Arachnoid granulations in the transverse and sigmoid sinuses: CT, MR, and MR angiographic appearance of a normal anatomic variation.

PURPOSE To investigate the imaging characteristics, prevalence, and clinical significance of arachnoid granulations in the transverse and sigmoid venous sinuses. METHODS We reviewed the imaging findings, clinical signs and symptoms, final diagnoses, and follow-up studies of 32 patients with 41 probable arachnoid granulations. RESULTS On CT scans, arachnoid granulations appear as well-defined filling defects, wholly or partly within a venous sinus, with the same density as cerebrospinal fluid. MR images show these entities as largely isointense with cerebrospinal fluid in all sequences. Linear variations of signal intensity within the granulations are thought to be fibrous septa or vessels. Calcification was present in 3 granulations and altered both CT density and MR signal intensity. The granulations appear as filling defects at MR angiography and at digital subtraction angiography. In some oblique MR angiographic projections, they appear elliptical and could be mistaken for thrombus. No clinical significance could be given to the existence of any of these arachnoid granulations. They occur in 0.3 to 1 of 100 adults in the population. CONCLUSION Arachnoid granulations in the transverse and sigmoid venous sinuses are common findings seen with thin-section imaging and are usually of no significance.