Imaging of Skeletal Muscle

Various diagnostic imaging techniques such as sonography, computed tomography, scintigraphy, radiography, and magnetic resonance imaging (MRI) have made possible the noninvasive evaluation of skeletal muscle injury and disease. Although these different modalities have roles to play, MRI is especially sensitive in the diagnosis of muscle disorders and injury and has proved to be useful in determining the extent of disease, in directing interventions, and in monitoring the response to therapies. This article describes how magnetic resonance images are formed and how the signal intensities in T1- and T2-weighted images may be used for diagnosis of the above-mentioned conditions and injuries.

A variety of diagnostic imaging techniques have made possible the noninvasive evaluation of skeletal muscle injury and disease. Sonography allows for the dynamic assessment of musculotendinous structures and tissue vascularity and is particularly useful in determining whether lesions are cystic or solid. Although inexpensive and widely available, the use of this technique is limited by operator experience, difficulty in imaging deep structures, a restricted field of view, and limited contrast resolution. Computed tomography acquires cross-sectional images that are valuable in imaging tissues deep in the surface that are not well displayed on routine radiographs. The technique, however, is limited by poor soft tissue contrast and the risk of radiation exposure. Radiography remains a relatively inexpensive and widely available technique for identifying calcified abnormalities such as phleboliths and heterotopic ossification. Scintigraphy is of some use in the evaluation of skeletal muscle disorders, but it lacks specificity.

Although all these different modalities have roles to play, magnetic resonance imaging (MRI) is especially sensitive in the diagnosis of muscle disorders and injury and has proved to be useful in determining the extent of disease, in directing interventions, and in monitoring the response to therapies. At the most basic level, MRI creates images by exploiting the predictable behavior of protons placed in a strong magnetic field. The images largely reflect the distribution of protons within fat and water. The signal intensity of any tissue is therefore largely a reflection of the proton density of a particular tissue. However, the behavior of protons in these tissues is strongly influenced by tissue structure and the behavior of nearby protons, and this behavior of protons influences the magnetic resonance (MR) image. T1 and T2 are constants that describe 2 behaviors of the protons of a given tissue in a magnetic field. The signal intensity of any tissue on an MR image can be altered by changing the manner in which it is created through the use of different imaging sequences. This alteration allows MR to create tissue contrast by exploiting differences in tissue composition and structure. Sequences that are T1 or T2 weighted are designed to emphasize the differences in T1 or T2, respectively.

On T1-weighted images, high–signal intensity adipose tissue can be seen surrounding intermediate–signal intensity skeletal muscle. These images are particularly useful for the evaluation of muscle size and the presence of anomalous muscles. Methemoglobin deposition secondary to intramuscular hematoma or a hemorrhagic neoplasm is also associated with increased signal intensity. In the setting of muscle atrophy caused by injury, inflammation, or denervation, T1-weighted images are of great value in assessing the presence and amount of intramuscular fat. Gadolinium-based intravascular contrast agents can be used in conjunction with fat-suppressed T1-weighted images ( Fig. 1 ). Although the value of contrast in skeletal muscle imaging is limited, it may be helpful in identifying areas of necrosis or abscess formation.

T2-weighted images are sensitive to the presence of water, which is displayed as high signal intensity. The finding of water signal, however, is not specific and may represent muscle exertion, inflammation, infection, subacute denervation, ischemia, myonecrosis, injury, or infiltrating neoplasm. Although fat signal intensity is low on classic conventional spin echo images, it remains high on the newer fast spin echo T2-weighted images, which have largely replaced spin echo imaging because of the decreased scanning time required ( Fig. 2 ). In order to produce truly fluid-sensitive images, it is therefore necessary to suppress the fat signal. Frequently, a saturation pulse is used to suppress the signal from protons with the frequency of fat. This fat saturation technique is routinely used with little difficulty. However, when imaging large fields of view, as is frequently the case when imaging skeletal muscle, suppression may fail at the margins of the image. This failure is the result of the diminished homogeneity of the magnetic field and associated variations in fat frequency. Frequency-selective fat suppression is also unreliable when imaging with low-field scanners. In these settings, inversion recovery (IR) fast spin-spin echo imaging, an updated version of short tau IR imaging (STIR) provides reliably uniform fat suppression.

When evaluating patients with suspected myositis, the authors acquire T1-weighted and IR images in the axial plane and supplement those images with coronal plane IR images. This protocol requires the acquisition of images at the thighs, followed by a change in the positioning of the patient and then a repeat imaging at the lower part of the legs. On some newer MR scanners, the entire body can be imaged without a change in position, allowing for the relatively rapid study of the total body. When infectious myositis is suspected, the use of gadolinium-based contrast may be useful in identifying abscesses.

Although the MRI findings of muscle inflammation are not specific, characteristic patterns of disease and injury have been described. For example, in the setting of suspected idiopathic inflammatory myopathy, little distinguishes polymyositis from dermatomyositis, but inclusion body myositis may affect the upper extremities more frequently and present with asymmetric distribution and distal involvement. MRI may be used to establish an early diagnosis, determine disease activity and extent, assess response to therapy, and direct biopsy. In the setting of active myositis, a high signal on T2-weighted images, often referred to as muscle edema reflects the accumulation of extracellular water and inflammatory infiltrate. In the chronic stage, muscle girth is decreased and replaced by adipose, which is well displayed on T1-weighted images. By combining fluid-sensitive images, such as IR images, with T1-weighted images, both the level of active disease and the extent of chronic disease can be evaluated. MR images can be used to identify areas of active inflammation, seen as high–signal intensity areas on fluid-sensitive sequences, and minimal atrophy ( Fig. 3 ). The findings of pyomyositis are similar to those of idiopathic disease, even though involvement is typically more localized and muscle enlargement is common. Using imaging to target the regions of concern improves the efficacy and cost-effectiveness of biopsy or, in the setting of infection, aspiration or abscess drainage.