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.

Accelerated T2*-compensated fat fraction quantification using a joint parallel imaging and compressed sensing framework

PURPOSE

To develop a T2*-compensated parallel imaging and compressed sensing framework for water-fat separation, and to demonstrate accelerated quantitative imaging of proton density fat fraction.

MATERIALS AND METHODS

The proposed method extends a previously developed framework for water-fat separation by additionally compensating for T2* decay. A two-stage estimation was formulated that first determines an approximation of the B0 field map and then jointly estimates and refines the R2* (=1/T2*) and B0 field maps, respectively. The method was tested using a set of water-fat phantoms as well as liver datasets that were acquired from seven asymptomatic adult volunteers. The fat fraction estimates were compared to those from a commonly used nonaccelerated water-fat imaging method and also to a sequential parallel imaging and water-fat imaging method.

RESULTS

The proposed method properly compensated for T2* decay to yield accurate fat fraction estimates in the water-fat phantoms. Further, linear regression analysis from the liver datasets showed that the proposed method accurately estimated fat fraction at acceleration factors that were higher than those achievable by the sequential parallel imaging and water-fat imaging method. Accurate fat fraction estimates were demonstrated at acceleration factors up to 4×, although some image artifacts were observed.

CONCLUSION

The proposed T2*-compensated parallel imaging and compressed sensing framework demonstrates the potential to further accelerate water-fat imaging while maintaining accurate estimates of proton density fat fraction.

Chemical shift encoded water-fat separation using parallel imaging and compressed sensing

Chemical shift encoded techniques have received considerable attention recently because they can reliably separate water and fat in the presence of off-resonance. The insensitivity to off-resonance requires that data be acquired at multiple echo times, which increases the scan time as compared to a single echo acquisition. The increased scan time often requires that a compromise be made between the spatial resolution, the volume coverage, and the tolerance to artifacts from subject motion. This work describes a combined parallel imaging and compressed sensing approach for accelerated water-fat separation. In addition, the use of multiscale cubic B-splines for B(0) field map estimation is introduced. The water and fat images and the B(0) field map are estimated via an alternating minimization. Coil sensitivity information is derived from a calculated k-space convolution kernel and l(1)-regularization is imposed on the coil-combined water and fat image estimates. Uniform water-fat separation is demonstrated from retrospectively undersampled data in the liver, brachial plexus, ankle, and knee as well as from a prospectively undersampled acquisition of the knee at 8.6x acceleration.

Chemical-shift water-fat MRI of white adipose depots: inability to resolve cell size differences

PURPOSE

Adipocyte cell size varies among individuals and importantly, is inversely correlated with insulin sensitivity, and modifiable by weight loss or pharmaceutical agents. However, there are no non-invasive, in vivo methods for adipocyte cell size determination. Here we apply Chemical-Shift Water-Fat MRI to in vivo measures of subcutaneous (inguinal) and visceral (gonadal) white adipose tissue (WAT) to determine whether the fat-signal fraction (FF) is a sensitive indicator of adipocyte cell size.

MATERIALS AND METHODS

C57BL/6J male mice (8 weeks old) were singly housed and fed a low-fat diet, high-fat diet or very high-fat diet (n = 16 or 15/group) for 8 weeks. Food intake, body weight and composition were measured; CS-MRI was performed on a 9.4 Tesla Bruker magnet with respiratory gating and anesthesia. Histology was acquired for gonadal WAT; both gonadal and inguinal WAT were fixed with osmium tetroxide and then measured through Image J for cell size.

RESULTS

Mice fed with higher fat content diets gained significantly more body weight, fat and lean mass while maintaining higher energy intakes over the 8 weeks. There was no significant difference in fat fraction for either gonadal (P = 0.1295) or inguinal (P = 0.4704) WAT among the three groups, despite significantly larger adipocytes (P <0.0001) in mice on high fat diets.

CONCLUSION

Although diet-induced obesity significantly increased the amount of fat mass, as well as mean and overall white adipocyte cell size, the CS-MRI measured fat fraction between groups were not significantly different. These results do not support the utility of CS-MRI measured FF for in vivo determination of adipocyte cell size.

