Milk cloud appearance-a characteristic sign of fibrous dysplasia on contrast-enhanced MR imaging.

OBJECTIVE: To evaluate contrast-enhanced T1-weighted magnetic resonance (MR) images in histologically proven fibrous dysplasia (FD) for the prevalence of "milk cloud appearance" and its association with ground-glass appearance (GGA) on radiography or computed tomography (CT).

METHODS: For this retrospective cohort study, 37 patients with histologically proven FD imaged preoperatively with contrast-enhanced MR imaging and radiography or CT were identified at our institution. Three radiologists independently evaluated MR images for the presence of milk cloud appearance on T1-weighted contrast-enhanced images, sites of skeletal involvement, type of bone involved, uni- vs. multifocality, mono- vs. polyostotic disease, maximum diameter, proportion of bone involved, expansile remodeling, and T2 homogeneity. The presence or absence of GGA on radiography or CT was determined in consensus. Inter-reader agreement was evaluated for milk cloud appearance using Cohen's kappa, and associations between milk cloud appearance and other imaging parameters were tested using Spearman's rho.

RESULTS: Among the 37 histologically proven FD lesions, GGA was identified in 70% of the lesions, while milk cloud appearance was found in 82% of the lesions. Inter-reader agreement for milk cloud appearance on MR imaging was good to excellent (κ 0.65, 0.82, and 0.8). A significant correlation was found between milk cloud appearance and GGA (ρ = 0.31, p < 0.001).

CONCLUSION: Milk cloud appearance is a characteristic sign of FD on contrast-enhanced T1-weighted MR images. Recognition of this feature may be helpful when radiographs are equivocal or unremarkable or when MR imaging is performed as the primary imaging modality in cases of FD.

KEY POINTS: • Fibrous dysplasia displays a characteristic feature on contrast-enhanced T1-weighted MR imaging: milk cloud appearance. • Milk cloud appearance correlates well with the radiographic or CT finding of ground-glass appearance. • Recognition of milk cloud appearance on contrast-enhanced MR imaging may be helpful when radiographs are equivocal or unremarkable or when MR imaging is performed as the primary imaging modality in cases of fibrous dysplasia.