Quantitative Magnetic Resonance Imaging (qMRI) is frequently used to map disease state and disease progression in lower extremity muscles of patients with Spinal Muscular Atrophy (SMA). This is in stark contrast with the almost complete lack of data on the upper extremity muscles which are essential for meaningful activities of daily living. The aim of this study was therefore to assess disease state in upper arm muscles of patients with SMA in comparison to controls by quantitative assessment of fat fraction, diffusion indices and water T2 relaxation times and to relate these measures to muscle force. We evaluated 13 patients with SMA and 15 controls with a 3T MRI protocol consisting of DIXON, DTI, and T2 sequences. qMRI measures were compared between groups and related to muscle force measured with quantitative myometry. Fat Fraction was significantly increased in all upper arm muscles of the patients with SMA compared to controls and correlated negatively with muscle force. Additionally, fat fraction was heterogeneously distributed within the Triceps Brachii (TB) and Brachialis (BR) muscle but not in the Biceps Brachii (BB) muscle. Diffusion indices and water T2 relaxation times were similar between patients with SMA and healthy controls but we did find a slightly reduced MD, λ1 and λ3 in the TB of patients with SMA. Furthermore, MD positively correlated with muscle force in the TB of patients with SMA. The variation in fat fraction further substantiates the selective vulnerability of muscles. The reduced DTI indices along with the positive correlation of MD with muscle force point to myofiber atrophy. Our results show the feasibility of qMRI to map disease state in the upper arm muscles of patients with SMA. Longitudinal data in a larger cohort is needed to further explore qMRI to map disease progression and to capture possible effects of therapeutic interventions.
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