAT in both lean and obese states.

In vivo triglyceride composition of abdominal adipose tissue measured by 1 H MRS at 3T

PURPOSE

To investigate the regional variability of adipose tissue triglyceride composition in vivo using ¹ H MRS, examining potential confounders and corrections for artifacts, to allow for adipose tissue spectrum estimation.

MATERIALS AND METHODS

¹ H magnetic resonance (MR) stimulated echo acquisition mode (STEAM) spectra were acquired in vivo at 3T from 340 adult patients (mean age 48.9 years, range 21-79 years; 172 males, 168 females; mean body mass index [BMI] 34.0, range 22-49 kg/m² ) with known or suspected nonalcoholic fatty liver disease (NAFLD) in deep (dSCAT), surface (sSCAT) subcutaneous adipose tissue, and visceral adipose tissue (VAT). Triglyceride composition was characterized by the number of double bonds (ndb) and number of methylene-interrupted double bonds (nmidb). A subset of patients (dSCAT n = 80, sSCAT n = 55, VAT n = 194) had the acquisition repeated three times to examine the repeatability of ndb and nmidb estimation.

RESULTS

Mean ndb and nmidb showed significant (P < 0.0001) differences between depots except for dSCAT and sSCAT nmidb (dSCAT ndb 2.797, nmidb 0.745; sSCAT ndb 2.826, nmidb 0.737; VAT ndb 2.723, nmidb 0.687). All ndb and nmidb estimates were highly repeatable (VAT ndb ICC = 0.888, nmidb ICC = 0.853; sSCAT: ndb ICC = 0.974, nmidb ICC = 0.964; dSCAT: ndb ICC = 0.959, nmidb ICC = 0.948).

CONCLUSION

Adipose tissue composition can be estimated repeatably using ¹ H MRS and different fat depots have different triglyceride compositions.

LEVEL OF EVIDENCE

2 J. MAGN. RESON. IMAGING 2017;45:1455-1463.

Thermogenic profiling using magnetic resonance imaging of dermal and other adipose tissues

Dermal white adipose tissue (dWAT) was recently recognized for its potential to modify whole body metabolism. Here, we show that dWAT can be quantified using a high-resolution, fat-specific magnetic resonance imaging (MRI) technique. Noninvasive MRI has been used to describe adipocyte depots for many years; the MRI technique we describe uses an advanced fat-specific method to measure the thickness of dWAT, together with the total volume of WAT and the relative activation/fat depletion of brown adipose tissues (BAT). Since skin-embedded adipocytes may provide natural insulation, they provide an important counterpoint to the activation of thermogenic brown and beige adipose tissues, whereby these distinct depots are functionally interrelated and require simultaneous assay. This method was validated using characterized mouse cohorts of a lipodystrophic, dWAT-deficient strain (syndecan-1 KO) and 2 obese models (diet-induced obese mice and genetically obese animals, ob/ob). Using a preliminary cohort of normal human subjects, we found the thickness of skin-associated fat varied 8-fold, from 0.13-1.10 cm; on average, this depot is calculated to weigh 8.8 kg.

Characterizing active and inactive brown adipose tissue in adult humans using PET-CT and MR imaging

Activated brown adipose tissue (BAT) plays an important role in thermogenesis and whole body metabolism in mammals. Positron emission tomography (PET)-computed tomography (CT) imaging has identified depots of BAT in adult humans, igniting scientific interest. The purpose of this study is to characterize both active and inactive supraclavicular BAT in adults and compare the values to those of subcutaneous white adipose tissue (WAT). We obtained [(18)F]fluorodeoxyglucose ([(18)F]FDG) PET-CT and magnetic resonance imaging (MRI) scans of 25 healthy adults. Unlike [(18)F]FDG PET, which can detect only active BAT, MRI is capable of detecting both active and inactive BAT. The MRI-derived fat signal fraction (FSF) of active BAT was significantly lower than that of inactive BAT (means ± SD; 60.2 ± 7.6 vs. 62.4 ± 6.8%, respectively). This change in tissue morphology was also reflected as a significant increase in Hounsfield units (HU; -69.4 ± 11.5 vs. -74.5 ± 9.7 HU, respectively). Additionally, the CT HU, MRI FSF, and MRI R2* values are significantly different between BAT and WAT, regardless of the activation status of BAT. To the best of our knowledge, this is the first study to quantify PET-CT and MRI FSF measurements and utilize a semiautomated algorithm to identify inactive and active BAT in the same adult subjects. Our findings support the use of these metrics to characterize and distinguish between BAT and WAT and lay the foundation for future MRI analysis with the hope that some day MRI-based delineation of BAT can stand on its own.

Separation and quantification of lactate and lipid at 1.3 ppm by diffusion-weighted magnetic resonance spectroscopy

PURPOSE

To separate the spectrally overlapped lactate and lipid signals at 1.3 ppm using diffusion-weighted magnetic resonance spectroscopy (DW-MRS) based on their large diffusivity difference.

METHODS

DW-MRS was applied to the gel phantoms containing lactate and lipid droplets, and to the rat brain tumors. Lactate and lipid signals and their apparent diffusion coefficients were computed from the diffusion-weighted proton spectra. Biexponential fitting and direct spectral subtraction approaches were employed and compared.

RESULTS

DW-MRS could effectively separate lactate and lipid signals both in phantoms and rat brain C6 glioma by biexponential fitting. In phantoms, lactate and lipid signals highly correlated with the known lactate concentration and lipid volume fractions. In C6 glioma, both lactate and lipid signals were detected, and the lipid signal was an order of magnitude higher than lactate signal. The spectral subtraction approach using three diffusion weightings also allowed the separation of lactate and lipid signals, yielding results comparable to those by the biexponential fitting approach.

