Tissues with very short T1 relaxation times, such as fat, methemoglobin after intracranial hemorrhage or in fresh thrombi, and contrast enhancing structures, pose a problem for TOF MRA. As a result of inefficient saturation, these tissues often produce a very bright signal which may render them undistinguishable from flowing blood. For example, methemoglobin in a hemorrhage (Fig.
Methemoglobin in thrombosed vessels (cavernous: sinus thrombosis) may mimic blood flow (i.e., vessel patency) Workaround: compare MIP with pre-contrast T1 images or use phase contrast MRA Short T1 tissues (fat, bleeding, tissue that take up contrast) may simulate vessels Pulsation artifacts in CSF may simulate vessel lesions
Signal loss occurring with turbulent or very slow flow causes overestimation of stenosis and artifacts in the depiction of aneurysms
Signal loss due to susceptibility artifacts (coils, clips) Signal loss in case of in-plane flow (2D) or slow flow (3D)
Overlap of arteries and veins after contrast administration, particularly in intracranial MRA
Fig. 18. Methemoglobin in an intracranial hemorrhage. Left. MIP reconstruction. Right. T1-weighted image
18) may be mistaken for an aneurysm while occluded vessels, such as sinus thrombosis (Fig. 19), may be misinterpreted as perfused. Confusions of this kind can be avoided by comparing the TOF data with precontrast T1-weighted acquisitions.
As shown in Fig. 20, pulsating cerebrospinal fluid (CSF) may cause artifacts on TOF MRA. In this case the 3D TOF multi-slab technique provokes strong inflow enhancement at the entrance of the upper slab, generating a bright signal of the CSF in the cerebral aqueduct of Sylvius on the sagittal reconstruction of the source images (Fig. 20 c). In the projection reconstructions obtained by MIP postprocessing (Fig. 20 a,b), this structure may be misinterpreted as a vessel lesion.
Distal to a stenosis or near arteriosclerotic alterations of the vessel wall, turbulent and accelerating flow may lead to signal loss due to spin de-
phasing. The degree of stenosis may therefore be overestimated (Fig. 21). The problem can be reduced, if not entirely eliminated, by the use of very short TE values. In addition, the low flow velocities that may exist in severely stenotic vessels may lead to increased saturation effects and hence reduced flow contrast.
Similarly, the evaluation of aneurysms is susceptible to artifacts caused by turbulence or very slow flow.
Further pitfalls associated with TOF MRA derive from the fact that GRE sequences in general are sensitive to distortions of the magnetic field originating from metallic implants (clips, coils, etc.). Likewise, signal loss due to magnetic susceptibility artifacts may simulate an interrupted vessel, while signal decrease resulting from saturation effects is always a major problem with TOF MRA.
In 2D TOF MRA, signal loss should be considered if vessels are running within the imaging plane. It is therefore essential to place the slices perpendicular to the vessel. In 3D TOF MRA, saturation occurs if thick slabs are applied to vessels with slow flow.
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