Skeletal muscle fat content based on echo intensity by ultrasonography

Ultrasonography is an easily applicable method for visualization of muscle. It is non-invasive, portable and very reliable for use, as it requires no sedation of the patient. Disruption of the normal muscle architecture and infiltration of fat causes increased reflections of the ultrasonography beam, resulting in an increase of muscle echo intensity (EI). EI can be quantified using histogram-based gray-scale analysis, which calculates the mean gray value of muscle in a region of interest. Previous studies have demonstrated the feasibility of muscle EI quantification. Quantitative analysis of EI and muscle thickness (MT) is a reliable method for discriminating between normal muscle and that affected by neuromuscular disease in children, or in middle-aged or elderly persons. However, it is unclear whether skeletal muscle fat content in healthy adults differs according to exercise habit. The purpose of this study was to compare MT and EI between exercising and non-exercising adults using ultrasound imaging.  Thirty healthy men (15 atheletes and 15 non-exercising men) with a mean age of 20.7 + 1.3 years participated. The subjects were free of serious disease, and had no family history of neuromuscular disorder or physical growth disorder. Anthropometry, body composition, and ultrasonographically determined MT and EI at the right upper arm and thigh were assessed. Ultrasound images of the biceps brachii muscle and quadriceps femoris muscle were obtained using a B-mode ultrasonography device (Aloka, SDD-Prosound2, Japan) based on the study of Pillen (2003). Mean echo intensities (EI) were determined using a computed 8-bit gray scale analyzer employing the standard histogram function of Adobe Photoshop Elements (Adobe Systems Inc., USA).  Percentage body fat (%Fat) and EI at the upper arm and thigh in non-exercising men were significantly higher than those in the athletes (%Fat: 17.4+5.8% vs 12.4+3.3%, EI: upper arm 18.6+2.5 vs 15.7+3.3, thigh 23.8+3.0 vs 19.7+3.1, p<0.05, respectively). Furthermore, EI at the thigh showed a significant positive correlation with %Fat (r=0.658, p<0.001). In contrast, MT showed no significant difference between the atheletes and the non-exercisers.  Our results suggest that accumulation of adipose tissue within skeletal muscle varies according to exercise habit in healthy adults. An increase in both body fat and skeletal muscle fat content is linked with an inactive lifestyle, which in turn is likely to cause qualitative changes in muscles.