Muscle quality: Intramuscular fat, collagen fibres, and mechanical properties in the triceps surae

Skeletal muscle architecture provides valuable insights for individuals with neuromuscular diseases, such as cerebral palsy (CP) [1]. Yet, to have a comprehensive view of muscle remodelling and better-informed clinical treatments, muscle quality (i.e. intramuscular fat, collagen fibres, and mechanical properties) should also be explored [2]. This comprehensive view can be achieved in a non-invasive image-based manner by combing magnetic resonance imaging (MRI) and shear wave elastography (SWE). What is the relationship between intramuscular fat fraction or T2 relaxation time and muscle mechanical properties? One individual with CP (13 years, male, GMFCS I) and four typically developing (TD, 17.3±7.9 years, 4 females) peers were enrolled in the study. Medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SOL) were assessed in neutral position (middle position between maximal dorsiflexed and plantarflexed position; CP -15.0°, TD -16.3±6.3°), while participants were laying prone with knee extended. SWE (Aixplorer, Supersonic Imagine) was recorded for MG and LG at mid-muscle belly, for SOL distally below the LG muscle-tendon junction. Shear modulus was estimated by means of an open-source software (ELASTOGUI, University of Nantes). Fat fraction and T2 relaxation times were estimated from modified Dixon and T2 mapping sequence using a 3.0-Tesla MR scanner (Ingenia CX, Philips Healthcare) at the same ankle position as SWE measurements. The intramuscular fat fraction was calculated based on 2-point fat-water separation [3]. T2 relaxation time is a quantitative parameter indicating collagen fibres content [4]. The correlation between shear modulus and fat fraction / T2 relaxation time was evaluated using linear correlation coefficient. Overall, the individual with CP showed higher muscle shear moduli than TD peers (Figure A) in all three muscles. The individual with CP had a similar fat content in MG and LG but higher fat content in SOL than TD peers (Figure B&F). Regarding the collagen fibres, the average T2 relaxation time for all three muscles were similar in both groups (Figure C). Overall, the correlation between muscle shear modulus and fat fraction / T2 relaxation time was weak (R=0.24 for fat fraction, R=-0.10 for T2 relaxation time, Figure D&E). Figure. (A-C) Average shear modulus, fat fraction, and T2 relaxation time. (D-E) Correlation between shear modulus and fat fraction / T2 relaxation time. The scatter points mean the imaging parameter and related shear modulus for all subjects. (F-G) Sample fat fraction and T2 maps. Download : Download high-res image (178KB)Download : Download full-size image This study is a first attempt to comprehensively analyze muscle quality in CP by combining MRI and SWE. It confirms the increased muscle fat fraction in CP [5], whilst no difference for T2 relaxation time was observed. The correlation results suggested higher passive muscle stiffness with higher fat content. These preliminary results need to be confirmed once a larger sample is collected.