(CASL) In order to take spins in a different magnetic state to that of the static tissue a train of RF pulses repeatedly saturates blood water spins flowing through the neck. The saturated (or 'labeled') spins flow into the brain and, assuming water is a freely diffusible tracer, exchange completely with brain tissue water, thus reducing the overall tissue magnetization. A steady state progresses where the regional magnetization in the brain is directly related to cerebral blood flow.

(PWI - Perfusion Weighted Imaging) Perfusion MRI techniques (e.g. PRESTO - Principles of Echo Shifting using a Train of Observations) are sensitive to microscopic levels of blood flow. Contrast enhanced relative cerebral blood volume (rCBV) is the most used perfusion imaging. Both, the ready availability and the T2* susceptibility effects of gadolinium, rather than the T1 shortening effects make gadolinium a suitable agent for use in perfusion imaging. Susceptibility here refers to the loss of MR signal, most marked on T2* (gradient echo)-weighted and T2 (spin echo)-weighted sequences, caused by the magnetic field-distorting effects of paramagnetic substances.
T2* perfusion uses dynamic sequences based on multi or single shot techniques. The T2* (T2) MRI signal drop within or across a brain region is caused by spin dephasing during the rapid passage of contrast agent through the capillary bed. The signal decrease is used to compute the relative perfusion to that region. The bolus through the tissue is only a few seconds, high temporal resolution imaging is required to obtain sequential images during the wash in and wash out of the contrast material and therefore, resolve the first pass of the tracer. Due to the high temporal resolution, processing and calculation of hemodynamic maps are available (including mean transit time (MTT), time to peak (TTP), time of arrival (T0), negative integral (N1) and index.
An important neuroradiological indication for MRI is the evaluation of incipient or acute stroke via perfusion and diffusion imaging. Diffusion imaging can demonstrate the central effect of a stroke on the brain, whereas perfusion imaging visualizes the larger 'second ring' delineating blood flow and blood volume. Qualitative and in some instances quantitative (e.g. quantitative imaging of perfusion using a single subtraction) maps of regional organ perfusion can thus be obtained.
Echo planar and potentially echo volume techniques together with appropriate computing power offer real time images of dynamic variations in water characteristics reflecting perfusion, diffusion, oxygenation (see also Oxygen Mapping) and flow.
Another type of perfusion MR imaging allows the evaluation of myocardial ischemia during pharmacologic stress. After e.g., adenosine infusion, multiple short axis views (see cardiac axes) of the heart are obtained during the administration of gadolinium contrast. Ischemic areas show up as areas of delayed and diminished enhancement. The MRI stress perfusion has been shown to be more accurate than nuclear SPECT exams. Myocardial late enhancement and stress perfusion imaging can also be performed during the same cardiac MRI examination.

A large network of interconnecting blood vessels at the base of the brain that when visualized resembles a circle, the arteries effectively act as anastomoses for each other. This means that if any one of the communicating arteries becomes blocked, blood can flow from another part of the circle to ensure that blood flow is not compromised.
The circle of Willis is formed by both the internal carotid arteries, entering the brain from each side and the basilar artery, entering posteriorly. The connection of the vertebral arteries forms the basilar artery. The basilar artery divides into the right and left posterior cerebral arteries. The internal carotid arteries trifurcate into the anterior cerebral artery, middle cerebral artery, and posterior communicating artery. The two anterior cerebral arteries are joined together anteriorly by the anterior communicating artery. The posterior communicating arteries join the posterior cerebral arteries, completing the circle of Willis.
The time of flight angiography MRI technique allows imaging of the circle of Willis without the need of a contrast medium (best results with high field MRI). A cerebrovasular contrast enhanced magnetic resonance angiography (MRA) depicts the circle of Willis in addition to the vessels of the neck (carotid and vertebral arteries) with one bolus injection of a contrast agent.

For Ultrasound Imaging (USI) see Cerebrovascular Ultrasonography at Medical-Ultrasound-Imaging.com.

MR imaging techniques capable to provide maps of cerebral activity. All these techniques are based on indirect assessment of local cerebral haemodynamics that have been demonstrated to be closely related to cerebral activity.
Two kinds of techniques have been developed:

T2 weighted imaging relies upon local dephasing of spins following the application of the transverse energy pulse. The contrast of a T2 weighted image is predominantly dependent on T2 and the T2 dependence will be increased by using a long echo time.
Fat has a shorter T2 time than water and relaxes or decays more readily than water. Since the amount of transverse magnetization in fat is small, fat generates very little signal on a strong T2 weighted contrast image and appears intermediate to dark. The T2 weighting is stronger with a longer TE. Water has a very high T2 constant, therefore has very high T2 signal and thus appears bright on a T2 contrast image. Cerebral white matter (fat containing) is less intense than grey matter. Flowing blood (flow effects) and haematomas (haemoglobin, haemosiderin) have a variable signal intensity on MR images.
Images created with TR's and TE's to enhance T2 contrast are referred to as T2 weighted images. Both T1 and T2 weighted images are acquired for most medical MRI examinations.