Measurement of skeletal muscle radiation attenuation and basis of its biological variation

<jats:title>Abstract</jats:title><jats:p>Skeletal muscle contains intramyocellular lipid droplets within the cytoplasm of myocytes as well as intermuscular adipocytes. These depots exhibit physiological and pathological variation which has been revealed with the advent of diagnostic imaging approaches: magnetic resonance (<jats:styled-content style="fixed-case">MR</jats:styled-content>) imaging, <jats:styled-content style="fixed-case">MR</jats:styled-content> spectroscopy and computed tomography (<jats:styled-content style="fixed-case">CT</jats:styled-content>). <jats:styled-content style="fixed-case">CT</jats:styled-content> uses computer‐processed X‐rays and is now being applied in muscle physiology research. The purpose of this review is to present <jats:styled-content style="fixed-case">CT</jats:styled-content> methodologies and summarize factors that influence muscle radiation attenuation, a parameter which is inversely related to muscle fat content. Pre‐defined radiation attenuation ranges are used to demarcate intermuscular adipose tissue [from −190 to −30 <jats:styled-content style="fixed-case">H</jats:styled-content>ounsfield units (<jats:styled-content style="fixed-case">HU</jats:styled-content>)] and muscle (−29 <jats:styled-content style="fixed-case">HU</jats:styled-content> to +150 <jats:styled-content style="fixed-case">HU</jats:styled-content>). Within the latter range, the mean muscle <jats:italic>radiation attenuation</jats:italic> [muscle (<jats:italic>radio</jats:italic>) density] is reported. Inconsistent criteria for the upper and lower <jats:styled-content style="fixed-case">HU</jats:styled-content> cut‐offs used to characterize muscle attenuation limit comparisons between investigations. This area of research would benefit from standardized criteria for reporting muscle attenuation. Available evidence suggests that muscle attenuation is plastic with physiological variation induced by the process of ageing, as well as by aerobic training, which probably reflects accumulation of lipids to fuel aerobic work. Pathological variation in muscle attenuation reflects excess fat deposition in the tissue and is observed in people with obesity, diabetes type <jats:styled-content style="fixed-case">II</jats:styled-content>, myositis, osteoarthritis, spinal stenosis and cancer. A poor prognosis and different types of morbidity are predicted by the presence of reduced mean muscle attenuation values in patients with these conditions; however, the biological features of muscle with these characteristics require further investigation.</jats:p>