MR Angiography

      The two primary methods for MRA are time- of-flight (TOF) and phase contrast (PC). Both use GRE pulse sequences and either 2D or 3D volume acquisitions. TOF makes use of the flow enhancement phenomenon to produce the vascular contrast. PC employs bipolar gradient pulses to tag moving protons with phase shifts. Both produce the MR angiogram by subjecting the data sets to a maximum intensity projection (MIP) ray tracing algorithm to display the vessels from multiple angled views around an axis.

      Imaging the neck vessels takes about 10 minutes and can be done in conjunction with a spin-echo scan of the brain. 2D TOF is very good for imaging this area. Overall, accuracy is quite good for estimating stenoses, although the method tends to overestimate the degree and length of stenosis. Very slow flow distal to a critical stenosis is difficult to detect and may be confused with a complete occlusion.

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      Gadolinium-enhanced MRA can be obtained with a large field of view to visualize the carotid and vertebral arteries from the aortic arch to the skull base. A 20 cc bolus injection of gadolinium is followed by a rapid 3D acquisition during passage of the contrast through the arch and neck arteries. The high contrast-to-noise images are very good for screening for cerebrovascular disease. Tight stenoses, slow flow, and areas of flow stasis are more accurately assessed with this technique. Endnote

      Another application of MRA is in cases of suspected dissection. Relatively minor trauma is sufficient to cause a dissection, or it can be spontaneous. The MRA may demonstrate complete occlusion or only narrowing of the arterial lumen. Spin-echo images should also be obtained because they are very sensitive for detecting the intramural hemorrhage. The typical appearance of an crescent-shaped hyperintensity with an eccentrically placed flow void may be more convincing for a dissection than the MRA. The MRA is very useful for following a dissection to look for recanalization of a complete occlusion or resolution of the vascular compromise caused by the intramural thrombus.

      MRA can also evaluate the major intracranial arteries about the circle of Willis. The resolution is lower, and flow artifacts limit accurate assessment of arteriosclerotic disease. Nevertheless, MRA is a very acceptable method for imaging the vertebrobasilar system, which is inaccessible to ultrasound and has no effective surgical therapy. The phase images of 2D PC MRA can be used to determine direction of collateral flow about the circle of Willis.

      TOF are PC MRA are very effective in evaluating intracranial veno-occlusive disease, such as superior sagittal sinus thrombosis. PC methods with lower velocity encoding (VENC) factors (15-20 cm/sec) work well for this disease and avoid any possible confusion with thrombus. Thin slab 2D PC can image the full extent of the sagittal sinus in only a few minutes. 

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