Neuroradiological diagnostic tools: new MRI perspectives.

MR angiography(5,17) is traditionally divided into "non-contrast enhanced" (NCE MRA) and "contrast-enhanced" (CE MRA) techniques. Contrast-enhanced techniques were initially developed to solve some of the problems encountered with non-contrast-enhanced methods, namely the long examination times, pulsatile flow artefacts, saturation when the mean blood flow is slow or when the imaging slice is parallel to the vessel, the characteristic problems of time of flight (TOF) acquisitions, and the need to define a priori beforehand the velocity sensitivity for phase contrast (PC) acquisitions. Currently used contrast-enhanced techniques(7.8.15.18) are based on the acquisition of 3D gradient-echo sequences synchronized at the first passage of contrast medium by different strategies (detection of a test bolus injection, automated triggering, fluoroscopic imaging). Basically, contrast medium is administered to counteract saturation induced by slow blood flow or flow directed parallel to the imaging slice. Contrast medium serves to obtain a "static" image of vascular structures with acquisitions times ranging from a few seconds to 1-2 minutes. To avoid venous contamination, different methods have been used to collect data (centric-approach, elliptical-centric phase encoding order, etc.) in an attempt to record first the information relating to low spatial frequencies (contrast data) with respect to high spatial frequencies (detail and contour data).