Brown adipose tissue localization using 18F-FDG PET/MRI in adult

A Novel PET Probe for Brown Adipose Tissue Imaging in Rodents

PURPOSE

Brown adipose tissue (BAT) has emerged as a promising target to counteract obesity and its associated metabolic disorders. However, the detection of this tissue remains one of the major roadblocks.

PROCEDURES

In this study, we assess the use of BODIPY 1 as a positron emission tomography (PET) imaging agent to image BAT depots in vivo in two mouse phenotypes: obesity-resistant BALB/c mice and the obesity-prone C57BL/6 mice. [¹⁸F]BODIPY 1 is a radioactive dye that processed both radioactivity for PET imaging and fluorescence signal for in vitro mechanism study.

RESULTS

Through the co-staining of cancer cells with BODIPY 1 and MitoTracker, we found BODIPY 1 mainly accumulated in cell mitochondria in vitro. Fluorescence imaging of primary brown and white adipocytes further confirmed BODIPY 1 had significantly higher accumulation in primary brown adipocytes compared with primary white adipocytes. We evaluated [¹⁸F]BODIPY 1 for BAT imaging in both obesity-resistant BALB/c mice and obesity-prone C57BL/6 mice. Indeed, [¹⁸F]BODIPY 1 was efficiently taken up by BAT in both mouse genotypes (6.40 ± 1.98 %ID/g in obesity-resistant BALB/c mice (n = 8) and 5.37 ± 0.82 %ID/g in obesity-prone C57BL/6 mice (n = 7)). Although norepinephrine stimulation could increase the absolute BAT uptake, the enhancement is not significant in both genotypes (p > 0.05) at current sample size. These results suggest BAT uptake of [¹⁸F]BODIPY 1 may be independent of BAT thermogenic activity. As a comparison, 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) PET imaging was performed in obesity-resistant BALB/c mice. Significantly increased uptake was observed in adrenergically activated BAT (10.08 ± 2.52 %ID/g, n = 3) but not in inactive BAT (3.803 ± 0.70 %ID/g; n = 3). Because [¹⁸F]BODIPY 1 maintained its fluorescent property, BAT tissue was excised and studied using fluorescence microscopy. Strong fluorescence signal was observed in BAT mouse that was injected with BODIPY 1.

CONCLUSIONS

Unlike [¹⁸F]FDG, [¹⁸F]BODIPY 1 showed prominent accumulation in BAT under both inactive and stimulated status. [¹⁸F]BODIPY 1 may serve as a valuable BAT PET agent to possibly assess BAT mitochondria density, thus BAT thermogenic capacity after further evaluation.

Techniques and Applications of Magnetic Resonance Imaging for Studying Brown Adipose Tissue Morphometry and Function

The present review reports on the current knowledge and recent findings in magnetic resonance imaging (MRI) and spectroscopy (MRS) of brown adipose tissue (BAT). The work summarizes the features and mechanisms that allow MRI to differentiate BAT from white adipose tissue (WAT) by making use of their distinct morphological appearance and the functional characteristics of BAT. MR is a versatile imaging modality with multiple contrast mechanisms as potential candidates in the study of BAT, targeting properties of ¹H, ¹³C, or ¹²⁹Xe nuclei. Techniques for assessing BAT morphometry based on fat fraction and markers of BAT microstructure, including intermolecular quantum coherence and diffusion imaging, are first described. Techniques for assessing BAT function based on the measurement of BAT metabolic activity, perfusion, oxygenation, and temperature are then presented. The application of the above methods in studies of BAT in animals and humans is described, and future directions in MR study of BAT are finally discussed.

Recent advances in the detection of brown adipose tissue in adult humans: a review

The activation of brown adipose tissue (BAT) is associated with reductions in circulating lipids and glucose in rodents and contributes to energy expenditure in humans indicating the potential therapeutic importance of targetting this tissue for the treatment of a variety of metabolic disorders. In order to evaluate the therapeutic potential of human BAT, a variety of methodologies for assessing the volume and metabolic activity of BAT are utilized. Cold exposure is often utilized to increase BAT activity but inconsistencies in the characteristics of the exposure protocols make it challenging to compare findings. The metabolic activity of BAT in response to cold exposure has most commonly been measured by static positron emission tomography of ¹⁸F-fluorodeoxyglucose in combination with computed tomography (¹⁸F-FDG PET-CT) imaging, but recent studies suggest that under some conditions this may not always reflect BAT thermogenic activity. Therefore, recent studies have used alternative positron emission tomography and computed tomography (PET-CT) imaging strategies and radiotracers that may offer important insights. In addition to PET-CT, there are numerous emerging techniques that may have utility for assessing BAT metabolic activity including magnetic resonance imaging (MRI), skin temperature measurements, near-infrared spectroscopy (NIRS) and contrast ultrasound (CU). In this review, we discuss and critically evaluate the various methodologies used to measure BAT metabolic activity in humans and provide a contemporary assessment of protocols which may be useful in interpreting research findings and guiding the development of future studies.

Functional imaging of brown adipose tissue in human

Obesity has become a major public health challenge and an increasing trend is seen in its prevalence worldwide. It is a complex disorder involving an excessive amount of body fat as a result of an energy imbalance leading to caloric overload. Since the discovery of functional brown adipose tissue (BAT) in adult humans, with energy dissipating properties, this thermogenic tissue has thus emerged as an attractive therapeutic target to combat obesity and related cardiometabolic disorders. The advancements in imaging modalities to evaluate organ-specific metabolism in humans is substantially contributing to understand the physiological role of BAT. This review presents an overview of the different imaging approaches implied in BAT assessment, with a special emphasis on adult human BAT. In this context, we also attempted to summarize the developmental origins and physiology of BAT.