Relevance of brown adipose tissue in infancy and adolescence

Brown adipose tissue (BAT) was thought to disappear after infancy. Recent findings of BAT in patients undergoing positron emission tomography/computed tomography (PET/CT) have renewed the interest in deciphering the relevance of this tissue in humans. Available data suggest that BAT is more prevalent in children than in adults and that its activation during adolescence is associated with significantly lower gains in weight and adiposity. Data also show that pediatric patients with metabolically active BAT on PET/CT examinations have significantly greater muscle volume than patients without identifiable BAT. Both the activity and the amount of BAT increase during puberty. The magnitude of the increase is higher in boys as compared with girls and is closely related to gains in muscle volume. Hence, concurrent with the gains in skeletal muscle during infancy and puberty, all infants and adolescents accumulate large amounts of BAT. These observations are consistent with in vitro investigations suggesting close interactions between brown adipocytes, white adipocytes, and myocytes. In this review, we discuss the potential role of this tissue in regulating weight and musculoskeletal development in children.

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.

Automatic intra-subject registration-based segmentation of abdominal fat from water-fat MRI

PURPOSE

To develop an automatic registration-based segmentation algorithm for measuring abdominal adipose tissue depot volumes and organ fat fraction content from three-dimensional (3D) water-fat MRI data, and to evaluate its performance against manual segmentation.

MATERIALS AND METHODS

Data were obtained from 11 subjects at two time points with intermediate repositioning, and from four subjects before and after a meal with repositioning. Imaging was performed on a 3 Tesla MRI, using the IDEAL chemical-shift water-fat pulse sequence. Adipose tissue (subcutaneous--SAT, visceral--VAT) and organs (liver, pancreas) were manually segmented twice for each scan by a single trained observer. Automated segmentations of each subject's second scan were generated using a nonrigid volume registration algorithm for water-fat MRI images that used a b-spline basis for deformation and minimized image dissimilarity after the deformation. Manual and automated segmentations were compared using Dice coefficients and linear regression of SAT and VAT volumes, organ volumes, and hepatic and pancreatic fat fractions (HFF, PFF).

RESULTS

Manual segmentations from the 11 repositioned subjects exhibited strong repeatability and set performance benchmarks. The average Dice coefficients were 0.9747 (SAT), 0.9424 (VAT), 0.9404 (liver), and 0.8205 (pancreas); the linear correlation coefficients were 0.9994 (SAT volume), 0.9974 (VAT volume), 0.9885 (liver volume), 0.9782 (pancreas volume), 0.9996 (HFF), and 0.9660 (PFF). When comparing manual and automated segmentations, the average Dice coefficients were 0.9043 (SAT volume), 0.8235 (VAT), 0.8942 (liver), and 0.7168 (pancreas); the linear correlation coefficients were 0.9493 (SAT volume), 0.9982 (VAT volume), 0.9326 (liver volume), 0.8876 (pancreas volume), 0.9972 (HFF), and 0.8617 (PFF). In the four pre- and post-prandial subjects, the Dice coefficients were 0.9024 (SAT), 0.7781 (VAT), 0.8799 (liver), and 0.5179 (pancreas); the linear correlation coefficients were 0.9889, 0.9902 (SAT, and VAT volume), 0.9523 (liver volume), 0.8760 (pancreas volume), 0.9991 (HFF), and 0.6338 (PFF).

CONCLUSION

Automated intra-subject registration-based segmentation is potentially suitable for the quantification of abdominal and organ fat and achieves comparable quantitative endpoints with respect to manual segmentation.

Brown adipose tissue and its relationship to bone structure in pediatric patients

CONTEXT

Emerging evidence suggests a possible link between brown adipose tissue (BAT) and bone metabolism.

OBJECTIVE

The objective of this study was to examine the relationships between BAT and bone cross-sectional dimensions in children and adolescents.

DESIGN

This was a cross-sectional study.

SETTING

The study was conducted at a pediatric referral center.

PATIENTS

Patients included 40 children and teenagers (21 males and 19 females) successfully treated for pediatric malignancies.

INTERVENTIONS

There were no interventions.

MAIN OUTCOME MEASURES

The volume of BAT was determined by fluorodeoxyglucose-positron emission tomography/computed tomography. Measures of the cross-sectional area and cortical bone area and measures of thigh musculature and sc fat were determined at the midshaft of the femur.

RESULTS

Regardless of sex, there were significant correlations seen between BAT volume and the cross-sectional dimensions of the bone (r values between 0.68 and 0.77; all P ≤ 0 .001). Multiple regression analyses indicated that the volume of BAT predicted femoral cross-sectional area and cortical bone area, even after accounting for height, weight, and gender. The addition of muscle as an independent variable increased the predictive power of the model but significantly decreased the contribution of BAT.

