Brown adipose tissue: Evaluation with201Tl and99mTc-sestamibi dual-tracer SPECT

18F-FDG PET/CT brown fat detection: Differences between adult and pediatric population in a 12 year experience

PURPOSE

To analyze epidemiological and anthropometric features of patients with brown adipose tissue (BAT) activation detected by fluorine18-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT).

MATERIAL AND METHODS

From 2005 to 2017, 818 ¹⁸F-FDG PET/CT studies positive for BAT detection were retrospectively included, 742 examinations performed on the adult population and 76 PET/CT on the pediatric population. A Chi-squared test was performed to compare features distribution between the adult and pediatric patients.

RESULTS

Adults showed a higher rate of BAT detection in females (79% vs. 61%, P<0.001) and in hyperglycaemic patients (>100mg/dL) (24% vs. 16%, P=0.02), no significant difference was found with regard to overweight patients (BMI>25kg/m²) (22% vs. 20%, P=.55). Considering females only, the adults showed a higher rate of BAT detection both in hyperglycaemic (83% vs. 42%, P<0.001) and overweight patients (80% vs. 67%, P=0.005). In both populations BAT activation happened more frequently in cold seasons; there was no significant distribution difference with regard to season of birth (P=0.2).

CONCLUSIONS

Sex, glycemia and BMI play a major role in predicting BAT activation, with significant differences between adults and pediatric patients. Cold exposure is confirmed as an important predicting factor, while season of birth is not significant.

Is Brown Adipose Tissue Visualization Reliable on 99mTc-Methoxyisobutylisonitrile Diagnostic SPECT Scintigraphy?

The 99mTc-MIBI has been used with great value as a diagnostic technique in patients with primary hyperparathyroidism. False-positive scans may occur due to misinterpretation of the physiologic distribution of the 99mTc-MIBI. Reviewing consecutive SPECT scans, we evaluated this possibility and assessed how frequently brown adipose tissue (BAT) is seen on 99mTc-MIBI scintigraphy. Here, we retrospectively reviewed scans of consecutive patients who were evaluated for parathyroid adenomas from March 2015 to June 2015, using dual-phase (early and delayed) planar imaging and SPECT. We identified 60 patients (48 female and 12 male; mean age, 52.25 ± 15.20 years; range, 22-86 years).We detected the presence of 99mTc-MIBI uptake in BAT in 20 of 60 patients (33.33%) in the neck. Although the patients with T99mc-MIBI uptake in BAT were younger (mean age, 48.85 ± 15.27 years, range, 26-73 years) than the patients with no 99mTc-MIBI uptake (mean age, 53.95 ± 15.07 years, range, 22-86 years), this difference was not statistically significant (P = 0.224). The percentage of female patients with BAT detection was higher (17/48 patients; 37.5%) than that of the male population (3/12 patients; 25%), this difference was not also statistically significant (P = 0.85).In patient population referred to 99mTc MIBI scintigraphy of the parathyroid glands, uptake of 99mTc-MIBI in BAT should not be misinterpreted with 99mTc-MIBI-avid-tumors. Fused SPECT/CT images (not SPECT-only) are necessary to distinguish BAT from bone, muscle, thyroid, myocardium, parathyroids, and other structures in the neck and chest.

Case 214: Adrenal pheochromocytoma with perirenal brown fat stimulation

History A 61-year-old woman with well-controlled diabetes presented with a 10-year history of hypertension, stifling sensation, and flushing. Her body mass index was 19.1 kg/m(2) (normal range, 18.5-25.0 kg/m(2)). She was being followed up for mild hypercalcemia (calcium level, 10.8 mg/dL [2.7 mmol/L]) (normal range, 8.5-10.5 mg/dL [2.12-2.62 mmol/L]) by the endocrinologist (S.J.M.), who decided to perform a technetium 99m sestamibi ((99m)Tc MIBI) parathyroid scan. The test showed an abnormal tracer deposit in the region of the clavicle and sternum; thus, unenhanced thoracic computed tomography (CT) was performed. No mass was seen in the region of abnormality. In light of these findings, the patient underwent contrast material-enhanced (120 mL of iopromide, Ultravist 300; Schering, Berlin, Germany) thoracic abdominopelvic CT. There was no history of underlying malignancy, and the complete blood counts were normal. The axial and appendicular skeleton showed no sign of lesions.

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.

Quantification of human and rodent brown adipose tissue function using 99mTc-methoxyisobutylisonitrile SPECT/CT and 18F-FDG PET/CT

For brown adipose tissue (BAT) to be effective at consuming calories, its blood flow must increase enough to provide sufficient fuel to sustain energy expenditure and also transfer the heat created to avoid thermal injury. Here we used a combination of human and rodent models to assess changes in BAT blood flow and glucose utilization.

METHODS

(99m)Tc-methoxyisobutylisonitrile (MIBI) SPECT (n = 7) and SPECT/CT (n = 74) scans done in adult humans for parathyroid imaging were reviewed for uptake in regions consistent with human BAT. Site-directed biopsies of subcutaneous and deep neck fat were obtained for electron microscopy and gene expression profiling. In mice, tissue perfusion was measured with (99m)Tc-MIBI (n = 16) and glucose uptake with (18)F-FDG (n = 16). Animals were kept fasting overnight, anesthetized with pentobarbital, and given intraperitoneally either the β3-adrenergic receptor agonist CL-316,243, 1 mg/kg (n = 8), or saline (n = 8) followed by radiotracer injection 5 min later. After 120 min, the mice were imaged using SPECT/CT or PET/CT. Vital signs were recorded over 30 min during the imaging. BAT, white adipose tissue (WAT), muscle, liver, and heart were resected, and tissue uptake of both (99m)Tc-MIBI and (18)F-FDG was quantified by percentage injected dose per gram of tissue and normalized to total body weight.

