Introduction: Determination of blood flow in the intracerebral collaterals is an essential step to investigate the effects of experimentally increased fluid shear stress (FSS) on the growth of cerebral collaterals in small animal models. Thus, the aim of this study was to establish the quantitative MRI-based flow measurement using the example of the rat circle of Willis. Materials and Methods: To increase blood flow in the rat circle of Willis we developed two different models. In the Solo-Shunt group the common carotid artery was anastomosed end-to-side with the jugular vein, thereby creating an arterio-venous (AV) fistula on the left side (n=10). In the Ligature-Shunt group AV fistula-creation on the left side was performed additionally to the ligature of the right common carotid artery (n=10). Blood flow changes were monitored by a MRI-Scanner (Bruker PharmaScan 70/16–7.0 Tesla; (300.51 MHz; 16 cm) 1 d, 7 d and 14 d after surgery under controlled isoflurane anaesthesia. First a coronal three-dimensional phase contrast angiogram of the whole rat brain was performed to detect the circle of Willis. Then, three points in the maximum intensity-projection (MIP) of the angiogram were selected, which were situated on a straight part of the target vessel, the A. cerebri prosterior. The connecting line of the two outer points defined the normal of the new slices and, consequently, the new slice angels, the included third point defined the position of the following flow quantification sequence. The calculation of the physical coordinates from the image points and the sequence parameters as well as the calculation of the parameters of the new slices were done with a program written by one of the authors. The maximum velocity encoding (venc) was adjusted to the expected flow in the target vessels. Usually, a venc of 50 cm/s was used. The quantitative flow values (ml/min) were then obtained by integrating across manually drawn regions of interest (ROI) that enclose the vessels using Paravision 4.1 ROI-Tool. Results: MRI-based flow measurement could verify the functionality of the fistula in both groups (Solo-Shunt: 3.28±1.34 ml/min, Ligature-Shunt: 1.37±0.61 ml/min). Flow quantification showed a significant increase of blood flow in the circle of Willis (preoperative: 0.48±0.36 ml/min): 0.83±0.26 (1 d); 1.76±0.41 (7 d); 2.06±0.83 (14 d) and sham-side (ml/min): 0.35±0.20 (1 d); 0.44±0.14 (7 d); 0.53±0.37 (14 d). Ligature-Shunt: fistula-side (ml/min): 1.18±0.10 (1 d); 3.60±0.59 (7 d); 4.95±0.45 (14 d) and ligature-side (ml/min): 1.06±0.20 (1 d); 2.38±0.38 (7 d); 2.61±0.70 (14 d). Conclusion: The combination of a phase contrast angiogram and a calculated flow quantification sequence provides via the Bruker PharmaScan 70/16 the possibility to determine the blood flow also in small, intracerebral collaterals The blood flow in the circle of Willis can be significantly enhanced by the creation of a carotid-jugular AV fistula.