CONCLUSION

DW-MRS presents a new approach to separate and quantify spectrally overlapped molecules and/or macromolecules, such as lactate and lipid, by using the diffusivity difference associated with their different sizes or mobility within tissue microstructure. Magn Reson Med 77:480-489, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Reliability and agreement of adipose tissue fat fraction measurements with water-fat MRI in patients with manifest cardiovascular disease

The supraclavicular fat depot is known for brown adipose tissue presence. To unravel adipose tissue physiology and metabolism, high quality and reproducible imaging is required. In this study we quantified the reliability and agreement of MRI fat fraction measurements in supraclavicular and subcutaneous adipose tissue of 25 adult patients with clinically manifest cardiovascular disease. MRI fat fraction measurements were made under ambient temperature conditions using a vendor supplied mDixon chemical-shift water-fat multi-echo pulse sequence at 1.5 T field strength. Supraclavicular fat fraction reliability (intraclass correlation coefficientagreement , ICCagreement ) was 0.97 for test-retest, 0.95 for intra-observer and 0.56 for inter-observer measurements, which increased to 0.88 when ICCconsistency was estimated. Supraclavicular fat fraction agreement displayed mean differences of 0.5% (limit of agreement (LoA) -1.7 to 2.6) for test-retest, -0.5% (LoA -2.9 to 2.0) for intra-observer and 5.6% (LoA 0.4 to 10.8) for inter-observer measurements. Median fat fraction in supraclavicular adipose tissue was 82.5% (interquartile range (IQR) 78.6-84.0) and 89.7% (IQR 87.2-91.5) in subcutaneous adipose tissue (p < 0.0001). In conclusion, water-fat MRI has good reliability and agreement to measure adipose tissue fat fraction in patients with manifest cardiovascular disease. These findings enable research on determinants of fat fraction and enable longitudinal monitoring of fat fraction within adipose tissue depots. Interestingly, even in adult patients with manifest cardiovascular disease, supraclavicular adipose tissue has a lower fat fraction compared with subcutaneous adipose tissue, suggestive of distinct morphologic characteristics, such as brown adipose tissue.

UTE imaging with simultaneous water and fat signal suppression using a time-efficient multispoke inversion recovery pulse sequence

PURPOSE

The long repetition time and inversion time with inversion recovery preparation ultrashort echo time (UTE) often causes prohibitively long scan times. We present an optimized method for long T2 signal suppression in which several k-space spokes are acquired after each inversion preparation.

THEORY AND METHODS

Using Bloch equations the sequence parameters such as TI and flip angle were optimized to suppress the long T2 water and fat signals and to maximize short T2 contrast. Volunteer imaging was performed on a healthy male volunteer. Inversion recovery preparation was performed using a Silver-Hoult adiabatic inversion pulse together with a three-dimensional (3D) UTE (3D Cones) acquisition.

RESULTS

The theoretical signal curves generally agreed with the experimentally measured region of interest curves. The multispoke inversion recovery method showed good muscle and fatty bone marrow suppression, and highlighted short T2 signals such as these from the femoral and tibial cortex.

CONCLUSION

Inversion recovery 3D UTE imaging with multiple spoke acquisitions can be used to effectively suppress long T2 signals and highlight short T2 signals within clinical scan times. Theoretical modeling can be used to determine sequence parameters to optimize long T2 signal suppression and maximize short T2 signals. Experimental results on a volunteer confirmed the theoretical predictions. Magn Reson Med 76:577-582, 2016. © 2015 Wiley Periodicals, Inc.

Brown adipose tissue triglyceride content is associated with decreased insulin sensitivity, independently of age and obesity

The aim of the present study was to determine whether single-voxel proton magnetic resonance spectroscopy ((1)H-MRS) can non-invasively assess triglyceride content in both supraclavicular fat depots and subcutaneous white adipose tissue (WAT) to determine whether these measurements correlate to metabolic variables. A total of 25 healthy volunteers were studied using (18)F-fluorodeoxyglucose positron emission tomography (PET) and (15)O-H2O PET perfusion during cold exposure, and (1)H-MRS at ambient temperature. Image-guided biopsies were collected from nine volunteers. The supraclavicular triglyceride content determined by (1)H-MRS varied between 60 and 91% [mean ± standard deviation (s.d.) 77 ± 10%]. It correlated positively with body mass index, waist circumference, subcutaneous and visceral fat masses and 8-year diabetes risk based on the Framingham risk score and inversely with HDL cholesterol and insulin sensitivity (M-value; euglycaemic-hyperinsulinaemic clamp). Subcutaneous WAT had a significantly higher triglyceride content, 76-95% (mean ± s.d. 87 ± 5%; p = 0.0002). In conclusion, the triglyceride content in supraclavicular fat deposits measured by (1)H-MRS may be an independent marker of whole-body insulin sensitivity, independent of brown adipose tissue metabolic activation.

