[The contribution of "time-of-flight" MRI-angiography in the study of neurovascular interactions (hemifacial spasm and trigeminal neuralgia)].

The purpose of this study was to investigate capabilities of Magnetic Resonance Angiography (MRA) in delineating neurovascular compression which has emerged as the most frequent etiology in cases of Hemi-Facial Spasm and Trigeminal Neuralgia. MR investigations were carried out at 1.5 t unit (Magnetom Vision, Siemens). The examination consisted of 3D time-of-flight MRA (3d TOF) protocol, with a FISP sequence (Fast Imaging Steady Precession) and magnetization transfer. Imaging parameters were; TR 39 ms, TE 7 ms; MX 256-512; FOV 210-210 mm. Time acquisition is 10 mm 30 s. The axially acquired angiographic slices were reformatted with a multiplanar reconstruction program (MPR) along and across the plane of facial and trigeminal nerves. Then millimetric raw data images were postprocessed with a maximum intensity projection (MIP) technique. 58 patients presented either with hemifacial spasm (36 cases, 16 women, 20 men, mean age: 58.5 y) or trigeminal neuralgia (22 cases, 13 women, 8 men, mean age: 51 y) were compared with 17 patients (9 women, 8 men, mean age: 57.5 y) as references. MRA with MIP and MPR techniques reliably demonstrates neurovascular compression at the root entry zone (REZ), and allows recognition of the implicated vascular loop in 34 the cases of hemifacial spasm investigated. Raw data were post-processed in an axially oblique plane parallel to the acoustico-facial bundle and secondarily reconstructed in coronally obliques views. These reconstructed images clearly delineate the cross-compression of the emerging facial nerve. The neurovascular conflict was related to an arterial loop from the postero-inferior cerebellar artery (PICA, 21 cases, 61%) and the antero-inferior cerebellar artery (AICA, 8 cases, 23%). Vertebral artery is implicated in conflicts in 5 cases. In 2 cases MRA exploration was negative. In HFS group distance between Vth nerve emergency and neuro vascular contact was measured and compared with control group; there is a significative difference (p < 0.01). In case of trigeminal neuralgia, MRA demonstrates neurovascular compression at the root entry zone of the Vth nerve of 16 patients. In theses cases, oblique and coronally reconstructed images are crucial in demonstrating neurovascular contacts. The vessel implicated in conflict is always the superior cerebellar artery (SCA). A supero-medial loop of the superior cerebellar artery was depicted in these 16 cases. Although 3 patients had negative MRA and MR studies, a case of compression of the origin of the Vth nerve by a enlarged draining vein in a case of brainstem arterio-venous malformation, a petrous apex meningioma was incidentally discovered and thoroughly investigated with gadolinium T1 weighted sequences and 1 demyelinating plaques located along the intraaxial course of the Vth nerve were noticed. In control group on 34 explanations only there are only 4 contacts between Vth nerve and SCA. MRA with a 3D-TOF FISP sequence allows recognition of neurovascular conflicts in case of hemifacial spasm and trigeminal neuralgia while MIP helps to characterize the implicated vessel. Sources images need to be carefully assessed in order to depict unsuspected pathology.