RESULTS

In 5.4% of patients (4/74), (99m)Tc-MIBI SPECT/CT showed increased retention in cervical and supraclavicular fat that displayed multilocular lipid droplets, dense capillary investment, and a high concentration of ovoid mitochondria. Expression levels of the tissue-specific uncoupling protein-1 were 180 times higher in BAT than in subcutaneous WAT (P < 0.001). In mice, BAT tissue perfusion increased by 61% (P < 0.01), with no significant changes in blood flow to WAT, muscle, heart, or liver. CL-316,243 increased glucose uptake in BAT even more, by 440% (P < 0.01).

CONCLUSION

Pharmacologic activation of BAT requires increased blood flow to deliver glucose and oxygen for thermogenesis. However, the glucose consumption far exceeds the vascular response. These findings demonstrate that activated BAT increases glucose uptake beyond what might occur by increased blood flow alone and suggest that activated BAT likely uses glucose for nonthermogenic purposes.

Effects of burn injury, cold stress and cutaneous wound injury on the morphology and energy metabolism of murine brown adipose tissue (BAT) in vivo

AIMS

Cold stress has been shown to produce dramatic increases in 2-fluoro-2-deoxy-D-Glucose ((18)FDG) accumulation by brown adipose tissue (BAT) in rodents. However, neither the effects of other types of stress on (18)FDG accumulation nor the effects of stressors on the accumulation of tracers of other aspects of energy metabolism have been evaluated. In this report we studied the effects of cold stress, burn injury and cutaneous wounds on murine BAT at the macroscopic, microscopic and metabolic level.

MAIN METHODS

Glucose metabolism was studied with (18)FDG, fatty acid accumulation was evaluated with trans-9(RS)-(18)F-fluoro-3,4(RS,RS)-methyleneheptadecanoic acid (FCPHA) and tricarboxcylic acid cycle (TCA) activity was evaluated with (3)H acetate.

KEY FINDINGS

All three stressors produced dramatic changes in BAT at the macroscopic and microscopic level. Macroscopically, BAT from the stressed animals appeared to be a much darker brown in color. Microscopically BAT of stressed animals demonstrated significantly fewer lipid droplets and an overall decrease in lipid content. Accumulation of (18)FDG by BAT was significantly (p<0.01) increased by all 3 treatments (Cold: ~16 fold, burn ~7 Fold and cutaneous wound ~14 fold) whereas uptake of FDG by white fat was unchanged. This effect was also demonstrated non invasively by μPET imaging. Although less prominent than with (18)FDG, BAT uptake of FCPHA and acetate were also significantly increased by all three treatments. These findings suggest that in addition to cold stress, burn injury and cutaneous wounds produce BAT activation in mice.

SIGNIFICANCE

This study demonstrates brown fat activated by several stressors leads to increased uptake of various substrates.

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

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

Potential false positive Tc-99m sestamibi parathyroid study due to uptake in brown adipose tissue

We report on a 55-year-old woman with suspected primary hyperparathyroidism who underwent dual phase Tc-99m sestamibi parathyroid imaging. Symmetric, patchy activity in the neck and shoulders was localized to low attenuation areas on integrated SPECT/CT and attributed to uptake in brown adipose tissue (BAT). Focal uptake in the anterior thorax on SPECT images, which potentially may have been misinterpreted as ectopic parathyroid tissue, was demonstrated on SPECT/CT as uptake in BAT. Recognition of this pattern on parathyroid SPECT/CT scintigraphy may avoid false positive reports. Our case provides further evidence that in addition to F-18 FDG, I-123 MIBG, and Tc-99m tetrofosmin, Tc-99m sestamibi may also accumulate in BAT.

Unexpected evidence for active brown adipose tissue in adult humans

The contention that brown adipose tissue is absent in adult man has meant that processes attributed to active brown adipose tissue in experimental animals (mainly rodents), i.e., classical nonshivering thermogenesis, adaptive adrenergic thermogenesis, diet-induced thermogenesis, and antiobesity, should be either absent or attributed to alternative (unknown) mechanisms in man. However, serendipidously, as a consequence of the use of fluorodeoxyglucose positron emission tomography (FDG PET) to trace tumor metastasis, observations that may change that notion have recently been made. These tomography scans have visualized symmetrical areas of increased tracer uptake in the upper parts of the human body; these areas of uptake correspond to brown adipose tissue. We examine here the published observations from a viewpoint of human physiology. The human depots are somewhat differently located from those in rodents, the main depots being found in the supraclavicular and the neck regions with some additional paravertebral, mediastinal, para-aortic, and suprarenal localizations (but no interscapular). Brown adipose tissue activity in man is acutely cold induced and is stimulated via the sympathetic nervous system. The prevalence of active brown adipose tissue in normal adult man can be only indirectly estimated, but it would seem that the prevalence of active brown adipose tissue in the population may be at least in the range of some tens of percent. We conclude that a substantial fraction of adult humans possess active brown adipose tissue that thus has the potential to be of metabolic significance for normal human physiology as well as to become pharmaceutically activated in efforts to combat obesity.