Magnetic resonance imaging of peri-aqueductal lesions.

Fourteen lesions in the peri-aqueductal region were evaluated by 0.5T magnetic resonance imaging (MRI) and computed tomography (CT). Fifty normal magnetic resonance studies of the same region were reviewed. The aqueduct of Sylvius is seen in the midline sagittal MRI sections in all normal patients. It is not usually visible in cases of benign stenosis but may be seen in membranous obstruction. Benign stenosis is distinguished from neoplastic obstruction by the lack of increased signal on T2 weighted techniques. Blood within pineal tumours is easily detected and may be differentiated from lipomas, cystic lesions and solid tumours of the brain stem. Arachnoid cysts are also visible in MRI scans.