Optoacoustic Molecular Signatures of Muscle Involvement in Facioscapulohumeral Dystrophy Compared to Conventional Imaging

Facioscapulohumeral muscular dystrophy (FSHD) is a genetic disorder characterized by asynchronous muscle degeneration and marked pathological heterogeneity. According to the National Institute of Neurological Disorders and Stroke (NINDS), FSHD affects approximately 1 in 20,000 individuals worldwide. While conventional MRI can detect fatty replacement and oedema-like abnormalities, it provides limited insight into other relevant biological processes such as extracellular matrix remodelling and tissue perfusion.

The Limitations of Conventional Imaging

Conventional MRI relies on the detection of structural changes in muscles, such as fatty replacement and muscle atrophy. However, these changes often occur after significant muscle damage has already occurred. In contrast, multispectral optoacoustic tomography (MSOT) is an emerging imaging technique that can non-invasively interrogate tissue composition through endogenous optical absorbers.

Multispectral Optoacoustic Tomography (MSOT)

MSOT has the potential to provide a unique window into the underlying biological processes that drive muscle degeneration in FSHD. This study investigated whether MSOT can provide complementary molecular information on muscle involvement in FSHD. The study used a standardized protocol to acquire MSOT images from 568 muscles from 15 genetically confirmed FSHD patients and 9 healthy volunteers.

Methodology and Study Design

The analysis included single-wavelength signal intensity and spectrally unmixed signals related to lipid, collagen, deoxygenated haemoglobin, oxygenated haemoglobin, total haemoglobin, and tissue oxygen saturation. Patients also underwent conventional muscle MRI together with clinical strength and disease severity assessments. Feasibility, reproducibility, discrimination between patients and controls, and associations with MRI and clinical measures were evaluated.

Key Findings and Data Analysis

MSOT acquisition was well tolerated and highly reproducible (intra- and inter-rater ICC = 0.88). FSHD muscles displayed a distinct optoacoustic profile characterized by reduced signal intensity at wavelengths below 900 nm, increased lipid signal, and reduced oxygenated and total haemoglobin signals. Lipid signal discriminated FSHD from control muscles (AUC 0.830, p = 0.008) and increased with MRI-defined fatty replacement (p < 0.0001).

Short Tau Inversion Recovery (STIR)-Positive Muscles

STIR-positive muscles on MRI exhibited a distinct molecular phenotype, with higher collagen-associated and lipid signals together with increased deoxygenated and total haemoglobin signals, consistent with active tissue remodelling. Although FSHD muscles classified as normal by conventional imaging showed group-level optoacoustic profiles comparable to controls, 13 of 141 (9%) demonstrated at least one abnormal MSOT-derived parameter.

Reduced Haemoglobin-Related Signals

Reduced haemoglobin-related signals correlated with greater global disease severity, whereas increased lipid signal was associated with lower muscle strength. These findings suggest that MSOT may be a valuable tool for monitoring FSHD progression and evaluating the efficacy of therapeutic interventions.

Clinical and Practical Implications

MSOT is a feasible and reproducible imaging technique in FSHD that provides complementary molecular information beyond conventional structural imaging. Its ability to detect signatures related to fatty replacement, STIR positivity, functional impairment, and isolated abnormalities in radiologically normal muscles supports further longitudinal studies of MSOT-derived readouts as potential biomarkers of disease activity, progression, and therapeutic response.

Future Directions

Future studies should aim to further establish the clinical utility of MSOT in FSHD, including the development of standardized imaging protocols and the evaluation of MSOT-derived readouts as potential biomarkers of disease activity and progression. Additionally, the use of MSOT in conjunction with other imaging modalities, such as conventional MRI and CT, may provide a more comprehensive understanding of the underlying biology of FSHD and offer improved diagnostic and monitoring capabilities.

In summary, this study demonstrates the potential of MSOT as a molecular imaging modality for FSHD. The technique provides a unique window into the underlying biological processes that drive muscle degeneration and offers a promising approach for the detection and monitoring of FSHD. The results of this study have important implications for the management of FSHD, particularly in the context of disease monitoring and therapeutic evaluation.

This study highlights the potential of MSOT as a valuable tool for the diagnosis and monitoring of FSHD. The use of MSOT-derived readouts as biomarkers of disease activity and progression may provide a more accurate and sensitive assessment of disease severity than conventional imaging. However, further research is needed to fully elucidate the clinical utility of MSOT in FSHD and to establish its place within the diagnostic and monitoring toolkit.