[Angiographically occult vascular malformations in the posterior fossa]

Three cases of spontaneous intraparenchymal hemorrhage in the posterior fossa were operated upon, and the diagnosis of thrombosed vascular malformation was histologically confirmed in two of them. So-called "angiographically occult" vascular malformations often present characteristic magnetic resonance (MR) images, namely, iso- to high intensity signal core surrounded by a low intensity signal rim or area on both T1 and T2 weighted images. Such findings are thought to indicate that subacute and chronic intraparenchymal hematoma has a relatively short T1 and long T2 relaxation times, resulting in an area of relatively increased signal intensity, and that hemosiderin deposits apparently decrease T2 relaxation time by paramagnetic effect. Most such lesions do not show significant perifocal edema. Occasionally, however, perifocal edema may be seen as in our Case 3, and the differential diagnosis from the tumor, particularly bleeding tumor, may become difficult even with the recent technical standards of high resolution CT and MRI. Reasons responsible for non-visualization of such vascular malformations at angiography are discussed. Small size of the lesion, slow blood flow through it, compression by adjacent clot or gliotic scar, spontaneous or secondary thrombosis with hyalin degeneration, and destruction at its rupture are possible factors.