Limb Salvage of the Diabetic Foot by Michael E. Edmonds & Bauer E. Sumpio

Author:Michael E. Edmonds & Bauer E. Sumpio
Language: eng
Format: epub
ISBN: 9783319179186
Publisher: Springer International Publishing

21.6 Non-Contrast Techniques

Newer unenhanced MR angiography sequences with use of flow-sensitive dephasing (FSD) –prepared steady-state free precession (SSFP) , and ECG gating use a subtractive approach similar to CE MRA but without contrast [20]. They rely on blood signal differences in arteries and veins in systole and diastole. Essentially arterial signal is only present in diastole with venous signal present in both phases. Thus the systolic venous mask can be subtracted from the diastolic acquisition to eliminate the diastolic venous signal. Several studies have shown adequate angiographic imaging using these techniques but images may be degraded by signal contamination from deep veins and soft tissues [21].

A further development of this technique known as Quiescent Interval Single Shot, or QISS, is outlined below and seems to offer promising results compared to standard 3D turbo spin echo (TSE) based flow/subtraction methods [22]. The technique relies on saturation pulses to annul the background signal, with a further pulse, inferior to the slice of interest, to eradicate venous flow signal. Following these pulses there is an interval (the quiescent interval), which allows arterial blood with unsaturated spins to flow into the slice and generate signal on sampling. An axial slice is sampled and then automatically stacked with adjacent slices to produce a volume of data. This axial acquisition technique negates the possibility of accidental exclusion of anatomical segments of artery as seen with coronal slab CE MRA techniques. Studies at 1.5 and 3 T have shown results comparable to CTA /CE MRA and DSA [23–25]. Importantly the calcific artefacts and difficulty in stenosis evaluation seen with CTA are less problematic with QISS, as expected from the differing imaging techniques.

QISS-MRA still suffers from specific artefacts and may have a lower rate of assessable segments compared to conventional CE MRA [26] as well having the inherent limitations of MRI in some patients. The technique also requires adequate fat-suppression which may be difficult to achieve in the feet. This requirement also makes the sequences more sensitive to magnetic susceptibility artefact. As venous flow is magnetically saturated, reversed or in plane arterial flow may not be well visualised, resulting in artefactual occlusions and overestimation of occlusion length (Fig. 21.9).

Fig. 21.9(a) Non contrast magnetic resonance quiescent interval technique (QISS-MRA) and (b) contrast enhanced MR angiography (CE MRA) in the same patient. (a) The proximal anterior tibial artery appears occluded (arrow) on the QISS sequence due to artefact likely due to horizontal/upwards orientation resulting in loss of arterial signal as flow is insufficient to fill the vessel with unsaturated spins due to the in plane orientation. (b) The artery is shown to be widely patent on the CE MRA acquisition (arrow) There is however evidence of motion artefact on the CE MRA with the subtraction mask and acquisition mismatch causing blurring at soft tissue/bone interfaces and of the vessel outlines

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