The fat-cap sign: an aid to MR evaluation of extradural spinal tumors.

The use of MR imaging in the evaluation of spinal tumors has been well documented [1-4]. Once a mass is recognized , images in multiple planes and multiple signal sequences are used for better definition, especially of its location. For the most part, the same neuroradiologic criteria used in CT, conventional myelography, and CT-myelography are used for this purpose. This report describes a new sign based on the ability of MR to show extradural fat as a bright signal on T1weighted images. A tumor arising in the extradural space may splay the fat , causing it to have a capped appearance. This fat-cap sign indicates that the epicenter of the tumor is extradural.