A 30-year-old marathon runner presented with intermittent posterior right knee pain. Symptoms had initiated after a long training session two months previously. On physical examination the popliteal fossa was sensitive to direct compression but no muscular weakness or neurological deficits were noted. His medical history was unremarkable. Radiographs of the right knee revealed no significant abnormalities and treatment with nonsteroidal anti-inflammatory drugs showed no noteworthy improvement. MRI examination demonstrated a multilocular hyperintense mass on T2 weighted imaging (WI) along the course of the tibial nerve in the popliteal fossa (Fig. A1). 3D Reconstruction showed extension of this lesion anteriorly into the muscular branches of the popliteus muscle and into an articular branch to the superior tibiofibular joint (Fig. A2). This cystic lesion caused an eccentric displacement of the nerve fascicles (signet ring sign) (Fig. B1). Appearing hypointense on T1 WI, there was no enhancement of the mass after contrast study (Fig. B2). Slightly increased T2 signal was noted in the popliteus muscle belly indicating denervation edema (Fig. C). Based on these MRI-findings, diagnosis of a tibial intraneural ganglion cyst was made.