Fatty degeneration (synonym: fatty replacement) is defined as the replacement of muscle tissue by fat. It can be a physiologic phenomenon like in normal aging muscle tissue (sarcopenia) but is more prominent in pathophysiologic conditions such as inactivity, disuse, denervation (chronic stage), drug use (steroids), and degenerative/dystrophic muscle diseases. However, the term fatty degeneration is more appropriate for primary degenerative disease processes like in chronic denervation and (inherited) dystrophic muscle diseases, whereas as the term fatty replacement should rather be used for normal muscle aging and other possibly reversible conditions.

Fatty degeneration as well as fatty replacement can present as focal lesions or as a diffuse process involving the entire muscle or even muscle groups. Since the volume of the muscle may decrease during muscle degeneration, fatty degeneration is frequently associated with atrophy. However, fat deposition may also lead to an increase in muscle volume and therefore to “pseudo hypertrophy”, most frequently observed in the calf muscles (Fischer et al. 2005). Structured and reproducible assessment and grading of fatty muscle degeneration is crucial for the diagnosis and disease monitoring. In particular, patterns of muscle involvement may be of value in diagnostic and differential diagnostic considerations. Although quantitative MR methods such as the three-point Dixon method might be more accurately describe the degree of fatty degeneration (Glover and Schneider 1991; Wren et al. 2008), several semi-quantitative rating scales have been developed, established and validated in clinical practice allowing a fast and reproducible rating of fatty degeneration (Kornblum et al. 2006; Fischer et al. 2008; Mercuri et al. 2002). Table 1 gives an overview of the most established and used rating scales on fatty degeneration in inherited muscle diseases. Fig. 2 shows different stages of fatty degeneration.

Skeletal muscle edema in MRI (synonym: myoedema) is defined as an increase in the amount of extracellular and/or intracellular water leading to a prolonged T2-tissue relaxation time and therefore to high signal intensities on T2-weighted images. Fat suppressed T2-weighted sequences are the most sensitive MR sequences in the detection of myoedema (Fig. 3). Similar to the assessment of fatty degeneration, muscle edema can present as focal areas of edema involving parts of a certain muscle, may involve the entire muscle and muscle groups or may present as diffuse edema involving major parts of the skeletal muscle system such as in systemic rhabdomyolysis. Compared to fatty degeneration, visual rating scales for the assessment and grading (quantification) of myoedema are less popular and not applied on regular basis in the clinical routine setting. One example of a rating scale in muscle edema by Poliachik and colleagues (2012) is presented in Table 2.

Myoedema is a rather unspecific finding on imaging and is not exclusively being found in degenerative muscle diseases accompanying fatty muscle degeneration. It can be observed in a broad and heterogeneous spectrum of diseases including muscle denervation (Chap. 11), autoimmune (aseptic) inflammatory myopathies (polymyositis, dermatomyositis) (Chap. 13), infectious muscle diseases or infections of the adjacent structures, muscle trauma (Chap. 10), treatment related (e.g., radiation), rhabdomyolysis (e.g., toxic, drug induced), and inherited myopathies/muscular dystrophies. Myoedema may also occur in physiologic conditions e.g., during or after exercise probably due to extracellular (intercellular) water accumulation.

In patients with inherited muscle diseases, the assessment of muscle edema is a crucial part of MR image analysis. Histopathological studies have conclusively shown that the corresponding histopathological findings include the loss of muscle fiber integrity leading to primarily intracellular water accumulation as a first sign of degeneration followed by degeneration of muscle fibers which will subsequently be replaced by fat and connective tissue in later disease stages. The exact pathophysiological mechanism is not well understood yet. However, results from immuno-histochemistry studies provided some arguments that inflammatory processes are involved in dystrophic muscle tissue with muscle edema. The development and degree of muscle edema in dystrophic muscle tissue can be initiated, aggravated, and accelerated by muscle exercise. From the clinical point of view, it is important to understand that muscle edema not only accompanies but also precedes degenerative findings such as fatty degeneration. Therefore, muscle cell edema in dystrophic muscle diseases detected by MRI might be the first imaging manifestation of muscle tissue degeneration. This is of particular importance since these findings on muscle MRI can guide muscle biopsy, in order to acquire muscle tissue with ongoing disease activity as a target tissue for diagnostic and treatment monitoring.

Fat depositions in the skeletal muscle have a broad differential diagnosis and do not exclusively represent fatty degeneration. In terms of the correct interpretation of fatty replacement and degeneration in the skeletal muscle, it is key to distinguish between real fatty muscle degeneration and other entities associated with increased fat deposition, such as soft tissues including lipomas, hemangiomas, and other soft tissue tumors containing lipomatous tissue. Another important pitfall is the increased amount of fat tissue in the muscle of older and/or immobile patients. From the clinical point of view, it is crucial to differentiate between muscle changes due to aging, misuse, and immobilization (e.g., atrophy and fatty replacement) in terms of sarcopenia and abnormalities due to a primary degenerative disease of the skeletal muscle such as late onset inherited muscle dystrophies or sporadic inclusion body myositis. In sarcopenia and “normal aging” skeletal muscle, atrophy and fatty replacement are rather homogenously and symmetrically distributed affecting almost the entire skeletal muscle. In immobilezed patients and patients with misuse, the underlying cause (e.g., degenerative changes of the spine, postoperative changes) must be assessed first to enable a correct interpretation. Degenerative muscle changes in patients with muscle dystrophies can be symmetrically but may also asymmetrical. Co-morbidity such as aging and misuse can make a correct estimation and differentiation of primary and secondary degenerative changes challenging.

As far as the assessment of muscle edema concerns, the greatest pitfall is the inhomogeneous and/or insufficient fat suppression of T2-weighted images due to magnetic field inhomogeneity particularly at the peripheral region of the field of view (Fig. 4). Identifying T2-hyperintense lesions in the skeletal muscle on fat suppressed T2-weighted images, the assessment of the adjacent tissues (e.g., subcutaneous fat) is useful. If the adjacent structures show inhomogeneous signal intensity and an insufficient suppression of the subcutaneous fat, these muscle changes should not be considered as “real” muscle edema. Other important reasons for insufficient fat suppression include susceptibility artifacts particularly close to material containing metal (e.g., joint prosthesis).