Pitfalls and Limitations

While pedal MRA is relatively straightforward to perform, several pitfalls await the unwary practitioner. The two most troublesome are venous enhancement and inadequate vessel visualization, both of which are related to timing (see discussion in Imaging Protocol above). While venous enhancement is not typically a concern with TOF, it can easily render CE-MRA uninterpretable (Fig. 4b). Venous enhancement tends to be much more problematic in patients with ischemic soft tissue disease - i.e. the exact patient population in need of pedal imaging [9]. Nonetheless, if a careful timing bolus is performed, significant venous enhancement can usually be avoided. Luckily, in cases where venous enhancement is most problematic (diabetics, soft tissue disease), we find the TOF images are of generally good quality, likely a reflection of the fast arterial flow. Thus we are, among others, proponents of always performing both TOF and CE-MRA for the pedal arteries [23].

Another pitfall lies with spatial resolution. Even with the ~1 mm isotropic resolution advocated here (and all published pedal MRA to date falls short of this), this remains much inferior to the 0.3-0.4 mm resolution routinely obtained with DSA (compare fine vessel detail between Fig. 2 and 3) [9, 14, 21]. As previously stated, resolution requirements are a minimum of 1/3 the size of the vessel of interest in order to accurately characterize a stenosis [33,34]. This assumes adequate SNR and CNR, and likely explains the poor inter-observer agreement in interpreting MRA studies in general, as if a vessel edge is poorly defined, it is extremely subjective where to place an electronic caliper to measure lumen size. Zero filling to as high a factor as possible (minimum of 512 in both frequency and phase and a factor of 2 in slice) helps to smooth out the vessel edges, and is particularly helpful for reformats, but is not a substitute for true spatial resolution [39]. Very often, MRA practitioners sacrifice resolution in the slice (z) direction as compared to the frequency and phase direction, the extreme example being true 2D MR DSA as advocated by Wang et al [24]. Sacrificing z spatial resolution becomes a problem when examining a vessel in a plane other than the acquired sagittal plane, something often required to sort out complicated pedal anatomy, and certainly one of the biggest advantages that 3D MRA has over planar DSA (Fig. 7c, 7d)

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