Magnetic Resonance Angiography. MRA is a generic name for different approaches and variations of vascular imaging by MRI. MRA refers to the computer-assisted generation of images (angiograms) by MRI created by the contrast of magnetically polarized flowing blood against a stationary magnetically saturated parenchyma. The resulting high-differential signal intensity creates a map that includes anatomic and physiological information. MRA techniques that avoid the risks and limitations of conventional invasive arteriographic methods are in development. Current techniques are based on inflow enhancement (time-of-flight methods) or rely on velocity-induced phase shifts (phase-contrast methods). Time-of-flight acquisitions emphasize vessel morphology, whereas phase-contrast methods provide additional information about velocity, direction of blood flow, and volume flow rates. Slice-by-slice (two-dimensional), volumetric (three-dimensional) acquisitions, or a combination of these allow greater flow sensitivity and/or spatial resolution.
The best images are obtained in normal vessels or those with minimal stenotic lesions, using multiple overlapping thin slabs or three-dimensional-phase contrast techniques that are widely available. Vessels with a large lumen give excellent signals that can be measured and used for MRA display.
Advanced degrees of arterial narrowing will be detected by MRA as luminal narrowing and by an overall drop-off in signal intensity due to associated turbulence as lesions become more severe. There is an inherent bias in interpretation of current MRA images to overestimate degree of stenosis. The image of the true stenotic lumen is reduced by a combination of factors: turbulence that causes countercurrent blood flow, decreasing or negating the signal; loss or absence of laminar flow that produces better signals than turbulent flow; and volume of flow too small to generate a signal. A small volume of blood moving through a tight stenosis produces little detectable signal; in the most severe stenoses or near- occlusions with scarce flow of blood, a signal may not be detected at all. In 16 studies that were not uniformly blinded in which MRA was compared with carotid angiography and reported in the English literature, the concurrence rate in depicting lesion size ranged from 39% to 98%36 (Classes II and III). Overestimation of degree of stenosis was frequently noted and accounted for most disagreements (Class II).36 37
A meta-analytic review of MRA pooled as a noninvasive test with carotid duplex ultrasonography and carotid Doppler ultrasonography compared with conventional carotid arteriography has shown sensitivities between 0.82 and 0.86, specificities at 0.98, and test-effectiveness measures at or exceeding 3.0 when predicting occlusion32 (Class II). At ≥70% stenosis of the extracranial ICA, these tests have sensitivities of 0.83 to 0.86, specificities of 0.89 to 0.94, and test-effectiveness measures approaching 3.032 (Class II). At 50% stenosis, sensitivity of all three noninvasive tests ranges from 0.85 to 0.93, with a specificity of 0.9232 (Class II).
There is satisfactory anatomic correlation with conventional arteriography, but MRA generally overrepresents arterial stenosis, especially in high-grade narrowing.38 39 40 41 42 Carotid atheromatous ulceration is not reliably visualized with MRA.43 Research concerning ways to reduce signal loss as a result of stenosis is ongoing; contrast media, for example, might sufficiently increase the signal of flowing blood through stenoses to improve specificity.44 MRA has the advantage of not being operator dependent.
In patients with vertebrobasilar ischemia, vascular imaging may identify the source vessel of ischemic attacks such as the subclavian artery, extracranial or intracranial vertebral arteries, basilar artery, or their branches. A specific surgical intervention analogous to carotid endarterectomy as a verified therapy in the posterior territory does not exist; therefore, precise imaging and measurement of vascular stenoses as obtained with conventional carotid arteriography has not reached the level of importance attained in carotid disease. In this setting a technique such as high-quality MRA that provides a vascular overview45 of the extracranial and intracranial circulations is acceptable for evaluation of vertebrobasilar ischemia. Various MRA techniques produce anatomic images of the vertebral and basilar arteries and their main branches, including the posterior inferior cerebellar, superior cerebellar, and posterior cerebral arteries; the anterior inferior cerebellar artery appears with less consistency. An associated overview of the carotid arteries and the main intracranial branches can be obtained at the same time. Depending on vascular tortuosity and field placement, some vessel segments may not be seen on MRA because of planar exclusion, not disease. In the special case of fibromuscular dysplasia, MRA lacks the capability to distinguish this condition from long segmental atherosclerosis or dissection and may not cover the entire vertebrobasilar and subclavian system.
The agreement of MRA with conventional carotid arteriography in evaluating intracerebral vascular pathology reaches a mean of only 62%36 (Classes II and III), which is less than with extracerebral carotid pathology.
MRA alone is often not sufficient for study and analysis of blood flow and blood vessel anatomy. When MRA is combined with duplex ultrasound, sensitivity and specificity improve but still result in misclassification of 3% of patients showing negative noninvasive test results but carotid stenosis ≥70% on carotid angiography. Misclassification occurs in 9% of patients with occlusion on noninvasive test results but some degree of luminal patency on carotid angiography32 (Class II). When MRA and duplex ultrasound findings agree, some practitioners suspend use of radiographic angiography, reserving this technique for disparate results46 (Class III). Overestimation of degree of carotid stenosis without concurrent conventional angiographic measurements could lead to an excessive number of surgical procedures.
In summary, the limitations of MRA are relative unavailability, high cost compared with other noninvasive tests, sensitivity and specificity insufficient to establish an indication for carotid endarterectomy, and claustrophobic reactions.
Exclusions: MRA is not indicated for patients with intraorbital or intracranial ferromagnetic fragments, aneurysm clips, otic or cochlear implants, old prosthetic heart valves, pacemakers, and neurostimulators, or when agitation and severe claustrophobia cannot be resolved.
Source: AHA Journal
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