MR Angiography

      MRA can be used to screen for intracranial aneurysms in asymptomatic patients who have a family history of aneurysms or who have polycystic kidneys, coarctation of the aorta, or collagen vascular disease, putting them at a higher risk for aneurysms. Endnote 3D time-of-flight (TOF) or phase-contrast (PC) is recommended for screening for aneurysms around the circle of Willis. Relatively small imaging volumes can be used to keep imaging times short. Aneurysms 5 mm and larger are detected reliably on good quality studies. Depending on the size of the aneurysm neck and local hemodynamics, the entire aneurysm lumen may be rendered hyperintense, or an internal flow jet may be observed similar to the first film of a conventional angiographic sequence. Endnote The flow jets can be seen as high or low signal intensity, depending on the flow dynamics and the MRA parameters used. Potentially slow flow areas, such as the anterior and posterior communicating arteries, may not be visualized. The resolution of MRA is insufficient to accurately define the aneurysm neck or adjacent perforating arteries for presurgical planning.

      To avoid errors in diagnosis, all images should be reviewed, including the individual 2D sections or partitions, the collapse image, and the projection images of the MRA, as well as any available spin-echo scans. Using this approach to screen for aneurysms about the circle of Willis, a sensitivity of 86-95% can be achieved for aneurysms 5 mm or larger, Endnote but the sensitivity decreases to 56% for smaller aneurysms. Endnote Patients must be informed that a normal MRA does not absolutely exclude an aneurysm. Conventional angiography remains the definitive procedure. For the same reason, MRA is not recommended in patients with documented subarachnoid hemorrhage.

      Giant aneurysms are usually readily apparent on spin-echo images. MRA can help identify any residual patent lumen and the parent artery. As mentioned before, intraluminal clot and flow stasis may cause signal loss and obscure the vascular anatomy. Spin saturation is especially a problem for TOF sequences in the larger aneurysms, but spin-phase shifts from swirling blood within the patent lumen also cause signal drop-out and flow-related artifacts in PC images. Also, since the MIP algorithm selects for maximum intensities, an aneurysm that is faintly visible on the collapse image, may not be displayed on the projection images. 

{To return to cases, use the "Back " button on the Toolbar}