MRI of nontumorous skeletal muscle disease: case-based review.

Objective The educational objectives for this case-based review are to use case examples to improve skills in diagnostic radiology with regard to the interpretation of MRI of nontumorous skeletal muscle disease and to improve understanding of the pathophysiology and clinical features of each scenario. Conclusion MRI is a useful tool for assessing muscle disease. In this article, multiple cases illustrating common and a few uncommon nontumorous skeletal muscle diseases will be presented. At the end of this article, readers will be able to generate a concise list of differential diagnoses for nontumorous skeletal muscle disease to make the correct diagnosis or to guide appropriate treatment. Scenario I Clinical History Three days after a minor trauma, a 20-year-old man presented with continuous right thigh pain and swelling. The patient had a history of hypertension and had recently experienced acute gastric ulcer bleeding associated with non-steroidal antiinflammatory drug use. An MRI examination of his right thigh was performed. Description of Images The axial contrast-enhanced T1-weighted fat-saturated MR image (Fig. 1A) shows a rim-enhancing cystic lesion in the adductor brevis, which shows a corresponding dark-signal-intensity rim that appears to bloom on the gradient-echo image (arrow in Fig. 1C). The axial T2-weighted fat-saturated image (Fig. 1B) shows increased signal intensity not only of the cystic lesion but also at the insertion of the adductor brevis at the posterior femoral shaft (linea aspera). The surrounding muscle fibers within the adductor brevis show mild enhancement and mildly increased T2 signal intensity (Figs. 1A and 1B). Discussion Muscle injuries that result in hemorrhage or the disruption of muscle fibers may reveal a masslike pattern on MR images. Moderate to severe muscle strain, laceration, and contusion are examples of such injuries. Intramuscular or intermuscular hematomas may be seen with any of these lesions or may occur spontaneously, especially in patients receiving anticoagulant therapy [1]. Most of the intramuscular hematomas that are evaluated with MRI between 2 days and 5 months after injury display characteristics of methemoglobin, with increased signal intensity on both T1- and T2-weighted images. Occasionally, serous-appearing fluid from a hematoma may linger within a connective tissue sheath, creating an intramuscular pseudocyst [2]. In this case, a low-signal-intensity rim in all sequences (hemosiderin) may suggest the diagnosis of intramuscular hematoma. An intramuscular hematoma may mimic an intramuscular abscess, hemorrhagic neoplasm, or myonecrosis on MR images because all of these lesions may contain fluid-fluid levels and show surrounding muscle edema and enhancement. However, these conditions can be differentiated by clinical settings because intramuscular hematomas are usually seen after a muscle injury or in patients receiving anticoagulant therapy; the clinical history usually allows distinction among these conditions [1]. Hematomas are common after a myotendinous injury and may be predominantly intramuscular or intermuscular in location. Intramuscular hematomas often resorb spontaneously over a period of 6—8 weeks. With an equivocal or remote history of trauma, imaging may be indicated to assess a soft-tissue mass that is clinically suspected of being neoplastic. When the diagnosis of a probably benign hematoma is in doubt, clinical correlation and a follow-up MRI examination may be indicated to establish the appropriate evolution of the abnormality [2]. Differentiation between simple hematoma and hemorrhagic neoplasm may be difficult in some patients. Both of these lesions may contain fluid-fluid levels and show surrounding muscle edema and enhancement. Three potential diagnostic pitfalls must be recognized when interpreting the enhancement of a focal lesion after the IV administration of gadolinium contrast material. First, contrast enhancement is possible in the fibrovascular tissue of an evolving hematoma, potentially making differentiation from neoplasm difficult. Second, gaddolinium may diffuse slowly into a fluid-filled space, such as a hematoma or an abscess. Consequently, imaging should be performed promptly after contrast administration to avoid spurious enhancement within a mass that might falsely suggst that is is solid. Third, minimal or no appreciable enhancement may be observed in myxoid lesions, which then may be confused with cysts or lesious with a cystie component. When the lesion in question shows enhancing modules, however, it may suggest the diagnosis of a neoplasm rather than a hematoma. Conclusion The diagnosis is adductor brevis hematoma due to injury.