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

Abstract

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 (MR) imaging, MR spectroscopy and computed tomography (CT). CT uses computer-processed X-rays and is now being applied in muscle physiology research. The purpose of this review is to present CT 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 Hounsfield units (HU)] and muscle (−29 HU to +150 HU). Within the latter range, the mean muscle radiation attenuation [muscle (radio) density] is reported. Inconsistent criteria for the upper and lower HU 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 II, 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.

Circulating Activin A predicts survival in cancer patients

“…The most common and accepted HU range for skeletal muscle tissue is −29 to +150 HU and for adipose tissue is −190 to −30 HU. Cross‐sectional area for muscle and adipose tissues was normalized for stature and expressed as muscle and fat indexes (cm 2 /m 2 ) 2, 23. All CT‐scan images were analysed by a single trained observer, who was blinded to patient's status.…”

“…The generally accepted lower limit of SMD is 30 HU, a value defined as two standard definitions below the mean attenuation value of muscles of young healthy persons 2, 24…”

“…Despite intense research, mechanisms of cachexia are still poorly characterized, and therapeutic targets are desperately lacking. The muscle‐wasting process in cachexia is demonstrated by a reduction of muscle size and also an increase in muscle fat infiltration,2 as assessed by the muscle density measured by CT scan. Interestingly, low muscularity and muscle fat infiltration are recognized as independent risk factors of poor survival in cancer patients, regardless of body weight 3, 4, 5, 6.…”

Sarcopenia and cachexia in the era of obesity: clinical and nutritional impact

“…Reduced muscle attenuation is reflective of intermuscular adipose tissue or poor 'quality' skeletal muscle. Aubrey et al (59) introduced a radiation attenuation map of paraspinal/psoas muscles depicting the myoesteatosis phenomenon, a concept illustrated in Fig. 6.…”

“…Conversely, the patient on the left (A 1 ) has the majority of tissue area within normal muscle radiation attenuation values. This concept is presented in Aubrey et al (59) . by cancer patients is variable (0·2-2·7 g/kg) (69) and many do not meet current dietary guidelines of 0·8 g/kg for healthy individuals (70) or 1·0-1·5 g/kg for those with cancer (71)(72)(73) .…”