Thrombus simulating flow void: a pitfall in diagnosing aqueductal patency by high-field MR imaging.

A 21-year-old man with tuberous sclerosis presented with a 2-day history of progressive nausea, vomiting, ataxia, headache, and mild spasticity. The patient was known to have a large giant-cell astrocytoma occupying the third ventricle, and he had previously had ventriculoperitoneal shunts placed into both lateral ventricles . A multiecho 20FT MR scan was obtained on a 1.5-T unit' with TEs of 20 and 90 msec and a TR of 2000 msec (Fig. 1). In addition to showing distortion of structures adjacent to the third ventricle by the astrocytoma, this scan revealed enlargement of the fourth ventricle, which had not been present on prior studies . This finding, along with the posterior fossa symptomatology, raised the possibility of fourth-ventricle entrapment [1 , 2) by aqueductal obstruction. However, the MR examination showed absence of signal from the cerebral aqueduct and superior aspect of the fourth ventricle, which was interpreted as indicative of aqueductal patency. Since this apparent patency did not correlate with the enlargement of the fourth ventricle or the patient's rapid clinical deterioration, a posterior fossa craniectomy was performed for decompression in spite of this MR finding. After incision of the vermis and separation of the cerebellar hemispheres, fresh thrombus was discovered in the superior recess of the fourth ventricle suspended from the aperture of the cerebral aqueduct in stalactite fashion . A "tail " of this thrombus extended into the aqueduct and was removed. Microscopic examination of the pathologic specimen showed a typical blood clot with evidence of early lysis and organization, and no evidence of tumor cells.