CONCLUSIONS

The volume of BAT is positively associated with the amount of bone and the cross-sectional size of the femur in children and adolescents. This relation between BAT and bone structure could, at least in part, be mediated by muscle.

Comparison of catalytical activity and stereoselectivity between the recombinant human cytochrome P450 2D6.1 and 2D6.10

Polymorphisms of the cytochrome P450 2D6 (CYP2D6) gene play a major role in pharmacokinetic variability in human, while CYP2D6*10 is an important subtype in Asian people. In this study, the co-expression enzyme of human recombinant CYPOR, CYPb5 and CYP2D6.1 or CYP2D6.10 with the Bac-to-Bac system in baculovirus-infected insect cells was used to study the catalytical activity to imipramine metabolism and stereoselective metabolism of propranolol. The metabolites of imipramine were identified of hydroxyl imipramine and desipramine by LC-MS/MS. There are some differences between CYP2D6.1 and CYP2D6.10 activity. The kinetics parameters K(m), V(max), and CL(int) are 11.77 +/- 0.91 micromol/L, 0.4235 +/- 0.05 nmol/nmol CYP2D6.1/min and 3.60 x 10(-5) ml/min/nmol CYP2D6.1 (n = 3) for CYP2D6.1, respectively, and 9.05 +/- 0.87 micromol/L, 0.42 +/- 0.03 nmol/nmol CYP2D6.10/min, and 4.60 x 10(-5) ml/min/nmol CYP2D6.10 (n = 3) for CYP2D6.10. For propranolol, two metabolites were identified to be hydroxyl and N-desisopropylation propranolol by LC-MS/MS. When the substrate concentration was 0.20 micromol/L, CYP2D6.1 and CYP2D6.10 exhibited significant stereoseletivity. Furthermore, enantioselective formation has been detected. Both of CYP2D6.1 and CYP2D6.10 produced more hydroxyl propranolol from the R-(+)-isomer than from the S-(-)-isomer while there was no obvious difference for N-desisopropylation propranolol production between R-(+)- and S-(-)- isomer. In summary, there is a somewhat different catalytical activity and stereoselectivity between the human recombinant CYP2D6.1 and CYP2D6.10. The data we got will be helpful in preclinical research and clinical use of CYP2D6 substrates.

Inverse association between brown adipose tissue activation and white adipose tissue accumulation in successfully treated pediatric malignancy

BACKGROUND

Although the accumulation of white adipose tissue (WAT) is a risk factor for disease, brown adipose tissue (BAT) has been suggested to have a protective role against obesity.

OBJECTIVE

We studied whether changes in BAT were related to changes in the amounts of subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) in children treated for malignancy.

DESIGN

We examined the effect of BAT activity on weight, SAT, and VAT in 32 pediatric patients with cancer whose positron emission tomography-computed tomography (PET-CT) scans at diagnosis showed no BAT activity. Changes in weight, SAT, and VAT from diagnosis to remission for children with metabolically active BAT at disease-free follow-up (BAT+) were compared with those in children without visualized BAT when free of disease (BAT-).

RESULTS

Follow-up PET-CT studies (4.7 ± 2.4 mo later) after successful treatment of the cancer showed BAT+ in 19 patients but no active BAT (BAT-) in 13 patients. BAT+ patients, in comparison with BAT- patients, gained significantly less weight (3.3 ± 6.6% compared with 11.0 ± 11.6%; P = 0.02) and had significantly less SAT (18.2 ± 26.5% compared with 67.4 ± 71.7%; P = 0.01) and VAT (22.6 ± 33.5% compared with 131.6 ± 171.8%; P = 0.01) during treatment. Multiple regression analysis indicated that the inverse relations between BAT activation and measures of weight, SAT, and VAT persisted even after age, glucocorticoid treatment, and the season when the PET-CT scans were obtained were accounted for.

CONCLUSION

The activation of BAT in pediatric patients undergoing treatment of malignancy is associated with significantly less adipose accumulation. This trial was registered at clinicaltrials.gov as NCT01517581.

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.

Changes in brown adipose tissue in boys and girls during childhood and puberty

OBJECTIVE

To characterize the changes in brown adipose tissue (BAT) occurring during puberty in boys and girls.

STUDY DESIGN

We examined the prevalence and the volume of BAT at different stages of sexual development in 73 pediatric patients who underwent positron emission tomography (PET)/computed tomography (CT) studies.

