Dystrophic changes such as fatty degeneration can be easily and sensitively detected using the T1W and T2W sequences. In addition, inflammatory changes such as muscle oedema can be depicted on the T2WFS sequences (Fig. 2). It has been conclusively shown that MRI has a higher sensitivity in the detection of dystrophic changes compared with CT [22, 23]. MRI can be performed and rated in a standardised manner suggesting a good inter-rater agreement and intra-rater (during follow-up) agreement. The degree of muscular dystrophy in inherited muscle diseases is rated according to rating scales [25–27]. Most of the established rating scales are based on the amount of fatty degeneration ranging from normal appearance to complete fatty degeneration (Table 1). The evaluation of the muscle MRI using standardised rating scales allows a fast and reproducible assessment of the degree of involvement of each muscle. Initially, muscle MRI protocols were developed to evaluate certain anatomical areas such as the lower extremities and pelvis. More recent whole-body imaging protocols have been established allowing the evaluation of almost all relevant striated muscle groups. Pattern recognition of muscle involvement is sometimes helpful in narrowing the differential diagnosis and leading to the most probable diagnosis before muscle biopsy. Because of the lack of any radiation exposure, muscle MRI has become an important diagnostic technique for the evaluation of children [3]. In addition, whole-body MR imaging protocols allow evaluation of tissues and organs beyond the striated muscle such as the parenchymatous organs in the abdomen, the oesophagus and heart, all of which can be affected in patients with inherited neuromuscular diseases [28, 29] (Fig. 3).
Fig. 2 Transverse T1-weighted (upper row) and spectral fat-suppressed T2-weighted MR image (bottom row) of the thighs of a 48-year-old woman presenting with myotonic dystrophy type 1. Note the different degrees of fatty degeneration within the gastrocnemius muscle. The medial head shows an end-stage fatty degeneration (grade 4 according to the Mercuri and Fischer scale, grade 3 according to the Kornblum scale, Table 1). The muscle tissue is completely replaced by fat. The lateral head shows a moth-eaten appearance with scattered small areas of increased signal (fatty degeneration grade 2 according to the rating scales established by Mercuri et al., Kornblum et al. and Fischer et al., Table 1). The fat-suppressed T2-weighted image shows a high signal in the medial head of the gastrocnemius muscle indicating oedema because of inflammatory changes before and during the degenerative disease stages
Table 1 Summary of the well-established rating scales on MRI concerning the visual rating of dystrophic change of striated muscle tissue
Grade Mercuri et al. 2002 [25] Kornblum et al. 2006 [27] Fischer et a. 2008 [26]
0 Normal appearance Normal appearance
1 Normal appearance Discrete moth-eaten appearance with sporadic T1 hyperintense areas Mild: traces of increased signal intensity on the T1-weighted MR sequences
2 Mild involvement: Early moth-eaten appearance with scattered small areas of increased signal or with numerous discrete areas of increased signal with beginning confluence, comprising less than 30% of the volume of the individual muscle a. Moderate moth-eaten appearance with numerous scattered T1 hyperintense areas Moderate: increased T1-weighted signal intensity with beginning confluence in less than 50% of the muscle
b. Late moth-eaten appearance with numerous confluent T1 hyper-intense areas
3 Moderate involvement: Late moth-eaten appearance with numerous discrete areas of increased signal with beginning confluence, comprising 30-60% of the volume of the individual muscle Complete fatty degeneration, replacement of muscle by connective tissue and fat Severe: increased T1-weighted signal intensity with beginning confluence in more than 50% of the muscle
4 Severe involvement: Washed-out appearance, fuzzy appearance due to confluent areas of increased signal or an end-stage appearance, with muscle replaced by increased density connective tissue and fat, and only a rim of fascia and neurovascular structures distinguishable End-stage appearance, entire muscle replaced by increased density of connective tissue and fat
Fig. 3 Transverse spectral fat-suppressed T2-weighted images obtained from two patients presenting with a myotonic dystrophy type 1 (A: 37-year-old woman; B: 16-year-old boy). The images were obtained during a multi-sequence whole-body muscle MRI protocol. In both patients, dilatation of the oesophagus in the proximal segment with the air-fluid level could be diagnosed, which was clinically reflected by dysphagia