MRI Findings of Early Myositis Ossificans without Calcification or Ossification

Abstract

Purpose: To characterize and evaluate the MR imaging features of early myositis ossificans (MO) without calcification or ossification.

Methods: The MRI manifestations of seven patients with pathologically proven early MO were retrospectively analyzed with regard to tumor location, size, margins, signal intensity, and enhancement appearance in MR images. Additionally, the surrounding soft-tissue edema and adjacent bone change were assessed.

Results: All cases (n=7) had intramuscular tumor-like masses without calcifications. The lesions appeared as isointense in T1-weighted images (T1-WI) and inhomogeneous hyperintense in T2-weighted MR images (T2-WI). On T2-WI and postcontrast T1-WI, the heterogeneously high signal intensity in the expanded muscle interspersed with a few hypointense linear structures consistent with intact muscle fibers showed "striate pattern" in the plane parallel with muscle fibers. The relatively hypointense areas with geometrical pattern consistent with the bundles of intact muscle fibers are found within the lesion with diffuse high signal intensity, displaying the "checkerboard-like pattern" in the plane vertical to muscle fibers. A "striate pattern" (n = 7) and "checkerboard-like pattern" (n = 3) in the lesion appeared in T2-WI. In contrast-enhanced MRI images, all cases showed diffuse "striate pattern" enhancement. Among them, one case demonstrated "checkerboard-like pattern" enhancement. All cases had diffuse and prominent muscle edema that preserved the muscle fascicles. For two lesions located in the deep muscle group, the adjacent bone showed bone marrow edema.

Conclusion: MR imaging has unique advantages for diagnosis of early MO without calcification or ossification: the "striate pattern" and "checkerboard-like pattern" appearance shown in T2-WI and contrast-enhanced MRI images can be helpful for differential diagnosis. MRI can delineate the extent of the tumor and provides accurate anatomical information, which is important in diagnosis, treatment, and follow-up.

Figure 1

CT and MR imaging of the left forearm. (a) Axial plain CT image reveals an ovular, low-density, intramuscular mass-like lesion in the brachialis and triceps muscles. (b) Axial T1-weighted image shows an ill-defined isointense lesion. (c,d) Axial and coronal fat-suppressed T2-weighted image reveals a hyperintense lesion (large white arrow) with a “striate pattern” in the brachialis and triceps muscles with a pattern of edema in the brachialis and biceps muscles (arrowheads). (d) Enhanced fat-suppressed axial T1-weighted image shows that the lesion enhances intensely with a “striate pattern” in the muscle (large white arrow). Preservation of the muscle fascicles is noted in the lesion. (e) One month later, follow-up axial CT image shows a low-density lesion with a rim of well-defined focal and linear calcification (black arrow).

Figure 2

MR imaging of the right thigh. (a) Coronal T1-weighted image shows an ill-defined isointense lesion in the iliopsoas muscle (large black arrow). (b-d) Sagittal, coronal, and axial fat-suppressed T2-weighted images reveal a hyperintense lesion with a “striate pattern” in the iliopsoas muscle (large black arrow) with a pattern of edema in the iliopsoas and sartorius muscles (black arrowheads). The femur bone shows bone marrow edema (white arrowhead). The femur periosteum displays patchy areas of hyperintensity in FSE T2WI images (small white arrow). (e and f) Axial DWI and ADC mapping display hyperintensity in DWI and high ADC values of the lesion, which indicate a T2 shine-through effect.

Figure 3

MR, DR, and histopathology imaging of the right thigh. (a) Axial T1-weighted image shows an ill-defined isointense lesion in the vastus intermedius muscle (large arrow). (b) Coronal fat-suppressed T2-weighted image reveals a hyperintense lesion with a “striate pattern” in the vastus intermedius muscle (large arrow). (c) Axial fat-suppressed T2-weighted image shows a hyperintense lesion with a “checkerboard-like pattern” in the vastus intermedius muscle (large arrow). Surrounding soft-tissue edema in the vastus intermedius and vastus medialis muscles (arrowheads) and overlying fascia (small arrow) is seen. (d) Enhanced fat-suppressed axial T1-weighted image shows that the lesion enhances intensely with a “checkerboard-like pattern” (large arrow). Preservation of the muscle fascicles is shown within the lesion. Enhancement of the overlying fascia (small arrow) is seen. (e) Lateral DR shows the anterior femoral soft tissue without any calcification or ossification. (f) The specimen mainly includes loose, immature textured fibroblasts with mild cellular pleomorphism. Some portions of the lesion contained osteoid formation. The entrapped atrophic or necrotic muscle fibers are also shown in the lesion (H&E staining, ×200).