MRI in muscle disease
Pathology: Muscle edema, myositis, inflammatory myopathies: T1WI: hypointense; T2WI: hyperintense; STIR: hyperintense.
Pathology: Fatty infiltration (chronic muscle injury), muscle dystrophies: T1WI: hyperintense; T2WI: hyperintense; STIR: hypointense.
MRI in muscle disease: Imaging is used in evaluation of muscle diseases, with MRI and ultrasound playing important role in the detection and characterization of skeletal muscle abnormalities.
Detection of muscles that are difficult to examine or are not clinically weak.
Fascioscapulohumeral muscular dystrophy will have atrophic and dystrophic changes seen (fatty infiltration) in the serratus anterior.
MRI can be helpful in identifying an appropriate target for biopsy, particulary in myopathies that have a patchy involvement. It also helps avoiding end-stage muscles with extensive fibrosis or fatty replacement that will likely have a low diagnostic yield.
Udd (tibial) myopathy may present with fatty replacement of the anterior tibialis.
sIBM, atrophy and edema involving VL and VM of the quadriceps with relative sparing of the rectus femoris may help distinguish IBM form other myopathies. Axial proton density MRI of distal leg is helpful.
Collagen VI-related myopathies (Ullrich and Bethlem).
Bethlem myopathy is often associated with rimming of T1 hyperintensity around the outer part of the VL with relative sparing of center of the muscle, while the rectus femoris may have prominent central T1 hyperintensity (called as a central shadow). This can be seen as a hypointensity in the middle of rectus femoris on axial STIR.
Ullrich myopathy can also have similar rim of T1 hyperintensity around the VL with a similar but less prominent central hyperintensity in the rectus femoris. There is relative sparing of sartorius, AL, and gracilis, but diffuse involvement of other muscles are seen.
Desminopathy. Semitendinosus is affected at least as much as tibialis anterior.
Myotilinopathy. Adductor magnus is more affected than gracilis and sartorius is least affected as the semitendinosus.
Filaminopathy. Biceps femoris and semitendinosus are least affected as the sartorius, and medial gastrocnemius is more affected than the lateral gastrocnemius.