Limitations of Tof Mra

Despite the fact that "time-of-flight" MRA gave encouraging results compared to DSA (Tables 1-3) [ 13-22], it never became universally accepted as an acceptable alternative to arteriography primarily

Table 2. Indications for MRA

Potentially any patient in whom catheter angiography is requested

Patients in whom catheter angiography has failed to demonstrate all of the relevant arterial segments Patients with poor or absent arterial access Patients with prosthetic vascular grafts Patients with impalpable femoral pulses

Patients with common-femoral, iliac or aortic occlusion which prevents appropriate placement of a catheter Patients with any other contra-indication to arteriography Patients with renal impairement

Patients with hypersensitivity to iodinated contrast material Female patients of child-bearing age because of the inherent limitations of an "inflow" based technique, as follows.

1. Studies, although non-invasive, were extremely time consuming. Long scan times were enforced by three limitations, all related to the need to maximize "inflow" as follows:

• Mandatory use of the axial scan plane (gives "worst-fit" geometry for the lower extremity arteries which predominantly run in a head-foot direction)

• A relatively long TR is essential (i.e. substantially greater than the shortest TR available and "long" in relation to that used for CE-MRA) because the "inflow" requirement mandates use of TR of 30msec or greater (adversely affects scan time compared to use of shortest available TR of 5msec or less on most systems)

• Slow, absent, or in the case of severely diseased arteries, reversed diastolic flow mandates use of cardiac triggering with a further time penalty.

2. Images were prone to artefactual over-estimation of the degree and length of stenosis, because of intra-voxel dephasing secondary to turbulent, slow and pulsatile flow. This is one of the inherent drawbacks of the technique, however, it is particularly troublesome both in regions of stenoses and also in locations where the vessels follow an "in-plane" course (common iliac artery and horizontal initial part of the anterior tibial artery).

3. The length of occlusions was frequently overestimated due to elimination of the retrograde component of flow in patients with distally reconstituted vessels by the trailing inferior saturation pulses designed to suppress venous flow.

Thus, TOF MRA has failed to offer a viable non-invasive screening test to conventional arteri-ography, and has not had a major impact on clinical practice.

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