Magnetic Resonance Imaging of Human Tissue-Engineered Adipose Substitutes

Adipose tissue (AT) substitutes are being developed to answer the strong demand in reconstructive surgery. To facilitate the validation of their functional performance in vivo, and to avoid resorting to excessive number of animals, it is crucial at this stage to develop biomedical imaging methodologies, enabling the follow-up of reconstructed AT substitutes. Until now, biomedical imaging of AT substitutes has scarcely been reported in the literature. Therefore, the optimal parameters enabling good resolution, appropriate contrast, and graft delineation, as well as blood perfusion validation, must be studied and reported. In this study, human adipose substitutes produced from adipose-derived stem/stromal cells using the self-assembly approach of tissue engineering were implanted into athymic mice. The fate of the reconstructed AT substitutes implanted in vivo was successfully followed by magnetic resonance imaging (MRI), which is the imaging modality of choice for visualizing soft ATs. T1-weighted images allowed clear delineation of the grafts, followed by volume integration. The magnetic resonance (MR) signal of reconstructed AT was studied in vitro by proton nuclear magnetic resonance ((1)H-NMR). This confirmed the presence of a strong triglyceride peak of short longitudinal proton relaxation time (T1) values (200 ± 53 ms) in reconstructed AT substitutes (total T1=813 ± 76 ms), which establishes a clear signal difference between adjacent muscle, connective tissue, and native fat (total T1 ~300 ms). Graft volume retention was followed up to 6 weeks after implantation, revealing a gradual resorption rate averaging at 44% of initial substitute's volume. In addition, vascular perfusion measured by dynamic contrast-enhanced-MRI confirmed the graft's vascularization postimplantation (14 and 21 days after grafting). Histological analysis of the grafted tissues revealed the persistence of numerous adipocytes without evidence of cysts or tissue necrosis. This study describes the in vivo grafting of human adipose substitutes devoid of exogenous matrix components, and for the first time, the optimal parameters necessary to achieve efficient MRI visualization of grafted tissue-engineered adipose substitutes.

Evaluating invasive and non-invasive methods to determine fat content in the laboratory mouse

In the midst of an obesity epidemic in humans, diet induced obesity studies in rodents are fundamental to unravel the complex mechanisms underlying this disease, ultimately resulting in the identification of new preventative and therapeutic strategies. The current study was designed to determine if high throughput multiobject CT scanning was capable of providing precise quantification of adipose tissue in C57BL/6 mice when benchmarked to the gold standard method for evaluating fat mass (freeze drying). We report a strong correlation between body weight alone and fat percentage in our mouse cohort (20 g-40 g, r = 0.95). The gonadal fat depot was identified as the most accurate single predictor of total fat mass (r = 0.931). Importantly, we observed a high positive correlation between both live tissue weight and dissected adipose tissue when correlated to CT predictions (r ≥ 0.862), suggesting CT can accurately be used to predict total fat mass/percentage and non-fat mass/percentage in our cohort. We conclude that the use of multi-object in vivo CT fat quantification is cost effective, accurate and minimally invasive technique in the genetic manipulation era to exploit lean/obese genes in the study of diet induced obesity, allowing longitudinal studies to be completed in a high throughput manner.

Insights into Brown Adipose Tissue Physiology as Revealed by Imaging Studies

There has been resurgence in interest in brown adipose tissue (BAT) following radiological and histological identification of metabolically active BAT in adult humans. Imaging enables BAT to be studied non-invasively and therefore imaging studies have contributed a significant amount to what is known about BAT function in humans. In this review the current knowledge (derived from imaging studies) about the prevalence, function, activity and regulation of BAT in humans (as well as relevant rodent studies), will be summarized.

Cold-activated brown adipose tissue in human adults: methodological issues

The relevance of functional brown adipose tissue (BAT) depots in human adults was undisputedly proven approximately seven years ago. Here we give an overview of all dedicated studies that were published on cold-induced BAT activity in adult humans that appeared since then. Different cooling protocols and imaging techniques to determine BAT activity are reviewed. BAT activation can be achieved by means of air- or water-cooling protocols. The most promising approach is individualized cooling, during which subjects are studied at the lowest temperature for nonshivering condition, probably revealing maximal nonshivering thermogenesis. The highest BAT prevalence (i.e., close to 100%) is observed using the individualized cooling protocol. Currently, the most widely used technique to study the metabolic activity of BAT is deoxy-2-[18F]fluoro-d-glucose ([18F]FDG)-positron emission tomography/computed tomography (PET/CT) imaging. Dynamic imaging provides quantitative information about glucose uptake rates, whereas static imaging reflects overall BAT glucose uptake, localization, and distribution. In general, standardized uptake values (SUV) are used to quantify BAT activity. An accurate determination of total BAT volume is hampered by the limited spatial resolution of the PET image, leading to spillover. Different research groups use different SUV threshold values, which make it difficult to directly compare BAT activity levels between studies. Another issue is the comparison of [18F]FDG uptake in BAT with respect to other tissues or upon with baseline values. This comparison can be performed by using the “fixed volume” methodology. Finally, the potential use of other relatively noninvasive methods to quantify BAT, like magnetic resonance imaging or thermography, is discussed.

Brown fat in humans: consensus points and experimental guidelines

As part of a current worldwide effort to understand the physiology of human BAT (hBAT) and whether its thermogenic activity can be manipulated to treat obesity, the workshop "Exploring the Roles of Brown Fat in Humans" was convened at the National Institutes of Health on February 25-26, 2014. Presentations and discussion indicated that hBAT and its physiological roles are highly complex, and research is needed to understand the health impact of hBAT beyond thermogenesis and body weight regulation, and to define its interactions with core physiological processes like glucose homeostasis, cachexia, physical activity, bone structure, sleep, and circadian rhythms.

Presence of brown adipose tissue in an adolescent with severe primary hypothyroidism

CONTEXT

Brown adipose tissue (BAT) generates heat during adaptive thermogenesis in response to cold temperature. Thyroid hormone (TH) receptors, type 2 deiodinase, and TSH receptors are present on brown adipocytes, indicating that the thyroid axis regulates BAT. It is unknown whether absent TH in humans would down-regulate development of BAT and its thermogenic function.

OBJECTIVE

The objective of the study was to examine BAT by magnetic resonance imaging (MRI) and infrared thermal imaging (IRT) in a pediatric patient with severe primary hypothyroidism before and after TH treatment.

DESIGN/SETTING

This study was a case report with longitudinal follow-up in a tertiary center.

MAIN OUTCOME MEASURES

BAT fat fraction (FF) by MRI and skin temperature by IRT were measured.

