1. A method for assessing vascular reactivity during an arterial occlusion, comprising the steps of: using a magnetic resonance imaging scanner to obtain images; locating an artery using coronal scout images; positioning the subject or patient, such that the artery is parallel to a magnet bore of the magnetic resonance imaging scanner and cross sectional images can be thus obtained; constricting the artery for a time period, whereby the artery is fully occluded; acquiring images of the artery prior to occlusion (at baseline), as well as one or more times during occlusion; and using the acquired image data and measuring the shear rate and/or radius of the artery at baseline and for each of the one or more times during occlusion.