Nuclear MedicineE2250. Recognition of Hypermetabolic Fat in Imaging
Massie A1,2, Junewick J.3 1. Spectrum Health, Grand Rapids , US; 2. Grand Rapids Medical Education Partners, Grand Rapids, US; 3. Advanced Radiology Services, Grand Rapids, 49512
Address correspondence to A. Massie (email@example.com)
Background Information: Hypermetabolic fat (brown adipose tissue) is a highly vascular and metabolically active tissue with the purpose of generating heat to maintain body temperature via nonshivering thermogenesis. Previously believed to be present only in infants, activity has recently been shown in many adults as well. Brown fat has the potential to confound the results of FDG PET imaging, but also visible on many other examinations. Brown fat biology, mechanisms of visualization, and recognition on clinical imaging are discussed.
Educational Goals/Teaching Points: We discuss basic brown fat physiology and demonstrate brown fat with multiple imaging modalities. CT and MRI imaging features are discussed. Uptake in nuclear medicine imaging (Xenon, sestamibi, MIBG, FDG) and mechanism of localization are discussed.
Key Anatomic/Physiologic Issues and Imaging Findings/Techniques: Brown fat can be found in many subcutaneous and visceral locations. It has abundant mitochondria and the inner mitochondrial membrane uniquely contains the protein UCP1, which uncouples substrate oxidation and adenosine triphosphate production by favoring proton loss and consequently heat generation. High metabolic demands of brown fat require increased blood perfusion. As a result, brown fat hyperenhances on contrast-enhanced CT and MRI. FDG is a glucose analog and while the primary metabolic substrates of brown fat are fatty acids, approximately 10% is glucose, with increasing glucose uptake after stimulation. Metaiodobenzylguanidine (MIBG) is a molecule similar to norepinephrine radiolabeled with I-123. Since brown fat is sympathetically innervated, uptake is believed due to binding of tracer to adrenergic receptors. Technetium-99m sestamibi is a lipophilic cation that crosses cell membranes by diffusion, and is retained by electrostatic binding to negative electrical charges on the mitochondrial membranes. Localization to brown fat is presumed due to its high blood flow and active mitochondria. Xenon-133 is an inert gas and is poorly soluble, but trace amounts enter the bloodstream. It is lipophilic and while commonly uptake is seen with fatty liver, it can localize to any fatty tissue receiving high blood flow, such as brown fat.
Conclusion: Brown fat is a unique tissue visible with many imaging modalities and is important to recognize so as not to be mistaken for a pathologic process. Its imaging features can be attributed to its high vascularity, metabolic demand, and myocyte origin.