RESULTS

An 11.5-year-old female was severely hypothyroid (TSH, 989 μIU/mL; free T4, 0.10 ng/dL; low thyroglobulin, 3.0 ng/mL). Low MRI measures of FF (56.1% ± 3.7%) indicated that BAT was abundantly present in the supraclavicular fossa. IRT showed higher supraclavicular temperature (36.0°C ±0.16°C) than the suprasternal area (34.3°C ± 0.19°C). After 2 months of TH replacement, she was euthyroid (TSH, 4.3 μIU/mL; free T4, 1.49 ng/dL; T3, 102 ng/dL) at which time supraclavicular BAT decreased (increased FF 60.7% ± 3.8%). IRT showed a higher, more homogeneous skin temperature throughout the upper thorax (supraclavicular, 37.1°C ± 0.23°C; suprasternal, 36.4°C ± 0.13°C). The overall size of the supraclavicular fat depot decreased from 84.79 cm(3) to 41.21 cm(3).

CONCLUSIONS

These findings document the presence of BAT and thermogenesis in profound hypothyroidism and suggest a role for TSH and/or TRH as a potential regulator of BAT.

Two-point magnitude MRI for rapid mapping of brown adipose tissue and its application to the R6/2 mouse model of Huntington disease

The recent discovery of active brown fat in human adults has led to renewed interest in the role of this key metabolic tissue. This is particularly true for neurodegenerative conditions like Huntington disease (HD), an adult-onset heritable disorder with a prominent energy deficit phenotype. Current methods for imaging brown adipose tissue (BAT) are in limited use because they are equipment-wise demanding and often prohibitively expensive. This prompted us to explore how a standard MRI set-up can be modified to visualize BAT in situ by taking advantage of its characteristic fat/water content ratio to differentiate it from surrounding white fat. We present a modified MRI protocol for use on an 11.7 T small animal MRI scanner to visualize and quantify BAT in wild-type and disease model laboratory mice. In this application study using the R6/2 transgenic mouse model of HD we demonstrate a significantly reduced BAT volume in HD mice vs. matched controls (n = 5 per group). This finding provides a plausible structural explanation for the previously described temperature phenotype of HD mice and underscores the significance of peripheral tissue pathology for the HD phenotype. On a more general level, the results demonstrate the feasibility of MR-based BAT imaging in rodents and open the path towards transferring this imaging approach to human patients. Future studies are needed to determine if this method can be used to track disease progression in HD and other disease entities associated with BAT abnormalities, including metabolic conditions such as obesity, cachexia, and diabetes.

Fat and iron quantification in the liver: past, present, and future

Liver fat, iron, and combined overload are common manifestations of diffuse liver disease and may cause lipotoxicity and iron toxicity via oxidative hepatocellular injury, leading to progressive fibrosis, cirrhosis, and eventually, liver failure. Intracellular fat and iron cause characteristic changes in the tissue magnetic properties in predictable dose-dependent manners. Using dedicated magnetic resonance pulse sequences and postprocessing algorithms, fat and iron can be objectively quantified on a continuous scale. In this article, we will describe the basic physical principles of magnetic resonance fat and iron quantification and review the imaging techniques of the "past, present, and future." Standardized radiological metrics of fat and iron are introduced for numerical reporting of overload severity, which can be used toward objective diagnosis, grading, and longitudinal disease monitoring. These noninvasive imaging techniques serve an alternative or complimentary role to invasive liver biopsy. Commercial solutions are increasingly available, and liver fat and iron quantitative imaging is now within reach for routine clinical use and may soon become standard of care.

Brown adipose tissue in the buccal fat pad during infancy

BACKGROUND

The buccal fat pad (BFP) is an encapsulated mass of adipose tissue thought to enhance the sucking capabilities of the masticatory muscles during infancy. To date, no conclusive evidence has been provided as to the composition of the BFP in early postnatal life.

OBJECTIVE

The purpose of this study was to examine whether the BFP of neonates and infants is primarily composed of white adipose tissue (WAT) or brown adipose tissue (BAT).

MATERIALS AND METHODS

The percentage of fat in the BFP in 32 full-term infants (16 boys and 16 girls), aged one day to 10.6 months, was measured using magnetic resonance imaging (MRI) determinations of fat fraction.

RESULTS

BFP fat fraction increased with age (r = 0.67; P<.0001) and neonates had significantly lower values when compared to older infants; 72.6 ± 9.6 vs. 91.8 ± 2.4, P<.0001. Multiple regression analysis indicated that the age-dependent relationship persisted after accounting for gender, gestational age, and weight percentile (P = .001). Two subjects (aged one and six days) depicted a change in the MRI characteristics of the BFP from primarily BAT to WAT at follow-up examinations two to six weeks later, respectively. Histological post-mortem studies of a 3 day and 1.1 month old revealed predominantly BAT and WAT in the BFP, respectively.

CONCLUSION

The BFP is primarily composed of BAT during the first weeks of life, but of WAT thereafter. Studies are needed to investigate the contributions of BAT in the BFP to infant feeding and how it is altered by postnatal nutrition.

Relaxation effects in MRI-based quantification of fat content and fatty acid composition

PURPOSE

To investigate various sources of bias in MRI-based quantification of fat fraction (FF) and fatty acid composition (FAC) using chemical shift-encoded techniques.

METHODS

Signals from various FFs and FACs and individual relaxation rates of all signal components were simulated. From these signals, FF and FAC parameters were estimated with and without correction for differences in individual relaxation rates. In addition, phantom experiments were conducted with various flip angles and number of echoes to validate the simulations.

RESULTS

As expected, T(1) weighting resulted in an overestimation of the FF, but had much smaller impact on the FAC parameters. Differences in T(2) values of the signal components resulted in overestimation of the FAC parameters in fat/water mixtures, whereas the estimation in pure oil was largely unaffected. This bias was corrected using a simplified signal model with different T(2) values of water and fat, where the accuracy of the modeled T(2) of water was critical. The results of the phantom experiment were in agreement with simulations.

