Noninvasive technique to evaluate the muscle fiber characteristics using q-space imaging

Background: Skeletal muscles include fast and slow muscle fibers. The tibialis anterior muscle (TA) is mainly composed of fast muscle fibers, whereas the soleus muscle (SOL) is mainly composed of slow muscle fibers. However, a noninvasive approach for appropriately investigating the characteristics of muscles is not available. Monitoring of skeletal muscle characteristics can help in the evaluation of the effects of strength training and diseases on skeletal muscles. Purpose: The present study aimed to determine whether q-space imaging can distinguish between TA and SOL in in vivo mice. Methods: In vivo magnetic resonance imaging of the right calves of mice (n = 8) was performed using a 7-Tesla magnetic resonance imaging system with a cryogenic probe. TA and SOL were assessed. q-space imaging was performed with a field of view of 10 mm × 10 mm, matrix of 48 × 48, and section thickness of 1000 μm. There were ten b-values ranging from 0 to 4244 s/mm2, and each b-value had diffusion encoding in three directions. Magnetic resonance imaging findings were compared with immunohistological findings. Results: Full width at half maximum and Kurtosis maps of q-space imaging showed signal intensities consistent with immunohistological findings for both fast (myosin heavy chain II) and slow (myosin heavy chain I) muscle fibers. With regard to quantification, both full width at half maximum and Kurtosis could represent the immunohistological findings that the cell diameter of TA was larger than that of SOL (P < 0.01). Conclusion: q-space imaging could clearly differentiate TA from SOL using differences in cell diameters. This technique is a promising method to noninvasively estimate the fiber type ratio in skeletal muscles, and it can be further developed as an indicator of muscle characteristics.

背景：骨骼肌包含快肌纤维与慢肌纤维两类。胫骨前肌（tibialis anterior muscle, TA）主要由快肌纤维构成，而比目鱼肌（soleus muscle, SOL）则以慢肌纤维为主。但目前尚无合适的非侵入性方法可用于精准研究肌肉特性。对骨骼肌特性的监测，有助于评估力量训练与疾病对骨骼肌产生的影响。目的：本研究旨在探究q空间成像（q-space imaging）能否在活体小鼠体内区分胫骨前肌与比目鱼肌。方法：采用配备低温探头的7特斯拉磁共振成像（magnetic resonance imaging, MRI）系统，对8只小鼠的右侧小腿开展活体磁共振成像，对胫骨前肌与比目鱼肌进行分析。q空间成像的视野设置为10 mm × 10 mm，矩阵为48 × 48，层厚为1000 μm。共设置10个b值，取值范围为0至4244 s/mm²，且每个b值均进行三个方向的扩散编码。将磁共振成像结果与免疫组织化学（immunohistological）检测结果进行对比。结果：q空间成像的半高全宽（full width at half maximum, FWHM）与峰度（Kurtosis）图谱的信号强度，与快肌纤维（肌球蛋白重链II，myosin heavy chain II）、慢肌纤维（肌球蛋白重链I，myosin heavy chain I）的免疫组织化学结果一致。定量分析结果显示，半高全宽与峰度均可反映胫骨前肌的细胞直径大于比目鱼肌这一结论（P < 0.01）。结论：q空间成像可通过细胞直径的差异，清晰区分胫骨前肌与比目鱼肌。该技术有望实现无创评估骨骼肌的肌纤维类型比例，可进一步开发为肌肉特性的检测指标。