sensitive to turbulent flow, which causes a variety of different magnetized proton spins to lose phase coherence (intra-voxel dephasing phenomenon), resulting in a loss of signal. this phenomenon may result in the overestimation of arterial stenosis. other artifacts observed in mra include: phase-contrast mra: phase wrapping caused by the underestimation of maximum blood velocity in the image. the fast-moving blood about maximum set velocity for phase-contrast mra gets aliased and the signal wraps from pi to -pi instead, making flow information unreliable. this can be avoided by using velocity encoding (venc) values above the maximum measured velocity. it can also be corrected with the so-called phase-unwrapping. maxwell terms: caused by the switching of the gradients field in the main field b0. this causes the over magnetic field to be distort and give inaccurate phase information for the flow. acceleration: accelerating blood flow is not properly encoded by phase-contrast technique, which can lead to errors in quantifying blood flow. time-of-flight mra: saturation artifact due to laminar flow: in many vessels, blood flow is slower near the vessel walls than near the center of the vessel. this causes blood near the vessel walls to become saturated and can reduce the apparent caliber of the vessel. venetian blind artifact: because the technique acquires images in slabs (as in multiple overlapping thin-slab acquisition, motsa), a non-uniform flip angle across the slab can appear as horizontal stripe in the composed images. visualization maximum intensity projection of an mra covering from the aortic arch to just below the circle of willis occasionally, mra directly produces (thick) slices that contain the entire vessel of interest. more commonly, however, the acquisition results in a stack of slices representing a 3d volume in the body. to display this 3d dataset on a 2d device such as a computer monitor, some rendering method has to be used. the most common method is maximum intensity projection (mip), where the computer simulates rays through the volume and selects the highest value for display on the screen. the resulting images resemble conventional catheter angiography images. if several such projections are combined into a cine loop or quicktime vr object, the depth impression is improved, and the observer can get a good perception of 3d structure. an alternative to mip is direct volume rendering where the mr signal is translated to properties like brightness, opacity and color and then used in an optical model. clinical use mra has been successful in studying many arteries in the body, including cerebral and other vessels in the head and neck, the aorta and its major branches in the thorax and abdomen, the renal arteries, and the arteries in the lower limbs. for the coronary arteries, however, mra has been less successful than ct angiography or invasive catheter angiography. most often, the underlying disease is atherosclerosis, but medical conditions like aneurysms or abnormal vascular anatomy can also be diagnosed. an advantage of mra compared to invasive catheter angiography is the non-invasive character of the examination (no catheters have to be introduced in the body). another advantage, compared to ct angiography and catheter angiography, is that the patient is not exposed to any ionizing radiation. also, contrast media used for mri tend to be less toxic than those used for ct angiography and catheter angiography, with fewer people having any risk of allergy. also far less is needed to be injected into the patient. the greatest drawbacks of the method are its comparatively high cost and its somewhat limited spatial resolution. the length of time the scans take can also be an issue, with ct being far quicker. it is also ruled out in patients for whom mri exams may be unsafe (such as having a pacemaker or metal in the eyes or certain surgical clips). mra procedures for visualizing cranial circulation are no different from the positioning for a normal mri brain. immobilization within the head coil will be required. mra is usually a part of the total mri brain examination and adds approximately 10 minutes to the normal mri protocol.