MAGNETIC-RESONANCE ANGIOGRAPHY AND TOMOGR APHY OF VERTEBROBASILAR DISSECTIONS

Vertebral artery dissection (VAD) is an important cause of posterior c irculation stroke in young adults. Initial symptoms are often non-spec ific and diagnostic arteriography is not performed until neurological deficits are obvious. Since magnetic resonance tomography (MRT) is sup erior in the diagnosis of vertebrobasilar ischemia, we retrospectively analyzed the role of MRT and MR angiography (MRA) in the detection of dissections of the vertebral artery. Between 1989 and 1995 we identif ied 24 patients with a vertebral artery dissection and 1 patient with a basilar artery dissection (8 females and 17 males, 23-60 years of ag e, mean 41.2 years). The diagnosis of VAD (14 left VAD, 9 right VAD, 1 bilateral VAD, 1 basilar artery dissection) was established by specif ic arteriographical findings (DSA) or clinical and neuroradiological c ourse. All patients underwent a combined MRT/MRA examination protocol at 1.5T that consisted of spin-echo imaging and time of flight MRA of the intra- and extracranial arteries using 2D Flash and 3D Fisp sequen ces. The MRT/MRA findings were correlated to DSA and ultrasound result s. During the acute and subacute stage, MRT/MRA revealed abnormal find ings in 21 of 22 dissected vessels (95.5%). There was one false-negati ve MRT/MRA in a patient with a V1 dissection (intimal flap without per ipheral flow disturbances). In 7/22 VAD MRT/MRA findings were specific (double lumen n = 1, mural hematoma n = 4, pseudoaneurysm n = 2). DAS was sensitive in 100% and ultrasound in 77.3%. Specific results were obtained by DSA in 8/22 VAD (36.4 %) and in 7/22 VAD (30.4%) by MRT/MR A. When MRT/MRA and DSA results were combined, the specific findings i ncreased to 43.5%. Follow-up examinations revealed recanalization in 5 2% of initially stenosed or occluded vertebral arteries; four patients developed a pseudoaneurysm, and two of them underwent ligation of the VAD. With this retrospective approach, we were able to show a high se nsitivity of MRT/MRA for the presence of disturbed flow in the dissect ed vertebral artery. The MRA projections tended to overestimate stenos is and were inferior to DSA in the appreciation of irregularities of t he vessel wall. Identification of high-grade stenosis, especially in t he presence of distal occlusion, was improved on the MRA source images . During the acute and subacute stage, the diagnosis of luminal thromb us can be difficult, because signal ambiguities exist between hemoglob in breakdown products and flow effects and adjacent fat tissues. The d ifferentiation between luminal thrombus and mural hematoma requires in terpretation of MRA source images, together with flow compensated spin -echo images. Additional fat suppressed images and flow presaturation may be required at the appropriate levels. The identification of mural hematoma is important, because this finding is considered specific an d cannot be obtained with DSA. There is a complementary role of MRT/MR A and DSA for an improved overall specificity for vertebral artery dis section. A negative MRT/MRA result in a patient with appropriate sympt oms, however, cannot exclude a dissection and should prompt DSA. On th e other hand, a suggestive MRT/MRA result in the appropriate clinical context can replace DSA. The advantage of MRT/MRA is that the method o ffers a simultaneous diagnosis of posterior fossa ischemia and vertebr al artery abnormalities. Therefore, MRT/MRA should be recommended in p atients with suspected VAD and especially in those who have no definit e neurological deficit. These patients will benefit greatly from early diagnosis and therapy. The fact that all our patients were diagnosed after neurological symptoms and that 64% of them have residual deficit s gives an ethical and economical rationale for advocating early MRT/M RA in these patients.