Hyperintense thrombus on GRASS MR images: potential pitfall in flow evaluation.

Gradient-recalled acquisition in the steady state (GRASS) MR images, obtained in four patients with angiographic evidence of successful occlusion of cerebral arteriovenous malformations, demonstrated hyperintense signal intraluminally. Although this was initially mistaken as evidence of persistent blood flow in the arteriovenous malformation, the short TR/TE spin-echo images showed hyperintense signal rather than flow void, thereby indicating the presence of subacute thrombus. GRASS images alone should not be used to determine the success of embolotherapy of cerebral arteriovenous malformations or to determine aneurysm patency, since the hyperintense signal is a potential pitfall that may mislead the radiologist in the absence of corroborative images, particularly the short TR/TE spin-echo sequences.