Ultrasound and MR imaging of fibromatosis colli (sternomastoid tumor of infancy).

The sonographic and CT findings of fibromatosis colli (sternomastoid tumor of infancy) have been described, but the MRI appearance has been reported in only one case in which the mass resolved over time. This case describes the detailed MRI findings in a biopsy-proven case of fibromatosis colli; the signal intensity of the mass on T2-weighted images was slightly less than on gradient-recalled T1-weighted images, consistent with the presence of some fibrous tissue within the muscle mass. The involved portion of the muscle was better defined on MRI than sonography. MRI was helpful in demonstrating the signal characteristics of the mass; localizing the mass to within the sternocleidomastoid muscle; and demonstrating clear surrounding fascial planes with lack of associated lymphadenopathy, airway compression, vascular encasement, bone involvement or intracranial/intraspinal extension associated with other neck masses.