CONCLUSION

T(1) weighting has only a minor effect on FAC quantification in both fat/water mixtures and pure oils. T(2) weighting is mainly a concern in fat/water mixtures but may be corrected using a simplified model.

Brown adipose tissue mapping in rats with combined intermolecular double-quantum coherence and Dixon water-fat MRI

Brown adipose tissue (BAT) is a promising therapeutic target in obesity studies. Recently, MRI has been proposed for the mapping of BAT. However, because of the limitation of spatial resolution, similar to the existing positron emission tomography and computed tomography techniques for BAT detection, it fails to distinguish BAT cells when they are mixed with other cells. In this work, a new MRI method is proposed, combining intermolecular double-quantum coherence and the chemical shift-encoded Dixon method. Its contrast depends on the water to fat ratio at the cellular scale, which is smaller than the imaging voxel size. The feasibility of this MRI method was shown with computer simulations and phantoms, and preliminary imaging of BAT of rats at 7 T. Both computer simulations and experimental results are consistent with theoretical predictions. The method provides a novel contrast mechanism and can map BAT distribution exclusively. In particular, a mixture of BAT cells and white adipose tissue cells was detected in an older rat, which was undetectable by other noninvasive methods. This method may be applicable to a wide range of uses in BAT-related studies, including the formation and variation of BAT.

Quantitative proton MR techniques for measuring fat

Accurate, precise and reliable techniques for the quantification of body and organ fat distributions are important tools in physiology research. They are critically needed in studies of obesity and diseases involving excess fat accumulation. Proton MR methods address this need by providing an array of relaxometry-based (T1, T2) and chemical shift-based approaches. These techniques can generate informative visualizations of regional and whole-body fat distributions, yield measurements of fat volumes within specific body depots and quantify fat accumulation in abdominal organs and muscles. MR methods are commonly used to investigate the role of fat in nutrition and metabolism, to measure the efficacy of short- and long-term dietary and exercise interventions, to study the implications of fat in organ steatosis and muscular dystrophies and to elucidate pathophysiological mechanisms in the context of obesity and its comorbidities. The purpose of this review is to provide a summary of mainstream MR strategies for fat quantification. The article succinctly describes the principles that differentiate water and fat proton signals, summarizes the advantages and limitations of various techniques and offers a few illustrative examples. The article also highlights recent efforts in the MR of brown adipose tissue and concludes by briefly discussing some future research directions.

Brown adipose tissue quantification in human neonates using water-fat separated MRI

There is a major resurgence of interest in brown adipose tissue (BAT) biology, particularly regarding its determinants and consequences in newborns and infants. Reliable methods for non-invasive BAT measurement in human infants have yet to be demonstrated. The current study first validates methods for quantitative BAT imaging of rodents post mortem followed by BAT excision and re-imaging of excised tissues. Identical methods are then employed in a cohort of in vivo infants to establish the reliability of these measures and provide normative statistics for BAT depot volume and fat fraction. Using multi-echo water-fat MRI, fat- and water-based images of rodents and neonates were acquired and ratios of fat to the combined signal from fat and water (fat signal fraction) were calculated. Neonatal scans (n = 22) were acquired during natural sleep to quantify BAT and WAT deposits for depot volume and fat fraction. Acquisition repeatability was assessed based on multiple scans from the same neonate. Intra- and inter-rater measures of reliability in regional BAT depot volume and fat fraction quantification were determined based on multiple segmentations by two raters. Rodent BAT was characterized as having significantly higher water content than WAT in both in situ as well as ex vivo imaging assessments. Human neonate deposits indicative of bilateral BAT in spinal, supraclavicular and axillary regions were observed. Pairwise, WAT fat fraction was significantly greater than BAT fat fraction throughout the sample (ΔWAT-BAT = 38 %, p<10(-4)). Repeated scans demonstrated a high voxelwise correlation for fat fraction (Rall = 0.99). BAT depot volume and fat fraction measurements showed high intra-rater (ICCBAT,VOL = 0.93, ICCBAT,FF = 0.93) and inter-rater reliability (ICCBAT,VOL = 0.86, ICCBAT,FF = 0.93). This study demonstrates the reliability of using multi-echo water-fat MRI in human neonates for quantification throughout the torso of BAT depot volume and fat fraction measurements.

MRI detection of brown adipose tissue with low fat content in newborns with hypothermia

PURPOSE

To report the observation of brown adipose tissue (BAT) with low fat content in neonates with hypoxic-ischemic encephalopathy (HIE) after they have undergone hypothermia therapy.

MATERIALS AND METHODS

The local ethics committee approved the imaging study. Ten HIE neonates (3 males, 7 females, age range: 2-3days) were studied on a 3-T MRI system using a low-flip-angle (3°) six-echo proton-density-weighted chemical-shift-encoded water-fat pulse sequence. Fat-signal fraction (FF) measurements of supraclavicular and interscapular (nape) BAT and adjacent subcutaneous white adipose tissues (WAT) were compared to those from five non-HIE neonates, two recruited for the present investigation and three from a previous study.

RESULTS

In HIE neonates, the FF range for the supraclavicular, interscapular, and subcutaneous regions was 10.3%-29.9%, 28.0%-57.9%, and 62.6%-88.0%, respectively. In non-HIE neonates, the values were 23.7%-42.2% (p=0.01), 45.4%-59.5% (p=0.06), and 67.8%-86.3% (p=0.38), respectively. On an individual basis, supraclavicular BAT FF was consistently the lowest, interscapular BAT values were higher, and subcutaneous WAT values were the highest (p<0.01).

