CASE 20: 12 y/o girl with left arm palpable mass

This case remarks the importance of the ultrasound. Patient came with history of palpable mass in the medial aspect of distal arm. Physician suspected an abscess but no recent trauma or infection whatsoever.

Sorry for the poor quality but in this US device the only way I could extract images was through a discontinued diskette (yep, old model). Anyways here are the US images:

Ignore the anatomic reference (right rather than left side)

I suggested an MR due to US findings, so here are the relevant slices:

Findings and differentials of both studyes please...soon, the conclusion.