Test-retest Reliability Of Ultrasound Derived Functional Cross-sectional Area And Echo-intensity Values In Older Men

Abstract

Ultrasound (US) evaluations of muscle quality have used echo intensity (EI) values to determine the amount of fat and/or connective tissue infiltration. However, these values provide only arbitrary units from 0 - 255. Magnetic resonance imaging has quantified tissue composition by a grey-scale threshold technique to determine a functional cross-sectional area (FCSA) or a CSA that excludes the non-contractile tissue. It’s possible that these techniques may be useful in US imaging; however the consistency of these measures warrants further research. PURPOSE: To examine the test-retest reliability of US derived EI and FCSA values in older adults. METHODS: Twenty-two older men (mean ± SD age 69.0 ± 3.1 years; height 171.2 ± 5.9 cm; mass 81.7 ± 10.7 kg) visited the laboratory 2 times separated by 2 - 7 days. Measurements were taken in the prone position with leg fully extended and the right foot attached to a vertical post at an ankle joint angle of 90°. The US probe was positioned perpendicular to the skin and moved along the transverse plane of the medial gastrocnemius with transmission gel applied to the skin to enhance acoustic coupling. Panoramic images were obtained using a portable B-mode US imaging device and were analyzed off-line using Image-J software. The mean EI values were assessed by computer-aided gray-scale analyses. Using the same region of interest, FCSA was calculated from a threshold signal range (0- 60) that would include just lean muscle tissue. Test-retest reliability for the mean EI and FCSA values were determined using the intraclass correlation coefficient (ICC) and standard error of measurement (SEM). Systematic error was examined using separate one-way repeated measures analyses of variance. P-value was set a priori at 0.05. RESULTS: There was no systematic error between days for EI (P=0.336) or FCSA (P=0.765). The ICC values for EI and FCSA were 0.843 and 0.860, respectively. The SEM values (expressed as a percentage of the mean) for EI and FCSA were 3.70 and 28.17%, respectively. CONCLUSIONS: These findings suggest that EI is a reliable measure of muscle quality in older adults and FCSA measures demonstrate acceptable relative consistency. However, due to the larger SEM% values, FCSA should be interpreted with caution. Future studies should examine more specific threshold signal ranges for older adults.