AUTHOR=Hamaoka Takafumi , Nirengi Shinsuke , Fuse Sayuri , Amagasa Shiho , Kime Ryotaro , Kuroiwa Miyuki , Endo Tasuki , Sakane Naoki , Matsushita Mami , Saito Masayuki , Yoneshiro Takeshi , Kurosawa Yuko 

TITLE=Near-Infrared Time-Resolved Spectroscopy for Assessing Brown Adipose Tissue Density in Humans: A Review

JOURNAL=Frontiers in Endocrinology

VOLUME=11

YEAR=2020

URL=https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2020.00261

DOI=10.3389/fendo.2020.00261

ISSN=1664-2392

ABSTRACT=<p>Brown adipose tissue (BAT) mediates adaptive thermogenesis upon food intake and cold exposure, thus potentially contributing to the prevention of lifestyle-related diseases. <sup>18</sup>F-fluorodeoxyglucose (FDG)–positron emission tomography (PET) with computed tomography (CT) (<sup>18</sup>FDG–PET/CT) is a standard method for assessing BAT activity and volume in humans. <sup>18</sup>FDG–PET/CT has several limitations, including high device cost and ionizing radiation and acute cold exposure necessary to maximally stimulate BAT activity. In contrast, near-infrared spectroscopy (NIRS) has been used for measuring changes in O<sub>2</sub>-dependent light absorption in the tissue in a non-invasive manner, without using radiation. Among NIRS, time-resolved NIRS (NIR<sub>TRS</sub>) can quantify the concentrations of oxygenated and deoxygenated hemoglobin ([oxy-Hb] and [deoxy-Hb], respectively) by emitting ultrashort (100 ps) light pulses and counts photons, which are scattered and absorbed in the tissue. The basis for assessing BAT density (BAT-d) using NIR<sub>TRS</sub> is that the vascular density in the supraclavicular region, as estimated using Hb concentration, is higher in BAT than in white adipose tissue. In contrast, relatively low-cost continuous wavelength NIRS (NIR<sub>CWS</sub>) is employed for measuring relative changes in oxygenation in tissues. In this review, we provide evidence for the validity of NIR<sub>TRS</sub> and NIR<sub>CWS</sub> in estimating human BAT characteristics. The indicators (Ind<sub>NIRS</sub>) examined were [oxy-Hb]<sub>sup</sub>, [deoxy-Hb]<sub>sup</sub>, total hemoglobin [total-Hb]<sub>sup</sub>, Hb O<sub>2</sub> saturation (StO<sub>2sup</sub>), and reduced scattering coefficient (<inline-formula><mml:math id="M1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mi>μ</mml:mi><mml:mrow><mml:mtext>s sup</mml:mtext></mml:mrow><mml:mo>′</mml:mo></mml:msubsup></mml:mrow></mml:math></inline-formula>) in the supraclavicular region, as determined by NIR<sub>TRS</sub>, and relative changes in corresponding parameters, as determined by NIR<sub>CWS</sub>. The evidence comprises the relationships between the Ind<sub>NIRS</sub> investigated and those determined by <sup>18</sup>FDG–PET/CT; the correlation between the Ind<sub>NIRS</sub> and cold-induced thermogenesis; the relationship of the Ind<sub>NIRS</sub> to parameters measured by <sup>18</sup>FDG–PET/CT, which responded to seasonal temperature fluctuations; the relationship of the Ind<sub>NIRS</sub> and plasma lipid metabolites; the analogy of the Ind<sub>NIRS</sub> to chronological and anthropometric data; and changes in the Ind<sub>NIRS</sub> following thermogenic food supplementation. The [total-Hb]<sub>sup</sub> and [oxy-Hb]<sub>sup</sub> determined by NIR<sub>TRS</sub>, but not parameters determined by NIR<sub>CWS</sub>, exhibited significant correlations with cold-induced thermogenesis parameters and plasma androgens in men in winter or analogies to <sup>18</sup>FDG–PET. We conclude that NIR<sub>TRS</sub> can provide useful information for assessing BAT-d in a simple, rapid, non-invasive way, although further validation study is still needed.</p>