CONCLUSION

We speculate that hypothermia therapy in HIE neonates likely promotes BAT-mediated non-shivering thermogenesis, which subsequently leads to a depletion of the tissue's intracellular fat stores. We believe that this is consequently reflected in lower FF values, particularly in the supraclavicular BAT depot, in contrast to non-HIE neonates.

Measurement of brown adipose tissue mass using a novel dual-echo magnetic resonance imaging approach: a validation study

OBJECTIVE

The aim of this study was to evaluate and validate magnetic resonance imaging (MRI) for the visualization and quantification of brown adipose tissue (BAT) in vivo in a rat model. We hypothesized that, based on differences in tissue water and lipid content, MRI could reliably differentiate between BAT and white adipose tissue (WAT) and could therefore be a possible alternative for (18)F-Fluorodeoxyglucose Positron Emission Tomography ((18)FDG-PET), the current gold standard for non-invasive BAT quantification.

MATERIALS/METHODS

Eleven rats were studied using both (18)FDG-PET/CT and MRI (1.5 T). A dual echo (in-and-out-of-phase) sequence was used, both with and without spectral presaturation inversion recovery (SPIR) fat suppression (DUAL-SPIR) to visualize BAT, after which all BAT was surgically excised. The BAT volume measurements obtained via (18)FDG-PET/CT and DUAL-SPIR MR were quantitatively compared with the histological findings. All study protocols were reviewed and approved by the local ethics committee.

RESULTS

The BAT mass measurements that were obtained using DUAL-SPIR MR subtraction images correlated better with the histological findings (P=0.017, R=0.89) than did the measurements obtained using (18)FDG-PET/CT (P=0.78, R=0.15), regardless of the BAT metabolic activation state. Additionally, the basic feasibility of the DUAL-SPIR method was demonstrated in three human pilot subjects.

CONCLUSIONS

This study demonstrates the potential for MRI to reliably detect and quantify BAT in vivo. MRI can provide information beyond that provided by (18)FDG-PET imaging, and its ability to detect BAT is independent of its metabolic activation state. Additionally, MRI is a low-cost alternative that does not require radiation.

Measurement of human brown adipose tissue volume and activity using anatomic MR imaging and functional MR imaging

The aim of this study was to assess the volume and function of human brown adipose tissue (BAT) in vivo using MR imaging.

METHODS

BAT volumes under thermoneutral conditions in the cervical areas were assessed via water-fat contrast using the Dixon method and via water-saturation efficiency using fast spin-echo and T2-weighted images. The existence of cervical BAT was also assessed by (18)F-FDG PET/CT scans in the same subjects. BAT functionality was assessed via functional MR imaging (fMRI) blood oxygenation level-dependent (BOLD) signal changes in response to a mild cold challenge.

RESULTS

Under thermoneutral conditions, we were able to distinguish BAT from white adipose tissue in the cervical and supraclavicular fat. BAT showed higher water-to-fat contrast and higher water-saturation efficiency in MR imaging scans. The location and volume of BAT assessed by MR imaging were comparable to the measurements by (18)F-FDG PET/CT scans. During mild cold challenge, BOLD fMRI signal increased in BAT by 10.7% ± 1.8% (P < 0.01).

CONCLUSION

We demonstrated the feasibility of using MR imaging and fMRI to assess BAT volume and BAT responses to mild cold stimulation in the cervical areas of human subjects.

Measurement of interscapular brown adipose tissue of mice in differentially housed temperatures by chemical-shift-encoded water-fat MRI

PURPOSE

To determine differences in fat-signal fraction (FF) from chemical-shift-encoded water-fat MRI of interscapular BAT in mice housed at different ambient temperatures (Ta ).

MATERIALS AND METHODS

C57BL/6J male mice (8 weeks old) were singly housed at 16°C, 23°C, or 30°C (n = 16/group) for 4 weeks. Measures included food intake, body weight (both measured weekly) and body composition (at baseline, 2, and 4 weeks post-thermal exposure); chemical-shift-encoded water-fat MRI was performed on a 9.4 Tesla Bruker magnet with respiratory gating and anesthesia at 4 weeks post-thermal exposure.

RESULTS

A significant inverse relationship between food intake and Ta was evidenced (P < 0.0001). Lean mass was similar among groups, while total fat mass was significantly different among groups ([mean ± SE]: 30°C = 5.10 ± 0.19 g; 23°C = 4.18 ± 0.16 g; 16°C = 3.48 ± 0.54 g; P < 0.0001). Mean BAT-FF was positively related to Ta (means: 30°C = 79.4%; 23°C = 61.8%; 16°C = 50.9%; P < 0.0001).

CONCLUSION

These cross-sectional results demonstrate that MRI measurement of FF within the interscapular BAT in mice reflects recent functional status of the tissue, with a lower Ta leading to a significantly reduced BAT-FF, indicative of the tissue's involvement in thermogenesis.

Comparison of brown and white adipose tissues in infants and children with chemical-shift-encoded water-fat MRI

PURPOSE

To compare fat-signal fractions (FFs) and T2* values between brown (BAT) and white (WAT) adipose tissue located within the supraclavicular fossa and subcutaneous depots, respectively.

MATERIALS AND METHODS

Twelve infants and 39 children were studied. Children were divided into lean and overweight/obese subgroups. Chemical-shift-encoded water-fat magnetic resonance imaging (MRI) was used to quantify FFs and T2* metrics in the supraclavicular and adjacent subcutaneous adipose tissue depots. Linear regression and t-tests were performed.