RESULTS

Of the 73 patients studied, 43 (59%) had BAT depicted on PET/CT. The presence of BAT was detected significantly less frequently on PET/CT in prepubertal subjects (Tanner stage 1) than in pubertal subjects (Tanner stages 2-5) (15% vs 75%). BAT volume also increased during puberty, with a significantly greater magnitude of the increase in the final 2 stages of puberty (Tanner stages 4 and 5) than in earlier stages (Tanner stages 1-3) (boys: 499 ± 246 vs 50 ± 36, P < .0001; girls: 286 ± 139 vs 36 ± 29, P = .024). Changes in BAT volume were also significantly greater in boys than in girls (P = .004) and were closely related to muscle volume (r = 0.52, P < .01 for boys; r = 0.64, P < .01 for girls).

CONCLUSION

The presence and volume of BAT increase rapidly during puberty. Metabolic and hormonal events related to the achievement of sexual maturity are likely responsible for this increase.

MR properties of brown and white adipose tissues

PURPOSE

To explore the MR signatures of brown adipose tissue (BAT) compared with white adipose tissue (WAT) using single-voxel MR spectroscopy.

MATERIALS AND METHODS

(1) H MR STEAM spectra were acquired from a 3 Tesla clinical whole body scanner from seven excised murine adipose tissue samples of BAT (n=4) and WAT (n=3). Spectra were acquired at multiple echo times (TEs) and inversion times (TIs) to measure the T1, T2, and T2-corrected peak areas. A theoretical triglyceride model characterized the fat in terms of number of double bonds (ndb) and number of methylene-interrupted double bonds (nmidb).

RESULTS

Negligible differences between WAT and BAT were seen in the T1 and T2 of fat and the T2 of water. However, the water fraction in BAT was higher (48.5%) compared with WAT (7.1%) and the T1 of water was lower in BAT (618 ms) compared with WAT (1053 ms). The fat spectrum also differed, indicating lower levels of unsaturated triglycerides in BAT (ndb=2.7, nmidb=0.7) compared with WAT (ndb=3.3, nmidb=1.0).

CONCLUSION

We have demonstrated that there are several key MR-based signatures of BAT and WAT that may allow differentiation on MR imaging.

Short-term physical activity intervention decreases femoral bone marrow adipose tissue in young children: a pilot study

Mechanical stimulation is necessary for maximization of geometrical properties of bone mineralization contributing to long-term strength. The amount of mineralization in bones has been reciprocally related to volume of bone marrow adipose tissue and this relationship is suggested to be an independent predictor of fracture. Physical activity represents an extrinsic factor that impacts both mineralization and marrow volume exerting permissive capacity of the growing skeleton to achieve its full genetic potential. Because geometry- and shape-determining processes primarily manifest during the linear growth period, the accelerated structural changes accompanying early childhood (ages 3 to 6 y) may have profound impact on lifelong bone health. The objective of this pilot study was to determine if a short-term physical activity intervention in young children would result in augmentation of geometric properties of bone. Three days per week the intervention group (n=10) participated in 30 min of moderate intensity physical activity, such as jumping, hopping and running, and stretching activities, whereas controls (n=10) underwent usual activities during the 10-week intervention period. Femoral bone marrow adipose tissue volume and total body composition were assessed by magnetic resonance imaging and dual-energy X-ray absorptiometry, respectively, at baseline and after 10 weeks. Although after 10-weeks, intergroup differences were not observed, a significant decrease in femoral marrow adipose tissue volume was observed in those participating in physical activity intervention. Our findings suggest that physical activity may improve bone quality via antagonistic effects on femoral bone marrow adipose tissue and possibly long-term agonistic effects on bone mineralization.

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.

Accelerated water-fat imaging using restricted subspace field map estimation and compressed sensing

Water-fat separation techniques play an important role in a variety of clinical and research applications. In particular, multiecho separation methods remain a topic of great interest due to their ability to resolve water and fat images in the presence of B(0)-field inhomogeneity. However, these methods are inherently slow as they require multiple measurements. An accelerated technique with reduced k-space sampling is desirable to decrease the scan time. This work presents a new method for water-fat separation from accelerated multiecho acquisitions. The proposed approach does not require the region-growing or region-merging schemes that are typically used for field map estimation. Instead, the water, fat, and field map signals are estimated directly from the undersampled k-space measurements. In this work, up to 2.5×-acceleration is demonstrated in a water-fat phantom, ankle, knee, and liver.

