ABSTRACT: Obesity is a leading cause of preventable death and morbidity. To elucidate the mechanisms connecting metabolically active brown adipose tissue (BAT) and metabolic health may provide insights into methods of treatment for obesity-related conditions. ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸FDG-PET/CT) is traditionally used to image human BAT activity. However, the primary energy source of BAT is derived from intracellular fatty acids and not glucose. Beta-methyl-p-iodophenylpentadecanoic acid (BMIPP) is a fatty acid analogue amenable to in vivo imaging by single photon emission computed tomography/CT (SPECT/CT) when radiolabeled with iodine isotopes. In this study, we compare the use of ¹⁸FDG-PET/CT and ¹²⁵I-BMIPP-SPECT/CT for fat imaging to ascertain whether BMIPP is a more robust candidate for the non-invasive evaluation of metabolically active adipose depots. Interscapular BAT, inguinal white adipose tissue (iWAT), and gonadal white adipose tissue (gWAT) uptake of ¹⁸FDG and ¹²⁵I-BMIPP was quantified in mice following treatment with the BAT-stimulating drug CL-316,243 or saline vehicle control. After CL-316,243 treatment, uptake of both radiotracers increased in BAT and iWAT. The standard uptake value (SUV_mean) for ¹⁸FDG and ¹²⁵I-BMIPP significantly correlated in these depots, although uptake of ¹²⁵I-BMIPP in BAT and iWAT more closely mimicked the fold-change in metabolic rate as measured by an extracellular flux analyzer. Herein, we find that imaging BAT with the radioiodinated fatty acid analogue BMIPP yields more physiologically relevant data than ¹⁸FDG-PET/CT, and its conventional use may be a pivotal tool for evaluating BAT in both mice and humans.

Results from nanoScan SPECT/CT

• Authours find that imaging BAT with the radioiodinated fatty acid analogue BMIPP yields more physiologically relevant data than ¹⁸FDG-PET/CT, and its conventional use may be a pivotal tool for evaluating BAT in both mice and humans

Representative sagittal ¹⁸FDG-PET/CT of a CL-316,243 treated (a) and vehicle control (b) mouse and ¹²⁵I-BMIPP-SPECT/CT of a CL-316,243 (c) and vehicle (d) mouse (n = 5). The classical interscapular fat pad is circled in image (a), cardiac uptake is indicated with orange arrows in (a,d), and prevertebral and anterior abdominal fat uptake indicated with yellow arrows in (c). Extensive hepatic, gastrointestinal, and mediastinal background uptake was seen with BMIPP but not FDG.

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