On ultrasound, normal muscle is relatively anechoic (black) or hypoechoic (gray) and interspersed with hyperechoic (white) perimysial septa, which gives it a starry sky appearance on cross-section (see Figure 1E). Longitudinally, the parallel orientation of muscle fibers is appreciated. Myopathic muscle is marked by an increase in muscle echointensity, mild in acute muscle edema, with a more marked increase with fatty fibrous replacement. Changes in muscle architecture are also seen, as well as loss of visualization of structures deeper to the muscle (attenuation) when pathology is severe (see Figure 1D).
In cases of acute edema, a see-through echogenicity is described, in which there is no attenuation of ultrasound waves despite an increase in muscle echointensity.9 Notably, acute myopathic changes can be subtle, and detecting edema may be inconsistent on ultrasound. Ultrasound is very useful, however, to pick up chronic changes given its sensitivity for fat/fibrosis and atrophy. Unlike MRI where different sequences can delineate either edema or fatty replacement, ultrasound shows a combination of all pathology in one B mode image. In terms of vascularity, a few studies suggest that higher Doppler scores are seen with acute myositis.10,11 Although the use of Power Doppler is a must in inflamed joints, a need exists for better standardization for muscle because perfusion is affected by multiple factors, including activity and muscle contraction.
Ultrasound has been gaining traction for diagnostic purposes especially in chronic myositis, such as inclusion body myositis (IBM) in which the specificity of the affected muscles can help distinguish it from mimics.12 It may find more use as a follow-up tool in treatable cases of myositis by showing the successful resolution of increased muscle echogenicity or the development of fatty, fibrous replacement. The routine use of such modalities as elastography, evaluating tissue stiffness, may provide further information about muscle quality in the future. Despite the formidability of MRI for myositis, ultrasound is a viable alternative in the hands of experienced operators.
PET
On the other end of the innovation spectrum is metabolic imaging—the nuclear imaging technique of positron emission tomography (PET), which is most often combined with computed tomography (CT) or MRI to provide concurrent metabolic and anatomic information of tissues. The most commonly used tracer of 18F-FDG picks up an increase in glycolysis that occurs in the setting of inflammation.
Other tracers—such as 15O-water, which detects blood flow, and 11C-L-methylmethionine, which targets amino acid transport—have been used to evaluate skeletal muscle; however, their considerably shorter half-lives compared with 18F-FDG translate to shorter—and thus less optimal—post-injection imaging windows.13,14 For IBM specifically, the amyloid markers 11C-PIB[97] and 18F-florbetapir and the tau marker 18F-THK5317 may have value in differentiating IBM from other IIMs.15-17