Assessment of abdominal adipose tissue and organ fat content by magnetic resonance imaging

As the prevalence of obesity continues to rise, rapid and accurate tools for assessing abdominal body and organ fat quantity and distribution are critically needed to assist researchers investigating therapeutic and preventive measures against obesity and its comorbidities. Magnetic resonance imaging (MRI) is the most promising modality to address such need. It is non-invasive, utilizes no ionizing radiation, provides unmatched 3-D visualization, is repeatable, and is applicable to subject cohorts of all ages. This article is aimed to provide the reader with an overview of current and state-of-the-art techniques in MRI and associated image analysis methods for fat quantification. The principles underlying traditional approaches such as T(1) -weighted imaging and magnetic resonance spectroscopy as well as more modern chemical-shift imaging techniques are discussed and compared. The benefits of contiguous 3-D acquisitions over 2-D multislice approaches are highlighted. Typical post-processing procedures for extracting adipose tissue depot volumes and percent organ fat content from abdominal MRI data sets are explained. Furthermore, the advantages and disadvantages of each MRI approach with respect to imaging parameters, spatial resolution, subject motion, scan time and appropriate fat quantitative endpoints are also provided. Practical considerations in implementing these methods are also presented.

Functional brown adipose tissue is related to muscle volume in children and adolescents

OBJECTIVE

We examined whether the depiction of brown adipose tissue (BAT) with positron emission tomography/computed tomography (PET/CT) in pediatric patients is associated with anthropometric measures.

STUDY DESIGN

We determined measures of body mass, adiposity, and musculature in 71 children and adolescents who underwent PET/CT examinations and compared patients with and without BAT. We used regression analyses to assess the relation between BAT and anthropometric measures.

RESULTS

A total of 30 patients (42%) had BAT depicted on PET/CT, 10 of 26 girls (38%) and 20 of 45 boys (44%). Compared with patients without functional BAT, patients with BAT had significantly greater neck musculature (1880 ± 908 cm(3) versus 1299 ± 806 cm(3); P = .028 for boys and 1295 ± 586 cm(3) versus 854 ± 392 cm(3); P = .030 for girls) and gluteus musculature (1359 ± 373 cm(3) versus 1061 ± 500 cm(3); P = .032 for boys and 1138 ± 425 cm(3) versus 827 ± 297 cm(3); P = .038 for girls), but no differences in age, body mass index, or measures of subcutaneous fat. With logistic regression analyses, neck and pelvic musculature predicted the presence of BAT independently of age, sex, body size, and season of scan (P = .018 and .009, respectively).

CONCLUSION

Pediatric patients with visualized BAT on PET/CT examinations had significantly greater muscle volume than patients with no visualized BAT.

Ethnic differences in pancreatic fat accumulation and its relationship with other fat depots and inflammatory markers

OBJECTIVE

Visceral adipose tissue (VAT) and hepatic fat are associated with insulin resistance and vary by sex and ethnicity. Recently, pancreatic fat fraction (PFF) has also been linked with increasing obesity. Our aim was to assess ethnic and sex differences in PFF and its relationship to other fat depots, circulating free fatty acids (FFA), insulin secretion and sensitivity, and inflammation in obese adolescents and young adults.

RESEARCH DESIGN AND METHODS

We examined 138 (40 males, 98 females) obese Hispanics and African Americans (13-25 years). Subcutaneous adipose tissue and VAT volumes, hepatic fat fraction (HFF), and PFF were determined by magnetic resonance imaging. Insulin sensitivity and β-cell function were assessed during an intravenous glucose tolerance test.

RESULTS

Hispanics had higher PFF than African Americans (7.3 ± 3.8 vs. 6.2 ± 2.6%, P = 0.03); this ethnic difference was higher in young adults compared with children and adolescents (ethnicity × age: P = 0.01). Males had higher PFF than females (P < 0.0001). PFF was positively correlated with VAT (r = 0.45, P < 0.0001), HFF (r = 0.29, P < 0.0001), and FFA (r = 0.32, P = 0.001). PFF positively correlated with inflammatory markers but lost significance when adjusted for VAT. In multiple stepwise regression analysis, VAT and FFA were the best predictors of PFF (adjusted R(2) = 0.40). There were no significant correlations between PFF and markers of insulin sensitivity or β-cell function.

