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.

Keywords: Hounsfield units; computed tomography; muscle attenuation; muscle density; myosteatosis; skeletal muscle.

Figure 1

Radiation attenuation map of paraspinal muscles with and without myosteatosis. (a, c, e, g): Subject 1 is a 63-year-old male with a body mass index of 26.0 kg m⁻². Paraspinal and psoas muscles of Subject 1 show visible fat within the fascia surrounding skeletal muscle (intermuscular fat, light blue) making up 4.6% of total tissue area. Exclusive of the intermuscular fat, the mean overall radiation attenuation is 42.3 HU with 77.2% of the total muscle cross-sectional area falling into the normal attenuation range for muscle [red]. (b, d, f, h): Subject 2 is similar in age [65 years] and body mass index [26.7 kg m⁻²] to Subject 1. Subject 2 exhibits extensive visible regions of intermuscular fat infiltration (light blue) comprising 14.1% of total area, a value threefold higher than Subject 1. Exclusive of the macroscopic fat infiltration, paraspinal and psoas muscles show abnormally low overall mean attenuation [20.4 HU]. In this subject, less than half [44.4%; annotated in red] of the total tissue cross-sectional area falls within the normal range of muscle radiation attenuation values.