Ultrasound vs. MRI: Which is the Best Way to Measure Muscle Volume?
"A new study compares 3D ultrasound to MRI for measuring gastrocnemius muscle volume in adults and children with and without cerebral palsy, revealing surprising insights for athletes and patients."
Understanding muscle volume is crucial for assessing muscle health, tracking adaptations to training, and diagnosing various conditions. Historically, MRI has been considered the gold standard for measuring muscle volume due to its high-resolution imaging capabilities. However, MRI has limitations, including high costs, long scanning times, and contraindications for individuals with metal implants or epilepsy. These limitations have spurred the search for alternative, more accessible methods for assessing muscle volume.
Ultrasound, particularly freehand 3D ultrasound (3DUS), has emerged as a promising alternative. 3DUS offers several advantages over MRI, including lower cost, portability, and shorter scanning times. However, questions remain about the accuracy and reliability of 3DUS compared to MRI, especially in diverse populations such as children and individuals with neuromuscular conditions like cerebral palsy.
A recent study published in 2023 directly compared 3DUS to MRI for measuring the gastrocnemius muscle volume in adults and children, with and without cerebral palsy. The study aimed to evaluate the validity of 3DUS and explore its potential as a practical tool for assessing muscle health in various clinical and research settings.
Growing Precision in Muscle Measurement
Ultrasound-based muscle assessment has progressed from measuring cross-sectional area to reconstructing three-dimensional muscle volume. A 2022 study synchronized probe motion-tracking data with B-mode ultrasound scans to reconstruct 3-D muscle volume and used statistical shape modeling to examine segmentation repeatability. Another approach estimates vastus lateralis volume from a muscle shape factor, muscle length, and a single vastus lateralis anatomical cross-sectional area measurement. Recent research reports that freehand 3-D ultrasound provides accurate skeletal-muscle-volume estimates relative to MRI across varied ages, health statuses, and muscle groups.
Methods Still Seeking Standardization
MRI is described as a reference standard for quantifying muscle volume, but its limited accessibility has encouraged interest in freehand 3-D ultrasound. Freehand 3-D ultrasound combines B-mode imaging with 3-D tracking technology as a more accessible alternative. Conventional skeletal-muscle ultrasound commonly evaluates cross-sectional area, muscle thickness, and echogenicity, although a 2026 ICU review reports that heterogeneous methods, limited outcome validation, and variable measurement reliability currently prevent peripheral muscle ultrasound from being classified as standard care.
From Cross-Sectional Area to Muscle Volume
Ultrasound muscle measurement traces back to 1968, when Ikai and Fukunaga used a 2.25–5 MHz scanner to assess arm-flexor muscle cross-sectional area. Decades of technological development and studies involving more than 27,500 subjects and 107 different skeletal muscles have supported ultrasonography as a valid tool for quantifying muscle size. Its use for one-time and repeated measurements has extended across young and older adults, athletes, healthy populations, and people with disease. Musculoskeletal ultrasound has also become popular because of its low cost, accessibility, and lack of ionizing radiation.
How Accurate is 3D Ultrasound Compared to MRI?
The study meticulously compared muscle volume measurements obtained from 3DUS and MRI. The results indicated a high level of agreement between the two methods. Specifically, the mean difference in muscle volume measurements between MRI and 3DUS was minimal, with strong correlation coefficients, suggesting that 3DUS can accurately measure muscle volume. Furthermore, digitizing errors for 3DUS were minimal, demonstrating the reliability of the technique.
- Accuracy: High agreement between 3DUS and MRI measurements.
- Reliability: Minimal digitizing errors for 3DUS.
- Versatility: Accurate in adults, typically developing children, and children with cerebral palsy.
An Evolving Evidence Base
Current research is expanding ultrasound muscle assessment beyond isolated thickness or cross-sectional-area measurements toward three-dimensional volume estimation. The field appears promising, but the evidence remains dependent on measurement protocols, imaging technology, and validation against established methods. Further research is needed to clarify how newer approaches perform across muscles, populations, and clinical settings.
The Field-of-View Problem
A major limitation of ultrasound is its restricted field of view. The reported consequence is that ultrasound cannot measure the quantity and quality of certain muscles, which limits its usefulness when complete muscle assessment is required. This limitation is particularly relevant when evaluating muscle loss and quality in older people, where those measurements are associated with adverse effects.
Comparing 3-D Ultrasound With MRI
A study published in 2025 set out to validate a custom 3-D ultrasonography setup for measuring lower-limb muscle volumes against MRI, identified as the current gold standard. It also aimed to assess test–retest and inter-rater reliability for the ultrasound measurements. This comparison frames 3-D ultrasound as a potentially more accessible method whose practical value depends on how closely its volume estimates and repeatability match MRI.
The Future of Muscle Assessment: Broader Implications
The findings of this study have significant implications for clinical practice and sports medicine. 3DUS offers a cost-effective, portable, and time-efficient alternative to MRI for assessing muscle volume. This is particularly valuable in settings where MRI is not readily available or feasible. The ability to accurately measure muscle volume with 3DUS can aid in the diagnosis and monitoring of various conditions, as well as in tracking training adaptations in athletes. Further research is needed to explore the use of 3DUS in different muscle groups and populations.
Choosing the Method for the Question
The available evidence suggests that ultrasound and MRI should not be treated as interchangeable in every context. MRI remains an important reference for muscle-volume measurement, while ultrasound offers a potentially more accessible route to repeated or targeted assessment. The appropriate choice is therefore likely to depend on the required coverage, clinical context, equipment access, and need for repeatability.
Toward More Practical Volume Estimates
Future work will likely focus on making three-dimensional ultrasound measurements more consistent, repeatable, and broadly applicable. Progress will depend on improving imaging and tracking workflows while testing them against established reference methods. Research across more muscles and populations may clarify where ultrasound can provide a reliable alternative to MRI.
Access, Consistency, and Validation
The broader challenge is balancing measurement accessibility with methodological rigor. Ultrasound may be easier to deploy than MRI, but differences in protocols and operator technique can affect reliability. Establishing consistent procedures and validating clinically meaningful outcomes will be important before newer ultrasound approaches can be used uniformly across healthcare and research.
Connecting Measurements With Patient Outcomes
Skeletal-muscle ultrasound is being studied in acute and chronic clinical conditions to measure muscle mass and quality. The reviewed literature examines whether these measurements can help predict functional capacity, severity of malnutrition, hospital admission, and survival. This focus connects imaging results to outcomes that matter directly to patients and clinical decision-making.