Problems and pitfalls of the MRI angiogram technique

      As with any imaging technique, MRA has its own artifacts and problems which must be recognized in order to avoid misdiagnosis. In general, MRA displays beautiful normal anatomy, but in the presence of significant vascular disease, technical shortcomings may surface. Pulse sequences can be adjusted to minimize these adverse effects, or a combination of MRA techniques can be used to distinguish artifacts from real disease.

Complex Flow

      Turbulence, nonlaminar flow, flow separation, vortex flow, eddy motion, collectively called "complex flow", result in intravoxel phase dispersion and loss of signal on both TOF and PC MRA. The signal dropout causes defects in the vascular image that simulate disease. This effect is often observed in the petrous and cavernous segments of the carotid arteries. It is more of a problem with 2D methods because of larger voxel size and longer minimum TEs. Unfortunately, complex flow is also common at sites of stenosis, areas that must be visualized in order to make sound clinical judgments. Of the four MRA methods, 3D TOF is affected least by complex flow signal loss because of small voxel size and the shortest TE.

Slow Flow

      Significant vascular stenoses often result in slow flow distal to the site of stenosis. The reduced velocities produce increased saturation effects within the imaging volume and decreased signal intensity within the vessel lumen. In cases of severe stenosis with markedly reduced distal flow, intraluminal signal may become imperceptible and simulate a complete occlusion. Slow-flow signal loss is particularly a problem with 3D TOF imaging of vessels deep within the imaging volume. Similarly with 2D TOF, vessels flowing transversely within the slice plane are more likely to show saturation effects. The MIP processing technique contributes to the problem because it ignores low signal intensities that fall below a certain threshold. With PC techniques, normally effective arterial VENC factors may fail to image slow flow distal to a stenosis.

Flow Stasis and Recirculation

      These flow phenomena occur normally within the carotid bulb, and also in diseased vessels, such as beyond an area of stenosis, in large ulcerations, and aneurysms. They result in signal loss due to saturation effects and intravoxel dephasing. Corrective measures mentioned above are partially successful at imaging these difficult areas of vascular anatomy.

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Thrombus

      Blood degradation products can interfere with vascular imaging. Methemoglobin within thrombus has a short T1 relaxation time and generates high signal intensity on TOF images, simulating intraluminal flow. PC contrast methods are not affected by methemoglobin and can be used to distinguish thrombus from flow. Both TOF and PC MRA are subject to signal loss from the magnetic susceptibility effects of deoxyhemoglobin and hemosiderin/ferritin. This is mostly a problem in partially thrombosed aneurysms. The patent lumen of the aneurysm may be obscured or the margins of the residual lumen may appear indistinct.

Advantages of Contrast MRA

      Gd-enhanced MRA is a very robust technique that solves many of the problems associated with noncontrast time-of-flight and phase-contrast methods. Large fields of view can be used to image the cervical-cranial circulation from the aortic arch all the way superiorly to the circle of Willis. The addition of gadolinium essentially eliminates problem of saturation effects from slow flow and stasis. The very short TE markedly reduces signal loss due to complex flow and intravoxel phase dispersion.