The vessles can be displayed by MR angiography without contrast medium, after contrast administration with higher dose than routine the vascular contrast especially in the small vessles would be improved. But what about the lower dose ? Trshortening agents always were used in three-dimensional time-of-flight (3DTOF) technique, What about three-dimensional phase-contrast (3DPC) technique? Both 3DTOF and 3DPC can show the circle of Willis with maximum intensity projection (MIP). To 3DTOF one major drawback is its progressive signal loss due to the saturation effect. Furthermore its background suppression of MIP images is poorer than that of 3DPC. But the source images of the 3DTOF have reasonable spatial resolution and can show more small vessels than MIP images. It can also show intracranial nonvascular diseases because of its TiWI characteristics. What about combination of 3DTOF and 3DPC techniques to show the circle of Willis? Whether the sensitivity and specificity in detection of vessels can be improved after that? What about the morphology of the circle of Willis on MRA. As one of the most important collateral pathways, what about its relationship to ischemic cerebrovascular diseases? These questions are the aimof this study.Part I: Experimental study: effects of low concentration contrast medium, flow velocity and tube diameter on 3DTOF and 3DPC MR angiographyPurpose: To evaluate the effects of low concentration contrast medium, flow velocity and tube diameter on 3DTOF and 3DPC MR angiography respectively.Materials and methods: Pump was used to control flow velocity (15cm/s, 30cm/s,-6-50cm/s respectively). Medical plastic tubes with different diameters (0.8mm. 1.6mm, 3mm respectively) were used to mimic vessels. Distilled water was used as fluid and the Gd-DTPA was diluted to three different concentrations (O.OOlmmol/l, 0.005mmol/l, O.Olmmol/1). Tap water in plastic bottle was used as control fluid. The flow fluid with different concentration contrast medium, flow velocity and tube diameter was scanned by 1.5T MR machine with 3DPC and 3DTOF MR angiography respectively. Then the contrast noise ratio (CNR) was calculated at five partitions(10mm, 20m, 30mm, 40mm. 50mm distance from the entry point).Results: Progressive signal loss could be seen in 3DTOF MRA due to saturation of inflowing spins. But it was not obvious in 3DPC MRA. Lou concentration of Gd-DTPA can improve the CNR of flow fluid, but it must reach a certain level about O.Olmmol/1 to gain dramatic statistics significance compared with that of no contrast medium (P>0.05). The CNR of flow fluid in 3DTOF and 3DPC was seemed to be improved with increasing of flow velocities and tube diameters. The flow velocities, contrast medium concentrations and tube diameters were all positively correlated to the CNR in every partitions after tested by partial correlation analysis to 3DTOF MRA. To 3DPC MRA the velocity and diameter gained positive correlation to the CNR in every partitions, but the low concentration of contrast medium was not.Part II: Comparison of different post-contrast MR angiography techniques with digital subtraction angiography in the detection of the circle of WillisPurpose: To display the circle of Willis by 3DTOF, 3DPC and 3D FSPGR and evaluate abilities of every sequences and their combination to detect the components of the circle of Willis, then to compare with the results of DSA. To determine the sensitivity, specificity and positive predictive value and negative predictive value of every MRA sequence in displaying the circle of Willis.Materials and Methods: 55 patients which were examined by MRA with contrast enhancement and DSA were included. MRA examination included 3DTOF, 3DPC. All patients also accepted examination by 3D FSPGR sequence. All images of 3DPC MIP, 3DTOF MIP, and 3D FSPGR and 3DTOF source images and their combination were evaluated respectively in display of the circle of Willis. T