CONCLUSIONS

PFF is higher in Hispanics than African Americans, and this difference increases with age. In young obese individuals, PFF is related to VAT, HFF, and circulating FFA, thus possibly contributing to their increased risk for type 2 diabetes and related metabolic disorders.

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.

Cloning and expression of three fatty acid desaturase genes from cold-sensitive lima bean (Phaseolus lunatus L.)

The critical chilling temperature of cold-sensitive lima bean (Phaseolus lunatus L.) seedlings was determined to be approx. 8°C. The full-length cDNAs of PlSAD, PlFAD2, and PlFAD3 encoding three fatty acid desaturases were isolated from lima bean leaves. The PlSAD open reading frame (ORF) had 1,317 bp and a single intron of 601 bp, encoding a polypeptide of 438 amino acids that showing 86% homology with the plastidial stearoyl-acyl carrier protein desaturase in soybean. The PlFAD2 ORF contained 1,059 bp and was uninterrupted, encoding a polypeptide of 352 amino acids having 85% identity with the microsomal omega-6 desaturase GmFAD2-3 in soybean. PlFAD3 ORF had 1,116 bp and seven introns, encoding a polypeptide of 371 amino acids showing 91% homology with the microsomal omega-3 desaturase FAD3 in cowpea. PlSAD and PlFAD2 were expressed highly in leaves but poorly in roots and stems, while PlFAD3 was expressed in three tissues. All three genes were significantly induced in leaves by drought. PlSAD and PlFAD3 in leaves were down-regulated by high temperature and salinity, whereas PlFAD2 was up-regulated by the two stresses. The expression patterns of the three genes in lima bean leaves under suboptimal temperature, 18°C, were different from those under 8°C. The down-regulation of PlSAD transcript at chilling temperature might be an important factor contributing to chilling susceptibility for lima bean plants.

Identification of brown adipose tissue in mice with fat-water IDEAL-MRI

PURPOSE

To investigate the feasibility of using IDEAL (Iterative Decomposition with Echo Asymmetry and Least squares estimation) fat-water imaging and the resultant fat fraction metric in detecting brown adipose tissue (BAT) in mice, and in differentiating BAT from white adipose tissue (WAT).

MATERIALS AND METHODS

Excised WAT and BAT samples and whole-mice carcasses were imaged with a rapid three-dimensional fat-water IDEAL-SPGR sequence on a 3 Tesla scanner using a single-channel wrist coil. An isotropic voxel size of 0.6 mm was used. Excised samples were also scanned with single-voxel proton spectroscopy. Fat fraction images from IDEAL were reconstructed online using research software, and regions of WAT and BAT were quantified.

RESULTS

A broad fat fraction range for BAT was observed (40-80%), in comparison to a tighter and higher WAT range of 90-93%, in both excised tissue samples and in situ. Using the fat fraction metric, the interscapular BAT depot in each carcass could be clearly identified, as well as peri-renal and inguinal depots that exhibited a mixed BAT and WAT phenotype appearance.

CONCLUSION

Due to BAT's multi-locular fat distribution and extensive mitochondrial, cytoplasm, and vascular supply, its fat content is significantly less than that of WAT. We have demonstrated that the fat fraction metric from IDEAL-MRI is a sensitive and quantitative approach to noninvasively characterize BAT.

Change in the proton T(1) of fat and water in mixture

This work describes observed changes in the proton T(1) relaxation time of both water and lipid when they are in relatively homogeneous mixtures. Results obtained from vegetable oil-water emulsions, pork kidney and lard mixtures, and excised samples of white and brown adipose tissues are presented to demonstrate this change in T(1) as a function of mixture fat fraction. As an initial proof of concept, a simpler acetone-water experiment was performed to take advantage of complete miscibility between acetone and water and both components' single chemical shift peaks. Single-voxel MR spectroscopy was used to measure the T(1) of predominant methylene spins in fat and the T(1) of water spins in each setup. In the vegetable oil-water emulsions, the T(1) of fat varied by as much as 3-fold when water was the dominant mixture component. The T(1) of pure lard increased by 170 msec (+37%) when it was blended with lean kidney tissue in a 16% fatty mixture. The fat T(1) of lipid-rich white adipose tissue was 312 msec. In contrast, the fat T(1) of leaner brown adipose tissue (fat fraction 53%) was 460 msec. A change in the water T(1) from that of pure water was also observed in the experiments.