RESULTS

Infants had lower supraclavicular FFs than children (P < 0.01) but T2* values were similar (P = 0.5). Lean children exhibited lower supraclavicular FFs and T2* values than overweight children (P < 0.01). In each individual infant and child, supraclavicular FFs were consistently lower than adjacent subcutaneous FFs. Supraclavicular T2* values were consistently lower than subcutaneous T2* values in children, but not in infants. FFs in both depots were positively correlated with age and weight in infants (P < 0.01). In children, they were correlated with weight and body mass index (BMI) (P < 0.01), but not age. Correlations between T2* and anthropometric variables existed in children (P < 0.01), but were absent in infants.

CONCLUSION

Cross-sectional comparisons suggest variations in FF and T2* values in the supraclavicular and subcutaneous depots of infants and children, which are potentially indicative of physiological differences in adipose tissue fat content, amount, and metabolic activity.

Characterization of human brown adipose tissue by chemical-shift water-fat MRI

OBJECTIVE

The purpose of this study was to characterize human brown adipose tissue (BAT) with chemical-shift water-fat MRI and to determine whether trends and differences in fat-signal fractions and T2(*) relaxation times between BAT and white adipose tissue (WAT) are consistently observed postmortem and in vivo in infants, adolescents, and adults.

MATERIALS AND METHODS

A postmortem body and eight patients were studied. A six-echo spoiled gradient-echo chemical-shift water-fat MRI sequence was performed at 3 T to jointly quantify fat-signal fraction and T2(*) in interscapular-supraclavicular BAT and subcutaneous WAT. To confirm BAT identity, biopsy and histology served as the reference in the postmortem study and PET/CT was used in five of the eight patients who required examination for medical care.

RESULTS

Fat-signal fractions and T2(*) times were lower in BAT than in WAT in the postmortem example and in seven of eight patients. With the exception of one case, nominal comparisons between brown and white adipose tissues were statistically significant (p < 0.05). Between subjects, a large range of fat-signal fraction values was observed in BAT but not in WAT.

CONCLUSION

We have shown that fat-signal fractions and T2(*) values jointly derived from chemical-shift water-fat MRI are lower in BAT than in WAT likely because of differences in cellular structures, triglyceride content, and vascularization. The two metrics can serve as complementary biomarkers in the detection of BAT.

Adiposity in children and adolescents: correlates and clinical consequences of fat stored in specific body depots

The 2011 Pennington Biomedical Research Center's Scientific Symposium focused on adiposity in children and adolescents. The symposium was attended by 15 speakers and other invited experts. The specific objectives of the symposium were to (i) integrate the latest published and unpublished findings on the laboratory and clinical assessment of depot-specific adiposity in children and adolescents, (ii) understand the variation in depot-specific adiposity and related health outcomes associated with age, sex, maturation, ethnicity and other factors and (iii) identify opportunities for incorporating new markers of abdominal obesity into clinical practice guidelines for obesity in children and adolescents. This symposium provided an overview of important new advances in the field and identified directions for future research. The long-term goal of the symposium is to aid in the early identification of children and adolescents who are at increased health risk because of obesity and obesity-related conditions.

Anatomical and functional assessment of brown adipose tissue by magnetic resonance imaging

Brown adipose tissue (BAT) is the primary tissue responsible for nonshivering thermogenesis in mammals. The amount of BAT and its level of activation help regulate the utilization of excessive calories for thermogenesis as opposed to storage in white adipose tissue (WAT) which would lead to weight gain. Over the past several years, BAT activity in vivo has been primarily assessed by positron emission tomography-computed tomography (PET-CT) scan using 2-[18F]-fluoro-2-deoxy-D-glucose (18F-FDG) to measure glucose utilization associated with BAT mitochondrial respiration. In this study, we demonstrate the feasibility of mapping and estimating BAT volume and metabolic function in vivo in rats at a 9.4T magnetic resonance imaging (MRI) scanner using sequences available from clinical MR scanners. Based on the morphological characteristics of BAT, we measured the volume distribution of BAT with MRI sequences that have strong fat-water contrast. We also investigated BAT volume by utilizing spin-echo MRI sequences. The in vivo MRI-estimated BAT volumes were correlated with direct measurement of BAT mass from dissected samples. Using MRI, we also were able to map hemodynamic responses to changes in BAT metabolism induced pharmacologically by β3-adrenergic receptor agonist, CL-316,243 and compare this to BAT activity in response to CL-316,243 assessed by PET 18F-FDG. In conclusion, we demonstrate the feasibility of measuring BAT volume and function in vivo using routine MRI sequences. The MRI measurement of BAT volume is consistent with quantitative measurement of the tissue ex vivo.

How to diagnose a lipodystrophy syndrome

The spectrum of adipose tissue diseases ranges from obesity to lipodystrophy, and is accompanied by insulin resistance syndrome, which promotes the occurrence of type 2 diabetes, dyslipidemia and cardiovascular complications. Lipodystrophy refers to a group of rare diseases characterized by the generalized or partial absence of adipose tissue, and occurs with or without hypertrophy of adipose tissue in other sites. They are classified as being familial or acquired, and generalized or partial. The genetically determined partial forms usually occur as Dunnigan syndrome, which is a type of laminopathy that can also manifest as muscle, cardiac, neuropathic or progeroid involvement. Gene mutations encoding for PPAR-gamma, Akt2, CIDEC, perilipin and the ZMPSTE 24 enzyme are much more rare. The genetically determined generalized forms are also very rare and are linked to mutations of seipin AGPAT2, FBN1, which is accompanied by Marfan syndrome, or of BANF1, which is characterized by a progeroid syndrome without insulin resistance and with early bone complications. Glycosylation disorders are sometimes involved. Some genetically determined forms have recently been found to be due to autoinflammatory syndromes linked to a proteasome anomaly (PSMB8). They result in a lipodystrophy syndrome that occurs secondarily with fever, dermatosis and panniculitis. Then there are forms that are considered to be acquired. They may be iatrogenic (protease inhibitors in HIV patients, glucocorticosteroids, insulin, graft-versus-host disease, etc.), related to an immune system disease (sequelae of dermatopolymyositis, autoimmune polyendocrine syndromes, particularly associated with type 1 diabetes, Barraquer-Simons and Lawrence syndromes), which are promoted by anomalies of the complement system. Finally, lipomatosis is currently classified as a painful form (adiposis dolorosa or Dercum's disease) or benign symmetric multiple form, also known as Launois-Bensaude syndrome or Madelung's disease, which are sometimes related to mitochondrial DNA mutations, but are usually promoted by alcohol. In addition to the medical management of metabolic syndrome and the sometimes surgical treatment of lipodystrophy, recombinant leptin provides hope for genetically determined lipodystrophy syndromes, whereas modifications in antiretroviral treatment and tesamorelin, a GHRH analog, is effective in the metabolic syndrome of HIV patients. Other therapeutic options will undoubtedly be developed, dependent on pathophysiological advances, which today tend to classify genetically determined lipodystrophy as being related to laminopathy or to lipid droplet disorders.

