An unusual cause of persistent subcutaneous fluid collection

A 44-year-old man was involved in a motorcycle crash. His left lower extremity was impacted between the motorcycle and a car. He suffered left-sided rib fractures, capsular injury to his left fifth proximal interphalangeal joint and had a large haematoma in the medial aspect of the left thigh. Ultrasound of the left thigh was performed 1 week after the accident (Figure 1). The subsequent clinical course was characterised by a persistent fluid collection in the medial aspect of the left thigh that was painful and bothersome. Further evaluation with MRI of the left thigh was performed 2 months after the time of injury (​(​FigureFigure 2, 3 and 4). Laboratory findings revealed a normal coagulation panel. Figure 1 Longitudinal greyscale ultrasound image of the left upper thigh shows a subcutaneous fluid collection containing avascular fat lobules (arrow). Figure 2 T1 weighted axial MRI of the left upper thigh shows a subcutaneous fluid collection containing globular hyperintense fat lobules (arrows) and hypointense capsule. Figure 3 Short tau inversion-recovery (STIR) coronal MRI of the left upper thigh confirms the subcutaneous location of the fluid collection. Note the typical ovoid margins and hypointense capsule. Figure 4 T2 weighted axial MRI of the left upper thigh shows a subcutaneous fluid collection containing multiple fluid–fluid levels. What are the characteristics of the fluid collection on ultrasound? What observations can you make from the MRI? What is your diagnosis?