# Ultrasound and MRI Are Not Interchangeable for Measuring Muscle Cross‐Sectional Area: Analysis of Reliability and Validity in 10 Lower Limb Muscles Considering Sex Differences

## TL;DR

Ultrasound is reliable for measuring muscle cross-sectional area but consistently underestimates values compared to MRI, making the two methods non-interchangeable.

## Contribution

The study evaluates reliability and validity of ultrasound in 10 lower limb muscles across sexes, revealing systematic bias not previously emphasized.

## Key findings

- US-ACSA showed high reliability (ICC2,3 0.992%–0.999%) across muscle sizes and sexes.
- US-ACSA systematically underestimated MRI-ACSA by 11.5% to 23.7%.
- Despite strong correlations (r = 0.73–0.95), US and MRI measurements are not interchangeable due to bias.

## Abstract

Ultrasound‐based anatomical cross‐sectional area (US‐ACSA) measurement using extended‐field‐of‐view (EFOV) imaging offers a practical alternative to magnetic resonance imaging (MRI‐ASCA) measurements. Although its reliability and validity have previously been assessed, previous studies have focused only on a single or limited number of muscles with little attention to effects of muscle size and sex. This study was conducted in ten lower limb muscles of varying sizes in males and females in order to assess the reliability and validity of US‐ACSA measurements using MRI as the gold standard method. Twelve males and twelve females participated in this study. US‐ and MRI‐ACSA were measured twice, one day apart. Reliability was assessed using intraclass correlation coefficients (ICC2,3), standard error of measurement (SEM) and minimal detectable change (MDC). Validity was examined using Pearson product‐moment correlation and Bland–Altman analysis. A linear model was used to evaluate the effects of muscle size and sex. ICC2,3, SEM and MDC ranged from 0.992% to 0.999%, 0.50%–1.90% and 1.25%–5.36% for US‐ACSA and from 0.952% to 0.998%, 0.90%–4.76% and 2.51%–13.19% for MRI‐ACSA. US‐ACSA showed strong correlations with MRI‐ACSA (r = 0.73–0.95). A systematic underestimation of MRI values (relative bias: −11.5% to −23.7%) was identified, with no systematic effect of muscle size or sex. These findings suggest that US‐ACSA measurements of lower limb muscles are highly reliable regardless of muscle size and sex. However, due to the large systematic bias, US‐ and MRI‐based ACSAs measurements are not interchangeable.

Ultrasound‐based ACSA measurements using the extended‐field‐of‐view method are highly reliable for a wide range of lower limb muscles, regardless of muscle size or participant sex.Ultrasound‐ and MRI‐based ACSA measurements were strongly correlated for most muscles; however, ultrasound systematically underestimated ACSA values.Findings suggest that, regardless of muscle size or sex, ultrasound‐ and MRI‐based ACSA measurements are not interchangeable.

Ultrasound‐based ACSA measurements using the extended‐field‐of‐view method are highly reliable for a wide range of lower limb muscles, regardless of muscle size or participant sex.

Ultrasound‐ and MRI‐based ACSA measurements were strongly correlated for most muscles; however, ultrasound systematically underestimated ACSA values.

Findings suggest that, regardless of muscle size or sex, ultrasound‐ and MRI‐based ACSA measurements are not interchangeable.