Variations in T(2)* and fat content of murine brown and white adipose tissues by chemical-shift MRI

PURPOSE

The purpose was to compare T(2)* relaxation times and proton density fat-fraction (PDFF) values between brown (BAT) and white (WAT) adipose tissue in lean and ob/ob mice.

MATERIALS AND METHODS

A group of lean male mice (n=6) and two groups of ob/ob male mice placed on similar 4-week (n=6) and 8-week (n=8) ad libitum diets were utilized. The animals were imaged at 3 T using a T(2)*-corrected chemical-shift-based water-fat magnetic resonance imaging (MRI) method that provides simultaneous estimation of T(2)* and PDFF on a voxel-wise basis. Regions of interest were drawn within the interscapular BAT and gonadal WAT depots on co-registered T(2)* and PDFF maps. Measurements were assessed using analysis of variance, Bonferroni-adjusted t test for multigroup comparisons and the Tukey post hoc test.

RESULTS

Significant differences (P<.01) in BAT T(2)* and PDFF were observed between the lean and ob/ob groups. The ob/ob animals exhibited longer BAT T(2)* and greater PDFF than lean animals. However, only BAT PDFF was significantly different (P<.01) between the two ob/ob groups. When comparing BAT to WAT within each group, T(2)* and PDFF values were consistently lower in BAT than WAT (P<.01). The difference was most prominent in the lean animals. In both ob/ob groups, BAT exhibited very WAT-like appearances and properties on the MRI images.

CONCLUSION

T(2)* and PDFF are lower in BAT than WAT. This is likely due to variations in tissue composition. The values were consistently lower in lean mice than in ob/ob mice, suggestive of the former's greater demand for BAT thermogenesis and reflective of leptin hormone deficiencies and diminished BAT metabolic activity in the latter.

Ambient Temperature and Obesity

Homeotherms maintain an optimal body temperature that is most often above their environment or ambient temperature. As ambient temperature decreases, energy expenditure (and energy intake) must increase to maintain thermal homeostasis. With the wide spread adoption of climate control, humans in modern society are buffered from temperature extremes and spend an increasing amount of time in a thermally comfortable state where energetic demands are minimized. This is hypothesized to contribute to the contemporary increase in obesity rates. Studies reporting exposures of animals and humans to different ambient temperatures are discussed. Additional consideration is given to the potentially altered metabolic and physiologic responses in obese versus lean subjects at a given temperature. The data suggest that ambient temperature is a significant contributor to both energy intake and energy expenditure, and that this variable should be more thoroughly explored in future studies as a potential contributor to obesity susceptibility.

Unequivocal identification of brown adipose tissue in a human infant

We report the unique depiction of brown adipose tissue (BAT) by magnetic resonance imaging (MRI) and computed tomography (CT) in a human 3-month-old infant. Based on cellular differences between BAT and more lipid-rich white adipose tissue (WAT), chemical-shift MRI and CT were both capable of generating distinct signal contrasts between the two tissues and against surrounding anatomy, utilizing fat-signal fraction metrics in the former and x-ray attenuation values in the latter. While numerous BAT imaging experiments have been performed previously in rodents, the identification of BAT in humans has only recently been described with fusion positron emission and computed tomography in adults. The imaging of BAT in children has not been widely reported and, furthermore, MRI of human BAT in general has not been demonstrated. In the present work, large bilateral supraclavicular BAT depots were clearly visualized with MRI and CT. Tissue identity was subsequently confirmed by histology. BAT has important implications in regulating energy metabolism and nonshivering thermogenesis and has the potential to combat the onset of weight gain and the development of obesity. Current findings suggest that BAT is present in significant amounts in children and that MRI and CT can differentiate BAT from WAT based on intrinsic tissue properties.

Developments in the imaging of brown adipose tissue and its associations with muscle, puberty, and health in children

Fusion positron emission and computed tomography (PET/CT) remains the gold-standard imaging modality to non-invasively study metabolically active brown adipose tissue (BAT). It has been widely applied to studies in adult cohorts. In contrast, the number of BAT studies in children has been few. This is largely limited by the elevated risk of ionizing radiation and radionuclide tracer usage by PET/CT and the ethical restriction of performing such exams on healthy children. However, metabolically active BAT has a significantly higher prevalence in pediatric patients, according to recent literature. Young cohorts thus represent an ideal population to examine the potential relationships of BAT to muscle development, puberty, disease state, and the accumulation of white adipose tissue. In turn, magnetic resonance imaging (MRI) represents the most promising modality to overcome the limitations of PET/CT. The development of rapid, repeatable MRI techniques to identify and quantify both metabolically active and inactive BAT non-invasively and without the use of exogenous contrast agents or the need for sedation in pediatric patients are critically needed to advance our knowledge of this tissue's physiology.