Parameningeal cyst (Spinal Extradural Meningeal Cyst)
HSS 1250237 (3/4/13)
Case in Point (1/2/08)
http://3s.acr.org/cip/ArchiveCaseView.aspx?CaseId=GMgeHtHlU2Q%3d
Also
for completeness K. York Chynn's article from SL
http://www.ajronline.org/doi/pdf/10.2214/ajr.101.1.204