Validity of ultrasound imaging for intrinsic foot muscle cross-sectional area measurements demonstrated by strong agreement with MRI

PURPOSE: Intrinsic foot muscles maintain foot structural integrity and contribute to functional movement, posture and balance. Thus, assessing intrinsic foot muscle size and strength are important. Magnetic resonance imaging (MRI) has been shown to accurately image the individual muscles but is costly and time consuming. Ultrasound (US) imaging may provide an alternative that is less costly and more readily available. The purpose of this study was to investigate the validity and intratester reliability of US imaging in measuring intrinsic foot muscle size in comparison to MRI. METHODS: US and MRI were employed to measure the intrinsic foot muscle size involving 35 participants (females = 13; males = 22). The scanned intrinsic foot muscles included the flexor hallucis brevis (FHB), abductor hallucis (ABDH), flexor digitorum brevis (FDB), quadratus plantae (QP) and abductor digiti minimi (ADM). Pearson product correlation (r), intraclass correlation coefficients (ICC), standard error of the measurement (SEm) and minimal detectable difference (MDD) were calculated. RESULTS: High correlations were detected between the US and MRI cross-sectional area (CSA) measurements (r = .971 to 0.995). Test reliability was excellent for both MRI and US (ICC = 0.994 to 0.999). Limits of agreement between MRI and US measurements from ranged from 5.7 to 12.2% of muscle size. SEm values for US ranged from 0.026 to 0.044 cm2, while the SEm for MRI ranged from 0.018 to 0.023 cm2. MDD values for US ranged from 0.073 to 0.122 cm2, while MRI ranged from 0.045 to 0.064 cm2. CONCLUSIONS: US appears to be a valid and reliable alternative to MRI when measuring intrinsic foot muscle CSA. While US is less costly and more readily available, the MRI results were shown to be slightly more precise.